What are "secondary precessional forces" that apply to boomerangs in flight?

The aerodynamic forces on the boomerang are roughly like the springs that limit the (primary) precession of a rate gyroscope.  These forces introduce a secondary torque, which in turn produces the secondary precession in question, which is needed to tilt aka "lay over" the boomerang.  (My physics is rusty; the physics SE may be more fruitful.)


Does increased fineness ratio decrease skin friction drag?

Basically, your confusion comes from not knowing what needs to be held constant.  When you increase the fineness ratio without increasing wetted-area, i.e. you hold wetted-area constant, the overall drag goes down.  You can see this in racing shells (sculling).  The shells (boats) have an absurd fineness ratio, very narrow and very long.  This decreases the overall drag (somewhat, there are other forces at work there).

As you increase the finess ratio, at the expense of increasing the wetted area, you will need to do a trade-study to determine if you gain an advantage.  You probably will, up to a fineness ratio of 6:1.  Much beyond that the extra drag from the increase in wetted-area starts to dominate any savings in drag due to body fineness.

The inital statement is true, presuming you keep the wetted-area constant as you adjust the fineness ratio.

T


Why did Britain develop three different bombers (V-bombers) after WW2 instead of one?

I've heard numerous reasons cited - perhaps due to the secrecy around all things nuclear - but from what I know, there are a few reasons:

Speed of deployment

Of the three designs, the Valiant was simplest, and could therefore be built and deployed the soonest. The initial Valiant prototype didn't actually meet the requirements that had been set forth for a nuclear-capable bomber aircraft, but it was better than nothing and with the mounting threat that the Cold War might turn hot, this speed was an asset, so the requirements were changed to allow the Valiant to enter service. This would fill the gap in the UK's forces, and more advanced aircraft could follow later.

Specialisation

Each of the V-bombers had slightly different performance, meaning that they could perform slightly different roles.

The Victor had the greatest payload capacity (up to 35 thousand-pound bombs)
The Vulcan was smaller and more agile and could therefore use smaller runways, and could just about break Mach 1 in a slight dive. It could also be refuelled in flight.
The Valiant was the simplest design, and therefore the most reliable (although it later emerged that a newly-developed and poorly-understood alloy introduced some serious design flaws)

Spreading the risk

Overall, the differences between the various aircraft were pretty minor, and the main reason that three contracts were allowed to progress to delivery was simple: the UK didn't want to put all their eggs in one aeroplane. The need for a nuclear deterrent - and a means to deliver it - was so great that it could not be jeopardised by a poorly-designed bomber, a mismanaged project, unforeseen failure of the company building the aircraft, etc. By having three aircraft from three different manufacturers, the UK greatly improved their chances of having at least one aircraft that actually worked, was delivered on time, and was able to remain in service for a reasonable period of time.


What would happen if I had a type rating for an Airbus A320 and applied for an airline that has an A320 and other aircraft?

No. You'd take the same initial course as everybody else, learning that airline's procedures & checklists for that aircraft.  Having a type rating shows the hiring people that you're able to pass the training course & checkride for that level of aircraft, and that's good to have. But beyond making the aircraft systems part of the syllabus easier for you, having the type won't change your training.

It also probably won't affect which aircaft you initially get. Those are based on where the airline needs pilots, and then (typically) on some sort of bid process. The airline presumably expects all its new pilots to be able to complete whichever training course they get, so your type and/or prior experience is unlikely to drive your initial assignment.

For a new, small airline in need of hiring captains off the street, things might well be different.


Why don’t large aircraft (larger than ~737-size) have even partial manual-reversion capability?

Adding in manual control mechanics for a large aircraft has serious drawbacks:

Weight: For a large aircraft, the weight of pulleys, cables, etc., will add up quickly. This costs money in fuel and by reducing the useful load of the aircraft.
Complexity: The manual system would have to be routed through the entire aircraft, including passing through pressure bulkheads and other structure. This takes a lot of time to design, install, and maintain. While there are certainly ways to deal with the high forces involved, that just adds more complexity.
Vulnerability: There have been multiple accidents in which manual control systems were improperly set up or jammed. The manual system would have to be designed so that fly-by-wire system would still work even if the manual system develops an issue. If an event is catastrophic enough to cripple all of the hydraulic/electrical systems, there's a good chance a manual system would be disabled as well.

The alternative is to design a hydraulic system that can match or exceed the reliability of a manual system. Of course this is not without its own complexities and issues, but virtually all modern airliners have gone this route. The weight and complexity of a manual system are not worth the remote probability that such a system would be both necessary and useful.


Why are many jet aircraft designed to cruise around FL350-370?

In a word, the tropopause.

Gas turbine engine efficiency improves with colder & denser air. As an airplane climbs through the troposphere, the density & temperature both drop, and the loss of density is more than offset by the lower temperature. Above the tropopause, however, the density continues to drop while the temperature holds (approximately) constant.

In the U.S. Standard Atmosphere model, this occurs at 36,089 feet. This represents a local (possibly global) optimum altitude for efficiency (and if there is a better optimum at a higher altitude, it's inaccessible for other reasons).


Can state or local laws regulate where aircraft can take off and land?

As any licensed pilot knows, the FAA and the US military are the "de-facto" regulators of 99.9% of what goes on in the air over the US (see: https://aviation.uslegal.com/government-regulation-and-control/state-aviation-regulations/ ).  Still, state and local governments are not entirely powerless here.  The general theory being more or less that, in cases where there is no obvious conflict with the FAA, or the US military, then local jurisdictions may still have some small level of control over certain very limited "air-spaces" (see: http://caselaw.findlaw.com/us-6th-circuit/1140229.html ).  Suggestion: 99.9% of the time, the FAA wins, but hey, don't let that slow you down!  (Still, in view of the findings of the second case I just listed above, if I were you, I might try keeping peace with my neighbors over trying to save just a few steps on the way home.)


Who invented the cabin chime? What aircraft was the first to use it?

I can't definitively identify the true origin of the chimes; but the earliest reference to them I can find is in:

ARINC Characteristic 715-3 Airborne Passenger Address Amplifier (PA AMP)
This standard describes the characteristics of a PA AMP designed for
installation in commercial aircraft. Its function is to amplify control voice,  music and internally generated chime audio signals to drive the aircraft cabin  loudspeaker system.

The original ARINC 715 was published in 1978.
The standard they defined is in section 3.4 of the document:

3.4 Chimes
The PA Amplifier equipment should include a chime generator capable of
providing two tones in the four remote controlled sequences that may
be deployed through either the auxiliary or main amplifier utilizing
aircraft strapping. Internal controls shall be provided to adjust the
individual tone levels for balancing the related outputs.
3.4.1 Tone Frequencies
The high tone should be 587 Hz (musical Note D) and the low tone
should be 494 Hz (Musical Note B) signals of a pleasant sounding
musical nature.
3.4.2 Tone Harmonics
The chime tone harmonics should be minimized with the second harmonic
level at least 30 dB below that of the fundamental and each succeeding
harmonic should be a further 3 dB below the preceding harmonic.
3.4.3 Tone Decay
The single high and low tone levels should decay to a level 30 ±6 dB
below the peak level within 900 milliseconds from the peak. The second
tone of the high/low combination should begin 900 ± 100 milliseconds
after the peak of the first tone.
3.4.4 Sequence Control
Five tone sequence control lines should be provided to actuate
individual or two tone sequences as follows:
a. Passenger Call (High Tone)
b. Attendant Call (High/Low Tone)
c. Attendant Emergency Call (High/Low Tone Repeated Three Times)
d. Seat Belt Sign (Low Tone)
e. Smoking Sign (Low Tone)


Is it allowed to let the engine of an aircraft idle without a pilot in the plane. (For both helicopters and aeroplanes)

You won't see it done in the fixed wing world unless the aircraft is tied down or otherwise securely restrained (like when you tie off the tail to something when hand starting your no-starter taildragger; some pilots just use chocks or parking brakes to hand bomb their airplane, but it's a terrible idea).

However, it's common in the helicopter world especially in bush operations.  With the collective and cyclic friction locks tightened down and the engine in ground idle, it can't really go anywhere and a pilot who lands in a remote area without assistance to hook up a sling load or lug something on board will often do it without shutting down.


What are the ICAO PANS? What are their contents?

ICAO Procedures for Air Navigation Services (PANS) are documents approved by the ICAO counsel and recommended to states for worldwide application. As such they attempt to make air navigation services uniform across the world.

There are five PANS:

Procedures for Air Navigation Services - Air Traffic Management (ICAO document 4444, a.k.a. PANS-ATM)

Specifies the actual procedures to be used by air traffic service units in providing the various air traffic services to air traffic.

Procedures for Air Navigation Services - Aircraft Operations  (ICAO document 8168, a.k.a. PANS-OPS)

Lays down rules for designing instrument approach and departure procedures. Such procedures are used to allow aircraft to land and take off when instrument meteorological conditions (IMC) impose instrument flight rules (IFR).

Procedures for Air Navigation Services - Abbreviations and Codes  (ICAO document 8400, a.k.a. PANS-ABC)

Is a list of abbreviations and codes commonly used in aviation.

Procedures for Air Navigation Services - Aerodromes  (ICAO document 9981, a.k.a. PANS-ADR)

This details aerodrome licensing and Aerodrome Manuals

Finally there is the PANS Training which details ISD (instructional Systems Deisgn and Systems Approach to Training (SAT) which enables criterion based and material dependent methodical learning


Can a deployable ILS operate in CAT II or CAT III?

This article only states Cat 1.  Deployable ILS project has been ongoing for 7+ years, so -1 to you for not posting anything relevant about it in your question.

