Looks like a wiki answer would be possible, several of us can provide a part of the full answer. I initiate this one... (before saving your edit, copy your text somewhere, e.g. in your clipboard, as in case of simultaneous updates, you may lose it.)

Feel free to edit and complement!

See also:

Wikipedia article on Direction Finding
Wikipedia article on Direction Finder
Wikipedia article on Bellini–Tosi direction finder

Principle: Radiogoniometry, aka direction finding

Available techniques

Determining the bearing of an aircraft from the ground by radio can be done using multiple principles:

With no cooperation from the target: Primary surveillance radar, which means sending a radio wave to the aircraft, and looking for the bearing of the echo returned by the aircraft skin materials.
With automatic cooperation from the target: Secondary surveillance radar, the ground SSR sends an interrogation to the aircraft SSR transponder, which in turn replies by sending pulses which bearing is determined by the ground SSR interrogator.
With manual cooperation from the pilot: The pilot sends a continuous wave using any of the transmitters on-board, e.g. the voice communication transmitter, and the ground station determines the bearing of this signal.

In the documentary video the last technique is used, so let's focus on this one.

Bearing determination techniques

There are several principles which can be used to determine the bearing of the waves from the aircraft:

Peak / null of signal amplitude with a directional antenna privileging reception in one direction. We just turn the antenna until the maximum or minimum signal is found. The maximum gives directly the bearing, the minimum gives the direction +/- 90°. This last option is generally used, as the amplitude rate of change near the null is generally much larger than near the maximum, which increases precision. See How does the ADF null positions find where the NDB is?
Doppler shift: When a radio source moves, the frequency of the source is shifted according to the relative speed between the source and the observer. With some (complex) signal processing, the bearing and trajectory of the source can be extracted.

In 1955, only the first technique was used.

By using two distant stations, two intersecting bearings are available, the distance to the source can also be determined. The technique is known as triangulation (the two stations and the source being the three apexes of the triangle).

Receiver on the ground: Antenna

We know the principle to use: The pilot triggers a continuous wave transmission the time for the ground station to determine the maximum or null direction.

The first generation of direction finders used rotating antennas. Using an antenna with an asymmetrical sensitivity, the voltage at the antenna output is dependent on the direction of the source, according to the antenna radiation pattern (which is also the reception pattern). The antenna can be rotated until the maximum or minimum is found.

A first improvement has been to use the principle of interferometry. With two or more antennas. The same signal reaches the antennas with a difference of phase due to the different distances traveled, unless the source is perpendicular to the antenna array plane. With this technique, the antennas mustn't be directional, they can be more simple: a vertical pole. A system using four antennas, creating two reception directions (North-South and West-East), has been deployed in UK on 1920-1930 (the Adcock antenna network and the Huff-Duff/HD-DF receiver)).

Adcock, Huff-Duff/HD-DF and Bellini-Tosi/BT-DF

The principle of the Huff-Duff was to use the N-S and W-E antenna outputs to drive the electronic beam of an oscilloscope. An oscilloscope is (was) a cathodic ray tube, with a source of electrons and two pairs of deflecting electrodes, one to determine the horizontal position of the electron beam on the front face, and one to determine its vertical position. By ensuring the displacements were a sine/cosine transformation of the Adcock array output, the beam was directed to a spot indicating the bearing of the source.

With this system, it is not required to turn the antennas, and a bearing can be determined instantaneously.

A similar direction finder was also built in the US by Marconi: Bellini–Tosi direction finder (B–T or BTDF) using a loop antenna and two vertical antennas (sense antennas) to remove the front-back ambiguity. The output of the antennas are quite the same than the Adcock system, except being not referenced to the North direction (so technically speaking, the Adcock system provides an azimuth, and the B-T system provides a bearing).

Ground direction finder for US Navy

...

From ground direction finder to airborne direction finder

...

Ground radiogoniometry circa 1955 in the US

...

Time to get a bearing

...

Ground radiogoniometry today

...


What are the 40 hours requirements for a PPL?

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.


Why does a banked aircraft create sideslip?

Incorrect explanations abound for why an aircraft tends to sideslip when banked, in the absence of any "coordinating" rudder input.  Often the fact that the weight or gravity vector has a component pointing towards one wingtip is invoked.  See for example the diagram in Martin Simons' "Model Airplane Design".
The truth is that this situation exists in a coordinated turn as well as in a sideslip, and so this cannot be the reason for the sideslip. According to Newton's principles, a steady sideways force tends to drive a curvature in a trajectory, not a linear sideways motion.  It is not correct to say that the weight or gravity vector tends to cause the aircraft to slide toward the low wingtip whenever the aircraft is banked, just because the weight or gravity vector contains a component that points toward the low wingtip.
Here is one reason that an aircraft tends to sideslip when banked-- banking tends to cause a turn (a curvature in the flight path), and the resulting curvature in the flight path "bends" the airflow or relative wind.  In other words, as the aircraft starts to yaw in response to the changing direction of travel, different parts of the aircraft are moving through the airmass in different directions at different speeds at any given instant in time, and so different parts of the aircraft experience different velocities (speed and direction) in the local relative wind, which can be described as a "curve" or "bend" in the direction of the relative wind.  In the absence of any rudder input, the vertical tail experiences a relative wind component that tends to oppose the turn and slow the yaw rotation rate.  This tends to cause a sideslip.
If this is unclear, consider a pinwheel, or an aircraft in a flat spin.  As the body rotates, clearly, different parts of the body are moving through the airmass in different directions at any instant in time.
This phenomenon can also be described as "yaw damping".
This is also the phenomenon that is described as the "long-tail slip" effect in Section 8.10 of John S. Denker's "See How It Flies" website.
If the aircraft's fuselage could bend like a banana to conform to the curving path of the turn, then every part of the aircraft from the nose to the tail could be streamlined to the local direction of the curving relative wind, and this phenomenon would not occur.
Rotational inertia in the yaw axis also plays some small role in driving sideslip as the bank angle and turn rate increase.
Explanations that attempt to account for the relationship between banking and and sideslip, without discussing the curvature in the flight path that generally results from banking, are invariably incorrect.


I can answer part of this by experience, as a non-professional hobbyist pilot, and partly by information which is fairly easy to come by. So here goes:

Are fractions of an hour taken into account or only ever full hours? I.e. if some pilot flew 1:27 h on one flight and 1:33 h on another how does that count towards the logged flight hours?

Fractions of hours are absolutely taken into account. Typically accuracy is not to the minute, as this would be fairly pointless but typical schemes are to round to the nearest 1/10th of an hour (6 minutes) or 5 minutes. Personally, a long time ago I chose to go for 5 minute rounding and have regretted it ever since - 6 minutes is much easier to add up when dealing with hours & minutes!

Given many regulations surrounding aviation seem to be global, is the way flight hours are logged uniform around the world?

Nothing in aviation is global, or very little. Although there are bodies such as ICAO (International Civil Aviation Organisation) which most member states tend to derive most of their rules from. For the purpose of logging flight time, I expect most civil pilots log on the basis of "Chock to chock" time. Which is to say when the aircraft moves under it's own power until it stops.

Any differences between commercial, private and military aviation?

From what I can tell, the only difference is military which seem to log in-air time. This, I'm told, is why military airmen who intend to pursue a commercial pilot career after they're discharged maintain a separate record of their flight time.


What gas is used in airships to give them buoyancy?

There are only 2 lifting gasses that are lighter than air and practical for airships;

helium: this is a noble gas which means it's inert. Safest option all around except that US has a monopoly on the production of it. That is also the reason Zeppelins didn't use it because of an embargo the US had at the time against the Germans.
hydrogen: even more buoyant than helium but flammable. It is however easier to get by running a current through water or dropping some metals in an acid.

The third option for providing buoyancy is heated air, this requires a heat source to counteract the loss of heat over time.


My question was due to the confusing mention of ADS-B in what was supposed to be either TIS-B or ADS-R.

I've now confirmed that the text on ICAO Doc 9871 Version 2 I quoted was due to a error.

In fact DO-260B, and even Doc 9871 Version 2 at table C-37 specify that TIS-B CF=4 is "Fine TIS-B message, AA field contains a 24-bit anonymous aircraft address".

ICAO e-mailed me confirming that this has been fixed into their working document.


What is the difference between conventional VOR and Doppler VOR?

Short answer:
There are several types of conventional VORs using either rotating or static antennas. What they have in common is:

They are relatively compact.
Bearing is determined from a HF signal rotating 30 rounds per second.
They are very sensitive to multipath interference created by reflection on nearby obstacles, and must be located at isolated places. They are often used for airway beaconing.

There are also several types of Doppler VORs, single or double sideband:

They requires a large 14 m diameter circular array of antennas.
Bearing is determined from the Doppler effect created by scanning the array at 1,300 m/s.
They are less efficient, inactive antennas from the array absorb a significant part of the signal emitted from the active antennas, reducing the effective range.
They are less sensitive to multipath, and are easier to install at airfields. They are commonly used for runway approach.

Bearing is more accurate with a Doppler VOR unless the conventional VOR site neighborhood is exempt of obstructions.
That said both types have the same tolerances from an ICAO standpoint:

The transmitting elements must be accurate to 2°. Most stations are able transmit a signal with an accuracy better than 0.5°. A perfectly sited VOR could deliver this precision to the receivers.

However errors are mostly created by the environment, by reflection and other phenomena changing the signal phase, similarly to a mirror inverting left and right on its reflected image. When direct and reflected waves interfere, an error can be introduced. ICAO limits this error to 6.5° in the volume of service. In practical most VOR stations are well below this limit, possibly using Doppler VORs in most cluttered sites.

