When is turbulence most dangerous to an aircraft?

Wake turbulence can affect aircraft in the most critical phases of flight such as landing approach.

For example, consider this AOPA report

  On June 12, 2006, while on visual approach at Kansas City International Airport in Kansas City, Mo., the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway. The Saratoga encountered wake turbulence so violent that it tore apart the aircraft in flight. The pilot and his passenger were killed.

The effects are most severe for small aircraft entering the wake of a large aircraft but large aircraft can be affected too.

Skybrary lists several examples of incidents involving wake turbulance.


Why is ground effect greater over smooth hard surfaces?

From a conservation of energy perspective, the rotor is transferring energy to the air, which then moves down and is deflected by the ground. A soft or drag-inducing surface will absorb more of that energy, whereas a smooth hard surface will deflect it.

Think of it like running: If you run on a hard surface, your down force creates an equal force up. If you run on sand, some of the down force is lost by moving the sand, and you are not propelled up/forward as much. This is why it is harder to run on sand than a solid surface.


In some cases the prevailing winds change and certain runways fall out of use. Since airports tend to be land limited they may wish to use the land for hangars or other facilities so the runways are eventually decommissioned.
In other cases it could a noise abatement issue, many airports predate the property development that often surrounds them. It may be in the best interest of the local neighborhood to change the runway orientation to one that creates less noise and pollution over homes or businesses.
In some cases runways may be moved to comply with changing regulations as was the case at Aniak Airport, where an existing runway no longer complied with obstacle clearance regulations.
In some cases it also becomes expensive to maintain a runway that may not see much use and it will be shut down.

Magnetic shift generally wont cause a runway move but it may cause a runway re-number since the runways are numbered as per their magnetic heading.


Am I allowed to replace my own Model A-30 Altitude Digitizer?

Per 14 CFR 43.3(d):

  A person working under the supervision of a holder of a mechanic
  or repairman certificate may perform the maintenance, preventive
  maintenance, and alterations that his supervisor is authorized to
  perform, if the supervisor personally observes the work being done to
  the extent necessary to ensure that it is being done properly and if
  the supervisor is readily available, in person, for consultation.
  However, this paragraph does not authorize the performance of any
  inspection required by Part 91 or Part 125 of this chapter or any
  inspection performed after a major repair or alteration.

This means if you are not a mechanic yourself, you can still perform this work as long as it is supervised by someone who is authorized to perform the work. Based on your question, I assume that you actually want to perform this work yourself.  Further down in 14 CFR 43.3(g), the FARs state the type of work you can perform without supervision:

  Except for holders of a sport pilot certificate, the holder of a pilot
  certificate issued under part 61 may perform preventive maintenance on
  any aircraft owned or operated by that pilot which is not used under
  part 121, 129, or 135 of this chapter. The holder of a sport pilot
  certificate may perform preventive maintenance on an aircraft owned or
  operated by that pilot and issued a special airworthiness certificate
  in the light-sport category.

Preventive maintenance is defined in  Appendix A to 14 CFR 43, paragraph C. If a function does not appear in this list, it is not preventive maintenance, and as such must be performed by personnel authorized to perform repair work. The only item in this list that is even remotely relevant to your situation is item 31:

  Removing and replacing self-contained, front instrument panel-mounted
  navigation and communication devices that employ tray-mounted
  connectors that connect the unit when the unit is installed into the
  instrument panel, (excluding automatic flight control systems,
  transponders, and microwave frequency distance measuring equipment
  (DME)). The approved unit must be designed to be readily and
  repeatedly removed and replaced, and pertinent instructions must be
  provided. Prior to the unit's intended use, and operational check must
  be performed in accordance with the applicable sections of part 91 of
  this chapter.

Note that your situation does not appear to qualify, because the item you are replacing is not part of a self-contained, tray-mounted device.  Even if it were, you would still not be authorized to perform the work because this item specifically excludes work on transponders.

Of course, I am not an attorney nor do I perform aircraft repairs, so my interpretation is completely non-qualified. For the best answer, I would suggest contacting your local Flight Standards District Office for guidance.


Is line training (pay to fly) allowed in the USA?

It is not prohibited in the US, but I don't think it's currently practiced anywhere.  There used to be a school called Tab Express that did something like this, but the most notorious one was an airline called Gulfstream International Airlines that operated as United Express in Florida.  Gulfstream is now operating as Silver Airways and, as far as I know, has discontinued the practice of having the FO pay to be there.


Can a kite take advantage of crosswinds to gain speed?

Why do you need a kite? A glider is much better suited to the task.

Again, nature has invented this long ago. Albatrosses use a technique called dynamic soaring to cover wide distances over water, using the windshear close to the surface.

Humans have tried to imitate this with gliders, albeit with limited success. The few who tried it could indeed stay in the air by harvesting energy from windshear, but the dynamic loads (up to 4 g) were too much to do this over an extended time.


Does it exist? Environmentally-friendly/safe fluorescent liquid to make sea crashes easier to find?

Thanks to those who commented and provided answer-worthy comments. Thanks!

What is already in existance today:

@Andy commented:

  For spacecraft splashdowns (certainly Gemini, maybe others) they used
  a green marker dye, but I don't know how long it lasted. That would be
  a good thing to start with as they must have tested how long the
  traces were visible when choosing it.

@RogerLipscombe commented:

  prestodye.com/sea-dye-markers says "30 to 40 minutes", depending on
  sea conditions. Apparently there's a standard: MIL-S-17980D.

And for the challenges:

@abelenky commented:

  Rough approximation: How many gallons of this stuff would you need for
  it to not dissipate in minutes? How much would that weigh? How often
  to airplanes crash in the sea? The cost and complexity just doesn't
  even come close to the benefit.


why are regional cruising altitudes higher than those of international much longer flights?

Flight altitudes have relatively little to do with whether a flight is regional or international per se. More important factors include the winds aloft and the individual aircraft capability.

In the case of the Winnipeg to Manchester flight, they were probably using the North Atlantic Track System (NATS). The winds aloft over the North Atlantic are generally out of the west. The routes of the tracks are arranged each day to take advantage of the those winds. The track system puts the eastbound aircraft on tracks and at altitudes that would maximize the tailwinds, and westbound aircraft on tracks and at altitudes that minimize the headwind.

Since the winds aloft in the northern hemisphere are more often than not westerly, the Winnipeg to Vancouver flight might well have been fighting a headwind, but the strongest winds aloft tend to occur in the mid 30s, which can mean that you can lessen the headwind by climbing higher.

Aircraft capability also plays a major role, and aircraft vary widely in what they can do altitude wise. For example, a 747-200 up against its max takeoff weight of 800,000+ lbs can initially climb to only about 31,000 feet (FL310), but towards the end of the flight can reach FL390 and above.


How are crews managed after a diversion (far from hub)?

I'd say you've answered your question yourself, by listing the various scenario's, any or all of which could be of concern in a particular situation. Handling a diverted flight is a juggle between, safety, time, comfort factor (for crew, for passengers), cost, etc. In turn these are affected by feasibility, location of base (for replacement of crew/airplane), ease of availability of replacements, number of passengers, break up of their final destinations (likely factor when flying to long-haul to a hub), fuel and tech facilities available at the diversionary airfield, whether local 'diversion handling' contracts are in place, or the situation is being handled on-the-fly at an obscure airport. Airlines tend to have their own tailor-made SOPs, or guides.
These issues are ideal ones to be addressed by Airline Alliances such as STAR or 'One World' and traditionally all help is given to such flights. "You today, could be me tomorrow"
Assuming the airplane is ok, The change of crew may not be required if the remainder of the flight fits within the DTL's (Duty Time Limits) and the extra landing for recovery falls within the ambit of the rules. Crew are expected to be rested enough, when reporting for Flight Duty, to operate to their limiting DTL's. There are allowable extensions to these limits too, provided every crew member is upto it, ie not fatigued, and the Captain judges it to be a true reflection of the situation. The Captain would have to document all this in the Regulatory/Company paperwork. Note that Long-haul flights carry more crew than the minimum for shorter flights, so all crew get some time-out and an opportunity for horizontal rest, ideally in the form of a 180deg reclinable seat or bunks, normally in a private crew rest area.
For Ultra Long Haul flights, the hours, and crew numbers are ideally decided on a case by case, 'city-pair' basis, along with the Regulator and related procedures documented by the Operator, acceptable to crew, and approved by the Regulator.
In case a layover has to be declared by the Captain, many Operators/Regulators have a concept of Minimum Rest, which maybe typically 8 or 6 hrs of undisturbed rest, sometimes called (hotel) "key to key" time. This may provide more certainty than organizing a relief airplane that has to fly 12 hours to get there.


Why does the APU Master Switch flash during a normal APU shutdown?

If it's the flap closing, you should be able to verify that from the APU ECAM page.

Based on the time frame you mentioned, most likely it's the simulated overspeed, which is part of the shutdown sequence and is basically the signal that closes the fuel solenoid (which you can also verify from the APU page).

APU flap closing comes later. For more on the shutdown sequence, see: Why would smoke enter the cabin if the batteries are switched off before the APU flap closes?

  The cool-down cycle lets the APU operate for 60 seconds at a low stabilized temperature. (...) Overspeed test occurs when the cool-down cycle is complete. The ECB supplies a signal to simulate an overspeed condition, that causes the fuel solenoid to close, and starts the APU shutdown. [emphasis added]

Source: APU training manual


Is defacing the airline logo of a wrecked airplane illegal?

