What are the differences between ACARS and CPDLC?

CPDLC
CPDLC is used for communication to/from ATC facilities. As the name suggests, it's a controller—pilot link. An aircraft with CPDLC has one of the following panels/interfaces:
787 example:

The pilot can accept, cancel, or reject an instruction.

Uploaded instructions are automatically fed into the different MCP windows, for example UL (uplink) 250 for the speed. And a transfer (XFR) button to activate it. The RL in the heading window is the turn direction.

For route changes, those are fed into the CDU. There's also a text reply option via the CDU (multi function display on the 787). The message also appears on the aux display (outboard side of the display unit for the PFD):

The pilots can also send requests.

A330 example:
(wikimedia.org) The Datalink Control and Display Unit (DCDU) on an Airbus A330, the pilot interface for CPDLC messages.
Similar system but without integration into the autopilot control panel.
ACARS
ACARS is accessed via the control display unit, the same unit for controlling the FMC. Messages can be printed. It's mainly for company communication (between an airline and its pilots). The company uses the ACARS for Aeronautical Operational Control.

Sending wind aloft data, expected delays, METAR, load sheets, etc.
Receiving position reports, systems status, etc.

Also, messages can be relayed to/from ATC via the company during communication difficulties, for receiving delivery clearances, submitting oceanic position reports, etc.

Summary:

CPDLC is an ATC tool, and ACARS is a company tool but it can be used to relay messages to/from ATC.
CPDLC coverage is not global yet, it depends on the FIR to have it.
CPDLC utilizes VHF Data Link (VDL) and satcom. ACARS uses VHF, VDL, HF, HFDL, and satcom.


How to execute a gear up landing on aircraft with low propeller?

Not only some, but in fact most GA propeller-driven aircraft have propellers that extend below the bottom of the fuselage. (The only exception that quickly comes to mind is the Cessna Skymaster; I'm sure there are plenty of others.)

Note that the aircraft will have some secondary method of gear extension, such as gravity drop, a manual crank, or a blow-down bottle. But there are still times where the gear cannot be extended and locked.

In this situation, you've got one emergency. Don't make it two emergencies by shutting down your engine. There's an aphorism - "If the gear fails, the insurance company just bought the airplane."1 Without gear, you're going to do some damage: scraped belly, probably tearing off some antennas. The aircraft will already require maintenance; the extra time and cost of an engine teardown is insignificant compared to the increased risk from a higher workload and a landing that the pilot may not have ever practiced before. (The approach with gear up and engine out will be different than that with gear down and engine at idle in a simulated engine failure.) Don't risk your life to save the insurance company money.

Additionally, just stopping the engine isn't sufficient to prevent a teardown. You must also rotate the prop so that it's horizontal, which requires slowing substantially as you pull the mixture to stop it from windmilling, and then using the starter to move the prop.2 You'd need to prevent any substantial contact between the propeller and runway, as Lycoming's Service Bulletin on propstrikes that require inspection and repair (Continental's is similar) includes the situation of

  Any incident, whether or not the engine is operating, where repair of the propeller is necessary

After all of that, there are some best practices:

Declare an emergency. ATC will help you, have equipment ready on the ground, and clear traffic around you - but they need to know about it first.
Use your checklist. An airplane with retractable gear will have an entry for gear failure; follow it. In particular, you'll shut down the electrical system before landing, and probably brace open a door for exit.
Land on a paved surface. Dirt or grass may seem softer, but they aren't as smooth. According to an AOPA flight-training newsletter, "Statistics suggest that putting the airplane on the asphalt is likely to cause less damage to you and your airplane than putting it on the grass."

(1) I've also heard "The insurance company buys the aircraft when you take off; when you land safely, you buy it back."

(2) Or you could do it in the flare - a suggestion so dangerous I include it only for completeness.


According to the image below it seems that the APU fuel pump at the aft side of the center fuel tank.

And this is the view from inside the center fuel tank looking aft:

(instagram.com)

Note the pump still draws fuel from the left line, and not the center tank, it activates automatically if the left side engine fuel pumps don't provide the required pressure.


What damage can occur and what penalties will you face for flying a quad rotor into a restricted airspace?

The legal issues are covered in another question. The FAA doesn't currently regulate recreational model aircraft operations, which includes quadcopters.

Were you to actually fly it over the fence, you could be charged with trespassing. If you had any intent to cause harm, other charges might apply (up to attempted murder if you were trying to bring down an airliner).

  Can I crash a plane with it?

Not likely. If you flew the quadcopter into a jet engine intake, you might be able to shut down the engine (similar to ingestion of a bird). This would really annoy the flight crew but would normally not result in a crash. A strike in any other place is probably going to merely dent something on the plane, and utterly destroy your quadcopter. For a small plane, shattering the windshield is a possibility, and this would be a serious hazard that could cause a crash, but is quite unlikely. Without some kind of advanced guidance system, it's going to be very hard to hand-fly the copter into a plane at all, let alone precisely target a vulnerable point.

The bottom line, though, is don't do it.

  Is this a new terrorist attack vector?

Maybe! The use of unmanned aerial systems by terrorists is a subject of concern and research in the defense community. The RAND Institute has published a whole book about it.

Drones are already being used by drug cartels to smuggle drugs across borders. There have also been some high-profile incidents where terrorists were able to tamper with, and in at least one case eavesdrop on, a U.S. military drone by obtaining and decoding its live video feed. I don't know of any publicized instances of someone intentionally causing damage with a UAV or model airplane, but I'm sure the terrorists are thinking about it.


Why have different flights with the same flight number?

As far as I know, this practice is mostly just limited to very large airlines in the U.S. The reason for this is that the airlines are so large that they'd literally run out of flight numbers in some of the allocated ranges. Usually the 9000 range is reserved for special flights (e.g. repositioning flights, or, as discussed in the answer to the linked question, when a different callsign is needed,) but other ranges are also reserved for things like codeshares onto the flights of partner airlines and for flights operated by regional carriers instead of by mainline.

For example, this post from this flyertalk thread lists the following ranges used by Delta observed by a frequent flier on that airline:

  GoJet: 6000s
   Endeavor: 3000s
   Shuttle America: upper 2000s to lower 3000s.
   Compass: 5000s
   SkyWest: 4000s, 7000s
   Mainline: 0001-2999

From personal experience, I've noticed that partner airline codeshares on Delta are usually up in the 8000 range (maybe also part of the 7000 range?) Delta has a lot of partner airline codeshares, so quite a lot of the flight numbers go to this. Delta mainline operates a lot of flights. According to Wikipedia, Delta mainline and its regional carriers operated 5,400 flights per day as of October 2016. Having only 3,000 flight numbers available to mainline means that they'd almost certainly not have enough if they didn't reuse some of the numbers.

In total, according to Delta, their route network, include partner airline flights, totals over 15,000 flights per day. There are only 10,000 possible flight numbers and, when the reserved 9000 range is removed from consideration, only 9,000 possible flight numbers (or 8,999 if "Delta Flight 0" isn't allocated, which I expect it isn't.) As more and more of Delta's partner flights are given codeshares to Delta flight numbers, it would be very easy to exhaust the entire range of possible flight numbers.

An additional reason (possibly more relevant historically than today) is that it allows the airline to have a "direct" service to a city it doesn't actually fly to non-stop. Nowadays, people commonly use the term "direct flight" to mean "non-stop flight," but that's not the technical use of the term in airline-speak. The term "direct flight" really just means "we have a flight that gets you from A to B, but it might stop in C and D along the way."

Back in the day, this made more sense, as it wasn't as common for aircraft to have enough range to connect cities non-stop. So, a flight from New York to Seoul, for example, would typically stop in Anchorage to refuel before crossing the Pacific. Going even farther back, the first trans-Pacific flight (from San Francisco to Manila) had stops in Honolulu, Midway, Wake Island, and Guam. Since almost all of the passengers were going from San Francisco to Manila (not to, say, Wake Island or Midway, which were previously uninhabited,) it made sense to have a single flight number rather than using a different one for each segment. Nowadays, however, due to the much longer range of aircraft, there's not as much need for 'direct' flights with stops.

Pan-Am's SFO-MNL route via HNL, Midway, Wake, and Guam


Can Bell-lift distribution increase glider efficiency(L/D)?

Try to fly fast and you will see who hasn't understood the concept.
Just one datapoint: The lack of a rudder might be acceptable in a glider (but Heinz Scheidhauer, the pilot with most experience on Horten gliders, clearly said that at high speed the Horten gliders would be directionally indifferent) but not in powered flight. The fatal crash of the Ho-IX V2 can be attributed to loss of directional control after one engine failed.
On the SB-13 the vertical was replaced by two winglets which gave sufficient directional stability. However, when operating the plane, we often wished it had a conventional tail. Flight testing showed no clear advantage over current standard class gliders but operational restrictions were many.
All flying wings are susceptible to flutter because of their low pitch damping. In some Horten designs, flutter would limit the speed range at both ends.
Next: The low pitch stability forces you to use airfoils with low pitching moments. Trailing edge flaps can only be used in a very limited way, nowhere near what is usual on airliners. This limits the speed range almost as much as flutter does already.
And concerning the bell lift distribution: That lesson has been learned since maybe 70 years. Just look at the actual planforms: They are not elliptic and have little lift towards the tips.


How to record Intercom / ATC in a small aircraft like a Piper Archer?

For the GoPro specifically there are audio recording cables you can buy which connect to your intercom jack and provide a pass-thru connection for your headset.
There are other cable options available to support recording on different devices (e.g. iPhone/Android phones and tablets).

As is often the case with "aviation" accessories these can be somewhat expensive, but they're also pretty reliable drop-in solutions. As mah noted if you have a spare headset jack you're not using plugging into that can be much cheaper and just as effective.


How to log pattern work time during X-country?

You can log it all as XC. This is quite common. XC is all flight time to an airport more than 50 NM away from origin, even if you don't spend it flying in a straight line.


What is this aircraft (Le Bourget airshow 1965)

It's a Nord 500 Cadet. From Wikipedia:

  A model kit presenting the concept was first shown at the Paris Air
  Show at Le Bourget in 1965. The aircraft was driven by two ducted
  fans, with three blades per fan, mounted on short wings that were able
  to pivot between providing vertical and horizontal thrust.

