Are the total hours of an aircraft based on flight hours or operational hours?

Cycle
One takeoff and one landing makes up one cycle. Counting cycles is important to keep the number of times the cabin was pressurized (with the assumption that the airplane flies at high altitude). It is also significant for engine maintenance. A cycle and flight/operation time are not related.

Hours
This is the time when an airplane is in operation (more on this below).

Flight Time
Generally speaking, it is the time when an airplane is flying. It is the duration from when landing gears leave the ground to when they touch the ground again. Pilots are not paid by this. In my opinion, it is mostly used by flight trackers to inform people if an airplane has landed or not.

Operation Time
For GA airplanes, when the airplanes is started, Hobbs meter starts and it stops when the engine is turned off. Most commonly, this is how rental GA airplanes' rent is calculated.

For commercial airplanes, pilots pay clock starts when parking brake is released and ends when the parking brake is set.

Hence, total hours is more closely related to operation time.

If you are talking about total time and total cycles in reference of retiring an airplane, that is a different discussion. Each part of an airplane has a certain life. Though most of the time it can be predicted, but sometimes a certain component fails earlier than the expected life and therefore needs to be replaced. Airplane manufactures do specify a design life limit, but that is normally changed because of regular maintenance.


How will a prolonged series of steep turns produce a stall in subsequent level flight?

A prolonged series of steep turns will not produce a stall in subsequent straight and level flight.

  "after perhaps twenty turns have been completed, it will stall: stall, mark you, out of level flight with cruising throttle!"

In this case "level flight" means not climbing or descending while still in a steep turn.

Stopping the turn by rolling level would unload the wings and prevent the stall. Nosing down would also unload the wings and increase airspeed, also preventing a stall.


Should an aircraft's registration match with its tail number?

The registration number painted on the aircraft is required to match the aircraft's registration certificate (and airworthiness certificate). If the numbers don't match the aircraft's paperwork isn't in order, and it can't legally be flown.

There is a procedure for changing your N number which is usually used to put a "special" (reserved) number on an aircraft:

Send the FAA a letter requesting an N number change.
(It probably helps to have a good reason - like say United Airlines buying a used jet and wanting to re-register it with a N___UA tail number from their reserved block).
Wait for the FAA to send you paperwork authorizing the N number change.
You will get 3 copies of form 8050-64 - two for the FAA, one for you.
Put the new N number on the plane.
Usually you'd do this by repainting the aircraft (or the portion of the tail where the number is), but you could also do vinyl stickers or something similar)

  One of the copies of form 8050-64 becomes your temporary registration certificate at this time, and you must carry that along with your original registration certificate showing the old registraton number.
  Within 5 days of applying the new N number return one of the copies of form 8050-64 to the FAA to obtain a new registration certificate with the new N number.
  Within 10 days of applying the new N number return the other copy of form 8050-64 to the FAA to obtain a new airworthiness certificate with the new N number.

  I expected to find something stating how long you have between the time the FAA grants the request to change registration numbers and when you must apply the new one to the plane, but as far as I can tell you can take your time doing this: there doesn't seem to be an expiration on the authorization to change N numbers.

As long as you comply with the requirements for the registration number change the aircraft is only "grounded" for as long as it takes to apply the new registration number (if you're repainting this could take a while ; if you apply a large sticker with the new number on it it could be done in less than a day).


In what way are the Concorde's engines considered efficient?

You compare SFCs at different speeds. That is like comparing payloads for differently sized aircraft. SFC goes up with speed and, therefore, must be compared at the same speed. The work performed by an engine is thrust times distance, and higher speed means that the same thrust will perform more work per unit of time when the engine moves faster. The moving engine needs to slow down the airflow for combustion to take place, and then needs to accelerate the air by more than it has been slowed down to have positive thrust. Hence, SFC goes up in parallel with speed.

To have a meaningful comparison, we need to define efficiency. There are several, and two are of major importance for air-breathing aircraft engines: Thermal efficiency and propulsive efficiency.

Thermal efficiency

This describes how efficiently the chemical energy in the fuel $Q$ is converted into an impulse change of the air flowing through the engine. Formulated using the mass flow per unit of time $\dot{m}$, the impulse is $\dot{m}\cdot\dfrac{\Delta v^2}{2}$. Using $v_{\infty}$ for the incoming air speed and $v_{\infty} + \Delta v$ for the exit flow speed, the thermal efficiency is $$\eta_{therm} = \frac{\dot{m}\cdot \left((v_{\infty} + \Delta v)^2 - v_{\infty}^2\right)}{2\cdot Q}$$
To achieve good efficiency at high speed, a high $\Delta v$ is helpful. This explains why efficiency drops more over speed for high-bypass ratio engines and especially propellers.
Since the thermal energy in fuel is the same for all engines in your question, because all run on kerosene, and we can assume a similar efficiency of combustion, we can neglect $Q$ in the comparison.

Propulsive efficiency

This describes how well the conversion is performed. Using the same variables as above, propulsive efficiency is $$\eta_{prop} = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}}$$

This equation explains the better efficiency of high-bypass ratio engines and propellers at the same speed, because propulsive efficiency is proportional to the inverse of $\Delta v$.

Overall efficiency

This is the product of thermal and propulsive efficiency, and the equation is $$\eta_{total} = \frac{T\cdot v_{\infty}}{Q}$$
where $T = \dot{m}\cdot\Delta v$ denotes the thrust. Conveniently, $\Delta v$ is eliminated in the product, allowing turbojet engines like the Olympus 593 to look much better in comparison to other engines.

Intake efficiency

This answer would be incomplete without a look at the intake of the Concorde. At cruise, it would lift the pressure of the air at the compressor face by a factor of more than six over ambient by efficiently decelerating the flow. The compressor added a compression ratio of 12, so the pressure in the combustion chamber was 80 times higher than ambient. This high pressure makes the engine so efficient, but is also needed to maintain combustion. Remember, ambient pressure in 18 km is just 76 mbar, so the absolute pressure in the combustion chamber at cruise was only 6 bar.

The full answer would be like this: The combination of intake and Olympus 593 at Mach 2.02 had a very good total efficiency, and comparisons with other engines at static conditions are misleading.

The comparison of results from a test stand on the ground would yield a very different picture, however.


Can roll be controlled through elevators in a fixed-wing aircraft?

Yes.
Fighter jets with split elevators can command a roll in the way you describe, see this answer for more info.

For an aircraft like a delta wing (tail-less) that lacks a traditional empennage these control surfaces are combined into an elevon to produce a mix use elevator aileron.

The Concorde is a classic implementation of this design. Flying wings like the B-2 also use this design.


How does the nose wheel lock on the Robin DR400 work?

It looks like the "How do they work?" question has already been addressed pretty well, so I'll answer some of the other parts of the question:

  What is gained by having the nose wheel lock itself?

The nose gear is prevented from turning during flight when the rudder pedals are actuated. If the nose gear turned when the rudder was being used in flight, it would add a lot of form drag.
The nose gear remains straight until it touches down on landing, which prevents it from landing sideways in a crosswind landing where the rudder is being used to maintain runway alignment. This allows the pilot to continue to use the rudder inputs to maintain the centerline through the landing roll without damaging the tires or having a sudden yaw in the direction that the tire is pointing when the nose wheel touches down.

  Rather than for instance a spring-like mechanism that just keep the wheel aligned with the plane when the plane is in the air.

You mean like a Cessna-style spring linkage, where there is not a hard linkage between the rudder pedals and the nose gear? One advantage of not using the Cessna-style springs is that it's easier to taxi with direct linkages rather than spring linkages. Cessnas have a lag between rudder pedal input and the nose wheel actually turning because of the spring linkages, so you have to give directional inputs ahead of when you actually want to turn. With direct linkages, the nose wheel turns immediately when you push the pedals, which makes taxiing a little easier, especially when you're first learning.

  Are there planes with a different mechanism to keep the nose wheel from moving in flight?

Yes. See above for Cessnas. Also, in larger aircraft (e.g. passenger jets and similar,) there are no mechanical linkages to the nose wheel at all and the nose wheel is turned (mostly) by a separate control from the rudder. This control is called a tiller and it's moved by hand instead of by foot. It looks like this:


What are the regulations for lighting fixtures around an airport?

To start you off there's these two:

14 CFR Part 139 - Certification of Airports and

14 CFR Part 157 - Notice of Construction, Alteration, Activation and Deactivation of Airports.

Part 139.311 covers all the lighting related requirements.

Part 157 is there just so you can inform the FAA about the changes you're going to make in the airport.

for specifics about LED lights, I would suggest looking for Advisory Circulars with LED lighting for airports in particular.


How is separation managed when there are only two parallel runways in a very busy airport?

With two runways, usually, one is dedicated to arrivals, and the other is dedicated to departures. That's the simplest way to handle separation -- only in-trail separation is involved.

That's usually the way traffic is managed at Heathrow (LHR) and Dubai (DXB), two of the world's busiest two-parallel-runway airports. At Heathrow, when in a westward flow (70% of the time) the runway functions are swapped about 3pm, to equalize noise impacts.

San Francisco (SFO) has two parallels, 28L and 28R, that are 750 ft apart -- very close. Under visual conditions, both runways are used for VISUAL approaches. Generally, aircraft arrive side-by-side, for wake avoidance.

For about an hour a day. 6am to 7am, when arrivals dominate, LHR allows arrivals on both runways. I don't know what separation rules are used. At SFO, under marginal conditions, both 28s are used for arrivals but aircraft are staggered and capacity is less than for visual operations.


Is Electronic Flight Bag(EFB) any different from our day-to-day tablet?

