What are the customs decal and eAPIS requirements for flying to the Bahamas? I'm planning to rent an aircraft and fly to the Bahamas. I've got all the requirements including the Restricted Radio license.

However, there was another requirement which concerns paying of the Annual decal fee through CBP DTOPs and filling the form through eAPIS which kind of got me confused. Now I'm about to pay the annual user decal fee for a private aircraft but is this required if its a rental aircraft?

And could someone please briefly explain the difference between the two and how they should be used for my trip?

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

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


What is the current state of regulation regarding VTOL aircraft in the LSA category? With quite a few concept designs in progress, I am curious what the current FAA regulations are regarding vertical takeoff and landing vehicles in the light sport aircraft category.

The FAA term for VTOL [other than helicopters and lighter-than-air craft] is "Powered Lift". Also of note, gyroplanes by nature may be able to land vertical but cannot takeoff vertical so are not VTOL.

LSA excludes powered lift. Not much more to it, unless maybe it launches from a catapult and lands with a parachute.

title 14 CFR section 1.1 "General definitions":

  Light-sport aircraft means an aircraft, other than a helicopter or
  powered-lift that, since its original certification, has continued to
  meet the following:

  (1) A maximum takeoff weight of not more than—

  (i) 1,320 pounds (600 kilograms) for aircraft not intended for
  operation on water; or

  (ii) 1,430 pounds (650 kilograms) for an aircraft intended for
  operation on water.

  (2) A maximum airspeed in level flight with maximum continuous power
  (VH) of not more than 120 knots CAS under standard atmospheric
  conditions at sea level.

  (3) A maximum never-exceed speed (VNE) of not more than 120 knots CAS
  for a glider.

  (4) A maximum stalling speed or minimum steady flight speed without
  the use of lift-enhancing devices (VS1) of not more than 45 knots CAS
  at the aircraft's maximum certificated takeoff weight and most
  critical center of gravity.

  (5) A maximum seating capacity of no more than two persons, including
  the pilot.

  (6) A single, reciprocating engine, if powered.

  (7) A fixed or ground-adjustable propeller if a powered aircraft other
  than a powered glider.

  (8) A fixed or feathering propeller system if a powered glider.

  (9) A fixed-pitch, semi-rigid, teetering, two-blade rotor system, if a
  gyroplane.

  (10) A nonpressurized cabin, if equipped with a cabin.

  (11) Fixed landing gear, except for an aircraft intended for operation
  on water or a glider.

  (12) Fixed or retractable landing gear, or a hull, for an aircraft
  intended for operation on water.

  (13) Fixed or retractable landing gear for a glider.


How dangerous are Polar Routes, if an aircraft must land on water or wild land? Should an emergency landing be necessary for a commercial aircraft on a Polar Route on water or in the unoccupied, secluded Arctic outback (e.g. Siberia, Far Northern Canada).

What is the time interval for aid to arrive (I write 'interval' because I know that this question depends on the location)?
How likely would the passengers survive in the freezing cold (for brevity, I call this 'frigidity') in their cabin clothes? Assume that they are not equipped with clothes or gear for frigidity.

Depending on where you are it will take around 4 hours to 3 days, if the location is know. Depending on how remote your location is, and if the SAR team knows your location (Sometimes forgotten by pilots after a emergency landing.). If the location isn't known, the search can take up to 2 weeks, finding you and your passengers dead.
Let's say temperature is -10 degrees Celsius, it will take around 1-3 hours without any clothing or external heat source (like a camp-fire) to start hypothermia. Although the critical point of Hypothermia is 25 degrees Celsius, people survived 14 degrees Celsius. The best thing you can do during this plane crash is stay closely together and if possible start a camp-fire. Drink hot water (just take some ice and put it on the fire) with a lot of sugar (or anything else that gives you energy) and just do anything to keep yourself heated until help arrives.

If possible, keep the navigation lights, strobe lights on of  the plane until battery runs out of fuel. This increases the chance being found in time.


Why does the Piper Cherokee (PA-28-140) engine have such low horsepower despite the very large displacement? I'm sure this question could apply to many other plane engines, but specifically I'm looking at the Piper Cherokee PA-28-140.

Surprisingly, it appears that the engine used, the Lycoming O-320, has ~320 cubic inches of displacement but only puts out 150hp.

When comparing the horsepower per cubic inch of displacement (CID), the ratio is quite low. We have the technology (and already used it in vehicles in the 50's, 60's etc.) to achieve a better efficiency than that.

Why is this engine (and perhaps other models) so inefficient?

Aviation engines run at near max RPM through out the flight. A car on the other hand doesn't use the full RPM spectrum except in bursts.1

If a car engine was utilized the same way an aviation engine is, it won't last long.

So an aviation engine is sturdier, heavier, and weaker (hp) for the same displacement, but also provides higher torque (big cylinders).2

Formula One engines have small displacement, very light weight, yet deliver near a 1,000 hp. But they don't last long either. Something like 15 hours of racing and practice sessions.

Further reading: Do Car Engines Make Good Airplane Engines?

1 Piston planes run at near max RPM because they don't have and don't need gearboxes.

2 Same era alloys and technology.


What are the pros and cons of having canard control surfaces versus a horizontal tail control surface? For a normal civil aviation aircraft, what are the pros and cons of having Canard control surfaces or horizontal tail control surface?

For statically stable aircraft, the canard is a spoiler in disguise. It will create a strong downwash right behind itself, coupled with an upwash outward of $\pi$/4 of its semispan. With changing angle of attack and lift coefficient, the vertical position and the strength of both up- and downwash will change, so the wing incidence variation over wingspan can be made right for just one angle of attack.

Pros

The force to effect flight path changes works the right way, so pitch response times of canards are slightly better than those of a conventional configuration.
The wing can be further aft, so in short business jets the cabin might not interfere with the wing spar.

Cons

The induced drag at off-design points is higher than that of a comparable conventional configuration.
The bigger lifting surface must keep some stall margin, so the total lift coefficient will be smaller than that of a comparable conventional configuration. In the end, a canard will need more wing area.
Control surface deflections will add to the already high lift coefficient of the canard plane, increasing lift demands on the surface with the smallest margins.
The highest lift coefficient is needed at the surface with the smaller Reynolds number. Normally, the reverse would be better.
If the engines are mounted below and ahead of the wing, they will be hit by the wake of the canard, reducing intake efficiency and compressor stall margins.
To make sure that the control surface will be the last thing on the plane to run into compressibility problems, it should have less lift coefficient and more sweep than the wing. In the end, a statically stable canard will have a lower cruise Mach number than a comparable conventional configuration.
The landing gear must be made to retract into the fuselage, because the wing is too far behind the center of gravity to allow the gear to retract into the wing.

I could go on, but I guess you see by now where this is heading …


How is lift provided by a wing affected by propeller wake? Does the lift of that section of the wing which is in the wake of the propeller increase (because the airspeed in that section is higher)?

If yes, is this fact used actively to improve the lift capability?

Or does it decrease significantly because of turbulence and an overall disturbed airflow on the wing surface?

It both increases and decreases, depending on the local direction of the propellor (e.g. on the upgoing side, or the downgoing side) . See for example this PhD thesis: Propeller Wing Aerodynamic Interference.

This thesis shows the following image:

Simply said, the velocity of the propellor locally changes the angle of attack, and thus the lift generated by the wing.
It is interesting to know that it is possible to locally optimize the shape of the wing to take advantage of this effect. Again the thesis shows the result, and it looks like this:

If you look closely, you can see that the wing is changed at the propellor location to accomodate the adjusted flow.


Will an aircraft always crash on take-off if de-icing is not performed correctly? I recognise the Daily Mail's propensity to adulterate and colour news, but are these user comments about the UAL951 flight on 29 Dec 2014 correct?
The Daily Mail claims:

A United Airlines plane aborted its transatlantic journey and diverted to London today because the flight crew apparently wasn’t satisfied with the de-icing effort it received in Brussels.

But some commenters under the article regard this official reason as concealment or falsehood:

If the de-icing job wasn't good the plane would have crashed on take off. There must be more to this story.
Correct. If the de-icing was not done correctly, the aircraft would have crashed on take-off. Since it was able to fly, climb to cruise altitude and travel several hundred miles, no further de-icing would be required. There was some other unreported reason for the diversion.

Is there any truth in these claims? Will an aircraft crash on take-off if the de-icing is not done correctly?

The claim that de-icing must have been done perfectly in Brussels or the plane would have crashed on take-off is patent nonsense. Ice on the aircraft degrades performance. A little bit of ice degrades performance a little bit; a lot of ice degrades performance a lot. It's perfectly possible that they took off from Brussels with slightly degraded performance, realised that there was still some ice on the plane and decided they needed to get rid of it.

There's no reason to suspect a cover-up. If there was even a hint of terrorism, the plane would have diverted to Stansted, not Heathrow. If the plane had some mechanical problem, they'd have made a vague statement about the plane having mechanical problems, landed, fixed the problem (which requires maintenance technicians, not a de-icing crew), and then said something much less vague to reassure the 227 passengers that the plane was safe. If there was a problem with the passengers, luggage or cargo, the plane would have gone to a terminal. If a vice-president of the company had been served macadamia nuts in the wrong way, they'd have dealt with that in Brussels.


How close is X-Plane experience to real flying? I started using X-Plane and am surprised at the easiness of flying (using a flight controller). Does this simulation software reflect how it is to fly (in any way) in reality?

From using the simulator, I understand that all I need to do is to switch the engines on, turn the brakes off and fly. Are there any additional things apart from a pre-flight checklist that an actual airline pilot has to do?

In addition, I understand that I haven't dove into the details of navigation in X-Plane as I am just wondering if the basics of the simulator software reflect real life conditions.