In the US (you tagged your question FAA) it would most likely be illegal. 49 CFR 830.10 says (note that the NTSB investigates US accidents, not the FAA):

  (b) Prior to the time the Board or its authorized representative takes custody of aircraft wreckage, mail, or cargo, such wreckage, mail, or cargo may not be disturbed or moved except to the extent necessary:

  (1) To remove persons injured or trapped;

  (2) To protect the wreckage from further damage; or

  (3) To protect the public from injury.

It seems highly unlikely that painting over logos would be justified for any of those reasons. On the other hand, this wording leaves the question open of what might happen after the NTSB takes custody, and in theory an airline could do it with permission.

Personally, I find it hard to see how this would help the airline because the story would be all over the news anyway, but laws and attitudes vary widely in different parts of the world.


To paraphrase GySgt Foley from the movie An Officer and a Gentleman:   “My grandmother is an aspiring Naval Aviator!”
Anyway, I am a former LSO, and I pulled out my old LSO NATOPS manual to see if I could find book definitions of the criteria for each grade.  I can’t seem to find them, but they are as close to verbatim as I can remember as described in the answer here.
Worth noting, the grade is not always about the wire you catch.  In other words, OKs aren’t limited to the 3 wire, you can still get an OK for a 2 or 4 wire.  It all depends on how and why you got that particular wire.  For example, if you get low, just miss the ace and taxi into the 2, that would likely be a "No-grade".  However, if you fly a sweet pass and dip just enough at the ramp to barely scoop the 2, that could still be an OK.  And anything in between is probably a "Fair".
The same thing would apply to being way long and just barely grabbing the 4 to avoid a bolter, vs just barely missing the 3 after an otherwise solid pass.
Really though, the question is not really answerable in a useful manner to a lay person.  Judging any “sport” is subjective, and often the LSOs don’t even agree on a grade.  After watching literally thousands of landings over a few years and being mentored by senior LSOs you just develop an eye and a feel for what a good pass and bad pass look like.  (and there are always plenty of “tweeners” to argue over!)
It really boils down to subjectively assessing the duration and magnitude of the pilot's deviations from perfect, and the appropriateness and timeliness of corrections.  In other words. how much the airplane is "moving around" out there...


Where can I find details of the holding models for JFK, Schiphol, Heathrow and Dubai?

Most chars can be found in the local AIP. Most charts containing such data are the 'Standard Arrival' charts, such as location, heading and holding time.

For the  Netherlands (Because you asked for Schiphol) it would be http://www.ais-netherlands.nl/aim/index.html


There are some types of landings, can you, please, clarify them? Or even add some other types

From European Action Plan for the Prevention of Runway Excursions 1.0, Appendix E- Aircraft Operators:

Deep landing – a certain distance behind the glide slope touchdown point
Short landing – touching down before the glide slope touchdown point

Belly landing (or pancake landing or gear-up landing)-  The aircraft lands without its landing gear fully extended and uses its underside, or belly, as its primary landing device.

"Thunderbolt II 080325" by U.S. Air Force photo/Brad White - Official USAF Website 1. Licensed under Public Domain via Commons.
Partial gear up landing- One/or more of the landing gears fail to extend. Similar to belly landing, but in that case, all the gears fail usually.
Hard Landing- The aircraft impacts the ground with a greater vertical speed and force than in a normal landing. The actual speed varies from aircraft to aircraft. This kind of landing usually requires some inspection afterwards.
Nose pointed down landing- Aircraft usually land at a positive attitude (i.e. with nose pointed up). However, in some cases (usually due to some problem), the landing can be carried out with nose pointed down. This is normally in violation of standard procedures.
Arrested landing- Usually done in carriers; an arrester cable is used to bring the aircraft to stop.

"FA-18 Trap". Licensed under Public Domain via Commons.
Smooth landing- If the landing has no problems like given above, it can be called a smooth landing. It is subjective though.


Does FAA have any info on army air corps pilots during wwII (know the FAA was formed later)?

Unfortunately no, the FAA does not certify or track military pilots. The only records the FAA has or carried over from CAA would have is if he converted his hours for a certificate, but I would be surprised it would be anything more than a number.


How do you determine whether you can us LNAV/VNAV minima on an RNAV approach?

The STC documentation provided with the GPS / FMS installation will indicate the type of minima it can go down to.

Typically, GA aircraft like a Garmin 430/530 will only be certified down to LNAV minima.

Business type aircraft with FMS's may be certified to LNAV/VNAV minima.  These installations use a barometric altitude input to calculate a pseudo glidepath to be displayed.


How do pilots detect and respond to windshear while landing and taking off?

The most dangerous form of windshear for planes is the microburst. This is a downdraft that after it hits the ground spreads out in all directions. It is dangerous particularly for planes low to the ground (on takeoff and landing)

When a plane coming in for a landing approaches the microburst it will enter a headwind and see and increase of airspeed and lift, the inexperienced and unsuspecting pilot may then reduce engine power to slow down and keep himself aligned. After the plane passes through the burst it will enter a tailwind decreasing lift which may cause the plane to crash into the ground short of the runway.

To prevent crashing the pilot would need to keep power and possibly abort the approach to wait out the burst.

Detecting windshear from the ground is done with doppler radars (TDWR) and multiple wind sensors (LLWAS) which can detect the changes in wind which will be relayed to pilots in accordance with regulations (3-1-8).

All turbine powered airplanes used in §121 operations, except for turbopropeller powered airplanes, must have a windshear detection system installed, this is detected in one of 2 ways, using the onboard weather radar (predictive) or using the wind sensors on the aircraft like the angle of attack and airspeed (reactive).


What airfoil is used on the Lazair ultralight?

That looks close but I think there is too much camber in it as depicted above, and that is only the root airfoil.  There are significant changes along the span.

I used to jokingly call it my 'Lie Back' airfoil because I usually design things at night in bed :)

At some point I hope to get a 3D CAD model published of the whole Lazair series of aircraft but I can't promise when.

The Lazair is to be featured on a Canadian stamp this year and there is a Lazair in the Smithsonian :)

Dale Kramer

PS.  Peter Corley was killed in the Toucan of his own design and I never flew any aircraft with Peter hanging on my waist.


What is the maximum number of planes a controller can control?

There is no fix maximum a controller is allowed to have under control, as the other answer already pointed out. The airspace under control of ATC is divided up into sectors containing the airways to and from airports within that sector or in upper sectors, airways just for en-route traffic. (see also: Why do pilots use airways instead of GPS direct?).

Sectors can be merged during off-peak hours, split back into separate sectors during peak hours and in the case of TMA or approach sectors, the controller owning the sector can receive additional help by splitting the approach sector further and using different controllers for different tasks within that sector: Feeder/Pickup - Director - Departure Controller. The thresholds of when a sector is split is dependent on many factors, like weather, runway in use, outside influences like adjacent sectors having more traffic that usual, special occurances within that sector like parachute jumper drops.

Sectors also vary in size and capacity based on their intended use. While TMA/Approach sectors are rather small and need to accomodate arriving traffic into an airfield, an enroute or upper sector can be significantly larger in size, lateral and vertical. Again, this is individual to each sector, because some factors which hold true for TMA sectors do not apply to enroute upper sectors and vice versa. It also really depends on the controller working the sector and his personal limits, favorite procedures and work practice. A few examples:

A TMA controller using vectors to the approach instead of standard arrival routes or transitions will have more workload to monitor and sequence aircraft.
The minimum radar separation in TMA sectors and upper sectors can be either 3NM or 5NM. While upper sectors are larger in size, they have to maintain a greater separation, so in some cases, an upper sector twice or multiple times the size of a TMA sector will only be able to accomodate the same number of aircraft (ceteris paribus) before reaching saturation.
A controller having to work with special situations such as aircraft in distress or student pilots requiring more attention than experienced pilots, will reduce the controllers personal capacity.

In summary, to answer your question: There is a situation and controller based threshold, at which saturation will be reached. Controllers are trained to anticipate this threshold and make their supervisor aware of the situation beforehand, so that measures can be taken. There is however no fixed numerical maximum, unless specified in the ARTCC/ACC's operating procedures.


Do airliners have any cockpit indication of open doors?

The Airbus A320 has an ECAM (Electronic Centralized Aircraft Monitor) Door Page:

(A320 FCOM - Doors - Controls and Indicators)
When a door is opened, this will also trigger a caution. According to the FCOM, a MASTER CAUT is triggered together with a SINGLE CHIME. The lower ECAM page will also automatically switch to the Door page.
Other aircraft may have similar pages or individual warning lights, like e.g. the Boeing 737 on the overhead panel:

(Boeing 737 NG FCOMv2 - 1.30.15 - Airplane General, Doors - Controls and Indicators)

I can only speculate why the pilots did not recognize the open door earlier in the incident you linked. My best guess would be that they were preoccupied with the engine failure that had just occurred, which will obviously create a lot of other warnings. It is easy to miss the door warning in all of those.


What are maximum G forces humans can survive?

I read that very high g forces could kill a pilot, brain pushing into
  the skull.

High G-Forces cause blood flow to the brain to be impeded due to blood pooling low in the body under high acceleration. This causes the pilot to black out eventually.

  Is there a way of decreasing or surviving these forces and how would
  it work if you ignore aircraft capabilities.

Yes, most if not all modern fighter pilots wear G-Suits. The basic principal of operation is to constrict blood flow to the lower body to prevent pooling in a high G maneuver to increase blood flow to the brain. The g-suit typically buys the pilot about 1G of increased tolerance. The average human tolerance is between 3G and 5G. Aside from possibly some aerobatic planes I dont know of any civilian or commercial aircraft equipped with G-Suits. Generally speaking good heart health can help mitigate the effects in sustained G situations. This why it is so vital that fighter pilots be in top physical shape as fighter planes are capable of very high G maneuvers.