The FAA (unsurprisingly) has definitions of different types of Electronic Flight Bag (EFB) systems, which you can find in AC 120-76C.  It's targeted at airlines, but the concepts are generally applicable.
They break them down into Class 1, Class 2, and Class 3 EFBs, and the answer is a little different for the different classes:

Class 1 EFB Hardware.
This is what most of us General Aviation Folks use: A portable computer or tablet considered "portable electronic devices" with no specific FAA approvals.
Per the FAA a Class 1 EFB can't be "mounted to the aircraft" or connected to the aircraft systems or a dedicated power supply (i.e. they can't be permanently wired into anything), but you can plug them in to an "existing aircraft power supply" (like the cigarette lighter port) for battery recharging.

This is what most of us GA folks are using - an iPad with Foreflight and Stratus is technically a "Class 1 EFB".
This is just a regular computer, so it can in fact be the same device you play Plants vs Zombies on (though if you're using it in an airline environment they've probably locked it down and restricted what you can do: Plants vs Zombies is probably not available in the cockpit of your Southwest flight).

Class 2 EFB Hardware.
Class 2 EFBs are essentially Class 1 EFBs that get mounted to the aircraft, but can be easily removed by the flight crew (for example a GPS or iPad in a panel-mounted cradle) and may connect to aircraft data systems via cables or wireless connections (for example Garmin's Flightstream system).
Class 2 EFBs have the same battery-charging provision as Class 1 EFBs, but can't be directly wired into aircraft power. As far as the FAA is concerned they're still "portable electronic devices".

This is what the more advanced bits of GA are using - An iPad with Garmin Pilot communicating with your GTN stack via flightstream to get its GPS position & transfer flight plans.
Again this can be the same device you play Plants vs Zombies on, but the operator may have locked it down to keep their crews from flying past their destination while playing with their laptops.

Class 3 EFB Hardware.
Class 3 EFBs are installed systems with FAA approvals (for type-certificated aircraft).
The chart data stored on e.g. a GTN650 or GMX200 with charts and plates loaded on it would qualify here, as would flight management systems on larger aircraft.

Generally you won't be playing Plants vs Zombies on Class 3 EFB hardware because the approvals for it wouldn't include that capability.
The exception here is the Experimental Aircraft community: Such aircraft may have a consumer tablet installed in the panel and wired into ship's power in such a way as to qualify for all practical purposes as a "Class 3 EFB", and it's possible you could play Plants vs Zombies on your installed iPad in such an aircraft.


Which system on the 737NG decides the use of roll spoilers?

That would be the "spoiler mixer". See this document, refer to pages 3, 4 and 20.


If you know what you're looking (sniffing) for, yes.

Avgas 100LL smells like gasoline (a little different than what you pump into your car, but similar).

Jet-A smells like kerosene (not quite "like diesel", but definitely not like gasoline).

If you are familiar with how both smell you can tell them apart by odor, and a significant contamination of Avgas with Jet-A could be detected by smell. (Other tests, like an evaporation test, are more reliable for lower levels of contamination however).


Why is the aviation industry taking so long to move away from the tube with wings model?

In aviation there are simply configurations that work the best when working out all the compromises caused by competing and contradicting objectives, and these become timeless and continue for many decades until some completely new physics paradigm comes along to upset the apple cart.

So, the tube with wings is the most efficient way to do it, not from some optimized theoretical single minded objective standpoint, but from the real world need to balance this goal against that goal.  You need to minimize frontal area, and to get the most people in the smallest frontal area, you have to arrange them lengthwise.  But not too long, so at some point you have to make the tube wider instead of longer.  A tube, low in drag, efficient structurally and easy to make, can't be beat.

The various radical approaches almost always fall short in key ways, which is the real reason you don't see them.  That doesn't mean stopping experimentation, because some day someone will come up with a radical configuration that sets a new standard while managing to meet all sorts of secondary needs, and then everybody will start to copy that configuration.

I always like to point out the ultimate engineering sweet spot as the pinnacle of this sort of thing, and really, probably the originator of the "tube with wings" concept - the Douglas DC-3, the greatest engineered conveyance of modern times.  This is a set of engineering compromises in such perfect balance, that eighty five years after it was designed, at the end of the second decade of the 21st century, they are still purchased and operated as working machines to earn a living for somebody, pretty much as they were originally built, as opposed to being operated for show or fun or as a museum piece.  That's engineering perfection, the grandfather of all modern airliners, still making money for people, the original tube with wings.


What is the force to change the pitch of a variable pitch propeller?

In a well-designed propeller the hinge axis is very close to the line connecting the centers of pressure, so the aerodynamic pitch moment is rather small. The main force is friction; note that the centrifugal pull of the blades at the hub will put a considerable load on the blade bearings, so friction goes up with rotation speed. Regular propellers have plain bearings; if you know the mass of the blade, the rotation speed and the coefficient of friction, you can calculate the breakaway torque of one blade.

Since propeller blades have some (but not much) camber, the center of pressure moves forward with increasing angle of attack. This will drive the blade away from a medium angle of attack; in other words, the aerodynamic moment is destabilizing the propeller pitch.


Why don't commercial aircraft have drag chutes for aborted takeoffs?

The answer to this is much the same as they answer to any "Why don't airliners have [XYZ safety equipment]?"; the answer is generally along the lines of questioning whether the occurrence of such incidents (and the danger presented by them), outweigh the extra weight/certification/maintenance/cost of the equipment.

I'd argue that the danger presented by, and the relatively infrequent nature of, late aborts does not require braking parachutes to be added.

See also

Do any commercial airline models have a parachute at rear to help in landings?
Could ejector seats save lives in commercial aircraft?
Airframe parachute on commercial airlines and Why don't commercial jet aircraft have a break-apart, parachute escape system?


Are solid-state attitude indicators subject to gimbal lock?

Yes and no.  Gimbal lock is a physical AND mathematical phenomenon.  Physically, gimbals sharing the same axis can cause a lock, and mathematically, a representation of a multi-axis rotation can be in a condition which is singular, depending on how it is represented and calculated internally.

Normally, we discuss and think about aircraft attitude intuitively as set of angles: roll, pitch and heading.  In mathematical circles, this is known as a set or 3-2-1 Euler angles.  If you look into the boring (or exciting, if you are that kind of person) math, you can see a vector-matrix equation relating the change in Euler angles given a set of angular rates given in the frame of the aircraft, by common convention these are referred to as p, q and r for roll rate, pitch rate, and yaw rate respectively.  The way these rates affect the change in the angles depends on the current state of the the angles, e.g. when the aircraft is pitched, a body-frame yaw rate will alter the pitch angle.  Mathematically, this means that the "state matrix" is state-dependent and must be constantly updated using the freshest set of states available.  Any set of Euler angles will have a set of states which causes the state matrix to become poorly conditioned, resulting in numerical instability at those conditions.  For the roll-pitch-heading set of angles, this happens when pitch is + or - 90 degrees.

The way that we escape the singularities when writing the software for an AHRS is by representing the attitude in a non-singular form.  This done by using more numbers to create the representation, then adding constraints which preserve uniqueness.  A 3x3 direction cosine matrix (DCM) can do this,  It has constraints which cause it's eigenvalues to lie on a unit circle in the complex plane.  My favourite representation, however, is a quaternion constrained to have unit length.  The quaternion can be thought of in terms of Euler's rotation theorem.  There's that Euler guy again, he must have been some kind of genius.  Anyhow, the idea is that you can define an axis of rotation with three numbers, and use a fourth to represent an angle about that axis to rotate.  This gives you a system free of "mathematical gimbal lock"  The state matrix remains adequately conditioned under all states.  All you need to do to keep things friendly is to normalize the quaternion from time to time, and you always have a "background" state that you can translate into aviator-friendly Euler angles.


What is the top speed of the twin seat Hawker Hunter?

The manufacturer
lists over half a dozen variants of Hunter two-seaters.
You probably mean the T.7.

For a fair comparison, its closest single seat variant is the F.6,
maximum speed Mach 0.94
(715 mph) at sea level.

The T.7A, a T.7 with different instruments, has a maximum speed of
Mach 0.92 (690 mph) at 36,000 ft.

So the T.7 is a few per cent slower than the F.6.
But it's no slowpoke: in 1959 it even won
the flying leg of a complicated race against 200 other entries.

Your empty-weight test suggests that these numbers are trustworthy:  the weights posted there say that the T.7 is 762 pounds (5 per cent) heavier.


Steering angle, according to the text you bolded:

  pointing at perpendicular to the flight heading (90 degrees counterclockwise)

means the steering angle 090 is the angle, relative to the flight heading counterclockwise, that the data sample was taken.

If the flight heading was 345 degrees, 90 degrees counter clockwise (CCW) is 255 degrees.  This steering angle has no aviation context and is just a relative offset from the aircraft heading for data acquisition (which in this case sounds like a radar sample).


What are these panels underneath the wing root of a A380?

They are indeed the Ram Air Outlet Doors.

(source, showing Ram Air Inlet (RAI) and Ram Air Outlets (RAO))

From the A380 FCOM (21 - Air Conditioning):

  PACK DESCRIPTION
  The hot bleed air flows into each pack, via the two pack valves, then enters the heat exchanger. This
  heat exchanger precools the air using external air. This external air enters through a ram air inlet door,
  and is then discharged overboard through two ram air outlet doors.

The reason they are located in the wing root is that the entire packs are in the wings. From this article:

  The A380's wings are big enough that they can hold the plane's air conditioning packs. That, says Airbus, is an advantage over most other large planes because the packs don't have to be stored in the A380's belly fairing. That, in turn, means the plane's fuselage is more streamlined than it would otherwise be, reducing its drag.


What are the effects of communicating on VHF/UHF on a RMI's ADF needle?

NDBs in the U.S. transmit between  90 kHz and 535 kHz, and VHF is between 108 and 137 MHz. Of the VHF frequencies, the nav/com is only capable of transmitting between 118 MHz and 137.

So aside from electromagnet interference, no VHF radio should transmit on a frequency that he ADF receives.


How can I correctly decode CPR positions from ADS-B frames?

We were decoding Airborne Position frames having TypeCode 0, 9-18 and 20-22.
Apparently, frames with TypeCode 20-22 and possibly 0 have to be decode a different way.