X-Plane is a much truer simulation of flying than that flight game Microsoft used to make. In fact, when coupled with certified hardware and a CFI, X-Plane can power a sim that you can actually log FTD time in.

Since you ask about airline flying for your comparison, there is a lot more to that than just the flying. An airline pilot is, with few exceptions, always operating under instrument flight rules (IFR). This means flying in the clouds with no reference to anything but instruments, weather as bad as a few hundred feet visibility and strong crosswinds for landing and takeoff are all part of normal ops. You are also operating under ATC, so radio ops are normal. The only comparable computer simulation of ATC to reality is VATSIM (as mentioned in comments), though it doesn't approach the level of busyness that you'll find in some airspace and airports (with the exception of some of their organized fly-ins).

A transport category jet is also a bit more complicated than "turn the engines on, release the brakes and go". The only computer sims that capture the level of detail properly are ones like the A-10 simulator by DCS. Before you can start the engines, you need to first power the aircraft and perform your receiving checklists, and if you are the first flight of the day, additional first-flight checks. These cover:

Thorough pre-flight walkaround inspection
Powering the airplane on batteries
Transition to ground power (GPU) or start the APU for power
Verifying the state of all systems
Fire test
Stall warning test
Anti-ice system test
Checking oxygen levels and masks
Autopilot test
Backup instrumentation checks

Before pushback:

Verifying fuel load
Get load information from flight attendant/gate agent/dispatch/ramp supervisor
Calculate weight and balance, move passengers and/or bags if needed
Get ATC clearance and program FMS
Flight crew and cabin crew briefings
Weather briefing
Start APU and disconnect ground power and ground air conditioning
Get clearance for pushback

During pushback:

"Before engine start" checklist
Set bleeds / packs for engine start
Communicate with ground crew for engine start

Now you can start an engine. Unlike a small airplane where you start the engine by turning a key in an ignition switch, in bigger airplanes its a series of knobs and buttons to press, sometimes dependent on certain events during the start. Engine parameters must also be monitored in case an abort becomes necessary.

At this point you can get a taxi clearance and start moving toward the runway. You'll go through a couple more checklists, "after start/taxi" and "before departure", to prepare the plane for taxi and takeoff. Once you are on the runway, with all checks completed, with more fuel than your minimum dispatch release, and an ATC clearance you may then release the brakes and go flying. One pilot will fly and the other will monitor instruments.

Much of the above is beyond the typical scope of X-Plane, though some can be addressed with specific aircraft add-ons, manuals and software like VATSIM.


Is my understanding of a gas turbine engine compressor correct? So I understand that the rotors add dynamic pressure to the airflow and this dynamic pressure airflow is transformed into static pressure in the stator vanes. But now does the airflow still have dynamic pressure once it leaves the stator vanes, or does it get completely transferred into static pressure?

And the static pressure stays in the airflow and keeps increasing as it goes on, it doesn't get transformed into dynamic pressure because the movement of the rotor vanes create the dynamic pressure, is that correct?

..the rotors add dynamic pressure to the airflow..

  ..this dynamic pressure airflow is transformed into static pressure in the stator vanes.

are both incorrect partially correct statements, but can be open to confusion. Dynamic pressure is transformed into static pressure continuously, by narrowing the passage through the compressor. The rotors add total pressure to the airflow, some of which is extra dynamic pressure and some extra static pressure.

The rotor vanes increase the energy contents of the gas stream. Another name for the energy contents of a gas stream is enthalpy.
Energy contents can be transformed into gas speed when allowed to expand (dynamic pressure) or into static pressure when you keep the volume confined.

For ease of reference, just take a car tyre as an example. Some little rotating implements in an air pump at the gas station provide a pressurised air stream into the tyre. Stop pumping and measure the tyre pressure: what you're measuring is static pressure. The weather news talks about high pressure and low pressure systems, that is static pressure as well. It's energy contents that has not been converted into a velocity yet, like increasing potential energy of a rock by lifting it up further.

Now get a match stick and press on the tyre valve. The air streams out of the tyre, this air stream can be called dynamic pressure. You can use the outflowing air to inflate another flat tyre, you're then converting static pressure in tyre 1 into dynamic pressure into static pressure in tyre 2. End pressure in tyre 1 is lower than when you opened the valve: total energy contents of tyre 1 is now divided over 2 tyres. No energy was added, only pumps can do that, by converting electrical energy into air pressure energy.

For the axial compressor in a jet engine:
* The rotor vanes act as the air pump, and add energy into the air stream: total pressure is increased.
* The stator vanes do nothing with the energy content of the air stream: total pressure stays the same. The swirl is removed, and some dynamic pressure is converted into static pressure.

From The Jet Engine issue 5 by Rolls Royce: notice how the total pressure rises in the rotors, and the static pressure rises as a function of the passage narrowing.


How can aircraft land on choppy water? What plane can land the slowest on the water? What plane can land on the choppiest water? Is there a way to land between the waves?

Depends on how bad is the sea state, but the general technique for seaplanes is to land parallel to the swell, preferably on top of a wave:

  When landing on a swell system with large, widely
  spaced crests more than four times the length of the
  floats, the best landing heading parallels the crests and
  has the most favorable headwind component. In this
  situation, it makes little difference whether touchdown
  is on top of the crest or in the trough.

If forced to land against the swell, it is recommended to land on the backside of the wave:

  If crosswind limits would be exceeded by landing parallel to the swell, landing perpendicular to the swell
  might be the only option. Landing in closely spaced
  swells less than four times the length of the floats
  should be considered an emergency procedure only,
  since damage or loss of the seaplane can be expected.
  If the distance between crests is less than half the length
  of the floats, the touchdown may be smooth, since the
  floats will always be supported by at least two waves,
  but expect severe motion and forces as the seaplane
  slows.
  A downswell landing on the back of the swell is preferred.


How likely are flares to be fire hazards once touching the ground? According to the Wikipedia page of flares, flares create intense heat.

  A flare, also sometimes called a fusee, is a type of pyrotechnic that produces a brilliant light or intense heat without an explosion.

Could the flares launched by an aircraft as a defensive countermeasure against a heat-seeking missile cause a fire once they reach the ground?

I have no idea how likely it is general, but it seems that flares are indeed sometimes blamed for causing wildfires on the ground.

In New Jersey (2007):

  A massive wildfire that has already burned thousands of acres in the
  Pinelands and forced the evacuation of residents in two towns likely
  started this afternoon when an F-16 fighter jet dropped flares as part
  of a maneuver over a gunnery range, New Jersey National Guard
  officials said tonight.

  [...]

  "We believe it was one of our F-16s on a routine training mission that
  dropped the flare that started the fire," said Lt. Col. James Garcia,
  a spokesman for the state Department of Military and Veterans Affairs.
  "There's going to be an investigation."

In Oregon (2017):

  A remnant of a military flare was found at one of the areas burned,
  triggering a call to the Air National Guard, which uses flares in
  self-defense training exercises over those same areas that burned.
  Flares are deployed in air combat to draw heat-seeking missiles away
  from their targets.

  After being contacted by investigators about the fires, the Oregon
  National Guard agreed to suspend the use of flares. Agencies also
  engaged them about mitigation techniques to employ if it is determined
  that the guard was the cause.


Why does a sharp leading edge result in a larger pressure peak? I know in the real world the sharp leading edge (LE) will results in a larger pressure peak compared to a large radius LE.

And that there are three types of stall.

But I want the explanation in theory. I can't overcome the problem why the smaller the radius of the leading edge, the larger the pressure peak will be.

Envisioning the air flow at 15 degrees AoA is very helpful in understanding the negative pressure distribution on the upper wing.  Smoke in wind tunnels is used so the airflow pattern can be seen.  Yes, it is illogical for a thinner wing to have a higher pressure peak,
but notice the TOTAL negative pressure area of both 4408 and 4415 is very close.

What is going on here is the larger frontal area of the 4415 creates a larger, but not as low pressure bubble at 15 degrees AOA (look at graph) than the 4408.  Try superimposing the two airfoils at 15 degrees angle of attack and also throw in a flat plate too.

Remember, a higher pressure peak does not mean higher total negative pressure.  It means, like @Jan Hudec says, air molecules, which have mass and velocity (inertia), can not turn a sharp corner as easily as a small turn.

Imagine 20 race cars going down the track together.  They will make a small turn together, but if too fast into a very sharp curve, there will be a vacuum peak (lack) of cars on the near side of the turn!


What is the approximate thrust at sea level for a C-130J Hercules? I am looking for the approximate thrust of a C-130J Hercules under ISA sea level conditions. Also the efficiency of its propeller would be of help.
An answer to this question would really help my calculations for an assignment, so I don't have to guess values. If you could direct me to your sources as well, that would be highly appreciated.
The C-130J is equipped with  Rolls Royce AE-2100 D3 engines and the propellers would be the 6-bladed Dowty R391.

Okay guys, so I found some data from an old service publication from Lockheed. It says that the new C-130J propulsion system under standard conditions and flying 80 kts is rated "4,637 SHP, but produces 10,200 lbs".
So using the equation for thrust available for a propeller aircraft I get:
$T_a = \frac{\eta \cdot P_s }{V}$
with:
$T_a = $Thrust available
$\eta =$propeller efficiency
$P_s =$shaft power
$V=$air speed
$\eta = \frac{T_a \cdot V}{P_s}$
$\eta = \frac{10200 \textrm{lbf} \cdot 80 \textrm{kts}[ \textrm{aka. }135 \textrm{ ft/s}]}{4637 \textrm{shp} [\cdot 550 \textrm{ conversion}]}$
$\eta = 0.54$
This seems quite low but its what I got for anyone wondering. The service publication I got the info from is this:
https://www.sikorskyaerospace.com/content/dam/lockheed-martin/aero/documents/sustainment/csc/service-news/sn-mag-v21-v30/V25N03.pdf


Why do UCAV/UAV designs mostly use lambda wings instead of pure delta wings? Why do recent UCAV/UAV designs mostly use lambda wings (such as Taranis and Dassault nEUROn) instead of pure delta/diamond wings (such as X-47A Pegasus and Lockheed D-21)?