  Two prototypes were constructed, one making its first (tethered)
  flight in July 1968.

This early VTOL concept aircraft was designed by Nord Aviation (formely Potez), the French manufacturer. They were known for their military Transall and Noratlas seen everywhere in Western Europe in the 60s.

This aircraft is similar to CL-84 and XC-142 designed at the same period. The only one to have flown:

(Source)

(Source)

Powered by Allison T63 (aka Model 250) turboshaft, 316 hp.
Maximum speed: 350 km/h.
Ceiling 4,101 ft.

The prototype used for static demonstration:

(Source)

Bonus: Trying to fly the prototype on Youtube.

A quite funny video I may say. With nearly all the attributes of my fellow countrymen of this era: beret, cigarette... just missing a baguette under the arm.


How far ahead does an airline know which aircraft you will be flying on?

It depends on the airline and their planning capacity. The simple answer is that they can't be sure of the tail number "of record" for a flight until the plane pulls into the gate at its destination. It's rare for a plane that departs the gate for a flight to not run that flight (it's pretty bad business for it to be common for flights to depart the gate and then come right back, or to be diverted, or otherwise not finish the route they left the gate to fly); it indicates a mechanical issue exists that did not develop and/or was not discovered until taxi or takeoff, and at some point in the flight (probably when departure control hands them off the the ARTCC) the plane could be said to have "run" the flight even if it ended up diverted.

Most of the equipment schedules (which tail number will fly which route at exactly what departure and arrival times) are generated by computer and can be set up months in advance if they really wanted to. However, that's usually an exercise in futility along the lines of predicting the weather more than a few days ahead. Airline scheduling, like weather, is susceptible to "chaos theory", because small deviations from the initial plan tend to compound over time instead of factoring out.

Ticket sales, rebooks and cancellations will affect the schedule days and weeks in advance of the flight. It's technically illegal to cancel a flight less than a certain number of days before its scheduled departure if it has even a single paying customer on the roster, but if a route is underperforming, the scheduler can reduce the scheduled daily flights along that route which could bump people who bought a ticket far in advance. Also, an underbooked flight can legally be cancelled if the plane is needed for an equipment change on a fuller flight, as cancelling the underbooked flight would cause less overall disruption than cancelling the full flight because that plane can't make its next hop.

I just mentioned mechanicals; that's a big reality check for a computer-generated schedule. If a plane cannot continue because of a mechanical defect, it gets pulled for maintenance. Airlines typically have a few planes parked strategically on the tarmac of various airports for a day, which can be activated to run a route, but they don't have too many of these since a parked aircraft isn't making money, and there isn't always one of these standby aircraft readily available as a substitute (though if a defect is found in flight the replacement can be on the way before the defective one lands to reduce the delay). Meanwhile, mechanicals are typically such a localized problem that the schedulers can't get away with just delaying flights systemwide, like they can often do with severe weather. They have to run the rest of their fleet on time, and that means either getting a replacement plane to the gate, pronto, or else sending that replacement plane empty to the cancelled flight's next destination, so the new plane can get those passengers on to their destination.

Obviously, severe weather can cause major ripples through an airline's schedule for days to come. If ORD or DFW has a "ground stop" due to severe precipitation or high winds, that will delay not only the flights scheduled to arrive or depart in that time, but every flight those tail numbers were scheduled to run for the rest of the day, as well as various scheduled connectors (those connecting flights, if they're not also under severe weather, often have to depart with whatever passengers they have otherwise there will be even more delays; then the passengers who have missed that connectot will have to be rebooked, on other airlines if need be).

That delay time also counts as "active" time for a flight crew; if a flight is delayed beyond a crew's end of shift, they often must get off the plane to comply with FAA regs and union CBAs. The scheduled replacement crew, however, was supposed to get on at the next stop hundreds of miles away; so, it's not just the planes that have to be replaced on a route when a flight is cancelled. In cases of a severe delay extending beyond a normal shift change, the crew either flies the plane if it's legal and are paid overtime, or if it would be illegal for that crew to continue, an on-call crew is brought in, flies the plane to the scheduled changeover point, then rides a standby seat (or in the extreme an empty plane) back to their home terminal or somewhere else they'll be useful when their shift really starts.

All of these considerations and more are weighed by the sophisticated algorithms now used to generate flight schedules. American Airlines calls theirs the "cancellator", and when a major hub like DFW, ORD or ATL sees a weather shutdown that's pretty much exactly what it does, sacrificing flights to and from that hub and some of its connectors to try to save what it can of its passenger load for the day (and thus the airline's profitability for the quarter).


How are the TRACON location IDs determined?

An "up/down" facility is where the tower and TRACON are in the same building and most of the traffic is to/from that airport (typically one class C or TRSA), so they share the identifier. They do provide services for satellite (class D or untowered) airports, but their traffic levels are usually negligible.

A "standalone" facility is where the TRACON is separate building, often on the grounds of a large airport but not the same building, and serves multiple airports (typically one or more class Bs plus any underlying class Cs, plus any satellites) so it gets a distinct identifier with numbers, which can't be confused with an up/down. I've never found a source for where the specific numbers come from; they don't seem random, but there isn't any obvious pattern either.

Recently, the FAA has created "consolidated" TRACONs that merge several neighboring standalone facilities. I'm not sure why the FAA has assigned them all-letter identifiers (e.g. PCT for Potomac Consolidated TRACON), but perhaps it's because there are so few of them (so far) and they cover such large areas that they're relatively well known and there isn't much risk of confusion.


Why does an aircraft not spin out of control right after touch-down?

how does it not immediately start spinning?

Physics, and engineering. But I guess that you would like a bit more detail, so let's try dive in.

  wheel brakes

They tend to have a stabilizing effect by shifting some of the weight on the front wheel and allowing the pilots to control the aircraft with more effectiveness.

  spoilers

These are automatically controlled by the Fly-by-Wire system, so if required they would be retracted.

  thrust reversers

If they are functioning properly, the resulting force will go through the vertical plane containing the C.o.G. (but a bit below it), so they do not pose much of a problem.

Another thing you have not mentioned, but I am going to add, is crabbed landings (such as this one), since in these situations the aircraft DO tend to spin a little.
In these cases, in the timeframe between touchdown and braking, 2 things help the aircraft remain on the runway: the vertical tail (with the rudder), and the position of the main landing gear w.r.t. the center of gravity of the aircraft. As it can be seen in this image taken from the lecture notes of the course I followed (the title translates to "Braking run in crabbed landing"), normal tricycle gear aircraft tend to be stabilized by their own landing gear in this situation, while the same cannot be said for old school taildraggers.

So, what really could cause a problem? The answer is wind gusts (especially if the wind is perpendicular to the runway), uneven conditions of the runway (say, a puddle or a thin layer of ice on one side of the runway will cause one set of main gear to have less grip than its counterpart on the other side, causing a yawing towards the side with more grip), asymmetrical failures in the thrust reversers, excessive braking while also turning the aircraft to realign to the centerline (particularly a problem if the distance between the two main gears is not sufficient).

How does the aircraft remain on the runway despite these possible problems? At high speeds, thanks to the vertical tail; and at low speeds thanks to the front nose gear. At in-between speeds, the situation is slightly more complicated: the tail is not much useful anymore, while the front gear would risk breaking if used to correct too much. So, for example, a FbW system can help through the asymmetrical use of spoilers.
An example of what could happen, but seen during a take-off, is illustrated in this video (video kindly provided by MichaelK in chat).


Is this photo of an aircraft in a flood genuine?

Yes, the photo is a "Fake".

The photo was taken by Gary Watt of Aberdeen Scotland and then photo-shopped using the  "Flood Filter".

PPRUNE explanation

  Hi all
  Amazing how this image gets around after posting it to someone some months back.
  Yes, it's a fake. It's my photograph taken at ABZ and the 'Flood Filter' done in Photoshop.
  Great tool and can produce some fantastic effects.
  For all the doubters, the engines ARE running as it was taxiing to 34 for departure.
  I can still confirm it\'s fake and if anyone else thinks different, ye need yer heids examined
  Cheers
  Gary Watt


Do thrust reversers make more noise than engine at same RPM?

Yes. Most of the noise you hear from aircraft is not mechanical noise from engines but disturbances in airflow around the airframe and engine outlets and inlets.
The airflow from reversing jet engine is much more turbulent than when the engine is forward pushing and thus more noisy.


What are the pros and cons of sealing the gap between the wing and the aileron?

This will become a long answer again.
The most important benefit of an internally sealed aileron is reduced control forces. Ailerons require lots of attention to keep forces small while still avoiding a negative force gradient (that is when the aileron runs into its stops all by itself). In most manually controlled airplanes the ailerons become stiff as a brick at high speed and the designers of airplanes like the P-47 and the Do-335 even used telescopic sticks which would give the pilot a longer lever arm at high speed, albeit at the price of a smaller deflection range. But the larger deflections were impossible to steer anyway …
Only when much care is put into aileron design will control forces be manageable at high speed. Examples are the Supermarine Spitfire or the Junkers Ju-88. To be able to control the plane with only two fingers on the yoke at 500 km/h required several hours of flight testing and adjustable aileron hinges in case of the Ju-88. Another fine design is the English Electric Canberra (known as the Martin B-57 to you Yanks) which had a spring between stick and ailerons. The pilot would only move one side of the spring while the other would move the control rod running to the ailerons. On the ground, you could move the yoke easily ±60° while someone else would keep the aileron from moving - that is how soft that spring was. Now you have a moving mass and a spring - what comes to mind? Flutter! This was a very risky design and it shows how desperate the engineers were to get the aileron forces down. And of course those ailerons were sealed. With an internal seal.
You can either use a brush seal or a cloth seal, but in all cases you need something protruding forward of the hinge line into a cavity inside the rear wing. This restriction will keep maximum deflections down to less than ±20° but will produce the best aerodynamic balance. In case of the Canberra, the chord of this forward plate was almost 40% of the aileron chord, measured from hinge to trailing edge. This is what it takes to compensate >90% of the aerodynamic force. The F-86 Sabre ailerons were similar and used a cloth seal while the Canberra used a brush seal (IIRC). The downside is the maintenance and care to keep the cavity clean. Especially water trapped there which freezes as the airplane climbs into colder air can produce unpleasant surprises.
If you only fly slowly, an external seal is also possible. This is popular on gliders where sometimes the hinge line is placed as closely as possible to the lower skin in order to seal the aileron gap with a sticky tape. More elaborate seals use a thin metal ribbon like in the picture below on a PA-28. The main benefit is a higher aileron effectiveness and a very slight drag reduction. The downside is a bit more friction and more care to keep the seal in shape.