You can decrease the overall G-Force experienced by altering your maneuver (i.e. not undergoing as much acceleration). However in a combat situation some high G maneuvers may be warranted and thus the suits are used. In civilian flying all deliberate maneuvers are well within human limits.

  If not what is the maximum survivable g force with current
  technology.

This depends on the exposure time to the G-Force, see this question/answer.


What is the airflow across and below an elevator?

When an elevator moves (trailing edge) up, it partly moves in the way of the airflow. That means pressure goes up.The elevator is an airfoil itself, in the shade of another airfoil. If it moves up, the same thing happens when a wing moves trailing edge up: lift works downwards.

Image source

Picture above shows an airfoil with trailing edge down: lift is upwards.

Upper aft side of the airfoil: air only moves faster if the surface moves out of the way of the stream, if the shape moves away gently enough the air still tries to follow. Note that the nose of the shape had pushed air out of the way, so there is less air along the upper aft side of the picture: lower pressure.
Lower side of the airfoil: air tries to stream like it did before, but now there is a shape in the way of the airstream. The air needs to stream through a narrower stream tube: higher pressure. When the stream bumps into the airfoil at a shallow angle, it gets deflected downwards: impulse lift.


Is the Bede BD5 actually a tandem wing airplane, and not a mono wing?

No. The BD-5 uses a stabilator for the horizontal tail with a single wing. When parked and the gust locks are in place the stabilator is in the 'pitch down' position you see in the picture. As the whole surface moves, the apparent incidence is not present when in normal flight.


Qualification Requirement of Aircraft taxi and landing lights?

There is no mention of any lighting regulations in the DGCA schedules however you should take a look at PART VI - AIRWORTHINESS as it will depend a bit on if the aircraft is Indian made or foreign built. From my reading of it if the aircraft is Indian built there is no regulation covering it but if the aircraft is foreign built, in order to receive and airworthy certificate you will need to cary over the foreign airworthy info. This may in turn mean that the light must adhere to the regulations of the certifying body of the aircraft such that the airworthiness can be carried over.  The actual usage of the light does not seem to be regulated under DGCA regulations.


Can cold thrusters/roll posts be used to control/stabilize an aircraft (like a satellite)?

There are several ways to do it, rockets or turbojets. Rockets have a problem with fueling them, turbojets are very difficult to find a place for that offers reasonable leverage.
Roll posts on the F-35B are not really thrusters, but rather use redirected engine thrust. They're not light either. Engine thrust at high altitude and airspeed is considerably lower than at sea level, while required control forces are larger.

Short version: The reason why control surfaces are better for control than cold thrusters is Lift coefficient, or more precisely the lift-to-drag or lift-to-thrust ratio.

An engine directed in the direction you want to apply force in has a lift-to-thrust ratio of 1. Each newton of engine thrust, gets you 1 newton of force. To produce 1 kN of force from the roll post, you lose 1 kN of thrust.

An airfoil at some angle of attack has a lift-to-drag ratio generally between 5 and 25. By deflecting an airfoil to get 1 kN of force, you only lose 40-200 N of thrust to drag. Thus, it's a much more efficient way to change your direction.

All in all, using thrusters of either type would result in...

Less efficiency: Thrusters require fuel, control surfaces don't. And if they're rocket type, they even need to carry the oxidizer.
Cold thrusters don't require extra fuel directly, but they "steal" main engine thrust, so you get slightly less efficiency.

More weight: An engine can produce 10 times its weight in thrust for 1 unit weight, but an airfoil at high angles of attack can produce a force 100 times its weight. The big weight problem will the be the added fuel, especially for rocket engines that need a lot of oxidizer. Plus, you still need the control surfaces, because they're the only way to adjust your aerodynamics for sustained flight. Thrusters could only be used to accelerate the initial maneuver.

Better responsiveness: with rockets, but a lot of fuel is required for those.
Worse responsiveness: if you reduce control surface size and use turbojets (they take a long time to spin up). The same if control surface size is unchanged.
Same responsiveness: with roll posts/cold thrusters.

Less friction drag: if control surface size is reduced. But since you still have to keep your control surfaces, just can get them smaller (the largest deflections are used at low airspeeds), not much less. Also, the added induced drag from the extra fuel may make up for it.

Overall: There's a reason that even rockets typically use fins (control surfaces) when possible. The only things an aircraft may need thrusters for is things that can't be done with control surfaces.
For instance, roll posts are used on the F-35B because control surfaces don't work at zero airspeed. Thrust vectoring has enabled some maneuvers beyond the normal angle of attack envelope.

If not for the oxidizer problem, there could be some applications for thrusters for unusual maneuvers, strictly in addition to control surfaces. As it stands, their thrust is too expensive to get, compared to force from the control surfaces and even to main engine thrust.


How quickly can a hot air balloon climb?

It very much depends on the balloon, but 2 metres/second in actual flight is pretty typical, while most are capable of something like 3-5 m/s ascent/descent if required. Ascent and descent speeds are normally about the same for a particular balloon, but this isn't a hard and fast rule

"sport" balloons being are capable of moderately less comfortable ascent/descent speeds in the range of 5 to 10 m/s


Why, during a magneto check, does manifold pressure increase as RPM decreases?

On the intake side, the engine is effectively a vacuum pump; it's sucking in air past a restriction in a duct - the venturi and throttle plate, creating a partial vacuum in the intake passage, and measured as total pressure in inches of mercury (In cars, the MP gauge is labelled "vacuum"; similar, but expressed as a differential between ambient and duct pressure instead.  The problem with car style vacuum gauges is you can't use it to measure horsepower because you don't know the absolute pressure in the intake duct, only the differential, and you'd have to know the actual ambient pressure to work out what is the actual intake duct pressure).
With the restriction being fixed (not moving the throttle), anything that changes the pumping rate (directly tied to piston speed, therefore RPM) affects the amount of suction created between throttle plate and cylinder.  Decrease the pumping rate (or sucking rate you might say), and the vacuum goes down/MP goes up.  And vice versa.
So anything that raises RPM without moving the throttle plate moves MP down a bit because the piston is sucking harder because it's going faster, and anything the reduces RPM without moving the throttle plate moves MP up a bit, because the piston's moving slower.  As you get familiar with this, you learn to drop MP about half an inch below target when following up with an RPM reduction, knowing it will come up as you reduce RPM. And vice versa on power increases.


If I have multi-engine land, can I learn multi-engine sea directly?

As shown on page 77 of this FAA document, the various tests needed for an AMES (air multi engine sea) rating depend on the ratings you already hold and your area of operation. Depending on your area of operation, holding an AMEL (air multi engine land) rating can allow you to skip most or all of the required tasks for an AMES rating. In many cases, an AMEL rating is actually better for getting an AMES rating than a single engine sea (ASES) rating will be. So in short, you do not need an ASES rating first, but having one can make the process of getting an AMES rating easier in certain areas of operation.


What happens when autopilot and propulsion settings do not match?

It all depends on the autopilot, but generally no. If for example, you tell the autopilot to climb at 300 fpm, it will keep adjusting pitch to do just that.   As you climb higher (without changing the power) the autopilot will keep increasing pitch to maintain the set vertical speed.  Eventually the airplane will get too slow because of the increased pitch and can even stall.  Some autopilots have pitch limits or monitor the stall inputs and automatically disconnect before it gets to this point, but others don't.  It is very important to know your autopilot and to monitor it at all times to make sure that it is doing what you want.


Is it possible to predict how busy an airport will be in a given time? (are there existing services that to this?)

This is a bit of a different approach than you were thinking, but Google estimates how relatively busy a place is based on location data:

This can be found in the right-side bar if you search for an airport code or many other locations. It's based on their location data, not flight schedules.


Is the official weather report for an airport in Class D airspace controlling for its Class E surface extension?

The ATC Orders 7-5-1(a)(4) say:

  a. SVFR operations in weather conditions less than basic VFR minima
  are authorized:

  [...]

  Only within the lateral boundaries of Class B, Class C, Class D, or Class E surface areas, below 10,000 feet MSL.
  Only when requested by the pilot.
  On the basis of weather conditions reported at the airport of intended landing/departure.

That means, ATC will issue SVFR clearances - or not - in the class E surface area according to the official airport weather.

The 'twist' here is that ATC can't issue an SVFR clearance anyway unless the pilot requests it. The AIM 4-4-6 simply says that an SVFR clearance is required "when the weather is less than that required for VFR flight". So as far as I can see, if you're in VMC in a class E surface area and you expect to remain in VMC then you aren't required to request SVFR, whatever the reported airport conditions are.

I'm actually not sure what your exact scenario is here (in contact with ATC or not, within 4NM or not, etc.) but in reality I suspect that ATC would say "N12345, the field is IFR, say intentions". If you tell them you're in VMC then they have no way to challenge that but they might hint strongly that you should request SVFR - "N12345, is there anything special you'd like to request?" - because it would clarify the situation for everyone and help keep you safe and legal.


Who's in charge of running VOLMET stations worldwide?

This is an interesting question. According to the Air Navigation Plan of the ICAO (Part 6), all member countries have to broadcast these volmets, therefore it is extremely likely, that all Volmet stations are run by the government.

It also states that this service has to be available all the time, but I doubt that fines are involved. There will be a Notam issued though.

Source: http://aviadocs.net/icaodocs/Docs/9673_vol1_en.pdf


Turborockets are hybrid systems that aim to use atmospheric oxygen as oxidizer instead of carrying it on board and thereby improve performance. While they do have a real chance of replacing the conventional rockets in use today, there are a number of engineering challenges to be overcome before they become viable.