As for stealth characteristics, both wings have a direct advantage in radar wave reflection.

The image was taken from Technology challenges of stealth Unmanned Combat Aerial Vehicles (Sepulveda & Smith, 2017).

Aspect ratio.

Look at your two images: both wings offer the same wingspan, but the former requires a lot more wing area for it. Wing area is weight. More wingspan for less wing area means better fuel efficiency and thus better range.

Poor aspect ratio, leading to excess induced drag, is a major disadvantage of delta wings. Much of modern stealth design is an effort to mitigate the poor flight characteristics of early "stealth-only" solutions.

Neither UAV nor UCAV get meaningful benefit from supersonic flight, since they can't realistically oppose manned fighters. Large UAV and UCAV are meant for long-range predictable stealth missions, where slow and steady wins the race. Better fuel efficiency means more payload or longer-range missions, both directly improving the vehicle's usefulness.


What is the relation between flap setting and aileron droop on the A350? The A350 has a aileron droop function, which is linked to the flap setting. In contrast to other aircraft with aileron droop, the  A350s ailerons do not droop when the flaps are fully extended.

I would like to know why the ailerons don´t droop when the flaps are fully deployed, and if possible what the scheduling for the aileron droop looks like in relation to the selected flap setting.

Some of the gliders I fly do the same: They do not droop ailerons in full landing configuration but do in intermediate settings. There the reason is aileron effectiveness at high angle of attack and prevention of a tip stall.

If you droop the ailerons it yields more lift but the angle of attack at which the airfoil stalls is decreased, which might give you a more violent stall. And if you suddenly deflect the aileron down further the flow could separate and cause "the wrong wing to go down". To avoid this airliners are often fitted with vortex generators on the outboard section (e.g. Boeing 737) but that was probably a no-go for the clean A350 wings.

Regarding droop vs. flap setting, the FCOM probably has the answer for that. Couldn't find it online yet. A380 does 5 degrees when flaps reach position 1+F or greater, A320 does 5 degrees when flaps reach 1+F or greater so it might be similar with the A350


What would be the process to approve a minimal, standardized flight recorder for commercial balloons? According to this article about the recent balloon event, the NSTB will try to interrogate personal electronic devices for any related data. Since some of these mentioned in the briefing do not contain cameras, they may look for accelerometer, gyro, and altimeter data that may have been captured automatically. Chips for these are commonly found in cell phones, tablets and other personal monitoring devices.

Standardization, performance and reliability are only some of the things that a NTSB-recognized voluntary flight recording device would need. Is there any mechanism or regulatory pathway for a standardized yet voluntary flight recorder for balloons, or would it have to be all-or-nothing - required, regulated, and therefore in the end substantially more expensive?

According to this recent WSJ article:

  More than two years ago, the NTSB urged tighter regulation of balloons carrying passengers. Without such changes, the safety board said in a April 7, 2014, letter, “the potential for a high number of fatalities in a single air tour balloon accident is of particular concern.”

Currently, passenger balloons do not file flight plans (see this question and associated answers), and while transponders for balloons may exist they are not required, nor is any passive reflection enhancement for radar.

As Ron mentioned, if you want to produce a "standardized" voice recorder for balloons you'd be complying with FAA Technical Standard Order (TSO) C123 (currently revision C).

Along the same lines if you wanted to make a "standardized" flight data recorder it would need to comply with TSO C124 (and you would have to figure out what data would be useful to record, and how to get that data to the recorder).

If we're not sticking with an established actual standard then the next best thing are the personal portable electronic devices used in the flight - particularly if the balloon operators are using something like Foreflight, Garmin Pilot, WingX, etc. on a tablet. The NTSB lab has a good amount of experience in recovering useful data from these devices, and they can be almost as useful as an actual flight data recorder. Combined with an onboard camera (say a GoPro) capturing audio/video of the flight you can make an excellent reconstruction of events (and since many balloon operators offer their customers a flight video there's a good chance the cameras are already installed...)

It's worth noting however that none of these devices improves the safety of a given flight: CVRs, FDRs and even cockpit video are forensic tools used after a crash or incident to help us understand what happened, but their value is after an incident, and thus usually after loss of life.

The other NTSB recommendations (including carrying transponders) are aimed at actively improving the safety of individual flights, and may prevent incidents.


Why does the U.S. have so many general-aviation aircraft? In a comment on my previous question about the paucity of South Korean general-aviation aircraft in the 1990s, @Pondlife pointed out (in response to my comparison between the numbers of GA aircraft per capita in South Korea and the United States) that, in their words:

  Considering that just over 50% of the world's GA aircraft are in the US perhaps you should be asking why the US has so many, rather than why South Korea has so few? :-)

Why does the U.S. have such an extreme abundance of general-aviation aircraft (210,000 out of the total world population of 416,000 GA aircraft in 2016, according to the provided link), amounting to one GA aircraft for every 1,540 U.S. citizens (a per-capita abundance more than ten times the global average of one GA aircraft per 17,950 humans)?

Part of the reason is also historical. After World War II was over there was a large number of veterans with flying experience returning to a country that suddenly had a massive surplus of small trainer aircraft.

While the US expended huge amounts of resources on World War II, it was largely untouched by the destruction that many other countries suffered. This meant that instead of having to rebuild, the US could focus on growing the economy. Though GA declined in the Great Depression and was suspended during World War II, there had been a similar situation after World War I that had gotten the industry started.

Small aircraft were much simpler then, with a much lower cost to build and certify. This made general aviation relatively much cheaper. Also, while the US certainly had a highway system, the interstate highway system had yet to be built. Commercial aviation service was not nearly at the scale it is today and was still fairly expensive. This made general aviation a much more reasonable option for traveling around a large country.


Is it normal practice to refuel commerical planes whilst boarding is taking place? The other day, I was on a KLM Boeing 737. I had just sat down in my seat, and the captain was making announcement. He said something along the lines of,

  As we are refuelling at this moment, please take your seat quickly, leaving your seat belt open, and follow any instructions by the cabin crew immediately

This suggests to me that refuelling whilst boarding is not normal, as I've never heard this before, and it sounds like they want us to be ready to evacuate if something goes wrong with the refuelling.

Refueling with passengers onboard is quite normal and is done according to the recommendations of regulatory bodies/manufacturers and operators. Refueling with some fuel types are not permitted, while others are. From Airbus Flight Operations Briefing notes:

Refueling  with  wide  cut  gasoline  type  fuel (JET  B,  JP4  or  equivalent)  or  when  a  mixture  with  these  types  of  fuel  might  occur,  is  not  permitted  with  passengers  boarding, on board or disembarking.
Refueling with Kerosene (JET A, JET A1 JP8, TS1, RT, TH or equivalent, as approved by  the  approved  Aircraft  Flight  Manual),  is  allowed  when  passengers  are  boarding,  on board, or disembarking.

Some precautions are required to be followed in case refueling is done with passengers on board. The announcement you heard is simply following that. From Appendix 1 to JAR-OPS 1.305 Refuelling/defuelling with passengers
embarking, on board or disembarking:

(a)   An  operator  must  establish  operational procedures   for   e/defuelling   with   passengers embarking,  on  board  or  disembarking  o  ensure  the following precautions are taken:
(1)   One qualified person must remain at a specified location during fuelling operations with passengers  n  board.  This  qualified  person  must be  capable  of  handling  emergency  procedures concerning  fire  protection  and  fire-fighting, handling  communications  and  initiating  and directing an evacuation;
(2) A  two-way  communication  shall  be established  and  shall  remain  available  by  the aeroplane’s  inter-communication  system  or  other suitable   means   between   the   ground   crew supervising  the  refuelling  and  the  qualified personnel on board the aeroplane;
(3)  Crew,  staff  and  passengers  must  be warned that re/defuelling will take place;
(4)  Fasten eat Belts’ signs must be off;
(5)  ‘NO  SMOKING’  signs  must  be  on, together   with   interior   lighting   to   enable emergency exits to be identified;
(6)  Passengers  must  be  instructed  to unfasten  their  seat  belts  and  refrain  from smoking;
(7)  Sufficient qualified personnel must e on  board  and  be  prepared  for  an  immediate emergency evacuation;
(8)  If  the  presence  of  fuel  vapour  is detected inside the aeroplane, or any other hazard arises  during  re/defuelling,  fuelling  must  be stopped immediately;
(9)  The  ground  area  beneath  the  exits intended  or  emergency  evacuation  and  slide deployment areas must be kept clear; and
(10) Provision is made for a safe and rapid evacuation.

You're right about the announcement and leaving seat belt open- it is to help in quick evacuation. The flight crew is just following the procedure.


Why would a plane descend rapidly in uncontrolled manner after a steep turn? Here's some footage from 2010 Red Bull Air Race.  At 0:17 the plane is passing through a gate and then makes a steep turn to the left and that requires the plane to have left wing pointing almost downwards and right wing pointing almost upwards - this is how such turns are typically done. Right after the turn ends - at about 0:18 - the plane just starts falling down. It's only the pilot's skill which prevents it from entering water with left wing pointing down - the pilot promptly levels the airplane such that wings are now horizontal and this mostly stops the incident from developing.

How is this turn different from "good" turns which don't end up with plane falling down?

Markus Voelter interviewed Matt Hall for an episode of his omega tau podcast. They discuss this incident in more detail at 1:13:00.