(For now I think that I get enough information out of TypeCode 9-18 frames so I'll just skip the other ones.)


Would it make sense to test the performance and handling of a prototype aircraft by mounting a free moving mock up on a track (like a rocket sled)?

No, it doesn't make sense because there are better alternatives:

Computer models: fluid dynamics programs are very good at simulating airflow over a 3d model
Wind tunnel testing: In a wind tunnel the airplane model stays in the same place and air is blown over it. Smoke can be added to show vortices, and cameras can record it

There are practical problems with rocket sleds:

A rocket sled would be much harder to get video from
It would be challenging to keep a rocket sled at a completely consistent speed
Rockets only last a few seconds, you'd only get a bit of data at a time


What is relation between Reynolds number-turbulent flow-flow separation?

In most commercial aircraft applications the Reynolds number per foot of chord length lies between 1.5 and 2 million per foot. Major operations of takeoff and cruise for turboprop and turbofan airliners fall in this range. Consider the following mean aerodynamic chords:

Turboprop Aircraft = 6.9 ft < cMAC < 10.6 ft
= Reynolds number of 10-20 million

Turbofan Airliners = 13.3 ft < cMAC < 26.9 ft
= Reynolds number of 20-54 million

B747/A380 size Aircraft = 30.6 ft < cMAC < 40.33 ft = Reynolds number of 45-80 million

Note that for highly tapered wings the outboard chords may be considerably smaller than the mean aerodynamic chord and will experience lower Reynolds numbers. During low-speed operations like landing and takeoff, high lift devices such as flaps and slats must be used, because the characteristic lengths of these elements are considerably smaller than the local wing chord they will be operating at lower Reynolds number and therefore more susceptible to flow separation and stalling. The general trend is for cl,max to increase slowly, if at all, with increases in Reynolds number and for that increase to be more substantial for thicker sections. This is understandable because as the Reynolds number increases the boundary layer effects become relatively weaker allowing the flow to remain attached to the airfoil for longer distances along the airfoil surface. The lift of an airfoil depends primarily on keeping the flow attached to the airfoil while friction drag itself weakly influences the lift of an airfoil.
A more detailed explanation is available here


How do airplanes act as corner reflectors despite lacking significant flat surfaces in one of the three necessary mutually-perpendicular planes?

A corner reflector is the most efficient way to generate a strong radar return from an incoming pulse. However, even if the plane has no corner reflectors in its overall structure, it will still generate a return because it is made of metal, and the characteristic impedance of the metal is sufficiently different from that of air to generate a reflection of the incoming electromagnetic wave. That reflection will not be collectively "aimed" straight back to the source is in the case of a corner reflector but if the incoming wave is strong enough and the radar receiver sensitive enough, the plane will still show up on the radar screen.


Why would flares be fired into the air after a flight lands?

I know that flares are used at Dover air force base in Delaware sometimes to deter birds from the airfield. I also know that flares are just a common bird deterrent anyway. So it may have been simply to clear the taxiways and runways of birds before the next departure or arrival.


As explained in Why do jet engines use kerosene rather than gasoline?, turbines can burn almost anything, but the lubricating properties of kerosene make the fuel pump simpler as it is otherwise hard to keep high-pressure pump lubricated.

Temperature limiting is simply a matter of sufficiently lean mixture and cooling air stream along the combustor walls.

And most efficient depends on criteria. For money it is definitely kerosene (Jet-A). Actually, Jet-A is an excellent match overall. It is relatively dense (~0.8 kg/l) while still having good specific energy (~43 MJ/kg; so fuels tanks are not too big), lubricates (good for fuel pumps), has quite high flash point (low risk of fire when handling) and is a large fraction of crude oil (C8–C16 (gasoline is just C6–C8) so it is cheap).


What kind of antique WWI aircraft model is this? Inventor's prototype?

Beautiful find..., an original scale model from the 20s.
At first glance, the rudder is a typical "Sturtevant" design..., but the rest of the fuselage appears to be from another aircraft, or a type of Sturtevant that I am not familiar with. It could also be a free work, a personal design by the modeler... It looks like an old rudder concept for a "modern" fuselage design.

Sturtevant S-4
Sturtevant S-2


What are the differences between C208 versions?

The civilian variants can be summed up in this table I made based on Wikipedia:

What to take out of this, is the different fuselage lengths, and that the amphibian comes in two fuselage lengths.

The belly-mounted cargo-hold is optional.

There are also military variants, even one that shoots missiles (an incentive to check out the Wikipedia page).


How does a true airspeed indicator work?

For a basic True Airspeed Indicator (TAS Meter) like the one shown below, you simply dial your pressure altitude into the top window using the knob (aligning it with the appropriate temperature mark at the bottom of that window), and then read your true airspeed in the other window at the bottom.

Your pressure altitude can be obtained by setting your altimeter to 29.92 and reading the indication, the air temperature can be obtained from an outside air temperature (OAT) gauge (or you can use one from the Winds Aloft forecast if your aircraft does not have an OAT gauge), and the combination of the two gives you your density altitude.
The meter itself is simply a mechanical slide rule, performing the same functions as you could do manually on an E6B flight computer.

The indicator above is showing a true airspeed of about 202 MPH, at a pressure altitude of 10,000 feet and a temperature of -15 degrees.

For a nice video tutorial check out this one from the University of Oklahoma.


How do I remind myself to run in-flight checklists?

In my view, the best tool for reminding yourself of recurring "household chores" is by using a timer, if the noise profile of your aircraft allows it. (Try it! You can probably hear more than you think, even in a loud prop). Either use a kitchen timer (which has a really simple physical interface, although two hands may be required) or your smartphone (a bit more complex interface, typically allows for single hand usage, with the added benefit that you can run multiple independent timers, and that the alarm will not stop sounding until you dismiss it). Modern GA EFIS (for example G1000) typically have timing facilities built in.

If you are unable to use an actual timer, determine the interval of the task (for example every 15 minutes), and use your normal watch. (For example "Check the tank every time the minute-hand passes the 0, 15, 30, 45 marks").


What license will one need to fly the Aeromobil flying car in 2022?

Short answer: You will almost certainly need a PPL at least.
On the subject of licensing for flying cars, nobody really knows, because no company has even come close to bringing one to market - and the term "bringing to market" for a flying car involves all the legalities of certification and licensing.

The U.S. based Chinese-owned firm Terrafugia and AeroMobil are also attempting to develop a two-seat flying car called the Transition, however, safety and development of the law for flying cars are just two of the challenges that must be met before they become a reality and not just a novelty. The Transition is designed to operate both on land and in the skies.

source: Aviation Law: An Analysis of the Development of Flying Cars for the 2020s Decade
In terms of aviation certification the Aeromobil specifications state

Aerospace: CS 23 Rev 5/ELA 1 (EU) & 14 CFR 23 Rev 5 (US)

That means for the US it is certified under Part 23 which will almost certainly need a minimum of a private pilots license. In the EU it is pretty much the same deal as many of the aviation standards/certifications around the world are based on ICAO norms.

Not that you asked for it, but in my opinion the only likely route to market for personal flying vehicles is ones where you're not doing the flying. One day I'm sure we'll have something like an automated Uber which ferries you A to B short distances. But something where you drive and fly is hugely unlikely.


Are there ILS approaches where descending by the glide slope outside final might be dangerous?

Are there ILS approaches out there where descending by the glide slope on a segment of the approach that's outside the final approach segment would cause the aircraft to descend below the no lower than altitude for the segment?

Yes.  See InFO 11009.

In part the InFO reads:

  On ILS approaches, stepdown fixes are established for obstacle or traffic separation.  For all practical purposes, the glide slope remains stationary regardless of atmospheric temperature and pressure.  Conversely, stepdown fixes are published for a pilot to fly using indicated altitude, which varies with temperature and pressure changes.  Therefore, the proximity of stepdown fixes in reference to the glide slope, changes with the weather.

We know the old adage:

  From high (hot) to low (cold) look out below.

What isn't obvious about that adage is the perspective of that statement.  It is from the perspective of the pilot to the ground.  A glide slope's perspective, if you will, is from the ground to the pilot.  Thus what you will experience is this.  In warm weather, the stepdown altitudes will be higher than the glideslope and you will need to level off to remain above the MEA until crossing the next stepdown fix.

You must remain above the MEA at all times till crossing the precision final approach fix.


Does wing sweep affect the direction of lift?

The Lift on an aircraft is the vector sum of one component of the aerodynamic forces pressing on the surface of the aircraft as they are distributed over the surface of the aircraft. At each and every point of the aircraft surface (top, bottom, sides, everywhere) the air is pressing on the surface normal, (perpendicular) to the 2-D surface. The vector sum of all these forces is the total aerodynamic force (**one component of which is lift). If the aircraft is laterally symmetrical, then of course the total vector sum will be straight up (as any lateral component from one side of the aircraft should more or less be counteracted by an equal but opposite lateral force from the other side.

The individual forces (acting on each point of the surface, including the wing), if pictured as a distribution of force over the surface, are always normal to or perpendicular to the surface. So if the wing has dihedral, then they are indeed angled in towards the fuselage. But in your "perhaps?" illustration, with a wing with zero dihedral, they would all be pointing straight up.

** The Lift component is the component of this aerodynamic force which is perpendicular to the flight path of the aircraft, the Drag component is the component parallel to the flight path.


Will the new airmen certification standards make it more difficult to add ratings?

Not significantly. The fundamentals of evaluation have not changed, and certainly the fundamentals of your next rating haven't changed. A few of the specific tasks have changed between PTS and ACS, like the point you raise. And, yes, you will need to review the ACS or PTS (whichever is in effect for your next rating -- not all have transitioned) in detail, so that you're aware of what's expected of you.

