When may I change my transponder code to 1200 when being handled by ATC? When am I allowed to change my transponder code to 1200?
Is it after clearing the boundary of regulatory airspace (ie. airspace requiring radio contact and an operating transponder)?

ATC will tell you something like, "N1234R radar service terminated. Squawk VFR. Frequency change approved; good day" which you will repeat back as an instruction.
You do not change from the assigned transponder code until instructed.
If you're in a hurry to get away from ATC, you may request to terminate radar service as soon as you are out of their regulatory airspace.


How is the terrain displayed on the ND in a A320? When you turn on "TERR ON ND" in a A320, you see terrain info on you navigation display (ND), but where does its source come from ?

if data comes from FMGS stored inside its memory, then how come it cannot display it in all bounds of the display (see picture) ?

if data comes from the nose radar, then how is it possible to see oceans when you're on the ground in an airport, say in JFK for example ? The radar beam would never be able to get to the water directly.

The terrain data is contained in a database within the EGPWS (Honeywell TM) or the Terrain Awareness and Warning System (TAWS) as defined in the regulations -- can't use a trademark in the regs.
The discussion in your link about "blockiness" when zooming in is a result of the database grid references.  To save memory space, the resolution of the world-wide terrain data varies based on terrain variation and it's relative hazard to flight operation.  There's greater resolution (smaller grid) near airports and in mountainous areas.
The A320 does have airborne radar that is capable of displaying "weather" which really means precipitation.  It can't be displayed at the same time as the terrain to avoid confusion.
Neither the EGPWS or the radar data are in the FMGS.  Their data is just underlayed on the navigation display.  (EGPWS does receive data from the FMGS to support it's warning functions.)
TAWS/EGPWS function is described in TSO-C151c


What is an "all-electric APU"? Understand that the APS5000 used in the B787 is the first all-electric APU for large commercial aircraft.

Source: http://www.pwc.ca/en/engines/auxiliary-power-units

Sorry if this sounds too amateurish, but what exactly is an 'all-electric APU' and how does it differ from past traditional APUs? Does the design allow it to generate the higher electric power required for the B787 and more electrical aircraft?

Any help would be appreciated, thanks!

Usually an APU provides electricity and bleed-air, just like the aircraft engines do. An all-electric APU only provides electricity. This can be done in the Boeing 787 because of the bleedless architecture where systems that would normally require hydraulic or pneumatic pressure are fully electrified.


Why do flaps retract? Why do flaps retract? I know, it might seem obvious at first, because of drag, but instead of retracting the flaps, can't they just make the flaps flexible enough to get into a reduced drag position, and act like an extended wing?

Maybe there is a reason we should look into this technology, since it could really help out in terms of climb rate. Or, was this already looked at, but engineers decided something was wrong about it, and threw the idea away?

And by the way most of the answers involved automatic retraction of the flaps, even though I never said anything about making the flaps being automatic. Just a disclaimer.

Flaps retract in order to reduce wing area. This has several advantages when flying fast:

The higher wing loading (weight per lift-producing area) reduces gust loads. When hit by a vertical gust, the angle of attack increases suddenly, and so does lift. If this happens at high speed (more precisely: At high dynamic pressure), the lift increase might overstress the wing. A reduced area will only produce so much extra lift before the wing stalls, which limits the extra lift from gusts.
Less exposed area also means less friction drag. Your flexible flap proposal would still leave both sides of the flap exposed, adding more surface for friction drag. Put another way, the higher wing loading shifts the lift coefficient up which generally improves L/D at high speed.
Retraction closes the gaps between wing and flap sections. While those gaps are important to maximize the lift coefficient for the lowest possible landing speed, they increase drag at all lift coefficients.

The retraction mechanism is heavy and increases maintenance cost, but it is worth it.


How safe are airplane tours of the Grand Canyon? I'm curious as a complete layperson who's looking at scenic flights with Scenic or Papillon. I'm not afraid of flying on commercial airplanes, but this is a private, single-engine flight around the Grand Canyon. I think the company has had a bit of a history of crashes with its helicopter program over the past 15 years as well.

The tour is two 45 minute flights, I haven't booked it yet. Should I be worried?

It used to be worse. (Comforting, I know)

Back in the day (1956), commercial airliners traveling coast to coast would often give their passengers a little tour of the Grand Canyon on the way to their destination (air travel was still a novelty).  When a mid-air collision between two passenger planes occurred, the government invested in a $250 million upgrade (1960 dollars!) of the air traffic control (ATC) system. One could say that the crash resulted in the 1958 creation of the Federal Aviation Agency (now Administration) to oversee air safety.

One of the things the FAA did was create a Special Flight Rules Area (SFRA) around the Grand Canyon and make a specific chart just for the area. Additionally, they created approved commercial tour routes (see the orange routes below)

In addition to commercial sight-seeing tours being more regulated, there is a lot more training, documentation and procedures available for pilots with the main interest being safety. For example, the following notice is printed on the chart (References 14 CFR Part 93 Subpart U which gives minimum altitude restrictions, both for safety and noise abatement!)

https://skyvector.com/?ll=35.7929449938479,-112.79126822365794&chart=231&zoom=2

And of course, as mentioned in another answer, commercial operators, even if they are operating what looks like a "small private plane" are subject to increased mechanical inspections and regulations. One thing to look for when you arrive is the level of professionalism exhibited by the crew (it might just be a single pilot), but also the people checking you in and working with you to get you on your flight. There is something called a "culture of safety" in aviation that, while the unforeseen can always happen, if there is a culture of safety, preventable incidents can be minimized or eliminated.


How does 220 kN of engine thrust create the 700 kN required to maintain the altitude of an A320? On the Airbus A320, the two engines produce about 220 kN in cruise, whose weight is about 700 kN (about 70 tons). The lift / drag ratio in cruise is about 15 (likely higher), meaning in cruise lift = 700 kN and drag = 47 kN (or less).

What is the accurate explanation of the figures above? How does 220 kN of thrust create the 700 kN required to maintain altitude and the 47 kN required to maintain speed?

The engines only need to overcome the induced drag to remain in the air, which is a result of the lift but usually much smaller on the order of 1/20 for a modern jet. This ratio is specified as lift to drag or L/D. It is a vital performance statistic of an aircraft. Why? Wings are weird and complicated, there is not a simple 100% correct answer.

Only a rocket needs thrust equaling gravitational force since it has no wings! (L/D = 1/1)


What is this WWI aircraft? I recently acquired what appears to be an extremely rare photograph. It was purchased at an estate sale auction in Europe. The photo was listed simply as "Flieger in seinem Doppeldecker "Bülow" (aviator in his biplane "Bülow"). No other I.D.

The "Flieger" is recognizable as Ace Leutnant Walter von Bülow, but he seems to be sitting in the rear seat, of a 2-seater biplane. This is evidently a photo from very early in his service (FA22? FA300?), before he joined Jasta 18 where he flew an Albatros D.III. This plane is clearly no Albatros D.III.

Based on the triangular bracing over the fuselage, I suspected this could be some AEG or Rumpler design.  What's leaving me completely confused, is that the wings have a third pair of struts, just visible at the left edge of the photo. Could this be a single-engine two-seater bomber of some type?

The photo doesn't give much to go on, but I know this aircraft must be from early in the war; probably pre-1917.

Can anyone help me identify this plane?

My best guess for the text below BÜLOW is "Deutsche Flugzeug Werke (newline) Leipzig".

Having three pairs of struts on each wing would make it a DFW B.I, C.I or C.II, as later models only had two pairs of them.

The small gap between the wings matches the photo too.

While some prototypes got a machine gun, the standard version of the DFW B.I was unarmed.

According to the book "German Aircraft of the First World War" (Peter Gray & Owen Thetford) the only difference between the C.I and C.II version is the position of the pilot (C.I: pilot in rear seat). Another source mentions the change of the machine gun from Parabellum to a synchronized one. A synchronization gear only makes sense attached to a fixed machine gun below the upper wing.

With a gun over the upper wing and assuming that Bülow is sitting in the pilot's seat, I'd say that the airplane is a DFW C.I.


What are the pros and cons of sealing the gap between the wing and the aileron? I read somewhere that sometimes gap between the wing and aileron is sealed and this is done to reduce drag. But somewhere else I read that "slotted" ailerons allows for better aileron effectiveness. So which design is beneficial? Is it possible to find an optimum solution or one design exclude other one? And second question - which type of drag unsealed ailerons generate?

This will become a long answer again.
The most important benefit of an internally sealed aileron is reduced control forces. Ailerons require lots of attention to keep forces small while still avoiding a negative force gradient (that is when the aileron runs into its stops all by itself). In most manually controlled airplanes the ailerons become stiff as a brick at high speed and the designers of airplanes like the P-47 and the Do-335 even used telescopic sticks which would give the pilot a longer lever arm at high speed, albeit at the price of a smaller deflection range. But the larger deflections were impossible to steer anyway …
Only when much care is put into aileron design will control forces be manageable at high speed. Examples are the Supermarine Spitfire or the Junkers Ju-88. To be able to control the plane with only two fingers on the yoke at 500 km/h required several hours of flight testing and adjustable aileron hinges in case of the Ju-88. Another fine design is the English Electric Canberra (known as the Martin B-57 to you Yanks) which had a spring between stick and ailerons. The pilot would only move one side of the spring while the other would move the control rod running to the ailerons. On the ground, you could move the yoke easily ±60° while someone else would keep the aileron from moving - that is how soft that spring was. Now you have a moving mass and a spring - what comes to mind? Flutter! This was a very risky design and it shows how desperate the engineers were to get the aileron forces down. And of course those ailerons were sealed. With an internal seal.
You can either use a brush seal or a cloth seal, but in all cases you need something protruding forward of the hinge line into a cavity inside the rear wing. This restriction will keep maximum deflections down to less than ±20° but will produce the best aerodynamic balance. In case of the Canberra, the chord of this forward plate was almost 40% of the aileron chord, measured from hinge to trailing edge. This is what it takes to compensate >90% of the aerodynamic force. The F-86 Sabre ailerons were similar and used a cloth seal while the Canberra used a brush seal (IIRC). The downside is the maintenance and care to keep the cavity clean. Especially water trapped there which freezes as the airplane climbs into colder air can produce unpleasant surprises.
If you only fly slowly, an external seal is also possible. This is popular on gliders where sometimes the hinge line is placed as closely as possible to the lower skin in order to seal the aileron gap with a sticky tape. More elaborate seals use a thin metal ribbon like in the picture below on a PA-28. The main benefit is a higher aileron effectiveness and a very slight drag reduction. The downside is a bit more friction and more care to keep the seal in shape.