A word about these multipath errors: The direct wave propagates along the line of sight (LoS), but the transmitting antenna radiates in other directions. The corresponding rays can be reflected, diffracted and refracted by obstacles and reach the receiver antenna as indirect waves. This can be beneficial, e.g. it extends considerably the range beyond LoS. But waves travel different distances before reaching the receiver:

This means their phase is different on the receiver antenna. Interferences occur, and they change the signal amplitude.
In a volume called the first Fresnel ellipsoid (FE1), centered on the LoS, where the length difference cannot exceed $\small \lambda / 2$, $\small \lambda$ being the VOR carrier wavelength, interferences are constructive, amplitude is increased.

In the second Fresnel ellipsoid where the difference is between $\small \lambda / 2$ and $\small \lambda$, interferences are destructive, amplitude is decreased, and this continues with other multiples of $\small \lambda / 2$. However 80% of energy is in FE1, and the rest is mostly in FE2. Therefore when planning a radio link, only FE1 is taken into account. The ratio between the obstructed areas in FE1 and FE2 determines the ratio of energy with constructive interferences, thus when siting radio stations, at least 60% of FE1 is kept clear of obstacles.
This can be a challenge. For an aircraft at a distance of 20 NM from a VOR located on the airfield, flying at 1000 ft QFE, the radius of FE1 at the middle point of the link is 500 ft. The ellipsoid is already tangent to the ground in the best case, and therefore the ellipsoid between the VOR and this middle point is slightly obstructed.
Limiting VOR errors consist first in removing obstacles from FE1. However as multipath first impacts amplitude, it hits conventional VOR harder, as this type of VOR depends more on a amplitude accuracy.

VOR principles
A VOR receiver, regardless of the technology behind, receives two sine signals repeating 30 times per second: Reference and variable signals, and determines the bearing by solely measuring the difference between their phases.
From a carrier standpoint, only amplitude modulation is used, but to minimize a possible influence of one signal onto the other, the VOR station uses a subcarrier, which frequency is low (9,960 Hz) but significantly distant from 30 Hz. One phase value is conveyed by normally modulating the carrier in amplitude, the other by modulating the subcarrier in frequency. The subcarrier then modulates the carrier in amplitude.
In addition, two antenna systems are used:

The carrier and the reference are send on one or more omnidirectional antenna(s) and all receivers see the same reference phase at the same time.

The variable signal is sent appart to appear different for receivers at different bearings. The type of antenna used depends on the type of VOR.

As explained below, the variable signal is not created by actual modulation of the carrier, but by synthetic sidebands. Modulation and sidebands are anyway equivalent by Fourier transform. Amplitude modulation changes the amplitude of the carrier over time (time concept), but also creates symmetrical sidebands on each side of the carrier (spectral concept):

12 kHz carrier with 500 Hz AM. Sidebands are created at a distance of 500 Hz from the carrier
Conversely, creating carrier sidebands by some method is the same as modulating the carrier.
Phases are equal when the receiver is located north of the VOR (360°M), and show a difference increasing by one degree for each degree of bearing counted clockwise. Therefore the phase difference directly gives the bearing value.
C-VOR and D-VOR differ by how the variable phase is made different for receivers at different bearings. The method relies on amplitude for a conventional VOR and on frequency for a Doppler VOR.
Conventional VOR (C-VOR)
Technologies have changed a lot in the first decades. This explains why C-VOR descriptions also vary a lot, and sometimes contain mixed principles from different generations. Early generations used a rotating dipole to broadcast the variable signal, but they were soon replaced by static antennas or slots on a cylinder, making VOR more simple and less demanding on maintenance.
The static system once used two pairs of Alford loop antennas, more commonly two pairs of vertical antennas. While a vertical antenna radiates equally in all azimuths, a vertical pair tends to increase the field in directions perpendicular to the pair plane.

Conventional VOR antennas for a Thomson CSF TAH 511. Source: STNA
In the aerial above, four vertical tubes form two crossed pairs and the rings below and above the pairs are omnidirectional antennas. A polarizer at the top of the tubes prevents them from radiating in vertical polarization, to increase accuracy.
Reference signal
The reference information is FM embedded into a 9.96 kHz subcarrier ("10k subcarrier") with a modulation index (modulation depth in ICAO parlance) of 16, meaning the subcarrier frequency varies by +/- 480 Hz.
Variable signal
The variable signal is created by artificially creating the sidebands corresponding to the subcarrier. Each pair of antennas receives a HF signal:

Which frequency is f-30 Hz or f+30 Hz (sideband frequencies)
Which phase is proportional to the sine or the cosine of the reference angle. This aspect is explained shortly.

Pairs are directional, sidebands are sent at right angle (image below, left), creating two individual figure-of-eight radiation patterns (below center), themselves equivalent to a larger composite  figure-of-eight along the symmetry axis (below right):

Two phenomena occurs:

Sidebands interfere with the carrier sent on the ring antennas. The result is a cardioid field.

With phase modulated by sine and cosine of the reference angle, the cardioid rotates at 30 revs per second, the frequency of the reference. This is equivalent to a rotating antenna broadcasting a constant signal (the technology used on earlier VORs).

The cardioid EM field which constantly increases and decreases in strength at a given point in space as the pattern rotates is sensed by the receiver as a constant signal modulated in amplitude at 30 Hz, this amplitude variation represents the variable signal.
Bearing determination
Carrier, reference and variable signal sidebands create a spectrum the receiver sees as a whole:

This spectrum has all the characteristics of a HF carrier modulated in amplitude by a 30 Hz signal and a subcarrier at 9960 Hz, itself modulated in frequency by the reference. Phases are finally extracted from this HF wave and compared:

Doppler VOR (D-VOR)
This VOR consists in an array of about 50 fixed Alford loop antennas located on a 14 m circle. Two opposite antennas are activated at a time, and are electronically switched so the active antennas seem to be moving along the circle at 30 rounds per second.

Seen from the receiver one source is moving forward, the other is moving backward, at a linear velocity between -1,300 m/s and +1,300 m/s (Mach 4).

Principle of the Doppler VOR: The variable signal generation
A 110 MHz source moving between -1,300 and +1,300 m/s creates a Doppler shift between -480 Hz and +480 Hz. This value, consecutive to the selection of the appropriate array diameter, is the same than the 480 Hz FM swing of the C-VOR, making the D-VOR compatible.
Reference
The 30 Hz reference AM modulates the carrier which is sent in all directions by an antenna at the center of the circular array.
Variable signal
The actual perceived Doppler shift depends on the direction of the displacement relatively to the receiver, maximum when "side" antennas are activated, null when "front" and "rear" antennas are activated.
Thus all receivers see a frequency variation repeating 30 times per second, and starting at a time varying with their own bearing. This shift therefore can be used as the variable signal.
The repeating Doppler shift being undistinguishable from frequency modulation, the variable signal must be conveyed in FM. However it doesn't FM modulate the carrier, but the subcarrier to be compatible with the C-VOR. To achieve this:

Each active antenna of the array transmit a pure frequency corresponding to a sideband of the subcarrier: f-9.96 kHz and f+9.96 kHz, f being the VOR carrier frequency. When associated with the carrier sent from the central antenna, these sidebands represent a carrier AM modulated by the 9.96 kHz signal.

When antennas radiating their pure frequency are given their angular velocity of 30 rps by the scan, a cyclic Doppler shift between -480 Hz and +480 Hz is automatically added. This shift is equivalent to a FM modulation of the subcarrier by a 30 Hz signal.

Spectrum created
The composite spectrum is identical to a C-VOR spectrum, the C-VOR receiver is able to continue working.

D-VOR frequency spectrum
Receiver
A VOR receiver is looking for 30 Hz and 9.96 kHz signals. We deliver both: 30 Hz signal on the central antenna and 9.96 kHz on the directional scanned array. We had to switch reference and variable signal functions on the carrier because the variable signal is naturally produced as FM, and the FM signal goes on the subcarrier in the standard.
Because of this inversion, the phase values are also inverted, and the result is a wrong bearing angle computation: When the angle is +a, the computation gives -a. The remediation consists in scanning the D-VOR array in reverse direction (counter-clockwise) and generating an opposite Doppler shift.
Accuracy
Both types of VOR (and their different subtypes) are required by ICAO to produce a bearing with a ±2° tolerance. The D-VOR is better in this aspect because it’s easier to produce an accurate Doppler shift than to form a precise pattern with space modulation.
Once the signals are emitted, they are subject to reflection on terrain and obstacles. The final composite error at the receiver location cannot exceed ±6.5°, as ICAO limits the error according to a bent (slow variation) tolerance of ±3.5° relative to theoretical course and to a superimposed scalloping (rapid variation) of ±3°.
On this aspect, the D-VOR is also superior, the variable signal is FM modulated and less subject to multipath errors. The smaller C-VOR is difficult to run on airfields due to reflections on close obstacles, the large D-VOR must be used for terminal VOR. The C-VOR is rather used for enroute navigation and implanted on isolated sites.
Large counterpoises under the antennas limit the reflection problem, and when the site used to built the VOR is carefully selected, the operational accuracy can be far better than the ICAO requirements in a large part of the service volume.
Answer to specific questions
Do they generate the same navigation signal?
There are two differences:

The C-VOR actually sends two modulated signal to generates a cardioid interference pattern which when steered creates an apparent AM modulation. The D-VOR sends only “moving” unmodulated sidebands which create by Doppler effect an apparent FM modulation.

To maintain compatibility with C-VOR, the D-VOR reference signal must be sent in AM, and the scan done in reverse direction of the cardioid rotation.

Can they be used by the same receiver, or do VOR receivers need to be specially adapted to D-VORs?
The C-VOR receiver is used for D-VOR. It computes a phase difference which is wrong (opposed cosine), but because the array is scanned counter-clockwise, the correct bearing is infered.
Is one more accurate than the other?
Wikipedia states, but with no reference, and I’m assuming it’s for a D-VOR: “The predicted accuracy of the VOR system is ±1.4°. However, test data indicates that 99.94% of the time a VOR system has less than ±0.35° of error”.
The same article states about ICAO Annex 10 volume 1: “This document sets the worst case bearing accuracy performance on a Conventional VOR (C-VOR) to be ±4°. A Doppler VOR (D-VOR) is required to be ±1°”. But I can’t find these figures in the version I have.
Annex 10 and Doc 8071 volume 1 (testing procedures) rather give three tolerances:

A maximum VOR equipment signal error of ±2°
A radial bend tolerance of ±3.5° from the perfect course.
A radial scalloping error of ±3° around the bend.

