What is the relationship between centre of gravity, centre of pressure and pitch?

Reading the heated discussion between Robert and JZYL makes me sad. Both are right and still they cannot agree. Maybe a longer and more detailed description might help.

  1) When the aircraft is taking off, is the tail down force required to pitch up the aircraft high? Because the aircraft always has a natural tendency to pitch down.

The aircraft should always have a natural tendency to return to its trimmed state. This might be by pitching up or down, depending on the actual state. There are conditions where that is not true, therefore I say 'should'.

In a take-off, the aircraft is rolling on its wheels and needs to rotate around the main wheels in order to increase angle of attack for liftoff. This means either to decrease tail lift (say, on a taildragger where the pilot has lifted the tail off the runway earlier in order to reduce drag) or to increase tail downforce. In order to pitch up the aircraft, the center of pressure needs to move forward of the main wheel location and the center of gravity, which is done by pulling on the stick. So what is needed is not an absolute tail downforce but a decrease in tail lift. This might well be a downforce, but could also be a lower positive tail lift. Most likely, it is a change from around zero to negative lift on the tail.

As soon as the correct pitch attitude is reached, the pitch rotation has to be stopped by pushing briefly on the stick. This shifts the center of pressure briefly back, behind the center of gravity. After that, the aircraft is in the correct attitude for its initial climb and the center of pressure is at the same lengthwise location as the center of gravity. If it isn't, the aircraft accelerates into some pitching motion which is not desired in straight flight.

Disclaimer: This all assumes no pitch moment contribution from either engines or drag. Any additional moment from other than lift forces adds to the momentum equilibrium and requires the pilot to shift the center of pressure a bit fore or aft of the center of gravity for straight flight.

  2) When the aircraft increases its AoA, the CP moves forward (towards the leading edge). Does this mean it would go ahead of the CG or move closer to the CG.

When the angle of attack of an airfoil with positive camber is increased, its center of pressure moves forward.

In potential flow theory, lift can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part of lift is constant, the angle-of-attack related part varies linearly with this parameter. The center of pressure of the camber part is somewhere at mid-chord (details depend on the camber line; with a Joukowski airfoil the center of pressure is precisely at mid chord). The center of pressure of the angle-of-attack dependent part is at the quarter chord (the center of the area below the chordwise Birnbaum distribution of lift). The important part is the self-similarity of Birnbaum distributions for different angles of attack: The center of pressure of the angle-of-attack dependent part is constant and at 25% of chord for 2D flow and wings of large aspect ratio. Since lift at zero angle of attack is solely from camber, the center of pressure is at around mid chord and moves forward towards the quarter chord point as angle of attack increases. But this would make an airplane with only a main wing using a positively cambered airfoil unstable in pitch.

Hence the addition of a tail surface with less loading than the main wing. Since it operates on a point of the lift curve slope that is lower than that of the wing, a change in angle of attack means a larger relative lift change on the tail which shifts the center of pressure of the whole aircraft back as angle of attack is increased. However, as this happens, the pilot will pull back on the stick, reducing some of the additional tail lift in order to keep the center of pressure close to the center of gravity.

In short: The center of pressure is always very close to the center of gravity unless the pilot commands some pitch acceleration.


What is an ATC clearance? Does a VFR flight have to get a ATC clearance?

All IFR flights require a clearance in controlled (class A-E) airspace. Clearances aren't available in uncontrolled (class F-G) airspace.

VFR flights in certain airspace (ICAO: class B-D, FAA: class B only) require a clearance and may be referred to as CVFR. This clearance will be much simpler than an IFR one, though.

The purpose of an IFR clearance having a limit, route and altitude is so ATC knows what the plane will do if communication is lost and therefore can maintain proper separation of other aircraft. This detail isn't required for a CVFR clearance because a VFR pilot is required to be able to provide his own separation anyway, so the VFR lost comms procedures are much simpler.

ATC can amend a clearance, in which case they only say the items to be changed, or they can cancel the old clearance and issue a new one from scratch. Which they choose depends on how much is changing and why.

An instruction may authorize a (usually brief) deviation from a clearance, e.g. to avoid traffic or weather. ATC can give instructions to VFR flights without a clearance to amend, though, and this doesn't create a new clearance, so they're something different.


Where can i find aircraft blueprints for study purpose?

Fifteen euros gets you what you need here: https://www.the-blueprints.com/vectordrawings/show/1750/embraer_190/

This one is six euros: http://www.lulu.com/shop/elodie-roux/3-view-drawing-plan-3-vues-embraer-190/ebook/product-20233147.html


Do leading edge flaps increase the critical angle of attack?

Yes leading edge devices indeed postpone stall to a higher Angle of Attack. They are particularly useful for:

Airfoils which are prone to leading edge stall. Deflection of the trailing edge flap creates upwash at the wing nose, which reduces stall AoA mainly on thinner wing profiles.
Aeroplanes which use lift dumpers as speed brakes. Deflection of the spoiler reduces the stall AoA, a leading edge device provides extra AoA margin.

Image source and source of info below

The four main types of leading edge devices are:

  LEADING-EDGE SLATS are small, highly cambered airfoils forward of the wing leading edge, which experience large suction forces per unit of area and reduce
  the suction forces on the basic airfoil.

Note that before deflection, the slat was part of the wing nose. Its area is mainly deflected to a new position, not extended as in the case of a trailing edge flap.

Slat deflection does indeed change the camber, by reducing the local AoA. This potential reduction in local lift coefficient is compensated for by the changed suction forces around the nose, with a net effect of around zero.

  KRUEGER FLAPS perform in the same way as slats, but they are thinner and more suitable for installation on thin wings. Krueger flaps are often used on the inboard part of wings, in combination with outboard slats, to obtain positive longitudinal stability in the stall.

  (PLAIN) LEADING-EDGE FLAPS (hinged noses) are less effective than slats. They are mechanically simple and rigid and partic- ularly suitable for thin airfoil sections.

  FIXED SLOTS are the simplest devices for postponing leading edge stall, but their profile drag penalty is generally prohibitive for effective cruising, except on some low-speed STOL aircraft.

The picture below plots what happens upon deflection of flaps (left) and slats (right). Flaps increase lift for a given AoA, but reduce critical AoA. Deflection of leading edge devices does not result in increase of lift at an AoA, but postpone the critical AoA - in order to obtain more lift, the AoA needs to be increased.


Why didn’t Boeing ramp 737NG production back up in response to the 737 MAX groundings?

There are several reasons why.

First, it takes an awfully long time to make that kind of switch. While one can switch an assembly line for a product, that works rather differently for (say) transistor radios and Boeing 737s. The assembly line for an airliner winds back very, very far, and nothing happens or can happen immediately, or even very fast.

That would have made it very difficult, even if Boeing wanted to do that.

And here's a second reason: they didn't want to. They had orders for the MAX. If they were late in delivering those aircraft, they could be subject to financial penalties. Boeing agreed to deliver the aircraft, so had to build them. At the same time, they weren't facing overwhelming demand for other 737s that could possibly have made it worthwhile to do it in the circumstances.

A third reason: Boeing had every expectation that the aircraft would soon be allowed to fly again, and continued to operate under that expectation (for much longer than many observers shared the same expectations, it has to be said).

A fourth reason: it would have been bad for business to do that. Doing what you suggest would have sent out the most appalling signals, i.e. that the company had lost faith in one of its most important products. In a way, they simply had to hope that things would turn their way sooner rather than later, and the option of turning away from that would have had tremendous costs. (You might argue that it was a gamble that didn't pay off; I think that would be fair enough to say.)

And finally, I think that Boeing's slow responses at many points in this story look like the behaviour of a company in shock. On multiple occasions, the company has found itself struggling to keep up with and make sense of events, even if they been events of their own making. Quite possibly the company's decision not to do as you suggest, or at least not to have done it much sooner, will turn out to have been a very bad one, but unfortunately, it will also have been only one in a long chain of them.

By the time I make several bad decisions in a row in a crisis, it's pretty much guaranteed that the next one I make will make everything even worse. Organisations also fall victim to that pattern.


How do oceanic lateral offset procedures work?

On high-altitude North Atlantic flights, planes can fly zero, one, or two miles to the right of the centerline of your assigned track. The offset is randomly selected. This is called Strategic Lateral Offset Procedures.

It's used in the oceanic flights or flights over remote land where there is typically no radar service, and ATC cannot "see" traffic separation. It provides, on average, additional horizontal separation between planes reducing the likelihood of midair collision and high altitude wake turbulence (which can occur as a result of RVSM).

Strategic lateral offsets can be authorized in en-route oceanic or remote continental airspace. The decision to apply a strategic lateral offset is the responsibility of the flight crew, and only where offsets have been authorized by the ATS and when the aircraft is equipped with automatic offset tracking capability.

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/enroute/oceanic/documents/WATRS_Plus/DOC_4444_16.5_SLOP_25Feb2011.docx


Why use jet engines instead of propellers when designing new airplanes?

Look into Propulsive efficiency (and maybe also this one).  Turboprops have a speed of peak efficiency, which is around mach 0.6 or so.  If you try to go faster, it gets significantly less efficient.  A high bypass turbofan, on the other hand, is most efficient at a higher speed, maybe mach 0.8 - 0.85.  So the simple answer is: we switched from props to jets because people wanted to fly faster but didn't want to burn a lot of extra fuel to do that.


Are there limitations on taking off or landing if airport lighting such as the VGSI (PAPI) is out of service?

There wouldn't be any limitation on taking off with the VASI/PAPI out of service, unless it involved a requirement to return to the airport where the VGSI would be required (see below).  That would be a very extreme case, because it would mean that

the rules + the existing weather require the ability to return to the departure airport (i.e. a "takeoff alternate" is either not authorized or none is available), and
no other runway (including the same strip of concrete, landing in the opposite direction) is available at the same airport (tailwind component could cause this), and
this is a case (discussed below) where a VGSI inop prevents an approach to 'this' runway.

That's going to be a very rare combination.