Exernal seal (picture source). I would have preferred to show that of the ASW-20C here, but could not find a decent picture online. The ASW-20C tries to extend the laminar run on the lower side across the flap and aileron hinge line with its seal.
External seals only improve the aerodynamics and aileron effectiveness, but don't help with the stick forces. Since only one side can be the one with the hinge, the opposite side still has a gap.
So why would you prefer to have a vented aileron? First, this allows for higher deflection angles. Second, now the aileron nose can be shaped such that it helps to tailor the stick forces. Case in point: The Frise aileron.

Frise aileron (picture source)
At very small deflection angles the nose of the aileron stays within the airfoil contour. This allows for a stiff centering of the ailerons, so you can feel when the ailerons are neutral. Note that a Frise aileron should be combined with aileron trim! When the deflection increases, the nose of the trailing-edge-up aileron will poke out of the contour and produce a suction peak which forces the aileron to an increased deflection. This force now is transmitted via the linkage to the opposite aileron and helps to force it trailing-edge down. This means at moderate deflection the stick forces become pleasantly low. Once the aileron deflection is too high for the flow to stay attached to the aileron's suction side, the pressure distribution over the aileron changes, the forces increase rapidly and the stick forces tell the pilot that he has reached the limit of the useable range. At high speed this helps to prevent overstressing the wing while at low speed the aileron can be deflected further, maximizing its effectiveness. The picture above mentions drag - this helps to reduce adverse yaw a bit, but the main reason for a Frise-type aileron should be lower mid-range stick forces.
Why else would a vented aileron be preferable? The gap reduces aileron effectiveness but keeps the flow attached at higher deflections. This gives more control authority at the price of increased drag. But there can be more: A clever way to install vortex generators can be found on the Short Belfast:

Tail of the Short Belfast from below (own work). Note the vortex generators hidden in the hinge gap - they only move out into the airflow when they are needed: When the elevator is moved to large negative deflections. This is not an aileron, admittedly, but the same can be done at the aileron, too.
To summarize:
Pros of internal seals:

much lower control forces at high speed
good aileron effectivity

Cons of internal seals:

limited deflection range
have to be kept clean all the time

Pros of external seals:

No gap on one side, lower drag
good aileron effectivity

Cons of external seals:

a bit more friction
care is needed when handling the delicate seals

Pros of vented ailerons

allow to tailor stick forces and reduce adverse yaw
higher deflection angles possible, especially at low speed

Cons of vented ailerons

more drag
lower effectivesss


How can I calculate the maximum range of an aircraft descending with power (not gliding)? Let's assume I have an aircraft, for which the thrust is not enough to keep the altitude. So over time I will lose some altitude. But it will still go further then it would without having an engine (gliding).

How can I calculate the maximum possible range for this case?

I know the Breguet equation, but I do not think this helps me in this case?

Some formulas would be great.

You can use the lift to drag ratio (L/D) to calculate for just about any stage of flight if you compensate it properly. Assuming your descent is steady-state, lift is equal to weight, using the L/D you can then determine drag. Next take drag and subtract the thrust produced by the engine to get the amount of excess drag need to be covered by gravity. You glide ratio will be lift divided by excess drag. This makes for a simple calculation of your variables remain constant, however you will need to use an integral if you want to compensate for changes such as differences in engine power at different altitudes, and reduction in weight as fuel is burned.


Why in helicopter autorotation phase the opposing torque is eliminated? As you many know, according to Newton's 3rd law for every action there is an equal and opposite reaction, as the main rotor of a helicopter turns in one direction, the fuselage tends to rotate in opposite direction but when there is no engine power during autorotation, there is no torque reaction, why???

Because the engine is not applying power to the rotors. No engine power means there will be no torque pushing the airframe in the opposite direction.

In auto, the main rotor essentially becomes a pinwheel, kept moving by the air rushing past it...

Auto doesn't really power the rotors, it keeps them moving so that the spinning rotor creates air resistance and slows the descent of the helicopter, and maintains enough inertia in the blades to flare and stop the descent.

Of course, that inertia is only sufficient for one attempt at flaring and stopping descent.


Can there be more than one airway with the same name in the world? I was working on a programmed simulation of an FMC. I was wondering, regarding airways, if there can be more than one airway with the same name in the world.

i.e can there be more than one airway named UL602?

No.

From ICAO SARPs Annex 11 'Air Traffic Services' Appendix 1:

  3.1.3 A basic designator assigned to one route shall not
  be assigned to any other route.

And to clarify what is meant by designator for the ATS routes (airways): a designator is a letter followed by 1 to 3 digits (1 to 999). In your case the L602 is the designator. The preceding U (for Upper) is a supplementary letter.

RE Is UL602 still part of the L602 airway or are they two separate airways?

They are the same route (lateral path), but not necessarily have the same navaids/waypoints:

The lower ATS route may require an extra VOR, for example, to compensate for the low altitude line-of-sight limitation.
It may also have extra waypoints for joining the SIDs and STARs to the airway (on/off ramp highway analogy). This is more applicable to the example given since it's an RNAV-only ATS route (see letter L below).

Bonus: The current letters for the designators are:

A, B, G, R (regional non-RNAV)
L, M, N, P (regional RNAV)
H, J, V, W (non-regional non-RNAV)
Q, T, Y, Z (non-regional RNAV)

A regional ATS route is usually one that passes through more than 1 FIR.


Is it worth it? Spending loads of money and years? I'm a high school student. Over here in India, there are too many people and less jobs. I have a passion and a dream to become a pilot in one of the distinguished commercial airlines of the world. I have researched all the data and know all the facts.

The question which comes to my mind is:
Is it worth spending more than US$80000 for all the training, type ratings and education and still not being capable to end up with a job?

The number of hours required for ATPL are 1500 here but we land up with approximately 310 after University and CPL. How do I increase my flight hours after that? How many years before I'll get a proper job? Is it worth spending so much money and time?

Please let me know guys! I'm not sure whether i should continue in this field or not.

I'm not a pilot myself, but can provide some links here:

Burdens of working as an airline pilot
Do pilots make a lot of money?
Why is being a commercial pilot stressful?

You may also ask on PPRuNe discussion board, which is more suited for this kind of questions that are a lot based on individual opinions.

Here is an interesting article from Vanity Fair, that I often cite, about the increasing automation of aircraft, and the related impact on crew members: The Human Factor. The article starts with "Airline pilots were once the heroes of the skies. Today, in the quest for safety, airplanes are meant to largely fly themselves", which gives the tone.


What are those things under the Concorde’s cockpit windows? I’ve noticed there are little bits sticking out under Concorde’s cockpit windows and they’re too small to be canards. Are they some kind of vortex generator or something?

(Source)

Those are nose (forebody) strakes, from Wikipedia:

On both supersonic and subsonic types, smaller strakes are sometimes applied to the forward fuselage to control the fuselage flow at high angles of attack; for example the Concorde SST has small nose strakes "to get a better directional stability".

You can also find them on an MD-88 (shown below) and the DC-9-50, for example.

Unique to the [DC-9] Series 50 were aerodynamic strakes mounted below the cockpit to assist in lateral stability at low speeds and high angles of attack by stabilizing the vortex pattern (Source: Great Airliners Series).

A 1998 NASA paper on the forebody strakes notes:

Results of this initial, exploratory investigation indicate that the use of strakes on transport aircraft configurations can have a noticeable effect on directional stability. The effectiveness of even very small strakes means that such a concept could be applied with perhaps only minor impacts on drag and performance. The requirement for minimum levels of crossflow, however, could mean that direct application of this strake concept may not be of great benefit in development of a control effector.
Recognizing the ability to alter the fuselage flowfield with relatively small surfaces, whether fixed or actuated, at low angles of attack should hopefully spur greater research toward the understanding of the flowfield behaviors that impact lateral/directional characteristics of transport aircraft. The STRATA program will continue along this course in improving flowfield understanding and in the development of innovative control effectors for transport aircraft configurations.

This is what it is and what it does. As for the how, my basic understanding from the paper is the high angles of attack (such as those experienced by the Concorde in slow flight) increase the crossflow of the fuselage (air flow around the circular cross section of the fuselage). Adding those strakes controls/limits this crossflow and improves the stability by way of vortex generation.
Another mention from a 1976 NASA research:

The use of nose strakes (currently in use on the Concorde) has been investigated. The data indicate large increases in directional stability due to the strakes. For example, at the approach angle of attack of 8°, the strakes approximately double the value of $C_{n_{\beta}}$.

$C_{n_{\beta}}$ being the "static directional stability derivative".


What is the aerodynamic purpose of an Acorn? This is an IL-62. On the tail of the aircraft,  there is a large cone, protruding out of the vertical and horizontal stabilizer which is called acorn. I am trying to figure out what is the aerodynamic explanation for its presence over there.

It's doing one or both of two things:

Fairing in some kind of projection as Ron said, may be an antenna or a mass balance.
Fixing an airflow disturbance at the T intersection.  Bullet fairings are a common band aid for that sort of thing.

Russian designs seem to be big on bullet fairings.