The aircraft experienced a high speed g stall, also called an accelerated stall. Stall is determined by the angle of attack, which is the angle of the airflow hitting the wing. Stall is most often thought of at low speed, because as the plane goes slower, it takes higher angle of attack to provide enough lift, until it reaches the stall angle of attack and stalls. But this can also happen at high speeds in an aircraft performing a high g turn. Much higher lift is required to make the airplane turn, meaning much higher angle of attack is needed. Trying to turn the aircraft too tightly can increase the angle of attack too far, and the airplane stalls at a high speed.

The other factor is that the airplane will tend to roll left due to torque from the engine and P-factor of the propeller. Since the plane was already in a left hand turn, this plus the stall made the airplane roll past 90 degrees and head down towards the water.

Another aggravating factor was the airplane's stability, which is discussed at 1:31:00. The CG was fairly aft, making the plane less stable. The elevators were also over-balanced, meaning they would tend to make the airplane pitch up even further as the airplane pulled more g's. These changes had helped to make the plane a bit faster but were corrected after the incident to make it safer.

On the human factors side, Matt also mentions that he had a head cold at the time, and made the decision to take a phone call right before his flight, meaning he was distracted and not at his best.

Elsewhere in the interview he mentions that when he flies in air shows, he is more conservative in his margin of safety than other pilots might be.

  You don't need the money if you're dead. So it's just a show.


Why do we commonly use a "/" in acronyms in aviation world? In non-aviation world, separation between letters in acronyms is either nothing (e.g. URL) or a point (e.g. A.M.). But in the aviation world, the "/" is common (e.g. G/S, V/S).

I'm puzzled as:

some acronyms never use "/" (e.g. ATC)
some acronyms almost always use the "/" (e.g. V/S")
some abbreviation use this separator but are not acronyms (e.g. A/THR, THR containing more than one letter)

Why is this separator so spread in this worlds ?
Why some acronyms does not use this separator ?

It's not just aviation, it's standard in English.

  The slash has become standard in several abbreviations. Generally, it is used to mark two-letter initialisms such as A/C (short for "air conditioner"), w/o ("without"), b/w ("black and white" or, less often, "between"), w/e ("whatever" or, less often, "weekend" or "week ending"), i/o ("input/output"), r/w ("read/write"), and n/a ("not applicable"). Other initialisms employing the slash include w/ ("with") and w/r/t ("with regard to"). Such slashed abbreviations are somewhat more common in British English and were more common around the Second World War (as with "S/E" to mean "single-engined"). The abbreviation 24/7 (denoting 24 hours a day, 7 days a week) describes a business that is always open or unceasing activity.

(Slash (punctuation) § Abbreviation, Wikipedia)

Note that there is a difference between abbreviation and acronym, though that line in recent usage is blurred.

Boeing uses VS for the 777, and V/S for the 737. Language is more about what's more common, preferred style, and/or trends (e-mail becoming email). For example, knowing when to use a hyphen, space, or no-space, for compound words requires checking a dictionary. There aren't fixed unchanging rules to follow really in language.

There are some questions on the topic on English Language & Usage Stack Exchange.


What is the aspect ratio of a wing? I've been trying to get my head around wing design and have been following a few YouTube videos. When people talk about aspect ratio (AR) on a wing and give a value (Piper PA28 having an AR of 5.7) what is the 5.7? Is it wingspan length? Square metre of coverage? Or something else? I'm confused.

The Aspect Ratio ($AR$) is defined as the ratio (hence the name) between the wingspan squared $b^2$ and the area of the wing $S$.

Since both $b^2$ and $S$ are measured with a length squared (e.g., $m^2$), $AR$ is an dimensionless quantity.

To answer the follow-on question of TomMcW in the comments, $b$ is the distance between the wingtips, no matter the shape of the wing.


What requirements aren't met, that would designate an airspace to be Class E instead of A, B, C, or D? Quoted from Transport Canada "Class E airspace is designated where an operational need exists for controlled airspace but does not meet the requirements for Class A, B, C, or D."

Class A airspace requires you to be at a certain altitude to actually be in Class A airspace.

Class B is the same, Class C requires ATC clearance for both IFR and VFR.

Class D just requires VFR to have dialogue (two way communcation) with ATC.

What requirements wouldn't be met in a particular airspace that would need to designate it as Class E instead of the other classes?

EDIT: After some further research, the easiest way I've thought about Class E is an area that requires some control strictly for IFR traffic, but to VFR aircraft it's pretty much uncontrolled airspace.

Thank you!

The simplest answer is that Class E doesn't have a control tower, and the others (Except A) do.

Think of Class E like it's "Class A", but just at the lower altitudes. Both A and E are basically "wide swaths of airspace, not really near a major airport" with control provided by the Area Control Center (similar to ARTCC in the USA). Whereas B, C, and D are designated at airports with Control Towers. The only real difference between A and E is that no VFR traffic is allowed to enter Class A.

Although you're right that VFR traffic does not need to communicate with ATC in Class E, be careful about using the phrase "to them, it's uncontrolled airspace".  It IS still controlled airspace. VFR traffic does not need (nor sometimes want) to be controlled, but that does not make the airspace "uncontrolled".  "Controlled Airspace" is a term one will likely encounter on exams, and one doesn't want to get into the habit of calling it "uncontrolled" just because one is a VFR pilot.


What are the procedures to get a revoked mechanics certificate reinstated? At one point I held a mechanic's certificate. However due to poor judgement it(the certificate) was revoked due to a narcotics conviction.

How can I get it reinstated? Is there refresher classes?

I think you've to apply for a certificate again after the required period and take the tests again. From 14 CFR §65.11 Application and issue:

(d) Unless the order of revocation provides otherwise—
(2) A person whose mechanic or repairman certificate is revoked may not apply for either of those kinds of certificates for 1 year after the date of revocation.

In your case, you have to wait for 1 year after the final conviction to apply. From 14 CFR §65.12 Offenses involving alcohol or drugs:

(a) A conviction for the violation of any Federal or state statute relating to the growing, processing, manufacture, sale, disposition, possession, transportation, or importation of narcotic drugs, marihuana, or depressant or stimulant drugs or substances is grounds for—
(1) Denial of an application for any certificate or rating issued under this part for a period of up to 1 year after the date of final conviction;


Sidestick safety concern I read about several aircraft crashes throughout aviation history and the investigations on wikipedia and also checked out some episodes of the documentary "Air Crash Investigation".

While of course I can only make conclusions of what I read, hear and see it appears, that problems related to communication are one typical reason to contribute to such disasters.

Something that attracted my attention is that the sidesticks used in the aircrafts of Airbus seem to be involved in not less than three crashes including Air France Flight 447.

Though there is the general distinction pilot flying (PF) and pilot not flying (PNF) failures, malfunctions and the triggered warnings tend to create a situation of stress and strain in the cockpit as comprehensible. Confusion comes up not seldom in such situations.

My point finally is, that

Sidesticks are smaller than yokes. To apply a control input one only needs to turn one's wrist while a yoke requires you to use both of your arms to make the big aircraft turn. I wonder if the inhibition threshold therefore is somehow lower to make extreme, exaggerated inputs in case of a sidestick?
Secondly, and this is the major question, as it appears, the sidesticks make it much harder, to be aware of the currently applied control input (especially) for the other pilot. While the yokes of both pilots are physically connected and both pilots have direct feedback of the currents state at any given time, the sidesticks

give no feedback to the other sidestick
are smaller, which makes it harder to see the current state their in already by the size alone
are more distant than yokes making it again harder to see for the other pilot, as this A380 cockpit shot shows

Question:

How is the general assessment of those sidesticks concerning safety?
Is there any feedback to the pilots which stick is giving what input?
Has there been any development in improving this situation?
What are the accidents/incidents sidesticks more or less contributed to confusion/lack of information?

Sidesticks are smaller than yokes. To apply a control input one only needs to turn one's wrist while a yoke requires you to use both of your arms to make the big aircraft turn.

Side-sticks are used together with control laws. In an Airbus Normal Law (and Alternate Law which differs only by not providing the flight envelope protections), the system automatically trims for 1 G wing loading (which equals constant flight path angle) and zero roll rate and the side-stick deflection commands wing loading in pitch and roll rate in roll. The springs are sufficient to provide all feedback there is to provide as the deflection and the force both correspond to the command given.

Due to the way side-sticks work, extended deflection is rarely necessary, nor are very fine adjustments normally needed to stabilize the plane on desired flight path, because the computer handles that.

  Secondly, and this is the major question, as it appears, the sidesticks make it much harder, to be aware of the currently applied control input (especially) for the other pilot.

Feedback, on any kind of control, is provided by position and force. The other pilot can only feel the position. This allows them to notice that the other pilot pulled up, but not whether it is dangerously close to stall, because the stick position is biased by aircraft balance and, if horizontal stabilizer is used for trim (which all airliners do), also by trim. So the information it provides is rather limited.

On Airbus the side-sticks are not connected, so even this limited information is not available. But they have an advantage too. If the pilot flying is incapacitated or confused, the pilot monitoring can push a “control priority” button on the side-stick and the computer then only responds to their side-stick, so they don't have to fight with the other pilot on the controls.

  How is the general assessment of those sidesticks concerning safety?

I've not seen any specific study. But they were approved and I am not aware of any accident on Airbus where similar accident wouldn't have happened with other type with traditional control column.

  Is there any feedback to the pilots which stick is giving what input?

If both side-sticks are giving input, there is aural and visual warning “dual input”. If one pilot presses the “control priority” button, there is an announcement “control priority left/right” and visual indication. There is no direct indication of what the command is, but the pilots are looking for what the plane is doing anyway and they can usually judge from that. There is also indicator of control surface positions.