So, when can an inop VGSI prevent an approach?  There are cases where US TERPS rules do require an operative VGSI in order to conduct certain operations -- mostly at night.  I've seen plates where a VGSI is required for circling operations to a given runway at night, and in one case where you couldn't fly any instrument approach, even an ILS, to the particular runway without the VGSI.

The latter fell in the category of TERPS rules that don't make intuitive sense... if I'm flying the ILS glideslope, by the time I'm going visual I'm so close to the runway that you'd think obstacle clearances have all been met & nothing remaining (in front of the aircraft) could possibly be a factor.  It was a memorable discussion because it seemed to make so little sense.  But, the restriction was published, and without the VGSI that runway wasn't usable.

The general case, however, is that the VGSI can be inop with no effect on the ability to fly an approach.  In 99.9% of the cases, it won't matter.  But for an obstacle in the right spot, you can get the note on the approach plate stating the requirement, and in that case, it matters.

But it's very uncommon.


What is the radius of a VOR compass rose on a VFR sectional chart?

I measured one on a sectional and it came out to approximately 19.4 NM. However, as mentioned, I'm not sure it has a fixed size. Symbols on a map can be resized by the cartographers (map makers) for ease of reading. I would not rely on this for anything other than trivia.


Can an engine on a Boeing 737 fly just using compressed air instead of jet fuel?

Where would you get the compressed air from? Jet engines "contain" compressed air because the engine itself compresses it using the energy from burning fuel. Asking if you could power the engine by that compressed air is like asking if you could power a car engine by the rotation of the gearbox: it's confusing cause and effect.


What material was/is used on the skin of the Blackbird SR71?

Titanium. Only the hottest portions of the fuselage (the chines) were cooled, using the fuel as a coolant. As far as I'm aware, there was no treatment to make the material stronger with age.


Are there any certification requirements for steering bypass or lock pins?

The steering bypass pin on modern airliners serves to isolate the nose wheel from the hydraulics so that movement of the nose wheel will not damage the hydraulic system. What the pin does is usually hold a bypass-valve open, allowing fluid in the actuator which is displaced by the movement of the nose wheel to 'bypass'.

This would be the most simple solution and most manufacturers have adopted this. Exceptions would be aircraft which have an electric nose wheel steering in which case the pin is not used, the pin being deactivated by a switch in the cockpit (ATR72).

In some older aircraft like the DC-8, the drag link (scissor-like link usually seen behind the nose-gear) has a quick disconnect. When disconnected the wheels are only fixed by the central oleo and can rotate thru 360 degrees. A bit of a pain for the push-back crew when reconnecting! On modern aircraft this can be done as well for non-normal operations like in the hangar or aircraft recovery. In normal conditions the actuators limit the angle of the nose wheel.


Why is runway friction measured at 65 km/h and 95 km/h?

A range of speeds were tested at NASA's Wallops Right Facility: 20, 40, 60, and 80 MPH.

20 was deemed too slow to cover a long runway in an acceptable time, and 80 was deemed too fast to achieve (accelerate to) in a short distance -- the test vehicle needs to reach the test speed by the touchdown zone (arriving from the other end also leaves no room for stopping).

So 40 and 60 MPH were chosen, which are 65 and 95 km/h. The two speeds measure different properties. The slow one is for the macrotexture of the runway, and the fast one for the microtexture.

  A complete survey should include tests at both speeds.

  Microtexture refers to the fine scale roughness contributed by small individual aggregate particles on pavement surfaces which are not readily discernible to the eye but are apparent to the touch, i.e., the feel of fine sandpaper. Macrotexture refers to visible roughness of the pavement surface as a whole. Microtexture provides frictional properties for aircraft operating at low speeds and macrotexture provides frictional properties for aircraft operating at high speeds. Together they provide adequate frictional properties for aircraft throughout their landing/takeoff speed range.

A good macrotexture result means the tires are able to drain the water and remain in contact and not aquaplane. A good microtexture is desired for airplane speeds below 100 MPH.

I haven't mixed macro/slow and micro/fast. Testing those properties has nothing to do with the actual airplane speeds. For example, if 20 MPH wasn't deemed too slow, it would have been the better speed for the macro testing for the aquaplaning at fast speeds.

  The lower speed determines the overall macrotexture/contaminant/drainage condition of the pavement surface. The higher speed provides an indication of the condition of the surface's microtexture.

  At speeds over 100 mph, the hysteresis component of friction governs. This component is the effect of damping or reacting elastic pressure of rubber when deformed around aggregate particles. The deformation is produced best by good macrotextured surfaces.

Summary:

References:

ICAO Doc 9137 Part 2 Pavement Surface Conditions
FAA Advisory Circular 150/5320-12C


Can the debris pattern from a crash site confirm explosion as the cause of a crash?

No, not an explosion, just break-up.

The debris field can prove if some parts of the aircraft broke off in the air, before the main impact. However the aircraft can break up also if the pressure vessel fails, if intense fire compromises the structural integrity, or due to mid-air collision.

And on the other hand, an explosion can make the plane uncontrollable without breaking off any large parts e.g. if it severes the hydraulic lines (which can also start fire as the hydraulic fluid is flammable).

An explosion can only be proven by collecting pieces of the wreckage and looking for traces of spalling, shrapnel damage and traces of an explosive.


How hard is it to certify a kit aircraft if it is based on a production design like a Cessna 180?

The FAA requires all aircraft above a certain weight (>ultralight) to be certified. You can certify the design and production process so that everything that comes out of the factory has a guaranteed level of safety, or you can certify the result. The former gets a standard certificate, the latter gets a special airworthiness certificate. Other jurisdictions make similar distinctions, with minor variations.

Kit planes, such as your 180 replica, get a special airworthiness - amateur built certificate. FAA AC 20-27G outlines the process for satisfying this requirement. It is sixty pages of instructions, limitations and forms. The two most commonly mentioned requirements are for majority construction by the amateur and for flight testing.

  It is important to document the entire fabrication and assembly process from the beginning to the end, in a continuous and sequential manner. This is because, at the time of certification, the FAA is required to ascertain whether the amateur builder(s) fabricated and assembled the major portion of the aircraft. Making this finding requires adequate, sufficient, and credible documentation.

For flight testing

It does not matter to the FAA whether or not it was based on some production certified aircraft.

  We do not certify amateur-built aircraft designs or require that you modify the design before airworthiness certification. However, we may deny airworthiness certification when we inspect your aircraft if we find it does not meet the requirements for the certification you request or is not in a condition for safe operation.

This is far less stringent that the requirements for a standard certificate. You will have to decide whether you consider this "a large amount of testing".


Why the ground effect has such a different outcome for wings and cars?

It actually has exactly the same outcome! Well, in reverse.

Wings generate lift by forcing air down (one way or another, let's not go into details). When the wings are close to the ground, the air that is forced down can't get out of the way, resulting in a higher pressure.

Cars generate downforce by forcing air up. When air close to the ground is forced up, it can't be replaced by 'new' air easily. This results in a low pressure, sucking the car down.

The ground effect is thus exactly the same in either case: air that is either pushed out or pulled in, cannot be replenished quickly enough due to the ground being in the way. The difference in outcome is because a wing is attached at a positive angle w.r.t the ground, whereas a car is shaped like a very thick wing at a negative angle.

A comparison could be a hovercraft on the one hand, which is lifted off the ground almost exclusively by ground effect (if you were to drop a hovercraft from a plane, it wouldn't matter much whether it was powered up or not). On the other hand, a vacuum cleaner, which is sucked down to the floor exclusively by ground effect (indeed, Feynman's sprinkler experiment will tell you it is not sucked down because of the upwards acceleration of air - only the 'engine' part of your vacuum is forced down by momentum exchange. Test this by holding the vacuum in the air - you won't feel it 'pull' at all).


Why do the A220 cockpit seats have a cutout?

As we have seen in a previous question, the A220 is a re-branded Bombardier CSeries.

The CSeries started flying in 2013. At the time Bombardier's other commercial jet was the CRJ, that started flying in 1999, 14 years prior.

CRJs do have a yoke:

(wikimedia.org)

If you look closely, you'll notice that the seats look identical, indicating that Bombardier probably decided to cut on costs by simply re-using something they had already (and minimizing the parts they would have to support).

Moreover, this answer mentions the need to have the straps as close to the pilot as possible, and this is achieved with the notch. A redesign of the seats has probably been deemed not worthy of the costs.


Is the number of relief pilots regulated?

Pilot (and other air crew) rest time is an important factor in aviation safety. I'm not sure where you picked up that it "isn't mandated", but that is quite false. For example:

United States: 14 CFR § 91.1059 - Flight time limitations and rest requirements: One or two pilot crews.
Europe: EASA welcomes new flight time limitations rules (older article, possibly out of date)

Other aviation authorities around the world have their own regulations on how much pilots can work at once. These limits are very strongly respected and enforced.

I am not familiar with the regulations in the Asian countries referenced in the article you linked to, but I would be very surprised if they did not have similar regulations.


How is the jet engine inlet danger area affected when airborne?

Let's assume the A320 is powered by V2500 engines, one of the options for it. This engine has an air mass flow of 355 kg/s. At sea level, air density is 1.225kg per cubic m. Hence, 355 kg/s = 355/1.225 = 290 cubic m/s.

Now, let's assume the aircraft is still at sea level, but now at Mach 0.8 (unrealistic, but we'll correct that next). The fan diameter is 1.6 m, which gives an intake area of 2.0 sq.m. Also, Mn 0.8 at 0ft is 272m/s. Hence, every second, the intake sweeps a volume of 272 x 2.0 = 544 cubic m. But, the engine only needs 290 cubic/m of air, per second assuming air density in the intake is the same as the air surrounding the aircraft. Therefore, the diameter of a streamline tube of the intake air will actually be smaller than the diameter of the engine inlet. It will be the area that sweeps 290 cubic m/s at 272m/s = 290/272 = 1.07 sq. m., or a diameter of 1.17m, not 1.6m.

Basically, the engine takes the airflow it wants, not what the intake area x forward speed provides. If the engine wants more (such as when the aircraft speed is low, or stationary, but the engine rpm is high, such as at the start of the takeoff roll), the engine will draw air from a large area in front of the engine (as per the max takeoff conditions diagram). Conversely, when the aircraft is at high speed, and the engine throttled back, the intake will spill the excess air it is providing (causing spillage drag).