What effect would an air-to-air GPS jamming system have? I just noticed an interesting AIC: NOTIFICATION OF GPS JAMMING EXERCISE – HEBRIDES RANGES DURING PERIOD 21 JULY - 1 AUGUST 2014.

PDF available on Eurocontrol (official) or Scribd (mirror).

The Ministry of Defence is planning a GPS jamming exercise:

  Jamming will occur across the 24 MHz bands centred around 1227.60 MHz and 1575.42 MHz. A Falcon 20 aircraft will be equipped with 2 high power jamming pods (1kw), one for each
  GPS frequency.

What effect would this have on a plane entering the jammed airspace?

Related: Can a GPS signal sent to a plane be spoofed?

Jamming GPS means that the GPS receiver is unable to lock-on to the signal from any satellite.

Presumably, pilots of small aircraft would have to revert to more traditional forms of navigation - assuming they have a GPS based navigational instrument or two.

Pilots of airliners would presumably see alerts and (as with most smaller aircraft?) their flight displays would include non GPS sources such as gyrocompasses or magnetic compasses.

I imagine redundant systems would include other sources of position information such as reception of radio-based omnidirectional navigation beacons, inertial navigation systems and looking out the window.


Why do we often have more than one method of slowing down (retarding) aircraft on the ground? Why do we often have more than (thrust reversers and brake chute) one method of slowing down (retarding) aircraft on the ground?

I would suspect that it is due to redundance, safety and usefulness in different circumstances.

Thrust reversers are useful to slow down from high speeds, but you wouldn't want to use them when trying to stop at a gate.
Wheel brakes are powerful both at high speed and when parking, but can catch fire if the temperature resulting from the braking gets too high. Also less efficient on slippery runways.
Some (primarily) military aircraft have tailhooks despite not being designed to land on aircraft carriers. The hooks provide an alternative to ejecting and loosing an otherwise good aircraft if the systems powering the brakes fail.
Braking chutes are as far as I know mainly used on aircraft without thrust reversers; these provide an alternative to wheel brakes which is useful in certain parts of the world. Norwegian F-16 and F-35 are equipped with braking chutes as they regularly operate in areas with lots of ice and snow. Braking chutes must be jettisoned before taxiing, and has to be repacked before they can be used again, so they are less common than the other methods.

There are probably other means of slowing down I've forgotten to list. If you are on an aircraft with only one means of slowing down, what do you do when it doesn't work? Having multiple means to slow down means that any passengers or crew has a higher chance of surviving, and also that a failure can result in less damage to the aircraft.


How can you find AGL if the altimeter is set to MSL? Without doing the math, how do you know your true altitude AGL when your altimeter is set to MSL

I usually don't care a lot about my AGL altitude.

When I'm doing my flight planning on the ground, I do take into account terrain and obstacles, and pick appropriate MSL altitudes to be well clear of anything on the ground.

But by the time I get in my plane, I'm thinking in terms of MSL (altimeter) and no longer about AGL.

That even applies to approaching my destination field.  If I know the field is at 1,500 ft. and I want to do a traffic pattern at 1,000ft. AGL, then I plan for my altimeter to read 2,500 (MSL).  And I think about if I'm at the proper MSL altitude (2,500), not what my AGL altitude is.


What is the optimal wing sweep angle for a passenger airliner? There are many different types passenger planes, but is there a optimal wing angle for these planes, that improves the plane's take off speed for example, or the fuel efficiency

The Boeing 747 has a wing seep angle of 37.5 degrees, the Airbus A380 has a wing sweep angle of 33.5 degrees. These are two of the largest planes in the world. So is the optimal range for a wing sweep angle between 33.5 and 37.5 degrees, or are there certain regulations stating different sweep back angles that a passenger airliner can use.

Generally, sweep makes the wing less efficient in creating lift and complicates structural design. Therefore, aircraft designers try to use as little sweep as they can get away with. However, sweep delays the onset of compressibility effects and allows jet aircraft to improve their maximum transport performance (the product of speed and payload).

Forward and backward sweep both help to fly faster, but backward sweep is preferred because it makes stalling less dangerous, improves directional stability and allows to make the wing lighter. The shortest take-off distance will be achieved with zero sweep - if it were not for the need to limit compressibility effects, sweep would only be used in tailless aircraft.

The main factor for choosing wing sweep is the desired cruise Mach number, and Mach 0.85 has become the de-facto standard. Other factors are:

Relative wing thickness: A thicker wing needs more sweep, because thickness adds its own compressibility effects.
Maximum local Mach number of the airfoil where recompression is shock-free: So-called supercritical airfoils help to reduce sweep, and older designs (before the A310) had generally more sweep because their critical Mach number would had been lower at equal sweep.

Regulations have nothing to do with it, only engineering determines the sweep angle. Since the design cruise Mach number is quasi-fixed at Mach 0.85 for long-range airliners, the sweep angle of different aircraft is very similar. Regional jets normally fly a little slower to improve fuel efficiency, and their sweep angle is less than that of their big, long-range cousins.


What is this little door on the upper deck of the A380? I found a little "door" on the upper deck of this A380:

Source: Airliners.net

What is the purpose of that door, hatch, whatever?

It's for the emergency slide.

You can see it deployed here:

Source:  Airliners.Net


Why doesn't EVA Air BR087 (Taipei to Paris) follow the shortest path? I've noticed that the flight BR087 from TPE to CDG doesn't follow an orthodrome.

Instead, it seems to take great pains to avoid the Chinese airspace entirely.

Why? EVA Air has other flights connecting Taiwan to China.

For instance, a similar flight KLM808 follows the orthodrome more closely :

EVA Air is Taiwanese. Taiwan and People's Republic of China are not on the best of terms (publicly).

  The relations between China and Taiwan have been characterized by limited contact, tensions, and instability, due to the fact that the Civil War merely stopped without formal signing of any peace treaty and the two sides are technically still in a state of war.

The details of the bilateral Air Service Agreements (ASA's) are not public though. Treating airlines differently is supported by a discussion on airliners.net.

EVA Air does land in Mainland China, but that's only recently. That's because ASA's are not black and white. (Both economies have been on good terms lately.) An example of the limited liberalization:

  The first direct commercial flights from China (from Guangzhou) to Taiwan since 1949 arrived in Taipei on January 29, 2005.

About 70% of the Chinese airspace is military controlled, i.e., the military may choose to allow KLM, but not EVA Air.

  Aviation experts agree that one of the main problems in China is the fact the country’s airspace is largely controlled by the military, leaving little room for civilian aircraft even as the domestic airline industry booms. According to the state-run China Daily newspaper, less than 30% of China’s airspace can be used by commercial airlines, compared to about 80% of the airspace in the United States (BBC, April 2016).

As DeltaLima mentioned, KLM operates a subsidiary on those routes, KLM Asia:

  A wholly KLM-owned subsidiary registered in Taiwan was established in 1995 to operate flights to Taipei without compromising the traffic rights held by KLM for destinations in the People's Republic of China.

Here is another example a couple of hours apart (do note China Airlines is Taiwanese).

(flightaware.com)


Why do some non-towered airports have class E airspace from the surface up? I've noticed that when it comes to non-towered airports, some are in class G airspace and others have class E airspace starting from the surface. At first I thought the airports with class E at the surface might be in that configuration because they have precision approaches, but then I found exceptions where the airport has class G at the surface along with a precision approach. Is there a particular reason some non-towered airports are in class E while other are in class G?

This answer is specific to the United States (heck, I don't know if this is an even an issue elsewhere in the world).

The airspace classification actually has nothing to do with whether or not a particular airport has a precision approach. It is based on two factors: communications capability and weather observations.

For a non-towered airport to have Class E (surface) airspace, ATC must have communications capability with aircraft down to the runway surface. Second, weather observations must be provided by a Federally certificated observer and/or Federally commissioned weather observing system. Reference: FAA JO 7400.2K, paragraphs 18-1-1 and 18-1-2.

If one (or both) of these conditions are not satisfied, the surface airspace will be class G with a class E floor of 700 ft. AGL if there is an instrument approach into the airport and 1200 ft. AGL if not.

Now, here's a twist: if the airport has a part-time control tower, the airspace may become Class G when the tower is closed, even if both communication and weather reporting requirements are satisfied. Be sure to check the Chart Supplement (fka A/FD).


What is the yellow marking underneath Delta A220 doors? I've recently noticed a yellow marking underneath the left door of Delta A220s. It's a yellow rectangle with a black T on it and it is exclusive to Delta and no other airlines on the A220. Any ideas what it's for exactly?

Thanks!

This is almost certainly an alignment marking for the passenger boarding bridge (jetway). You can find the same markings on other Delta aircraft as well:

(image sources: A320, B737, B767)

The dead giveaway is that these markers are found on the second door on the long haul jets, which are typically boarded through this door instead of the first one:

(image sources: A330, B777)

The jetways used by Delta will have a similar marking on them which aids in properly aligning them with the aircraft door such that the door can still be opened and passengers can safely board and disembark.

See also: What are these markings under the passenger door?


Are there any privately owned V-22 Ospreys? Could someone buy a V-22 Osprey and convert it to a luxury interior rather than a military one? Has anyone done that? Are there any images available of a private Osprey made to be luxurious inside? What does the interior of the US President's Osprey look like?

There are no 'private' Ospreys thus far (all are operated by USAF, USMC and JSDF)- the Bell Boeing MV-22 Osprey used by the US Marine Corps's HMX-1 presidential helicopter squadron:

  ... will conduct presidential support missions, which means these aircraft will carry presidential support staff and news media representatives travelling with the president," the USMC says. "The Osprey mission at HMX-1 does not include carrying the President of the United States."

so I'm not counting on 'luxury' interiors. When (and if) a civil version of V-22 is produced (or if the Japanese feel like selling theirs), you can buy it and make the interior whichever way you like.