The two last errors are due to the environment rather than to the instrument. VOR are located at places where these errors can be minimized.
So the acceptable total deviation of ±6.5°, as attested by Doc 8071 volume 1 §2.3.48, when the radial is bent by ±3.5° and a scalloping of ±3° is superimposed. However scalloping is a rapid oscillation around the course, and in most cases is ignored by the pilot who keeps working on a mean deviation value (but there are limitations of scalloping when the autopilot is used, because the A/P can react to oscillations).

ICAO VOR tolerances on bearing value
The D-VOR was designed to minimize the multipath effect, the variable signal is frequency modulated for this reason. The size of the counterpoise was increased so that reflection occurs on a controlled surface, rather than randomly. This way VOR could be located on airfields, allowing better VOR approaches and easier maintenance.
However D-VOR introduces a new error: The array antennas are close to each other, therefore the inactive antennas on each side of the active one interact on the signal. First they delay the wave, introducing an error in the phase before the wave is Doppler-shifted. Second they absorb a large portion of the energy, reducing the range. As explained, the 14 m diameter can't be changed to space further the antennas, and they cannot be spaced vertically without impacting the maximum elevation of the signals and increasing the cone of silence, which is already relatively large (100°).
Another error is the discretization of the bearing information due to a scan over only 46-50 antennas (the matter is complex), a fix is the so-called power blending: The two or four antennas around the active antenna are fed with a smaller power HF signal and the transition from one pair to the next is smoothed.
The last error, which has been partially corrected by the DSB D-VOR (the one with two sideband antennas described here) is related to the portion of counterpoise between the antenna and the receiver which varies with the antenna location. If a reflection occurs on the ground, and the one from the reference antenna occurs on the counterpoise, then their path is not equal and the phase difference is altered (however the modulation wavelength being 10,000 km, a path length error of 1 km is not very significant).
It's difficult to quantify a mean error, it really depends on the conditions the VOR is used and at which elevation angle the receiver operates. There are tools for simulation like the Ohio University Navaid Performance Prediction Model (OUNPPM) you may already know. Here is a study of the C-VOR/D-VOR accuracy with this tool.
Is one more susceptible to interference than the other?
For interference from signals or from obstacles, the C-VOR is more prone to errors because of the use of AM for the variable signal. In the D-VOR the reference is AM modulated, but a receiver can maintain a local reference using a PLL oscillator synchronized with the VOR reference when the signal is good, and reject transient spurious values (it’s like maintaining a local time reference, synchronized from time to time with a master clock).
Here is a study of wind turbines interference on D-VOR.
That said:

EUROCAE ED-52 provides a guidance for VOR siting.

For foreign signals, VOR are subject to a requirement from ICAO (Annnex 10, volume 1, §3.3.8.1): "The VOR receiving system shall provide adequate immunity to interference from two signal, third-order intermodulation products caused by VHF FM broadcast […]".


Can I get an EASA class 1 medical if I am red/green color blind?

For an applicant failing the Ishihara test, the next step is a medical assessor. If they're deemed "colour safe" (defined below), they can get Class 1 medical.
In your case, presumably, you were not deemed colour safe since you're daytime-only Class 2, if that's indeed the case, then unfortunately Class 1 will not be possible according to the EASA regulations below:

MED.B.075 Colour vision
(a) Applicants shall be assessed as unfit, where they cannot
demonstrate their ability to readily perceive the colours that are
necessary for the safe exercise of the privileges of the licence.
(b) Examination and assessment

Applicants shall be subjected to the Ishihara test for the initial issue of a medical certificate. Applicants who pass that test may be
assessed as fit.

For a class 1 medical certificate:

Applicants who do not pass the Ishihara test shall be referred to the medical assessor of the licensing authority and shall undergo
further colour perception testing to establish whether they are colour
safe.

Applicants shall be normal trichromats or shall be colour safe.

Applicants who fail further colour perception testing shall be assessed as unfit.

For a class 2 medical certificate:

Applicants who do not pass the Ishihara test shall undergo further colour perception testing to establish whether they are colour safe.

Applicants who do not have satisfactory perception of colours shall be limited to exercising the privileges of the applicable licence in
daytime only.

— Commission Regulation (EU) No 1178/2011

"Colour safe" from the same source means:

... the ability of an applicant to readily distinguish the colours used in air navigation and to correctly identify aviation coloured lights.


What is this plastic device inside the plane window?

I've found it on the 737, both the -800 and Max as far as I've looked. It's even noticeable from the outside.
The related patent from 2005 is US7267302B2 Window airflow damper. It solves an issue with severe window fogging. Basically it's a device that replaces the usual breather hole by putting the hole on top and covering it with a damper.
There is a blog post about it called New cabin window design & installation:

A new cabin window assy has been introduced due to the fogging problems we all encounter on the 737NG’s. The new cabin window assy has a breather hole at the TOP, compared to the bottom vent hole at the BOTTOM on the old panes.
The new window assemblies use thicker outer panes, redesigned seals with integral stiffening ring, and an airflow damper over the middle pane vent hole (...)


Would it be feasible for a typical passenger jet to carry along its own Oxidizer?

Short answer: Yes for a short-range airliner, but it would cost a couple of seat rows. No for long-range travel.

Longer answer: To burn 1 kg of Kerosene you need 15.6 kg of air or 3.59 kg of O$_2$. Its density as a liquid is 1.14 kg/l, so it will use 2.65 times the volume of fuel. A cryogenic tank with sufficient insulation is required, though. This means it should have as little surface area per volume as possible. You could not put the LOX into the wing tanks, but need something with a spherical shape somewhere in the fuselage.

If a large commercial airliner needs 100 tons of fuel, it would need to add 359 tons of oxygen. This would be already close to its own takeoff mass, so adding the oxygen will make the airplane too heavy. If, however, you are content with a short-range version of the same airliner that consumes just 20 tons of kerosene for its trip, the needed 72 tons of LOX would require a volume of 63 m³ which is equal to a sphere of 5 m diameter. This would fit nicely into the cabin of a wide body airliner and allow for some insulation. For cg reasons the tank would be right after the wing spar carrythrough, so the airliner would have two short cabins, one ahead and one aft of the tank.

Sorry, the 747 hump cannot be used for LOX storage - the cg shift would be too big.

For running regular jet engines you would still need lots of process gas. To carry that along as well is impossible, even for a short hop. Process gas is all the nitrogen and extra oxygen which is compressed, heated and then ducted through the turbines and accelerated on the way out, but does not actively take part in the combustion. Without that mass of gas thrust would be much lower, or the exhaust speed would need to be much higher for the same thrust, turning the engine into a rocket.


Is Doppler navigation still in use for civilian aircraft?

Only in military applications nowadays, primarily helicopters as the system doesn't require the alignment process of the INS system, is not subject to GPS jamming, is very accurate when flying low (as opposed to high and fast over oceans), and most helicopters don't do the occasional barrel roll (the system is belly mounted to track the ground below).

(Calm seas messed with the Doppler early-on due to the double reflection – surface and sub-surface.)

However for the airliners, as you ask, it was short-lived, but it helped usher in a new era.

TWA in c. 1962 fitted its Boeing 707 fleet with dual redundant Bendix DRA-12 Doppler systems and CNA-24A computers (pictured above). After demonstration flights to the FAA for technology proving, TWA was the first to let go of the navigator (the fourth crew member back then). It was the first time long-haul over-water flights were operated without a dedicated navigator.

The inertial navigation system in that decade was still a top military secret. But the technology and computing power were making their way into the civilian world.

First was the sub-inertial navigation system (part INS; part Doppler), such as the Sperry SGP 500 gyro platform coupled with a Decca Type 72 Doppler. It was cheaper than a pure-INS system and had improved accuracy over a pure-Doppler system.

Initially operators favored the sub-INS system, because of the much quicker alignment time. In that era, any disturbance to the aircraft – while being refueled or boarded – would add to the alignment time. That was c. 1969.

1971 – just under a decade from TWA's initiative – was when the INS first made it onto an aircraft as default equipment straight from the factory. The plane was the Boeing 747, the INS system was the Carousel IV, and the rest is history.

(Doppler navigation was also used on general aviation aircraft, and much like the VOR-based RNAV equipment, such as the King KNS 80, both yielded to the GPS.)

References:

TWA Relies on Doppler, Flight International, 11 October 1962
New sub-inertial navigation system, Flight International, 6 March 1969
Carousel IV in the 747, Flight International, 1 July 1971


When has China said it will mandate head up displays (HUD)?

2025
Sometime in 2012 CAAC are alleged to have suggested 100% of Chinese airlines should have adopted HUDs by 2025.
It seems the Rockwell Collins Heads-Up Guidance System (HGS) has been trialled and that HGS equipped aircraft are permitted to land at certain airports in low visibility conditions - lower landing minima for HGS equipped aircraft.
Reports mention enhanced vision systems (EFV) and enhanced flight vision systems (AFVS) where the HUD is used to project data, including runway outlines, over the pilot's view of the runway ahead.
The CAAC seem to be actively encouraging this, but I don't find the  word mandatory used in this context.

November 14, 2012, 5:07 PM
Rockwell Collins China managing director Ron Ho believes business for aircraft head-up displays (HUD) in China will soon be looking up. “We have been working with the Civil Aviation Administration of China (CAAC) for the past 10 years on the HUD project, and now it has reached the stage where CAAC will require the Chinese airlines to implement the HUDs into their operations,” he explained.
“This will be done in three phases, I understand,” Ho noted. “During the first phase, 10 percent of all the airlines will be equipped and operating with HUDs. In the [s]econd phase, this will be 50 percent. The third phase will be complete fleet equipage by the year 2025.”