Now, let's correct the fact the aircraft can't do Mn 0.8 at sea level. Let's re do the calculation at 35,000ft (10,700m). Here, air density is 0.38 kg per cubic m, and pressure and temperature are 3.46 psi and  219 Kelvin (-54C), compared with 14.7psi and 288 Kelvin (15C) at sea level. Thus our 355 kg/s, which is actually a corrected airflow, is a physical (real) air flow of 95.8 kg/s at 35,000ft since theta = 219/288 = 0.76 and delta = 3.46/14.7 = 0.235. Now 95.8 kg/s at 0.38 kg/ cubic m = 252 cubic m per second. Also, Mn 0.8 at 35,000ft is now 237m/s, not 272 as it was at seal level. Hence, every seond the intake area of 2 sq.m is sweeping through 2 x 237 = 474 cubic m per second. But we only want it to sweep 252, hence we need to find the diameter of a streamline tube that will cause this, at 237m/s. Therefore, we need an area of 252/237 = 1.06 sq. m., which occurs with a diameter of 1.16m. This compares with the physical inlet diameter of 1.6m.

Thus, at Mn 0.8, 35,000ft, max power, the green area at the engine intake is now a tube, of 1.16m dia, that extends in front of the aircraft. For how far this extends, depends as suggested, on what reaction time is required for the aircraft to manoeuvre away from an obstacle in this region, or for the object (a bird?) to manoeuvre out of the aircraft's way.

The exact figure of 1.16 is questionable, as the assumption of air density in the intake not changing from the surrounding air is not entirely realistic. But the general result, is I believe, reasonable.


Does an aircraft carrier have a VOR onboard?

Aircraft carriers use a TACAN, which stands for Tactical Aid to Navigation.  (The Navy is really good at making acronyms that don't follow normal conventions for abbreviation)

Functionally the TACAN is like a VOR with DME, providing bearing to and from the ship, as well as range.  The system is not susceptible to rocking motion, and least not perceptibly.  (and if you are already familiar with VORs, a VORTAC is a landbased co-located VOR and TACAN station.)

And true, an enemy could pick up the signal.  For this reason carrier airwings need to prove themselves proficient in EMCON (Emission Controlled) launch and recovery operations before being certified ready for deployment.  During an EMCON recovery crews are told where to expect the ship, what the expected recovery course will be, and  will be given a marshalling lat/long.

At the appointed time, and with no radio communication, formation flights will peel off the bottom of the holding stack, drop to break altitude,  head towards where the ship should be, and sequence themselves in with traffic in the pattern.  It is a pretty amazing thing when it works.


What should a pilot do in this lost communication scenario?

It means to expect radar vectors between PHX and DRAKE because ATC expects to have to move you around to accommodate the traffic situation.  If you had a comm failure, once you squawk 7600, ATC will expect you to find your way between PHX and DRAKE by the most logical path.

In this case, because you had filed to fly along V105 you would take that route and that's what the controller would expect.  The controller would quickly figure out what you are doing as you proceed down V105 and would be able to route other traffic to maintain separation to accommodate your track.

The only thing that would create a problem is if you headed off in some direction that made no sense to the controller once at PHX.


Is there a proper term to use when you're referring to the total number of people on an aircraft, including both the passengers and crew/pilots?

CAP 419 (the UK CAA radio telephony manual, see the glossary) uses 'POB' which stands for '(total) persons on board'


How is Altitude/QNH defined state when passing transition altitude?

In Canada at least, you are supposed to make the change between QNH (alt setting) and QNE (standard pressure) while in QNE airspace, whether climbing or descending; that is, right after passing the transition altitude while climbing, or just before passing transition altitude while descending.  The variable overlap zone you describe resulting from the change in barometric pressure is not critical since you are almost always just passing through on a climb or descent clearance to an altitude a couple thousand feet or more above or below. ATC sees your altitude in 100 ft increments as broadcast by your transponder, computed baro alt (transponder's pressure alt signal corrected for local baro pressure) when below the transition, then pressure alt when above, but they don't know or care what you are seeing on your altimeter while climbing through the transition as long as you level off where you are supposed to be.
So I would expect to see the altimeter setting change shortly after passing transition altitude when climbing, say within the next 500 to 1000 ft, but the pilot doesn't have to work out exactly what QNH altitude to observe to be able to make the switch right at transition alt.  Shortly following is good enough and if you are in a climb to, say, FL100, if you forget to set to 1013.25 until the TA is 8000 ft, it's not that big a deal (nobody's going to know - except for the Flight Data Recorder, and your pilot monitor nanny software that is... lol). All that matters is that you level off at your cleared FL. ATC only sees your FL, or FL corrected internally to baro, on radar so the controller isn't interested in where you switch exactly while in a climb as long as you level at the correct FL, although you are supposed to do it in a timely way.
On a descent, with a TA of FL50, I would expect to see the change from QNE to QNH while still above the TFL, say between FL50 and 60.  Even there, if you forgot to set to QNH until below FL50, if you are descending to a lower altitude, it's not critical as long as you level off at the cleared QNH altitude, monitoring software notwithstanding.
Note that Europe may have variations on this protocol, since in North America the transition level is FL180 everywhere except the high arctic (called the Standard Pressure Region in Canada), where the FLs start at the surface except where near airports.
This is the Canadian regulation on moving between Standard Pressure and Altimeter Setting zones:

Transition – CAR 602.37 – Altimeter Setting and Operating Procedures in Transition between Regions, specifies that except as otherwise authorized by ATC, aircraft progressing from one region to another shall make the change in the altimeter setting while within the standard pressure region prior to entering, or after leaving, the altimeter setting region. If the transition is to be made into the altimeter setting region while in level cruising flight, the pilot should obtain the current altimeter setting from the nearest station along the route of flight as far as practical before reaching the point at which the transition is to be made. When climbing from the altimeter setting region into the standard pressure region, pilots shall set their altimeters to standard pressure (29.92 inches of mercury or 1013.2 mbs) immediately after entering the standard pressure region. When descending into the altimeter setting region, pilots shall set their altimeters to the appropriate station altimeter setting immediately prior to descending into the altimeter setting region. Normally, the pilot will receive the appropriate altimeter setting as part of the ATC clearance prior to descent. If it is not incorporated in the clearance, it should be requested by the pilot.


How do you know ELT's batteries used more than 50% of their capacity?

There are two ways:

First, when you test the ELT you should make a note in the aircraft logbook of when it was tested and the results of that test. You can also note how long the transmitter was on during the test. You can then look at all the logbook entries to verify that the cumulative time is less than one hour.
Some ELT's have a self-test feature that presents error codes via an LED or audible beep when they are powered on. For example, here is the self-test page from the Artex 345:

The ELT will beep/flash 7 times when the battery operating time is >1 hour.

As far as the useful life of the battery goes, that is determined during the manufacturer testing for the ELT and will be marked on the ELT as an expiration date. Here is an example:

Source: Aerotechservices.com


Why does aviation use Zulu time instead of the local time?

Zulu means the letter Z in radio communication. The letter Z designates UTC time.

There is actually a list of time zones for each letter of the alphabet (except J):

Alpha time zone: UTC + 1
Bravo time zone: UTC + 2
...
India time zone: UTC + 9
Kilo time zone: UTC + 10
Lima time zone: UTC + 11
Mike time zone: UTC + 12
November time zone: UTC - 1
...
X-ray time zone: UTC - 11
Yankee time zone: UTC - 12
Zulu time zone: UTC + 0

However, the other time zones are seldom referenced in aviation.

Why do we use UTC time instead of local time?

Because flights often cross time zones.

Imagine this: the time now is 11:59. We will takeoff at 12:30 and fly west. Afterwards, at around 11:48, we will be 15 nautical miles away from the airport.....................Huh?

Even more confusing: We takeoff at 12:30 and land at 13:00 ==> we only need 30 minutes of fuel. Wrong! We actually cross a time zone and the actual flight time is 90 minutes. But we've taken off with 60 minutes less fuel than we need. What do we do?


Would it be a benefit to install winglets on a Hawker 400XP?

Hawker 400XP with winglets.

  HAWKER WINGLET BENEFITS

  3-4% range increase and precise handling characteristics
  9% reduction in time to climb
  Fast cruise speeds at same fuel flows
  Increases resale value

  — txtav.com

Related: Is a winglet better than an equal span extension?


Is it possible for a fighter jet to shoot itself down with an IR missile?

Not really, it would take any of these missiles so long to turn 235 degrees (way more than 6-10 secs), which is generally the maximum burn time of most of their rocket engines. Plus, if it did somehow, get turned around 235 degrees, the energy (airspeed) bleed from such a large turn would leave it without any energy to intercept the launch aircraft even if by some miracle it acquired an IR lock


What are the Autopilot Minimums for a C-172 G1000?

Typically, the aircraft flight manual will identify this. In your particular case (Cessna 172 G1000), it is 200 feet AGL for all approach types.

The C-172 can be fitted with either a Garmin GFC 700, or a Bendix/King KAP 140.

For the GFC 700, it is listed in the aircraft operating limitations:

In the case of the KAP 140, it is listed in the autopilot supplement:


Who manufactures the landing gear on the Airbus A350 series aircraft?

The landing gears are manufactured by Messier-Dowty, a Safran group company. According to their site,

  Safran Landing Systems is responsible for the design, development, qualification testing, manufacture and support of the main landing gear for the A350-900 aircraft.

Also,

  Safran Landing Systems also provides wheel and carbon brakes for the A350, as well as the landing gear extension-retraction system, monitoring system, steering system and braking system.

The landing gear design does use large amounts of titanium:

  The A350-900 main landing gear ... includes the increased use of advanced materials, in particular high strength titanium, corrosion resistant steels
  and HVOF coatings.

A350 landing gear materials, from A350 XWB family & technologies


How do you choose CAT II vs. CATIII ILS?