Exernal seal (picture source). I would have preferred to show that of the ASW-20C here, but could not find a decent picture online. The ASW-20C tries to extend the laminar run on the lower side across the flap and aileron hinge line with its seal.
External seals only improve the aerodynamics and aileron effectiveness, but don't help with the stick forces. Since only one side can be the one with the hinge, the opposite side still has a gap.
So why would you prefer to have a vented aileron? First, this allows for higher deflection angles. Second, now the aileron nose can be shaped such that it helps to tailor the stick forces. Case in point: The Frise aileron.

Frise aileron (picture source)
At very small deflection angles the nose of the aileron stays within the airfoil contour. This allows for a stiff centering of the ailerons, so you can feel when the ailerons are neutral. Note that a Frise aileron should be combined with aileron trim! When the deflection increases, the nose of the trailing-edge-up aileron will poke out of the contour and produce a suction peak which forces the aileron to an increased deflection. This force now is transmitted via the linkage to the opposite aileron and helps to force it trailing-edge down. This means at moderate deflection the stick forces become pleasantly low. Once the aileron deflection is too high for the flow to stay attached to the aileron's suction side, the pressure distribution over the aileron changes, the forces increase rapidly and the stick forces tell the pilot that he has reached the limit of the useable range. At high speed this helps to prevent overstressing the wing while at low speed the aileron can be deflected further, maximizing its effectiveness. The picture above mentions drag - this helps to reduce adverse yaw a bit, but the main reason for a Frise-type aileron should be lower mid-range stick forces.
Why else would a vented aileron be preferable? The gap reduces aileron effectiveness but keeps the flow attached at higher deflections. This gives more control authority at the price of increased drag. But there can be more: A clever way to install vortex generators can be found on the Short Belfast:

Tail of the Short Belfast from below (own work). Note the vortex generators hidden in the hinge gap - they only move out into the airflow when they are needed: When the elevator is moved to large negative deflections. This is not an aileron, admittedly, but the same can be done at the aileron, too.
To summarize:
Pros of internal seals:

much lower control forces at high speed
good aileron effectivity

Cons of internal seals:

limited deflection range
have to be kept clean all the time

Pros of external seals:

No gap on one side, lower drag
good aileron effectivity

Cons of external seals:

a bit more friction
care is needed when handling the delicate seals

Pros of vented ailerons

allow to tailor stick forces and reduce adverse yaw
higher deflection angles possible, especially at low speed

Cons of vented ailerons

more drag
lower effectivesss


Would it be safer if Drones carried transponders?

It is of course a good time to begin considering this, and in truth, it's already a consideration. It's also an assumption by many in the private sector that the FAA will mandate this for commercial drones. Recreational drones will not be allowed to fly at high altitudes any time in the near future, however, as companies that aim to autonomously transport clients with drones are starting to test their aircraft, it will be very necessary.

The difference between recreational drones and commercial drones is largely their operating space. A recreational drone pilot has no good reason to fly anywhere near 1000 feet, though the technology is becoming more easily available to do so. If you operate under the assumption that the majority of drones are likely to be under 1000 feet, you can somewhat rule them out as factors in commercial operations, for now. Recreational drones are also very small, and they are generally looked at as having similarities to bird strikes - the difference is, there are effective ways to get rid of drones, and not as many effective ways to get rid of birds!

Beyond the fact that commercial transport drones would be flying at higher altitudes, they would also be much larger, and could cause much more damage in an accident. ADS-B will no doubt be a requirement for commercial drones (perhaps of a certain size or above a certain altitude), in the near future. The FAA actually broached this topic with the industry at EAA this past summer.


General dimensions for a uav type rc plane?

You can start by looking at plans here to get a general idea. When it comes to planes you can't just get some "general" measurements and wing it (pun intended). There are a lot of considerations when it comes to making things fly. With out going into specifics it would be best to use known working plans to build something as apposed to trying to throw something together that looks right.

To Answer your question directly that picture looks like the wing span is the the 3.5 foot range, the center section is about 2.5 feet long and tail poles are about 18 inches.


What simulation software do airlines use?

X-Plane offers a "Professional Level" which is mostly about licensing. However X-Plane also has an FAA certified version that if paired with proper controls is legal for certain training/instructional hours. You can find more info on the FAA's approved simulators here and here. The flight school I trained at had a certified sim from the 80's that was pretty miserable by todays standards. It was certified and had no screen, I would say it was "custom built" FWIW.

Many airlines may turn back to the manufacturer for recurrent training. Both Boeing and Airbus run facilities with full motion simulators. I would think they built the software in-house but I don't have any hard evidence on that. As the OEM builder they have access to genuine interior parts to build the sims.

For smaller aircraft places like SIMCOM offer training in full motion sims but do not list their software of choice. SIMCOM uses actual aircraft interiors (most likely from defunct airframes) for their sims.

If your really adventurous you can build a full cockpit sim yourself.


Are flexible pavements not recommended in hot climates?

Answering for ICAO, no there aren't such regulations/recommendations.

All what matters to the regulatory bodies is the ACN-PCN. If seasonal changes affect the PCN, then different PCN's are to be reported.

  The bearing strength of a pavement intended for
  aircraft of apron (ramp) mass greater than 5 700 kg shall be
  made available using the aircraft classification number —
  pavement classification number (ACN-PCN) method.

  [...]

  Different PCNs may be reported if the strength of the
  pavement is subject to significant seasonal variation.

  — ICAO Annex 14 Chapter 2

Flexible pavements (asphalt) are actually very common in hot climates, see here: Why are most runways made of asphalt and not concrete?


Is it possible to isolate COM1 and COM2 on a Garmin G1000?

(wikimedia.org)

On the audio panel installed between the two G1000s, there should be a COM 1/2 key. That will enable the split COM operation with COM1 going to the pilot and COM2 going to the copilot.  What avionics are in the aircraft that you have?

  Split COM allows the pilot to use COM1 while the copilot uses COM2. This proves very helpful for talking to an FSS en route without leaving the ARTCC frequency. Pressing the COM 1/2 key on the audio panel turns on Split COM mode (Garmin).


Is it possible to have an angle of incidence for a vertical stabilizer?

Some vertical stabilizers will have a slight angle of incidence to counter left turning tendencies. Older model Cessna 172s and 150/2s have a little metal trim tab that can only be adjusted by manually bending it from the outside. This will serve to adjust the vertical airfoil chord line angle in the aircraft slipstream or relative wind. The degree of incidence will be manufacturer and model specific.


Why manufacturers does not producing variable sweep wing in our current scenario?

This was, as you are likely aware, the swing-wing was a major design element in the 1960s. Everything from the F-111 to the B-1 to the 2707 used them.

The design does have the advantages claimed for it, notably the better performance over a wider performance envelope. But the weight was a killer - not directly (although that may be the case in the 2707) but in relative terms.

Invariably what happened was that other parts of the system were improving too, engines, avionics, etc. At some point the performance improvement of the swing-wing required more weight than the same improvement from a new engine.

So it's not simply that it was too heavy, but too heavy compared to other moving goalposts.


Do any aircraft use throttle steering during normal flight?

Yes, The Northrop Grumman B-2 Spirit Stealth Bomber.

  The leading edge of the wing has an internal structure that helps it
  absorb radar energy. The outermost wing segment features a "rudderon"
  or "deceleron", a vertically-split airbrake / rudder that
  simultaneously opens up and down. To act as an airbrake, both the
  decelerons are opened, while to act as a rudder only one is. This
  clever gimmick goes back to the original Northrop flying wings. There
  is an elevon inboard of the deceleron on the outermost segment of each
  wing, and then two elevons further inboard, on the next segment.
  Finally, there is a single control surface for pitch control on the
  "beavertail" at the center end of the aircraft, giving a total of nine
  control surfaces.

  The decelerons have to be opened about five degrees before they are
  effective, and in normal cruising flight they are left slightly open.
  However, this undermines stealth, so when the bomber is in hostile
  airspace, it uses differential engine thrust for yaw control.


Is there any FAA Regulation who may wash a private aircraft such as Cessna Skyhawk or Citation?

There is no regulation on who can wash a plane. You'll probably find that quite a few planes (at least of those used for instruction) are in fact washed by student pilots.


What vertical separation is required in a holding stack to avoid wake turbulence?

The minimum vertical separation in a holding stack is no different from the minimal vertical separation used elsewhere. This is almost universally 1000 feet. In countries where Reduced Vertical Separation Minima (RVSM) has not been implemented, 2000 feet of vertical separation is required above FL290. 2000 feet is also needed if a non-RVSM equipped aircraft (such as certain older military planes) is flying above FL290. However, not many holding stacks grow to above FL290.

A point to consider is the fact that aircraft produce the most wake turbulence when they are flying slowly for landing. So since 1000 feet is enough during approach, it should also be plenty in a holding stack.

There has been some evidence that 1000 feet of vertical separation may not be enough for certain, heavy aircraft types. In 2017, a Challenger jet had an accident after passing 1000 feet below an Airbus A380 during cruise, presumably because the smaller business jet hit the wake turbulence from the A380. See AVHerald for details. However, this has not (yet) resulted in changed separation minima.

1000 feet is not enough to prevent planes from "flying in each other's wake turbulence" as you say, but in most cases it should be enough to ensure that the wake turbulence has weakened enough to not pose any threat to the lower aircraft. Also remember that wind can have a significant effect on the spread and dissipation of wake turbulence, so on days with a certain wind direction and velocity, the wake turbulence may survive longer than other days.

But the answer to your question remains the same: the vertical separation to be applied in a holding stack (below FL290 and during RVSM) is 1000 feet.