These hybrid systems basically use a gas turbine inside the atmosphere (ramjets/scramjets have also been proposed) to produce thrust. The issue is that the jet engine becomes dead weight outside the atmosphere, so its weight has to be minimized. Skylon uses a slightly different method with a heat exchanger using helium, but the problem is the same.
The air intake has to operate over a wide range of speeds, from subsonic to hypersonic; the best option is to have a variable air intake, which adds weight and complexity.
The nozzle has the same problem. The area ratio of the nozzle has to vary significantly to cope with widely differing flow conditions.
If the vehicle has to take off under its own power, it has to have lifting surfaces, which are dead weight (eating into payload) once out of the atmosphere.
The integration of all these extra systems also causes a significant weight penalty.

The addition of all these systems increases weight more than the weight of the oxidizer they replace. This demands extremely high performance materials and very high efficiency systems for the vehicle to be commercially viable.


Is there a formula to convert SFC to air-fuel equivalence ratio λ?

No, there is no formula to calculate the air-fuel equivalence ratio from specific fuel consumption (SFC). To calculate the equivalence ratio, you need the air mass flow, and the fuel mass flow. From this data, you can calculate the actual AFR (air fuel ratio), and lookup the stoichiometric AFR for your given fuel.

$AFR = \frac{m_{air}}{m_{fuel}}$

The air-fuel equivalence ratio is then simply calculated from these two AFR values.

Air-fuel equivalnce ratio: $\lambda = \frac{AFR}{AFR_{stoich}}$

You don't have a value for the air mass flow. You only have fuel flow from the SFC and current power.

If you had the equivalence ratio and SFC at two different operating points, not just one, you could assume linear interpolation, or extrapolation, to the SFC in question.

With the equivalence ratio and SFC at just one point, you would have to find a characteristic curve for SFC vs equivalence ratio for a similar type of piston engine and the same fuel type, and assume the shape of this curve is applicable to your particular engine.


Why isn't an LPV SBAS approach considered a precision approach according to ICAO?

Precision Approaches (PA), as you mention, have specific performance requirements, one of which is how "good" the vertical position measurement must be during an approach. In the case of a PA, ground systems either directly measure the aircraft's vertical position on the glide path (Precision Approach Radar in the case of parallel approaches), or provide a means to follow a precise vertical glide slope via a gauge in the cockpit.

For Approaches with Vertical Guidance (APV), there is vertical guidance, but the aircraft's vertical position is measured onboard the aircraft by either a GPS position source or a barometric sensor. Either of those sensors are subject to measurement errors:

Barometric Measurement Errors: ICAO Annex 10 section 4.4.2.4 defines the error model that is used for barometric altitude reporting systems. The 95% bound of the worst case is +/-282ft.
Barometric Setting Errors: Additional errors are introduced depending on what barometric MSL pressure is selected on the altimeter and how that compares to current (actual) atmospheric pressures.
GPS Measurement Errors: While GPS position sensors provide very good horizontal position, GPS vertical position is generally poor. According to the FAA, the 95% vertical error of GPS systems used in civilian contexts is on the order of 300ft and according to this Advisory Circular (section 5-7), geometric/GPS altitude does not meet requirements for use in air traffic control internationally.

Combining these errors with flight technical errors while flying the approach, the total error you can expect can add up quickly. Compare that to the fact that in the vertical guidance of an ILS improves the closer you get to the ground, you can quickly see why the alternatives of GPS or altimetry don't quite cut it...

So specifically to your question: For an approach to be a precision approach, you need precise vertical position measurements, which an SBAS cannot provide.


What is the purpose of the serrated trailing edge spoilers on the B52?

The comments above by John K. and Maury Markowitz are on the right track. The serrated trailing edge of the spoilers has an effect on vortex shedding. It is called that because it is a vortex-shaped airflow that is shed from the rear of an airfoil or other surface.

On modern jet engine nacelles the serrated edges at the rear reduce noise. However, on the B-52 the most likely reason for this feature on the spoilers is to reduce the possibility of vortex shedding inducing vibrations that could cause the wing structure to resonate due to the harmonic oscillation of the vortex. This is also known as flutter. My guess is that at some point in the life of the aircraft design it was determined that this was (or could be) an issue, and this was the solution.

If you think about the last time you flew on an airliner where the pilots deployed the spoilers while still in flight, you may remember the vibration generated by them. That is the consequence of vortex shedding, and that is what the serrated edges are designed to minimize on the B-52.


In theory, can jetliners remain controllable after losing the entire tail section?

No - though the vertical stabilizer might be worked around, the same is not true for the horizonal stablizer. Without it you will loose stability in the pitch axis so much that I doubt there will be any chance for a controlled flight on nearly all aircaft (though there is a possibility that a few ones might remain some kind of controllability)


Why did most airliners have black noses in the 60's and 70's?

Under the nose of modern airliners is weather radar that helps the crew to see and avoid hazardous weather conditions. This is required by FAA CFR §121.357 for transport category aircraft. The radar signal would be blocked by the aluminum that makes up the rest of the fuselage, so the radome is made of a non-metallic material, generally a fiberglass composite.

These threads discuss the topic. The last post in the first linked thread suggests that most paint needed lead to prevent fading and/or a zinc-based primer, which would interfere with the radar. Black was one color that did not require these. Modern paint has done away with these metal additives, allowing more color options without interfering with the radar.

The Museum of Flight claims that the black color is only to remind crews to not paint over the radome. Painting over it would affect the radar signals.

According to this thread, the color of the radome affects the operation of the weather radar underneath. They were painted black because it has the lowest reflectivity. Modern radars are able to deal with different colors much better.


Why didn't Air India always fly DEL-SFO over the Pacific?

(gcmap.com) Great circle route.

The polar route is mostly over land and is in close proximity to airports and navigational aids. Flying over long stretches over water requires (among other things) enhanced navigation, using specific communication protocols, and adhering to criteria set by the countries overseeing the region. Picking the new Pacific route is not easy (authorization, training, and the associated costs).

The Pacific communication, navigation, and surveillance requirements also allow the utilization of flex routes to adapt to the changing jet stream, as the linked document below explains. (The flight's history also shows the different routes.)

The relevant operational circular—Operational Authorization Process for Pacific Operations—can be found on the DGCA website (.pdf). The document covers all the requirements, of which, the carrier seeking the authorization may need to demonstrate to the authorities a flight or more:

  (...) the final step of the approval process may require a validation flight through Pacific airspace by a DGCA Flight Operations Inspector to verify that all relevant procedures are applied effectively.

In short, when an airline selects a new challenging route, it must prove it is prepared for it, and to set up new training procedures. It's like a pilot who has never flown over the Atlantic, when that time comes, they undergo special training for it. In this case, flying over the Pacific was new grounds for Air India.

The requirements (published August 2016) were put in place after Air India sought this route. Air India has been undergoing major changes recently due to large amounts of debt, so they've been streamlining their operations.

Related (question about same flight): Could winds of up to 150 km/h impact the structural loads on a Boeing 777?


How does the X-59 acheive quiet supersonic flight?

The X-59 has not yet flown, but according to the designers...

  The ground noise is expected to be around 60 dB(A), about 1/1000 as
  loud as current supersonic aircraft. This is achieved by using a long,
  narrow airframe and canards to keep the shock waves from coalescing.
  It should create a 75 Perceived Level decibel (PLdB) thump on ground,
  as loud as closing a car door, compared with 105-110 PLdB for the
  Concorde. The central engine has a top-mounted intake for low boom,
  but inlet flow distortion due to vortices is a concern.

Source: Wikipedia

Additional information on this NASA project is available here.


What is this on top of the Air Force One?

The aircraft is the Boeing E-4B Advanced Airborne Command Post 'Nightwatch', which serves as the survivable mobile command post for the US National Command Authority. The aircraft are based on the same Boeing 747s as the VC-25s used by US President.
There are four identical aircraft performing this function in the US. The "bulge" on the top is a radome that houses a steerable SHF SATCOM antenna, as can be seen in the following image.

Image from cnet.com
The following image shows the aircraft systems in a little more detail.

Image from http://everythingisahoax.tumblr.com
As a side note, Air Force One refers specifically to USAF aircraft carrying the US President. From US DoT ORDER
JO 7110.65W

When the President is aboard a military aircraft, state the name of the military service, followed by the word "One".
EXAMPLE−
“Air Force One.”
“Army One.”
“Marine One.”

For civil aircraft, it is Executive One.


How to land the aircraft when the flight deck is full of smoke?

Smoke is a real challenge on an aircraft. The first course of action, other than extinguishing the source of the smoke, if possible, is to evacuate the smoke. In some aircraft windows can be opened. Some have a system to vent the smoke outside. In the crash of SAA flight 295 it was mentioned that the smoke evacuation procedure on the 747 Combi was to open the entry door in flight.

When UPS Flight 6 had an onboard fire the smoke evacuation system was inadequate and the pilots were unable to land safely. They were unable to see any of the instruments or the runway. It ended in disaster. Many lessons were learned from it and advances were made to help deal with smoke in the cockpit.