If both are available for the runway you're using, it's often easiest just to set up the Cat III, because you have the least risk that way of a random cloud between the Cat II DA and the Cat III DA interfering with your approach.

That said, there may be reasons to prefer the slightly higher mins of the Cat II, and those are typically specific to your equipment and your procedures.  Perhaps crosswinds, either steady or gusts, make the Cat II more desirable, or if your equipment has a limitation on "XYZ must be operative for Cat III approaches" and that happens to be deferred on MEL for you, then the Cat II is the better (only) option.  But generally, if both are available, setting up for the Cat III is usually preferred unless some specific factor drives you to the Cat II.

Separate from this, some runways only have a Cat II approach and not a Cat III available.  This is typically driven by runway lighting differences, although other equipment may drive this as well (specifically, a backup Glideslope transmitter, IIRC).

As much as I hesitate to answer "always," I can't think of any Cat IIIA DA that's other than 50' RA, and I can't think of a reason it would be, since you're over the runway itself by that point.  Cat II DA's can be slightly above or below 100' RA since the terrain you're over when you reach 100' above the TDZ (on glideslope) may be slightly above or below the runway elevation, but that's a Cat II thing, not a Cat IIIA deal.


What is the relationship between the design lift coefficient and the section lift coefficient of a NACA5 airfoil?

The design lift coefficient determines the camber line of the airfoil. It is chosen such that the flow hits the airfoil exactly parallel to the start of the camber line when at the design lift coefficient. For a NACA 23015 this would be a lift coefficient of 0.15 x 2 = 0.3. The index i only denotes that this was held to be the ideal lift coefficient.
At this ideal lift coefficient the highest local curvature of the airfoil contour is right at the stagnation point and the pressure distribution downstream from there is smooth without the suction peaks which occur when the high curvature of the leading edge has to be negotiated by the upper (higher lift coefficient) or lower (smaller lift coefficient) side flow.


What happens to an airborne plane when its type is grounded by an authority?

You can find the full wording of the EASA grounding order here which does not specify a course of action for planes mid air but does call for the immediate grounding at the time the order goes into effect (March 12th 2019 @ 19:00Z)

  For the reasons described above, pending the availability of more
  information, EASA has decided to suspend all flight operations of the
  two affected models.

  This AD is considered an interim action and
  further AD action is expected to follow.

The AD points to sections 301 and 303 of the EASA regulations which does not seem to shed much more light on the situation other than;

  Any applicable airworthiness directive must be carried out within the
  requirements of that airworthiness directive, unless otherwise
  specified by the Agency.

Which points us back to the AD, which again offers no specific instructions on immediate grounding.

However according to FlightRadar24's Twitter they are seeing some diversions or at least possible course changes. As people point out in the comments its possible the airlines are diverting the aircraft to airports they can better provide support/maintenance at.


Definitions always depend on their context, that is why documents often included a list of definitions. It allows authors to define terms in the context of the document. A definition context is not always a document, it could for example be a field of expertise as well.

The Commercial Aviation Safety Team / International Civil Aviation Organization (CAST/ICAO) Common Taxonomy Team (CICTT) taxi is defined as:

  The aircraft is moving on the aerodrome surface under its own power
  prior to takeoff or  after landing

More detail can be found in the phase of flight definition document.
This is definition can be applied in many situations.

The Eurocontrol definition you link to comes from the Air Traffic Flow & Capacity  Management (ATFCM) and Airport Collaborative Decision Making (A-CDM) domains where statistics about gate-to-runway time play an important role for planning purposes.

In that context it is not really interesting to distinguish the time that was spend in pushback from the time spend actually taxiing to the runway. The taxi-out time is hence defined as the time between off-blocks and take-off.


Book recommendations for learning how to fly?

As you pointed out in your question, no book is a substitute for actual instruction in an aircraft with a flight instructor. That being said, doing your homework and knowing what your instructor is going to expect goes a long way toward completing your training in a timely manner.
The books here are all great reference texts to be combined with a flight instruction curriculum.

FAA Publications / Books for initial training

A surprising number of new student pilots are not aware of the wealth of information the FAA has compiled and published to help people learn to fly. There is a whole series of FAA Handbooks and Manuals that you can take advantage of which are available from the FAA as well as from a variety of publishers like ASA. While all of the regulatory information is geared toward US/FAA regulations much of the technique information, signs & markings, etc. will be universal, in conformance with ICAO rules.
I list a whole bunch of them here, but you can find a wealth of information by googling what you want to do and the words "FAA Handbook".

Books on Flying the aircraft - maneuvers, etc.
Quite simply if it flies the FAA probably has a book about how you should fly it.  These books cover basic flying technique and the maneuvers the FAA expects pilots to be able to demonstrate.
The popular ones include:
The Airplane Flying Handbook (FAA-H-8083-3A)
The Helicopter Flying Handbook (FAA-H-8083-21A)
The Glider Flying Handbook (FAA-H-8083-13A)
The Balloon Flying Handbook (FAA-H-8083-11A)
Books on "essential aviation knowledge" - Airport signs & markings, "rules of the air", etc.
The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A) is the starting point here.
The Aeronautical Information Manual (AIM) is another key resource (you will often find this as a combined "FAR/AIM" printed publication).
Books for the more advanced stuff - Instrument ratings and beyond
The Instrument Flying Handbook (FAA-H-8083-15B) and Instrument Procedures Handbook (FAA-H-8083-16) are excellent references for pilots pursuing an instrument rating.


Do modern aircrafts like 747/787/A320/A380 have some Plug and Play subsystems/sensors?

It wouldn't make much sense.

By definition "plug and play" would means that the avionics automatically and silently adapts to whether whether the optional device is present or not.

This would mean that if the device is physically fitted, but not in contact with the main avionics -- say, a wire might have broken or shaken loose recently -- then the cockpit instrumentation would look exactly as if it had never been there. This is not desirable: when something the pilot thinks will be available for the flight isn't actually available you want to tell him explicitly so he can decide whether the problem needs to be planned around or reacted to.

Of course there are limits to how much the plane can itself verify that it is whole and in working order -- that's why there are preflight checklists. But to spend engineering effort specifically on allowing it to fail silently when communication with one of the sensors is lost, that would be counterproductive.


I have an idea about how to resolve a conflict between the pilots and an automated flight control system. Does it make sense?

You actually bring up an interesting point.
The force gradients in the elevator system in a non FBW airplane are set by  some form of bungee system, like springs, that you are pulling against.  The hydraulics drive the control surfaces and there is no actual tactile feel, beyond what the feel force system imposes (think of fully hydraulic power steering in a big earth mover where there is no mechanical feedback to the steering wheel other than some form of spring centering unit).
The feel force is typically set to increase to about 50lbs at full travel, less at lower speeds and with flaps out, maybe 30lbs.  So the "two hand pull" required to get full elevator is there by design.  The force gradient is necessary to give the pilot a sense of how hard to pull and tries to approximate the forces on an airplane with mechanical controls.
You could easily design the feel force unit in the airplane to reduce the spring load to make the stick lighter for whatever purpose you want, and modern airplanes with the newer flight data recorders already have force sensors in the control linkage to provide data to the Flight Data Recorder, so the hardware is already there.
Problem is, the force gradient is needed to keep from overcontrolling, and either overstressing the airframe when above "maneuvering speed" or going into an accelerated stall when below it, from making large control inputs.
Where that kind of feature could be useful however, is during stab trim runaways, especially runaways to full nose down, and what happened in the MAX events, which was more or less similar to a ND stab runaway (the MAX's inadequate MCAS architecture got away with it by taking credit for the existing stab runaway procedure as a mitigation).
I've done those sorts of runaway scenarios in the simulator during recurrent training for the CRJs and the full nose down runaway leaves you wishing the feel force could be reduced as you have to continuously hold 20 or 30 or 40 pounds to keep the nose up.  It's very unpleasant (in the RJ, the fix was to get the flaps out one way or another, which drastically reduced the required feel force thanks to the pitching moment of the flaps reducing trim speed,  plus the feel force reduction that results from flap extension).
To implement what you suggest, you would have to have the feel force system set up to respond to an uncontrolled stab trim runaway by reducing the feel forces.  I'm not aware of anyone who has done this, and certification would probably be quite difficult (there will be many many unintended consequences that have to be ironed out, such as how to get the system to decide what a legitimate runaway is in a reliable way, and then how to get around the basic feel force requirements in a temporary way).  The certification hurdles are the likely reason this concept has never been done.


What to be considered in designing airplane tail (vertical and horizontal stabilizer, elevators and rudder)?

The measurement of overall tail authority for stability and control (a function of tail surface area and its leverage or arm) is called Tail Volume and the link goes to a handy calculator page for working it out.


Which means, roughly 5 times more passengers can be accommodated by compromising half of the top speed. So, why does speed loses out so much to capacity that it ultimately becomes unfeasible to operate a supersonic airliner?

I think you are asking, "why do supersonic aircraft have such low capacity"?

To travel faster than the speed of sound you need to have a very sharp nose and minimal area exposed to the forward direction, to minimize the generation of shockwaves. Shockwaves are bad for people on the ground (the 'sonic boom' effect) and they are terrible for fuel efficiency. Second you really do not want to trap air on flat surfaces facing forwards, because the air will get squashed in the supersonic flow and heat up a lot (like when you blow up a bicycle tyre or a football it is warm afterwards, or when the space shuttle re-enters the atmosphere). Again this is an enormous drag and having hot air flowing around your aircraft causes problems with the structure (Concorde used to grow by 300 mm during flight because of the heat).

Since you cannot be wide and you cannot be tall (without being aerodynamically inefficient), you need to be very long if you want lots of people. And then you have other problems.

The design and construction of Concorde was never a commercial project, it was funded by two governments almost as a vanity project, as a prototype or as an experiment. Then it was given for free to two airlines to use as they wanted.

So unlike the A380 and the B747, no one has ever said "now we try to maximize the economics and the capacity of our design": that never happened. All they ever asked was "How do we make this work!?"

Whereas the subsonic aircraft get a lot of time, money and thought spent on how to make them more economical to use by filling them with more passengers.