The D-ILS is to provide a system of equal performance to existing Category I fixed-based systems that provide aircraft guidance on final approach in low-visibility and low-ceiling weather conditions.

https://www.militaryaerospace.com/articles/print/volume-27/issue-11/rf-microwave/thales-developing-deployable-instrument-landing-system-for-air-force.html
March 2010 article on expected May 2010 Request For Proposal release:

The final RFP is anticipated to be out in May with source selection in June, and an award by December. Initial operational capability is scheduled to occur in early 2013.

http://www.aero-news.net/index.cfm?do=main.textpost&id=c873f7b8-2dd6-408f-9da2-b85f5b36a7ea
Here is another that says it is available in a transportable configuration and says it can do Cat II.  Is it available?  I don't know, the AF seems to be buying the Thales version currently.

Transportable and permanent configurations
Meets icao cat II ILS specifications
For Ground Controlled Approach (gca) operations, the ILS provides
controllers with par-like displays for up to four aircraft. All ILS glide slope
and localizer signal parameters comply with icao Annex 10 standards
for Category II ils, including accuracy, integrity, displacement sensitivity,
clearance, alignment, modulation and frequency. The fixed-based ILS has
been approved by the faa for use in the National Airspace System.

http://www.anpc.com/wp-content/uploads/2011/08/ILS-ANPC.pdf


Heaviest Boeing vs Airbus aircraft: Why large (150 ton) difference in Weights?

Short Answer:

Difference in corporate mindset between the two companies and incompatible interpretation of market conditions and projections of future market demand.

Long Answer:

@Anilv answers your question to an extent but you should really understand the total scope of the commercial aviation sector. Airbus is a relatively newer company that has a widely different mission goal than Boeing: Airbus is an exclusively EUROPEAN owned multinational company with a majority share under EADS, the European Aeronautic Defence and Space Company. It was primarily brought together to give Europe and its smaller more specialized companies another chance against massive, government backed corporations from America (the big three: McDonnell Douglas, Lockheed Martin, and our big daddy Boeing). This actually caused quite a bit of a political sh*tstorm, with Boeing complaining that Europe's explicit backing of this new Airbus being far too pointed in corporate tie-in with government, but besides for some concessions made to ease relations with America, Airbus was allowed to continue receiving grants and other national backing from its parent countries.

Now to explain specifically why Airbus doesn't compete directly ALL the time with Boeing's aircraft is because frankly, leadership of both companies cuts projects all the time in different sectors because of their individual personas and visage for the companies' futures. I'm glad you mentioned the A380 because except for a few other situations continuing the A3XX (in-house title for A380 prototype) was by and large the most controversial internal decision made by any Airbus executive. The situation was this: Both Boeing and Airbus were looking at the VLA (Very Large Aircraft) concept, an idea becoming popular in the 90's due to burgeoning flight routes and demand for efficient carriers of mass amounts of passengers. Also, as the Gulf War ended Airbus had begun to reap profits with their A330 and A340 series aircraft. However, not too long ago Boeing had released its jumbo-jet 747-100 and -200, which were extremely popular and Airbus leadership was getting worried about their potential loss in this possibly goldmine niche. Alan Boyd, at the time president of Airbus North America reported after a joint study with Boeing of the market that Airbus predicted a market for about 500 to 600 airplanes, while Boeing envisioned a market for only 300 to 350. Interestingly both saw a profit margin of about $15 billion. Boeing saw the risk/reward too high, and at the time (I would argue it still is) was very encumbered with bureaucracy and promises to keep to the American government. It was a large project no matter what, and Boeing to an extent was on America's payroll after all. However, although risks were high for Airbus as well, the company exec at the time, the legendary Jean Pierson saw that the risk was worth taking, not only because of projected efficiency and prototype designs, but also because he needed to protect the rest of Airbus' lineup.

To even comprehend the complexity of the issues facing these men, you MUST understand that decisions to make or scrap a new plane isn't an isolated choice: an airline buying a new plane from one maker is highly unlikely to mix and match to buy a different size plane from another maker just to fit a smaller or larger route i.e. buy the A380 for its long-haul high-demand routes and the 787 for its longer but lower-demand routes. Boeing on the other hand had a relatively fleshed-out spread of planes and due to a HIGHLY incorrect mindset of complacency and EVOlution rather than REVOlution, they did not seize this oppurtunity to expand and improve. This was common knowledge to all business-minded men in the sector who were worth their jumping beans (sadly few at Boeing after , so the most prominent factor in Pierson's mind was probably his defense of the also-new A340 series, a small long-haul aircraft that fills a similar role to the new 787. This, as the chart you provided shows, filled a spot that was projected to be VERY profitable (as Boeing found out too late and rushed in the 787 to fill) but at immediate time was a little too hard to swallow for most medium sized airlines. Keep in mind that Airbus was still trying to prove itself to the world.

So, by Pierson's logic introducing a superior 747 in the form of the A380 would not only draw attention to the plane itself, but also help establish the A340 and start the ball rolling for future development of the wide-body long range medium size airliner. So, that's pretty much why the A380 was a (mostly) no-brainer for the Airbus group, and from the late 80's on, why Boeing and Airbus' line of airliners has considerably diverged. Hope this answers your question, and if you'd like more information or just clarification feel free to comment.

[Edit]
The book Boeing Versus Airbus: The Inside Story of the Greatest International Competition In Business gives a wonderful account of everything I just detailed and more. I've read it multiple times and I never cease to learn something new from something under 300 words. Give it a try if you have time!


What is the phraseology when leaving a TRSA?

You can say "cancel radar service" (probably the most standard under ICAO), "cancel flight following," "request frequency change," or anything else that conveys the same general idea. If all else fails, plain English is always acceptable too.

Regardless of which you say, ATC's response should be "radar service terminated, squawk VFR, frequency change approved," same as in any other class E airspace.


What defines the end of a "cross-country flight" for ppl aeronautical experience purposes and for general logbook purposes?

It is not in the interests of the FAA to provide a specific definition of how long an aircraft can spend on the ground before a flight "ends".
Pilots are expected to use their judgment to deal with weather, fuel planning, air traffic control, etc.  Pilots are also expected to use their judgment when deciding what should and should not be logged as a single cross country flight.
This is good for pilots. The FAA trusts us to do the right thing.
Ok, but what about my questions?

For general logbook purposes you can do anything you want. Exactly nobody cares what's in your logbook except you (and maybe the person interviewing you for a job in your future).

Private, commercial, and ATP experience require hours, not flights. Therefore you may log case 1 as one flight, two flights, or three flights, depending on which is most reasonable in your judgment as a pilot and based on the unique circumstances of the flight.  For the most part this simply means you need to decide whether you want to fill out one, two, or three lines in your logbook.

Similarly, you may log all of case 2 on one enormous line in your logbook, if for some reason the unique circumstances of the flight indicate that that's the best way to do it. (Maybe you're going to the south pole, and the month-long break is because you had mechanical trouble along the way.) Similarly, you may choose to log each leg or even each landing as a separate line in your logbook (for a total of 38 lines), if that makes more sense in your judgment.

The longer the stop, the more difficult is it to argue that it should be considered one flight.  I think it would be exceptionally difficult (though not impossible, see above) to make the case that two hours worth of flying separated by a month of not flying should be considered one flight.

As guidance, remember that your logbook is a reflection of you as a pilot.  If there's uncertainty, ask yourself "How would I like to be perceived, if someone were to audit this logbook?"  If there's still uncertainty, ask an instructor.  If there's still uncertainty, ask an FAA inspector.  They're there to help.


Why don't airbus aircraft have raked wingtips?

Probably because Boeing holds a number of patents related to raked wingtips, like US5039032 A High taper wing tip extension and US6089502 A Blunt-leading-edge raked wingtips, among others.


Why is there a cutout in the pilot seat (looking as if it had separate "legs")?

I can't speak to all aircraft, but on the 727-100 and 747-100/200 aircraft I flew, the seat cutout accommodated the strap on 5-point seat belt systems that went down between your legs. See

https://pitstopusa.com/c-134853-safety-equipment-seat-belts-harnesses-cam-lock-restraint-systems-5-point-camlock-restraints.html.

I don't recall for certain whether the control yoke could be brought all the way back into the cutout, but I don't think it could. I may be misremembering though.

As I remember, the camlock into which the other 4 straps (left and right across the seat and left and right shoulders) was permanently attached to the strap going down between the legs. On sitting in the seat, you reached down and pulled that strap up with the camlock on the end and then put the other 4 straps into their slots.

I greatly disliked the shoulder restraints, so once we reached cruise climb, I would rotate the camlock while holding the left and right seats straps in. This would release the shoulder straps.


Can spatial disorientation in low visibility situations lead a pilot to fly upside down?

The short answer is "Yes", if the pilot is not properly trained, doesn't trust their instruments, or has an instrumentation failure.

Without a visual reference (such as the real horizon or artificial horizon) it's very easy for a pilot to place their aircraft in an "unusual attitude" -- something other than the expected straight-and-level flight or commanded turn.
As humans we naturally rely on our inner ear and sense of balance to tell us what's "up" or "down", but in a moving aircraft you can move through a whole range of attitudes in "1G maneuvers" (where your body thinks down is where your seat is).
Little micro-movements we unconsciously make on the controls (or larger movements we may make consciously in response to turbulence or other perceved disturbances in the aircraft's attitude) based on what our ears and butts are telling us will eventually get us into trouble.

It's so easy that often instructors will have students create their own unusual attitudes in training by having the student close their eyes and simply try to fly straight and level - within a few minutes at most the student will invariably no longer be in level flight.

There's an exercise you can do on the ground to illustrate how imperfect your sense of balance and direction is, but it requires a large, flat, open area. We'll use an American Football field for this example:

Stand on one of the goal lines
Have a friend stand at the 50 yard line (about 45 meters away)
Close your eyes or put on a blindfold, count to ten, and walk directly toward your friend
Stop when you reach your friend (or your friend yells "STOP" because you're about to walk into a goal post)

Most people will not make it to their friend on the 50 yard line, because without a visual reference it's very hard to walk a straight line: You will veer off one way or the other, and some folks will even manage to walk in a complete circle (which is what eventually tends to happen over a large enough distance).
The same principles apply to flight, except there's a third dimension (vertical) to contend with.