There are three distinct problems with smoke:

Smoke inhalation Having an adequate supply of breathable air or oxygen is key. On UPS 6 the captain's oxygen supply ran out. He attempted to get to another mask, but succumbed to smoke inhalation before he could get there. There can still be plenty of oxygen in smoke-filled air, but smoke contains many toxins that quickly damage the lungs making it impossible to get the oxygen into the blood. Some toxins damage the hemoglobin in the blood so that it can't pass oxygen to tissues. Some types of material can even produce cyanide when burning.
Eye irritation. I was in a house fire once and I can tell you that my biggest problem was the eye irritants in the smoke. Even a moderate amount of smoke contains enough irritants to be debilitating. The smoke in my house was nowhere near being thick enough to prevent seeing through it but my eyes teared up and there was an almost irresistible reflex for them to close. It's very painful and it takes a lot of effort to keep them even partly open. Finding the fire extinguisher was a difficult task. One of the lessons learned from the UPS accident was that pilots needed to have protection for their eyes also. The best solution is to have full face oxygen masks similar to what firefighters wear.
Opacity Since a cockpit is enclosed, if smoke builds up faster than it can be evacuated then it can become thick enough to prevent seeing through it. I've read stories of pilots of light aircraft actually sticking their head out the window to land. Seeing instruments is also a problem. One development made in the wake of UPS 6 is the Inflatable Vision Unit. This is a transparent inflatable bag that fills up with clear air and displaces thick smoke to allow pilots to see instruments and out the window.

source
I'm not sure how many aircraft are equipped with such a system, but combined with oxygen and eye protection it could make a big difference in a severe smoke situation. (As was apparently experienced by Santa this Christmas Eve!)


Briefly, "overhaul" isn't a strictly defined term but it's generally used to refer to the intensive inspection, maintenance and repair of an aircraft component, frequently - but not exclusively - the engines.

The word overhaul by itself is a general term that's used in all sorts of places but it's not defined in 14 CFR 1.1, for example. The most specific definition I could find is in this agreement between the FAA and EASA:

  "Overhaul" means a process that ensures the aeronautical article is
  in complete conformity  with the applicable service tolerances
  specified  in the type certificate  holder's, or equipment
  manufacturer's instructions for  continued airworthiness, or in  the
  data which is approved or  accepted by the Authority.

  No person may
  describe an article as being overhauled unless it  has been at least
  disassembled, cleaned, inspected, repaired as necessary, reassembled
  and tested in accordance  with the above-specified data.

Note that that definition only mentions "aeronautical articles", which is very broad indeed and not limited to just engines, airframes or anything else.

In the general aviation world, a very common usage of "overhaul" is in the context of piston engines, which have a Time Between Overhaul (TBO or TBOH) set by the manufacturer. Once an engine reaches that number of hours, it needs to be overhauled. But even there, the term isn't precise and the FAA has a circular called Reciprocating Engine Overhaul Terminology and Standards that explains what it usually means:

  c. Overhaul.  In the general aviation industry, the term engine
  overhaul has two  identifications that make a  distinction between the
  degrees of work on an engine:

  (1) Major Overhaul.  A major overhaul
  consists of the complete disassembly of an  engine. The overhaul
  facility inspects the engine, repairs it as necessary, reassembles,
  tests, and  approves it for return to service within the  fits and
  limits specified by the manufacturer’s  overhaul data. This could be
  to new fits and limits or serviceable limits. The engine owner  should
  clearly understand what fits and limits should be used when  the
  engine is presented for  overhaul. The owner should also be aware of
  any replaced parts, regardless of condition, as a  result of a
  manufacturer’s overhaul data,  SB, or an Airworthiness Directive (AD).

  (2) Top Overhaul.  Top overhaul consists of repair to  parts outside
  of the crankcase, and  can be accomplished without completely
  disassembling the entire engine. It can include the  removal of
  cylinders, inspection and repair to cylinders, inspection and repair
  to cylinder walls,  pistons, valve-operation mechanisms, valve guides,
  valve seats, and the replacement of piston  and piston rings. All
  manufacturers do not recommend a top overhaul. Some manufacturers
  indicate that a powerplant requiring work to this extent  should
  receive a complete overhaul.

Apart from that usage, the term MRO (Maintenance, Repair and Overhaul) seems to be used in the airline industry to describe maintenance facilities, but there isn't any specific definition that I could find.

So there's no single definition of "overhaul". It just means that you've very thoroughly inspected and tested something on an aircraft, whether it's an engine or a wheel, and you've verified that it's fit to be used in flight, i.e. it's airworthy. The reasons why it's important are probably obvious; they're the same reasons that your car needs regular servicing to run safely, efficiently and reliably.


Why can't I exactly match the same points on different VFR sectional charts?

This is by no means specific to aviation charts. Most likely it's simply general property of local maps.

Maps are flat, but earth surface is (approximately) spherical. So to represent ground features on the flat map a projection is needed and this projection can't preserve all relations—angles, distances and areas—precisely. So there are many different projections that distort different properties.

Small area maps are usually drawn in azimuthal projection with the central point in the centre (or significant point near the centre) of the mapped area. While such projection does not preserve exactly any properties out of the centre, the distortion grows with distance from the centre, so for small areas it's appropriate.

This however means, that each section uses different projection and that near the edges the maps will be distorted each in a slightly different way. That's why the maps can't be easily laid out to form a bigger map. You would have to lay them out on a sphere.

You could try using some software for stitching panoramatic photos (here is one such you can use for free). That should be able to apply suitable transformation to make the maps match. At the cost of increasing the distortion.


What units should use for velocity prediction equations?

In principle it does not matter which system of units you choose to use as long as you are consistent. It is however always easier to do everything in SI units and then convert the result of your calculation into whatever unit system you prefer. Using inconsistent units has resulted in many accidents including the crash of the Mars Climate Orbiter. That is why NASA is using SI units now:

Public Law 94-168, §2 requires use of the International System of Units for measurement in U.S. Government programs, "except where impractical." That requirement is reflected in NASA policy, NPD 7120.4. That measurement system, formally known as the "SI" system," after its name in French, Systeme Internationale, is almost universally used by all countries except the United States. Use of SI measurement is growing in importance for international trade.

In your case, the mistake is using Watts for power, which is inconsistent with the remaining units you chose:
$$ 1 \, \text{W} = 1 \, \text{kg} \, \text{m}^2 \, \text{s}^{-3} \neq 1 \, \text{lbm} \; \text{ft}^2 \, \text{s}^{-3} $$
If you want to use imperial units, you need to convert your power to $ \text{lbm} \; \text{ft}^2 \, \text{s}^{-3} $, which can also be expressed in units $ \text{lbf} \; \text{ft} \, \text{s}^{-1} $ where lbf is the pound-force (lbm is the pound-mass)1. Horsepower is different again and I would advise against using it because there are many different definitions, the most common one being that 1 horsepower is equal to $ 550 \, \text{lbf} \; \text{ft} \, \text{s}^{-1} $.
If you stick to SI units (kilogram, meter and second), you can use Watts for power and you will get your velocity in meters per second, which is easy to convert to any unit for speed you want.

1 $ 1 \, \text{lbf} \approx 32 \, \text{lbm} \; \text{ft} \, \text{s}^{-1} $ (Thanks to Michael Seifert for the clarification), see also this question on engineering.SE.


What are the differences between Normal and Utiltiy Categories

From a regulatory standpoint the governing regulation is FAR 23 (airworthiness standards for what we'd generally call "small" airplanes - Normal, Utility, Acrobatic, and Commuter category).

FAR 23.3 is probably what you're interested in, and tells us:

Normal Category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for nonacrobatic operation.
They go on to specify that nonacrobatic operation includes:

Any maneuver incident to normal flying
(for very vanilla definitions of "normal" - they expect you to use some common sense here!)
Stalls (except "whip stalls")
What most pilots recognize as the "commercial flight test maneuvers": Lazy eights, chandelles, and steep turns, but with an angle of bank not greater than 60 degrees in any of these maneuvers.

Utility Category is the same type and size of airplane, but approved for "limited acrobatic operations" - this may include intentional spins, as well as the "commercial maneuvers" with higher bank angles (greater than 60 degrees, up to 90 degrees).
Acrobatic Category is the same type and size of airplane, but approved for acrobatics  without any restrictions "other than those shown to be necessary as a result of required flight tests" -- basically "airplanes that can go upside-down".

The commuter category is boring: It's basically the normal category, but with more engines (at least 2) and more seats (up to a maximum of 19). Commuter category aircraft can also have a higher maximum takeoff weight (up to 19,000 pounds).

An aircraft can be certificated in any combination of the Normal, Utility, and Acrobatic categories if the aircraft meets the requirements of the appropriate category (as specified later in FAR 23).  An aircraft certificated in the Commuter category may not be certificated in any of the other categories however.

Generally the restrictions you find on aircraft are not regulatory ones (from the FARs) but Operating Limitations (in the POH or placarded on the aircraft) which you must comply with (per Part 91 of the FARs).

For example, the Cessna 172 and the Piper Warrior are both dual-category aircraft (Normal and Utility), and specify a "Utility Category CG Envelope" in their weight and balance charts.

The Piper Warrior is not approved for spins in the utility category (they are placarded with SPINS PROHIBITED). they also have other restrictions for utility category operations which are found in the POH and on aircraft placards (e.g. UTILITY CATEGORY - NO BAGGAGE OR AFT PASSENGERS, and maximum entry speeds for other acrobatic maneuvers).

Most Cessna 172s are approved for spins in the utility category and are not placarded against them. (Certain aircraft with specific STOL kits installed are disapproved as a result of the kit, and have placards prohibiting them).


Does the minimum power required for level flight depend on CD0?

Thank you for the excellent question!

The whole expression is only valid for the optimum polar point where the ratio $\frac{c^2_D}{c^3_L}$ reaches its maximum. If you deflect spoilers, this optimum will shift to a higher lift coefficient, so both aircraft will not fly at the same lift coefficient or the same speed any more! Always keep in mind that in this special condition the lift coefficient can also be expressed as
$$c_L=\sqrt{3\cdot\pi\cdot AR\cdot e\cdot c_{D0}}$$
Just plug that back into the equation and the minimum power expression will make a whole lot more sense:
$$P_{min}=\frac{4}{3\cdot\pi\cdot AR\cdot e}\cdot\sqrt{\frac{2\cdot W^3\cdot\sqrt{3\cdot\pi\cdot AR\cdot e\cdot c_{D0}}}{\rho\cdot S}}$$
This is still the same equation, but now it should be obvious that opening the spoilers will require more power.