For more information I recommend this interview with a former Concorde pilot: http://omegataupodcast.net/2015/02/166-flying-the-concorde/

  Also, Concorde was developed 40 years ago. Are some more efficient supersonic Airliners possible today?

I am sure that these days a far more fuel efficient supersonic engine could be built for passenger operations. And I do believe there would be a market for it on say LON-NYC or even maybe LON-DXB. But designing supersonic aircraft is quite different to designing subsonic ones; and then you have all the difficulty of modern safety regulations without being able to demonstrate you are relying on old, proven designs; and then there are very few routes where it could work profitably so you have very high development and maintenance costs spread over a small number of airframes.

Basically, it isn't worth the risk to anyone to make it.


Indeed, when an aileron moves upward, it locally generates less lift and less drag.

Assume we are talking about the aileron on the right wing.
The reduced lift drops that wing, rolling the aircraft right. But the reduced drag accelerates that wing, yawing the aircraft left.

On the left side the opposite happens, but it has the same effect. The aileron moves down, increasing lift to roll the aircraft right. But the added lift increases drag, decelerating the left wing to yaw the aircraft left.

That is the adverse yaw effect.


Yes, that is indeed a C-17. As to why it's in the area, only the Air Force would know.

https://www.boeing.com/defense/c-17-globemaster-iii/#/overview


How exactly did engineers come to the final design of jets like the F-16 or SR-71?

The short answer is that if we can program it into a computer then we can do it manually, it just takes a lot longer.
Back in the day, a "computer" was a clerical professional or mathematician and with paper, pencil, log tables, slide rule and the like on their desk. The data centre of the day was a large hall filled with rows of desks with a computer working industriously at each one, each carrying out a precisely specified routine. Supervisors collected their calculations and passed them on to the next calculator in the chain, who carried out the next routine. It could take a couple of years to fully calculate out a large airframe for production.
The mechanical desktop calculator looked rather like a small old-style cash register and made their lives a lot easier. Electronic desktop calculators raised their game another step.
The computer was one of the first professions to be wholly usurped by the electronics revolution. Programmers long afterwards talked of writing their code in a modular fashion, as routines and sub-routines.
Nowadays we can figure out the basic airframe in a few weeks at worst, while the wind tunnel models are being 3D printed, and iterate until we achieve joy unbounded. However design for manufacturing has become immensely more sophisticated, not to mention standards-bound, and the timescale has if anything increased.
But all computers suffer from one particular phenomenon, known in the trade as GIGO - Garbage In, Garbage Out. You really do need a competent and experienced designer at the top of the food chain. A GA designer turning their hand to a warplane has a steep learning curve; the likes of SR-71 design lead Kelly Johnson do not turn up still wet behind the ears. (It was not always so; UK company de Havilland badgered the Air Ministry for years, were fobbed off and insulted at every turn for being mere civilian designers, so they built their brainchild as a private venture and called it the Mosquito....)


Could an electric engine provide the same performance as jet engines on current aircraft?

Not yet.

To look at a medium-range aircraft, let's base the engine on the CFM56 or the IAE V2500. Those engines produce between 100 and 150 kN static thrust. In cruise, their thrust is considerably lower due to the low density in cruise altitude and due to them moving at Mach 0.8. Let's use a value of 25 kN - this is enough so that two of them will comfortably push an A320 class airframe through the thin air at altitude.

The power to produce such an amount of thrust is force times speed. The speed when flying with Mach 0.8 in 35.000 ft is 240 m/s, so the power produced by one engine is 6.0 MW. Now let's look up how big and heavy an electric motor has to be to produce 6 MW continuously. As you can see from the linked Wikipedia page, the results are all over the place. Big, industrial motors come in at less than 1 kW/kg, so our motor would weigh more than 6 tons. The smaller motors for electric airplanes are pushing 10 kW/kg, the power-to-weight ratio of the GE90 turbofan, but will lose some of that when scaled up to size. Remember, even at 98% efficiency the motor will generate 120 kW of heat - this needs to be removed, and operating in thin air doesn't make this easy.

With current technology the motor could achieve maybe 2 to 3 kW/kg - this means the motor driving our hypothetical engine comes in at 2 to 3 tons. Add to this the fan and the fairing of the jet engine (we will not need the high-pressure part and all the turbines), but double the fan weight because we need to compensate for the missing high-energy core flow. This will maybe weigh 50% of the CFM56 / V2500, so we need to add another 1.2 tons.

The electric motor will be twice as heavy as the parts it replaces. There is still some work to do before it can gain an advantage over current jet engines, but it has some potential because it does not dump half of the energy supplied to it overboard in a hot, fast-moving, noisy gas stream.

EDIT:

Since so many people get excited about me omitting the energy density aspect of electric propulsion, even though the question did expressly desire to leave this out, here are two things to consider. Energy density is only half of the problem of electric storage.

The energy density of jet fuel is around 43 MJ/kg, while Lithium polymer batteries achieve not even one MJ/kg. But this comparison is linear thinking - realistically, the current will be produced either by a high-efficiency turbine-generator combination, or by fuel cells, burning hydrogen at twice the efficiency of a conventional jet engine. Since hydrogen packs 142 MJ per kilogram, at twice the efficiency the electric airliner would need only 162 kg of hydrogen for every ton of kerosene in a conventional jet. Yes, I know, even then its volume will still be a problem.
If any form of batteries is used, the fact that empty batteries weigh as much as full ones is the final nail in the coffin of battery-powered flight. While your average long-range jet lands at 60% of its take-off weight, the battery-powered jet would need to schlepp those heavy batteries all the way to the final destination. To be competitive, those hypothetical batteries would need to have twice the energy density of kerosene.


In an unpressurized cabin, why is the cockpit ambient pressure lower than the outside pressure?

The air moves around the fuselage, which causes a localized increase in velocity around the aircraft.  This increase in velocity causes a slight decrease in the outside air pressure, which effectively sucks air out of the fuselage, making the cabin at a slightly higher pressure altitude.

The effect is localized.

While the reduction in pressure, and increase in pressure altitude inside the cabin is small, many GA aircraft display a 40 foot or so difference in pressure altitude between the alternate static port (inside the aircraft) and the pressure altitude with the factory static port.


Did WWII fighters have inverted airfoils for horizontal stabilizers?

From the Longitudinal Stability (Pitching) chapter in the FAA Pilot's Handbook, two things stand out to me:
One:

Regardless of the airfoil shape, nose-heavy airplanes like WWII fighters will almost always have downforce on the horizontal stabilizer.
Even if the airfoil is not inverted, it will be attached at a negative angle of attack.

Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative angle of attack.

Cutaway drawings for the P-47 seem to suggest they're not inverted, example:
(Source)
Two:
Unless it's a T-tail, the propwash combined with the downwash results in downforce on the horizontal stabilizer.
(Source)

P.S. Do not rush to accept, wait for votes or wait for our resident aerodynamicists to answer.


When does the mandatory minimum climb gradient during an Instrument Departure Procedure end?

I believe that now I am able to provide an answer to my own question.

I think that the "minimum of 200 ft/NM" rule applies during the whole climb phase. In other words, until the end of the SID, where either the enroute phase begins or the SID itself provides some enroute transitions as a passageway to the enroute phase.

FAA's TERPS manual clearly states:

  Chapter 2, "Sloping OCS" 2-1-4b. The concept of providing obstacle clearance in the climb segment of an IFP is based on the aircraft maintaining a minimum climb gradient. [...] The minimum climb gradient that will provide adequate ROC in the climb segment is 200 ft/nautical mile (NM), unless a higher gradient is specified.

In addition, the rule is also repeated in the PBN manual:

  Chapter 1, "Common Criteria" 1-3-1g(2). Climb Gradient (CG). Obstacle clearance in a climb segment is dependent on the aircraft maintaining a minimum CG to a specified altitude. The minimum CG (also referred to as standard CG) is 200 ft/NM.


How meaningful is the total number of flight hours?

As far as what hours are logged, there are a couple of different ways to do the counting.  On a light aircraft, time on the Hobbes meter is generally used, which equates to "engine running" time.  In the military, it's takeoff to landing, usually plus a small amount for taxi time (which may or may not account for all the time that the pilot is actually operating the aircraft).  In the airlines, it's "block to block" time, so pushing back from the gate until stopping at the gate.

As far as the time all being equal, it isn't.  Not even close.  2000 hours flown from Big Airport A to Big Airport B, 8 hours at a shot, means about 125 takeoffs, approaches, and landings flown, and an equal number monitored.  Almost all of those are radar vectors to an ILS approach -- very standard & predictable.  Five hundred hours as a pilot training instructor probably translates into about 500 landings performed, and many, many more supervised, with a great variety of approaches flown (visual, precision, non-precision, practice emergency patterns, etc)  The "hands on" time on the controls of a military fighter pilot in 500 hours logged is far greater than the hands on time during 2000 hour trans-Atlantic airline flying.

The problem is, "hours" is the currency that we have, and so that's how experience is measured.  Not because it's a great measure, but because anything more granular becomes very complicated and subjective.  Does my 1000 hours flying Lear jets count for more than your 1200 hours flying a King Air?  One's a jet, the other a turboprop.  Did one of us fly 3-hour legs mostly on autopilot on clear days, while the other flew 30 minutes at a time, much by hand, with lots of instrument approaches in bad weather?  Who flew with veteran copilots, and who flew with copilots brand new in the airplane?  Who flew to lots of unfamiliar destinations, and who flew the same canned route(s) day after day?  And how does any of that compare with 3000 hours in a C-5?  Or 800 hours in a crop duster?

All of this to say, that "hours is hours", while it overlooks a LOT of very relevant distinctions between some very diverse types of flying, is still the quick and easy comparison between pilots.  Not because it's wonderfully accurate, but because it's possible (we all log hours, after all), and the more detailed comparisons quickly become hard to objectively make.


Are you required to report leaving an altitude if you have been given a crossing restriction?

I’ve been a controller in both the en route and terminal environments. While I agree with your interpretation of the AIM that it appears to be required, reorting leaving your altitude is not expected and I did not usually hear it done.