  Has there been any development in improving this situation?

I am not aware of any aviation authority thinking there is a problem to fix. Manufacturers are coming up with new schemes they consider good, but that may be as much marketing as genuine improvement.

  What are the accidents/incidents sidesticks more or less contributed to confusion/lack of information?

I can't think of any where it could be said the situation would have been significantly better if the controls were linked. If both pilots are confused, the connection would not help much and if only one is confused, the other can use the control priority to take over.

Even AF447, where some dual input was involved, is not really unique. There have been several crashes where pilots didn't realize a stall and didn't attempt recovery all the way from cruise level down to the ground (the most recent one) and all the others happened in planes with mechanically connected control column.


Is it possible to stall a stationary helicopter through excessive collective input? I understand that helicopters can suffer from "retreating blade stall" where the retreating side of the rotor has low airspeed and stalls in a typical way that airplane wings stall. This happens when the helicopter is underway.
By contrast I am interested if it's possible to stall in a hover or at very low speed, with normal rotor rotation speed, by pulling up too hard on the collective controls, generating an excessive angle of attack for the blades. Is it impossible because of the lever effort that would be required? Are there any safety systems that prevent it? Is it simply impossible to dial in enough collective to cause the blades to stall, even with the lever pulled all the way?
If it is possible, how do pilots know how much collective input is excessive and could cause a stall?

Helicopters use speed of the wing as a proxy for angle of attack in a similar manner to airplanes; in the airplane's case it's forward airspeed and in the helicopter's case it's "rotary airspeed", expressed as rotor rpm.  In the airplane you have the bottom of the green arc on the airspeed indicator; in the helicopter it's the bottom of the green arc on the rotor tach.  In the airplane there's a stall warning horn; in the helicopter there's a low rpm warning horn (that's the only "safety system" to prevent blade stall).
If you're in a hover, there's nothing stopping you from yanking the collective pitch all the way to the up stop.  This shouldn't be sufficient to induce blade stall because the machine will respond by both climbing and increasing the vertical downwash of air, both of which will make the actual blade AOA increase less than the physical pitch change made by pulling collective, as long as rotor rpm stays in the green arc.  Whether this action keeps the rpm in the green is another matter, and on turbine machines it's likely to overtorque/overtemp the engine(s) even if rpm is kept in the green.
If you pull pitch like that and the engine can't deal with the load even at maximum power, the rpm will quickly decay and once it goes below the green arc, blade stall could (will) occur at some margin below the green arc.  When blade stall occurs from low rotor rpm while in a hover or at very low speed, the rpm decays even faster, the blade drag is too high for the engine to fight against, and the machine starts to settle vertically, causing blade AOA to increase further.
Before you know it the blades are WAY above stalling AOA, and you become While-E-Coyote strapped to an anvil and down you go; there's no recovery once a vertical descent with a stalled rotor starts, engine or not (if this happens in an engine out autorotation at altitude, the rotor will be close to stationary by the time you hit the ground - I recall a Robinson R22 incident from the 90s where the pilot lost engine power due to suspected carb ice, while flying along at 1000 ft, and didn't lower collective in time to get a proper autorotation started, the rotor rpm quickly decayed, the rotor stalled, and the machine just fell out of the sky like a wounded goose).
So the limitations are not so much about collective movement, it's the effect of collective movement on rotor rpm (and engine limits) that is critical, and the pilot has to monitor rotor rpm and respect the bottom of the green arc and the rotor rpm warning horn to stay safe, knowing that if the limits are exceeded and a stall happens, there is probably no recovery.
So, if you are hovering 10 feet in the air and you let it happen, it's a hard landing with maybe the tail boom getting chopped off.  If it happens while hovering at, say, 100 ft, the result is fatal.
To answer each question specifically:

Yes you can pull full collective any time. But the key safety parameter isn't collective position, but rotor rpm.
The only safety system is the rotor tachometer and the low rpm warning horn.
Yes, IF the engine has sufficient power to maintain rotor rpm in the green while pulling full collective. Some helicopters may be able to do this some may not.  If it can't, the rotor stall won't occur until the rpm decays to some point below the green arc, and once that happens, you are done, even if the engine is still making power.
Excessive collective input is revealed not by the actual lever movement, but by its effect on rotor rpm, which is the most critical parameter for the pilot to monitor. See here for a Robinson safety letter that talks about low rotor rpm rotor stall to see just how critical this parameter is.


Does holding an MEI entitle you to instruct in ASEL/ASES as well? Suppose someone earned a commercial pilot certificate in both ASEL and AMEL as well as only a  Multi Engine Instructor license.  Contingent upon both the MEI being valid and a valid Class II or better medical certificate, does the MEI allow this person to exercise the privilege of flight instruction in a single engine aircraft as well?

No. See FAR 61.195 (b):

Aircraft Ratings. A flight instructor may not conduct flight training in any aircraft for which the flight instructor does not hold:
(1) A pilot certificate and flight instructor certificate with the applicable category and class rating...

Without an Airplane - Single Engine rating on the flight instructor certificate, an instructor may not conduct flight training in a single-engine airplane.
As an aside, a CFI/I/MEI needs only a third class medical. See FAR 61.23(a)(3)(iv):

(3) [A person] must hold at least a third-class medical certificate— ...
(iv) When exercising the privileges of a flight instructor certificate and acting as the pilot in command;
(v) When exercising the privileges of a flight certificate and serving as a required pilot flight crewmember

In addition, note that these requirements are for an instructor also acting as a PIC or required pilot. When the student is the only required crewmember for a given flight, the instructor is not required to hold a medical certificate at all.


Have Eddy current brakes ever been used/tested on an aircraft? Eddy current brakes (wiki) --- a.k.a. magnetic brakes --- get quite some attention from train engineers. They are used on Shinkansens, roller coasters, and even (as far as I know) metro trains on Lane C in Prague. They basically consist of a magnet and a rotating disk, where these two never touch. The kinetic energy is converted to heat, as in conventional brakes. While Shinkansens give them up, from what I view it is in favour of recuperating brakes and whence it is irrelevant with respect to aviation.

I wonder whether anyone considered their use in aviation. To me it seems that it could be a good option.

Eddy current brake is a misnomer: it's a damper, not a brake as in a device that works on friction forces and can completely stop motion. Eddy current dampers do provide deceleration forces, by inducing whirly electromagnetic currents in a rotating wheel - they could provide extra emergency braking power without installing extra brake pads.

However, aircraft wheel brakes are dimensioned for emergency deceleration, can provide this adequately using friction pads, and their main problem is getting rid of the heat that is created. Eddy current dampers create the same amount of heat...


How does a Fenestron remove or reduce the tip vortex losses in a helicopter? How are Fenestrons (or fantails) able to reduce the tip vortex losses in a helicopter?

There are basically two effects here:

One is the ducted fan effect. Ducted fans are more efficient than open fans, specifically creating smaller vortices at the blade tips.
The other is the fact that the tail rotor of a helicopter is not situated in an area with almost linear airflow along the rotor axis. It is also submitted to:

A steady flow due to helicopter forward motion. This is coming from the side of the rotor, so affects opposite rotor blades differently.
A buffeting flow from the h/c main rotor. This is an unsteady flow, also coming laterally from the point of view of the tail rotor (actually, the top).

Fenestron tail-rotor ducts can be considered a fairing that reduces airflow across the rotor. The end result is that the rotor blades are in a much smoother flow that helps reduce unwanted turbulence. This is one of the reasons such rotors are quieter than regular open tail-rotors (the other being the fairing material absorbing some of the noise).


Why burn fuel at departure airport instead of flying to destination before emergency landing due to deflated tire? A Southwest airliner recently had an issue where a "deflated tire indicator" showed on one of the landing gear tires after takeoff from Oakland, CA. They circled nearby for 4 hours while burning off fuel before landing back at Oakland.

In a circumstance like that, where there's no flight control or aerodynamic problem (i.e. landing gear door stuck open), why wouldn't the flight continue on to the destination airport and burn fuel along the way? The flight time is close to the amount of time they spent circling to burn off fuel, so it's not like they get on the ground any faster by staying near their source airport.

Note that this particular plane landed safely and inspection showed no indication of any real issue with the tire, but even if it did have a flat tire, would it be any worse to go on to the destination?

I suspect that it may be just in case another in-flight emergency forces the plane to divert to an alternate airport that may not have the full emergency equipment to deal with a landing gear problem, which raises the question: if the flight had departed from a small airport with limited emergency support, would the plane travel to an alternate airport with better emergency support?

I have no idea what a deflated tire indication is on a 737, but perhaps the news media got something wrong in their reporting. (Nah... that never happens, does it???)

That said, the cardinal rule of landing gear problems is to leave a good configuration alone, so don't retract a gear with a possible problem, since it might not come back down again.

With the landing gear extended, your fuel burn per mile is far greater than with the gear up, so "enough fuel to get to XYZ" (based on normal gear-up cruise) isn't nearly enough to get there with the wheels down.

Any time an airliner takes off from a field they couldn't return to (most often, this comes up because the weather isn't good enough for a published approach back in), they have a "takeoff alternate" that they could get in to within a prescribed distance -- typically an hour's flying time.


What are the differences between air brakes, spoilers and lift dumpers? So many people use these words interchangeably, but after a little searching on Google, I found they are not quite the same thing. All the links I read explain things in a misleading way which pushed my confusion even further.

I'm looking for a simple explanation on the effect of these parts on The Four Forces of Flight

Air brakes increase drag with little change in lift. The following image shows the air brake being used in BAe 146

"Eurowings bae146-300 d-aewb arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.