Why was the A380 built with a gull-wing design? I've noticed that the wings on the A380 are very curved in comparison to the 747. I mean the 747 has a lot of wing flex but the A380 has less wing flex and the wings are flexed down in comparison to the 747. Here there are two photos that might help you understand what I'm writing. So why are the A380 wings like that and what are the benefits?
Source

Source

The wings are designed like that. You can see them in the picture below.

Image from airliners.net

The A380 wings are designed to be the most efficient while still inside the 80m 'box' for efficient airport operations. The result is a clean slate design which used CFD extensively to get the most efficient aerodynamic design possible.
The 'curve' near the root means that the landing gear in the wings could be as short as possible while being long enough to prevent a tail strike during rotation and allowing the spar to pass below the floor.
The shape also allows enough ground clearance for the inboard engines. This assumes importance with the advent of engines with ever higher bypass ratios.
Of course, the use of modern composite materials means the the wing 'flexes' up to be more or less straight once the aircraft is airborne.

(wikimedia.org) In-flight 'flex'.


Why locate engines far from the fuselage in some aircraft, such as the SR-71? There are some plane designs where the engines are placed quite far away from the fuselage, one example is the SR-71.

This amplifies the effects of any asymmetry of the engine thrusts, the worst case being zero thrust on one side. What are the reasons that justify or force those designs?

From Wikipedia:

  Placing engines on the wing provides beneficial wing bending relief in
  flight. The further the engines are away from the fuselage the greater
  the wing bending relief so engines buried in the wing root provide
  little relief. Almost all modern large jet airplanes use engines in
  pods located a significant distance from the wing root for substantial
  wing bending relief.

One could imagine that, in flight, the wings are holding up the aircraft, but this causes a massive bending moment at the wing roots (just visualize  an airplane suspended by ropes from the wings). As such, is favourable to have the airplane body be as light as possible, and put as much mass (e.g., fuel) as possible on the wings, where the lift forces coincide (or at least, are closer to) the weight of the engine, reducing bending moment on the wing roots. This allows for smaller (and thus more aerodynamically efficient) wing structures.

As noted in the comments, placing the engines further away of course increases downwards bending moment when the plane has landed. However, this will be 1g by definition (at touchdown, the wings still provide lift (generally)), whereas during the flight, an airplane is designed to take up to 2.5g (and, I suppose not completely coincidentally, -1g).


As a Brazilian pilot, can I buy an aircraft in the US then fly it to Brazil to register it? I have a pilot license in Brazil and would like to purchase an aircraft in the US and then register it in my country.

My license allows me to fly the aircraft back to my country?

I'm not sure if you're asking about general export regulations, in which case Farhan's link is useful, or the more specific licensing question.

But regarding licensing it's clear that you may not fly a US aircraft from the US to Brazil with only a Brazilian license. 14 CFR 61.3 says:

  (a) Required pilot certificate for operating a civil aircraft of the
  United States. No person may serve as a required pilot flight
  crewmember of a civil aircraft of the United States, unless that
  person:

  (1) Has in the person's physical possession or readily accessible in
  the aircraft when exercising the privileges of that pilot certificate
  or authorization—

  (i) A pilot certificate issued under this part and in accordance with
  §61.19;

  (ii) A special purpose pilot authorization issued under §61.77;

  (iii) A temporary certificate issued under §61.17;

  (iv) A document conveying temporary authority to exercise certificate
  privileges issued by the Airmen Certification Branch under §61.29(e);
  or

  (v) When operating an aircraft within a foreign country, a pilot
  license issued by that country may be used.

In other words, you can use your Brazilian license to fly a US aircraft in Brazil (see 61.3(a)(1)(v) above) but otherwise you need a US license or a special authorization (61.77 covers leasing and commercial operations so it almost certainly won't apply to you).

I know very little about this, but at least in theory it looks like your options are:

Get a US license yourself, perhaps a foreign-based license based on your Brazilian one (see 61.75)
Get some special permission from the FAA
Pay a ferry company to fly or transport the aircraft to Brazil
Register the aircraft as Brazilian before it leaves the US (this may not even be possible)

And those are only the FAA/aviation issues, you have to think about customs and general import/export regulations too, both in the US and Brazil. I would look for some specialized help with this.


Is FAA approval required in order to fly an RNAV SID/STAR for a private aircraft operated under FAR Part 91? I know that under FAR Part 135, specific approval is required for an RNAV SID/STAR (given via an OpSpec), but what about Part 91?  Does the aircraft/pilot have to be approved, and if so how is the approval obtained?

The best answer I can give is generally, no.

In relatively few cases – for aircraft operated under Part 91 Subpart K, which applies to fractional ownership like Netjets – the operator must possess an Mspec, which would be applied for at the local FSDO. I'm not sure what that process looks like, but the FAA's Commercial Operations Branch has some good information on that front.

For all other Part 91 operations the aircraft simply needs to be appropriately equipped.


Are there any IFR-certificated and equipped aerobatic aircraft? Are there any acrobatic-category aircraft that are also type-certificated/rated and equipped for flight under the instrument flight rules?  What limitations would be placed on such an airplane when flying as an IFR aircraft vs. a VFR aircraft?

Yes: Consider the Zlin Z 242. The type certificate shows that it is certified in the Acrobatic, Utility, and Normal categories. In the section for Optional Equipment, the aircraft can be sold with

  Communication and navigation equipment VFR DAY, VFR NIGHT OR IFR according to actual requirements of the customers

Generally, if the aircraft meets the minimum equipment requirements and inspections, and the POH, Type Certificate or other manufacturer's documents allow it, an aircraft may be flown IFR in IMC.

There would be no particular limitation on operations specifically because the aircraft is capable of aerobatics, but the pilot would still need to follow their clearance. This would significantly limit the benefit of the instrument certification. (Note that even if deviations in altitude and flight path were approved, aerobatics still wouldn't really be possible.)


When two pilots are required for a private aircraft, is it a requirement for the PIC to be ATPL? After discussion on who could be a captain, I would like to clarify, is having ATPL a requirement for PIC on an aircraft that requires two pilots to fly?

Negative.  A person holding a private pilot certificate (at least in the USA) is allowed to add a type rating granting both PIC and SIC privileges in an aircraft type certified for a two person flight crew provided the flights are not carrying passengers or property for compensation or hire.  See 14 CFR 61.117.

An ATPL would be required if said type rated pilot was conducting Part 121 or Part 135 operations in such an aircraft.


What are the various fees associated with flying? I am aware of airport landing fees, fuel fees etc, but it was brought to my attention that there is airspace charges? And especially in Canada, the fees are horrendous. What are the various types of fees airlines and private planes have to pay in terms of airspace and air traffic control fee around the world, and Canada and the US?

I will answer this for the US as that is the jurisdiction I know best.

There are no airspace or ATC usage fees here in the US. The airspace and its resources are free for any plane of any size that conforms to the FAA's FAR's and is legal to fly in a given airspace.

The FAA is in the process of adding overflight charges for aircraft that fly through our airspace but neither land nor depart from the US.

The only fee (somewhat indirectly related but not a component of the airspace its self) is for planes coming in from abroad who will need to pay some customs fees to enter the country.

Strictly speaking you may be required to carry IFR charts or VFR charts on board to conduct the flight in question in US airspace. These must be purchased from an official source. The FAA maintains them but the Pilot must pay for them. Until recently the FAA also physically printed them and they were purchased directly from them but this is no longer the case.

If you extend the question to include airports the answer will drastically change for the US.


How do airlines pay for aviation fuel and services at an airport they don't normally use? On flights of a regular nature to set destination airports, I assume that there is a system is in place between airlines and airports/providers to pay invoices for fuel/services.

What do airlines do if diverted to an airport that the airline never normally serves or a freight airline doing a one-off flight to an airport? How do they pay? Does the captain carry a company credit card to pay for these one off charges?

In the United States most places take credit cards.

Large carriers generally get billed, or for smaller airports they use a company credit card. There are even specialized credit cards that are designed specifically for this use.

There is also what is called "contract fuel". Basically there is a company that is a middleman, a fuel supplier who pays the airport fuel company and then rebills the airline. The pilot has a card that identifies them as a client of the fuel contractor.

For example, check out AEG Fuels, a typical fuel contractor. (Please click on that link. I get 25 cents every time you do. Just kidding.)


How do I determine odd or even when working with altitudes? I am always confused about how to calculate safety altitude, especially when it comes to even heights and odd heights, e.g. is 7500ft even or odd?

Look at the thousands of feet only.
Is 7(thousand) even or odd?

I'm sure a mathematician views 7,500 as equally close to 8,000 as 7,000.

But in aviation terms, it is 7,000 plus another 500 feet, and therefore an odd altitude.


Why are many early gliders high-wing designs as opposed to newer designs? Many early gliders were designed with a high-wing configuration. An example of this is the K 8, but many more of the same and previous generations follow this configuration.

Later generations (e.g., ASK 21) mostly follow a mid-wing design.

Is there a simple reason for this (including the change of material for fuselage and wings)?

It has to do with improvements in the understanding of aerodynamics and the various tradeoffs discovered over time. High wing has less visibility in turns, especially during thermaling. Low wing has better ground effect, but gives you less options when landing in a crosswind. High wing has better ground clearance, which helps in off-field landings.


Why was the Akron’s spy basket installation so remarkably unstable? A spy basket (also known as a cloud car) is a type of airship appurtenance consisting of a streamlined one-(usually)-person gondola attached to its host airship by a long cable; to use it, the host airship hides in or above a cloud layer, and the spy basket is then lowered, cum crewmember,1 from the airship down through the base of the cloud deck, and serves as an observation station in the (hopefully) clearer lower-altitude air, allowing the airship to know what happens to be below it without exposing its gigantic soft underbelly beneath the clouds.2

It was pioneered by the German armed forces during World War I, who used (or at least tried to use) it to drop bombs on Londoners3 without getting shot down; they eventually discarded the whole concept, but it was picked back up, five seconds over a decade later, by the United States Navy (the only post-1918 military user of rigid airships),4 when the U.S.S. Akron (ZRS-4) was built with provisions for the fitting and use of a spy basket.  The Akron’s spy basket was first deployed in April 1932.