The major parts of any checkride will be your basic knowledge of the subject, your ability to move the aircraft through the air to standard, your ability to demonstrate sound judgment including a thorough evaluation of pre-flight risks and your overall attitude toward flying. That really hasn't changed with the ACS. The regulations have not changed either. The ACS is meant to give instructors and examiners a more holistic approach to evaluate the above factors.


Can a barrel roll really be executed at a constant 1G?

You are absolutely right, a load factor of greater than 1 is impossible to avoid in a proper barrel roll. The barrel part of its name comes from the spiral path the aircraft needs to perform in order to add a centrifugal acceleration which is greater than gravitational acceleration at the top of the roll. This is the condition to ensure a still positive normal load factor in the reference system of the aircraft through the whole roll. Therefore, at the bottom of the roll the normal acceleration needs to be greater than 2 g.

I took the liberty to edit the wrong answer. Given that it has received 14 upvotes at this time, it should teach everyone around here a lesson how much to rely on the number of votes to assess the correctness of an answer ;-).


Google "NACA 65-110" and the first entry is airfoiltools.


There certainly are; it's called "Business Class" and you'll find such areas with seats that recline into what are essentially beds on most long-haul aircraft. However, you simply can't fit as many people on board when they use up so much room, so airlines have to charge much more for these to make it economically viable. It's often twice the price of a normal ticket or even more.


What certificates/medical are required to fly in Japan?

With a US license, you are allowed to fly US registered (n-registered) airplanes everywhere in the world.
Find a US CFI (in Japan) that can get you current and start flying.

The bigger problem might be, to find a Medical Examiner in Japan that is allowed to do medicals in Japan. My first search, at the FAA website (https://designee.faa.gov/SelectTypeOfSearch/SelectTypeOfSearch), didn't show anything.

Not sure if BasicMed works for you though.


If "all flying rudders" for LSA aircraft need up to 40% less area, what about "all flying elevators (stabilators)"?

Zenith gets away with the little All Flying Rudder without an anti-servo tab because the fuselage itself is used to provide sufficient fixed fin area for acceptable passive weathervaning. If you look at their designs they all have slab sided fuselages that are quite deep all the way back.  So there is a kind of "fin" with the rudder; it's just built into the fuselage.  I got to fly the Zenith 750 demonstrator once in Mexico Missouri and I recall that its yaw stability was perfectly acceptable even though the rudder just wanted to trail if if left it to do its own thing.

Horizontal tails have more work to do and horizontal surface area of the fuselage aft won't cut it without a fixed stabilizer surface; unless you use a stabilator/anti-servo tab. If you wanted to use an all-flying surface without an anti-servo tab, you have to have a means to "fix" the surface to provide stick free static stability.  If you're in an F-86, the all flying tail is hydraulically powered and can be fixed easily with a bungee that controls where the hydraulics wants to center the surface when you're not applying force.  Some gliders use all flying tails with no tabs of any kind, and they depend on bungee springs in the pitch control circuit to provide a tendency of the surface to want to hold a certain angle if you aren't hanging on to it.

You could design your ultralight with an all flying tail with no anti-servo tab by using a fairly stiff spring bungee as your "stick free" stabilization device, and for trim. You may find the anti-servo tab method provides better handling characteristics however and if it was me I would go that way.

Note that airplanes sometimes use bungee springs in the rudder control circuit to provide a passive centering force that has the effect of increasing the fin area to improve passive weathervaning. Cessna 180/185s do this to improve yaw stability so that when fitted with floats they don't need extra ventral fins.  Problem is, it makes the rudder unpleasantly heavy.


What are the advantages of dual counter-rotating rotors vs coaxial rotors?

Counter-rotate Picture above from this answer, which compares the Chinook with the V-22 Osprey, another counter rotating twin rotor.

For a given weight, the larger the rotor the less power is required for generating the necessary thrust. And here is where a single rotors' advantage is, one single rotor requires less power than two rotors with half size each. In the hover, the single rotor throws away about 10-20% of the engine power to drive the tail rotor, but in cruise flight much of that torque compensation is generated by the vertical tail.

Yes the payload location range of the Chinook config is without equal. In case of one engine failure, the remaining engine can drive both rotors since they are coupled by a shaft. Autorotation is not an issue either - from this thread:

  Chinook is extremely docile in handling, yet extremely maneuverable when light. Autorotation is of the thistledown type, rotor inertia is massive and the beggar wont come down fast even if you want it too. Engine off landing is merely a continuous flare to a running landing on the rear wheels which requires quite a lot of runway. I dont know if the military use other techniques.

So in short, the Chinook config has pros and cons. The two rotors' wakes intermingle with a reduced power efficiency in forward flight. From same thread as above:

  Due to the dynamics the Chinook develops maximum translational lift in sideways flight to the left, hence the antics of the logging machines plugung sideways on the end of theit long cables - gives them greatly increased lift capacity.

So yes the twin counter rotating rotor does not spill tail rotor power during the hover and has a large payload range. It has a longer but narrower footprint than a similar MTOW single rotor. But it is not universally more power frugal than the single rotor config.

Co-axial. Picture above from this question.

The co-axial setup main drawback is its complexity. Counter rotating shafts, one in the other. One rotor quite high over the other one to allow relative flapping. Super complicated control actuation.

Power requirements: the coaxial configuration has power requirements according to this answer.

All in all, the counter rotating setup is simpler than the coaxial setup, which does have the advantage of a very small footprint. But the single rotor set-up has proven to be the configuration with the best design balance.


Why is the angle of attack not proportional to the stagnation point?

From this site:

Notice that at low AoA the stagnation point is smack on the nose of the wing profile, while at higher AoA it is situated on the lower wing surface.

Hence: stagnation point location lowers with increasing AoA.


Which engine type requires the shortest take-off run: a turboprop or a turbofan?

Even with similar weight, turboprop and turbofan aircraft will have different design characteristics. A turboprop will fly at lower altitudes and airspeeds, resulting in different aerodynamic designs. A turboprop also burns less fuel per hour, so it will need less fuel capacity.

That being said, we can look at some aircraft to get an idea of what the trends might be. The best example is probably the Dornier 328. It's a turboprop aircraft that seats about 30 passengers. There also happens to be a version of the plane with jet engines. This is probably about as similar as you can get in both weight and aerodynamics. According to Skybrary, the figures are as follows:

Dornier 328
MTOW 13990 kg
Distance 1000 m

Dornier 328JET
MTOW 15200 kg
Distance 1382 m

You can see the the jet derivative is a bit heavier but requires a much longer takeoff distance.

The trend is similar between other aircraft.

Dash 8-300 Turboprop
MTOW 18600 kg
Distance 1085 m

ERJ-135 Jet
MTOW 20000 kg
Distance 1580 m

CRJ-200 Jet
MTOW 21523 kg
Distance 1527 m


What kind of information is shown on airport surface traffic surveillance displays?

Advanced Surface Movement Guidance and Control System (A-SMGCS) provides controllers (eventually pilots and  vehicle  drivers)  with  situational
awareness on the movement area and  normally  consists of three type of sensors. Surface movement radar, MLAT and ADS-B.

The kind of information is displayed to the controller is dependent on what are the sensors used and the capability of aircraft and vehicles on the ground.

If the A-SMGCS is equipped with Surface Movement Radar (SMR) it can display the moving vehicles and aircraft on the maneuvering area. (except in areas where tracking is inhibited ) to the controller. But manual correlation is required to identify the target.
If ASMGCS is ADS-B capable, controllers display will show Aircraft or vehicle  Identification (ACID), Essential Flight Plan details like destination etc and type of aircraft also in addition to the exact location of the aircrat or vehicle. For this to work, Aircraft/vehicle should be ADS-B capable.
using  MLAT (Multilateration ) ASMGCS system displays postion and ACID and other FPL details to the controller if the aircraft SSR transponder is in operating mode. System correlate the transponder code with the flightplan.

Depending upon the level of ASMGCS operation , it can also display the taxiway route, SID, Runway assigned etc to the controller.

If the target is obtained from ADS-B or MLAT then it is called a cooperative sensor target. It will be displayed in a different symbol.


Could a blown wing ever be powerful enough to lift an aircraft at zero forward velocity?

Technically speaking, yes, it's possible, but wildly impractical.

Of course, as usual, Jan Hudec's explanation of the physics is completely correct. Whatever is blowing enough air over the wings to fly would generate an insane amount of forward thrust, meaning that you wouldn't stay at zero forward velocity for long.

However, it's completely possible to apply another force in the opposite direction equal in magnitude to what would otherwise be the net forward force produced by the engine(s) blowing the wings. This would yield a total net forward force of zero, thus no forward acceleration would happen, allowing you to take off at zero forward velocity. Of course, you'd want to make sure that whatever is applying this force isn't also blowing the wings in the opposite direction, otherwise you lose your lift.

Some possibilities for providing this force:

Normal jet engines mounted backwards
Normal propeller engines with negative pitch
Rocket engines mounted backwards
A (really strong) tether
Wheel brakes (this only works as long as you're on the ground, of course... you'd start accelerating forwards very quickly as the wheels break free from the ground, but you could technically leave the ground with zero forward velocity.)
A really big machine gun, such as the GAU-8 Avenger or Gryazev-Shipunov GSh-6-30
Lots of AK-47s ('lots' being a number roughly equal to the net forward thrust of the engines in pounds divided by 13)

This is roughly what the backwards-mounted rockets would look like:

C-130 Rocket Assisted Landing
Source: YouTube

Another possibility to technically meet your requirement of taking off at zero forward velocity would be to simply start off rolling backwards fast enough that you would accelerate forwards to zero forward velocity at the moment the tires leave the pavement. :)

Of course, as you've hopefully figured out by now, that all of the above are wildly inefficient ways to take off. They would, however, technically work. And, of course, in the case of the GAU-8, whatever is in front of you during the takeoff is going to have a very bad day.


How is ACS different from PTS and when does it replace PTS for FAA certification?