According to Wikipedia there are still 8 JU-52's flying today. Which one do you remember seeing flying?

List of airworthy Ju 52s


How to design the blade twist angle in propeller design?

You should be twisting the profile chord, like in the picture. Why is your tip so small, that is where most thrust is created.

 Source


How are Cd and Cl calculated from Cp data?

Without the local inclination of the airfoil surface you will not be able to achieve exact results for drag. You need to integrate just the component orthogonal to the flow direction at infinity for the lift coefficient (because it is defined that way) and parallel to that direction for the pressure part of the drag coefficient.

While it is OK to neglect friction for lift, it does contribute about half of total drag when the flow is attached, so it should not be neglected for calculating total drag.

To illustrate how important the local inclination is, here the c$_{\text p}$ plot of the Eppler 502 at a moderate angle of attack (3°):

And now the same thing, plotted with vectors. Arrows pointing away from the surface denote suction and vice versa.

Since the c$_{\text p}$ plot should use the projected length along the X-axis, this inclination effect is already considered when you integrate the pressure coefficient over chord. Treat chord length as a dimensionless number that runs from 0 to 1. Since the angle of attack is 0°, you need no other correction and the net surface under the c$_{\text p}$ plot is your lift coefficient. Use the results at the six known span positions and interpolate between them.

In principle, the same applies to the drag coefficient, but now you would need a plot which projects the pressure coefficient on a coordinate which runs orthogonal to the flow direction. In inviscid flow, the pressure differences equal out, so it is only the friction-induced changes in the pressure distribution between inviscid and viscous flow which cause drag.

My best advice is to find airfoils with the same pressure distribution and use their drag coefficient values. Make sure you know the Reynolds and Mach numbers, because both have a higher effect on drag than on lift. Some quirks in the pressure distribution can give them away, like the location of the laminar separation bubble on the lower side of the c$_{\text p}$ plot above or the magnitude of the stagnation point pressure coefficient. Regarding the relative thickness of the airfoil: The amount of suction on both sides of an airfoil grows with its relative thickness.


Does high density altitude affect your landing speed?

Your $v_\mathrm{ref}$ does not depend on density altitude since it is given in Indicated Airspeed, which already accounts for density effects. However, the True Airspeed and therefore also Groundspeed will be higher at a higher density altitude, resulting in more runway required to stop.

The Flight Safety Foundation has a nice summary of factors influencing $v_\mathrm{ref}$:

  Factors Affecting the Final Approach Speed

  The following airspeed corrections usually are not cummulative;
  only the highest airspeed correction should be added to VREF
  (unless otherwise stated in the AOM/QRH):

  Airspeed correction for wind;
  Airspeed correction for ice accretion;
  Airspeed correction for autothrottle speed mode or autoland; or,
  Airspeed correction for forecast turbulence/wind shear conditions.

  Gross Weight

  Because VREF is derived from the stall speed, the VREF value
  depends directly on aircraft gross weight.

  The AOM/QRH usually provides VREF values as a function of
  gross weight in a table or graphical format for normal landings
  and for overweight landings.

(FSF ALAR Briefing Note 8.2 - The Final Approach Speed)

Altitude, Density Altitude or Pressure are not listed as factors affecting $v_\mathrm{ref}$.


What is the maximum height of USAF fighter pilot?

To become an Air Force pilot, you have to be a commissioned officer, there are few ways you can do that. First you can go through the Air Force Academy, which is probably the most common. Another way is to be in an ROTC program through your college of choice. The last way I know of is to graduate college and join the Air Force through the officer candidate school (OCS).

Regardless of the method you choose, there are certain requirements you must meet:

Be a U.S. Citizen
Have any 4 year college degree or be within 365 days of attaining it
Minimum 2.5 GPA (although I'd be surprised if they took you that low)
Under the age of 28 by the board convening date
Have a standing height of 64-77 inches and a sitting height of 34-40 inches
Have no history of hay fever, asthma, or allergies after the age of 12
Meet USAF weight and physical conditioning requirements
Normal color vision
Meet refraction, accommodation, and astigmatism requirements
Distance vision cannot exceed 20/20 uncorrected or must be corrected to 20/20 or better
Near vision cannot exceed 20/40 uncorrected or must be corrected to 20/20 or better

If you've met those requirements you can then go through the Air Force Officer Qualifying Test (AFOQT) and through Military Entrance Processing Station (MEPS) for your intellectual and physical evaluation. If you pass those tests, you may be selected for the Officer Candidate School (if not joining through ROTC or Academy).

After that you go into undergraduate pilot training (UPT) where you spend a year learning to fly through academic and hands-on training. Depending on how you perform at UPT you will receive a seat assignment. Higher scores are selected for fighter training while lower scores move towards transport aircraft. You can voice your opinion on what you want to fly, but ultimately the decision is driven by the needs of the Air Force at the time you graduate UPT.

After UPT, you will move to Advanced Flight Training (AFT) which is between 6 months and a year long depending on the aircraft you've been assigned. After AFT you'll be assigned a squadron and location.

If you don't meet height requirements

There is a waiver process, although getting a waiver for height requirements is extremely rare (if not completely unheard of). The issue is that the ejection systems are designed for a specific height. This process starts by appealing a medical review board which you are allowed to write a letter to argue your position, however even with a waiver you would probably not be assigned a fighter aircraft. You would probably be assigned an aircraft that does not have an ejection system like transport or mid-air refueling aircraft.  As RhinoDriver mentions in the comments, the T6 training aircraft is also equipped with an ejection seat and if you can't fly the trainer, you can't progress on.

The only other alternative to be a US Air Force pilot outside of height requirements is to be an unmanned aircraft pilot.

Source 1: U.S. Air Force ROTC Website
Source 2: Department of Defense Article


Although that specific information may be impossible to get without starting up a Mojave and standing underneath with a sound level meter, it is generally understood that helicopter noise can easily exceed 100db. The CDC indicates that permanent hearing loss can occur after 15 minutes of exposure to this level of sound.
Here is a link to a study on helicopter decibel levels.
Here is a link to the CDC page on hearing loss.
Here is a link to a Purdue University chart, listing a Bell helicopter’s sound level at 100db flying 100 feet overhead.
I hope this helps at the VA. Thank you for your service.


Can small children be seated away from their parents/guardians on a flight?

This is what I could find. I looked on the Civil Aviation Authority's website and found this article.

Seating Allocation

  Families, children and infants

  The seating of children close by their parents or guardians should be
  the aim of airline seat allocation procedures for family groups and
  large parties of children.

  Young children and infants who are accompanied by adults, should
  ideally be seated in the same seat row as the adult. Children and
  accompanying adults should not be separated by more than one aisle.
  Where this is not possible, children should be separated by no more
  than one seat row from accompanying adults. This is because the speed
  of an emergency evacuation may be affected by adults trying to reach
  their children.

  Whenever a number of infants and children are travelling together the
  airline should make every effort to ensure that they can be readily
  supervised by the responsible accompanying adults.

From what I read, it doesn't look like there is a law set in stone, but instead it is recommended by the CAA to be put in regular practice by airlines.

If there was a law then Ryanair would have been in serious trouble for charging people extra  money to sit next to their kids.

Ryannair booking separates mum and tot


How does a TACAN signal change with bearing from the station?

Change in bearing doesn't change frequency. The ground station always transmits  pulses on a single assigned frequency. The illustrations in the link from @CrossRoads only show what would be seen by a receiver at 000 magnetic. For other bearings the whole envelope shifts while the reference bursts stay still. The antenna illustration shows reflectors on spinning drums that create an envelope whose phase difference from the reference bursts depends on bearing. My markup of the diagrasms in the link (MRBs are blue):


What kind of damage would low-level supersonic flight inflict?

The video is highly inaccurate in that area.

The actual damage the missile would have caused would have been largely radiological, from ejected radioactive material in its exhaust plume.

While windows and weak structures may receive damage from extremely low flying Mach 3 bodies, this would probably be limited to say some broken glass.

The plan was also never really considered. What was considered is having the weapon, now without its warheads, fly to a final target area and do a high speed dive into the ground, the kinetic energy alone combined with the spread of radioactive material from the rupturing nuclear reactor causing the damage there.

Mind that the damage (both physical and radiological) of the weapon flying overhead isn't clear. Different sources list different predictions.
Some way the damage would be minor, others that it would be severe.
But rest assured, the noise would shatter eardrums, the supersonic shockwave flatten crops and lightly constructed buildings, and the radioactive exhaust plume wouldn't have been healthy either.

I've always found this to be an excellent article on the project and the people surrounding it.

Edit: a very interesting article by the designer of the reactor about the problems inherent in the reactor design, and how he thought about the radiation hazard for people on the ground working with the missile:
Merkle public paper


What is the maximum crosswind limit for a C150/2 aircraft?

Section 4  of the C152 POH lists the demonstrated crosswind component as 12KIAS. The 150 had a few variants so finding a published number is a bit more tricky. The Aerobat's are sometimes listed as 13KIAS, sometimes 15KIAS - but in general its the same sort of range as the 152(ish!).

You will only ever see "demonstrated" next to these numbers, it is not the maximum possible - I regularly hear of people landing 152's over that 12KIAS.

The number does not change due to your license privileges, it changes based on what the pilot is comfortable with /able to handle safely. Personally I would be wary of any crosswind over about 9/10KIAS.

In an emergency almost everything is an exception. Except aviate.


Why is the seat belt mechanism on airplanes so different from the the one on cars?

As Roe pointed out you don't necessarily pull a flap on airplane seatbelts -- Sometimes (particularly in newer light GA planes from the 1990s and on) you push a button just like you do in a car, other times it's the "utility buckle" (flap buckle) - sometimes with a shoulder harness attach peg or other attach mechanism, and sometimes you'll find rotary buckles, where you turn a knob to release the belt (particularly common in aerobatic aircraft, with 4-point or 5-point restraints).  I've attached a little gallery of belt types below (and this isn't even complete - military aircraft hafe some downright funky buckles!).

This variety is why the FAA requires flight crew to instruct passengers in the operation of the seat belts.

So, why are the belts in airplanes different?  And why are there so many types?
Each belt is good at something different, and each belt has its own set of drawbacks.