It did not go well:

  Soon after returning to Lakehurst to disembark her distinguished passengers, Akron took off again to conduct a test of the "spy basket"—something like a small airplane fuselage suspended beneath the airship that would enable an observer to serve as the ship's "eyes" below the clouds while the ship herself remained out of sight above them. The first time the basket was tried (with sandbags aboard instead of a man), it oscillated so violently that it put the whole ship in danger. The basket proved "frighteningly unstable", swooping from one side of the airship to the other before the startled gaze of Akron's officers and men and reaching as high as the ship's equator.[18] Though later improved by adding a ventral stabilizing fin, the spybasket was never used again.[26]

This was not, so far as I can tell, a problem faced by the German airship bomber crews of World War I (who were blessed with spy baskets that dangled placidly from their mounts), and I’m having trouble seeing why the Akron’s spy basket would exhibit such remarkably unstable behaviour, unless the first (and only) trial of her spy basket was held in severe turbulence (which, given the purpose and method of use of the spy basket, I’m having great trouble believing that her commanding officers would have allowed).  Can someone clarify this for me?

1: Presumably the one with the most favourable combination of good vision, non-acrophobia, and low rank.

2: It can also, if necessary, be used to determine if the cloud deck does, in fact, have a base, or if it extends all the way to ground level.  Obviously, due to the likelihood of eventual ground/water/tree/automobile/watership/Paul Bunyan contact in the event that the cloud layer is continuous to ground level, this is only advisable when the airship is in a stationary hover, unless you really don’t like the person manning the spy basket.

3: And probably some other people, too.

4: The German military, having lost World War I fair and square, was forbidden to build, use, possess, rent, touch, or think dreamily about any sort of aircraft at all, lighter-than-air or heavier-than-air, while the British military, in order to save money, scrapped its fleet of rigid airships at the end of the war and cancelled all those not yet delivered (with the sole exceptions of one they were going to sell to the Americans but which, due to poor judgement on both the designers’ and the pilot’s parts, crashed while they were showing it off for its prospective customers and one which was hastily reconfigured for passenger service but proved too small for the role).

Very interesting bit of physics going on here with the cable car swinging "up to the equator" of the airship.  Perhaps something similar to what happened with the "Galloping Gertie" Tacoma Narrows suspension bridge in 1940.

The suspension cable could be hundreds of feet long.  In a cross wind, if the mass to surface area (wind pressure) of the airship and the cable car did not match, the cable car would swing to the side and then back.  If the wind and the swing motion became harmonicly re-inforceing, each swing becomes a little higher, eventually reaching a full 180 degree sweep.

The elasticity of a cable several hundred feet long may have contributed, stretching a little at the bottom of the arc and contracting a little at the top of the swing, helping propell the object back in the other direction.

Another factor have been a very slight difference in airspeed as the car swung away from the wind, pulling the airship with it, as compared with the return swing back into the wind.  Each swing adds more energy to the system.

Without "dampers", the cable and car are wildly out of control.  The fix, amazingly, may have been to fashion adjustable "sails" for the cable car. Some wind tunnel testing may have been
helpful as well.


How to find flying directions (similar to driving directions) for navigation? Similar to driving directions, is there a service from where we can get flying directions? Let's suppose an airplane does not have a GPS and it has to rely on VORs for navigation.

I know about SkyVector. But if I create a flight plan there, it is a straight line. They also specify that this is not for navigation.

The traditional (i.e., "old") method is to spread out aviation charts on a large table, and use a plotter to hand-craft a flight plan on paper. Victor airways (the VOR airway network) are depicted both on VFR sectionals and on IFR low & high altitude enroute charts. The pilot/navigator will mark his or her origin and destination, and eyeball the available airways to come up with a suitable route, much like a driver would use a national atlas to find what interstates/highways to take for a long road trip. These maps depict all the information needed about distances, courses, minimum altitudes and airspace classifications.

Planning by hand may also involve the use of the Airport/Facility Directory (A/FD), which has information about airports, and the Terminal Procedures Publication which shows what instrument approaches are available at each airport and any necessary IFR departure procedures, takeoff weather minimums, etc.

Nowadays, people will more often use computer planning tools like AOPA Flight Planner, fltplan.com, etc. There are a large number of sites and software programs for this, some free, some requiring paid subscription / purchase. Most recently, mobile apps for smartphones and tablets have emerged with the same capabilities.

The best such sites/software will also provide the necessary information about terrain, takeoff minimums, departure procedures and so on, and even give you current NOTAMs, weather forecasts, etc. Many also have aircraft performance databases (or allow you to create one yourself for your airplane) and can do performance calculations for you. Some will even find the cheapest places along your route to purchase fuel, and also give you information about local transportation and accomodations.

Note, though, that neither the FAA nor any other civil aviation authority authorizes the pilot to rely on software resources. The pilot is still always responsible for verifying that he or she has complete, accurate information from official sources. In other words, if you're using software to plan your flight, and you violate restricted airspace that your flight planning software didn't warn you about, you'll be held responsible for not verifying the information and catching the error. So be careful what you use and rely on!


Explained intuitively, why does aspect ratio affect induced drag? Over the last few hours, I've been trying to intuitively understand induced drag, and how aspect ratio affects it. I already know the equations, but while they explain the relationship, they do not really explain anything, I want to understand what is happening physically. I've read on many answers in this community, specially by Peter Kämpf, but I've been unable to attain an intuitive understanding.
I will explain my line of thinking, hopefully I can be corrected on any misconceptions, or wrong assumptions.
First of all, my way of thinking about lift is simply airflow being redirected. If airflow is redirected by a surface, a force in the opposite direction is applied on said surface. This means that for lift to be generated, downwash must exist, therefore induced drag is an inseparable side-effect of generating lift.

Let's imagine 2 theoretical wings, both of which have the same area, but differ in aspect ratio.
If induced drag depends on the downwash angle, why would a longer wingspan reduce the angle? If the downwash angle is reduced, then the amount of air being redirected (total mass) must increase to keep lift equal, how can that happen when the wing area is equal?
All of the sources I am able to find blame induced drag solely on wingtip vortexes, since they cause downwash, but the entire wing is creating downwash, so what gives?
If tip vortexes are what cause induced drag, how does a longer wingspan reduce the induced drag coefficient? As an example: If the span of rectangular a wing is increased without changing the chord length, the aspect ratio will increase, which according to the math, will lower the induced drag coefficient. How does a longer wingspan affect the vortexes such as to reduce the downwash?

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

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

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

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

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

how can that happen when the wing area is equal?

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

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

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


How much does the landing gear compress on a fully loaded airliner on the ground? When an empty airliner is loaded with passengers, cargo and fuel, the extra weight compresses the gear strut and it sinks. Jet bridges are designed with this in mind.

How much height does it actually sink?

For the purpose of the question, I am interested in any typical commercial airliner models.

The value will obviously vary for different aircraft models (and also with the loading, c.g. location etc), but the usual range can be found in the airport planning document for the particular airliner, where the values for ground clearance for the aircraft at the OEW (max. value) and MTW (min. value) would be given. The difference should give you the compression.

For example, for Boeing 737-800, the quoted value is around 15 cm (plus or minus 7cm).

Image from 737 Airplane characteristics for Airport Planning document

Note that the different parts of the aircraft 'sink' different amounts- the fuselage sinks around 15cm, while the engines (mounted on the wings) sink more- by 16cm due to the fuel in the wings.

Another 'typical' aircraft, the A320neo sinks by about 10cm.

Image from A320 Aircraft characteristics for Airport Planning document

Note that the values are given here for max. ramp weight.


Tyre wear and land gear shock reduced by wheel spin up From the answer to the question 'How are forces on the landing gear reduced or compensated for at touchdown?' I note:

  "For what it's worth you do not necessarily want to spin up the gear
  before landing for a few reasons. First off you would need to spin up
  the gear to precisely (or at least very close to) the speed that the
  plane was going to touch down at."

And:

  "Spinning the gear up can also have a serious gyroscopic effect that
  will affect the handling of the aircraft."

So why is this not a takeoff problem?

You do not need to spin to landing speed to have a dramatic effect. If you start the spin, you have added angular momentum to the wheel. It is much easier to increase angular momentum in a spinning wheel than it is to instantaneously impart it to a stationary wheel. A few scupper cups on the rim of built into the tyre edge would do the job.

  "This is a great research paper on the topic that suggests (in their
  conclusion) a pre spin situation would see 1.07% of the wear of a
  static situation."

Sounds like a worthwhile saving on so many aircraft.

I don't believe there is problem with gyro precession in spinning wheels; it's more of a vibration issue.  On most airliners the brakes are applied automatically after liftoff to stop the mains and the main reason is unpleasant vibrations as the wheels slow down without the damping effect of being pushed into the pavement.  On less complex aircraft pilots are taught to tap the brakes after liftoff to stop the main wheels from shaking (to the extent that they do) as they wind down.

Airliner wheels get statically balanced after tire installation but they don't usually get balanced dynamically like car tires, and there is often minor vibration that is not noticeable while rolling but can be quite noticeable while spinning on a gear leg dangling in the open when the wheel's natural frequency comes into phase with the gear, scaring the crap out of the pax.

Nose wheels are typically stopped by a snubber in the nose gear well upon retraction so they spin down until then.  Being so much smaller, they aren't as big a problem but if the static balance is not good you can get a pretty noticeable vibration under your feet that grows and then declines as the wheels slow down, then stops as soon as the gear is up. If it's bad enough crews will snag it.

Anyway, to have all that mass spinning all the time the gear is out?  Even if there was a simple way to do it, without the usual weight/cost/complexity issues, airlines would probably be forced to dynamically balance their wheel assemblies and purchase hugely expensive equipement.