To give the perspective from the other side of the mic, an altitude crossing restriction is functionally equivalent to a pilot’s discretion climb/descent to that point. ATC will protect all altitudes between your current one and the restriction, and you are free to hit that restriction as you see fit (barring the violation of other regulations, of course). You may not return to a previously vacated altitude as part of your climb/descent, but you may level.

You are also correct in that it can tie up the freq as an “unnecessary” call, but if you are unsure, better to over inform rather than under inform.

Incidentally, very few controllers read the AIM. The 7110.65 takes precedence, and I’ve actually had a supervisor yell at me for referring to the AIM once. I know, I know.

So, legally, it should be done.  In practice? Unnecessary and I preferred that it not be done when I was working.


What is the maximum age limit for getting a pilot licence?

As far as I know there are no maximum age restrictions on getting a pilots license anywhere in the world. Many countries have a minimum age restriction for getting a license, that usually does not prevent pilot training from taking place. 26 is fine, in fact 76 is fine as long as you pass whatever medical is required.


Why does directional stability decrease at supersonic speeds?

It is a combination of several effects:

Aeroelasticity: With the higher forces at high speed, the structure deforms such that the effective flow angle at the tail surface is reduced.
The supersonic lift curve slope of the tail surface decreases with Mach while that of the fuselage stays roughly constant, so with higher Mach the tail contribution to stability shrinks while the destabilizing contribution of the fuselage stays the same.
Tail location: At supersonic speed the density of air on the upper fuselage is reduced while that on the bottom is increased. Since the vertical sits on top of the fuselage, it flies in rarefied air. Ventral fins, on the other hand, become more effective.

The supersonic lift curve slope is approximately $$c_{L\alpha}=\frac{4}{\sqrt{Ma^2-1}}\cdot\left(1-\frac{\lambda}{2\cdot AR\cdot\sqrt{Ma^2-1}}\right)$$
Note that the second term is for the influence of the triangular surface on the tip where the pressure difference is reduced. Nevertheless, it shows that lift curve slope of the vertical decreases with increasing Mach number.

For the fuselage, the lift curve slope is that of a slender body:$$c_{L\alpha}=\pi\cdot\frac{AR}{2}$$
Note that this equation is independent of Mach, so the destabilizing fuselage contribution stays constant over Mach.

For the beneficial influence of ventral fins see the diagram below which has been published by Lockheed in The F-104 Strafighter Test Pilot's Notebook by Glenn L. Reaves (source):


How does a linear acuator in flight control work?

There are several types of linear actuators. The main three types are Pneumatic, Lead Screw Driven, and Hydraulic Linear actuators. In addition there are also solenoids and stuff but the three main types are used in heavy mechanical applications.
The one in the picture looks like a Hydraulic Linear actuator. The way hydraulics work is that the pressurized non-compressible liquid is forced into a cylinder using a pump. The pressurized liquid then moves a sliding wall inside of the cylinder. The sliding wall is also connected to the main shaft. When the liquid is pushed from one side, the sliding wall inside of the cylinder moves as the liquid pushes onto it, thus moving the main rod in and outside depending on the configuration.
Hydraulic linear actuators are used for mechanisms that reqiuire precision and high force / torque. İn this case, we want the rudder to move on a hinge that is placed on the root of the rudder. The linear actuator is placed on an lever that is offset compared to the hinge. Then, when the linear actuator moves, the linear movement converts to rotational movement and rotates the rudder surface.
Here is an image to help.


How does the range of a single- and twin-engine for small aircraft compare?

(Cessna 185 Skywagon and Cessna 340)

See here for the major points: What are the pros and cons of single-engine vs. twin-engine? Regarding your specific question:

Wouldn’t a single engine plane weigh less and thus can fly longer on a specific amount of fuel comparative to a twin engine plane?

Both airplanes above are powered by the same 300 hp engine, so yes the twin will be heavier. But it can also lift more weight (i.e. more fuel), and cruise at a higher speed for the same power setting, or cruise at the same speed for lower power setting.

Either the higher speed or lower power, combined with the more fuel, gives the twin a longer range.

In fact, double. 2,603 km compared to 1,333 km.

If you wanted to compare the twin to a single with a 600 hp engine, then there is no weight advantage. Only the bigger engine is harder to maintain.

On a technical note, weight is not a big issue in cruise. The power to overcome the induced drag from the lift (which counters the weight), decreases with speed, i.e., it takes less power to create lift in cruise, than at slow speeds. (See graph below for the induced drag.)

Same fuel

Twins are designed to fly on one engine, which means half the power potential is enough, which is helped by the bigger span wing. So the same fuel load will give the same or more range for the twin. Unless, you want 600 hp vs 2x300 hp, in that case the single will win because of its smaller form.

Some numbers:

For the two planes above, here are the gallons-per-hour and cruise speeds:

             gph             cruise speed kts
Twin         26.0-34.0       202-229
Single       11.7-15.5       122-140

As you can see, the higher fuel consumption is offset nicely by the speed, and note that the twin will reach the cruise altitude in less time as well.

Sources: Cessna 185, Cessna 340

If we compare a twin-piston to a single-turbine then it won't be an apples-to-apples comparison, but the single-turbine plane will be faster, and will have longer range, but that's mainly due to the different engine types (combined with different cruise speeds). For the Cessna Caravan, the turboprop engine (minus the accessories) weighs as half as a single 300 hp piston engine, which can allow for 200-400 kg more fuel, but the turboprop engines are very, very expensive. So depending on the operation, a fuel saving here might be overdone by the initial cost.

(wikimedia.org)


Is it possible to start another engine of Mil Mi 8 with one engine started and APU off?

From that I see in the quite detailed description of the helicopter systems in Russian Wikipedia, looks like both engines have independent electric starters (starters-generators) so could be started independently.

There is no mentioning of APU in the description of all systems. They say power plant consists of the two engines. However as mentioned in comments, the helicopter does have the control panel for starting the "Турбоагрегат". It is generally a name for the device that combines turbine engine and electricity generator. APU is called "Вспомогательная силовая установка (ВСУ)" in Russian.


Why does the QNH of an aircraft change or need to be changed?

The QNH is the reference pressure of the aerodrome, not the aircraft. It is calculated such that an altimeter, which assumes ISA conditions otherwise, shows the correct altitude above mean sea level at the aerodrome:

  Airfield QNH is obtained by correcting a measured QFE to sea level using ISA regardless of the temperature structure of the atmosphere. As your altimeter is calibrated using ISA, it will indicate altitude correctly at the airfield reference point. At other altitudes, the indicated altitude is likely to be in error, depending on the temperature of the atmosphere.

(skybrary.aero)

The altimeter uses the following formula to compute altitude:

$$ h = c T \log \left( \frac{P_0}{P} \right) $$

Here, $c$ is a constant, $T$ is the temperature (which is assumed to be the ISA temperature, not the actual temperature here) and $P_0$ is the QNH reference pressure. The altitude $h$ is then exactly correct at the aerodrome and also quite accurate above if the actual temperature does not deviate from ISA too much.

The QNH value is typically obtained from the ATIS or from ATC. It is given in units of hectopascals (average value of 1013 hPa) or inches of mercury (average value of 29.92 inHg).

The QNH changes whenever the pressure at the aerodrome changes because the altimeter is using the pressure to determine altitude. These pressure changes are a normal part of changes in weather. You can see an example surface pressure map of the US here:

(image source: Wikimedia)

Note that the pressure values (in hPa) on this map are not the QNH values. These are the current actual surface pressures.


How many Angle of Attack sensor vanes should a commercial passenger airliner have?

This depends very much on what the AOA sensors are used for.

Traditional airplanes

Traditionally, AOA sensors are only used for stall warning and stall barrier (i.e. stick pusher). In this case, you can get away with only two AOA sensors. A sensible design would trigger the pilot/copilot side stall warning with the onside (or opposite side) AOA sensor, which would confer the largest availability on stall warning. This is sensible because traditional stall warning does not effect flight controls and a nuissance stall warning can be inhibited manually by the flight crew.

Stick pusher, on the other hand, should require more prudence. By regulation, the activation of the stick pusher must be beyond the stall warning activation; and the airplane is designed to be stall-free if appropriate actions are taken from the recognition of stall warning activation. Therefore, the use of stick pusher is beyond the operational envelope of the airplane. A sensible design would be to require both AOA sensors to be functional and agree for the stick pusher to activate.

Fly-by-wire airplanes

For FBW or even partial FBW airplanes, AOA information is often directly in the loop of flight controls for gain scheduling and envelope protection (e.g. MCAS). This becomes a completely different problem. By regulation, continued safe flight and landing must be possible without exceptional pilot skills following any single point failure in any flight control related system.

This would require at least three AOA sensors for these airplanes. It's not enough to know that the AOA measurements disagree, but it is also necessary to detect which measurement is erroneous. With three sensors, if a single one fails and disagrees with the other two, you can shut off the failed sensor and continue flying as if nothing happened. Of course, the airplane would not be dispatchable until that failed sensor has been serviced once landed.

Some airplanes (e.g. A220) have four primary AOA sensors. But this is more for dispatch availability (MMEL) than for safety. Note that the A220 has two redundant AOA vanes on-top of the primary AOA sensors (SmartProbes) for Direct Mode operation.


Is it possible to fly with throttles staggered?

Yes it is possible - as mentioned in a comment, most multi-engine aircraft can (fairly) happily fly with one engine at idle or disabled entirely.

Some autopilots will use some asymmetric thrust for yaw trim, and others will use asymmetric throttle settings to compensate for the engines producing different amounts of thrust. Those sound similar, but aren't quite the same thing.

For example here's a description of that happening on the 777. Neither are ubiquitous and both depend on the aircraft.

http://www.flight.org/the-boeing-777-thrust-asymmetry-compensation-tac

Note that two engines can never be identical, there will always be some level of asymmetric thrust. Traditionally this was countered with rudder trim, but newer aircraft sometimes adjust the throttle instead, as per the above link


How many hydraulic systems does the Boeing 757 series have?