The Airman Certification Standards (ACS) is basically an 'enhanced' version of the Practical Test Standards (PTS). According to FAA briefing on ACS,

  It (ACS) adds task-specific knowledge and risk management elements to each
  PTS Area of Operation and Task.

In general, while the PTS concentrates on what the applicant must 'do', the ACS specifies what he/she must 'know' and 'consider' (in addition to the 'do' part) to qualify for a given airman certificate or rating. The differences (in draft) are shown in this slide.

Image from Update on the ACS by FAA

According to the FAA briefing,

  • The “skills” section of the ACS covers the flight proficiency performance
  metrics in today’s PTS.

  • For each PTS Area of Operation/Task, the ACS lists the elements of
  aeronautical knowledge that support that skill.

  • In addition, for each PTS Area of Operation/Task, the ACS lists the risk
  management elements or behaviors associated with it.

The ACS is still under development, but FAA is targeting a June 2016 implementation (for airplane) depending on results of prototype (there is one running at the Embry-Riddle Aeronautical University) and some other factors.


Explained intuitively, why does aspect ratio affect induced drag?

Let's imagine 2 theoretical wings, both of which have the same area, but differ in aspect ratio.

Then the wing with the higher aspect ratio also has more span. This is what counts.

If induced drag depends on the downwash angle, why would a longer wingspan reduce the angle?

Because the wider wing will affect more air. Think of the air affected by the wing as what flows through a tube, the diameter of which equals wing span. Like in the picture below.

Of course, for clarity, the amount of bending the stream tube undergoes due to the wing is greatly exaggerated here. In reality it is a few degrees at most.

how can that happen when the wing area is equal?

Wing chord does not count here as long as the amount of stream tube bending can be done by the skinnier wing. More chord will not affect the result. Only when too much being is asked of a limited chord will the wing stall. At lower lift coefficients, more chord will only increase friction drag.
In reality the influence of the wing fades the more you move away from it. This picture of a stream tube is a simplification which you will also find in propeller theory, and it gives the correct result, because what you neglect in the far field you compensate by assuming a constant influence over the near field.

All of the sources I am able to find blame induced drag solely on wingtip vortexes

You do well to disregard those sources. They have the causality backwards.


What weapons do A-10 Warthogs typically carry?

The 30 mm GAU-8/A Avenger  with over 1000 rounds is in itself a potent anti-tank weapon, which can fire depleted uranium rounds, among others. It carries a number of anti-armor weapons like the AGM-65 Maverick, Hydra 70 rockets and cluster bombs in 11 external hardpoints with a maximum load of 7200 kgs.

The 'typical' operational layout of aircraft depends on the mission. For example, the image below shows one configuration.

By U.S. Air Force photo by Master Sgt. Blake R. Borsic - http://www.af.mil/shared/media/photodb/photos/000217-F-0656B-004.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=505075

In the above picture, the aircraft can be seen carrying a ALQ-131 ECM pod under one wing and two AIM-9 Sidewinder air-to-air missiles under the other wing for self-defense, along with AGM-65 and bombs. If these are not required (for example, where there is little danger from anti-aircraft fire), they can be replaced with others or the aircraft is flown without one so as to increase range/time on target.

By U.S. Air Forces Central Public Affairs Photo by Master Sgt. William Greer - http://www.flickr.com/photos/dvids/5510306058, Public Domain, https://commons.wikimedia.org/w/index.php?curid=37832505

In this photo, we can see the aircraft carrying only precision munitions and targeting pods.


What factors are included in the moment coefficient of a wing?

As much as I try to explain aerodynamics without resorting to potential theory, sometimes it is indeed helpful. If you interpret the pressure distribution around a wing as the sum of

a zero angle of attack distribution due to camber and
the Birnbaum pressure distribution due to angle of attack,

things should become clearer.
On the left I show a cambered airfoil at zero angle of attack and on the right an uncambered airfoil at different angles of attack. The first cambered airfoil is a Joukowsky-type which has an arc section for its camber line. The one below it has an irregular camber line that is simply the center between its two surfaces. If you draw circles inside the airfoil at different chord locations, the camber line is the connection of the centers of those circles. The higher the local curvature of the camber line, the higher the local lift at that chord station.

The remarkable thing about the uncambered airfoil is that its lift distribution over chord is geometrically similar between different angles of attack. It only scales in magnitude, but keeps its general shape. Therefore, its center of pressure is at the quarter point for all angles.
The lift due to camber does not change with angle of attack and can be added to the lift of the symmetric airfoil in order to arrive at the total lift for a cambered airfoil at a nonzero angle of attack. Since the lift due to camber dominates the total lift at small angles of attack and becomes more and more insignificant as the angle of attack increases, the center of pressure will move from the point due to camber to the quarter point as the angle of attack rises.
If the lift is composed of a force acting at the quarter point plus an offset moment, this moment is only determined by the camber lift. Since that is constant over angle of attack, so is the offset moment.
This is all strictly true only in inviscid 2D flow, but gives a good approximation for real wings with enough aspect ratio and no flow separation. Viscosity will muddle the picture a little and with smaller aspect ratio the center of pressure of the symmetric airfoil will move forward, but for most airplanes the above captures reality rather well.

For a positive cambered wing, I understand that there is a moment due to the lift force (mentioned above) and also another intrinsic nose-down moment (which I think it is constant for all angles of attack) solely due to the cambered shape of the wing.

Only if you decide to let your lift force attack outside of the quarter point. Then the lift moment caused by the distance between the force vector and the quarter point will produce that moment due to the lift force. If, however, the lift attacks in the quarter point, only the intrinsic nose-down moment which is constant for all angles of attack will be left.

This intrinsic moment is nonzero when the lift is nonzero or even zero.

Yes, indeed. At the right negative angle of attack, the downforce from that angle of attack will exactly compensate the camber lift, leaving the whole airfoil at zero lift but still with the constant moment.

Does the value of the moment coefficient take this intrinsic moment into account as well besides the moment due to the lift force?

Again, only if you (against all convention) decide to define your lift force in a point different from the quarter point. As the moment coefficient is defined, it contains only the "intrinsic moment".
When separation starts (and even with a thicker boundary layer) this will gradually break down and the moment will fluctuate. But then we move outside of the linear range of aerodynamics and potential theory will no longer apply.


How does the glide ratio change with velocity in a glider with air brakes deployed?

This can be easily answered with a spreadsheet. Start with speed $v$ going from 20 m/s to 50 m/s. Add a column for the zero-lift drag coefficient and keep in mind that the increasing Reynolds number will change that value over speed, like that: $$c_{D0}=c_{D0_{Ref.}}\cdot\left(\frac{v_{Ref.}}{v}\right)^{-0.3}$$
I picked as reference values $c_{D0_{Ref.}}$ = 0.01 and $v_{Ref.}$ = 25 m/s. Adjust as required.
Next, add a column for the zero-lift drag coefficient with speed brakes deployed and add a constant value. I picked 0.015 for the example plot below.

Now we need to add induced drag and start with a new column for the lift coefficient $c_L$. Since weight does not change with speed brake setting, one will do for both configurations:$$c_L=\frac{2\cdot m\cdot g}{\rho\cdot v^2\cdot S}$$
I used 350 kg for the mass $m$, 1.225 for air density $\rho$ and 10 m² for wing area $S$. The induced drag coefficient $c_{Di}$ will be $$c_{Di}=\frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$$
My sample glider has an aspect ratio $AR$ of 20 and an Oswald factor $\epsilon$ of 0.98. Again, adjust as required. For extra correctness you might want to add an extra column for the real-life lift coefficient with beginning stall. Keep the "clean" lift coefficient for induced drag but plot all drag coefficients over that extra column where the lift coefficient grows less than the linear value once it is above the threshold where flow separation starts on the wing, say 1.25. Now your speed will not be correct for lift coefficients above that threshold, but your induced drag will become very realistic.

Now add another column for induced drag with speed brakes deployed. In oder to model the distorted lift distribution over span with speed brakes, reduce the Oswald factor to maybe 0.7. Add both drag coefficients for the total drag coefficient: $$c_D=c_{D0}+c{Di}$$

The result should look something like that. The index wF is for the configuration with speed brakes deployed:

The glide ratio E is the ratio $\frac{c_L}{c_D}$ and plotted on the right Y axis. Note that the best glide is cut almost in half and moves from $c_L$ = 0.7 to about $c_L$ = 1.0 for speed brakes deployed. I did not add the extra drag at high lift, so your result might look slightly different.


Is there a way to calculate the perfect angle of incidence of a horizontal stab for a given airspeed?

One clue may be increase in power and speed.  It is not unusual to trim down as plane accelerates from climb to cruise.  Since you are faster, the plane is generating more lift at a given AOA.  Greater speed will require more downtrim.  Your engine may be too powerful for your current airframe.

If the plane is safely flyable, you may try to isolate the problem by gliding power off to see if Hstab, elevator, and trim are working properly.  CG of course must be within limits.

If everything is ok, you may need a larger trim tab rather than resetting the Hstab angle.
Remember, the Hstab incidence compared to the wing AOA is critical.  I would not mess
with it.

If the problem isolates to the new engine, you may also check the thrust line.  A few degrees can make a big difference.  Your more powerful engine may need to be mounted with more downthrust.  That, and a larger trim tab, may do it.

I would definitely seek out people who have done similar modifications for expert advice.
Please, be safe.


Which part of aircraft is contributing the most weight of airplane?

Aerospace Engineering e-Mega Reference shows the following table for two aircraft (the large commercial jet, the B747-100, and the military jet, the C-5A). You can see that it differs quite a lot between the B747 and the C-5 already, so it's hard to give a definitive answer for all aircraft in general.

Also, it depends a lot on how you break down the components, for example, here the Nacelle and Propulsion system are shown separately, but if you combine the two (and some others as is done in the dry engine weight, I think) things could change.
The Aerospace Engineering e-Mega Reference points to Beltramo as the source for the numbers. Perhaps you could find some information there about what belongs to which group.