Spoilers both increase drag and reduce lift- basically, they change the lift-to-drag ratio. Sometimes, they are also used as control surfaces in lieu of ailerons. This image shows a Boeing 777 spoilers being used in flight.

Screengrab from youtube.com

The lift dumpers are basically ground spoilers that are used to 'dump' lift, especially immediately after landing, where you want rapid reduction in lift. The following image shows lift dumpers being used in BAe 146.


Can I apply for a commercial-utility licence for an auto-gyro? Can an auto-gyro apply for commercial purposes? Utility in remote areas, Alaska for example.

What license is needed (airworthiness, etc.)? How much does it cost approximately?

Can I build my own Auto-gyro and license it to this purpose? Applying for the required FAA license surely will improve the design's safety.

Really this is a moot point...

14 CFR 91.319 (a) states:

  (a) No person may operate an aircraft that has an experimental certificate—
     (1) For other than the purpose for which the certificate was issued; or
     (2) Carrying persons or property for compensation or hire.

And again (e) states:

  (e) No person may operate an aircraft that is issued an experimental certificate under§ 21.191(i) of this chapter for compensation or hire, except a person may operate an aircraft issued an experimental certificate under § 21.191(i)(1) for compensation or hire to—
     (1) Tow a glider that is a light-sport aircraft or unpowered ultralight vehicle in accordance with§ 91.309; or
     (2) Conduct flight training in an aircraft which that person provides prior to January 31, 2010.

21.191 (i) is in reference to operating light-sport aircraft and how it obtains an experimental certificate.

There are very few auto-gyros that are factory built and certified, and none that I can find that are currently being built.

Anything you build in your home (home-built) needs to pass certain FAA inspections before it is issued an airworthiness certificate and/or experimental certificate. All home-builts are experimental since they are not built under the manufacturers quality control system. Either way, the FAA certifies that you build it to a safe standard and a standard in which the manufacturer intended it to fly. Being used for a commercial purpose, even if that were possible (which it is not) would not change the certification process for that aircraft. It would not require you to abide by any different rules or build standards. That is implying that personal aircraft would be built to a lower safety standard than commercial ones, which isn't the case.


Can I log PIC instrument time in a sim without a current medical or flight review? Just wondering if one is on land and in a simulator, if you need a current medical and flight review to train and log PIC time?

You can log training time in a simulator but you can't log PIC time.

The rules on logging are in 14 CFR 61.51. There's no requirement anywhere that I can see to have a medical and/or flight review in order to log time.

Logging training time in a simulator is allowed by 61.51(h):

  (h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, full flight simulator, flight training device, or aviation
  training device.

But, you cannot log PIC time in a simulator. 61.51(e) says (emphasis mine):

  (e) Logging pilot-in-command flight time. (1) A sport,
  recreational, private, commercial, or airline transport pilot may log
  pilot in command flight time for flights-

That means, the only way to log PIC time is in an aircraft.


Why don't aircraft fly direct via oceanic airspace from UK to Iceland? Whenever I watch the routes of aircraft flying between BIKF (Keflavik, Iceland) and e.g. EGLL (Heathrow, UK), they make a small turn in between to avoid oceanic airspace. See for example this flight route:

(Source: Screenshot of https://www.flightradar24.com/data/flights/ba894#236858ea)

The turn is made at the point where Shanwick Oceanic, Reykjavik FIR and UK airspace border each other. A direct routing would have a segment of around 220NM in Shanwick Oceanic airspace.

What's the reason, these flights don't go through Oceanic airspace? I observed that it's operator independent (ICE, BAW, EZY all fly this dented route) and it doesn't correlate with aircraft capabilities (ICE flies the same airctaft through Gander Oceanic on the way to North America all the time). So why is this routing?

I don't think they are specifically avoiding Shanwick Oceanic airspace. The reason seems to be related to the airways in the Scottish airspace. In general, you have to file a route using airways when operating in this airspace and there simply isn't one available that provides a more direct route. I recreated what looks like the route you show on skyvector.com:

This route would use UL612 between DCS (Dean Cross) and STN (Stornoway) and then continues via the N610 to RATSU, which marks the border between Scottish, Shanwick Oceanic and Reykjavik airspace. There are no airways from RATSU in the Reykjavik airspace, so they probably filed a direct route from there. The total length of this route would 1049 NM.

You can see alternative airways to use in this zoom (or go to skyvector.com, select "World Hi" for high altitude airways and zoom in):

There are two waypoints South of RATSU where one could enter Shanwick Oceanic airspace: ATSIX and BALIX. The shortest routes I could find with skyvector were 1042 NM and 1035 NM long respectively. This is basically the same distance and the optimal route would likely depend on the current winds.


How safe is the Boeing 737-800 in high winds? I'm not good with flying. However, on Wednesday morning I'm flying from Gatwick to Bergen, Norway, traveling alone with two young children.
I'm trying desperately to not show my kids how afraid I am of flying - with families in two countries we have to do this all the time. So I have to be mentally prepared for this: the forecast is showing high winds.
Around the time of take-off (10:50) it shows 16-18mps, SSW.

As far as I've been able to understand, this is at the limit of what a Boeing 737 can take?

Basically: Will we be safe? How can I know? Will it be bumpy? What will happen? Should we cancel?

Can you help me understand?

Extremely safe.

Firstly, your pilots will have access to much more detailed and real-time weather information than you can get. They want to get home safely as much as you do, and will not fly if it is not safe to do so.

If the wind is coming from straight ahead, there is no maximum limit, which is good as aircraft takeoff and land into the wind whenever possible. If the wind is coming from the side - known as a crosswind - the limit in a 737 is about 36 knots/19 mps (the specific number can vary but that's a guide). The reason for this limit is to avoid the winnd blowing the plane off the side of the runway. So if the wind is that strong, hopefully the runway faces into wind, otherwise your pilot may have to delay the takeoff until it calms a bit.

Once the aircraft lifts off the ground, the wind speed is irrelevant (except for navigation purposes). Imagine a fast flowing river. Placing your canoe into the river when it is at such speed is difficult, but once you are in the middle of the water it is smooth as silk. The transition from ground to cruising altitude is similar. The initial climb will probably be turbulent but the aircraft is built to sustain more than nature can throw at it. Once at altitude the wind speed will almost certainly be much more than 100 knots - but just like the fast flowing  river, if you close your eyes it will feel nice and smooth.

You should only cancel your flights if you want to throw away your money. Otherwise just try to relax, and understand that many airline delays that you hear of are caused by a pilot choosing safety over schedule.


What are the procedures for flying supersonic over international waters? Supposing I owned a supersonic aircraft eg a MiG-21 and wanted to fly the jet out into international waters for a supersonic speed run.  Are there any particular ATC procedures for doing so?

As a non-commercial civilian, the high seas (international waters) do not exempt you from the local governing rules: the civilian FIR boundaries meet over water, and there is no free-play gap in between. This goes as far back as 1948:

  Flight over the high seas. It should be noted that the
  Council resolved, in adopting Annex 2 in April 1948 and
  Amendment 1 to the said Annex in November 1951, that the
  Annex constitutes Rules relating to the flight and manoeuvre
  of aircraft within the meaning of Article 12 of the Convention.
  Over the high seas, therefore, these rules apply without
  exception.

If your plan is to fly VFR and not file an IFR plan with the appropriate ATS authority, then you can't fly supersonic (unless authorized):

  4.4 Unless authorized by the appropriate ATS authority,
  VFR flights shall not be operated:
  a) above FL 200;
  b) at transonic and supersonic speeds.

Reference: ICAO Annex 2 (Rules of the Air).

Note: Consult the local AIP for listed deviations from Annex 2 if the country of operation is an ICAO member.


Which kind of stability do aerobatic aircraft have? I'm designing an aerobatic aircraft similar to EXTRA 330SC.
I would like to know about stability: lateral and longitudinal, static and dynamic.
I read that aerobatic aircraft have neutral static stability.
Some books relate Cm to AoA.  Others relate Cm to CL.
Please detail more about this or provide some bibliography.

If you are serious about designing an airplane that will one day fly and certify, you should, to the very least, read up Raymer, Aircraft Design: A Conceptual Approach. It has a chapter on stability & control, and would be a good introduction to the lingos.

More in depth coverage can be found in Etkins, Dynamics of Flight. It's pretty much the bible for S&C engineers.

Once you feel like you have the theory down, design guidelines for conceptual design (even preliminary design) can be found in Roskam, Airplane Design.

Since design goes hand-in-hand with certification requirements, a knowledge of 14CFR Part 23 and AC 23-8C wouldn't hurt.


How do pilots set the exact amount of thrust needed for a reduced power takeoff? I know jet-powered airplanes very rarely use 100% available power for takeoff, and most often reduce power to increase engine life. How does a pilot set thrust to an exact level?

The airplanes I'm referring to are common commercial jets such as A320/B737 types.

If they'd set it manually with thrust levers, there are some complications: there are no placards on them. If you try setting revs via the displays, revs and temperatures do not change instantaneously, and looking at the indicators and adjusting the lever would be a dangerous distraction during take-off (imagine: the engines roaring, the brakes are applied and tires are rubbing against the asphalt; or the plane is rolling, and the pilot is looking at the gauges).

On the other hand, if a limit is set via FMS, how does it know when to add more power if it's urgently needed?

The EMB-145 used a combination of FADEC settings and the thrust levers to select the desired thrust and to signify the need for additional thrust.

The thrust levers had a detent that selected "100%" thrust for the given mode the FADEC is operating in.  If you pushed the thrust levers past the detent to "max" this would give you extra thrust.  The mode the FADEC is operating in is selected by a button in the cockpit that cycles through the available modes.