The automotive style push-button belt is very familiar to infrequent flyers.
Most people who are going to get on a plane have been in a car (they probably rode in one to the airport), so familiarity with automotive-style seatbelts can generally be assumed. This is great when you take someone for a flight who isn't a frequent airline passenger.
The drawback to the automotive belt is it's not as easy for rescuers to operate, and the push-button mechanism can jam - particularly in fires where the plastic shell can melt/fuse (in cars the belts are often cut by rescuers because it's easier/faster than dealing with the button). If your passengers are injured in an emergency and can't operate the belt it may be difficult for you to locate and press the button to assist in evacuation.
The "utility buckle" flap belt is easy for rescuers & cabin crew to operate.
The exact opposite of the push-button belt's chief drawback: It's really easy for cabin crew or rescurers to find a utility buckle on a belt by feel and yank the flap to release it (try it some time: Buckle someone into an airplane seat, close your eyes, and find/release the buckle. The hardest part is finding the belt if they're wearing bulky clothes). It's also relatively hard to jam a utility buckle, and relatively easy to inspect them to determine if they're damaged.
The drawback to the utility buckle is that not everyone has been on an airplane (or in a really old car that uses them), so they might not be familiar with "Lift the flap to release the belt", and in a panic situation it may take them a few seconds to remember what to do.
The rotary buckle is unique and kind of awesome for what it's built for.
Rotary buckles make it easy to attach 4 or 5 point restraints, and to release all or part of them quickly.  (The buckle shown in the photos is single-function - turn to release - but dual-function buckles exist where you can press the button in the center of the buckle to release the shoulder straps, or turn the knob to release everything.)
The big drawbacks to rotary buckles are their novelty: On the ground you usually only find them on 5-point racing harnesses, and in the air they're pretty uncommon too (mainly aerobatic aircraft as I mentioned earlier). They're probably the last type of buckle a passenger or rescuer would expect to have to deal with, and particularly with the dual-action buttons they can present a challenge in a panic egress (passengers will hit the button and release the shoulder harnesses, but they're still restrained by the lower portion of the belt - this can cost precious time for them to realize they have to turn the knob).


How is varying modulation depth achieved by localizer ground transmitters?

The beam strength decreases as you move away from it's own centreline,
so is it actually that the entire modulated signal strength decreases which
when de-modulated is effectively a difference in amplitude modulation depth?

Your question is a really good one. The depth of modulation is the ratio between the modulation amplitude and the carrier amplitude. Changing the relative strength of a signal changes both amplitudes in the ratio, but keeps the ratio unchanged. No DDM can indeed appear this way.
The 90 and 150 Hz tones are not used directly. The heart of the system creating the DDM is two signals: CSB (carrier and sidebands, see below what is a sideband) and SBO (sidebands only) and surprise! they are containing the same quantity of 90 and 150 Hz tones and are transmitted in equal quantity on both sides of the runway.
Now the key points of ILS:

SBO phase depends on the runway side (+/-90°, CSB is kept unchanged)
Phase of the 150 Hz tone is inverted in SBO (90 Hz is kept unchanged).

As SBO contains only the sidebands when CSB and SBO interfere in space, only the sidebands can actually interfere, the carrier found in CSB is unaltered.
When the sidebands interfere, the way they are constructed (by tweaking phases) makes 150 Hz more prominent on the left of the runway, and less prominent on the right side (details below, drawings will clarify). So:

The carrier amplitude hasn't changed in the process
The sidebands amplitude has been altered.

As the sidebands are the modulation, in effect there is a change in the DM (the ratio between modulation and carrier amplitude), and a DDM appears magically, increasing as we move away from CSB, that is as we move away from the centerline.
Countless oversimplified descriptions put the focus on 90 and 150 Hz, each one allegedly transmitted on a specific side of the runway (cf. image above). They miss the point the ILS is not based on amplitude comparison, but on depth of modulation comparison.
Two last points:

A precise location cannot be determined by simply using received signal strength (RSS) of two or more signals. RSS-based positioning improved by averaging algorithms is commonly found for rough positioning (e.g. in cellular networks) because the cost is marginal, but RSS depends on many factors which cannot be controlled.

On the other hand, the phase of a signal remains unaltered until the signal is reflected, diffracted or enters a material with a different refractive index. By taking appropriate protection measures in the Fresnel zone, the phase-based ILS is therefore very reliable. Phase is used in many sophisticated radio applications. Missing the phase role in the ILS (and conventional VOR) is actually missing the cleverness of these technologies.

If this summary increased your curiosity for more details, just follow on, but for a good understanding, we'll need to first clarify a few notions: Phase, modulation and sidebands.
Phase and phase comparison
When representing a signal, e.g. a sine signal, we can look at different parameters. Often we draw the amplitude as a function of the time (right side):

Two representations of a signal: Phase/amplitude and time/amplitude
We can also imagine a circle with a scale from 0° to 360°, a point traveling on this scale in the time of a signal cycle (left side above).
The phase is the value of the angle. We are usually interested in the phase difference of two signals with the same frequency:

Phase difference
The two signals can be in phase (no difference), in phase opposition (180°), in quadrature (90°) or have any phase difference. The phase difference plays a major role when two signals interfere:

If the signal are in phase, the amplitude of the result is the sum of the individual amplitudes.
If the signals are in phase opposition, the amplitudes subtract.
For any another phase difference, the amplitude of the result is between zero and the sum of individual amplitudes.

In the ILS, basically one signal is sent for reference (CSB) and one signal is sent for comparison (SBO). SBO on one side creates additive interferences with CSB, and substractive interferences on the other side. But actually because 150 Hz is inverted in SBO, on the additive side, at the center of the interference area, 150 Hz cancels itself, 90 Hz doubles its amplitude. On the other side 150 Hz doubles its amplitude, 90 Hz is cancelled. As we move away from the center of the interference area, this additive/subtractive effect decreases until SBO is left unaltered because we are out of CSB area.
Now let's clarify the part related to the modulation and depth of modulation.
Amplitude modulation, depth of modulation, sidebands
Modulation relates to a carrier, which is just a fancy name for a constant (usually sine) RF signal at a given frequency, therefore with all the wave energy concentrated at this frequency. The frequency of the carrier is usually very large compared to the modulation.
The modulating signal (or modulation) is the signal to transmit, it can have any shape, but in our case its shape is a 90 Hz or 150 Hz sine curve.
The amplitude modulation consists in changing the carrier instantaneous amplitude according to the modulation instantaneous amplitude. After it's done, the  carrier outer envelope reflects the modulation outer envelope:

Carrier modulated at 50%
The modulation can imprint itself at a variable depth (taller or smaller peaks). This is measured by the modulation index aka depth of modulation. The index value is an amplitude ratio: modulation amplitude / carrier amplitude.
The depth of modulation used for the LOC signal is only 20% (40% for the G/S signal) to leave some room for identification tones on 1020 Hz.
ICAO definition for ILS modulation depth:

The depth of modulation is the ratio of the amplitude of the modulation of
the 90 Hz or 150 Hz signal to the carrier amplitude. The DDM is the
modulation depth of the stronger signal minus the modulation depth of the
weaker signal.

Let's look at how energy is distributed according to frequency.

Spectrum of a carrier AM modulated by a sine tone
Before  modulation, all energy is concentrated at the carrier frequency (blue bar below). The modulation process creates two variable sidebands: lower (LSB) and upper (USB), depicted in magenta.
If the modulation index is 100%, the carrier contains 66% of the energy and each sideband 17% (mathematical explanation). When a 100 MHz carrier is modulated by a 150 Hz tone, the LSB is created at 100 MHz - 150 Hz, and the USB at 100 MHz + 150 Hz.
The larger the depth of modulation, the larger the energy transferred from the carrier to the  sidebands and the lower the energy remaining at the carrier frequency. Sideband energy and depth of modulation are the two sides of the same coin. In the next  paragraphs I'll talk about energy rather than depth, but it's the same.
Modulating the ILS signal with 90/150 Hz tones
The localizer signal is modulated by two tones, the reference at 90 Hz, the signal to compare at 150 Hz. Therefore each sideband energy is split between two  frequencies:

CSB signal spectrum
This signal, known as CSB (carrier and sidebands) will be used as-is in the localizer. However as explained earlier, the in-phase / out-of-phase element must be sent along with CSB.
From 90/150 Hz modulations to CSB/SBO modulations
The localizer transmitter sends an additional signal with an inverted 150 Hz, known as SBO (sidebands only). For this signal the carrier element is cancelled (DSB-SC modulation):

SBO signal spectrum with 150 Hz modulation inverted
The signal is sent in phase opposition with CSB on the left side of the runway, and in phase with CSB on the right side (this is done using phase shifters at the antenna level, see further down).
To summarize:

The localizer transmits CSB on the left and right sides of the array. CSB contains both 150 Hz and 90 Hz with no phase adjustment, and the carrier.

The localizer also transmits SBO which contains only the sidebands, but where the 150 Hz modulation is phase-shifted by 180° on both sides. The SBO signal itself is sent with a different phase on each half of the antenna array.

How CSB and SBO are sent by the antennas
The actual phase between SBO and CSB is adjusted to compensate for space modulation effects, and SBO is sent with a +90° phase relative to CSB for the antennas on the left of the runway centerline, and with a -90° phase relative to CSB for the antennas of the right of the enterline (the two sides of the array still have a 180° phase difference).
The combination of exact opposite phases for SBO signal cancels the EM field at the center, splitting the SBO radiation pattern in two portions, it seems there are two beams for SBO. CSB is radiated centered on this hole:

CSB + SBO radiation pattern
The elements compositing CSB and SBO must be synchrone. Phases are adjusted carefully to form the radiation pattern (i.e. align the loalizer with the centerline and set its width) and to ensure the linearity of the DDM in the central portion.
When spread in space, the three signals mix by space modulation, the principle the ILS is based on and which is more simply known as wave interference.
Space modulation
One signal (SBO) has its carrier suppressed preventing the carrier energy to be increased during the mix CSB+SBO. In contrary when mixing the sidebands from each signal, the space modulation adds the sidebands energy where the sidebands are in phase, and subtract the  sidebands energy when they are in phase opposition.
SBO is in phase opposition with CSB on the left side of the runway and is in phase with CSB on the right side of the runway.