How is the B737NG Autopilot controlling pitch?

This is from the B737NG airplane manual. Boeing doesn't offer the technical drawings in the pilot manuals, one would have to have access to the maintenance manuals. Boeing is very sticky about their material being published on the internet.

Autopilot Flight Director System (AFDS)
The AFDS is a dual system
consisting of two individual flight control computers (FCCs) and a
single mode control panel. The two FCCs are identified as A and B. For
A/P operation, they send control commands to their respective pitch
and roll hydraulic servos, which operate the flight controls through
two separate hydraulic systems.

This is a brief explanation of an autopilot system such as on the B737 from this website.

Aircraft with hydraulically actuated flight control systems have
autopilot servos that are electro-hydraulic. They are control valves
that direct fluid pressure as needed to move the control surfaces via
the control surface actuators. They are powered by signals from the
autopilot computer. When the autopilot is not engaged, the servos
allow hydraulic fluid to flow unrestricted in the flight control
system for normal operation. The servo valves can incorporate feedback
transducers to update the autopilot of progress during error
correction.


Why did the Mercure use cascade-type reversers?

Did Dassault have some magic trick up their britches

It was an era of innovations. Have you seen the Vickers VC10 thrust reversers? The clamshell / target doors can be made internal within the nacelle. Literature is lacking on the Mercure, but there is a nice hint from a cutaway drawing:

Source: Air Enthusiast magazine, via aviadejavu.ru

The highlighted item relates to the exhaust cone, and it's labeled "Thrust-reverser cone." I will use a different patent (for now) for illustrative purposes only:

Above shows a nacelle that cascades, revealing vane panels to direct the gases forward, and blocker doors that pivot radially inwards.
The Mercure's thrust reversers were supplied by Snecma, now Safran, and indeed I found a Safran patent for a thrust-reversing cone of similar embodiment that was filed in 1974 (the year of Mercure's entry into service). It does not have cool drawings, but the text is clear:

The jet propulsion nozzle 7 is here of the needle or plug type, or in other words provided with a central body 30 [i.e. cone], and can be closed off completely by means of flaps (not shown) which seat (in the closed position) on said plug or central body 30. [emphasis mine]

Jumelle, Louis Francois. "Jet engine nacelle with drag augmenter auxiliary for thrust-reverser system." U.S. Patent No. 3,941,313. 2 Mar. 1976.


On modern commercial airliners, how much of the flight could be fully taken care of by the auto pilot?

This should apply to the most advanced modern airliners. Older aircraft may not have all of the same automation available. The autopilot can take care of most tasks between takeoff and landing.

The pilots have to take care of starting the airplane. This includes turning on electronics, bringing up the aircraft systems, and starting the engines.

Taxiing is pretty much on the pilot, there is no automatic taxi yet on commercial airliners.

Once the aircraft is lined up with the runway, the pilot can hit the TOGA (Takeoff/Go Around) button. The plane will automatically apply the right amount of thrust for takeoff. The pilot will then rotate the plane (lift the nose) at the rotation speed (calculated before the flight), and the plane lifts off. Shortly after that, the pilot will engage the autopilot.

If the autopilot is set up to follow GPS waypoints and proper altitudes, it will be able to take care of the climb, cruise, and descent portions of the flight. The autopilot can follow the best speeds and fly through each waypoint in the flight plan.

Once the aircraft is approaching the runway, the pilot will tune the proper ILS frequency for the runway into the navigation radio. They will then engage the Approach mode, which captures the localizer (lateral navigation left/right to align with the runway) and glideslope (veritical navigation up/down to have the proper approach path). The pilot then needs to deploy the flaps and landing gear as the plane slows down for landing.

In the lowest visibility conditions, the plane is capable of the approach and touchdown on the runway all by itself. The pilot will then apply reverse thrust and brakes as needed to slow down.

Now that the plane is back on the ground, it is once again the pilot's job to exit the runway and taxi to the gate. Shutting down the engines and then the electronics will also be up to the pilot.

What is the autopilot still missing?

Responding to ATC (readback clearances, change frequencies)
Reacting to nearby traffic
Following ATC instructions (climb/descend, turn, slow/accelerate)
Reacting to emergencies or system failures
Controlling lights, flaps, landing gear

This is of course a high level view and there is so much more that goes into the flights.

Also, keep in mind that we have the technology to do much more. It just hasn't made it into service for one reason or another (lack of development and testing, not seen as important, etc.). We have UAVs that are much more automated than this. However, they are still working on how to effectively avoid other aircraft.


Why are APU engine generators rated in kVA and not kW?

Current and voltage don't always vary at the same time, one usually lags, this lag is the root of evils found in electrical engineering.

Any device is a mix of resistive load (no lag), inductive load (current lags) and capacitive load (voltage lags).

The lag (phase shift angle) is denoted with Greek letter φ, and usually expressed as cos φ, or power factor. Power factor $=100 \cos φ$. Each device has its own individual φ, which may vary with conditions.

Lag effect is only visible when voltage or current changes. This happens transiently in DC when the circuit is closed or opened (effect used to create high DC voltage from low DC voltage in a contact breaker), but constantly in AC  (not mentioning other induced variations, e.g. changes in mechanical demand on an electric engine will change the impedance of the coil, so the current). The effect of the phase angle increases with circuit frequency, and is significant in aircraft circuitry, with major loads working at 400 Hz.

With this lag, the voltage and current mean (RMS) values are not sufficient to size electrical circuits. A phase shift introduces additional power in the circuit. This reactive component is not described by the RMS values, it's only visible when looking at instantaneous values, for example on this animation of the addition of two phasors, where the fixed circle radii indicate the mean values, and the moving dots show the instantaneous values. In simple cases one can determine the power required using the triangle of powers:

(Triangle of powers, source)

The apparent power is the sum of the real part of the power (resistive load in W) and the imaginary part (reactive load in VAr).

Work (Joule) is only produced by active power (the reactive power produces no actual work) and the active power rating of the generator must be higher or equal to the active power rating of the load. However the reactive power produces a reaction on the generator (by phase shifting the current) and must be accounted for when sizing the generator. The generator must have an apparent power rating higher or equal to the apparent power rating of the load, at the working phase angle.

A higher apparent power, regardless of the active power, requires a more robust generator and wiring to prevent overload.

One way to simplify the required power calculation is to just add the apparent power value of all devices (in VA). The result in VA or a multiple like kVA is conservative. This allows to select the generator required, provided we know its VA rating too. This is why generator manufacturers provide this value.

Notes:

All units, VA, VAr, W, are of the same kind (dimension of power, W). VA and VAr denote the fact that voltage, current and power are not varying in-phase in AC circuits. These units describe the kind of "power" we are taking about.
When the apparent power rating (VA) is the only rating known for a device, the active power rating (W) can be assumed to be no more than 60%, however the actual value depends on multiple factors and this is only a quick approximation. A generator must be provided with a chart showing the relationship between VA and W under different conditions of use.
A generator cannot work under any power factor, and cannot maintain the same efficiency across the valid power factor range. It is provided with a table or graph showing the acceptable conditions. For example, for this generator, the power factor must be kept in the green area (inductive load, between 80 and 100%), and will start being unreliable at some power factor, depending on the active power delivered and the type of reactance (red area).

(Source)
The explanations above are really simplifications, because in AC, the phase difference is only one side of the problem of reactive power. The other side is the presence of harmonics which are per definition out of phase.

The general principle in electricity is that voltage is set by the generator, and current is set by the load. The load can set a current which has the same frequency than the voltage, this is what we assumed for the answer. However most of the time the load sets a current variation which doesn't coincide with the voltage frequency and/or shape.

This non-linearity between current and voltage creates 'harmonics' and unbalances phases (so a current start flowing in the neutral, which is not something we want).

A little story about that relates to first datacenters in years 1990. Standby generators (diesel) not well sized were unable to sustain computers loads, due to a capacitive reactive power created by computer switching power supplies (non linearity and a cos φ as low as 0.6) Fires ensued, it was time to understand how to deal with apparent power.
The $φ$ angle just comes from the general definition of a sine wave current. Current at time $t$ is defined by $a = a _{peak} \space sin \space (\omega t \space + \varphi)$ where $a _{peak}$ is the maximum current, $ω$ the angular frequency (314 rd/s for the EU grid) and $φ$ the phase shift angle.


I live directly south in the line of a north-south landing strip of a low-traffic airport. What should I know?

1: At large international airports there has been at least some research to suggest that there may be affects on your health (http://www.latimes.com/local/la-me-0529-lax-pollution-20140529-story.html) but I'm assuming this is quite a small general aviation airport. With the low traffic, small planes, and far distance away frorm the airport, you are probably not experiencing much as a direct result of exhaust.

2: There is an almost no chance that an aircraft would land on your property. If one did happen to land on your property the NTSB does provide some guidance for public saftey personel: https://www.ntsb.gov/tda/TDADocuments/SPC0402.pdf.

This PDF is aimed at public safety personnel. If you want to be an active participant in this emergency, feel free to follow it and the rest of this post. If you don't want to get involved, please just stay at a safe distance away from the accident and report it to emergency services.

If you are going to try to help you probably want to receive training on first aid before anything happens. Once an accident occurs you can do several things. For the most part, you initially respond just as you would to any other emergency, first by securing the scene and making sure you are not in immediate danger and then calling emergency services. Be careful when approaching the plane, especially if anything is still moving (propeller spinning etc). There are also many dangerous substances that could be present at the site of a plane crash (listed in the NSTB PDF). No one on the ground needs to be injured unnecessarily after a plane crash.

Once everyone is out of the plane, get far away from it and wait for emergency services to arrive.

Administer first aid (if trained) to anyone who was injured
Call the NTSB at 1-844-373-9922 to report the accident (It is the pilots responsibility to report the accident)
Take photos and video of the crash and surrounding area so investigators have plenty of documentation to look through to determine the cause of the crash. This will also be useful for insurance purposes.
If you see anyone else around who may have witnessed the accident, try to get their name and contact information to emergency services or the NTSB so they can be contacted if needed in any investigation


How many lives would be saved if the airlines turned all the seats around?