From AINonline - Chinese Authorities Commit To Aircraft Head-Up Display Mandate

FAA has also been coordinating with CAAC. In Aug 2012 FAA hosted a Pacific Rim conference in Long Beach CA identifying key technologies, operations and regulations that could be improved with HUD, advanced PFDs and vision systems. Soon after the conference, the CAAC HUD application roadmap appeared. China has without hesitation laid forth a plan to move the country's civil aviation into a 100% HUD compliant operation by 2025.

From Professional Pilot Magazine - HUD gains favor in Asia Pacific

Start HGS from 2005
End of 2010, 52 B727-NGs
... Primary reference for maneuvering ... is natural vision
Promote airlines using HUD

From CAAC - HUD/EFVS systems in the implement(sic) in China


Wheel or skid? Which advantages does each one have when used as the landing gear system compared to the other one?

It really depends on how you intend on landing the vehicle, how you intend on using the vehicle, and generally any design limitations that may come into play.

For fixed wing aircraft that need to land with a forward velocity you can chose either skids, wheels or a combination of both.

Skids are beneficial as they are, simple, light and take up less space than conventional wheels as discussed in reference to the X-15

  The aft landing skids have been questioned many times. But if you can
  figure out how to put a conventional gear into a 5-percent-thick wing
  and struts long enough to reach the ground in the landing attitude
  without losing the back end of the fuselage, be my guest! The current
  skids were located, as you will note on other figures, as the best
  solution to a difficult problem from a weight, temperature, and
  simplicity point of view.

However with a front skid you lose the ability to have nose gear steering and must resort to differential braking (assuming rear tires). With skids all around you lose ground steering and may possible damage the runway. For space craft this is typically not an issue as they are usually towed off the runway anyway. In the case of the Dream Chaser their may have not been enough space to stow a nose wheel.

Skids generally dont allow for any mechanical braking method to be applied. This means that you need to rely on aerodynamic braking like a drogue chute or reverse thrust.

Below certain airspeeds, with only skids you also lose all ground steering which can be an issue.

Wheels preserve steering and generally allow the craft to be moved off the runway easily. However this comes at the cost of added complexity, added weight, and generally added size. Since the space shuttle we have gotten pretty decent at putting landing gear on orbiters and at this point we know how to do it. While the tires need to be a bit different than your average car tire many aircraft tires are highly specific anyway. This question may also be of interest on the topic.

Spinning wheels, generally speaking also allow for braking to slow the aircraft after touch down. For short runway operations this is a must, since rolling to a stop is often not an option.

Wheels are generally beneficial for your average powered aircraft since ground maneuverability is needed to get around the airport and tires allow good braking ability.

Helicopters are a bit of a unique case, since they have the ability to hover and land vertically they don't really need any landing gear beyond a flat bottom. Wheels make it easier to push or tow a helicopter around the ramp while skids are simple and require little service on a helicopter.


Is it required to contact approach when flying VFR into a Class D airfield?

No it is not required to contact approach. Unless you have to go through a larger D, C or B airspace to get there. I don't know of any larger Ds in the US, but there are plenty of D-airfields underneath C and B airspace.

In Germany (and I think around Europe, not sure though), there's a distinction between the tower-D and the approach-D (the former being called D-CTR, and the other just D), to enter the approach-D you'd have to talk to approach, but if you can sneak under/around it, go ahead and call the tower directly, let them know where you are, and they'll tell you how to enter their airspace. In the US, they'll probably just tell you to enter the pattern (like "N12345, enter and report the right base for runway 5").

In Europe you need an explicit clearance and they'll usually tell you how to enter (like "DEFGH, enter control zone via Echo, report Echo", Echo being a reporting point on the chart, Echo usually being to the east, with November, Whiskey and Sierra being north, west and south respectively (usually, that is, there may be more, and they may be placed differently, but they're always on the sectional or the terminal area chart)


Why is C-5 Galaxy not used for commercial airlines while Boeng 747 is?

C-5 Galaxy was designed to military requirements that for cargo aircraft include:

operating from unpaved runways (I am not sure in case of C-5, but probably yes)
operating from airfields without much support
operating from short runways
paradrop capability

These basically imply a high-wing design so that engines are high (to minimize risk of foreign object damage) and the deck is lower for easier loading, front and rear cargo door with loading ramps so loading is possible without any ground infrastructure and some excess thrust for sufficient performance on short fields.

However that design is less efficient. It has higher operating cost. The air force is willing to pay it, because it needs the capabilities it requested. But commercial operations don't need those things, because it has enough well equipped airports to operate from. So the higher cost of operating an L-500 (as civil version of C-5 would be called) makes it uninteresting.

B-747 only shares one design feature with C-5, the raised cockpit that allows full-height (well, almost) cargo door in the nose, but otherwise it is optimized to minimize operating cost, making it great cargo plane for operations between airports.

On a side note, there is some commercial demand for transporting heavy or large cargo to less equipped airports. These are currently covered by ex-soviet-military An-124 aircraft that look very similar to Galaxy, are a bit larger, are certainly able to land on unpaved runways and were simply available when Soviet Union collapsed.


What is the function of ECS in airplane?

The ECS system is the Environmental Control System.  It is the system that handles pressurization and thermal control of the aircraft. The FAA has a nice handbook on it here. There is some more information on how it works on the new 747-8 here that may be of interest.


Are there any passenger aircraft with no cabin access to/from the cockpit?

The only one I can think of is the Junkers F13. It has an open cockpit and an enclosed 6 passenger cabin. There's no access between the two areas; only a small window.

Credit:THOMAS LÜTHI / RIMOWA 2016

The CEO of Remowa is having recreations made of the aircraft which first flew in 1919. It was (according to Rimowa) the world’s first all-metal commercial aircraft.


What would prevent a solar powered aircraft staying aloft forever?

The most common failures in planes start with: a human mistake or a mechanical failure. Where, of course, mechanical failures are much more likely to happen on moving parts.

Satellites have very little moving parts: you get couple servo motors to adjust the angle of your equipment such as the solar panels and the antennas. Both happen in units of full rotations a day. You also often have some small rocket engines to correct the satellites course, but they are rocket engines whose moving parts comprise more or less just one valve.

In a plane, however, you have pieces that move very fast and fluids of all kinds (not only fuel which you need not have on a solar-powered plane, but also lubricants, hydraulics etc.), also very sensitive mechanical sensors whose non-stop operation is crucial for the flight. This all needs to be inspected, oil levels checked, hydraulics checked. Last but not least, the weather in the space is quite calm, compared to the atmosphere. Depending on how high you fly, you meet clouds, storms, birds, temperature changes etc., which all can add to the wear of things and make things require maintenance.

I'm not sure which would be the limiting factor, but I think that either some equipment wouldn't be operational without maintenance, or you would get a mechanical failure of some kind.


What is this WWII four-engine plane on skis?

Looks like a TB-3 or a variant thereof. According to Wikipedia, this four-engine all steel(!) heavy bomber first flew in 1930 and was the world's first cantilever aircraft in this class. The skis do not appear to have pertained exclusively to any specific variant, given that the first flight already used them.

It was used up to and including the Second World War, and a number of experimental variants were developed from it. One of the most interesting was the Zveno project, an experiment with parasite fighters. Here you can see the Zveno-SPB variant of the TB-3 with two I-16 fighters under the wings:

The Zveno-SPB saw operational usage and even success in raids against the Romanian oilfields during WW2.


What happens if a helicopter engine fails at 20,000 feet?

Yes. See the Wikipedia's article on autorotation!, which mentions a real-world example of the case you are describing:

  The longest autorotation in history was performed by Jean Boulet in 1972 when he reached a record altitude of 12,440 m (40,814 ft) in an Aérospatiale Lama. Because of a −63 °C (−81.4 °F) temperature at that altitude, as soon as he reduced power the engine flamed out and could not be restarted. By using autorotation he was able to land the aircraft safely.

The biggest concern here is the disk loading of the helicopter's rotor - the greater the disk loading (the mass of the aircraft related to the rotor's area) the worse the autorotation capabilities. See this Google Books link for an in-depth discussion of the autorotation capabilities of different aircraft based on their characteristics.

The weight of the very rotor also plays a significant role here - the heavier the rotor, the more kinetic energy it stores, and the greater the probability of a successful entry into the autorotation state. See the Osprey's design, for example - it has light propellers on each wing which are practically unable to autorotate because of their small mass.

As you can see, the aforementioned helicopter - Aérospatiale Lama - is a light aircraft which is geared towards high altitudes - it has three propeller blades, which translates to low disk loading, and sufficiently heavy blades to allow easy entry and execution of the autorotation stage.


Why might a too-wide runway be a problem?

The issue has always been human perception.  Pilots are tasked with trying to make great landings.  One way they do that is with peripheral vision.  If the runway is too wide, they lose that extra clue on when to roundout and flare.  They may flare too early and stall the aircraft too high above the runway.  This perceptual clue is more important for tailwheel pilots than tricycle gear pilots.
From the FAA AIM 8-1-5.

Illusions Leading to Landing Errors.

(a) Various surface features and atmospheric conditions encountered in landing can create illusions of incorrect height above and distance from the runway threshold. Landing errors from these illusions can be prevented by anticipating them during approaches, aerial visual inspection of unfamiliar airports before landing, using electronic glide slope or VASI systems when available, and maintaining optimum proficiency in landing procedures.

(b)Runway width illusion. A narrower‐than‐usual runway can create the illusion that the aircraft is at a higher altitude than it actually is. The pilot who does not recognize this illusion will fly a lower approach, with the risk of striking objects along the approach path or landing short. A wider‐than‐usual runway can have the opposite effect, with the risk of leveling out high and landing hard or overshooting the runway.