Cheaper to just let them wear down and recap them.  Aircraft tires can be retreaded indefinitely as long as the carcass is sound.


How much was the engine moved in the B737 Max w.r.t. older versions? In the wake of the sad crash of ET-302, I am reading on multiple sources that

  the 737 Max's "engine is a bit further forward and a bit higher in relation to the wing, compared to the previous version of the plane. That affects the balance of the plane"

(from the BBC)

Is there data available on how much these "bit further forward" and "bit higher" amount to?

In particular, to minimize misunderstandings, it would be ideal to know how much the CoG of the engines has moved w.r.t. the neutral point.

The Boeing 737-800 and 737 MAX 8 both have the exact same fuselage length of $ 39.47 \, \mathrm{m} $, so I chose these two models for comparison.

Boeing publishes CAD drawings for all aircraft models. I used the two CAD files for the 800 and MAX 8 to compare the engine size and location:

Since I do not know the units used in the CAD files, I used the known total fuselage length to scale the numbers:

Distance from the nose to the front of the engine nacelle:

$$ \text{737-800:} \; 13.544 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 13.180 \, \mathrm{m} $$

Length of the engine nacelle including exhaust:

$$ \text{737-800:} \; 5.075 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 5.345 \, \mathrm{m} $$

Diameter of the engine nacelle:

$$ \text{737-800:} \; 1.978 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 2.407 \, \mathrm{m} $$

Distance from the top of the engine nacelle to the bottom of the fuselage:

$$ \text{737-800:} \; 1.359 \, \mathrm{m} \; , \; \; \text{737 MAX 8:} \; 1.554 \, \mathrm{m} $$

Based on these measurements, the new engine nacelles are $ 270 \, \mathrm{mm} $ longer and have a $ 429 \, \mathrm{mm} $ larger diameter. They start $ 364 \, \mathrm{mm} $ further forwards and the top is $ 195 \, \mathrm{mm} $ higher.

Therefore, the center (geometric, not necessarily center of gravity) of the engines has moved $ 229 \, \mathrm{mm} $ forwards and $ 19 \, \mathrm{mm} $ downwards. While the geometric center may have moved downwards, the center of gravity probably moved upwards because the old nacelle was not symmetric (flattened at the bottom).


What is the exact wing chord length and thickness for Boeing 747? I can't find this info anywhere and all they give me is the ratio. However, I need the exact/approximate values

In general, you can get good general dimensions from the Airplane Characteristics for Airport Planning documents (Boeing). These documents include the overall dimensions and everything relevant for the airport. This does not include the chord length though.
To get more accurate dimensions, you can download CAD drawings from Boeing. I did this for the 747-400 and the 747-8I and measured the chord length in three places along the wing:

As you can see, the chord length varies between 14.84m at the root and 3.70m at the tip (before the winglet). The wing for the 747-8 has been slightly re-designed:

While keeping its basic structure and sweep, the wing is thicker and deeper, holding more fuel, and wider with raked wingtips.

(Wikipedia)
The differences in chord length are rather small (possibly less than the accuracy of these drawings):


Can airliners benefit from vertical take-off and landing? A vertical take-off and landing (VTOL) aircraft is one that can hover, take off, and land vertically. We have several military fighter jets, which can takeoff and land vertically (like the Harrier Jump Jet).

Would it be possible to design an airliner (for example as big as a A320 carrying over a hundred people and travelling at 0.85 mach) to perform VTOL?

The closest to what you ask was maybe the Do-31 experimental VTOL transport.

Dornier Do-31 in hover. Picture Source

It was developed to fly supplies to VTOL fighter bases. Makes no sense to have a VTOL fighter fleet if you cannot supply them, right? That was what the Do-31 was for, and the technically successful program folded when NATO abandoned the VTOL strategy in the Sixties.

Compared to regional airliners of its time, the table below shows how much the installation of sufficient thrust has cut performance:

I used the Fokker F-28 for comparison, because it is of similar size and age. As you can see, VTOL easily costs half the payload. 36 fully equipped troops may weigh more than 36 passengers, but they will sit on much lighter seats and have much more frugal accommodations. Note that I used the reduced take-off mass for VTOL flight, but have no information if that mass would have included the full payload. The range is valid only for the higher maximum take-off mass of 24.5 tons, for which a conventional take-off was necessary.

The Do-31 could have fitted the Belgian airline SABENA, which at the time (the Fifties and Sixities of the last century) had the most advanced VTOL strategy of all airlines. They operated regional connections with helicopters, but eventually reverted to regular aircraft well before the Do-31 would had been available. Heli-Air Monaco is still in operation after almost 40 years, but this is more a taxi service than an airline.

Considering not only the increased fuel consumption, but also the increased maintenance cost on more and diverse engines (most VTOL airplanes have dedicated lift engines), the operating cost even at the times of cheap fuel were too high to make the convenience worth it.

I think it is possible to develop a VTOL A320, but it would look very different, would cruise at a lower speed and have a much reduced payload and range. Tickets would be horrendously expensive, though.

EDIT: Thanks to @egid for his suggestion to use mass fractions for comparison. Unfortunately, I found detailed mass data of the F-28 only for a later version, but the expanded table shows the point clearly:

The useful load fraction is the result of dividing the difference between take-off mass and operating mass empty by the take-off mass, showing the sum of payload and fuel as a fraction of take-off mass. Adding lift engines, slow-speed controls and a rear loading ramp clearly reduced what could be carried in VTOL flight.


Why did Air India 139 fly so far south? (twitter.com)

An Air India flight from Delhi to Tel Aviv started very far to the south before turning west. Why did it do that? Which airspace did they have to circumvent for political reasons? The India-Pakistan border is way further up north, so that shouldn't be the issue, or is it?

(flightaware.com) Return flight.

AI 140 (the return flight) arrives closer to the Pakistani FIR border. The south route when flying westward can be explained by jet stream avoidance.

Apart from the recent exceptions given to Air India by Saudi Arabia and Oman, the countries shown below do not permit flights to enter their airspace if their destination/origin is Israel (flightservicebureau.org, March 2018).

This is also briefly covered on FR24's blog.


Can I land a plane in a field? If I had a plane that could take off and land in a short distance (eg., the SD-1 Minisport), do I have to use a recognized airport in order to land in the US?  Or can I just land anywhere so long as it's class G airspace?

FYI, this question is for non-emergencies.  IE., if I just wanted to land there because it was convenient.

Landowner's permission is always required.  Then check the municipal zoning regulations and the like - city, state regs etc.  Then your insurance company.  If all these points come up green, go do it.


What type of license do you need to solo a glider in the UK? What type of license do you need to solo a glider (cross-country) in the UK?

Wikipedia states that:

  There is no government-issued licence at present, but EASA regulations will require one by 2015.

This doesn't seem to have updated and I couldn't find anything on the  British Gliding Association (BGA) website.

This is on the BGA website under (unsurprisingly) Laws & Rules

  There is no requirement for glider pilots to comply with EASA regulations for pilot licencing until 8 April 2020. The relevant BGA requirements and guidance continue to apply.

You can further follow links to the BGA Certificates, including a Cross Country endorsement:

https://members.gliding.co.uk/library/bga-requirements-guidance/gliding-certificate-endorsements/

In part:

  The Cross-Country Endorsement can only be issued if the applicant has already qualified for a Bronze Endorsement or SPL/LAPL(S) and has the approval of theCFI.


Is there any size of tethered kite that is regulated by the FAA? Civilians fly kites for entertainment.

Is there a limit, after which the kite is considered an aircraft and becomes regulated?

Is a kite considered a balloon, after a certain size?  Do technology or other control apparatus make the distinction?  Is it purely height-based, and size is (mostly?) irrelevant?

As one of my instructors was fond of pointing out, there's pretty much a regulation for everything -- in the US kites and balloons would be covered under FAR part 101 which is pretty short and to the point as far as government regulations go.

The big rules regarding kites/balloons kick in for a kite "over 5 pounds" or a balloon "over 6 feet in diameter / 115 cubic feet of gas capacity", and are:

You have to keep them at least 500 feet below any clouds.
You have to keep them less than 500 feet above the ground.
You must be at least 5 miles from the boundary of any airport.

If your kite/balloon is going to be more than 150 feet above the surface you're also supposed to coordinate with the closest ATC facility (who would theoretically put out a NOTAM saying "John Doe is going to be flying a kite/balloon/thingy here, between these hours. Please try not to hit it.")

There are also some lighting requirements (if you fly the balloon/kite at night), and visibility requirements (but really, who wants to fly a kite in less than 3 miles visibility? That seems like pretty lousy kite weather.)

As a favor to your local pilot community (and to prevent having your wayward kite dragged off by a Cessna on short final) it's probably best to observe the three "big rules" above regardless of the size of your kite - a 6-ounce piece of balsa and cellophane can still make a lot of people's days more interesting if it appears in an unexpected location.


What is this delta-winged aircraft? I spotted this recently. The distance and poor camera makes it really hard to determine what it is. Any help appreciated. Thanks.

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

F-PDHV (D-Plane 1)

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

Picture from below:

Source

David Osborne/ Aeroprints Source


What is the background of the maximum altitude formula? This question What determines the maximum altitude a plane can reach? has a formula at Peter Kämpf's answer :

$$\rho_{min} = \frac{2\cdot m\cdot g}{(Mach^2 \cdot c_L)_{max}\cdot a^2\cdot S}$$

Does anybody know what's the name of the formula or who has developed it?
Could anyone explain why $Mach^2 \cdot c_L = 0.4$ is a good value? What determines good or bad values for airfoils or what type of airfoils is requested?