The evidence is not the most recent one, but

  According to an article in the December 1952 edition of Naval Aviation News, “Passengers in Navy transport planes have ten-fold better chances of coming out of crashes alive, thanks to backward-facing seats which are being installed in all new planes […]". The unsigned article cites two Royal Air Force accidents involving a four-engine Hastings and a two-engine Valetta. Both had rear-facing seats that were credited with minimizing injuries to passengers.

Unfortunately, the link to the 1952 edition ends in a 404. Thanks to the detective work by Gerald Schneider we now have a working link. The article is on page 27. Another source cites a 1957 study, without a traceable source:

  An Air Force study in 1957 reviewed a series of crashes and concluded
  that injuries were seven times greater among passengers facing
  forward.

That a backward-facing seat is safer is indisputable, but quantitative data is very hard to find. Given the much higher wing loadings and landing speeds of modern airliners, possibly those ratios cannot be translated to modern aviation. While in the days of propeller-driven transports the difference between the forward- and backward-facing position might indeed have meant that the latter provided a significant advantage in survivability, the deceleration level in a modern airliner crash might make both equally unsurvivable. In addition, the data from mostly young, fit and healthy subjects involved in crashes of military transports might not translate easily to todays untrained, middle-aged and overweight passengers.

Circumstantial evidence cited in this 2016 study, however, indicates that even for modern airliners seating position is important. For an A340 runway overrun accident in Toronto in August 2005 it cites the Canadian TSB as saying:

  ‘One of the cabin crew, seated in the same general area as the crew
  and passengers who incurred serious impact injuries, was not injured.
  This cabin crew’s seat was aft-facing; the other seats were
  forward-facing.’

Another more recent paper gives more modest advantages: This 1996 study by the European Transport Safety Council lists them in a table and awards rearward-facing seats with only a 5% advantage overall or 19% if based only on impact fatalities:

Commenting on the table, the numbers are based on the 1995 Cherry study (CHERRY, R.G.W. and Associates Ltd (1995) Analysis of factors influencing the survivability of passengers in aircraft accidents Vol. I-III. EU DG VII contract) and

  These particular figures are not quoted in the study, but have been
  calculated from the data provided in the study report.


What if an airspeed prescribed by a published procedure isn't safe for a particular aircraft's operations?

If you are unable to comply with a published procedure, you simply inform ATC and the controller will tell you what to do instead. The controller will most likely be authorized to make this decision, even if there is no written regulation covering the specific case.


Difference between multi slats and three slats on B737 ? (leading edge devices)

Prior to the NG, the total number of slats is 6, that is 3 slats per wing

Thus there are 6 slats numbered from 1 to 6, that is slats 1 and 6 are the outer slat

The NG's have an extra outboard slat on each wing giving 8 in total. They have the same sequencing as the classic

Thus a 3 slats per wing refers to 737 prior to the NG

more precisely

Leading edge devices are comprised of 4 Krueger flaps inboard of the engines and 6 slats outboard of the engines. The LE flaps are extended whenever the TE flaps are not up. The slats extend from 1 to 5 and fully extend when beyond 5. Slat numbers 1 & 6 (the outboard slats) move a few degrees less than slats 2 to 5 when at full extend, causing the leading edge to look slightly disjointed in this configuration, this is normal.


Is it a standard procedure to use emergency slides without imminent danger?

If an evacuation is called for the quickest method must be used, in this case slides. No exceptions no questions. Passengers can complain after they are out but they are not given a chance to argue during the evacuation, force can be used to clear an argumentative passenger so that the other passengers can get out. The 30 minute delay was before the decision to evacuate, it has nothing to do with the actual evacuation. Full scale evacuations require no more than 90 seconds for the aircraft to be certified.

What the police need is completely irrelevant to the evacuation.

Slides can be inspected, repacked, and reused, they are test deployed at regular D maintenance inspections.(maybe C as well. D is every 6 years C is every 2 years) Emergency doors are also routinely opened for testing.

A passenger needing medical attention could very well be nothing more than a stubbed toe, light scrape, or being out of breath.


How do I calculate the required heading for an aircraft carrier with an angled deck to sail into the wind?

A carrier heading of 010 degrees at a speed of 20 knots would give you an acceptable solution.  You could perform some vector addition to compute the resultant wind and perhaps optimize this somewhat, but in reality the winds are generally not constant enough to make minor adjustments worth chasing.

Even if the winds were calm and the ship was making all 30 knots the crosswind component is almost negligible, and well within limits for any carrier based aircraft.  This happens a lot.  There is a bigger negative effect from  turbulence caused by the island structure when the ship has to make wind than any effect of increased crosswind.


Can ADS-B used for voice communications?

Why dont we ditch the old radio system full of parasties and use ADS-B instead for voice communications instead?

Well, why don't we use televisions for washing dishes? Because televisions don't wash dishes.

Likewise, ADS-B doesn't carry an audio signal, so it can't be used for voice communications.

Perhaps ADS-B could be modified to carry voice signals, but this is probably not a good idea. The reason for this is that, while aviation voice communications currently use a band which is 19 MHz wide, the band that ADS-B uses is only about 1 MHz. (The Signal Identification Guide says, "There are two types of ADS-B: one [...] with about 50 kHz of bandwidth;
[and] one that transmits at 978 MHz [...] using a larger bandwidth of about 1.3 MHz".)

Transmitting voice signals digitally is a good idea, but I don't know of any reason why this would be done using ADS-B. It would make more sense to simply design a new protocol from scratch.


What runway is used in a 90 degree crosswind?

One would basically follow the same guidance as if the winds were calm:  If the field is tower controlled the controllers will pick a runway.  There may be a primary runway direction preferred in these circumstances for noise abatement, traffic routing, (i.e. Instrument Approach or Standard Instrument Departure procedures favoring one runway over another) or other considerations, otherwise they might consider whether the winds are forecast to shift to favor one or the other and base their decision on that.

At a non-tower controlled airport pilots would be expected to monitor a common traffic frequency and follow what others are doing.  And if you are the first or only person there, lucky you... take your pick!


Do GA gasoline engines have a tendency to cough, and splutter?

The short answer to your question is: Yes

However...

Its generally advised to avoid making abrupt power changes to aircraft engines (piston and jet for that matter) to avoid surge and potential starvation issues. You are usually taught to apply and reduce power smoothly when learning to fly. Aircraft piston engines are not really all that different than older air cooled car engines (think VW and early Porsche (there was an aircraft certified Porsche motor actually)). As such they can (and sometimes do) suffer from the same pitfalls.

The difference you may see in aircraft engines is that they tend to be a bit better set up and less tinkered with than car engines.

Like any gas engine aircraft engines have been known to, on occasion suffer from vapor lock and fuel pump failure as well. The most dangerous and potentially serious issues in aircraft carburetors is ice build up due to high moisture and low temperatures at higher altitudes. Airplanes have carb heating systems that are manually controlled from the cockpit to solve this problem.

  What would happen or what could be done if this scenario occurs?

Generally speaking this is a rare occurrence and often happens on the ground. Keep in mind that plane throttles are (during a routine flight) adjusted far less than a car throttle. In a car you may be going through a street with lots of stop lights causing you to go from idle to cruise/launch power frequently. In an airplane this really only happens once on take off and to an extend when reaching cruise. When you taxi out and line up you will advance the throttle to full/takeoff power to get in the air. You may then keep it there for your climb or elect for a slightly reduced climb power depending on the situation. You will climb to your cruising altitude and throttle back to between 55% - 75% power for cruise. Fly your flight and at some point you will get where you need to be and throttle back to your decent/landing power.

In the case of entering the runway and lining up if you starve the engine when you apply power you are assumed to be either not moving or barely rolling and should be safe. If you pull the power to quickly on decent and stall the engine you may be gliding distance from an airfield and or have time for a restart.

  When it happened in cars sometimes you could not restart until the
  residual fuel was burnt by cranking the engine for some time. Does
  this apply also to aviation engines?

One neat thing that planes have which most cars done is mixture control. As a result you can pull the mixture control all the way out on a plane (how the plane is shut down), crank the engine, and draw the fuel out of the carb. Some cars will keep pumping fuel and filling up the carb while you crank (since you cant over ride the fuel pump). Generally a flooded car engine you have to wait for it all to evaporate out.

  The question arises from the fact that the design of the engines are
  quite old from my understanding.

Age has little to do with these issues. Fundamentally the internal combustion has changed little since its inception in terms of the way it physically works. While they have gotten far more efficient and reliable the physics are more or less what they have always been. Its our manufacturing technology that has made the engines better. Surging/starving an old engine and surging/starving a new engine will produce a similar issue. Aircrafts use older engines due to the complexity and cost of certifying anything for use in an aircraft. For what its worth Ford has been using the same basic V-8 for as long as some aircraft engines have been around likewise Porsche squeezed a remarkable 30 years out of the same basic air-cooled engine design in the 911 (911 up through 993 variant).

Side Note: As mentioned its expensive to get a new engine put into service. That being said there are common airplane engines out there (like the IO-550) that have fuel injection. As computers become more common place in aviation FADEC systems are becoming more common in airplanes. In the end of the day however, some of these choices come down to manufacture decisions. Some makers prefer to leave things in the hands of the pilots, in other words you can pull and advance the throttle how ever you like and the fuel/air delivery system will behave as such. Other makers have begun to put safe guards in place to prevent this, and they will (no matter how you operate the throttle) ensure proper advance and retard of all mechanisms as fast as possible should you operate the throttle vigorously.


What does I1001 mean in a METAR remarks?