Would the Gee Bee be allowed to fly today, with so little forward visibility?

At least it has a forward looking windshield.

(Source) Spirit of St. Louis that was flown solo by Charles Lindbergh on May 20–21, 1927, on the first solo non-stop transatlantic flight from Long Island, New York, to Paris, France.

(Source)

(Source) A 777-300 needs a camera too to make cornering easier.

Like all tail-draggers, the pilot can be guided by a wing-sitter, or by taxiing in an S-shape.

For landing, a little cross-control to crab will allow a side view of what's ahead.

Above methods were/are very common for WWII-era planes with huge noses and a tail-dragger undercarriage.

(Left, right) Taxiing with wing-sitter assistance.

Landing on big grass-fields makes it easier too.

  Would the Gee Bee be allowed to fly today?

The Gee Bee was still flying airshows until 2002, so the answer is yes.

(Source) Gee Bee R2 flown by Delmar Benjamin at Oshkosh 2001. Benjamin flew an aerobatic routine in this aircraft at numerous airshows until he retired the aircraft in 2002.


We want to reduce (or simplify) equation 2, to get get dV/dt by itself, so we multiply both sides by g/W.  On the right hand side, this reduces to dV/dt.
On the left hand side it reduces to what is given in equation 3, but let me show you how:
 1. First term is T x g/W
 2. Second term is -D x g/W
 3. Third term is -u x g/W (W - L).  By the distributive property, we have -[u x g/W x W] + [u g/W x L]. The first part of this term simplifies to u x g.

Finally, I want to group like terms together.

The first term combined with the first part of the third term give me: T x g/W - ug = g(T/w - u).
The second term combined with the second part of the third term give me: -D x g/W + u g/W L = g/W(u L - D).

This yields equation 3: g(T/w - u) + g/W(u L - D) = dV/dt.

Finally, recall that L = 1/2 p S V2 Cl, with a similar equation for Drag, and we can substitute these directly into L and D to get equation 4.

S is the the wing planform area. Other sources use A, like this one: NASA - Lift equation.


Why does enriching the mixture ratio beyond stoichiometry give more power?

in practical terms, running rich means that despite the unevenness of the fuel-air mix both in a single cylinder firing and across all cylinders firing and across all firings in the engine, all cylinders are guaranteed to NOT be running lean on ANY firing, which means all of them will be pulling best power on EVERY firing.

In addition,  there is an effect that I know about from motorcycle engines which is probably true of airplane engines and that is the "quality" of combustion as a function of richness: If you run rich, preignition and lean misfire (both of which reduce combustion efficiency) are prevented, and the delivered power is smoother. You can hear this: a motorcycle engine sounds "happier" when running rich than it does running lean- and it pulls harder, even though the mix is nonstochiometric.


FAA design regulations require that the aircraft be equipped with fuel system drain valves that allow the entire fuel system to be drained.

"Fuel System Drains -
Aircraft fuel systems must be fitted with at least one drain to
allow safe drainage of the entire fuel system with the airplane
in its normal ground attitude."

According to a maintenance person: "The residual is vacuumed or absorbed and the negative ventilation is applied. You can make entry almost immediately, though air monitoring is constant. We keep air moving through the tank anytime there is someone in the tank. "

Doesn't clearly answer the question, but this article discusses the procedures for personal protection while entering aircraft fuel tanks.


The tail is quite unique. I thought about Embraer Phenom, Hondajet, and Cessna Citation, but it is none of these.

I believe that your images are showing a British Aerospace 125 (aka Hawker 800/1000, or DH125 Jet Dragon). They were produced from 1977 to 2013, and most of them have been in service for the air forces of Japan, Brazil, and Saudi Arabia.

Source: wikimedia.org


What part of the F-18's wing profile allows for high AoA flight?

You're correct that the leading edge extension (LEX) plays an important part in increasing the maximum angle of attack of the Hornet/Super Hornet. The LEX basically forms a vortex above the main wing, which improves the high angle of attack capabilities by delaying separation. See this question about vortex lift and also another related question.

You can see the LEXs forming vortices over the wing in the following image.

An F/A-18E Super Hornet assigned to Strike Fighter Squadron (VFA) 81 maneuvers over the Nimitz-class aircraft carrier USS Carl Vinson (CVN 70) during an air power demonstration. (U.S. Navy photo by Mass Communication Specialist 2nd Class James R. Evans/Released)

  Is high AoA associated with smaller wing width and high chord length?

There is no simple way to answer this question because what matters is not the aspect ratio of the wing per se, but the way the lift is created. In case the wings have high sweepback, the method of lift generation is by vortex lift, which operates in higher angle of attack compared to the normal (moderately swept) wings.

In other cases where the wing sweepback is less, the lift generation will be by conventional means and consequently, the high angle of attack performance will suffer, though this can be remedied by the use of canards, thrust vectoring etc.


Should the route be changed if the GPS RAIM prediction fails?

The answer to this question will probably depend on the equipment involved and type of route being flown; as asked, the question is open ended, and there are several types of operations where the answer is a definite "no" (i.e. you do not need to adjust your route), while in other situations, that answer might change.

If you are flying on Victor airways using VOR receivers, then the answer is certainly "no".  You aren't navigating with GPS, so you don't care about the RAIM.

If you are flying in RNP-3 or RNP-10 airspace (i.e. the required navigation performance is 3 NM or 10 NM -- so your INS can drift that far before you need to do something different) with an INS or multiple INS's, then you probably don't care about the RAIM... your INS's won't drift that far in several hours, and the RAIM should be back to acceptable limits well before then. (Maybe there are exceptions to this; I'm not aware of any.)

If you are flying an approach where GPS is your primary navigation, then YES, you absolutely do need to change your plans if the RAIM fails.  Break off that approach, go somewhere where the RAIM is good, use a conventional navaid for the approach, whatever the best option is for you.

I'm not aware of any enroute operations where RAIM is a factor; what I'm flying has dual INS's and the FMC can update its position off of not only the GPS but also from conventional (DME/DME, or VOR/DME) navaids, and we don't worry about the RAIM enroute, even when flying Q routes where GPS is required.  (Maybe that's a hole in our OpSpec, I dunno.)  We do look at it when flying an RNAV approach.


What is Full Fuel load and how does it differ from Fuel Capacity?

The payload does not include the fuel itself. Payload means the total weight of all passengers, their baggage and all cargo on board. If you add this payload weight to your OEW (Operating Empty Weight), you get the ZFW (Zero Fuel Weight). On top of that you have to add the fuel.
The payload you quote is the maximum payload you can have if you also have full fuel on board. This is to make sure that the total weight of the aircraft remains below MTOW (Maximum Takeoff Weight) or possibly maximum taxi / ramp weight. If you fly with less than full fuel, you might be able to load more payload (as long as you stay below maximum ZFW).


When testing the propellers for these quadcopters, would it be useful
  to test both the right handed and left handed rotation of the
  propellers or just one direction?

You may miss something about propellers rotation. A given propeller produces thrust (which is another name for lift) when rotation occurs in the designed direction. When direction is reversed, thrust may be produced, but not efficiently, and will be reversed compared to the regular lift.

A propeller is a wing in rotation. Wings are not used is reverse direction, with a plane going backwards.

Left-handed and right-handed propellers are different:

Thrust is determined in the intended direction of use.

  Why would a designer select a lower pitch to diameter ratio

For the same reason than wings on actual planes have an optimized size and incidence. Increasing this size or this incidence is not productive. The limit may come from different reasons, but essentially for weight and drag considerations.

In the quadcopters, the propellers are spun by electric motors that have a limited power and a preferred rotation speed. Propellers are selected to match the rotation speed and available torque of the motor.

In addition turbulence is created by the propeller, especially at high pitch. Turbulence can decrease efficiency of neighboring propellers, so a compromise has to be found between high individual and low overall lift produced.

From Studying the Aerodynamics of Multi-Rotor Drones (Nasa)

Also think about the fact that a propeller just converts a quantity of work (rotation) into another quantity of work (translation of air). The rate of conversion is dependent on the blade pitch. Air translation (lift) can be increased but not beyond the capabilities of the motor to create the equivalent rotation. If this is done, the motor just can't deliver, and it's efficiency decreases (battery will be empty more quickly).


Why is the 60-year-old NK-12 more fuel efficient than Europrop TP400?

The source of the Specific Fuel Consumption in Wikipedia is unclear. I have found a paper (PDF) that discusses the efficiency of shaft power off-take (that is using engine power to drive systems such a electric and hydraulic generators instead of delivering propulsion).

It mentions the TP400-D6 (which is installed on the Airbus A400M) has a SFC of 0.167 kg/(kW.h) for shaft power off-take and a SFC of 0.213 kg/(kW.h) for delivering propulsive power. This means that the TP400 is more efficient than the Wikipedia article presents and more efficient than the SFC of the NK-12 presented by Wikipedia.

It also shows that the TP400-D6 is more efficient in delivering shaft power than in delivering propulsive power.


Are test pilots who work for the FAA paid on the GSE pay tables?

Yes, FAA test pilots are paid based on the General Schedule for the levels of GS-1 through GS-18.

If you are interested in reading the job description, one can be found on AvJobs.

The relevant section for pay is this one:

  Wages and Benefits

  Most Federal Civil Service employees in the aviation field are covered by the General Schedule and their salaries vary according to their grade level (GS-1 through GS-18). Within each of the grades provided in the General Schedule, provision is made for periodic pay increases based on an acceptable level of performance. With an acceptable level of competence, the waiting period of advancement to steps two, three and four is one year, steps five, six and seven is two years, steps eight, nine and ten is three years. The starting grade is normally GS-7. Trainees are paid while learning their jobs.

  The entry level is GS-9. Entrance salary will vary with the degree of the applicant's experience and training.