Along the centerline CSB only is present, with equal energy for 90 Hz (green) and 150 Hz (magenta). The LOC indicator will be centered. In order to create a CSB prominent area along the centerline, SBO power is also reduced on the center antennas.

On the left side, 150 Hz modulation has been shifted by 180° in SBO, and SBO signal has been shifted 180° relative to CSB at antenna level. 150 Hz modulation is therefore in phase for CSB and SBO. Sideband energies are added, while energy for 90 Hz modulation is  decreased because 90 Hz modulation in CSB and SBO are out of phase. So there is more energy for 150 Hz (which means there is a larger depth of modulation). This effect is getting stronger as we move away from the centerline.

On the right side where 150 Hz modulation is out of phase for CSB and SBO, 150 Hz energy is reduced compared to 90 Hz  energy in the sidebands, and the difference is getting stronger as we move away from the centerline.

Past 10° from the centerline, CSB cannot be sensed, only SBO can, the LOC indicator will deviate to full scale left or full scale right depending on the phase of  SBO. Another signal is used to ensure a full scale deviation past this point (see clearance signal further down).
If you want to do some space modulation simulation, here are the details of the signals to mix (see details here):

$E_{CSB} = E_C \space \cos \space 2 \pi f_C t + E_{90} \space \sin \space 2 \pi f_{90} t \space \cos \space 2 \pi f_C t + E_{150} \space \sin \space 2 \pi f_{150} t \space \cos \space 2 \pi F_C t$

$E_{SBO \space 90} = K \space \sin \space 2 \pi f_{90} t \space \cos \space 2 \pi f_C t$

$E_{SBO \space 150} = K \space \sin \space 2 \pi f_{150} t \space \cos \space 2 \pi f_C t$

We talked about the sidebands energy. Regarding the carrier, which is only present in CSB, the space modulation result always contains the energy of the original CSB  carrier until reaching full scale (then the clearance signal provides the carrier because while all modulation information is in the sidebands, the constant carrier  is necessary to synchronize and lock the receiver and to retrieve the coherent CSB and SBO from the modulated radiofrequency resulting signal).
Where the DDM finally appears
Now let wrap up:

The carrier energy is not changed by the space modulation as it is present in only one signal (CSB).
The sidebands energy is increased or decreased in the resulting signal.

The energy of the sidebands varies relatively to the energy of the carrier, but wait... this is the same than saying the DM is varying (without actually changing anything at the transmitter level).
The DDM between 150 and 90 Hz, is a function of the receiver offset from the centerline (in the range -5° to +5° the function is quasi-linear).

The space modulation actually creates a DDM
The alignment offset is determined by sensing the DDM in the resulting signal.
Antenna array
The LOC antenna is an array system, usually made of a large number of individual log periodic antennas, to form a narrow directional beam.

Localizer array at Melbourne airport, source
CSB and SBO are not distributed equally to all individual antennas of the array. Before reaching the antennas, their amplitude is tuned to form the three sub-beams, and the SBO signal is phase-shifted:

Individual amplifier-phase shifter-mixer
We have now all the basic elements to draw a LOC system:

Components of the ILS LOC system
Example with an array of two antennas (and final answer to the question)
Let's imagine two antennas fed with our earlier CSB and SBO signals with SBO phase being shifted by +90° for antenna 1 and by 270° for antenna 2.

Two antennas with phase shift of 90° and -90° (270°)
Let's see what happens when the receiver is on the centerline at P0, and at any point P1. For this simplified explanation to work, distance to LOC array (D) must be much larger than distance between antennas and centerline (d).
Receiver on the centerline
The received signal is the sum of the signals sent by the two antennas. As the distance traveled is the same, the phase at the receiver location is also the same.  The signal at point P0 on the centerline is:
$$S_{P0} = K_1 \cdot (CSB - SBO) + K_2 \cdot (CSB + SBO)$$
With $K_1$ and $K_2$ being the attenuation in free space, as the attenuation is the same for both antennas, this can be simplified:
$$S_{P0} = K_1 \cdot [(CSB - SBO) + (CSB + SBO)] = 2K_1 \cdot CSB$$
In CSB, the 90 Hz depth of modulation is the same than the 150 Hz one (see previous figure) There is no difference in depth of modulation (DDM) between the  tones: DDM = 0%.
Receiver at point P1
The distance traveled is shorter for antenna 1, and longer for antenna 2. A phase lead and a phase lag respectively appear for antenna 1 and antenna 2. The phase  lead is also equal to the phase lag. After some simplification that can be read in the linked thesis:
$$S_{P1} (\beta) = 2K_1 \cdot (\cos \beta \cdot CSB - \sin \beta \cdot SBO)$$
with $\beta$ being the angular distance from the centerline. This formula shows:

when $\beta$ is null (receiver on the centerline), we have the result already seen in the previous case,

when $\beta$ is negative or positive, the SBO signal influence increases and the overall depth of modulation between 150 and 90 Hz tones follows the same trend  until reaching -100% or +100%, which correspond to the absence of CSB signal influence at the receiver side.

Actual principle of the ILS, from this source.
The range of difference of depth modulation obtained must be mapped on the actual standardized values for LOC needle deviation, reducing the full range to a  standardized guidance angle. The LOC specification are found in ICAO annex 10 to the Chicago Convention, volume 1, attachment C:

This setting is obtained by tuning the amplifier of the phase-shifter from each individual antenna (the amplifier is actually common to all antennas). This amplifier  reduces the SBO signal influence on the DDM.
Clearance signal
So far we have assumed the localizer signal is only composed of the course guidance mixing the two original signals CSB and SBO. But it is very difficult to obtain  an actual highly directional beam, like the course beam, without creating unwanted lobes:

Radiation pattern of beam antenna, source
The problem with the secondary lobes is an aircraft approaching aligned with one of them will see it as being the main lobe, and will approach the runway using a  wrong course. There are some techniques to minimize these unwanted lobes, but none is practical for large arrays like a LOC system. Alternatives are to block the  unwanted waves after they are radiated, or to flood them, the solution used for ILS.
A replica of the course, the clearance signal, is sent with the course signal to outshine the secondary lobes.
The clearance signal has a power level of 10 dB less than the course level (a tenth of the power) and is radiated with a less directional pattern (less individual  antennas to form the beam):

Course and clearance
Today's localizers use a dual frequency system, this means the course and clearance signals are sent on separate frequencies.
When an ILS has a published VHF frequency f, the localizer course signal is actually sent on f + 4.75 kHz and the clearance on f - 4.75 kHz. The two signals can use  the same aerials or dedicated antennas in the array. Example of signals send to each of the 14 antennas of a Thales LLZ array (refer to above for what are course,  clearance, CSB and SBO signals):

Thales Dual Frequency – Localizer Antenna

Most of the information about ILS come from the 1983 thesis presented by Capt Dennis M. McCollum, B.S.. Respectful thanks to the author.


Who would be allowed to operate with no navigation lights over a populated area?

Military aircraft actually aren't bound by FAA rules. They have their own set of rules to follow. Having said that, though, their rules are basically "Follow the FAA rules, with a few exceptions." And, yes, one of those exceptions is for lights. Military aircraft can turn off their lights after dark, but only if they're actually flying a mission in an active war zone, or if they're doing training in a restricted area that's been cleared of other aircraft.

According to FAR §91.209(b), "The anticollision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off." The "operating conditions" mentioned generally refer to a situation where the glare from the bright, flashing anticollison light reflecting off a cloud would blind the pilot. Note also that this only applies to the anticollision light, the position lights (the steady red, green, and white lights on the wings and tail) must be turned on from sundown to sunrise.

Since you didn't mention the sound of gunfire or bombs dropping, I think it's safe to rule out the "military plane in active war zone" explanation. And since there was a civilian 737 in the area, we can also rule out the "military plane on training exercise" explanation. Thus, I think its safe to assume that this was just some guy (probably practicing his night takeoffs and landings) who forgot to turn his lights on.


How do missiles steer (mainly air to surface)?

Take a look at the following image. What do you see?

The Fins, Wings, and Canards, as seen in the diagram, work in a similar fashion to rudders/elevators/ailerons to develop drag to yaw/pitch/roll respectively.
Fins: Most missiles use tail fins which give extreme maneuverability and high angles of attack. Often they are fitted with wings to give extra lift and improved range. Maverick, Harpoon, AIM-9X Sidewinder are good examples of tail fin controlled missiles.

Seen above: Grid fins
Wings: Generally, wings play the same role as for an aeroplane by providing lift, and are the oldest type of control surface. Moreover, wings cannot work independently and in most cases are used as vortex generators to improve the efficiency of the fins. In addition, they are mostly used in subsonic cruise missiles like Tomahawk and Sparrow AIM-7. The problem is that the wings must be long enough to produce necessary lift.

Canards: They work more effectively with great maneuverability on low angle of attack, and work the same way as fins. However, located near to the nose of a missile, canards can cause a missile to stall at high angles of attack, which limits them to short range. Almost all smart bombs and laser-guided missiles use canards.
Furthermore, the split canard is a modern development (found in missiles like Python-4 and AA-11) which uses two canards, one of which is fixed, and the second is located just behind the first set and is movable. The role of the fixed canard is to develop high power vortices for the movable canards, for better performance at high angle of attack.
Source


How can the breath of a pilot be recorded?

One of the channels of the Cockpit Voice Recorder is the microphone on each pilot's headset. This is much closer to the source of the breathing noise than your own ears. The microphone may also not have a wind filter so breathing on it will generate a lot of noise.


Its rare but it does happen. You can search the NTSB database here, the phrase "positive for alcohol" yields the best results as toxicology is fairly common in an accident so a lot of reports include "negative for alcohol" making "alcohol" alone a bad search term. I was able to find a few examples here, here, here, and here as well as some more report results.

according to this article

  ... Of over 10,000 pilots randomly tested for alcohol in 2010, only 12
  failed the test.

They don't cite any sources though.

This looks like it might be the original study and has slightly different numbers but this seems to include all "Aviation Employees" so pilots, crew, ATC and the related may have been included.