The Boeing 757 has three fully independent hydraulic systems. The three systems are "Left," "Center," and "Right."

The left system is powered by an engine driven pump (EDP) on the left hand engine, an alternating circuit motor pump (ACMP) located in the wheel well, and a power transfer unit (PTU) powered by the right hydraulic system.
The center system is powered by two ACMPs located just aft of the wheel well in the wing to body fairings, the center system can also be powered by a ram air turbine in an emergency.
The right system is powered by an EDP on the right hand engine and an ACMP located in the wheel well.

All three systems operate at a nominal pressure of 3000 psi.

Each system has its own reservoir. The reservoirs are air-on-oil type reservoirs and use engine bleed air to pressurize the fluid to provide proper head pressure for the pumps. The left and right reservoirs have a full fluid capacity of 6.6 gallons (on the ground), while the center reservoir is 3.5 gallons (on the ground). The reservoir itself is larger than these full volumes (roughly 50% larger) to allow for various effects, like thermal expansion of the fluid and different exchange volumes in actuators.

In addition to the 2 EDPs, 4 ACMPs, 1 RAT, and 1 PTU, there are also two small electrical pumps in the left system that are used to unlock the uplocks on the landing gear for alternate extension. These pumps only function in flight when alternate extend is required, but they are also used to release the gear doors on the ground for maintenance using switches on the belly.

Any one hydraulic system can fully control the aircraft, and further any one pump on any one hydraulic system can fully control the aircraft.

The image below gives a good block diagram of the 757 hydraulic systems. Another good resource for learning about the configuration of hydraulic systems on commercial aircraft is SAE AIR5005 "Aerospace - Commercial Aircraft Hydraulic Systems."


Why does Schiphol Airport have such a long taxi?

Schiphol indeed seems to have enough space to build a runway close to the terminal. However it is more the lack of airspace and laws preventing excessive sound levels that made it necessary to create the "polderbaan". It is also one of the two primary runways for nightflights (link in Dutch (sorry). Found a reference in the English wikipedia:

  Newest runway, opened 2003.
  Located to reduce the noise impact on the surrounding population; aircraft have a lengthy 15-minute taxi to and from the Terminal.


Meaning of XX below Slat FLap Trim on A350 with flap setting 1

The number below the flap indication is not the position of the flaps/slats, but the position of the flap lever. When the lever is moved, the number becomes cyan indicating that the flaps/slats are in transit and when they have reached the selected position the number becomes green.

Edit: As per FCOM, there is no mention of amber XX indication for flap lever position at all. Checked in a simulator that total computer failure will not cause indication to turn XX.


How do propeller planes suddenly make noise?

It is unusual to change prop speed quickly and only briefly, so I would suggest two possible explanations:

On the ground: The pilot tests the propeller pitch control. This he/she does by revving up the engine while the propeller is at a low pitch setting. Then the pitch is increased, which increases the aerodynamic forces on the prop and, consequently, the noise it radiates out. Since this test takes only a few seconds, the result is just what you described (but not while flying overhead).
In the air: Propellers don't radiate noise equally in all directions. This is especially pronounced on the T-6 "Texan", where the noise intensity increases dramatically when you stand directly abeam of the aircraft. The plot below is taken from this report on propeller noise. Probably what you hear is caused by the variation in noise (and dominant frequencies) while the plane flies overhead.

Directivity plot of propeller noise. The details depend a lot on the specific aircraft and propeller type.


Can I do the EASA part 66 theory exams before starting practical training?

I know very little about EASA regulations, but it looks like your first question is answered in EASA's Part-66 FAQ:

  I have completed my EASA Part-66 modules for B1 and I have passed all the exams, but I still lack experience to get my licence. Is there
  a time limit to get the licence ? Will the certificate expire in a few
  years if I do not get the experience?

  According to Commission Regulation (EU) No 1149/2011 of 21 October
  2011 (amending Regulation (EC) No 2042/2003), the basic examinations
  shall be passed and experience shall be acquired within the ten years
  preceding the application for an aircraft basic licence. The new
  regulation also states that for the purpose of time limits related to
  basic knowledge examinations, basic experience acquired before the
  Regulation applies, the origin of time shall be the date by which this
  Regulation applies, which is 01/08/2012 (which means until
  31/07/2022).

I couldn't find any clear answer for your second question, but another FAQ item might be relevant:

  Where do I gain the required basic maintenance experience? Is it mandatory to gain the required maintenance experience in an EASA
  approved Part-145 organisation?

  According to the AMC 66.A.30(a)(4), aircraft maintenance experience
  gained within different types of maintenance organisations (under
  Part-145, M.A. Subpart F, FAR-145, etc.) or under the supervision of
  independent certifying staff may be accepted by the competent
  authorities. This means that the aircraft maintenance experience may
  be accepted by the competent authority when such maintenance is
  performed in a maintenance organisation which does not necessarily
  hold an EASA Part-145 approval. However, it is on the competent
  authority to evaluate whether this experience is acceptable.

  Consequently, please contact the competent authority where you intend
  to apply for a licence, in order to check whether the basic experience
  would be acceptable, with detailed information on the type of
  aircraft, its operation and the nature of the work.

In other words, ask the authority in your country.


What is needed to operate a LSA aircraft at night?

Since you have your private, the LSA just has to be approved.

According to the FAA Order 8130.2G CHG 1 which covers LSA certification:

  (5) This aircraft is to be operated under VFR, day only, unless
  appropriately equipped for night and/or instrument flight in
  accordance with 14 CFR § 91.205, and when allowed by the
  manufacturer’s operating instructions.

So it must be properly equipped (see below) AND allowed by the manufacturer’s operating instructions.   You will need to check this with each manufacturer to see if they certified it for night flight.

91.205 says day VFR instruments (paragraph b) plus:

  (c) Visual flight rules (night). For VFR flight at night, the
  following instruments and equipment are required:

  (1) Instruments and equipment specified in paragraph (b) of this
  section.

  (2) Approved position lights.

  (3) An approved aviation red or aviation white anticollision light
  system on all U.S.-registered civil aircraft. Anticollision light
  systems initially installed after August 11, 1971, on aircraft for
  which a type certificate was issued or applied for before August 11,
  1971, must at least meet the anticollision light standards of part 23,
  25, 27, or 29 of this chapter, as applicable, that were in effect on
  August 10, 1971, except that the color may be either aviation red or
  aviation white. In the event of failure of any light of the
  anticollision light system, operations with the aircraft may be
  continued to a stop where repairs or replacement can be made.

  (4) If the aircraft is operated for hire, one electric landing light.

  (5) An adequate source of electrical energy for all installed
  electrical and radio equipment.

  (6) One spare set of fuses, or three spare fuses of each kind
  required, that are accessible to the pilot in flight.


What is the benefit of a curved up flight deck?

Because the method of launching aircraft are completely different in these carriers. Carriers with ski-jump, initially designed for RN Harriers (as @Simon already has explained), requires that the aircraft take-off under its own power.

On the other hand, large aircraft carriers having flat decks have their aircraft launched by catapults (with a notable exception) which basically throw the aircraft from the deck. Though this system is heavy, expensive and complicated, it brings significant advantages to the table:

It allows aircraft with higher t/o weight to operate from the carriers. USN operates F/A-18E/F Super Hornets, which weigh ~30 tons from its carriers (they even operated the heavier F-14 Tomcats). The ski jumps, on the other hand restrict the aircraft t/o weight (which means less ordnance), as the Chinese are finding out. Indian Navy operates Mig-29Ks from its carrier, but they are lighter (~25t).
The catapult launch offers more options - it can operate in wider range of sea conditions compared to the ski-jump and can even launch aircraft even if one of the engines fail during the launch, a feat usually not possible for the aircraft in ski-jump.
The catapult allows for a significantly higher takeoff rate, which means a higher sortie generation. As rusnavy.com notes:

  With its four catapults, a US carrier is capable to shoot one aircraft in every 15 seconds. Kuznetsov has only three takeoff tracks. Moreover, two bow tracks are not intended for fully-loaded aircrafts! They can take off Kuznetsov only from one track which starts far behind the midship, i.e. an aircraft must run almost the whole flight deck! Comparing to catapult, takeoff rate of ski-ramp is at least twice lower.

The catapult allows the launching of support aircraft (like AWACS), which have low thrust-weight ratio and would not be able to take off under their own power.
The ski jump takes up space in two ways- physically, it occupies front deck space (on the other hand, aircraft can be parked over catapults when not in use), while the take off run required by the aircraft (greater with more gross weight) eats up more space, leading to limitations of the number of aircraft on the deck.

As for the exception, the USN operates carriers without ski-jumps, but these are not called as such- they are called amphibious warfare ships an operate only VTOL aircraft like the F-35 Lightening II and V-22 Osprey.


Why are ethanol piston engines apparently cheaper to maintain than avgas piston engines?

Apparently a study was run for the FAA at Baylor University and it found that Ethanol caused less wear on the engines. You can trace the bibliography for quotes on the study. They concluded after testing for the O-235 engine

  The relatively low wear and general engine cleanliness indicates that
  the engine can operate on ethanol fuel for longer time periods than on
  100LL avgas. An additional test is planned in which the engine will
  run according to the schedule met in this test, except for 300 hours
  rather than 150 hours, to justify an increase in recommended TBO for
  the engine operating on ethanol fuel.

The FAA also ran a wear test on E-85 that you can find here. Their summary is mainly about whether or not it works which it seems to, they don't touch very much on wear. Still worth taking a look at on the topic.

It does seem that ethanol can increase engine life as has been seen in current testing.


Is there a tool to plot locations within X nm of a position?

At least a couple of tools exist to search for airports beyond a certain distance from your departure point. You didn’t specify, so I will use mine: KHUA.

Using gcmap.com, search for near:KHUA beyond:50nm within:70nm to get a list of airports in the specified range.

To plot some of them on a map, place a few of those in a comma-separated list, e.g., KHUA,KGAD,KBHM,KSYI,KBNA,KAYX,M22,TN50,KJFX,KMRC,KAPT,KPYP,1M4,M22, and select the Map tool. Plot range rings with one or more lines in the Ranges box under Map Controls. I used

  50nm@KHUA
  70nm@KHUA

Click the Draw Map button to update your map.