Why is managing CG (centre of gravity) important?

To balance the aircraft, the sum of the lift from all lifting surfaces has to be at the same position as the weight. If the c.g. is ahead of the center of lift, the aircraft will pitch down. The pilot can counteract this by pulling on the elevator, which in effect reduces lift at the tail and thus shifts the center of lift forward. As you can imagine, there is only so much lengthwise change which can be effected with the elevator. So the first limit for the c.g. position is given by controllability.
Another important consideration is stability. You want the airplane to return to the original flight attitude if it has been disturbed, say by jolt on the stick or yoke. This can be done by creating proportionally more lift with the forward surfaces (= the wing in a conventional configuration) than with the tail surfaces. Say the jolt pitches the aircraft up. Now the aerodynamic forces should change in a way which increases lift proportionally more on the tail surfaces, so the aircraft pitches down and returns to it's original attitude. By distributing lift the way I mentioned above, the same change in angle of attack on both wing and tail will create a proportionally higher lift increase on the tail, making the aircraft (statically) stable. Thus, the second important limit for c.g. location is stability.
As for tipping over …


Why does idling RPM increase with altitude in a jet engine?

For one there is less resistance to the fan and compressor due to the decreased air density -> more rpm out of the given power. On the other hand, the mass flow needs to be high enough to sustain the operation of the engine -> both lead to a higher rpm...


It sounds like "orbit" is UK-speak for making a 360-degree turn for spacing in the pattern.

I'm not sure what the custom is in the UK but in the US I was taught that you make a Rate One turn (3 degrees per second) for this, giving the tower two minutes between you and the aircraft in front of you (or the one on the ground they're trying to get rid of before you arrive).

The logic is that the maneuver is supposed to create spacing - flying a tighter circle takes less time, and thus creates less space between you and the aircraft in front of you. It also obviously requires more aggressive banking at low altitude compared to the relatively gentle bank required for a standard-rate turn at traffic pattern airspeeds, which as you noted will be easier to control in windy/gusty conditions.


What makes it so hard to land on an aircraft carrier?

Edit

Since another user, with actual carrier experience, has given a detailed answer, please refer to his answer.

It now appears that a good part of my answer was incorrect.  As I wrote in the beginning, my answer was based on flight-sim experience, and reading, but nothing formal or official.

I'll leave my answer up, solely for historical reference, unless the better answer gets accepted.

I've never done it IRL, but read about it lots, and Flight Simmed it many times, so I'll take a stab.(until someone who has actually done it says better)

Glide Path
When landing on a fixed runway, your glide-path is pretty much right towards the end of the runway.Its not like that at all on a Carrier.  From up high, you are actually aiming for a point in front of the carrier.  You're aiming for where the runway will be in 30 seconds.   As you get closer, the carrier moves closer into position.  But still not entirely in position until the last second.   So you're always aiming for an invisible/imaginary point where the runway will be at the last second.
This is the best picture I've found of what I'm describing:

Notice that the flight path indicator (the little circle in the HUD) is well beyond the front of the carrier.  If this plane was landing on a runway, the flight path would be aimed right at the numbers.
No Flare
In a typical runway landing you flare out just before touchdown, "float" for a second and drop on to the runway.In a carrier landing, you maintain your glide-path attitude all the way on to the deck.   There's no flare or float. This helps you hit exactly the spot you're aiming for (the 3-wire), but in some sense you really are crashing into the deck at a not-inconsiderable descent rate.
Pitching Deck
Remember the glide path to the imaginary touch-down point?  Because the carrier deck can be pitching or rolling in the waves, that imaginary touch-down point is not a fixed distance directly in front of the ship.  As the ship bobs and weaves around, your touchdown point is moving around too.   That makes it even harder to hit the point right.  It can well be at the last second, as your plane is dropping quickly, the deck suddenly jumps up to meet you, making the impact even more violent.
Sideways Movement
The landing part of a carrier deck is the diagonal part on the back of the deck.   It is angled about 20-degrees to the left, relative to the carrier.  But it is moving directly forward with the movement of the carrier.  This means the "runway" is not moving directly away from you, but slipping sideways.


What navigation publications are required when flying from KSNA to KAVX?

You'll need the navigation publications appropriate to your flight.  If you're flying under VFR, then you'll need the appropriate sectional chart, and should have airport diagrams.  You'll need to be aware of the airspace and how to communicate and navigate in it.  You should have taxi diagrams.  For VFR flying, I've always made a practice of having the appropriate terminal charts, sectionals, WAC's, and airport publications.  I have also always carried IFR enroute charts, as these provide useful information about communications enroute, as well as airspeed and navigation information.

These days, a subscription to Foreflight usually covers it, and I've flown all over using little else.

If flying IFR, you'll need appropriate IFR publications.

You should have a copy of the airport information, airport facility directory, or equivalent, etc.  If you don't carry and reference the current data, and end up getting busted, you'll have little to fall back on as an excuse.


Propeller effect on the L/D of an aircraft (Pusher Propeller configuration)

You need to fair in the tail with a carbon fibre fairing/spinner such that the prop folds back and nests into it.  Like below.


Why do some aircraft have pods on their wing tips?

Wingtip pods or winglets differ in installation reason from aircraft to aircraft. Here is a non exhaustive list of possible reasons:

Fuel tanks
Whitcomb bodies to enhance the area rule of the aircraft also known as Kuchemann Carrots
Vortex generators and relievers - used to enhance aircraft aerodynamics and modify fuel consumption
Mass balance - in particular on forward swept wing aircraft
Housings for sensors or landing gear


Where is the spanwise flow? How does the span wise flow point the air towards the wingtip?

The two pictures in the question only show how to decompose the speed vector into the normal and parallel components. They are nowhere near "real" flow vectors.
First I recommend to read this answer so you get an idea how air is accelerated and decelerated when flowing around an airfoil. Note that the low pressure area over the wing will suck in more air. This answer might also be helpful because it looks closer at the boundary layer.
With a swept wing the same happens, but now the flow will not stay at one wing section but show an added sideways component. Again, air is sucked into the low pressure area, but because that area is a little to the side of the direction of motion of the wing, the flow lines will initially move towards the center of the wing on the upper side in case of sweepback.
But that cannot last: The pressure recovery over the rear part of the airfoil reduces this bending, so the flow lines become almost parallel to the direction of motion of the wing. The lower the local pressure, the more the flow lines bend inward. Higher than ambient pressure at the trailing edge or on the lower side of the wing will bend the flow lines outward accordingly.

Let the red color denote suction: It bends the flow lines (black) towards it, creating a spanwise flow component. The higher the suction (the redder the color), the bigger the bending. The cyan arrows show the flow lines inside the boundary layer. By losing some of the early acceleration to friction but being subject to the same deceleration in the region of the pressure rise, the boundary layer flow shows a stronger tendency towards the tip. To be precise: The acceleration and deceleration acts in a direction perpendicular to lines of the local relative chord. The sketch above exaggerates this bending for the sake of clarity.
Now we must look at the boundary layer to get the full picture: Here the flow speed is reduced by friction, so the result of that initial inbound acceleration is worn away gradually. The result is a twist of the local speed vectors over the thickness of the boundary layer such that the inbound component is gradually reduced the closer you get to the wing's skin. Now the deceleration in the area of pressure recovery will slow down the already decelerated air close to the wing, and again that slowing will happen in a direction perpendicular to the lines of equal chord. If this deceleration is sufficient, it will leave the boundary layer flow with an outward speed component. Such a deceleration brings the flow close to separation so it occurs only at higher angle of attack when the wing approaches stall. Note that only the boundary layer will exhibit a tipwise speed component.
Only when the flow separates, the boundary layer thickens past the separation line (blue line in the sketch above) and the spanwise flow will become significant. Separated flow is characterized by a local forward speed component, and now, past the separation line, you get a layer of air which is left with mostly a spanwise component; outboard in case of a sweptback wing and inboard in case of a forward swept wing.

EDIT
This paper presents a CFD study on the PRANDTL-D P-3C model aircraft using the OVERFLOW software. Figures 10 and 11 show the boundary layer flow on the lower and upper surface and illustrate nicely what happens. Note that this particular wing has a washout of -10° towards the tips which is essential to keep tip flow attached.

Lower surface streamlines. Only the 12° AoA case is shown because all others look quite the same.

Upper surface streamlines for angles of attack from 0° to 17.5°. The development of spanwise flow with the ever steeper pressure recovery at higher angles of attack should be evident. Note that flow direction past the separation line (the area with yellow-greenish tint) is from back to front.


Do airline pilots routinely carry out "gross error" checks?

I agree, Malaysian MH132 was a Non-Event.   If the pilots had any concerns over the slight variation in their routing, they should have mentioned it before they loaded it into the Flight Management System (FMS).

I currently fly long haul B777 for a major airline. I can't speak for others, but at the airline I work for we do many "gross error checks". Some are done in flight planning, some during the loading of the FMC, and some are done during the flight.

While still in the flight planing area, the Captain and First Officer will look at the total routing in regards to wind, temperature, turbulence, volcanic activity, etc.  Often a call is made to the dispatcher who generated the Operational Flight Plan (OFP) to discuss the routing, expected levels of turbulence, and fuel reserves.  About 20% of the time the Captain will request a slight change to the routing or additional fuel be added.  A new OFP is then generated and it will be designated "Release 2".

Both the flight plan route and ICAO flight plan route are printed on the OFP and we compare the two.  I have never found an error in the 17 years I have been doing it, but we still do it every time.

Once the Captain, FO and augment pilots ALL agree, the OFP is officially accepted and we head to the aircraft. (up to 4 pilots on Ultra Long Haul flights)

When we get to the aircraft we run though the basic preflight checks, and then the flight plan is downloaded to the FMS using the Aircraft Communications Addressing and Reporting System (ACARS) digital datalink system. The departure runway and SID is also loaded at this time.