The formula can be derived equaling the weight ($W$) of an airplane with the maximum lift ($L$) generated by its wings. I'm not aware of any special name for it.

$$ W = L \\
m\cdot g= \frac{\rho_{min}}{2}\cdot V^2\cdot S\cdot c_{L,max}$$

Substituting the aircraft's speed with an expression containing the Mach number ($a$ is the local speed of sound),
$$V=Mach \cdot a$$
and solving for the density you get the formula in Peter's answer:

$$\rho_{min} = \frac{2\cdot m\cdot g}{(Mach^2 \cdot c_L)_{max}\cdot a^2\cdot S}$$

At high speeds the maximum lift coefficient is not constant. That's the reason why the Mach number and the maximum lift coefficient are written together in one term: $(Mach^2\cdot c_L)_{max}$. In order to fly as high as possible this term has to be maximized.

With supercritical airfoils you generate less drag and can therefore fly at higher Mach numbers, increasing this term.

I don't think the value 0.4 is a physical limit. Probably it is just an empirical value which takes many real-world constraints into account (if there is a physical limitation I'd be glad to read an answer explaining it). If you calculate $(Mach^2\cdot c_L)_{max}$ using U-2's data you get indeed a value close to it.


Descending on a given glide slope (e.g. ILS) at a given airspeed-- is the size of the lift vector different in headwind versus tailwind? We had a discussion with friend and we were talking about if the airplane is on approach and the one is landing with tailwind and the other one is landing with headwind, so both have the same value of IAS but the different value of ground speed. Will the airplanes produce the same value of lift in both cases or will be the value of lift be different? He said that value will be still the same and I was argueing that if you have tailwind you will have to increase angle of attack which means that parasite drag will increase so the value of lift will have to increase.

EDIT: Question is: If two same aircraft are on same glidepath (descending on ils glide slope) with same IAS but different ground speed, because of headwind acting on one aircraft and the tailwind acting on the other aircraft, will the amount of lift produced by one of the airplanes be different or will be both two values be the same?

EDIT: This question is really not about lift vectors. It is asking about the total amount or total value of lift needed with varying headwinds.

Everyone seems to be over complicating this very basic question which asks about two identical aircraft flying down an identical ILS glideslope with the exact same indicated airspeed. The only difference is the amount of headwind.

It is so simple. Rate of descent is a direct result of the amount of lift being produced.

It is a FACT that the aircraft with less headwind will need to produce LESS lift than the aircraft with more headwind, in order to maintain the same indicated airspeed, and follow the glide path.

The rate of descent with a headwind could be -500ft/min while the aircraft with a tailwind could be -1000ft/min.

The aircraft maintaining a -1000ft/min descent because of a tailwind will need to produce LESS lift than an aircraft maintaining a -500ft/min with a headwind.


If I am pushing my aircraft into the hangar with a tow bar, at night, with the engine off, am I required to activate my rotating beacon? I know there is a similar question here: When to use Beacon, Anti Collision, Strobe, Logo and Navigation lights?, but it doesn't answer my specific question.

Do I need to have my rotating beacon on at night while I park (push) my airplane after I've turned off the engine?

It sounds like an urban legend, but someone told me they heard of someone facing disciplinary action by the FAA due to moving their airplane at night without the rotating beacon turned on, even though they were just pushing it back into the parking spot with the tow bar.

I know the AIM 4-3-23 says

  Use of Aircraft Lights

  a. Aircraft position lights are required to be lighted on aircraft
  operated on the surface and in flight from sunset to sunrise. In
  addition, aircraft equipped with an anti-collision light system are
  required to operate that light system during all types of operations
  (day and night).

But is pushing an aircraft into parking with the engine stopped (in a non-movement area), considered "all types of operations?"

The AIM gives good advice, but that is really all it is. It is acceptable methods of complying with the regulations, but you are not required to comply with what the AIM says. In questions of what you are required to do, especially in questions of whether you might be at risk for a violation, we want to look at the regulations in 14 CFR.

14 CFR 91.209 states, in pertinent part:

  No person may:

  (a) During the period from sunset to sunrise [...]—

  (1) Operate an aircraft unless it has lighted position lights;

  (2) Park or move an aircraft in, or in dangerous proximity to, a night flight operations area of an airport unless the aircraft—

  (i) Is clearly illuminated;

  (ii) Has lighted position lights; or

  (iii) is in an area that is marked by obstruction lights;

  (b) Operate an aircraft that is equipped with an anticollision light system, unless it has lighted anticollision lights. However, the anticollision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off.

Note:
Position Lights are commonly called Nav Lights
Anticollision Lights can include what are more commonly called Strobe Lights and Beacon Lights (See AIM 4-3-23)

Lets first look at what the above regulation says about operating requirements. Is your given scenario (engine off, etc.) an example of operating the aircraft? 14 CFR 1.1 gives the following definition of operate:

  Operate, with respect to aircraft, means use, cause to use or authorize to use aircraft, for the purpose (except as provided in §91.13 of this chapter) of air navigation including the piloting of aircraft, with or without the right of legal control (as owner, lessee, or otherwise).

By that definition I would argue that towing, pushing, or otherwise moving an unoccupied aircraft with engine(s) shut off would not be operating that aircraft anymore than leaving the aircraft parked overnight would be operating it, since you are not using the aircraft for the purpose of air navigation. Thus I would say that you are not required to follow any rules that pertain to operating the aircraft, such as that any installed anticollision light be lighted.

(This is in contrast to an airliner pushing back with a load of passengers, flight crewmembers at their stations, electrical power on, APU running, etc., which would be operating, since the aircraft is being used.)

This leaves us with the Park or move paragraph. This paragraph specifically pertains to "in, or in dangerous proximity to, a night flight operations area of an airport".

What is a night flight operations area? Absent any proffered definition, the plain reading of the phrase—as I read it—would be an area used, or subject to use, for flight operations at night. In other words, any public use airport with an apron or taxiway that is open to flight operations at night could and should be considered—by my reading—a night flight operations area. Whether the area is a movement or non-movement area shouldn't matter. Could someone else reasonably taxi past you (or into you!) at night? That's probably a night flight operations area.

Thus, for your given scenario, if you are pushing your aircraft back into a tie-down spot on an apron that is open to night flight operations, by regulation it must be either:

a) clearly illuminated;
b) have lighted position lights; or
c) be in an area that is marked by obstruction lights.

I typically see airport ramps clearly illuminated by large overhead lights. This clearly meets the first requirement. Another means of compliance would be to turn on an internal hanger light to clearly illuminate the airplane when you push an airplane in off a taxiway into a hanger.

You are also good to go if you are in an area that is marked by obstruction lights. I don't think I have ever seen these demarcating an area where you would be pushing or towing an airplane (I'm thinking of those lighted construction barriers), but if you saw them I suppose you would know.

In the absence of clear illumination or an area marked by obstruction lights, if you are going to park or move the airplane at night, you had better turn on the position lights. And this, I think, is where you could be at risk for a violation.

Note that, even in that case, the requirement is for position lights, not the beacon. Unless the aircraft is being operated, there is no requirement that any installed anticollision light be lighted.


What are the 40 hours requirements for a PPL? According to 14 CFR 61.109, a private pilot applicant must log 40 hours of flight time.

  (...) that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training (...)

What are the requirements for the remaining 10 hours?

There is no requirement for the remaining 10 hours, just that those 10 hours are part of your instruction/training.

So really you can read it as "a private pilot applicant must log at least 40 hours, of which 20 must be flight training from an authorized instructor and 10 hours must be solo".

How you distribute the remaining 10 are up to you and your instructor, although the vast majority of students don't find it difficult to place those hours anyway since the national average is closer to 70 hours to get a PPL.


Would using a full span spoiler and a separate flap be a better design than a flaperon for a STOL ultralight? Would using a full span spoiler and a separate flap be a better design than a flaperon for a STOL ultralight?

I understand many ultralights use 50% flaps and 50% ailerons.  The problem with this, is that it greatly reduces your Clmax.

I understand Zenith uses a full length combination flap and aileron, called a flaperon.  However I imagine using the 50% split aileron with full flap would still effect total lift.

What about using a 100% spoiler and a separate 100% flap?  Would that be a better design?  If not, why not?

See pic below

There is an interesting NACA report on use of spoilers for roll control here.

I think if you want to do that the spoiler should just be around the outer 1/3rd of span (the rolling moment generated inboard is low - barely worth extending the surface inboard. And the spoilers should be at the rear spar.  For reasons I'm not entirely clear on, they work much better close to the trailing edge and the only time you see spoilers at the main spar is on gliders where they are being used only to increase sink rate.

A significant downside to roll spoilers you have to account for is a "dead zone" in the first, say, 10 degrees of extension due to it having to extend beyond the boundary layer to start to work, so you find the stick also has a dead zone a bit to each side of center.  This may be why you don't see more use of roll spoilers on ultralights and airplanes that have airfoils that are fairly turbulent aft of the spar.

On the  other hand, the report says, to my surprise, that roll authority is almost as good while in inverted flight (I guess in that case it acts like a split flap when the pressure distribution is flipped over and is able to move the wing while inverted in the same direction as while right side up - this is the first report I've found that directly addresses this).

I'll also point out that the use of roll spoilers and full span flaps is usually done on airplanes where you want to maximize the speed range like on the MU-2, where you have very large fowler flaps that increase the wing area by about 20% by being full span.  In the end, for an ultralight that flies down to 25mph and cruises at 55 mph, all that trouble to develop full span flaps and roll spoilers might only get you a couple mph reduction at the bottom compared to a large flap and smallish aileron at the tip, with negligible benefit at the top.


How can I get flying again after FAA medical rejection for anxiety two years ago? Post Military Service I was diagnosed with Generalized Anxiety Disorder and put on Zoloft (Sertraline). I had taken it for a while but had not refilled my prescription, and was off the meds with the approval of my doctor. During my FAA medical exam (3rd Class) they checked my insurance and saw that I had been prescribed the meds. I had not put it down as I misread the prompt as currently taking or being treated for those conditions.