What is the minimum separation for navaids to work properly without interfering?

VOR/DME and NDBs can't interfere with each other: they work on completely different wavelengths, so you could have a VOR right next to an NDB and they'll work just fine. Similarly, VORs on different frequencies won't interfere with each other; you could have them in close proximity as well. The only time you are likely to see navaids in close proximity is at an airport, which may have an NDB, VOR/DME, and multiple ILS in operation (an airport with 2 airstrips may have 4 ILS in operation: 1 per runway).

Where you do get interference is when stations are using the same frequency. A VOR has a service volume, a set distance it is designed to reach; in the US the high altitude service area is a minimum of 130 nautical miles at 45,000ft, but in practice in the right conditions you can pick them up further than that. Some stations are set at a lower power and have less range.


Which radio frequency is used during a flight interception?

An interception with an aircraft that's either on an unknown frequency, or non-responsive etc will be conducted on the emergency frequency of 121.5. Likewise, a pilot would be expected to tune into that frequency if they found themselves the subject of an interception.

There is no other standard "air to air" frequency that's likely to be utilised. I'm aware that some countries have special frequencies reserved for formation flight etc, but most pilots wouldn't have those memorised and wouldn't be expected to use them.

However, if the aircraft is/was communicating on a known frequency then there'd be no reason that an interceptor couldn't use that.


Despite all the 'grand scheme' described in the documents you found and the fact that it is all possible, MLS development ran into a few development issues along the way.  The biggest was that it was extremely expensive to build for both the ground systems and the airborne receivers.  That slowed its development as its benefits over ILS weren't justified by the costs.

One big benefit that your reference describes is that it actually allowed for area navigation.  Where ILS just gives you a deviation from a path, MLS gives you a 3-D position (Azimuth-Elevation-Range) that can allow for the RNAV paths.  But this was complicated and then the second big shift arrived -- GPS.  Since GPS gives you RNAV capability globally, why spend a fortune on something that would work only locally with very high infrastructure costs?  Based on this, the FAA pretty much abandoned MLS in the late 80's in favor of expanding GPS/GBAS/SBAS systems.

Europe kept on with MLS work because they had a different issue with multipath interference to the ILS systems.  The effort though just focused on the final approach segment.  GPS/RNAV systems can handle the inital portions of the approach just fine.  To keep things simple, and to make them retrofittable into existing aircraft at a reasonable cost, the MLS receivers were designed to provide an "ILS Look-alike" output.  And to avoid dealing with new procedures and training, they kept the ground systems and approach design limited to ILS-Like functionality -- just with higher precision and less interference issues.

The US does have some MLS approaches, but they're mostly 'private' approaches paid for by the airlines that want them.  The FAA won't fund 'Standard MLS approaches'.

As a result, current MLS equipped aircraft can fly MLS approaches that are operationally identical to an ILS CAT III approach.  Most aircraft use a multi-mode receiver (MMR) that allows for both ILS and MLS approaches (and some now also support GLS).  The difference is that instead of an ILS frequency, the pilot selects an MLS channel.  Approach procedures (and the approach plates) are basically the same as an ILS, so crew training is minimal.

So the answer is that today, you fly an MLS approach just like an ILS approach.


Why would a commercial flight make banked turns five minutes before landing / low altitude?

As denizhanedeer said, it was a circling approach. There is only one charted IFR approach to ESU. It comes in from the northwest and would line up for a straight-in approach to runway 16.

Depending on the wind the pilot may have needed runway 34 (same runway from the opposite end) so they would circle around to line up with it.

This flight did just that. (It might even be your actual flight, Easyjet 2039 on 5/17/2016) It did a short hold at the final approach fix to lose altitude then circled to land on runway 34. It landed at 4:09 UTC. It did the first left turn at 4:06 and 1850 ft. The image here shows it during the second turn at 4:07 and 1475 feet.

The altitudes given are MSL. The terrain rises pretty quickly there. I'm guesstimating from some topo maps that at the area of the turns it rises to 175-200 feet. So 1475 ft MSL would be as low as 1275 ft above ground level.


How many planes did the Soviet Union lose to Stinger missiles during Afghanistan War?

This quotes an estimate of 269 aircraft, but I think its hard to find the exact number:

Kuperman, Alan J. “The Stinger Missile and U.S. Intervention in Afghanistan.” Political Science Quarterly, vol. 114, no. 2, 1999, pp. 219–263.


How are different parts of airliners handled after the end of life-cycle?

Various parts of the airliners face different fates once it has been decided to breakup the aircraft. Most of the parts are reused in one way or the another. The dismantling of the aircraft starts after removal of hazardous liquids and radioactive components.

Engines- Usually, the aircraft and engines will be at different stages of their lives. The engines are removed and sold for operating in other aircraft. In case the engine life is also considered over, either its components will be used for spare parts or it too will be scrapped for materials.

Source: jalopnik.com

Avionics- The avionics components are usually valuable and are removed for use in other aircraft. Since they are line replaceable units anyway, this should pose no problem.

Stripped down cockpit of 747-400; Image from aviationweek.com

Interior- The interior of the aircraft, like the seats, entertainment systems are stripped and sold off. Items in the galley (like coffee makers), the air-conditioning systems also face the same fate.

Stripped cabin of 747-400; Image from aviationweek.com

Some items like the APU, actuators, landing gear, pipings, etc, if they have life are removed and sold for use in other aircraft (after appropriate checks and certification).
In case the aircraft has removable winglets, they are removed as well, for re-use.
Any hazardous materials like composites, depleted Uranium (used as counterweights in 747s, 727s etc.) are removed and disposed off.
The aircraft (structure) is then cut to pieces and metal parts are melted and reforged for reuse in industries, with unusable parts finding its way to landfill.

Source: tarmacaerosave.aero

The following figure from ICAO report Aircraft End-of-Life: Scrapping and Recycling shows how the materials are reused.

Image from What's new in aircraft materials recycling? ICAO workshop presentation

In general, stripping down and recycling of aircraft involves millions of dollars and is highly efficient, with over 90% reuse of materials being quoted. It remains to bee seen what happens to material reclamation as more and more aircraft components are made from composites.


Fine wire glow plugs, vulnerable to fouling, are not used on manned aircraft.  Glow plugs in most diesel engines have massive elements where the coil is protected from being fouled.


When would CAPT 3 or FO 3 ever be used on the A320?

"ECAM" is a typical terminology of Airbus aircraft.

From this accident report found quickly while searching for "CAPT3":

The panel:

The explanations:

  Normally all switch are positioned on NORM selection, mean that all of
  the source  are coming from co-location sources (i.e. system 1 for
  Captain, system 2 for F/O and  system 3 is standby).

  In case of
  failure of either of the related system sources for Captain or F/O
  side, they  can alternate it by selecting the switch to either CAPT 3
  or F/O 3 (CAPT or F/O for  ECAM/ND XFR).

The switches are used to designate which input to connect to equipment or display.

When everything is fine, the NORM position links a left side input to a left side equipment, and a right side input to a right side equipment. If an input is not operational, then the switch can be used to select the third input for either the left side or right side equipment.

The equipment and sensors we are talking here are:

Attitude / Heading reference
Air data reference
Electronic Instrument System / Display Management Computers
ECAM and ND XFR used to manage where appear PFD, ND and SD.

Detailed example

The aircraft attitude and heading are sensed using cooperating inertial sensors and air sensors which are combined in an ADIRU. As the attitude determination is key to pilot the aircraft, in particular when autopilot is used, there are 3 redundant ADIRUs on a A320, one for the Capt (#1), one for the F/O (#2) and a backup one (#3). Each ADIRU #n receives data from its ADR #n unit (air data) and IR #n unit (inertial).

The NORM position allow to send the left ADIRU to the Capt panels and the right ADIRU to the F/O panels. This allows the Capt and the F/O to have separate data, and they can compare them in case of doubt. Same for the flight control computers behind Airbus fly-by-wire.

If ADR or IR fails on the Capt ADIRU (#1):

ATT HDG on CAPT 3 position replaces IR #1 by IR #3 on the ADIRU #1
AIR DATA on CAPT 3 position replaces ADR #1 by ADR #3 on the ADIRU #1

If the failure occurs on F/O side, corresponding F/O 3 positions would be used to replace IR #2 and ADR #2 by IR #3 and ADR #3.


Where do websites like expertflyer.com get their information?

Simply put, ExpertFlyer gets its data from purchasing it from the GDSs (global distribution systems for airlines) these are the same systems that travel agents (online and off) use to book tickets/travel for their customers. This has a cost but has advantages vs free but less reliable means of acquiring flight data.

Other services may screen scrape (take) information from public airline websites, however airlines frown upon that. We have seen active campaigns by airlines to go after services that collect data in this manor.

Please note, this post is not meant to be "promotional" however fully answering the question posed requires explaining how different services may acquire flight data. Also, if the name "ExpertFlyer Voice" didn't indicate to you, I am a representative of ExpertFlyer.com


Why aren't variable swept wings being used in newer military aircraft?

The main advantage of variable sweep wings is that they reduce drag over a wide range of Mach numbers.

However, the variable sweep wings are not being used in present day aircraft for a variety of reasons.