Before we "activate" and "execute" the routing, the Pilot Monitoring (PM) reads the entire FMC routing and the Pilot Flying (PF) compares it to the OFP.

After runway takeoff performance data is loaded we then check the "Legs" page of the FMC for a logical sequence of legs. We generally just check to make sure there are no course reversals on the various legs of the route.  If there are any user defined waypoints (Lat/Long waypoints) we perform bearing and distance checks of  the FMC against the OFP. This is done with the the Heading Reference switched to TRUE heading instead of Magnetic for more accurate numbers.  These bearing and distance checks are not done for "Named" waypoints, only Lat/Long waypoints.

Wind data is then requested and then loaded via ACARS.

Next is a check of the FMC distance compared to both the OFP distance and Great Circle Route distance.  The FMC distance is almost always somewhere between the OFP distance and GCR distance. This check is one sure way to spot a routing entry error.

Finally a check of the FMC and OFP predicted fuel over destination is done. If there is a large difference, it means either the routing or winds were incorrectly entered.

Once airborne there are more checks:

In the old days before GPS we would check the accuracy of the Inertial Navigation System (INS) as we approached the coast for any over water flight. This was usually done using a VOR/DME fix.

Once airborne we again check bearing and distance as we approach each user defined waypoint (Lat/Long waypoint). Once past the waypoint by about 5 mins we then plot our current Lat/Long on a plotting chart to insure we are still on course.


What's the UX in modern airliner avionics for diversion choices, and does it integrate to autopilot?

There are a few options for this and it depends on the type of plane you are in, for the most part it looks like the information must at least be proactively navigated to in what ever NAV system you are using.

Every pilot should study their route before hand and identify possible diversions or at least understand where they are. For pilots flying common trans ocean routes there are some pre-arranged diversion airports that the pilot should know.

If you are using some kind of tablet as your source of navigation information (maps) most of the modern apps have a function for this.

If you have some kind of in panel GPS like a Garmin GNS system there is also a page for that very information.

According to this doc FMS systems may have this info as well. For example in the 737

  This extremely useful page takes a couple of minutes to calculate but
  will list the nearest airports in the database in order of DTG.

  Once again, line selection of 1R to 5R will give more useful diversion
  information as shown below.

(source)

You can also program in you alternates

  You can enter up to 5 alternates here, selecting 1R to 5R against any
  entered alternate will show the info below...

(source)

The FMS will compute data to those points from the aircrafts current location.

  All the diversion data is now shown based on you flying direct to this
  alternate from present position (VIA DIRECT). Selecting MISSED APP
  will show the same data but calculated from the missed approach point.
  Selecting nearest airports will give...

(source)

Even the more modern airbus cockpits contain a traditional FMS input or possibly a keyboard and mouse. To my knowledge airlines have not really adopted touch screens yet.

(source)

Some of the smaller panel avionics are starting to incorporate touch screens like Garmins GTN series however some pilots prefer the tactile feel of hard buttons.


What is the three-light indicator on carrier-borne airplanes?

Those are angle of attack indicator lights. The landing signal officer on the carrier can judge whether a pilot making an approach of the boat is on angle of attack using those. The center light will be amber and will indicate that the airplane is on angle of attack for the approach. The upper light is green and will indicate to the pilot is it too high of an angle of attack, that is going to slow. The lower light will be red and indicates that the pilot is it too low of an angle of attack i.e. is too shallow in approach angle and going to fast.  The lights correspond with those on the angle of attack indicator which is located to the left of the head up display in the cockpit.

Flying the proper approach speed is a key ingredient of any approach be it carrier or terrestrial.  Again due to the inherent tighter tolerances of a carrier landing, the AoA data is useful for the LSOs to spot deviations in the approach.  Second, since Vref varies for all aircraft approaching the ship based on type, gross weight, etc. a radar tack of the aircraft would require the LSOs compute the required Vref ahead of hand for every pass at the boat.  It’s much easier just to observe a simple light indicator on the jet from the LSO platform to gauge this by.

Regardless LSOs do also have access to the raw AoA information from an aircraft making a pass and it is displayed on the PLAT camera at the LSO’s platform.  Whether that data is based on a direct telemetry from the jet on approach or some other source is unknown.  One possible reason for retaining the lights is for Emission Control (EMCON). Radio signals known to be used for recovery procedures would be valuable to an enemy since a carrier is most vulnerable to attack during startup and recovery operations and a simple set of lights visible only in the immediate vicinity of the ship would be advantageous here.


Are there locations/altitudes with minimum speed requirements?

The way it works is you file your flight plan for the NAT with a proposed Mach #, but when you get your Oceanic Clearance they will assign Mach #s for each segment of the NAT and that's what you have to fly unless cleared for a different speed.  The adherence to assigned Mach #s are the only assurance of separation once out of radar coverage.

Perusing the NAT Operations Manual I couldn't find anything that gives a minimum Mach #, but if you were unable or didn't want to maintain the Mach # assigned, you would probably be given the option of flying down below RVSM airspace, or taking an off-NAT routing like the northerly "Blue Spruce" route (which airplanes use when they have to stop in Iceland for fuel because they don't have the range for the NATs).


It depends on the country and may depend on the airport - you have to check the AIP. The term "technical stop" or "tech stop" is often used instead of "refuelling stop", since I guess there could be other reasons than refuelling why you'd need to land and takeoff without exchanging passengers or cargo.

As an example from personal experience flying my 2-seat experimental across the pond, Kangerlussuaq and Narsarsuaq in Greenland do not require crew to go through any customs or immigration checks for a technical stop, while Reykjavik in Iceland does. They let me get out of my drysuit and take a pee break first though, unlike the US CBP!


What exactly are the "sealed" leading-edge slats in B737 short field performance aircraft?

A "sealed" slat has no slot between it and the main wing.  According to b737.org.uk, sealing flaps allows for more lift at a lower AOA (Angle of Attack).

Reference from Flightglobal.com "Short Field 737 goes into flight test" states that sealing of up to 25 degrees of slats will be available for take-off, an improvement from 1 and 5 degrees.  Slots (not sealed) are to be used for landings.

This information  indicates "sealing" creates a large single undercambered surface, which would have a lift to drag (less drag) profile more suited for take-off and climb out than the slotted settings.


Do the wheels of taxi-capable helicopters have power?

The wheels do not have power - there is no propulsion system linked to the wheels, they just spin freely.

Taxiing is achieved by adding just enough collective so that the helicopter starts moving, but not enough to lift it off the ground. A little forward cyclic also help.

To slow down, the main wheels have brakes, like fixed-wing aircraft.


Can we use the term CONTAMINATED RUNWAY for other meanings, such as: oil spill or debris on the runway?

From the FAA Pilot/Controllers Glossary Book:

  CONTAMINATED RUNWAY− A runway is
  considered contaminated whenever standing water,
  ice, snow, slush, frost in any form, heavy rubber, or
  other substances are present. A runway is contaminated
  with respect to rubber deposits or other
  friction-degrading substances when the average
  friction value for any 500-foot segment of the runway
  within the ALD fails below the recommended
  minimum friction level and the average friction value
  in the adjacent 500-foot segments falls below the
  maintenance planning friction level.

(See also ICAO Circular on Runway Surface Condition Assessment Measurement and Reporting) Thanks to mins.

So this covers rubber and oil, but not debris.

However, from searching the glossary there are no instances of the word "debris" in the document. Usually if debris or oil were noticed on a runway, ATC would close the runway (issue a NOTAM) and then use an alternate runway until the debris or oil were cleaned up.

From a pilots perspective, if you notice debris on the runway just tell ATC (provided you didn't hit it), and they will close the runway for a FOD (foreign object debris) check. My local airport does this usually twice a day or more, typically by running a car down the runway looking for debris.


What are the rules for Public-Use Operations?

Short answer: public (federal or state government) aircraft operations can be exempt - but aren't always - from most FAA regulations, provided they're non-commercial operations. Any regulation that specifically applies to a "civil aircraft" doesn't apply to a public aircraft operation.

First, some definitions from 14 CFR 1.1:

Civil aircraft means aircraft other than public aircraft.
Public aircraft means any of the following aircraft [...]

The definition of "public aircraft" basically comes down to 'any aircraft the government owns or has a long-term lease on'. That potentially covers everything from Air Force One down to the Podunk Sheriff Department's C172. With that in mind, a lot of regulations apply only to civil aircraft, e.g. 14 CFR 61.3 requires a pilot's certificate to operate an aircraft (emphasis mine):

(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:

So 61.3 doesn't apply to public aircraft. Similarly, 91.203 requires an airworthiness certificate for civil aircraft only (emphasis mine):

[...] no person may operate a civil aircraft unless it has within it the following:
(1) An appropriate and current airworthiness certificate

As for maintenance, part 43 only applies if an aircraft has that certificate, per 43.1:

this part prescribes rules governing the maintenance, preventive maintenance, rebuilding, and alteration of any—
(1) Aircraft having a U.S. airworthiness certificate;

In other words, if your aircraft is public use then you don't need a pilot's certificate to fly it, you don't need an airworthiness certificate, and therefore you don't need maintenance! See also 91.7, 91.9 and many other regulations that only apply to "civil aircraft".
Whether or not a specific flight is a public aircraft operation (PAO) or not is explained in AC 00-1.1A, Public Aircraft Operations. Pages 10 and 11 have flowcharts for determining whether a flight is a PAO but to simplify, if the aircraft is owned by the government and the flight isn't for commercial purposes (also defined in the AC) then it's a PAO. And that means many regulations don't apply to them:

What Oversight of PAO Does the FAA Have? The FAA has limited oversight of PAO,
though such operations must continue to comply with the regulations applicable to all aircraft
operating in the NAS.