  Validity of Suspected Alcohol and Drug Violations in Aviation Employees

  Results:
  During the 11-year study period, a total of 2,284 alcohol tests and
  2,015 drug tests were performed under the reasonable-cause testing
  program. The PPV was 37.7% [95% confidence interval (CI), 35.7–39.7%]
  for suspected alcohol violations and 12.6% (95% CI, 11.2–14.1%) for
  suspected drug violations. Random testing revealed an overall
  prevalence of 0.09% (601/649,796) for alcohol violations and 0.6%
  (7,211/1,130,922) for drug violations. The LR+ was 653.6 (95% CI,
  581.7–734.3) for suspected alcohol violations and 22.5 (95% CI, 19.6–25.7) for suspected drug violations.

The FAA also has an interesting fact sheet you can find here quoting some accident numbers from the late 80's and early 90's as well as the pilots BAC.

Its worth noting that the regulations have changed a fair bit over the years. The current regulation for this is FAR 91.17 which establishes an 8 hour "bottle to throttle" time frame as well as a maximum .04 BAC.

  § 91.17 Alcohol or drugs.

  (1) Within 8 hours after the consumption of any alcoholic beverage;

  (2) While under the influence of alcohol;

  (3) While using any drug that affects the person's faculties in any
  way contrary to safety; or

  (4) While having an alcohol concentration of 0.04 or greater in a
  blood or breath specimen. Alcohol concentration means grams of alcohol
  per deciliter of blood or grams of alcohol per 210 liters of breath.

If you take a look at the original amendment creating 91.17 it reads

  Sec. 91.17 Liquor and drugs.
  (a) No person may act as a crewmember of a civil aircraft -

  (1) Within 8 hours after the consumption of any alcoholic beverage;

  (2) While under the influence of alcohol; or

  (3) While using any drug that affects his or her faculties in any way contrary to safety.

the important difference being that it does not establish a maximum BAC but leaves it open to the interpretation of "under the influence" which potentially left room for intoxicated pilots prior to the current verbiage.


Can large airliners do aerobatics such as loops?

Design

The key point is that these vehicles are not designed to perform such maneuvers.

A coast-guard lifeboat is designed to self-recover from a capsize and can roll through 360 degrees without significant structural damage. A cruise-liner would not survive being rolled over completely.

If a vehicle is not designed to perform a high stress maneuver it is safe to assume an unmodified vehicle cannot do so without incurring severe damage.

Inside loop

Regarding an inside loop, the world's smartest man says

  Looping a 747 or a DC-10 would be trickier [than rolling one ...]  Boeing suspects its planes could make it, but since no one has ever been silly enough to try, there's no way of knowing for sure.

Outside loop

An outside loop is much much more stressful on airframe and pilots as it involves negative G. Airliners have flight envelopes that have much less capability under negative G. There must be a very high probability the aircraft would break up when attempting an outside loop.

Roll

A barrel roll is a less stressful maneuver.

60 years ago, in 1955, "Tex" Johnston, a Boeing test pilot performed a barrel roll in a Boeing 707. So far as I can find out, no one has since deliberately attempted to replicate this stunt in a commercial airliner. At the time of publishing his autobiography, the pilot claimed in a newspaper report that the stunt was planned by him but unknown to his boss.

in 1985 China Airlines 006, a 747, inadvertently executed a half roll but suffered structural damage. It rolled to 60 degrees before the pilot disengaged the autopilot, the pilots suffered spatial disorientation and the roll continued into a dive. The pilots obviously weren't flying the aircraft within its limits - meaning you can't read a lot into this, other than it might be easy to break up the aircraft while attempting this sort of maneuver.


What did a maintenance person put into the engines after landing?

It was probably Engine Oil.

On modern jet engines, oil is a consumable in the sense that it gets burned up and needs topping up. A new engine will require less, a high time engine more. The JT-9s on older 747 were notorious oil-guzzlers. CF-6s were a bit better but they still needed topping up after an 8 hour flight.

Jet engines never get an oil change. The regular topping-up of burnt oil keeps the oil fresh and regular filter changes ensures material from engine wear is not circulated. Oil samples are taken at set intervals and sent for spectrographic analysis. Too much of a particular element will focus attention on a particular component. There are also 'Magnetic Chip Detectors' which trap ferrous metals and this also helps to pinpoint wear in the engine.


Was wood used in the construction of the TU-144?

You're looking at a typical metal wing's innards with stringers (small lateral stiffeners made from aluminum angle) riveted to aluminum sheet, and the remnants of wing ribs also attached.  The colour is zinc-chromate primer which ranges in colour from bright green (epoxy zinc chromate), to olive drab, to, that that case, yellow/brown.
There is no wood there.  If any wood can be found in an airplane like the  TU-144, it will be in the interior finishings or panels, or possibly internal bits or fittings that are usually made from plastic, say like a saddle that holds some kind of cylinder in place (in lieu of a plastic or rubber material).  But as far as that picture is concerned, you are definetely looking at metal.


Is it true that the red lights on building antennas help the aircraft "detect/visualize" ground objects?

Those lights are used to show obstacles that may interfere with the aircraft's course, such as antennas, buildings, wind turbines, and they are mandatory.
The name of those lights is Aircraft Warning Lights, and here is an article from Wikipedia about them.


Why was a flight delayed because of a bird in the cockpit?

Having a small bird in a cockpit, or anywhere on board, would not be a problem during a normal flight because a normal flight should have a wide safety margin.

However, a situation can change extremely rapidly from safe-and-normal to heavy workload to full-emergency due to any number of factors, including weather, mechanical issues, airspace/congestion, etc.   In a high stress situation, a small bird could be the proverbial tipping point.  So I see and understand the decision to turn back.

It is remotely possible that a small bird could get somewhere it shouldn't, like behind a panel or near electrical lines and cause a short or other serious problem.  Its not terribly likely, but with 100+ lives at stake, why would you risk it?

As for why they didn't just kill it:  Killing it involves catching it, and catching a small bird while still flying a plane is not easy!!


How a customer could choose "A330-Neo" or "A350 Regional" on shorter routes?

This probably just boils down to the economics of the route. An aircraft for a route is chosen based on more factors than just fuel efficiency. Airlines have to weigh in other factors like:

Number of landing/take-off slots available at the airport

This is because an airport can only support so much traffic. It may be more economic to fly a larger aircraft with more seats than multiple flights.

Cost of gates/landing fees, etc

Airports make money by charging per-passenger and per-aircraft fees. If an aircraft is parked at a gate, it is costing the operator to be there, so the more aircraft the more expensive it gets. Airports may charge depending on the size of the aircraft as well.

If the airport is equipped to handle an aircraft of that size. Some regional airports can't handle larger aircraft (either from a gate/facility perspective or a runway/taxiway surface one).
How many passengers need to travel that route on a daily/weekly basis
Regional fuel costs
Training/currency requirements for pilots and how easy it is to move between them

If going from the A320 to the A330 is a simple course vs the A350 being a whole new endorsement, they may choose the A330 to be able to have higher utilization of the existing pilots vs having specialized crew.

Same as above except substitute mechanics, spare parts, etc. This is a major reason that Southwest only fly's 737's, short or long-haul.
Ability for the aircraft to service other routes.

All said and done they need to pick the aircraft that is the most economic for the intended mission and fits into the business model of the company more. Airbus isn't going to take a customer that wants the A330 and push them into an A350, it isn't like buying a car where the salesman upsells you, the customer by the time they approach Airbus for a purchase contract already knows exactly what they need. Think of it more like buying tools (where you need exactly the right tool for the job) than it is about buying a car (the Corvette is nice, but have you looked at the Prius?)...


What kind of sunglasses are good for flying?

No polarizing glasses, they make reading anti-glare instruments difficult and sometimes even limit your outside view if the cockpit windows are polarized as well.
Good fit, you don't want to loose them in the slightest of turbulence. Consider fitting a strap so you can temporarily hang them around your neck. They also need to be comfortable if you wear a headset over them.
Neutral colour (grey, grey-green, brown) of the lenses, which does not distort the natural colours too much as that would make distinguishing navigation lights more difficult.

Actually the FAA have published a safety brochure on sunglasses which covers many aspects.


Is there any current and/or upcoming technology that alerts Air Traffic Controllers in case of loss of aircraft separation?

Most air traffic control systems nowaday have various safety net functions built into them. Short Term Conflict Alerting is one of these functions.

Short Term Conflict Alerting (STCA) works on the basis of extrapolating the current position using velocity and turn rate to predict imminent separation violations. If such a imminent separation violation is detected, the system will alert the air traffic controller.

The next level of conflict alering is Medium Term Conflict Detection (MTCD); this is not considered a safety net, but a tool that assists the Controller in detecting conflicts early on (up to 20 minutes in advance). Instead of relying on simple position extrapolation, MTCD uses more advance trajectory predictions that combine knowledge of the current aircraft position and velocity with information from aircraft performance models, flight plan, weather and given ATC instructions.
MTCD development started 20 years ago, and the tool is now being implemented in various ATC systems.


What are the names and the function of these aircraft components?

1 - nutplate holding something on the other side of the firewall

2 - starter or master contactor (a relay operated by low-current signal from cockpit switch)

4 - positive heavy-gauge cable to battery or starter

5 and 6 might be the alternator voltage regulator to hold the output voltage at 14.4V

10 - oil filter to remove carbon and other crud from oil

11 and 23 - magneto to provide carefully-timed high voltage pulses at for ignition

12 - alternator to charge battery and run electric instruments / radios

14 - vacuum pump to power gyroscopic flight instruments

15 - carburetor to meter air and fuel

21 - crankcase vent (?) to prevent blow-by gases from forcing oil out of the engine

22 - fuel strainer drain valve to drain water / crud from low point in fuel system

Better pictures would help to identify the other parts!


Could the wing struts on the SUGARVolt cause problems?

Struts like those on the SUGAR Volt and SUGAR Freeze have been used on gliders and propeller aircraft without problems. Since they are loaded in tension, they can oscillate like the string on a guitar. Note that they are braced at two thirds of their span to reduce their free length and to increase their resonance frequencies. A coupling between the strut bending motion and oscillating aerodynamic forces can be expected, but only at speeds above the SUGAR Volt and SUGAR Freeze design speed.