I'm totally fine now, currently an EMT, been off the meds for 3 years now, and hoping to start flying again.

My hopes are low, but is there any hope for me getting flying again?

In general, as I do not have the guidance in front of me presently, if you have been diagnosed with an anxiety disorder, and later taken off the medication with no adverse effect, and the symptoms are absent, you have a high probability of obtaining a medical.

First, your primary care physician should document that you have been off the medication for x months.  Usually two years is good enough.

Then you should shop for an AME.  You want someone who is willing to work with you to get the documentation needed and submitted. Your initial application may be initially deferred, and forwarded to FAA Medical Headquarters in OKC for review. OKC has in the past taken calls on medical certification inquiries, but has also not taken them.  I would call them and ask for their guidance documentation.

If you are an AOPA member, their medical support people are excellent, and will give you specific guidance, as to what reports and findings will be most helpful.

It is possible that you might need an evaluation by a specialist, a psychiatrist, to assure that you are not walking around untreated, and do not show any signs of GAD.  GAD is often considered a long term chronic disorder, and it is not uncommon for people with GAD to have the condition for their lifetime.  In your case, you will want it to be that the diagnosis surfaced with situational circumstances, and the condition is no longer an issue, and the medication is not indicated at this time.

As a CFI, I have had quite a few primary students, and one CFI student who had similar or even more significant psychiatric disorders, and as those were addressed and they were able to drop the medications, they were again qualified for a medical.  Also keep in mind that your road to Basic Med will initially require a 3rd class or higher medical.  So it would be nice to get this administrative matter resolved.

In addition to AOPA, there are places which primarily help pro pilots, but will provide excellent services to help guide you through the process and if you are qualifying, will help you get a medical.  They can be several thousand dollars, and in your case a year of AOPA membership would be very cost effective.

As an example, I had a primary student who had a history of suicide attempts.  It turns out that she felt she was trapped in an abusive relationship.  A couple of years later the situation was different, and her PCP and therapist records were all that was needed to have the FAA issue an unrestricted medical.

For air ambulance, if you are not playing a role which requires a FAA medical, then you will not need one.  In air ambulance, the typical roles requiring a medical are pilots.  I don't know of any air ambulance outfit which requires a FAA medical for their EMTs.  You could always ask at a place that you might be interested in employment.

The FAA medical practices are in place to mitigate perceived risk.  People have situational circumstances in their life which may be outside medical issuance, but their situations also change.  To a degree, the FAA generally recognizes that.

Good luck pursuing your interests.


Swept back wing theory I'm a beginner in airfoil design. I just came across swept-back wings. There is a point that I couldn't understand that theory.
Why is the only perpendicular component of the airflow to the wing(component parallel to chord line) affects the shockwave formation? Why doesn't the total airflow vector(spanwise +chorwise vector component) contribute towards shockwave?
Why wouldn't the shockwave form with the total airflow vector in the swept wing?
I have attached the image to show my point better

Imagine a wing that is infinitely long, and stretches from east to west. It is moving north at 400 mph. Obiously, it does not produce a shockwave, as it is moving subsonically.
Now, imagine a copy of this wing  that is moving to the east at 800 mph (along its length) while still moving north at 400 mph. Other than skin effects, this wing is aerodynamically identical to the first, so it does not have shockwaves, yet it is very supersonic.
Of course, a real swept wing is not infintely long, and so there are shockwaves at nose and tip of the wing, but mid span it really does behave similarly to a wing moving much more slowly perpendicular to its leading edge.


Is there some functionality of an airliner which is not controlled by the cockpit? We have had a blockbuster question, asking about the complexity of the cockpit, which was answered very well. With so many controls, are there any flight- or safety-critical functions which cannot be controlled by the cockpit?

You cannot (as far as I know),

Flush the toilets
Control the IFE system (aside from possibly pulling the breaker to shut it off)(there seems to be some debate on this one and it could vary by plane/system)
Open the doors (of a passenger plane, cargo planes this may be possible)
Update the software for glass cockpit planes (if the computers are not located in the cockpit). I'm still looking for the link but FMS units are updated via USB on a unit usually housed with the other flight computers which is often below the cockpit. In some smaller planes these units are in the cockpit as are the USB ports so this may only hold for some aircraft.
General passenger tasks (reading lights etc). Although it should be noted that generally speaking the master breakers are in the cockpit so while they may not have control over individual system components (e.g. single reading lights) they can often shut them down completely if need be.
Ovens (for heating meals) and coffee machines. Only circuit breakers are in cockpit again (and they are needed; faults in these things cause relatively high fraction of diversions).


How wide is the cone of confusion above a VOR and NDB? I saw in book that the cone of confusion of VOR is 40 degrees, and NDB is 50 degrees of each side! Isn't it too much?
Maybe it's a mistake, or old information(book printed in 2004)
Clarify please:
What is the real amount of cone of confusion of VOR and NDB?
What should I do when flew above VOR/NDB (in cones) in terms of navigation?

The value +/-40° is correct, but need to be interpreted. The primary reason of the existence of a "silence" (confusion) cone is a design choice to increase navaid range. Range and "isotropy" are antagonistic.
However, when the aircraft approaches the confusion cone, say at 45° elevation, while flying at 9 km above the station, its slant distance to the station is about 13 km. Even with a small apparent power (say a few RF watts), the radio link being not attenuated by obstacles, is still very good. It should not prevent the signal from being demodulated.
There is a second phenomenon, which is more critical for a D-VOR: The Doppler shift is created by the antenna scan in the horizontal plane only, and is maximum when the receiver is on the station plane. As the relative elevation increases, the apparent displacement in the direction of the receiver is reduced. This reduction of the distance results in a reduced apparent velocity toward/away from the receiver, hence a smaller Doppler shift.

Without the correct shift, -480 Hz to +480 Hz, bearing determination is inaccurate (for a detailed explanation of VOR principles, see this answer). For conventional VOR there is a similar problem but related to the amplitude.

Detailed case: D-VOR
A VOR radiates a signal that can be interpreted in the horizontal plane, so the system is designed with antennas radiating horizontally. There are two types of VOR, the most modern is the D-VOR (Doppler VOR). A common antenna for D-VOR is the Alford loop antenna which is horizontally polarized:

Left: Source, right: Source
The D-VOR antenna system consists of a circular array of antennas:

Source
This array has a radiation pattern similar to this in the vertical plane:

How to interpret:

The straight lines are elevation angles, 90° being the zenith. The brown circles are a scale for power radiated, the large circle is the power measured in the direction where it is maximum. The next circle is where power is only -3dB of the maximum. Using decibels to measure ratios is frequent when working with radio signals because this is a logarithmic scale. -3dB is half the power, -6dB is 1/4 of the power, etc.

The red curve is the radiation power curve of the VOR for any vertical angle. It is maximum around 30° and minimum at 90°. For 60° its about -20dB which is a ratio of 1/100th only.

VOR cone of silence/confusion is where the signal is too small to be correctly interpreted by the VOR receiver, and you can see there is a strong signal fall around 50°-60° (that is 30°-40° from the VOR zenith)

As visible, power is mostly radiated to one side, the other side of the antenna array radiates only a parasitic power. This is due to the way the D-VOR works: Interfering pattern between all the antennas of the array. Power is destructed on the right side of the diagram, while it is cumulated on the left side.

The interfering pattern is rotated electronically around the vertical axis at 30 rounds per second (1,800 RPM, 30 Hz), so the antenna actually radiates power in an annular volume (donut).

Doppler cone of confusion
Specific to the D-VOR, due to the way this VOR works:

In the conventional VOR relative bearing is obtained by sensing the phase difference between a reference modulation (FM) and a varying modulation (AM) which depends on the direction the VOR antenna is currently pointing to.

In the Doppler VOR, the varying signal is actually obtained by electronically moving the center of the antenna array. This creates a frequency shift that is equivalent to the AM modulation of the C-VOR, but in frequency (and the D-VOR electronically "rotates" in the other direction so that the inversion between AM and FM modulation is not visible, ensuring compliance between C-VOR and D-VOR with the receiver).

However the Doppler effect can be sensed only when the receiver is on the same plane than the electronic "displacement", i.e. the horizontal plane. When the receiver is located too close to the VOR zenith, the frequency shift becomes small, and cannot be sensed correctly. This is why, even with an isotropic antenna, the system cannot work at high relative elevations. So there is actually no point to have an isotropic antenna for the D-VOR.

C-VOR and NDB
A conventional VOR (C-VOR) works with a vertical antenna which radiation pattern is naturally similar to the one above without the need for interferences. The antenna actually rotates physically.
NDB use long wire antennas of many kinds. The power is radiated with nearly no particular preference (omnidirectional antenna), but the radiation pattern is still similar to the C-VOR one, except that there is no need to rotate it. The donut volume is obtained with the antenna stationary.
As the NDB is omnidirectional, the energy in the horizontal plane is not focused as strongly as for the C-VOR and D-VOR. However NDB range is still reasonable because:

NDB uses the LF/MF portions of the radio spectrum which are less subject to propagation losses.

NDB receiver doesn't require a strong signal to locate the beacon, as simply detecting the carrier is in theory sufficient.

Why is that done this way?
You may ask why not using an antenna (or an antenna array) with a circular radiating pattern radiating the same power in any vertical direction. Such "isotropic" antennas exist in practical.
The reason is the same than for rotating the antenna: It's to concentrate power in one direction, to increase the range. In technical terms this is to increase the effective isotropic radiated power (EIRP).
A good similarity is the one of a light bulb and a torchlight:

A light bulb has an omnidirectional radiating pattern, it lights up in all directions, but the power received at a distant point is limited (unless the light bulb is a very powerful one).

A torchlight focuses light in a small solid angle, multiplying by a large factor the intensity in the privileged direction. A high intensity can be perceived in this direction without using a huge power, and the range is increased at equal power. However to scan all points in space, the torchlight must be rotated.