It indicates ice accretion (buildup). This seems to be very obscure, I finally found it in some NOAA release notes for ASOS software v3.07:

  (S01126) ADD ICE ACCRETION REMARK TO METAR/SPECI REPORTS

  Discussion: The amount of ice accreting on a flat surface (planar
  icing) as estimated by the data from the freezing rain sensor, will be
  included in the remarks section of the METAR / SPECI reports. The
  remark will have the format of “Ihnnn,” where “h” is the hourly time
  period, i.e., 1, 3 or 6 hours. The value “nnn” is the ice thickness
  accumulated to the nearest 0.01 inch.

  Please Note: In situations when FZRASN is manually entered in the
  Present Weather field, an inappropriate trace accretion amount (I1000,
  I3000, I6000) might be entered in the remarks section of the METAR /
  SPECI reports. This is a known issue and will be addressed at a later
  time.

Based on that, "I1001" means ice buildup of 0.01 inches in the last 1 hour.

And thanks to Jimmy for finding a more official source: FCM-H1-2017, Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports (p. 12-18) from OFCM, the Office of the Federal Coordinator for Meteorology.


What is the reason for changing the speed reference (IAS or Mach number) with altitude?

Aircraft are limited by both air speed (VMO, affects loads on the structures) and Mach (MMO, formation of shock waves resulting in buffet).

At low altitudes, the speed of sound is high so an aircraft is most limited by indicated airspeed (IAS). At higher altitudes, the speed of sound is lower so the aircraft will be limited by Mach number. Aircraft typically fly towards the upper limit of their speed, so at some point they will have to switch from remaining under the IAS limit to remaining under the Mach limit.

The chart below shows the changes in different speeds as altitude increases, assuming standard atmosphere.

Trend data source, speeds for 737-800

The aircraft climbs at 250 KIAS from sea level. An acceleration from 250 to 300 KIAS is included at 10,000 feet. You can see that as IAS is held constant, TAS (True Airspeed) and Mach both increase.

The switch from 300 KIAS to Mach 0.76 is done at FL280. You can see the Mach speed is approaching its MMO. From this point, both TAS and IAS decrease. After the tropopause, around 35,000 feet, temperature stops decreasing and TAS remains nearly constant.

At FL400 the aircraft levels off and cruises at Mach 0.765, and a step climb to FL410 is included at this speed to show the trends there.

More detailed charts and an explanation of IAS vs. Mach as it relates to climb and aircraft performance can be found here.


Took a fair bit of trial and error on the very vague registration on the port wing :)

F-PDHV (D-Plane 1)

  The Verhees D-Plane 1 is a Belgian homebuilt flying wing,
  designed by Verhees Engineering and supplied as plans for amateur
  construction.

Picture from below:

Source

David Osborne/ Aeroprints Source


Does the headwind component change noticeably when transitioning from a crab to a slip on short final?

Let's start by looking at a pure "kick out the crab" technique, where we don't lower the upwind wingtip at all.

Even though we don't usually use the word "sideslip" when we talk about "kicking out the crab", we do in fact cause a sideslip when we press that downwind rudder pedal, even if we don't lower the upwind wing.

When you kick out the crab you force the aircraft to slip (or we could say skid) sideways through the air.  A yaw string on the front of the windscreen would blow sideways, in the downwind direction (if there were no propwash).  This is not an efficient way to fly.  Initially, the slip angle is fully as large as the crab angle you just kicked out.  As the flight path starts to curve (because you didn't lower one wing to stabilize the situation)-- i.e. as the aircraft starts to "accumulate drift" (which in reality is actually a turn -- a curve in the flight path--a rudder-commanded "flat turn" or skidding turn)-- things change, but that's the initial situation.  So the more crab you kick out, the more the aircraft is forced to fly sideways.

For any given approach airspeed and any given crosswind component, as headwind increases, so will the required crab angle, so when you "kick out the crab", you'll have more sideways flow over the aircraft, and the sink rate will increase.  You may also notice an increased tendency to roll toward the downwind wingtip, due to slip-roll coupling.

It may help clarify things to understand that "kick" is a little bit of a misnomer.  After the "kick", if touchdown is delayed, to keep the plane aligned with the runway, you have to keep HOLDING some rudder pressure.  If you released it, the plane would tend to swing back into line with the original flight path-- the nose would swing back toward the original heading.  After a few seconds though, the plane has "accumulated enough drift"-- i.e. the flight path has TURNED or curved enough-- that the aircraft heading will be aligned with the runway even with no rudder pressure. You do not want to be in this situation, because it means you have TURNED your flight path through the airmass so much that it is fully aligned with the runway heading, and you have TURNED your ground track toward the downwind edge of the runway.  You'll soon be in the weeds.  That's the trouble with the pure "kick out the crab" technique-- if touchdown is delayed, you have a problem.

Up till now this answer addressed a pure "kick out the crab" technique, but you did in fact mention sideslip in your question.  If at the same time you give the downwind rudder input, you also lower the upwind wing, you'll have the same initial slip angle as described above, and a headwind will affect the required slip angle-- and the resulting sink rate-- in the same way as described above.  But if you lower the upwind wing to stop the rudder-commanded turn, then the situation will be stabilized and you'll see that same slip angle (and that same sink rate) all the way to the ground -- assuming of course that the crosswind component and the headwind component are both constant all the way to the ground, which may very well not be true in reality.

(We're ignoring here the effect that banking has on sink rate, which is real but but probably quite a bit smaller than the effect of flying sideways through the air (slipping/skidding) due to the rudder deflection.)

At any rate, whichever technique you use, for a given crosswind component and a given approach speed, more headwind = a larger crosswind component = a larger slip angle = a higher sink rate.

If you are crabbing along on final approach with NO wing-down correction, and you haven't yet "kicked out" any of the crab angle, then the plane doesn't "know" anything about the wind at all, for a given power setting and angle-of-attack.  It's only after you "kick out the crab", or after you initiate a wing-down cross-controlled correction, that the strength of the headwind matters, because of the way you must adjust your control inputs to match the required slip angle, which is affected by the strength of the headwind.

In actual practice, with the cross-controlled wing-down technique, it seems to work well to imagine that the lowered upwind wing is somehow "opposing the wind" and "stopping the downwind drift", while the rudder is controlling the heading.  The real truth though is that what the lowered upwind wing is really doing, is stopping the rudder-commanded flat (skidding) turn that would otherwise take place if we "kicked out the crab" and didn't touch down immediately after.


Can I get a pilot's license if my driver's license has been suspended?

The FAA has a guide on applying for a medical/student license

https://medxpress.faa.gov/MedXpress/Content/Docs/MedXPressUsersGuide.pdf

Near the end, pages 30-33. They ask for consent to review your National Driver Register.  Read those pages, see if any of it applies to you.


Why is the ratio maximum range speed/maximum endurance speed the same for any airplane?

The ratio is the same for all aeroplanes if you accept a number of assumptions:

The propulsion efficiency is constant, regardless of speed or power setting
Aerodynamic drag is the sum of parasite drag and induced drag
Parasite drag is proportional to the square of airspeed:$ D_p = k_p \cdot V^2$
Induced drag is inversely proportional to the square of airspeed: $D_i = \frac{k_i }{V^2}$
There is no wind

Since we assume the propulsion efficiency is constant, the fuel consumption rate is directly proportional to the power required. The power required is drag times airspeed:
$P = D\cdot V = D_p\cdot V + D_i\cdot V = k_p\cdot V^3 + \frac{k_i}{V}$
For the maximum endurance we need to minimise the fuel consumption and thus we need to find the speed that minimises the power.
$\frac{dP}{dV} = 3 k_p V^2 - \frac{k_i}{V^2} = 0$
Solving for $V$ results in
$V_{endurance} =  \sqrt[\uproot{1}4]{   \frac{k_i}{3k_p}} $
For the maximum range we need to find the speed that minimises the fuel consumption per distance travelled, which is found when the ratio of power to  speed over ground is minimal. As we assume there is no wind, the ground speed and the airspeed are equal.  Since the ratio of power to airspeed is drag, we have to find the speed for minimum drag:
$\frac{dD}{dV} = 2 k_p V - 2\frac{k_i}{V^3} = 0$
Solving for $V$ results in
$V_{range} =   \sqrt[\uproot{1}4]{   \frac{k_i}{k_p}} $
We can now show that the ratio of maximum range speed to maximum endurance speed is:
$\frac{V_{range}}{V_{endurance}} = \left. \sqrt[\uproot{1}4]{   \frac{k_i}{k_p}} \middle/  \sqrt[\uproot{1}4]{\frac{k_i}{3k_p}} \right. = \left. 1 \middle/  \sqrt[\uproot{1}4]{\frac{1}{3}} \right. =  \sqrt[\uproot{1}4]{ 3} = 1.316... $

Now as long as the assumptions hold, for any aircraft the ratio of the speed will be approximately 1.3.
For the F-35 the ratio may be approximately right in the subsonic domain. Effects of compressibility will cause the drag to be higher when the aircraft gets into the transonic domain so depending on the max-range speed the last two assumptions may not hold.
For an autogyro at higher speeds, the relation of induced drag to airspeed is inversely proportional to the square of velocity, just like conventional fixed wing aircraft. This is because the fundamental way of creating lift by deflecting the incoming airflow downwards is the same for autogyros and aeroplanes. Therefore autogyros have also a factor pf approximately 1.3 between their maximum endurance and maximum range speeds.


How can one enter an airliner without stairs or a jet bridge?

There are no rear entrances these days, but many smaller jets like the Avro-RJ, Bombardier CRJ series or Embraer regional jets have their own air-stair on the main (front left) door. On the smaller planes the door are hinged at the bottom and the stair are part of the inner side of the door, on the larger ones there may be folding stairs stowed beside the door. B737 can have those too, but it's optional, so many operators choose not to have them (and on second-hand aircraft the operator sometimes bother removing them, but rarely adding them back). I've never heard of them on A320 family as it's a lot higher.

You can get always out using the emergency slides and in cockpit they have a rope to climb down through the side window (where it's too high to jump from it) in case the cockpit door can't be used (happens sometimes).