Henschel 126 with a very similar strut arrangement. Only with the strut it was possible to reduce the wing chord at the root in order to give the pilot better upward visibility (picture source).

Those struts do indeed reduce drag. They cause friction drag but allow to build the wing much lighter, thus reducing the required lift and the inevitable induced drag.

Modern gliders have very similar wings but do not use struts. This might seem puzzling, but can be explained by the low payload of a glider, which is at most two humans and their equipment. In order to fly faster, gliders use water ballast to make their wings even heavier. Therefore, a glider will not benefit from a lighter wing.

Nimeta single-seat glider (picture source)

In contrast, a revenue-generating aircraft needs to pack as much payload as possible. It cannot afford to carry unnecessary weight around in its structure. Also, a lot of its mass is concentrated in the fuselage, which increases the wing's root bending moment. A strut will help to reduce the root bending moment greatly and will enable a much lighter wing structure.


What is required by and for flights that cross borders in terms of border control?

A famous US/Canada example is Niagara Falls.

As as student, part of my cross-country test involved flying a circuit around the Falls, which meant passing the international border.  No special permission was required.

If I'd gone anywhere near the American airport (top right of map), the situation would have been completely different though.

Note that even though there are no specific border control related rules, there are many safety regulations controlling this circuit.  (It's a lot more crowded now than when I flew it.)

GA Flying over Niagara Falls – Fly 'n Things


Is threshold crossing height and glide path angle the same for ALL airports and applied to ALL aircraft?

In the US, the requirements are specified in FAA Order 8260.3, United States Standard for Terminal Instrument Procedures (TERPS).

Section 2-3 states:

  2-3. General PA Requirements. See Order 8260.19 for requirements
  related to GPA/TCH coincidence.
  a. GPA. Utilize a standard 3-degree
  GPA where possible. GPAs greater than 3 degrees but not more than the
  maximum (table 2-1) are authorized without approval when needed to
  provide obstacle clearance or to meet simultaneous parallel approach
  standards. Other cases or GPAs less than 3 degrees require Flight
  Standards or military authority approval (USAF not applicable).

  b. TCH. The published TCH (nearest whole foot) should accommodate the
  largest aircraft height group normally expected to use the runway and
  must not be less than the minimum or exceed the maximum TCH.   Note:
  60 feet is the maximum TCH regardless of height group.
  (1) CAT I.
  The TCH is based on achieving an acceptable wheel crossing height
  (WCH). The WCH is the difference between the TCH and the approximate
  glidepath antenna-to-wheel height (see table 2-2).
  (a) The optimum
  TCH provides a 30-foot WCH. It must provide a WCH no less than 20 feet
  or greater than 50 feet.
  (b) Displaced Threshold Considerations. The
  TCH over a displaced threshold can result in a WCH of not less than 10
  feet if the height of the glide path over the beginning of the full
  strength runway pavement suitable for landing falls within the
  minimum/maximum TCH values.
  (2) CAT II/III. The optimum TCH is 55
  feet and must be between 50 and 60 feet regardless of height group.

      Note: To determine the
  minimum allowable TCH, add 20 feet to the glidepath-to-wheel height
  and to determine the maximum allowable TCH, add 50 feet to the
  glidepath-to-wheel height (not to exceed 60 feet).

So, the answer to the question is ; No, they are not all 3 degrees and 50 feet.


Is it unusual for an aircraft to flash its landing lights in sequence?

The flashing lights help to make the airplane more noticeable. They have also been suggested to reduce the risk of bird strikes. This is offered as an option or add-on for many types of aircraft.

In addition to helping the pilots to see ahead of them when on the ground in dark conditions, landing lights help to make planes more visible to others. The FAA requires landing lights to be on for commercial flights under 10,000 feet, and encourages pilots to turn them on at least when near an airport or in low visibility. Flashing the lights makes them less likely to be confused for something like a star at night.


On Sunday 4 October 1992, El Al Flight 1862 crashed into a suburb of Amsterdam after the right inboard engine separated from the wing and took part of the wing and the right outboard engine with it.

The separation was caused by fatigue fracture of the fuse-pins that connect the engine to the aircraft. The engines of the Boeing 747-200 are joined to the wing by four fuse-pins.

In the investigation report you can find detailed information about the construction that mounts the engine to the wing. Especially sections 1.6.3, 1.6.4 and 1.16 are of interest to you, although I recommend reading the entire report if you have the time.

Source: Raad voor de Luchtvaart, document AAR 92-11

Source: Raad voor de Luchtvaart, document AAR 92-11


How would you explain Bernoulli to a nine year old?

Bernoulli's principle is probably not the best starting point to explain lift generation, especially if you do not first explain the origin of the Bernoulli effect! Naïve invocations of Bernoulli have led to a number of incorrect 'explanations' that are quite widely circulated, such as the idea that air flows faster over the upper surface of a wing because it is more curved that the lower, and thus creates a longer path. If this explanation were correct, paper airplanes and inverted flight would be impossible.

Bernoulli's principle is a consequence of Newton's laws of motion applied to incompressible fluid flow. To explain lift, you can start by invoking Newton directly, by observing that lift is the reaction to the wing's downwards acceleration of the airflow around it. Bernoulli enters the picture when you get to calculating the changes in pressure and velocity within that airflow.

From this perspective, we can see that while blowing air over a wing would create some lift, as the wing will deflect at least some of that airflow downwards, you would get more lift by pointing the fan directly downwards. This is, of course, what a helicopter does, and it takes a lot of power, so wing-blowing, being a less-effective way of generating lift, would require even more. If, instead, you use the fan as a propeller, you only have to produce enough thrust to counter the drag, which may be as little as 1/50 of the lift.


Can/do commercial flights change their flight path during flight?

A flight plan is just that, a plan.  A flight can deviate from its cleared route for many reasons, the most common of which are:

Weather
Traffic
Shortcuts

Weather reroutes are often initiated by the crew.

  Center, N12345 request 15 right for weather
  N12345 15 right approved for 100 nm, report direct XYZ.

Or they can be initiated by ATC

  N12345, Atlanta, your arrival into EWR is in perpetual holding, I can re-route you over PA or NY or do you need to divert? state intentions.

Large scale re-routing for weather will occur when an organized line of convection impacts a major route.  Even a short deviation for weather may result in an amended route if the controller has an alternate clearance he can put you on that works for you (being on a clearance and on-route is slightly less work for a controller than being on a carte-blanche deviation).

Traffic conflicts are usually short term vectors, but if you have an aircraft slowly overtaking you, you may be moved to a parallel route or have you altitude changed.  These, like the weather example above, would result in amended clearances.

Shortcuts are often requested to cut corners or try to make up some time lost in a long taxi on the ground.  These are usually crew initiated, but sometimes ATC will solicit them.  Regardless of what route is submitted in a flight plan, ATC will generally clear you via a preferred routing.  These routes aren't always the shortest distance between a departure and arrival and this is where shortcuts can be requested.  Usually this just means skipping a fix or getting direct routing to a later fix in your clearance.  The availability of shortcuts is a function of traffic, time of day, agreements between ATC sectors, what region you are in and what altitude you are at.

When you are on international flights, another issue you may run into is political and could result in some or all aircraft being routed to avoid certain airspace.

Lastly, your dispatcher may re-route you and ATC will clear you along a new route.

In all cases, whether deviation is pilot or controller initiated, both parties are in sync with what the airplane is doing, which will either be a vector or a new clearance.  Your dispatcher will be notified of this change automatically via their dispatch software (which is likely fed data from both ACARS and the FAA).


Is there any combination of aircraft and airports where no alternate landing airport would be possible?

For larger aircraft, likely no. Worst case they can carry enough fuel to return to their origin. If range does became an issue (e.g. Atlanta -> Anchorage) you would just schedule a technical stop somewhere like Yellowknife. Alaska has a ton of small airfields so short-range aircraft would have many alternatives. Wether the alternate has any services is another question.

As the question asks about US airports only, mainland -> Hawaii would be a possibility. Hawaiian Air's 717 planes already need extra tanks in the fuselage to get to the mainland, so a return flight would be weather-dependent. If you take off from Seattle and several airports close for bad weather after you pass the no-return point your options are rather limited. If the weather is bad enough to close every airport in the islands then channeling Sullenberger won't help much. Unlikely any sane pilot would take off with that kind of forecast, and Hawaii's weather is usually pretty good.


Are four-engined airplanes subject to ETOPS equivalent rules?

Yes, three- and four-engine airliners follow 'Long-Range Operations' restrictions just like twin-engine airliners.
From WikiPedia:

Airbus proposed to replace ETOPS by a newer system, referred to as LROPS or Long Range Operational Performance Standards, which would affect all civil airliners, not just those with a twin-engine configuration
(...)
The FAA has decided to use the single term, 'extended operations,' or ETOPS, for all affected operations regardless of the number of engines on the airplane.

The large 4-engined airliners can not reliably fly long distances on one engine only.  But  they can fly on three engines just fine.  When only two engines are available they divert to the nearest airport.
The ETOPS rules become relevant whenever a three- or four-engined jet diverts more than 180 minutes from the nearest airport.
For more information, refer to the airliners.net thread on 3 And 4 Engine Planes Requiring Etops.


What notification am I required to give when starting an engine?

shouted as loudly as I could - PROP CLEAR - waited a few seconds and
  then started the engine.

Something to note here. While your shouting of PROP CLEAR was correct you need to Look Up and ensure no one is around when starting your engine. This is a common mistake, however you can bust a check ride for only shouting and not looking.

  Was I supposed to walk up to him and notify him before starting the engine?

According to the FAA handbook, yes you should have alerted him and/or waited for him to clear the area.

  Prior to engine start, the pilot must ensure that the ramp area
  surrounding the airplane is clear of persons, equipment, and other
  hazards from coming into contact with the airplane or the propeller.
  Also, an awareness of what is behind the airplane prior to engine
  start is standard practice. A propeller or other engine thrust can
  produce substantial velocities, result in damage to property, and
  injure those on the ground. The hazard of debris being blown into
  persons or property must be mitigated by the pilot. A