The variable sweep wings are heavy and mechanically complex, affecting performance and maintenance.
The swing mechanism takes up too much space inside the wing that could be used for fuel.
It is difficult to position hard-points in the swinging part of the wing, affecting payload.
Advances in aerodynamics and better engines mean that the drag reduction using varying the wing sweep is not so critical anymore.
Also, the newer combat aircraft are not trying to outdo each other in speed, trying to gain the edge using better situational awareness, stealth etc.

However, the most important reason for variable sweep wings falling out of favor is that they are nearly impossible to be made stealthy. One way to make the aircraft stealthy is to reflect the Radar energy away from the source. In order to achieve this, the wings of stealth aircraft are designed in a specific way.

However, if the aircraft have a variable sweep wing, this condition is all but impossible to achieve. If stealth is still required, it has to be through more Radar absorbent materials, which further increases weight and maintenance requirements. The last variable sweep aircraft, B-1B, for example is designed to be a low level penetrator, unlike B2.

Because of these reasons, the variable sweep wings have fallen out of favor.

As for the second question, the wings of F22/F35 has basically a trapezoidal wing configuration, which gives a high performance at high speeds.

However, the main reason for using this type of wings is the excellent stealth characteristics they provide compared with other type of wings. One disadvantage of this type of wings is that their low speed performance is not up to the mark, but this has mostly been overcome by the use of improved control surfaces (like flaps, slats etc).

Interestingly, USN considered a variable sweep F22 for carrier operations, but the project was ultimately dropped.


Would reverse thrust increase with a decrease in aircraft forward speed when referring to propeller reverse thrust?

If the propeller keeps the pitch constant while in reverse thrust,  the component of the relative wind perpendicular to the prop disk goes down with decreasing aircraft speed. Hence, the AoA also goes down, the lift produced by the blade decreases, and the reverse thrust does also decrease...


What are these Lights on the Center Column between Cockpit Windows on the A320?

These are not lights, they are the Eye Reference Indicators. They are used to find the correct seating position. You adjust your seat such that the white ball is exactly hidden behind the red ball on your side:

Since the A300 Airbus has provided an eye reference indicator on the centre structure of the windshield in all Airbus aircraft (fig.3). It enables flight crew to adjust their seat position so that their eyes position matches the eye reference point. The indicator is a device that is fitted with 3 balls painted red or white. To achieve a correct seating position, pilots must align the red and white ball meaning that the white ball is hidden when in the correct position.

(Airbus - Are You Properly Seated?)
This YouTube video shows how to properly adjust your seat on the A320 (you can see the eye reference indicators at around 1:30): CORRECT COCKPIT SEAT POSITION! Eye-Position-Indicator explained by "CAPTAIN" Joe
They are not unique to Airbus though, some other aircraft have them too, like e.g. the ATR-72 (left) or the Dash-8 Q400 (right):

(image source: theflyingengineer.com)
Boeing uses a different method for seat adjustment, so you won't find such indicators there.


Why are even drone flights prohibited due to the earthquakes near Mt Keilir, Iceland?

The 5th of Mar NOTAM (valid to 1st of June)...
(A0100/21 NOTAMN
Q) BIRD/QWWXX/IV/NBO/W/000/999/6558N01748W999
A) BIRD
B) 2103051657 C) 2106011657
E) DUE TO POSSIBLE VOLCANIC ERUPTION ON REYKJANES, THE ICELANDIC
TRANSPORT AUTHORITY, IN COOPERATION WITH ISAVIA ANS, HAS DECIDED ON
A PLAN TO BE FOLLOWED TO MAINTAIN SAFETY OF FLIGHT WITHIN THE AREA.
FOR FURTHER INFORMATION SEE AIC A002/2021
REF AIP ICELAND ENR 5.2
F) SFC G) UNL)

... references AIC A002/2021. Which notes:

Pilots are urged to use extreme caution due to possible heavy traffic and risk of mid-air collision.
Expect drone flights below 120 meters. [emphasis mine]

That does not agree with the tweet, though permission is required to fly:

Contact Keflavík Tower for permission to enter or pass through the area.

Also, AIP GEN 3.5 (GEN 3.5.3.9) notes:

During volcanic eruption, a mobile radiosonde station can be deployed near the active volcano. Also it can be deployed for research and training purposes. Weather balloons with radiosonde will be launched by IMO scientists. Prior to launch, authorization must be given by Reykjavik ACC. When the launching site is near an aerodrome, further authorization must be given by the respective AFIS/ATC services. Information on mobile radiosonde launches shall be published in NOTAM.


It is the only Bede XBD-2, from the early '60s.

According to Wikipedia it was an experimental aircraft with STOL capabilities.


What are the regulations on flying UAVs over people on the ground?

The general rule of thumb is "don't do anything hazardous for people or property on the ground".

In the US, do not intentionally fly over unprotected persons or moving vehicles, and remain at least 25 feet away from individuals and vulnerable property.

Based on whether your project is for recreation or not, how heavy the drone is,... you may not be able to do the same things. The FAA website has a page titled ``What Can You Do With Your Unmanned Aircraft?'' that gives details on what you can and can't do based on these parameters.

A document published by EASA states that for low-risk drone operations:

  Flights above crowds are prohibited, but flights above people not related to the operation in cities or populated areas is allowed.

Refer also to local laws. A number of states and municipalities have passed or are considering limitations on unmanned aircraft.


What are the advantages and disadvantages of tail wheels that cause modern airplanes to not use them?

A tailwheel is a good choice for operation on unprepared surfaces with aircraft that have low wing loading and need to be as light as possible. Two main wheels and a small tail wheel weigh less and cause less drag than a tricycle gear, especially if they cannot be retracted. On the Bo-209 Monsun only the nose gear was made retractable because, being positioned right behind the prop, it caused 40% of gear drag all by itself.

On the other hand, high wing loading aircraft with their high landing speed would need very long landing runs due to their inability to brake hard. A rejected take-off close to the decision speed would be impossible without crashing through the airfield perimeter. Braking too hard with a tailwheel configuration will cause a headstand.

To summarise the reasons given in this answer:

A tricycle gear offers better visibility on the ground.
A tricycle gear allows full brake application.
A tricycle gear makes loading and unloading easier because the fuselage is horizontal.
A tricycle gear has less drag during the initial stage of a take-off run.

Cessna simply shifted priorities between the 140 and the 150.


What is a hypernym for the ascending and descending legs of a flight?

Since a hyperonym can be a word or phrase, I'd go for a non-cruise airborne segment. Or N-CAS, as it'll be known from this moment in history.

The ICAO Common Taxonomy Team (pdf) doesn't have a term for it though.

From that document you can call 4, 5, and 6 (from your diagram) just en-route. And I don't see harm in expanding that term to include 3 and 7 as well (for the purposes of the question).

Also during cruise, some planes continue to climb in steps, it's called a step-climb. In that case you end up with multiple climb and cruise phases.

Which makes me think of stepping.

Officially there's no answer to your question, "en-route" fits in fine and is official, and "stepping" (up or down) would exclude the cruise as you requested, it works from a technical (albeit very broad) standpoint. But it can't be used officially or otherwise if you ask me.


Why would the Speedbrake be required for such a long time on approach?

There are two reasons I can think of off the bat:

Its possible the controller asked them to hold a slow speed due to increased traffic ahead. The brakes may have been deployed to match the speed requested. I have heard this called on occasion over the radio in the terminal area I fly.
The controller cleared them for a steeper decent than usual this would cause them to potentially deploy the speedbrakes to make the descent rate.

This article discusses it in reference to small planes but idea is the same. And another article by a CFI here.


Why can't model aircraft be used to discover spin and stall behavior of manned aircraft designs?

Who says scale models cannot be used for spin modeling?

The key information is that the Reynolds number does not affect the behavior of separated airflow as much, so you do not need to increase speed to compensate for the smaller dimensions. Only when the stall behavior is studied would the Reynolds number matter, but here also meaningful results are possible at lower speeds.

Models are routinely used in spin tunnels. Here both the speed and the dimensions are smaller, but the results can be transferred to the original aircraft. There are two sorts of spin tunnels:

Free tunnels with an open, divergent section in which the model has to be thrown by a skilled operator so it will settle in a spin within the upwards flowing air stream. The section is divergent, so the speed changes with height and allows the model to find a matching speed.

Picture from spin tests on a Grumman E-2 model in the NACA 20 foot spin tunnel (picture source)

Closed tunnels in which the model is mounted on a sting. The sting is connected to a rotating balance, which can be set at different roll and yaw rates. The resulting matrix of coefficients over angle of attack, angle of sideslip and over the three rotation speeds is fed into a computer model which computes the equilibrium points.

The models used in free tunnels must be scaled both geometrically and inertially, so their mass distribution matches that of the original. If the spin tunnel uses a rotating balance, not even the dynamic scaling of masses and inertias is needed, and a regular tunnel model can be used. However, if the resulting spin is of an oscillating nature, the free spin tunnel is at an advantage, because this becomes readily apparent in the test. In a closed tunnel you just get two equilibrium points and must make the connection yourself.

A third way are free-flight tests of models, but they are much more expensive and allow less observations than wind tunnels.

The stall characteristic is a little harder to predict using models, but again you can draw conclusions from what can be seen in the tunnel. To arrive at the maximum lift coefficient starting from the measured value at Re = 1,000,000, you may use this scaling: $$\Delta c_{L_{max}} = \frac{log_{10}\left(Re^{\frac{1}{6}}\right)}{3.5}$$

The plot below shows the difference in lift coefficient between a small model and a full-scale airplane. The biggest differences are around stall, and at the high angles of attack seen in spins both show fairly similar behavior.

Lift coefficient over angle of attack for model and full-scale aircraft, taken from Joseph Chambers' monograph on testing with models (Modeling flight : the role of dynamically scaled free-flight models in support of
NASA’s aerospace programs).