The final result will resemble the image below.


Why does the Pilatus PC-12 go faster than the King Air C90?

The speed of an aircraft is not only dependent on the number of propellers. It is a function of (among other things),

Excess power available- The Pilatus PC-12 has more powerful engines compared to the King Air C90. The PW PT6 engine used in the Pilatus produces 895 kW, while (a different variant of) the same engine used in King Air produces 410 kW each. The power available for the PC-12 is more.
Propeller efficiency- The PC-12 uses a different propeller compared to the C90, which may have more efficiency at high speeds. Also, the PC-12 propeller diameter is more compared to the C90.
Aircraft design- The PC-12 flew around 30 years after the C90. The aircraft design has improved in many ways during the intervening years in aerodynamics, materials etc. These improvements can be used to increase the cruise speed.
Power to weight ratio- The PC-12 has a better power-to-weight ratio compared to the C90, which translates into better speed.

It is difficult to pinpoint any single reason for increased speed. However, the increased (cruise) speed in PC-12 can be explained using a combination of the above reasons.


What are the differences between blue airports and green ones on the low en-route chart?

From the FAA Aeronautical Chart User's Guide:

  Facilities in BLUE or GREEN have an approved Instrument Approach
  Procedure and/or RADAR MINIMA published in either the FAA Terminal
  Procedures Publication or the DoD FLIPs. Those in BLUE have an
  Instrument Approach Procedure and/or RADAR MINIMA published at least
  in the High Altitude DoD FLIPs. Facilities in BROWN do not have a
  published Instrument Procedure or RADAR MINIMA.

So the green ones are in the FAA TPP but not in the DoD High Altitude FLIPs.


Do aircraft manufacturers like Airbus or Boeing charge the airline, when they assist in aircraft accident investigations?

Investigating agencies are in charge of most air accident investigations not involving criminal acts. In the US this is the NTSB, which is required to investigate air accidents per US Code Title 49, Chapter 11. These agencies exist to provide independent investigation of accidents and provide recommendations to regulators.

The investigating agents will do their best to understand the circumstances and causes of an accident. Agents of the NTSB have authority under §1134 of the above mentioned law to gather information about an accident.

  (a) Entry and Inspection.—An officer or employee of the National Transportation Safety Board—
  (1) on display of appropriate credentials and written notice of inspection authority, may enter property where a transportation accident has occurred or wreckage from the accident is located and do anything necessary to conduct an investigation; and
  (2) during reasonable hours, may inspect any record, process, control, or facility related to an accident investigation under this chapter.

Investigations can also benefit from working with people that are already familiar with the critical elements. As part of the investigation, the agency may invite various parties. This could include airlines, manufacturers, air traffic control, workers unions, other agencies, or anyone else with information or expertise that could be helpful. If these parties do not abide by terms for participation, this status can be revoked. The resources spent by parties during an investigation can in some cases be fairly intensive. For example, while supporting the investigation of the crash of British Airways Flight 38, Rolls-Royce and Boeing ran full-scale tests of the engine and the fuel system designs.

Investigating agencies are funded by their governments. The Aviation Safety department of the NTSB has an annual budget of about $50 million. Parties to the investigation represent their respective parties and are paid by their party to do so. Part of the party agreement acknowledges this:

  Participation as a party to an NTSB investigation is a privilege and confers no rights or benefits.

They "pay" that parties get is good press for assisting in the investigation, and helping to both prove and improve the safety of their products and services.


What was the average duration of a Me-262 combat mission during WWII?

This depended strongly on the flight altitude. Staying at low level for the full mission meant that the Me-262 would run out of fuel within 40 to 50 minutes.

Climbing above 6 km (20,000 ft) would extend that time to 90 minutes. Time to climb to 9 km was 13.2 minutes, so that would leave a time for engaging the enemy of 60 minutes. Considering that typical armament were 24 unguided rockets and 4 cannons with 80 or 100 rounds means that the 262 would run out of ammo in much less time.

One pilot told me that a typical training mission would start in Munich, continue to a radio station near Berlin and back to Munich, all within 60 minutes.


What trade-offs are being made in the design of the planform alignment of stealth aircraft?

To understand the functioning of planform alignment in befitting the fifth generation air-crafts in increasing their stealth, one first need to understand how Radars work:

Radar transmits a radio wave signal.
This signal when comes in contact with a surface (of an aircraft), it is reflected back. This reflection is a mixture of Specular and Diffused reflection.
The radar then after receiving the reflected signal, measures the speed (by computing the shift in the time period taken by the wave to be received over a period of time), and size (by computing the intensity of the reflected wave) of the air-craft.

But there is a problem, since the radar needs to get back its own transmitted wave back, if it is reflected off the surface of the plane only at a specific angle (specular reflection), then the receiver of the radar needs to be at specific places to receive the reflected wave.

Modern planes utilize this shortcoming of the radar by letting the incoming radar signal pass through them (using plan-form alignment), which then greatly reduces the reflected signal. The reduced reflected signal would be seen as an incoming signal of very low strength (intensity) by the radar, which then reads the size of the incoming air-craft as a very small object. This is called Stealth. The air-crafts also use special paints and composite coverings on the surface to again reduce the intensity of the reflected wave, to reduce the percieved size of the air-craft by the radar.

The shortcomings of such radars are removed by using radars such as AWACS, which is moving radar and can receive too directional signals (Specular reflection). Or, using radars such as AESA, which scans a large portion of area around it without having any moving parts.


Can I fly from the UK to Spain on an NPPL(A)?

The NPPL is a UK only license and is not recognized by other countries, however there is a letter of agreement between the UK and France that you can fly there provided you have a medical. There is no such agreement with Spain, however you can contact their version of the CAA and maybe get permission, I'm not sure what hoops you would need to jump through on that.


Does RTCA/DO-254 apply to power electronics?

The description of AC 20-152 answers your question:

  This advisory circular (AC) applies to manufacturers and installers of products or appliances incorporating complex custom micro-coded components with hardware design assurance levels of A, B, and C. These complex custom micro-coded components include application specific integrated circuits (ASIC), programmable logic devices (PLD), field programmable gate arrays (FPGA), or similar electronic components used in the design of aircraft systems and equipment. This AC provides a means (but not the only means) to gain Federal Aviation Administration (FAA) approval by showing the equipment design is appropriate for its intended function. Further, using this AC will help you satisfy airworthiness requirements when these types of electronic components are implemented

"Power Electronics" is a term so broad as to be nearly meaningless, but it doesn't matter: If the devices in question are designed to be installed on an airplane; have a DAL of A, B, or C (a failure would be Catastrophic, Hazardous, or Major respectively); and incorporate ASICs, FPGAs, or other circuitry with custom microcode then the FAA's guidance indicates you should comply with DO-254.

Note that anything you intend to install on an aircraft needs to demonstrate to the FAA's satisfaction that it is not hazardous to safety of flight in both normal and failure conditions, and that it does what you say it does.
Documented compliance with DO-254's guidance is one acceptable means of demonstrating the first of those elements (by showing robustness in the design, and graceful failure modes), though it is not the only means. (In other words if you're building a panel-mount cell phone charger or something equally ridiculous nothing indicates that you must comply with DO-254, but the FAA may ask you to comply if they feel your device is such that DAL A, B, or C is appropriate.)


How do you calculate the optimal cruising speed of an airfoil?

The cruising speed is only one parameter. For optimum flow conditions, you need to look at the Reynolds number and the lift coefficient at which the airfoil shows its best behavior and then tailor the aircraft to achieve those numbers at its design operating speed. Normally, a higher Reynolds number is better, but at low speed (= low Reynolds numbers) airfoil performance drops off a cliff. It is important to stay above this limit in order to get good performance.
This tailoring is done by selecting the wing area in order to achieve the right wing loading (the ratio of airplane mass to wing area) and the wing's aspect ratio (that is the ratio between span and chord) to arrive at the lowest possible drag. Selecting too high an aspect ratio (wing becomes too slender) will drive up the mass of the wing structure and reduce its Reynolds number. Conversely, picking too little wing span will give you plenty of wing chord, but induced drag will be higher than necessary.


Does the "follow" instruction give an aircraft clearance to turn onto a taxiway and/or cross a runway?

The controller is required to give explicit clearance for crossing a runway.  "Follow" is not clearance to cross a runway.
From the ATC phraseology:

7-3-2
d. When an aircraft/vehicle is instructed to “follow” traffic and
requires a runway crossing, issue a runway crossing clearance in
addition to the follow instructions and/or hold short instructions, as
applicable.
EXAMPLE−
“Follow (traffic), cross Runway Two−Seven Right.”
or
“Follow (traffic), cross Runway Two Seven−Right, hold short Runway
Two−Seven Left.”

The AIM doesn't spell this out as explicitly.  AIM 4-3-18-5

A clearance must be obtained prior to crossing any runway. ATC will issue an explicit clearance for all runway crossings.

The explicit requirement for runway crossings is somewhat new (2010 I think) and was the more interesting part.  I neglected the question about crossing taxiways:

Does this [follow...] instruction mean N456 can turn right at Kilo without further ATC instruction?

Yes.  Also from 7-3-2

a. Note 1. The absence of holding instructions authorizes an
aircraft/vehicle to cross all taxiways that intersect the taxi
route.

That said, I would not expect the only instruction from the controller to be "follow (traffic)".  I would expect to be cleared to a specific destination as well.

"N456, taxi to 36.  Follow N123"


Is a VOR check required to legally fly an ILS approach under IFR?

This regulation does not apply to an ILS approach.

You are right is saying that "the VOR system of radio navigation" refers to actual VORs.

If the flight is conducted using GPS to an ILS approach then you aren't using VOR for navigation. It does not matter that ILS uses the same instrument display.

Intuitively, though ILS and VOR use the same display and frequency band, they are functionally different. The VOR beacon sends out a omnidirectional signal, from which a bearing (0-359 degrees) is calculated. The ILS sends out a focused, directional signal, which the receiver centers on.