For takeoff power the modes were "ALT T/O", "T/O" and "E-T/O" which gave you 90%, 100% and 107% thrust for takeoff when the thrust levers were in the detent.  If advanced past the detent this would put the mode into a reserve mode and depending on model would give you up to 110% or 117% thrust.

Setting the exact amount of thrust needed was up to the engine FADEC units, our job was just to tell it what thrust mode it was in and it scaled the output of the thrust levels to that mode.  Flying a 90% thrust takeoff, a normal rated climb and then reducing to cruise levels only required initially setting the thrust levers into the detent and adjusting the FADEC mode.


What does induced flow exactly means on helicopters flight? How is it different from downward deflection of air? I believe all airfoils deflect air downwards at an angle at a velocity V. What is the difference between the downward-vertical component of the deflected wind and the induced velocity in Helicopter flight?

Corollary: Why the airplane doesn't face this induced velocity and induced drag?

These are referring to the same thing.  The induced velocity is the change in air velocity due to aerodynamic interaction with an object.  The downwash from a helicopter is the vertical component of the main rotor's induced velocity.  This is the induced velocity of a bunch of rotating wings.

A wing on a fixed wing aircraft produces induced velocity too, but it's located in a region of lower induced velocity during flight.  The induced velocity varies in space and time.  As a fixed wing induces velocity in the flow field, it "flies away" from that region.  So it's flying in "cleaner" air.  An exception would be if a fixed wing could fly in a tight circle.  In this case it would fly through more of its induced velocity, experience more induced drag like a helicopter, and have a large downwash below the circle.

Another way to say this is that the fixed wing's induced velocity is more spread out in space, and hence not as concentrated at any one point.  The hovering helicopter's induced velocity is highly concentrated in the region around the helicopter.

You can probably find visualizations or animations of vortex lines on the web.  In those, you will see that a fixed wing primarily trails vortex lines behind it, while a hovering helicopter produces a helix of vortex lines below it, but close by, which keep the induced velocity at the rotor disk high.


How are the limits of the center of gravity chart established? When one views a center of gravity limit chart, in this case a C-182T chart, one notices that the forward C.G. line has two small bends in it, one around 2250# mark and the other around the 2675# mark. Why the two bends in the forward C.G. limit line? What has happened at these two points?

Image from: C-182T POH pg. 211

These "bends" (or sloped straight lines) are due to various structural constrains on the aircraft.

In level flight, the sum of forces must be zero:

LiftWing - Weight - LiftTail = 0

The summation of moment should also be zero, otherwise the plane will pitch up or down.

The wing has a structural limitation, i.e. a maximum loading. Solving the above equations for constant LiftWing yields a slopped straight line. This is the "bend" portion you see in the graph.

Drawing these lines (shown as colored lines here) on top of a weight and balance graph yields:

The resulted graph is the intersection of the various constraints.

In the C-182 graph, there are five constraints (five edges). Wing loading and tail loading is used as an example here, since I do not know what is the actual constraint used by the engineers. Another possibility is strength of the nose gear.

Main landing gear loading and aft tipping limit would have resulted in a downward slope, which does not correspond to the given graph.

Source: Center of Gravity Limitations (PDF)


Why don't commercial aircraft adopt a slightly more seaplane-like design to allow safer ditching in case of emergency? For example :

Why don't they have a more hydrodynamically shaped fuselage underside ?
Specifically for the aircraft that have their engines mounted under the wings, isn't this too dangerous for ditching ? As far as I understand when an engine touches water during ditching the whole aircraft would/could break apart ?
Or, couldn't they have some kind of (retractable) floats ?

I'm far from being an aviation expert myself, but I imagine some things like the above (or possibly others) could be done, in order to allow safer ditchings for commercial aircraft. Would all these be too costly for airlines ?

Because ditching is extremely, extremely rare, so the costs of redesigning aircraft along with the extra drag and weight (increased fuel burn) it would no doubt add to the airframe far outweighs the potential benefits.

That argument might sound weird to someone, but think about it this way: would it seem reasonable to redesign every single car on the planet so that they can float, just because every year a few people around the world accidentally drive their car into a lake?

A quick search on The Aviation Herald suggests that, since 2005, a total of four flights worldwide have been forced to land on water. To give you an idea of the scale, there are about 100,000 commercial flights worldwide daily. So that's something like 4 flights out of half a billion, which is practically nothing. Also, in those 4 accidents, more than half of the people onboard actually survived. Actually, the increased greenhouse gas emissions caused by adding extra weight, such as floats, to aircraft worldwide would probably kill many more people than such safety equipment would ever save.

It's not that the aviation industry isn't investing heavily to improve safety. It's just that making ditching safer is an area where there is almost nothing to be gained. So the money can be much better spent elsewhere.


Why do we always use circular nozzles for commercial aircraft engines? Why do we always use circular nozzles for commercial aircraft engines? Other shapes like rectangular nozzles are used in military aircraft, e.g., Raptor, Nighthawk.

Simple geometry: Circles are strong, and provide a large area for the amount of material used. That means you get more intake/nozzle for the cost of the materials, while giving you a strong and (relatively) light engine.

A circle also helps to keep the airflow smooth, because it's (again, relatively) uniform: no weird edges, corners, or angled things to cause turbulence, vortices, or otherwise disturb or make the airflow "complicated". This is ideal at subsonic speeds.

Overall, this makes a circular engine the most efficient, and when it comes to commercial aircraft efficiency is king: the lighter and simpler you can make a safe engine, the better.

In some aircraft we will see "off-circle" (eg 737 engines, which have a not-quite-circular intake) or oval intake, but they're basically just circles where the dimensions have been adjusted to take a slight efficiency hit for the purpose of fitting the shape of the aircraft better.

Military aircraft which use rectangular air intakes tend to do so for two different reasons, rather than because it's actually any more efficient.

Stealth. Circles scatter radar waves in a "bad" way for stealth. Angles are much better for stealthy aircraft
Because that air intake is actually an Intake Ramp designed to slow the airflow into the engine by deliberately creating a shockwave to slow the air down. This is because jet engines need subsonic air to work properly: this is why you tend to see square intakes on supersonic aircraft. Commercial aircraft don't usually fly supersonic, though, with the notable exceptions of Concorde and the Tu-144 which both had square intakes for this reasons.

If you don't care about subsonic efficiency (Concorde/Tu-144, which were both designed to spend most of their working lives supersonic), or if you just want to be able to go supersonic at all, a square intake can make sense - but for those of us living life under Mach 1, and for airlines who care about fuel efficiency above pretty much anything else, a circle is best.


An instrument applicant must have 40 hours of actual or simulated time. Can some of this be flown without an instructor? 61.65 says an instrument applicant must have 40 hours of actual or simulated instrument time, 15 of which must be from an authorized instructor. What about the remaining 25 hours? Someone without an instrument rating can't file and go fly an IFR flight, right?

The 25 hours that are not required to be "instrument instruction" may be flown under the hood with a safety pilot. This is the usual way to accumulate some of the simulated instrument time.

I believe this time may also be flown & logged in actual IMC with an instrument rated pilot acting as PIC (their name on the flight plan, etc.) and the instrument student acting as sole manipulator of the controls, but you may prefer to conduct your actual IMC practice with an instructor.


What are the most significant risks to life for the airline passenger? Suppose that I start taking long-distance airline flights, and that after each flight I immediately jump on to another aircraft that takes off for another faraway destination - and continue doing this, indefinitely.

In other words, I'm going to live the rest of my life travelling on airliners.

What's going to kill me first:

an airline accident or incident
excessive radiation exposure
some other flying-related health risk
old age?

If the answer is old age, then let's suppose for the sake of the question that I'm also going to live indefinitely, until one of the other factors does me in.

Exclusions

I'm only interested in risks that are specific to flying. I suspect that breaking my neck falling down the steps or choking to death on an in-flight peanut might be higher, but they don't count, because I use stairs and eat peanuts regularly anyway.

I guess exposure to infectious diseases (and other risks associated with the airline cabin environment) should count, since air travel does  increase those significantly.

The health risks of an airline food diet are excluded though, especially if I decide to fly economy class. So is dying of despair.

However, the risk of severe food poisoning should be included.

Finally, I'm only flying on scheduled flights on reputable airlines.

The comparison

As you can probably gather, the device of the hypothetical permanent travel  is simply a way of framing the risks for the sake of comparison. I'm not really interested in what it might be like to fly forever, but in the quantum of risks in airline travel.

On the other hand there presumably come a point where the frequency of travel means that some risks become disproportionately more significant (e.g. eating 70 airline meals a year might be simply unpleasant, but eating 700 would be to suffer from a genuinely unhealthy diet).

However I can't think of any risks of that kind that would start to increase at a rate greater than frequency of flying, within any normal flying frequencies.

Future risk changes

I assume that flying is going to continue to become ever-safer, which could mean that the risk of an incident or accident will fall over time while (say) health risks don't. For the sake of a fair comparison, let's say that even into the far future the risks will remain as they are today.

Risks of dying of air travel are tiny. We can find data on direct risks of air travel, and there is an app for calculating how many years of continuous flying would lead to known aviation related chance of dying. Tiny.

Some risks ore harder to quantify though. If you fly economy class, you may be exposing yourself to the health risk of excessive sitting. It damages blood vessels and may lead to Deep Vein Thrombosis which can cause sudden death. From this news article of 23 Oct 2000:

  Emma Christoffersen, 28, died from deep vein thrombosis (DVT) believed
  to have been caused by sitting for several hours on the 12,000-mile
  flight to Heathrow from Sydney. Medical experts said she was one of
  the youngest victims of DVT, a sudden illness caused by lack of
  movement over a sustained period that claims the lives of a number of
  airline passengers each year.

Also, you're excluding the effects of airline food. An economy class meal costs between USD 2.- and 10.-, how good is that for someone to eat day in day out?