However the only way in is using stairs. It's hard to imagine you wouldn't be able to scramble at least a ladder even at very remote landing strip where the plane has made an emergency landing. Regular landing will obviously not be done to a place without regular stairs (unless the plane has integral airstairs, of course) and if it was emergency landing you need to bring in mechanics and fuel to get the plane out, brining a ladder is not much extra trouble.

Edit: As Kevin noted below, many aircraft have engineering access hatch in or behind nose wheel well. Not an option for boarding passengers, but the crew can use it when they don't have stairs/ladder handy.


Why do model aircraft fly and maneuver so differently from real aircraft?

It's partly inherent in the way things scale.

If you double the length of the model, then the wing area (length times width) increases by a factor of 4, but the weight and volume (length times width times height) will increase by a factor of 8 ... so doubling the size means halving the weight-to-lift ratio.

In the most extreme cases, a tiny model will blow away on a puff of wind, and a huge model (bigger than the real plane) can't take off at all.

I suppose you might theoretically try to make small models harder to fly, by adding extra weight.

The above is theoretically true but maybe nonsense in practice: it assumes that structural materials become thinner when the model is scaled, in reality the structure isn't even the same material.

So let's look at it another way:

A full-scale A380 weighs let's say 500 tons, length about 70 metres.
Decrease that to 1 metre model and the surface area has decreased by (70x70=) 5000.
So for the model to have the same weight-to-area as the full-scale plane, it would need to weigh (500 tons / 5000 =) 100 kg.

Your 1 metre model presumably weighs much less than 100 kg, therefore it has much less weight-to-area ratio. QED.

It's also important to consider the Reynolds number, which depends on the air's viscosity and density, and on the size and speed of the model. The Reynolds number affects turbulence, which is very important to a wing's lift (for an example of how even a tiny change has a large effect, see Can a sandpaper-thick layer of ice reduce lift by 30 percent and increase drag up to 40 percent?).

To get the right Reynolds number for a small model you must increase the density (e.g. pressure) of the air, or increase its speed. But given the ordinary speed of aircraft, you couldn't increase (scale up) the air speed because it would become super-sonic, which would change the scenario.

Based on this answer to 'Understanding the Reynolds-number scaling problem', and the comments below it, I think that a 1-metre model of a 70-metre A380 (so a scale of 70:1) might behave like the full-scale model if it were flown under the following conditions:

air density is scaled up, so 70 atmospheres of air pressure
lift and drag are scaled down, so:

weight of the model is 7 tons (instead of 500 tons)
thrust of the model is 4,000 lb (instead of 300,000 lbs), i.e. about 2 tons

air speed is realistic (e.g. 150 knots to take off)

Obviously this would be quite unusual for a model airplane1.

1Air liquifies at 60 atmospheres; and the model would need a specific density of about 100, i.e. 5 times heavier than gold or uranium).


What extreme measures might a pilot take to avoid extreme weather?

Any which the pilot thinks is more dangerous than the avoidance measures?

If you were driving at 100mph towards a wall which you reckoned had a 90% chance of a head-on collision, or could attempt to turn which had a 50% chance of throwing you off a cliff, which would you choose?

Any decision the pilot makes will be made on the basis of "Taking this action is a better option than not taking it".

The main weather which would likely cause this would be high enough levels of icing to be dangerous, or such severe turbulence that the pilot believes the plane will be "upset" enough to throw it out of the performance envelope. A combination of these may be present in a severe storm.


How can I check that the GPS receiver is sending data to the transponder?

Your puck (GlobalSat BR355) runs on 5v and the UART line idles high (it is pulled high when there is no traffic).   The first thing you need to do is find the power lines that go to the gps puck -- connect wires to these lines (5v and gnd) and run them to the spot where you want the button/led.   Then make this circuit:

The value for R1 should be 150 ohms (which will yield under 20ma of current through the LED, assuming your led has a voltage drop of 2.1v (which the ones in the link I provided do have))

Here is a link to a transistor that will work for Q1:

https://www.digikey.com/product-detail/en/central-semiconductor-corp/2N4403/2N4403CS-ND/4806893

Here is a resistor that will work for R1:

https://www.digikey.com/product-detail/en/stackpole-electronics-inc/CF14JT150R/CF14JT150RCT-ND/1830603

And here is a link to a selection of LEDs you can use (all with voltage drop of 2.1v; Any of these should work; just choose a color and form factor that works for your application):

https://www.digikey.com/products/en/optoelectronics/led-indication-discrete/105?k=led&k=&pkeyword=led&pv16=2&FV=1140050%2C4fc0015%2C1f140000%2Cffe00069%2C87c001d&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Here is a link to a selection of pushbuttons that should work -- just choose a form factor you like (look at the datasheet for the ones with 4 pins -- these are just two pairs of pins (each pair is connected) for mounting ease -- two pins can be harder to mount on a circuit board):

https://www.digikey.com/products/en/switches/pushbutton-switches/199?k=pushbutton&k=&pkeyword=pushbutton&pv130=7&FV=1140050%2C2dc0017%2C3ac000e%2C1f140000%2Cffe000c7&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

I personally recommend you use something like this for the button:

https://www.digikey.com/product-detail/en/e-switch/KS-01Q-01/EG4791-ND/2116270

For this button here is a picture and a schematic (as you can see, the pins on either side of the flat side of the button are connected:

And something like this for the led:

https://www.digikey.com/product-detail/en/cree-inc/C503B-RAN-CZ0C0AA1/C503B-RAN-CZ0C0AA1-ND/6561758

Just wire these components up according to the picture and solder them together -- I'll leave how to mount the switch and the LED up to you but make sure once you are done you cover all the exposed wire with glue and/or electrical tape and/or liquid electrical tape so they don't accidentally short out.   Also make sure no exposed wires within the circuit are touching where they shouldn't touch (especially make sure you separate the pins of the transistor because if they accidentally touch the circuit will not work and you may damage your gps receiver -- long story short be careful in your wiring and make sure nothing is touching that shouldn't.


What is the youngest age allowed to fly any sort of plane in the USA?

The easiest way to figure this out for the US is to look at 14 CFR Part 61

  §61.83   Eligibility requirements for student pilots. To be eligible
  for a student pilot certificate, an applicant must:

  (a) Be at least 16 years of age for other than the operation of a
  glider or balloon.

  (b) Be at least 14 years of age for the operation of a glider or
  balloon.

That is the age at which you can solo. Lots of airplane pilots take lessons and solo on their 16th birthday. I don’t follow the gilder world and balloon world, but I assume lots of kids solo on their 14th birthday.

To get a license you must be a bit older:

  61.103   Eligibility requirements: General. To be eligible for a private pilot certificate, a person must:

  (a) Be at least 17 years of age for a rating in other than a glider or
  balloon.

  (b) Be at least 16 years of age for a rating in a glider or balloon.


What are some alternative benefits to electric propulsion?

For aircraft, none.

Hybrid drive trains are used because:

They offer, rather limited, efficiency improvement by being able to operate the engine closer to its optimal conditions.

Because spark-ignition engines are more efficient at higher power settings, the hybrid can get advantage by having smaller engine, running it at (relatively) high power or not at all and cover the variations in required power using the electric engine.

Cars accelerate and stop a lot and often cruise at small fraction of power, so they can make good use of this.

The advantage only applies to spark-ignition, i.e. gasoline, engines. Compression-ignition (diesel) engines have more uniform efficiency over their operating range, so there is no or only negligible advantage. Which is part of the reason you don't see hybrid diesel cars.
Batteries can be pre-charged with cheaper mains power. This is used in the hybrid cars, but it obviously requires lots of heavy batteries.
Electric motors can provide high torque from standstill. Piston engines can only provide limited torque from standstill and it stresses the clutch heavily.

This is particularly important for train engines. Train wheels have bearings lubricated by oil pumps attached to the axles that only operate when the train is moving, so when the train is stopped, the bearings are not lubricated and the force required to get it moving is much higher then when moving.
At high power outputs, electric generator and motor are simpler and more reliable than a gearbox and clutch. For big train engines and ships, mechanical clutch is out of question. Hydraulic one, which is a turbopump and turbine in oil, can be used, but that still needs variable gearbox. The electric drive-train acts as almost ideal continuously variable transmission, being able to deliver almost constant power over wide range or rotational speeds.

In either case the hybrid drive-train is heavier than mechanical one.

Now for aircraft:

The engine is run at constant speed and power setting, which is relatively high (usually cruise setting is 75% for piston and even higher for turbine planes). So there is no room to design a hybrid drive train to run the engine closer to optimal.
Weight is critical for aircraft, so the weight of electric generator and motor is a show-stopper there. For cars adding weight causes smaller increase in drag, so its less of a concern and trains even need to be heavy to have enough adhesion for their traction.
Since weight is critical, aircraft can't afford to take much batteries that could be pre-charged with cheaper mains electricity, the other benefit hybrid cars have.
Propellers need almost no force to start spinning and are operated at narrow range of RPM, so the ability of electric motor to provide torque from standstill and across wide range of RPM is of no use to aircraft.

So none of the benefits for which hybrid drive trains are used in other vehicles is applicable to aircraft, so there is no point in building one.


Can a flow which is initially supersonic, become subsonic without a shock wave?

Yes, it is possible . The shockless transition from supersonic to subsonic. It has been tested both experimentally and numerical solution. Read this AIAA Paper.

But in your case I would suggest you to be a bit cautious and recheck your mesh and boundary condition in CFD solver.


Why does IAS decrease with lower air density if power is constant?

Because even with constant power, thrust decreases with velocity, also known as TAS.

Drag depends mainly on the dynamic pressure, which is what IAS represents, so if you had the same thrust, you'd have the same IAS too.

However a piston engine produces approximately constant power (if your RPM is in the optimal range), and power is thrust times velocity. So the turbocharger lets you have the same power in the less dense air, but because your true airspeed is higher, the thrust is a bit lower and therefore your (maximum attainable) indicated airspeed is as well.