However for both aircraft, it can be seen that the Wing and the Airframe are significant contributors (and for the B747 furnishing is also a large part of the weight)

I think they included all the wiring in the Avionics part (that would make sense to me), but still it is a relatively small part of the total weight.


How many flying hours does one need to become a commercial pilot in the Philippines?

While CAAP seems to be pretty (or even very) vague on the topic, this is what I've been able to find:

To start with the private pilots license:

  You must also complete the required minimum of 40 hours of widespread flight training. The hours of flight time consists of at least 20 hours of dual flight instruction, at least 10 hours of solo flight, and 10 hours of which can be solo flights or dual.

Then you will need to build on this to get a commercial pilots license:

  You will need about 220 hours of total flight time to get your CPL.

* Another source suggests you need a minimum of 150 hours. Be aware however that this is not an exact science: Even for the PPL it can take 60 hours to 'get the feeling for it', so it is difficult to estimate exactly how much you will need. While I'm not a pilot myself I think it's a good idea to be on the conservative side and overestimate the hours you will need for a PPL and CPL.

Source

This is probably unfortunately what you have also been able to find. It is however a very good idea to go past the flight school in person and talk to them about this topic, since it's uncommon, and they can probably give you a better estimate. Be aware that there can be several more costs that are not visible at first sight (checkrides for instance) that can add to the tally so even the ones that appear the cheapest are not.


Can passengers tell (without GPS etc.) where the aircraft is heading to?

can a passenger really notice where the aircraft is heading to?

A wide variety of observable phenomena and general attentiveness can lead to an approximate understanding of which direction the aircraft is heading in.

Passengers can rely on visibly observed phenomena such as stars, moon, sun, ground topology, or major landmarks (interstates, cities, facilities etc.). General familiarity with flight routes and runways, coupled with attentiveness to significant changes in roll, pitch, and yaw can bolster an understanding of the flight's direction.

Items such as a compass (built into Casio calculator watches?) could also be used to get an approximation of direction. They may be affected by other instrumentation and mechanical features along with being really cheap.

Some passengers will be able to note which runway they took off from and a divergence from a typical path. While still ascending they can more easily notice landmarks that are likely familiar, and recognize that no significant change in direction has happened since orienting themselves.

It's possible too that cognitive bias may be another explanation. The media, and documentaries like this, seem to embrace an event as predictive or accurate despite a long history of inaccuracy or patterned behavior. That passenger who makes 98% incorrect assertions of plane direction is only recorded or listened to after the fact and appears to be 100% accurate in their vastly under sampled assertion set. How does it go? A broken clock is right twice a day?

Also, thanks to all the comments, tried to cover most of those notes/ideas here.


Is it possible to export NMEA data from a Garmin GDL-39 to a custom Android App?

Well, I think there's a way. Fair warning, it also looks like a fair bit of work, and I haven't tried any of this. Plus, you run the risk that the GDL 39 is going to refuse to connect or do something really weird or otherwise break/invalidate user agreements/warranty.
The user manual for the GDL 39 shows that the GDL 39 has Bluetooth, and this website purports to know the communication protocol and handshake sequence, with code for getting it to work written in Perl. Getting that to work in an Android application shouldn't be impossible, but I haven't looked at it thoroughly and I'm not enough of an Android expert to tell you if it would work off-hand.
You could also go over serial as an option to get NMEA data, which I investigated before I found the website above. I think the Bluetooth option is better, but here's the physical method since I already wrote it up.

You will need a cable like the one here to connect to the GDL over serial. From there, you'll also need a device to take the serial data and get it into the android app over Bluetooth. Here are a few options:

An Arduino or other micro-controller to handle the serial connection to the GDL 39, and then re-broadcast the NMEA data over a Bluetooth connection.

A ioio (pronounced "yo-yo"). Allows you to control a micro-controller with an android application over Bluetooth or via USB. I've used one before, and it's pretty easy to get running, but is a little old. Support still exists for it though.

Your next problem is to actually communicate with the GDL 39 over serial, which will be more complicated.  The GDL 39 appears to have a proprietary handshake protocol over serial, which in theory, should prevent you from using a non-Garmin device to get any data over serial. However, in practice, it may not be hopeless.
This website describes the handshake protocol to negotiate a connection over serial, and data from the GDL 39. I haven't tried to do this, but it looks like it might work. If you can get that to you work, it seems like you'll get a stream of NMEA data, and then you could send it over Bluetooth to an Android application.


Why does the US Marines continue to fly the AH-1 Cobra, retired by the US Army in the late 90s?

Well this ain't your father's Cobra.  Literally.

The current incarnation of the Huey cobra for the Marine Corps, which is called the AH-1Z Viper,  sometimes referred to as the King Cobra continues to be the backbone of the Marines attack helicopter force for several reasons.

Like all Huey cobra models, the AH-1Z and the USMC current utility helicopter, the UH-1Y Venom, make use of a common power plant, transmission and rotor set, tailboom and avionics architecture, saving quite a bit on spare parts.  The USMC quotes an 86% commonality between the UH-1Y and AH-1Z airframes.  And, as both the UH-1Y and the AH-1Z constitute a single acquisitions program, it saves a lot of time and costs trying to begin another weapons procurement program for the Corps.  This is also the reason the Huey Cobra even came into existence in the first place.

As helicopters have hit the upper limit in terms of performance, the procurement of a brand new airframe over an existing design doesn't offer many advantages and costs considerably more.  Simply upgrade the engines, add in a modular avionics bay with modern infrastructure facilitating upgrades to modern mission systems and modern weapons, and modern defensive countermeasures systems, and you have help that's as good or better than any existing attack platform out there.

The AH-1Z is comparable to the AH-64 Apache weapons system but remains slightly faster, carries a greater payload and has a 33% greater range than the Apache, all preferred attributes for shipboard operations.


Is there a preferred wind direction for a helicopter in hover?

In most helicopters, the wind direction when hovering is of concern to the pilot, as is which way you turn when hovering.

The following applies to a single rotor helicopter with the tail rotor mounted on the left and the rotor turning counter-clockwise.  The opposite applies to clockwise rotating rotors and mounting the tail rotor on the left versus on the right would take more pages of text!

  Hover with no wind.

As the rotor rotates counter-clockwise, the torque reaction on the fuselage wants to turn the nose to the right.  The tail rotor "pushes" the tail to the right to balance this torque.  It is by varying the angle of attack of the tail rotor that we can change the amount of thrust from the tail rotor and therefore have yaw control.

The tail rotor is no different to the main rotor, it just operates in the Y axis rather than the Z axis and is subject to vortex ring state.

The vortex from the tail rotor is to the left.  If the helicopter is yawed right, the tail moves left and the tail rotor moves into it's own vortex causing a reduction in angle of attack and therefore thrust.  If the turn is too fast, this reduction can lead to a loss of tail rotor effectiveness.

Also in a no wind hover, the vortex from the main rotor is forced downwards.

All of what follows is for a counter-clockwise rotating rotor and applies to all single rotor helicopters.

  Hover with the nose into wind, but not with enough wind to generate
  effective translation lift.

This state is more or less the same as for a no wind hover and is the easiest (and therefore preferred) direction in which to hover.  Turning the aircraft has the same risk as in a no wind hover in that turning too quickly to the right can lead to loss of tail rotor effectiveness.

  Hover into wind with enough wind speed to generate translational lift
  (let's say about 15 kts)

Now things start to get interesting.  If you turn to the left, the vortex from the front edge of the main rotor is blown backwards by the wind across the tail rotor and causes a decrease in the angle of attack of the tail rotor blades since the wind from the vortex is blown on to the right hand side of the tail rotor.  Therefore, a lot more left pedal is required as the helicopter turns from 0 degrees (relative to the wind) to 270 degrees.  As 270 degrees is reached and passed, the vortex is no longer interfering with the tail rotor and the angle of attack for the same pedal position is increased.  If the same pedal input is maintained, the rate of turn will increase rapidly and if not corrected, can lead to an uncontrolled spin.

If you turn to the right, the opposite happens.  More right pedal (less thrust) is required in the first quadrant of the turn from 0 degrees to 90 degrees since the vortex is blown into the face of the tail rotor causing an increase in angle of attack and therefore greater thrust.  As the nose passes 90 degrees, the angle of attack decreases and left pedal must be added to prevent a sudden acceleration in yaw.

You should be able to see that the amount of pedal required changes as the turn continues and without constant adjustment, a steady turn cannot be maintained and with enough wind, and a pilot who is not quick enough on the pedals, control can be lost.

Now let's take the turn out of the equation and consider steady hovers in wind that is not "on the nose".

  Wind from the left quarter, i.e. 270 - 0 degrees. This is effectively the same condition as that first quarter of a left turn with wind from the front.

This is the difficult one.  Wind is never constant. It is always changing slightly and turbulent.  As the main rotor vortex is blowing into the tail rotor, the tail rotor is operating in a very turbulent and unpredictable environment.  Rapid and constant pedal inputs are required to maintain control.  Any deviation not caught immediately can lead to a loss of control.  It is very difficult to hover with the wind from the left quadrant and is therefore usually avoided.

  Wind generally from the left (a left crosswind)

As the wind is blowing into the face of the tail rotor, its angle of attack is reduced as the airspeed through the rotor increases.  This can lead to the loss of tail rotor effectiveness or even a stall of the blades as the angle of attack exceeds the critical angle.

  Wind from behind.

The primary effect here is the tendency of the fuselage and vertical stabiliser to "weather cock" the aircraft.  It naturally wants to face into wind.  If a yaw begins and is not immediately corrected, due to the effects above, the yaw can increase rapidly and suddenly.  Additionally, the wind is never constant and varies between passing to the left and to the right of the tail rotor.  Therefore, hovering downwind needs immediate and constant corrections to yaw.