Air travel is not an activity that is good for health if undertaken on a daily basis. During a long economy class flight people should get up for a brief stroll every few hours or so, and once landed should be vigorously walking around in the sunshine over the course of a couple of days to reset their blood flow and health before taking another flight. Like pilots do.

From Mayoclinic.org:

  One study compared adults who spent less than two hours a day in front
  of the TV or other screen-based entertainment with those who logged
  more than four hours a day of recreational screen time. Those with
  greater screen time had:

  A nearly 50 percent increased risk of death from any cause
  About a 125 percent increased risk of events associated with cardiovascular disease, such as chest pain (angina) or heart attack

Air travel is very safe and mortality rates are low, as illustrated in this article from The Economist. It is a good thing though that most of us only take a couple of flights a year, with ample time in between flights. But it's not only economy class travel that is bad for health. I know people who used to be up in the air on an almost daily basis for years, flying business and first class. Unfortunately one of them is not here anymore, most probably due to DVT.


How much electrical power is available in a Gulfstream G550 and from which power sources? I'd like to understand what electrical power sources are available (engines? APU? GPU? Other?) on a larger jet like the G550, and how much power they can provide, depending on whether the airplane is flying at cruise speed or on the ground. I'm curious about how much electrical load can be added to the aircraft and how.

There are some answers to common electrical questions about the G550 here according to them the engine integrated drive generator (IDG) unit delivers 40KVA, 400HZ, 115V; they don't list a power factor but lets be conservative and assume .8 (it must be between 0 and 1). This would result in 32 Kilowatts per IDG. So you have 64 Kilowatts at your disposal. The minimum load will depends on the avionics installed and can vary from plane to plane. At night load will be increased as a result of the use of interior lights as well as landing lights during takeoff and approach. Your load will be highly variant on the interior of the aircraft. As this is a luxury plane there could be a whole host of things inside like TV's, lights, kitchen equipment etc. You can find specific equipment installations in the POH/loading info for a given airframe. You can find a nice breakdown of the G450's electrical system here.

As for how you can add it, you can simply add it as you would add any accessory, to the main bus (by an A&P of course). You will need to make sure the item can be added to the craft and in some cases there may be a stipulation along the lines of not using it during takeoff or landing. The amount of load will vary by aircraft and dependent on what else is installed.

  I'd like to understand what electrical power sources are available
  (engines? APU? GPU? Other?)

The engines (while running) are generally the main source of power. The APU is often used on the ground when the engines are off but the plane needs electronics on and can also serve to help start the engines. It does appear the G550 has an APU. According to this data sheet the G550 runs a Honeywell RE220 APU that can output 45 Kilowatts. According to the site it will operate up to 37,000+ Ft. so it in theory could be run for the duration of a flight.

A GPU is only available on the ground and only at airports that have them. Their output varies widely and a large variety of types may be available at any given airport.


Does a constant speed prop maintain the optimal angle of attack at all time? From an answer on this site:

The governor just knows: Speed too fast, flyweights move out, routes
pressure to move the blades more coarse; speed too slow, flyweights
move in, routes pressure to move the blades more fine. How coarse or
how fine, the governor does not know or care.

So a constant speed propeller will not maintain the best AoA through the flight, at the end of the day it will allow to fly at maximum prop. efficiency only selecting the appropriate MAP/RPM for the flight condition, otherwise it will just keep the rpm selected.
Am I correct? Or am I missing something?

You may be overthinking it.  The governor will set the blade angle that provides the load on the engine that keeps it from speeding up or slowing down from the setting determined by the spring preload that resists the flyweights.  You could connect the governor to a big disc brake in place of the prop and it would work the same.
So it sets the blade AOA that gives the required load without knowing what that AOA is. It's just a spindle with weights, a spring to hold them in, a mechanism to preload the spring and an oil metering valve - there's no AOA feedback to the governor, only a speed feedback, and the input command from the pilot is just pushing on a spring.
Whatever blade AOA that is achieved is the blade AOA required to achieve the load on the engine.  Whether that blade AOA is 100% optimized to get the maximum thrust from that particular condition is another story, and blade twist, airfoil selection, blade length, number of blades, total blade area, and all that all have to be balanced against various objectives.
As to what particular aircraft speed, prop rpm and engine load gives the highest possible thrust efficiency from the propeller, you'd have to get from the prop designer and there may still be some optimization of all the parameters I mentioned toward a cruise case, or a static thrust case (like seaplane props; desirable to get on the step as quickly as possible), or a maximum speed case.


Received ADS-B TC=31 message with non-existent version 3? On several dates, my ground station received some airborne operational status ADS-B messages (TC=31) where the version number was reported as 3. As far as I know, the only valid versions are 0, 1, and 2.

I don't think it was just a garbled reception because

I received more than 500 messages of the same type, all reporting 3 as the version number.
The messages came from three USAF aircraft of the same type, KC-10A Extenders.

Here is a sample message, in raw form: 8DAE022EF8000000206143635C24

The dates on which I saw the messages:

2018-04-13
2019-02-16

The aircraft associated with the messages:

ICAO AE020E, registration 79-1948
ICAO AE0212, registration 82-0193
ICAO AE022E, registration 86-0037

What's the most likely reason behind these messages having this (AFAIK non-existent) version number? A test? A misconfigured piece of equipment (it strikes me as unlikely that the version number can be set to 3, but maybe I'm wrong). Is it a bug in the decoder I'm using?

[Edited 2019-02-16 to include other aircraft I've received the anomalous messages from.]

I've decoded the message with my own decoder. I don't claim it is bug free, but it is designed to follow the 1090ES MOPS (ADS-B Minimum Operational Performance Standards) very closely and gives warnings when messages deviate from the standard. I forced decoding conforming to version 2 of the MOPS, which is the latest standard.

HEX: 8DAE022EF8000000206143635C24
Aircraft Operational status message (airborne format)
    Capability Class:
        TCAS op.          false
        1090ES IN         false
        Reserved [13,14]      0
        ARV rep. cap.     false
        TS report cap.    false
        TC rep. cap. lvl      0
        UAT IN            false
        ADS-R rsvd flag   false
        Reserved              0
    Airborne Operational Mode (OM)
        TCAS RA active    false
        IDENT             false
        Reserved [ATC]    false
        Single Antenna    false
        SDA               0
        Reserved [33-40]  32
     MOPS version     3
     NIC supp. A      false
     NACp             1
     GVA              1
     SIL              0
     SIL supp.        true
     NIC baro         0
     Heading ref.     True North
     Reserved [56]    true

The 1090ES MOPS version is indeed indicated as version 3, which is not yet published.
Also, the ME-bits 35 and 56 are set to 1. In version 2 of the 1090ES MOPS these are reserved bits, so they shall be 0.

Version 3 of the 1090ES MOPS is not yet published, so transponders should not use it.

Perhaps these messages are from an experimental transponder, which does not conform to the MOPS.


Why is fuel usage discussed per time, not per distance? I don't understand why sometimes the fuel used for a flight is discussed in terms of fuel used per time.
I think I have seen it a couple of times here - not sure whether it was in some common context.

I would think that it's much more relevant how much fuel is used per distance in almost every respect, except designing engines.

As an example, this answer to the recent question How do you compare private jet and commercial jet efficiency? uses "x/hr USD" as in "Hourly fuel costs for a private jet can run from 800-2000/hr USD." I'm sure I noticed similar cases a couple of times.
It's fuel cost per time, instead of fuel weight or volume, but that does not make a difference in regards to the question.

Maybe there is a reason based on some "tradition"?

Unlike with driving, in aviation the major variable when planning fuel consumption isn't how far you're traveling but how long you're traveling: Distance traveled in aircraft is a derived value (calculated from groundspeed and time - groundspeed also being a derived value affected by many factors including wind).

A pilot may not readily know their groundspeed (it can be quite different from airspeed), but at a given power setting they know an engine will burn a specific quantity of fuel per unit time, therefore fuel consumption is generally expressed that way (in gallons per hour or pounds per hour), and if they know how long they've been flying they can reliably estimate how much fuel they've burned, how much is left in the tanks, and how much longer they can keep flying before running out of fuel.

To illustrate why this is superior for flight planning consider the following (extremely simplified) flights fitting the following profile:

A 200 nautical mile trip
An airspeed of 100 knots
A fuel burn of 10 gallons per hour
A 25-gallon fuel tank.

Scenario 1: No Wind
With no wind the flight will take two hours to complete.
The groundspeed is 100 knots.
The aircraft will burn 20 gallons of fuel.
The fuel efficiency is 10 miles per gallon.

  In this scenario we land with 5 gallons of fuel left in the tank - that corresponds to 30 minutes, which is the minimum legal reserve for day VFR flight per FAA regulations.

Scenario 2: 25 Knot Headwind
With a 25 knot headwind the flight will take about 2:40 minutes to complete.
The groundspeed is 75 knots.
The aircraft will burn 27 gallons of fuel.
The fuel efficiency is 7.5 miles per gallon.

  In this scenario we run out of fuel 2:30 into the flight. Running out of fuel is generally considered to be a Bad Thing: The passengers complain, and the FAA asks a lot of uncomfortable questions.

Scenario 3: 25 Knot Tailwind
With a 25 knot tailwind the flight will take about 1:40 minutes to complete.
The groundspeed is 125 knots.
The aircraft will burn 16 gallons of fuel.
The fuel efficiency is 12.5 miles per gallon.

  In this scenario we land with almost an hour of fuel still in the tank - a very comfortable reserve.

On longer flights, and in slower aircraft, the effects of wind can be extremely pronounced, and a 5 or 10 knot difference from the forecast winds can mean the difference between arriving at your destination or having to add a fuel stop.