For small aircraft, the relative expense of a tunnel test is normally inadequate. Instead, the real thing is used. Since the speeds involved are small, spin tests are carried out with a spin chute or a releasable mass at the tail of the aircraft. On modern designs, even a full-aircraft ballistic parachute system can be employed to prevent a crash if the spin test ends in an unrecoverable situation.

Spin chute installation on a Columbia 400 (picture source)


How and why can multiple people control the Boeing B-29?

In aircraft of this vintage, all the controls are mechanically interconnected.  If the left-hand pilot's control yoke/ wheel moves, so does the right-hand pilot's.  Likewise for the rudder pedals.  And if a pilot or the flight engineer moves a throttle lever, the other throttle lever for that engine will also move.
If several people are exerting different inputs on the same control, then-- "strongest man wins!"
But normally the crew would have an understanding, at any given time, as to which pilot would be flying the plane, and who (which pilot, or the flight engineer) would be controlling the throttles.  Standard protocols would be established, and reinforced via training.  Normally only one pilot would be "flying", i.e. handling the primary aerodynamic controls (aileron, elevators, and rudder, via the control wheel and the rudder pedals), at any given time.  Normally the flight engineer would handle the throttles on this aircraft, in accordance with whatever requests the pilot flying the plane made via the intercom.


How complete is our understanding of lift?

Short answer: Yes, our understanding of lift is complete, but solving the equations for some practical cases needs more resources than what is technically sensible.

Lift is a matter of definition

First of all, lift is only one part of the aerodynamic forces. It is the component normal to the direction of airflow. Since the aircraft will distort the local flow around itself, this direction is taken ideally at an infinite distance where the air is undisturbed.

The other component is, of course, drag. It is defined as the part of the aerodynamic forces parallel to the direction of airflow.

The aerodynamic forces are the sum of all local pressures, which act orthogonally on the local surface of the airplane, and the shear forces, which act parallel to the local surface.

When aerodynamics was researched first, electric fields were new and exciting, and the same equations which help to calculate electromagnetic forces could be used to calculate aerodynamic forces. Therefore, abstract concepts like sources or sinks were used to explain aerodynamics. This made it not easier to understand, and many authors tried to find simpler explanations. Unfortunately, they were mostly too simple and not correct, but the next generation of authors would mostly copy what had been written before, so the wrong concepts were still bandied about.

To get to the bottom of it, it might help to look at lift at a molecular level:

Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects:

Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise.
Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they exert on their surroundings.
Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

All three contributions are well understood, and with the Navier-Stokes equations they can be completely mathematically expressed. What is still improving is our ability to solve these equations, and in turbulent flow the characteristic length required to capture all effects is so small that it is practically impossible to solve those equations fully with finite time and resources.

Flow over the upper side of the wing

Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly (because it has mass and, therefore, inertia), the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules overcome their inertia and change direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.

Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.

Flow over the lower side of the wing

A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.

Lift can be explained in several, equivalent ways

Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.

Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.

If you look at the wing as a black box and only pay attention to the impulse of the inflowing and outflowing air, the wing will change the impulse by adding a downward component. The reaction force of this impulse change is lift.

Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

Supersonic flow

When the aircraft moves faster than pressure changes propagate through air, the changes in pressure are no longer smooth, but sudden. The aircraft will push the air molecules aside, producing a compression shock. Behind the shock front pressure, temperature and density are higher than ahead of it, and the increase is proportional to the local change in flow direction. The incremental pressure change $\delta p$ due to the aircraft hitting air with an incremental angle of $\delta\vartheta$, expressed in terms of the undisturbed flow with the index $\infty$, is proportional to the change in the streamlines:
$$\delta p = -\frac{\rho_{\infty}\cdot v^2_{\infty}}{\sqrt{Ma^2_{\infty} - 1}}\cdot\delta\vartheta$$

Gas pressure on a molecular level is the number and severity of particle collisions. The air molecules experience more collisions on the downstream side of the shock, since air pressure is higher there. The average  direction of the additional collisions is indeed orthogonal to the shock, because it is the boundary between blissfully unaware molecules at ambient pressure ahead of the shock and their bruised brethren downstream which have just crossed that boundary. Once a molecule has passed the shock, the collisions are coming again equally from all sides and its speed does not change any more.

If the surface curves away from the local flow direction, the air produces an expansion fan which re-sets the old pressure and density values when the air flows again in its original direction.

Pure supersonic lift is only a matter of the angle of incidence, and any local curvature of the wing will not change overall lift (but increase drag). Now the total aerodynamic force is normal to the wing, and drag will become proportional to the angle of incidence. In hypersonic flow you will get good results with the venerable impact theory first formulated by Isaac Newton.

Separated flow

This happens when the air molecules are no longer able to follow the contour of the aircraft. Instead, you get a chaotic, oscillating flow pattern which is very hard to compute exactly. This is really the only part of aerodynamics which cannot be predicted precisely, even though the effects are well understood. Separated flow will produce lift, too, but less than attached flow. In delta wings, this separation is produced on purpose to create what is called vortex lift.


How do I track and visualize aircraft from home?

Yes, there is other signals broadcasted by aircraft, although not by all. In the General Aviation sector, at least in Europe, an on-board device known as FLARM is being used mostly in Gliders, Helicopters, Single-Engine Piston aircraft and touring motor gliders.

  FLARM is an electronic device to selectively alert pilots to potential collisions between aircraft. It is not formally an implementation of ADS-B, as it is optimised for the specific needs of small aircraft such as gliders, not for long-range communication or ATC interaction.

You can visualize these signals using some tools and you can also feed the data to sites like flightradar.

Related Answers to FLARM: What system is there in the UK to prevent collisions between small aircraft?
How does TCAS work?


Can small RC aircraft utilize a steel sheet monocoque construction?

Yes, it can be done, but to do it well is a lot of work. You will also need to use a thinner gauge than 1/32" to get the weight down.

Note that one of the very first all-metal aircraft was made of 0.2 mm (1/128") sheet steel and flew well. However, building it required a corrugated substructure and a welded steel tube frame for strength. Still, it was heavier than necessary so its builder switched to aluminium for later designs.

Generally, the tension length of steel (strength relative to density) is roughly equal with that of aluminium or titanium; however, you will need to use high-grade steel to be comparable with aircraft-grade aluminium, so some of the price advantage will be lost.

But there is a distinct disadvantage with higher-density materials. While their strength scales linearly with wall thickness, their buckling strength scales with the square of their thickness. You will end up with a very delicate structure which will require careful handling to avoid local dimples and a stiff substructure to avoid early failure in compression. Also, introducing loads locally (say, for a landing gear or an engine mount) will require local reinforcements which should ideally be multi-layered and taper out in several steps.

If you want to scale an existing structure down, make sure you place the stiffeners closer than what a scale construction would require. But I am sure you will prefer to bond the sheet steel to a styrofoam core. Use a higher-density styrofoam with better compression strength and be generous with local reinforcements, and the model should turn out fine.


Who operates the hoist in a Coast Guard helicopter SAR team?

who operates the hoist in the Coast Guard helicopter SAR teams?

This depends on country and other factors, perhaps aircraft type etc.

In general, the winchman descends on the end of the winch cable and does not operate the winch.

In the UK, helicopter search and rescue were, until recently, mostly provided by the RAF. HM Coastguard HMCG (now part of Maritime and CoastGuard Agency - MCA)  operate SAR helicopters too and will mostly take over the SAR role.

  The standard SAR crew is made up of four members: two pilots, one of whom is the aircraft captain, a radar operator who acts as the winch operator at the rescue scene and a winchman, normally trained to paramedic standard, who will supply immediate first-aid and recovery services at the rescue site.

From RAF Sea King

Sometimes the role is taken by aircrew normally responsible for loading/unloading:

  If you are part of aircrew, you would be responsible for loading and unloading aircraft payloads, ... You would also take on extra duties according to the aircraft type and its operational role. For example, ... you might train as a winch operator for search-and-rescue operations.

From RAF non-commissioned aircrew

Other References

Search and Rescue Training
Working with Search and Rescue Helicopters
Coastguard search and rescue helicopter programme


In a hydrogen fueled plane, could part of the fuel load be allowed to become lighter than air to offset or trim for the load?

The buoyancy of hydrogen is not as high as expected, and is even lower at high altitude due to the lower ratio of the density of hydrogen and air.
Just for comparison:
The tail tank of the A380 has a capacity of 23698 l.
When this tank is filled with kerosene / air / hydrogen, the content weights:
Substance           | Density [kg/l] | Weight of 23698 l [kg]
--------------------+----------------+-- -----------------
Kerosene            | 0.8            | 18958
Air (sealevel)      | 0.00122        | 28.91
Air (10km)          | 0.00041        | 9.72
Hydrogen (sealevel) | 0.00009        | 2.13
Hydrogen (10km)     | 0.00003        | 0.72

At sealevel, the lift of this tank filled with hydrogen is 28.91kg-2.13kg=26.78kg.
And at altitude, it's just 9.72kg-0.72kg=9.00kg
This is nothing compared to the 18958kg of kerosene, and shows that it makes no sense to fill any volume in an aircraft with hydrogen for trim.


Your reason 1 is correct. Without the ballast the aircraft would become a taildragger.

Why not pallets? This would produce the same center of gravity location, but a different mass distribution. Clearing this configuration even for being loaded and pushed around is more effort than simply placing the ballast where the mass of the engines would go. Now all clearances are valid and the aircraft can be handled much as one with the engines in place.

Next, engines are only installed shortly before delivery or first flight. The cost of capital is too high to install them any sooner.

(airbus.com) Example of clearances that would be affected by non-standard mass distribution.