  Summary

Hovering into wind is very much preferred and is always chosen unless operations require something different.  Hovering with the wind from the right is the next "most" preferred position with quartering winds from the left and hovering downwind to be avoided if possible.

All of this said, it's a standard part of training to learn how to hover in these conditions and how to maintain constant rate turns.  It does require a lot of learned skill and you must be quick and accurate on the pedals with constant corrections.

Incidentally, early models of the UH-1 were particularly difficult to handle in certain wind conditions and the tail rotor was moved from the right to the left on later models to improve handling.

In a light helicopter with little power reserve, e.g. the R22, hovering in strong gusty winds is an intense part of operations.  I had some close calls when learning how to do this including a loss of control which I only recovered by pulling in power and accelerating into forward flight.  Not good on an airfield with other aircraft operating since my "low hours brain" focussed only on regaining control.  I am ashamed to say that looking around took second place but I did learn an important lesson and with hindsight, the right thing to do would have been to land and discontinue the flight as the wind got more gusty.


Why are modern aircraft wings often pointed instead being more rectangular?

Rectangular wings are easy to build: You need just one rib size and can repeat making it several times, using the same jig. Early airplanes needed a lot of manual assembly by semi-skilled workers, and it made sense to simplify the task.

SE5a wing under construction (picture source)

However, to produce the least amount of induced drag, the downwash behind the wing should have the same angle over the whole wingspan. This can be achieved with washout, but then the washout needs to be tuned to one angle of attack. Change the angle of attack and your lift distribution over span is producing more drag than necessary. To overcome this problem, elliptic wing planforms were used, but they are complicated to build and prone to tip stall. This is when the wing tip loses lift first during a stall and causes the aircraft to roll violently. If this happens close to the ground, a crash is inevitable.

Therefore, designers soon settled for something wich resembles the elliptic planform, but leaves more area at the tip such that it has less loading and more stall margin. Especially when this wing is covered with sheet metal, it makes sense to have only one direction of curvature, thus trapezoidal planforms soon became the preferred way of building wings once aluminium became the primary structural material for aircraft.

The SB-11 glider uses a rectangular inner wing and a trapezoidal outer wing. This way, the mould for the inner wing could be used for the left and right side, making construction easier (picture source).

The trapezoidal wing is also helpful in another respect: Cantilever designs have high bending moments at the wing root. If the root has more chord, the spar can be made thicker, and if the area at the tips is smaller, less bending moment is created for the same lift. The bracing of biplanes takes care of root bending moment, but when monoplanes emerged, the rectangular wing lost most of its benefits.

Now for the rounded wingtips: They are an attempt to get the wing closer to the ideal elliptic planform without losing the benefit of a constant-chord wing over most of the span. Much of the wingtip design is also about aesthetics, and some people hold quasi-religious beliefs about the benefits of a particular wing tip shape. For metal wings, wingtips can be made by pressing the aluminium sheets for the top and bottom part in shape and riveting them together since the aerodynamic loads on the wing tip are low. Wingtips and fairings were also among the first parts on aircraft to be made with composite materials, which make three-dimensional shaping much easier. Closer to the root the wing needs much more strength, and here the trapezoidal planform is still the best compromise today.


How do military radars detect the type of an aircraft?

This depends on the type of radar system and its intended application. You can find a nice bit of history on it here. The early radars could not tell the difference between aircraft types, any kind of movement was coordinated beforehand on the friendly side and anything that was not pre-arranged was considered foe. The early Doppler systems that resided on the DEW Line could be trigger by as little as a dense flock of birds. The early radar data from the DEW Line was processed on the SAGE system a very early (very primitive) system for coordinating a great deal of weather, radar, and flight information.

After WWII and through out the cold war era newer tech was built to solve the issue of coordination. The modern Identification Friend Or Foe system that is now in use provides at least some way to pick apart radar targets.

With the advent of modern computer processing and the ability to analyze radar returns beyond staring at a scope it is possible to determine (with great certainty) the make of an aircraft by its radar signature (and in some cases thermal signature). Also known as Radar Cross Section. Everything (even stealth planes) have a radar cross section and once a cross section has been identified it could be used again to check against future pings.

One could go as far as to say there is some historical precedent around this. The general idea of "if it comes from over there is one of them, if it comes from over hear its one of us" is very much how radar worked for a long time and the identification of the actual plane was less important than identifying that it was there. If an unidentified target entered a radar area and did not identify its self, over the radio, with some kind of transponder and or at a preordained time it was intercepted. Not until visual identification was made was it not longer considered a threat.

Here in the states by 2020 the civilian world will be sending speed and position data via ADS-B OUT to supplement (and potentially replace) radar returns.


What features enable the Su-25 Frogfoot to operate with such a wide variety of fuels?

Being a ground attack aircraft, the Su-25 has an operational ceiling of 23,000 ft and will not encounter the low temperatures to which a Boeing 777 is regularly exposed. Therefore, the risk of diesel fuel gelling and clogging the fuel lines is much lower.

On the other side of the temperature scale, gasoline could vaporize in hot environments and equally endanger the fuel supply of the engines. Those extremes are rarely found in the countries of the former Eastern Bloc, and the Su-25 was primarily designed for operations in moderate climates.

Nevertheless, some modifications are advised in order to avoid trouble when exotic fuels are used:

Regular fuel pumps use kerosene for lubrication. Kerosene is a good lubricant and allows for a simpler pump design. By using better bearings, the fuel pump can also be used with gasoline or petrol without suffering long-term damage.
Running the fuel lines away from hot surfaces reduces the risk of vapour bubbles forming inside. I don't know the details of the Su-25 design, but I would be mightily surprised if any fuel line runs right below the upper skin of the aircraft where the sun can heat the surface to temperatures in excess of 70°C even in moderate latitudes. This is supported by a light grey paint scheme as shown by the Ukrainian Su-25 in your question.
Don't do fancy things with the fuel, like use it as a hydraulic fluid, but restrict its use to being burned in the engine, and maybe cool some engine oil on its way into the combustor. Then any liquid hydrocarbon can be used.

That turbofans can run on a wide variety of fuels has been established in the answers to this question.


How to determine if pilots in crashes flew again?

How can I determine if a driver who was involved in an automobile accident is still driving?
The answer is "They probably are."

Broadly, unless the pilot died or the FAA (or equivalent authority) revoked their license as a result of their actions there is no reason the pilot wouldn't still be flying unless they decide to hang up their wings for personal reasons.
They may not be flying for the same airline, and certainly will have had some sort of remedial training (particularly if they were involved in an incident where pilot error was the cause), but damaging an aircraft is usually not a career-killing move unless one makes a habit of it.

If you want to know for certain about a particular flight crew you would need to research each accident separately (locate the pilots' information - name or certificate number - and check something like the FAA airman records branch to see if the pilot still holds a valid certificate and medical.


How is a stalling speed not affected by airspeed?

Flying at too low an airspeed will result in a stall, but what "too low" is entirely depends on the situation. For example, if you try a very steep climbing turn, you can stall at an airspeed where you would be fine in level flight.

The answer is an incorrect answer, not because it's factually wrong, but just because it's not general enough. Exceeding the critical angle of attack always causes a stall, and it is the cause of every stall, so it's the best answer to the question. The examiner wants you to understand that the angle of attack is the mechanism that causes stalling, and that it can happen at different speeds.

It's quite common for these (and other multiple choice) exams to have an "incorrect" answer that isn't completely wrong, but isn't as good as the correct answer because it only applies in some conditions or is less general in some other way. In this case, another "incorrect" answer would be "pulling the stick back too far". That's an action that might result in a stall, if you're already at a high angle of attack, but it's not really the cause and isn't always true.


Is it legal to build and fly an international aid drone?

Is it legal to build and fly a scale 1m:20cm Boeing C-17 for use in flying from the U.K. [...]

Building an aircraft is likely not regulated (intellectual property laws still apply), it's probably like building a car in your garage, however using an aircraft, specially outside your own property is regulated. Drone use, in UK, is regulated by CAA as for any aircraft, the main reference is:

Air Navigation Order 2016 and Regulations (CAP 393).

Article 23 (Exceptions from application of provisions of the Order for certain classes of aircraft) specifies that some aircraft are excluded from the scope of the order, this includes:

  any small unmanned aircraft.

So some drones can be used quite freely. Indeed you are always responsible of what happens with your drone. Some articles still apply and in particular art. 94:

Art. 94 - Small unmanned aircraft: Need to fly safely, and can't drop anything that can endanger persons or property. Must maintain direct, unaided visual contact with the aircraft. Unless authorized by ATC, weight is limited to 7 kg when inside A, C, D or E airspace, when close to an airfield, or when flying higher than 400 ft.

This article alone prevents free use in your case.

Source

For the sake of reference, let's look at the other articles:

Art. 91 - Dropping articles for purposes of agriculture etc. and grant of aerial application certificate: This is not allowed without authorization.
Art. 92 - Mooring, tethering, towing, use of cables, etc.: Use of cables being a hazard for other aircraft, some cases are prohibited, e.g. using cables in a controlled airspace, or near an airfield.
Art. 95 - Small unmanned surveillance aircraft: Not applicable.
Art. 241: Endangering safety of any person or property: Don't, even recklessly or negligently!
Art. 239 and 257: Power to prohibit or restrict flying: You may be prohibited to fly if circumstances call for; Art. 265: Offences and penalties: How you'll be fined if you do something not allowed.

For uses not covered by art. 23, CAA says:

  If you want to use a small unmanned aircraft or drone outside of the
  operating limits set out in the Air Navigation Order then you will
  need Permission from the CAA, even if your activity is non-commercial.
  Applications for this type of Permission are considered on a case by
  case basis.

Permissions are granted on a case by case basis, they may be valid for one flight or for a period of up to 12 months.
An operating manual must be prepared and submitted to the CAA, and a designed pilot must be assessed by CAA as having the competences required to fly the missions.