And then there is the effect of RPM. Engine power (of any engine) is limited by RPM, growing up to some optimal speed and then declining (and then you get to the red line). If you have a fixed propeller, your available power grows with speed first as the RPM increases with decreasing effective pitch, but then you cross the optimum and the power starts reducing and then you hit the red line and have to retard the throttle. And effective pitch again depends on TAS.


How is the amount of time for an A check or daily maintenance determined?

This answer is not specific for A-checks, but covers the whole maintenance effort.

Try Eddins-Earles, Mary: Factors, Formulas and Structures for Life Cycle Costing, Eddins-Earles, 89 Lee Drive, Concord MA 01742, USA, 1981

It's a bit dated, but has a wealth of data.

Another source would be Fabrycky, Wolter J.; Blanchard, Benjamin S.: Life-Cycle Cost and Economic Analysis, Prentice-Hall International. ISBN 0135383234.

Also, the white volume (I think it is No 8) of Jan Roskam's collection is a good start. Roskam, Dr. Jan: Airplane Design, Roskam Aviation and Engineering Corporation, Ottawa, Kansas 66067, USA, 1990.

Generally, this is not exact science, so exact times cannot be expected. With experience, each operator will be able to estimate how long things take, but you do these checks to find possible damage which needs to be taken care of. A lot depends on the training and experience of the maintenance crew. The best way to approach this is with statistics.


Who pays for the damages in this Asiana-Aeroflot collision?

Short answer:
Someone's insurance company is writing a check.

Longer answer:
What usually happens in the insurance world is both insurance companies write checks to repair their respective clients' aircraft.  The insurance companies may then fight it out between themselves to determine who is responsible for what, if it makes financial sense to do so.

In cases where injuries or deaths result from an incident it almost always makes sense for the insurance companies to fight it out(those payouts can run in the millions of dollars).

In cases like this where there's only equipment damage the cost of the repair will determine how aggressively Aeroflot's insurance carrier pursues getting their money back from Asiana's carrier.  If we're talking about extensive and expensive structural repairs into the millions of dollars Aeroflot's insurance carrier will be motivated to get someone else to pay the bill. Conversely if the insurance company can make their money back through Aeroflot's premiums and pursuing Asiana for negligence or something along those lines to try to get them to pay for the repair would ultimately cost more (money and time pursuing the case) than just absorbing the loss the insurance carrier will just absorb the loss.


What is this hole on the left wing of the Eurofighter Typhoon?

The Auxiliary Power Unit (APU) exhaust

(Source)

The item 72 is what you are looking for and it is labelled "APU exhaust". The black residue is soot from incompletely burned fuel.

The Eurofighter Typhoon can use its APU to start the main engine or to provide power when on the ground. From airvectors.net:

  The Eurofighter is also fitted with an auxiliary power unit (APU) for self-starting and ground power.

See also:

What is the APU actually used for?
Do fighter jets have an auxiliary power unit?


What are the customs decal and eAPIS requirements for flying to the Bahamas?

AOPA has a useful guide to flying to/in the Caribbean, including the Bahamas. To answer your specific questions:

You do need a customs decal if your aircraft is considered private by the CBP, regardless of whether you own it or rent it (I've seen them myself on several aircraft that I've rented)
eAPIS is a system for reporting who is on board an aircraft every time it crosses the US border, it has nothing to do with the decal


Do Special MEA altitudes and routes in Alaska require WAAS?

The answer appears to be yes. The FAA document that describes this rule says:

  TSO C145a and TSO C146a GPS WAAS navigation systems are authorized to
  be used as the only means of navigation on Federal airways and other
  published ATS routes in lieu of ground-based navigation aids in Alaska

The IFR chart legend says:

  To utilize these routes aircraft will need to be equipped with IFR
  approved Global Navigation Satellite System (GNSS). In Alaska,
  TSO-145a and 146a equipment is required.

And both TSO C145 and C146 specifically mention WAAS in their titles.


Would the Aeroflot Flight 1492 have been able to dump its fuel first before landing?

This aircraft does not have fuel dumping capability. In general, only large wide body airliners have fuel dumping capability.

Aircraft can land overweight quite easily but getting stopped on the runway safely is the main problem.

Even with fuel dumping, aircraft will often still plan to land overweight. The amount of fuel to be dumped would be planned to make a safe landing on the available runway without excessively heating the brakes and tires.


How does this test flight escape aparatus work?

Qualification: I am an FAA master parachute rigger and taught sport skydiving for 10 years.

Most of the comments on the question are correct - the red slide will extend below the airplane and act as a wind buffer so you can clear the fuselage before the wind blast hits you. The yellow part is a cap that would be removed.

No one would clip a static line to the plane in an evacuation - you want to be well clear of the airplane before opening your own chute. The blue packs are the real thing, I've packed plenty of them. There's a 26-foot round parachute inside. Rough landing but usable by anyone with very little training.

How usable this contraption would be is a different question - skydiving is done at about 100 knots, well below the speed of jets. And it also requires a stable aircraft - if that plane loses it's tail and starts to spin no one will be using the escape slide simply because it will be impossible to reach it. The "bail out in the event of emergency" concept is a bit flawed - if the airplane is flying well enough to bail out then you don't need to. There's a very good reason the military spends a fortune on ejection seats.

Side note - the Space Shuttle had a crew escape process, but NASA considered landing a shuttle on anything other than a proper runway to be non-survivable.


How do engine manufacturers minimize damage from bird strikes?

Placement on aircraft that fly above birds. This is the #1 reason we don't hear about airliner bird strikes every day on the news; cruise altitude for airliners is around 30,000 feet, while only two species of bird have ever been recorded flying at that height, one in Central Africa, the other around the Himalayas, neither of these being common travel lanes for airliners. The critical time for most bird strikes is below the cloud ceiling and/or transition altitude, above which the air becomes thinner and birds have to work harder to stay aloft. Thus most birds fly below transition, while airliners spend as much of their flight time as feasible above it as the thinner air reduces drag and lowers fuel consumption (the "strata" of thicker air at the transition point, in addition to the simple reduction of SPL on the square of distance, also helps insulate people on the ground from jet noise).
Strong intake fan fins. The first set of turbine fins that you see looking into a nacelle are the ones in the direct line of fire. They're also the largest fins, but in their favor there are higher acceptable tolerances for those fins versus the ones in the compressor stages. So, they're built strong to hopefully survive ingesting your average goose. Here's a YouTube of the A380 engine's bird strike test; the engine continues to operate even though the blade that was struck is misaligned on the turbine hub for a few rotations.
Garbage disposer effect. At ascent throttle levels, an airliner's engines are rotating between 10,000 and 15,000 RPM. Even travelling at cruising speeds of 500-550 knots, the rotation speed of the turbine will chop most organic matter into mincemeat in a fraction of a second. In fact, bird strike tests show the bird carcasses literally exploding from the force of impact with the rotating blades. These smaller pieces cause fewer problems passing through the engine.
High bypass. About 80% of the air volume entering most underwing airliner engine nacelles passes around the turbine chamber instead of through it, increasing thrust while reducing fuel consumption. Fuselage-mounted engines on older aircraft had lower bypass ratios (one of the reasons those designs are being phased out of modern fleets). A bird entering the engine has a similar chance to pass right on through without encountering the combustion chamber.
Reinforced nacelle housing. In the event a bird strike does dislodge something in the engine, the nacelle housing is engineered specifically to contain the shrapnel, minimizing the chance of damage to fuel and control lines in the wings and of course to the fuselage. (Here's a YouTube of a "blade-out test" for the A380's Rolls-Royce engines) The pilots will be alerted the engine has failed, and will cut off the fuel supply, engage fire suppression and run on the remaining engine(s).
Overspecification of required engine thrust. One of the major tests an airliner design must pass before being FAA certified is an "engine cutout at decision" test. An aircraft is loaded to MTOW, taxis to the runway, the engines are spun up, the aircraft rolls out, then at the "decision speed" above which the pilot must commit to a takeoff, an engine is idled. The aircraft must still be able to get off the runway and attain a minimum climb rate. That test, for most modern two-engine jets, shows that the plane is perfectly capable of flying on one engine (if such a thing happened at takeoff in real life the jet would circle and land again but it would not have to risk an aborted takeoff above decision speed).

In short, these engines are built tough and tested tough, as is the rest of the aircraft (manufacturers build several planes' worth of components and subsystems for the express purpose of torture testing, and that's if they get it all right the first time). In addition to bird strikes, engines are tested for water ingestion, hail strikes, sand ingestion, smoke ingestion (a British Airways 747 lost all four engines after flying through the upper ejecta cloud of an erupting volcano; they got two restarted and landed safely, but future engine tests got even more stringent to avoid a repeat performance), and a host of other things that just aren't good for jet engines.

The most famous bird strike incident in recent memory, Sully Sullenberger's emergency water landing in the Hudson River, was caused by hitting not one or two, but an entire flock of birds, and not sparrows or pigeons, but Canada geese, which can weigh up to 20 pounds each. This incident is still a good example of how well engineered these engines are; both engines would have been at full revs as the plane was climbing out of LaGuardia, yet AFAIK the turbines, though deformed, didn't even part company with the hub, saving the control surfaces and lines for the ditch attempt, resulting in no fatalities and only one hospitalization.


Would a random passenger be allowed to fix a problem with a commercial aircraft?

I happen to have personal knowledge of this particular case. I was in that airplane, too, seated two rows away from him. I'm also a former pilot and was naturally very interested in what happened, so I asked him some questions during the flight back.

He had worked as service mechanic for this company before, servicing their planes in Iceland. Their technical staff and the pilot of this plane already knew him, which made things easier.

Tools and parts were available at the airport in Alicante, but no qualified technician certified for this type of aircraft. The technical staff of the airline contacted his boss in Iceland by phone, who agreed to sub-contract him to the airline for the duration of the repair and pay him overtime for the work.
They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair.


What does it mean to "firewall" an aircraft engine?