Again, if you read the regulations with the civil/public distinction in mind then you can see a lot of exemptions. For example, 91.131 is the regulation for operations in class B airspace. It says (emphasis mine):

(a) Operating rules. No person may operate an aircraft within a Class B airspace area except in compliance with §91.129 and the following rules:
(1) The operator must receive an ATC clearance from the ATC facility having jurisdiction for that area before operating an aircraft in that area.
[...]
(b) Pilot requirements. (1) No person may take off or land a civil aircraft at an airport within a Class B airspace area or operate a civil aircraft within a Class B airspace area unless—
(i) The pilot in command holds at least a private pilot certificate;

That means (a) applies to all flights but (b) applies only to civil flights, i.e. a PAO flight still needs a clearance for class B airspace, but the pilot doesn't need to have a pilot's certificate. It looks to me like the regulations are generally written so that everyone has to follow the same rules of the air (for obvious safety reasons) but public aircraft are often exempt from licensing, airworthiness and maintenance requirements.
Does this all mean that US airspace is full of unlicensed pilots flying non-airworthy aircraft that haven't been maintained properly, all under the cover of government business? Not really; the government has as much interest in safe and productive flying as anyone else, and that requires qualified pilots and well-maintained aircraft. There are some plausible reasons for the PAO exemptions, like allowing military pilots to operate civil flights, and federal and state governments are often exempt from their own legislation in all sorts of areas, not just aviation.


Are aviation regulators generally slower to issue orders affecting their "own" aircraft manufacturers?

I was many years in the industry with an OEM and have had various minor peripheral roles in certification and continuing airworthiness projects at the system level, giving me a kind of ring side seat.  And I would say that you tend to get more hair-trigger reactions from non-western regulators (China/Russia et.al), and that this is because there is a way lower level of trust between the regulator and OEM than with Western regulators.

Now, although it all went to hell in this case with respect to Boeing/FAA, there has to be some level of trust or else the regulator would have to constantly audit the manufacturer every time they scratched their butts.  It doesn't mean that the FAA just lets Boeing do whatever it feels like, but it does mean that they will tend to accept reports and analysis submitted without too much cross examination if things seem reasonable, and that's how situations like this can slip through.

Unfortunately the trust relationship broke down in the MAX case because Boeing perhaps could be said to have abused its trust relationship with the FAA.  Having been burned, you can be sure the FAA will tighten up its oversight of Boeing quite a bit, for a while anyway.

In non western countries the trust relationship is not very robust, so regulators will pull the trigger quite quickly.  This can cause its own problems.  You also have the tendency of non-western regulators to have much more severe punishment regimes, to the point of jail time for mistakes made in good faith.  This encourages people to hide their mistakes (a major problem in both China and Russia in manufacturing, maintenance and flight operations). So in the long run, in spite of the MAX fiasco, the low trust regime is much worse.


How to trigger warnings on an unplugged aural warning unit?

Answering my own question based on tips posted on comments:

As @Gerry suggested on comments, they are OPEN/GROUND, if i ground any of sensor cables it'll trigger the alarms. Those lines are 5V, and i think that most of sensors are just simple switches, except for the Fire Alarms, that i  couldn't trigger and has 6.8V over its line.

For now i got my AWU speaking.

Thank everybody who commented with their suggestions. Those were very very very useful,  you guys are awesome!


Why are there so many moving parts used in the landing gear?

One of the key points (circled in red) above shows the part that locks the gear in the extended position and requires that the linkage up top (circled in blue) is actuated to release the gear.

At the end of the clip you can see a glimpse of the linkage bar at the top returning to its previous position. That way if the gear is dropped by gravity it will lock itself once fully deployed.

If you strip those parts all that's left is a basic 4-bar linkage and a connection to the gear door.


Where Can I Find the Master Minimum Equipment List for the A320 Series (UK / EUR)?

The latest revision of the MMEL for the A320 family was approved a month ago by EASA. The EASA website only contains a PDF document with the MMEL approval signature for Airbus aircraft.  For some manufacturers (e.g. Cessna) the full MMEL is included on the EASA website.

The information page on MMELs says:

  Each MMEL document is the property of the (S)TC holder, and only made
  available here in full if requested by the (S)TC holder. If a full
  copy is not available here then please contact the (S)TC Holder to get
  a copy of the document in question.

So you have to contact Airbus in this case.


What is the reason for this discrepancy regarding the CRJ100/200's stickpusher-activation angle?

Your wrote:

a value which, according to the report, would not have been altered by the aircraft's rapidly-increasing AOA

The report did not say that (but I can see how that confusion can arise). It said the 1) flap retraction and 2) increasing g load -- would not alter the AOA trip point of 13.5 -- no mention of rapidly-increasing AOA:

the flaps had retracted a few degrees from 45 and the g was increasing, both of which would cause a slightly higher pusher activation speed; however, there would be no effect on the pusher activation AOA. [emphasis mine]

"Both of which" references the two conditions 1) and 2).

What would alter the trip point however is explained on the page before:

When the AOA is changing, the computer notes the rate of change and, if necessary, applies a correction to activate the protection system ahead of its normal trip points. [emphasis mine]

Which is what the computer did, as explained on page 49 (PDF page 59):

The left-hand and right-hand AOA vane angles as recorded on the FDR were compared with the shaker and pusher boundaries adjusted for the phase advance resulting from the rate of increase of aircraft AOA. This comparison revealed that both AOA vanes reached the shaker trip point, but only the right-hand vane reached the pusher trip point (...) [emphasis mine]

In other words, the trip point would be the AOA phase advanced to account for the AOA rate in the Stall Protection System computer. Which as you noted, was somewhere between 8.7° and 9.4° for the stick-pusher.


What is the most economical way to store a small aircraft in a place with severe weather?

Location matters.  Where are you located?  A hangar is not as important in a dry climate like Las Vegas as it is in, say, Miami.

It may make sense to pay more for a hangar.  Leaving a plane exposed to the elements you describe (0° - 100°, snow, wind, sun) will do damage over time.  The plane won't corrode into a heap but little things will start to require extra attention.  Those little things will add up at annual time.  It's unavoidable.  Having the hangar is another layer of protection that, in my opinion, is well worth the cost.  If a hangar is simply unworkable then think about a covered tie down. A covered tie down plus a decent cabin cover can accomplish about 75% of what a hangar offers at half the price.  You won't have complete protection from hail, vermin, or thieves but that's not a deal breaker.  Just lock the doors and keep the insurance up to date.

Are you considering a simple plane like a 150 (all metal, "affordable"), a tube and fabric plane, or something with more complexity like a Bonanza or Mooney?  Maybe a C-150 with rough paint, a worn interior, and old avionics that aren't worth stealing will do just fine outside.  If you buy something with good paint and interior (or fabric covering) you'll want to protect it.  New P&I costs thousands and thousands of dollars so it's much cheaper to keep a pretty plane pretty than to make an ugly plane pretty.  The costs to re-cover a plane in fabric can cost tens of thousands depending on the amount of fabric and complexity of the paint scheme.  Other airplane-specific considerations like corrosion on Mooney's steel structure (see Mooney Service Bulletin 208) or a Bonanza's magnesium control surfaces should be factors in your decision as well.  Finally, when you go to sell the plane a buyer will likely feel much better about a plane that was hangared than one that was parked outside.

One piece of (unsolicited) advice: draw up a conservative budget assuming that costs will be higher than expected.  Budget for a hangar, $1M smooth insurance, and a tough annual during the first year of ownership.  Multiply this number by 1.5 at the very least.  This is a "bad case" first-year cost (don't worry, it gets better as you settle into ownership).  If you can absorb that cost without feeling any pain then you're ready for ownership.  If you can't...who cares?  Buy the plane anyway!

We can give you a more complete answer if you know the answers to the following questions:

where are you based?
what kinds of planes are you looking at?
what is the current condition of this hypothetical plane?
what's your maximum annual ownership budget?


Do pilots get full credit hours when they fly double or augmented?

It really depends on the individual Air Regulations of each country, and the union contracts of each airline. Pilot union contracts are often more restrictive than the Air Regs of the country they operate in.

In my experience, North American airlines with "formula pay", both sets of pilots log the entire flight, because they are paid for the entire flight, even when they are sleeping in the bunk.

Other airlines who pay monthly salaries, with bonuses for extra overtime hours, do not credit the pilot for time in the bunk, and they tend not to log those hours either.


Could clouds on aircraft wings produce lightning?

Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by supercooled water droplets colliding with ice crystals near the freezing level. When a large enough charge is built up, a large discharge will occur and can be seen as lightning (from Wikipedia).

So you need two things for lightning to occur- a charge buildup in a place and a charge gradient (difference in charge, like between two clouds or cloud to surface) for lightning to occur.

Based on this, there is little chance of any charge buildup and discharge in the condensation over wings- it is too short lived for any charge buildup within itself and the charges over the aircraft wings are  dissipated by the static wicks in the trailing edges- so no chance of 'cloud' to wing surface either.


How do airlines handle uncertainty in passenger weight for weight and balance calculations?

They don't handle that uncertainty.

Safety is always the prominent factor in the aviation industry. That said, measurements need only to be "accurate enough" for the purpose of safety. Being off by small amount would not compromise safety.

Furthermore, the atmospheric condition is always different; it is so dynamic and cannot be precisely measured. As long as the measurement is accurate enough, the aircraft will fly.

Theoretically, you could argue that there's a chance:

calculated weight is already at MTOW
every passenger happens to weight way above average
they all bought a lot of souvenir at the airport shops
density altitude was extraordinary high
a 10 knots tailwind was present at takeoff

and the aircraft overrun the runway during its takeoff roll. However, such a chance is so small that we accept our current method as it is and expect the aircraft to tolerate that uncertainly within its designed safety factors.

Such believe was found to be wrong when a small aircraft crashed on takeoff in 2003. Regulations were then modified so that on a small enough aircraft, the actual weight of each passenger (plus luggage) must be used, rather than using a standard weight for estimation.