In addition, as mass increases, regulation becomes more stringent:

  Unlike small drones, unmanned aircraft with an operating mass of more
  than 20 kg are subject to the whole of the UK Aviation regulations (as
  listed within the UK Air Navigation Order – ANO), although they may be
  exempted from certain requirements by the CAA.

  Unmanned aircraft with a mass of more than 150 kg are also subject to
  additional certification requirements as determined by the European
  Aviation Safety Agency - EASA.

In the UK, "large" drones

  are not currently permitted to fly in any non-segregated airspace [...]
  without specific permission from the CAA.

Flying from the U.K. to other countries [...]

UK is not part of the Schengen Area, flying to a Schengen Area country will be considered as if flying to a foreign country.

Flying to (or over) any other country requires to follow local laws, which vary, but likely include:

Getting an authorization to fly a drone from the local agency equivalent to CAA. Regulations may be more limiting than in UK.
Getting a specific authorization if the pilot is a foreigner. In some countries this may be not allowed at all.
Getting an authorization to use a camera. Some countries don't welcome flying cameras, some just exclude their use at specific locations.

ICAO Appendix 4 to Annex 2 of the Chicago Convention (Remotely Piloted Aircraft Systems) lists the few agreed principles between ICAO members, including:

  An  RPA  shall  not  be  operated  across  the  territory  of  another  State,  without  special  authorization issued by each State
  in which the flight is to operate [...]
  An  RPA  shall  not  be  operated  over  the  high  seas  without  prior  coordination  with  the  appropriate ATS authority.

To deliver aid in places that are hard to reach, for example, on top of a large hill where it is hard to get aid workers on foot up there [...]

UPS-backed Rwandan blood deliveries (source)

Why not launching your drone close to the destination, instead of flying from the UK, think about aspects like:

The ability to maintain a radio link with a significant data rate for the camera. This can be difficult without using a satellite.
A drone limited range, even if fuel is used in place of batteries.

More on unmanned aircraft in UK airspace:

Drones are administratively known as UAS (unmanned aircraft system) and SUAS for the small ones.
CAA site
Unmanned Aircraft System Operations in UK Airspace – Guidance (CAP 722)
The Regulatory Framework for RPAs (EASA)


Can an aircraft owner legally replace an oil quick drain valve?

This is what the FAR allows you to do as preventive maintenance if you are not a certified mechanic:

(c) Preventive maintenance. Preventive
maintenance is limited to the following
work, provided it does not involve complex
assembly operations:
(1) Removal, installation, and repair of
landing gear tires.
(2) Replacing elastic shock absorber cords
on landing gear.
(3) Servicing landing gear shock struts by
adding oil, air, or both.
(4) Servicing landing gear wheel bearings,
such as cleaning and greasing.
(5) Replacing defective safety wiring or
cotter keys.
(6) Lubrication not requiring disassembly
other than removal of nonstructural items
such as cover plates, cowlings, and fairings.
(7) Making simple fabric patches not requiring
rib stitching or the removal of structural
parts or control surfaces. In the case of
balloons, the making of small fabric repairs
to envelopes (as defined in, and in accordance
with, the balloon manufacturers’ instructions)
not requiring load tape repair or
replacement.
(8) Replenishing hydraulic fluid in the hydraulic
reservoir.
(9) Refinishing decorative coating of fuselage,
balloon baskets, wings tail group surfaces
(excluding balanced control surfaces),
fairings, cowlings, landing gear, cabin, or
cockpit interior when removal or disassembly
of any primary structure or operating
system is not required.
(10) Applying preservative or protective
material to components where no disassembly
of any primary structure or operating
system is involved and where such
coating is not prohibited or is not contrary
to good practices.
(11) Repairing upholstery and decorative
furnishings of the cabin, cockpit, or balloon
basket interior when the repairing does not
require disassembly of any primary structure
or operating system or interfere with an
operating system or affect the primary
structure of the aircraft.
(12) Making small simple repairs to fairings,
nonstructural cover plates, cowlings,
and small patches and reinforcements not
changing the contour so as to interfere with
proper air flow.
(13) Replacing side windows where that
work does not interfere with the structure or
any operating system such as controls, electrical
equipment, etc.
(14) Replacing safety belts.
(15) Replacing seats or seat parts with replacement
parts approved for the aircraft,
not involving disassembly of any primary
structure or operating system.
(16) Trouble shooting and repairing broken
circuits in landing light wiring circuits.
(17) Replacing bulbs, reflectors, and lenses
of position and landing lights.
(18) Replacing wheels and skis where no
weight and balance computation is involved.
removal of the propeller or disconnection of
flight controls.
(19) ...
(20) Replacing or cleaning spark plugs and
setting of spark plug gap clearance.
(21) Replacing any hose connection except
hydraulic connections.
(22) Replacing prefabricated fuel lines.
(23) Cleaning or replacing fuel and oil
strainers or filter elements.
(24) Replacing and servicing batteries.
(25) Cleaning of balloon burner pilot and
main nozzles in accordance with the balloon
manufacturer’s instructions.
(26) Replacement or adjustment of nonstructural
standard fasteners incidental to
operations.
(27) The interchange of balloon baskets and
burners on envelopes when the basket or
burner is designated as interchangeable in
the balloon type certificate data and the baskets
and burners are specifically designed for
quick removal and installation.
(28) The installations of anti-misfueling devices
to reduce the diameter of fuel tank
filler openings provided the specific device
has been made a part of the aircraft type
certificiate data by the aircraft manufacturer,
the aircraft manufacturer has provided
FAA-approved instructions for installation
of the specific device, and installation
does not involve the disassembly of the existing
tank filler opening.
(29) Removing, checking, and replacing
magnetic chip detectors.
(30) The inspection and maintenance tasks
prescribed and specifically identified as preventive
maintenance in a primary category
aircraft type certificate or supplemental
type certificate holder’s approved special inspection
and preventive maintenance program
when accomplished on a primary category
aircraft provided:
(i) They are performed by the holder of at
least a private pilot certificate issued under
part 61 who is the registered owner (including
co-owners) of the affected aircraft and
who holds a certificate of competency for the
affected aircraft (1) issued by a school approved
under § 147.21(e) of this chapter; (2)
issued by the holder of the production certificate
for that primary category aircraft
that has a special training program approved
under § 21.24 of this subchapter; or (3) issued
by another entity that has a course approved
by the Administrator; and
(ii) The inspections and maintenance tasks
are performed in accordance with instructions
contained by the special inspection and
preventive maintenance program approved
as part of the aircraft’s type design or supplemental
type design.
(31) Removing and replacing self-contained,
front instrument panel-mounted
navigation and communication devices that
employ tray-mounted connectors that connect
the unit when the unit is installed into
the instrument panel, (excluding automatic
flight control systems, transponders, and
microwave frequency distance measuring
equipment (DME)). The approved unit must
be designed to be readily and repeatedly removed
and replaced, and pertinent instructions
must be provided. Prior to the unit’s
intended use, and operational check must be
performed in accordance with the applicable
sections of part 91 of this chapter.
(32) Updating self-contained, front instrument
panel-mounted Air Traffic Control
(ATC) navigational software data bases (excluding
those of automatic flight control
systems, transponders, and microwave frequency
distance measuring equipment
(DME)) provided no disassembly of the unit
is required and pertinent instructions are
provided. Prior to the unit’s intended use, an
operational check must be performed in accordance
with applicable sections of part 91
of this chapter.


There's a lengthy paper on the NASA web site that investigated this. There's a lot of detail covered in that paper so I'm not going to reproduce any of it here, but the main positive points that emerge are:

Improved safety margins on landing1
Reduced wear on brakes and tyres
Reduced brake temperature leading to faster aircraft turn around.

Even when setting this against additional engine maintenance and fuel consumption, the airlines surveyed overwhelmingly considered that the installation and use of thrust reversers gave a worthwhile return on investment.
1 Thrust reversers are not considered when assessing braking systems for certification.


Which civil airport has the longest runways in total?

Best I can find, within the US, is KDFW with 80,403' of total length. Second seems to be KDEN with 76,000'. Glanced at KORD, and with mental math it comes up to about 71,000'. Most other major airports are well behind all of these, mainly because they don't have as many long runways as DFW and DEN.

Somebody else can consider Russian, Chinese, and Middle Eastern airports -- I don't have the chart sets handy for those.  May be worth converting this to a community wiki answer if any of those places give interesting results.


Which regulation requires me to report entering a hold?

So, this is found in the AIM, para 5-3-3 (a)(1)(f) (PDF), assuming you're asking about FAA regulations.

  5-3-3. Additional Reports

  a. The following reports should be made to ATC or FSS facilities without a specific ATC request:

    1. At all times.

      [...]

      (f) The time and altitude or flight level upon reaching a holding fix or point to which cleared.

      (g) When leaving any assigned holding fix or point.

Now, the AIM is not regulatory. It uses terminology like "should" instead of "shall", which lawyers will happily jump on, and it in fact says this about itself:

  This publication, while not regulatory, provides
  information which reflects examples of operating
  techniques and procedures which may be requirements
  in other federal publications or
  regulations. It is made available solely to assist pilots
  in executing their responsibilities required by
  other publications.


Why do planes sometimes make a touch-and-go?

To practice approaches, traffic patterns and landings without the added downtime of taxiing back for another takeoff.  Pilots have to maintain proficiency in these skills by practicing them on a regular basis and touch and gos are some of the best ways to do this.  It can also be used for meeting regulatory currency for carrying passengers depending on local time and landing gear configuration of the airplane.  In terms of gas consumption, it’s no less wasteful than a stop and go or a taxi back.  It’s not just for small aircraft either.   I’ve seen plenty of large military aircraft doing pattern work like that as well as there is no substitute for the real thing.  Airliners occasionally do touch and gos, but generally as part of a test flight after major maintenance; company pilots will fly approaches and landings in a level D full motion simulator simply because it’s cheaper and can simulate a wide range of conditions and emergency contingencies as well.