"To firewall" is a phrase meaning to go to full power. Most aircraft throttle controls provide full power when moved to their furthest forward position - the direction towards the firewall separating the nose mounted engine from the cockpit in aircraft in the past. The phrase is still used, just as we "dial" a telephone even though the telephone dial is no longer used, either. A similar one for automobile driving is "pedal to the metal".


Do you need to fill out the MedXPress form every time you renew your medical?

Each time you renew you will need to complete the medical history questionnaire on the MedXPress website and obtain a fresh confirmation number.


May a pilot legally fly an ILS approach to ILS minimums with an inoperative Attitude Indicator without declaring an emergency?

For the record, the asker changed the question after I answered, specifying that not declaring an emergency is a condition of the question, so I will address this at the top without modifying the original part:

An attitude indicator is legally required for IFR flight.  If equipment becomes inoperative during IFR, the pilot should inform ATC.  I do not know that there is any specific legal text that addresses this particular situation you're describing with an ILS approach coupled with an AI failure, but it would be generally accepted by pilots that losing an attitude indicator is an emergency as opposed to losing other required IFR equipment such as the clock.  So, I'm going to go with no, not without declaring an emergency.

Original:

In that situation depending on the circumstances, declaring a missed approach, an emergency informing ATC of the system failure, and requesting vectors to VMC conditions would be the best option.

In an emergency, you can deviate from rules to meet the needs of the emergency at your discretion, so I don't believe that technically you can't still shoot to minimums for the ILS, but it would be dangerous, especially if the pilot isn't highly proficient, and should be a last resort with no primary gyros.

If an approach needed to be shot, and there was no VMC within range, a 'Precision Approach Radar' approach would likely be the best option if one existed within range.  This is can be a 'no-gyro' approach upon request.


When the velocity of the airflow locally exceeds speed of sound above the wing a shock wave forms and the flow detaches beyond this shock wave.

Similarly to stall, the supersonic separation of flow removes the component of lift produced by decrease of pressure on the upper surface of the wing and so the effects are similar.

I causes reduction of lift and because centre of pressure is about quarter chord on the upper surface, but midchord on the lower, it causes a significant pitch-down moment, which might be impossible to recover even if the post-stall lift is otherwise sufficient to keep the aircraft flying straight. This effect is often called Mach tuck. Supersonic planes often have all-moving elevators to have sufficient control authority to compensate for it.

A difference from normal stall is that after supersonic flow separation the lift remains proportional to angle of attack and so the aircraft continues to behave more or less normally except for the change in trim.

Mach tuck may occur as low as Mach 0.7 depending on aircraft design, because the air moves faster over the wing. It can be delayed by using swept wings, because the shock waves only form when the air velocity component perpendicular to the wing exceeds speed of sound.

Supersonic aircraft eventually encounter supersonic flow separation, but the lower surface lift is sufficient to balance the aircraft weight at that speed and altitude and the aircraft can continue flying with the upper surface flow separated.

That's not the case with most subsonic aircraft, which for efficiency reasons tend to cruise at altitudes where they have very small margin to stall. As the stall speed increases with altitude while speed of sound slightly decreases with the lower temperature there, the stall speed will eventually equal critical mach number, which creates the coffin corner and absolute ceiling for the aircraft. For most civilian transport aircraft the range between stall speed and critical mach number (where drag increases and supersonic flow separation would begin) reduces considerably at cruise altitude, but they usually don't have enough engine power to reach the actual coffin corner.


Dispensing with the math, because it makes my head hurt...

Azimuth refers to geographic direction relative to a compass reference point, so azimuth angle refers to the deviation between the body's longitudinal axis and its horizontal path parallel to the earth's surface (heading vs track along the ground).

Sideslip angle refers to the deviation, referenced along the lateral axis of the body (parallel to the wings in other words), between the body's longitudinal axis and the airflow, independently of direction relative to the earth.


Why can't aircraft be cleared to land before the preceding aircraft turns off the runway?

Actually, the FAA doesn't always require the preceding airplane to clear the runway if they have the required separation unless it is dark (when it is very hard to visually estimate distances).

According to the FAA Air Traffic Control Order Section 10. Arrival Procedures and Separation:

  3-10-6. ANTICIPATING SEPARATION

  a. Landing clearance to succeeding aircraft in a landing sequence need
  not be withheld if you observe the positions of the aircraft and
  determine that prescribed runway separation will exist when the
  aircraft crosses the landing threshold. Issue traffic information to
  the succeeding aircraft if a preceding arrival has not been previously
  reported and when traffic will be departing prior to their arrival.

  EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land,
  number two following a United Seven-Thirty-Seven two mile final.
  Traffic will depart prior to your arrival.”

  “American Two Forty-Five, Runway One-Eight, cleared to land. Traffic
  will depart prior to your arrival.”

  NOTE- Landing sequence number is optional at tower facilities where
  the arrival sequence to the runway is established by the approach
  control.

  b. Anticipating separation must not be applied when conducting LUAW
  operations, except as authorized in paragraph 3-10-5b2. Issue
  applicable traffic information when using this provision.

  EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land.
  Traffic will be a Boeing Seven-Fifty-Seven holding in position.”

And the separation is specified in the same document:

  3-10-3. SAME RUNWAY SEPARATION

  a. Separate an arriving aircraft from another aircraft using the same
  runway by ensuring that the arriving aircraft does not cross the
  landing threshold until one of the following conditions exists or
  unless authorized in para 3-10-10, Altitude Restricted Low Approach.

  The other aircraft has landed and is clear of the runway. (See FIG 3-10-1.) Between sunrise and sunset, if you can determine distances by
  reference to suitable landmarks and the other aircraft has landed, it
  need not be clear of the runway if the following minimum distance from
  the landing threshold exists:

  (a) When a Category I aircraft is landing behind a Category I or II-
  3,000 feet.

  (b) When a Category II aircraft is landing behind a Category I or II-
  4,500 feet.


Why does the local speed of sound decrease with temperature and not pressure?

It may be helpful to remember the ideal gas law here: $PV=Nk_BT$ (the physics version). If we divide both sides by $N$ we have a $V/N$ on the left which is just a $1/\rho$ where $\rho$ is the number density. Thus:

$\frac{P}{\rho}=k_BT$

In an ideal gas, to which air is close enough, the speed of sound is given by:

$c_s = \sqrt{\gamma \frac{P}{\rho}}$

Throwing in our first results, we see that:

$c_s = \sqrt{\gamma \frac{P}{\rho}} = \sqrt{\gamma k_BT}$

In other words, the speed of sound reduces with either pressure or temperature because those two are in turn connected by the ideal gas law. It's basically two different ways of looking at the same question.

Side note: throwing in the effect of density is another question all together since $c_s$ increases with an increase in temperature or pressure but decreases with an increase in density. The question then becomes whether the decrease due to the drop in pressure overcomes the increase due to the drop in density at higher altitude.


What do tower controllers do when the airport is quiet?

The same things people do when working jobs like night shift guard shack operator... catch up on paperwork, read, study, listen to music, etc.


Where did the "K" identifier prefix for US airports come from?

The letter K was simply assigned to the contiguous US by ICAO, in order to have a system with unique identifiers for world-wide use, instead of trying to adapt local system to match. The IATA codes had been in use already and possible duplicates could not be excluded. The K in US ICAO codes is there to mark the region the codes is in, see the chart below.

A good summary of how many of the airport codes have been created can be found on www.skygod.com.

Airlines use the three-letter codes internationally in their own network, Sita, for messages such as passenger loads and departure times. World ATC and weather agencies use a separate teleprinter network, the Aeronautical Fixed Telecommunications Network (AFTN), which uses a four-letter "location indicator." Going from large area to actual airport, the first letter relates to the part of the world and the second letter the country. The third letter is a group of airports within that country. Most countries who use this particular convention use a letter to denote the FIR in which the airport is located. So F is Frankfurt FIR in Germany, M is Munich; P is Paris FIR, M is Marseilles. Other ways to use the third letter include identifying a group of airports with a common factor. For example, A was used in Germany for all Canadian and American air force bases. The last letter positively identifies a specific airport.
Thus Aberdeen, Scotland, has the International Civil Aviation Organization (ICAO) location indicator of EGPD—E for Northern Europe, G for United Kingdom, P for Scottish region, and D for Dyce field. Want to figure out LFPG? It's L for southern Europe, F for France, P for Paris FIR, and G for Charles de Gaulle airport. Easy!  Some are more easy to remember. The Dominican Republic codes all start with M (Central America) and D (Dominican) and then, well, see if you spot the code for international airports in Santo Domingo (MDSD), La Romana (MDLR), Puerto Plata (MDPP), Punta Cana (MDPC) and Santiago (MDST).
So if London Heathrow has two codes — and it does, LHR and EGLL — how come I've heard Chicago O'Hare only called ORD? The answer is unique to the United States and Canada. In the 48 contiguous States the ICAO code is formed simply by adding a "K" to the three letter code (or by adding a "C" in Canada). This explains why international flight plans refer to KORD, KMIA, KJFK, etc. A meeting of two rules is Key West, the FAA code is EYW (lose the 'K') and the IATA code is KEYW (add a 'K') which works great for KEY West.
Other airports within FAA jurisdiction have ICAO codes usually formed by taking two letters from the FAA name and prefixing them with PA for Alaska, PH for Hawaii, or PG for Guam. PT appears to be Pacific Trust Territories (Pohnpei, Yap, Chuuk, etc.), and there are various random other P codes like PWAK for Wake Island.
While most U.S. airports have the same three-letter location identifier for the FAA and IATA, note I didn't say all do. Scenic Sedona Airport in Arizona is assigned SEZ by the FAA and SDX by the IATA. Because for international airlines SEZ is Seychelles International Airport in Mah�, Seychelles. Another is the Hilton Head airport in South Carolina mentioned earlier. It's IATA code is HHH, but it's FAA three letter code is HXD.

Related question:
What is the methodology for assigning airport codes?