Why does the Boeing 787 have fewer cockpit windows?

It is because of the carbon fibre. This is the same reason that the 787 has bigger passenger windows. Here you can find a link explaining the polarized windows on the 787. Because of its higher tensile strength, you can have bigger windows. Windows are considered weak points in the fuselage where there is no structural reinforcement. This means that the windows have to be supported by the fuselage and structural components around the window. Boeing Pilots do not fly with their elbow out the window during flight, so Boeing removes that feature. They added bigger windows because they could. There is there separation for the six windows because there is support needed.

Hope this helps, Charlie


Why do fighter aircraft manufacturers still focus on making their planes more dogfight-worthy?

That same thinking, "dogfighting is dead", got the USAF and USN in serious trouble in Vietnam. The U.S. Armed Forces were convinced that the next major war would be against the Soviet Union directly, in theaters including Europe, Alaska and Canada, and as a result, fighter designs succeeding the very successful Sabre and Super Sabre day fighters used in Korea began emphasizing standoff capabilities, believing the aerial threat would come from various classes of nuclear bomber, calling for fast, long-range missile-armed interceptors as a counter.

That war, thankfully, never materialized. Instead, the U.S. was pulled into more or less another Korea, a proxy war fought between Soviet-supplied and trained Communist forces against a U.S.-backed fledgling democracy. The big, heavy U.S. F-4 Phantom interceptor and F-105 Thunderchief light bomber, neither of which had internal guns, found themselves facing the older like-minded successor to the MiG-15s they faced in Korea, the MiG-17, coupled with the MiG-21 close-range interceptor. Instead of the long-range standoff air war the USAF was built for, US pilots were ambushed at close range by guns and infrared missiles, resulting in a piss-poor 2:1 kill/death ratio in aerial engagements, compared to the 14:1 ratio they enjoyed in Korea.

The result was increased attention paid to "air combat maneuvering", and an emphasis on this style of combat in the next generation of fighters adopted by the U.S. military. The F-14 and F-15, though intended to bring back dogfighting (and they did), turned out to be big, fast interceptor-style fighters like their predecessors, that still only had a clear advantage in BVR engagements. So a group of USAF generals, seeing what its allies had been doing with the F-5 Tiger that the Joint Chiefs had more or less passed on (a few trainers, a few aggressor planes, but no combat adoption), commissioned the LWF competition, which would produce the F-16 and F-18 lightweight multirole fighters with an emphasis on the close-quarters maneuverability that the Communist powers never really forgot.

The F-22 splits the difference between the F-15 and F-16's air combat strategies, producing a heavier but highly maneuverable fighter that excels at air combat at any range (but is so expensive that so far the USAF have only deployed them in situations where they would never face any real threat). The F-35, however, is rapidly becoming a disappointment; compared even to the F-16 or F-15E it will replace, it "can't climb, can't turn, and can't run", and the A variant is the only one with an internal cannon (which carries only 182 rounds of ammunition compared to the F-22s 480, the F-15's 650 and the F-16's 510); the STOVL B variant and the Navy's C variant won't have guns at all.

In today's military theater, the U.S. enjoys "air supremacy"; we're fighting largely paramilitary forces with practically zero air to air capability. Tomorrow's theater, however, may be quite different. Diplomatic relationships with a number of Russian-supplied nations including Russia itself are souring day by day, and the U.S. could easily find itself drawn into conflicts over Taiwan, the Korean peninsula, India/Pakistan, the Ukraine, etc, where they'd face militaries fielding Russian jets purpose-built to mix it up at visual range.

In short, while the U.S. hasn't directly fought a conflict against a serious air threat since Vietnam, there are a lot of potential near-future conflicts in which they would get their chance, facing the Sukhoi PAK-FA or Chengdu J-20 with skilled Chinese or Russian pilots at the stick. Even if these fighters aren't fielded in mass numbers, the U.S. would still face the Russians' direct answers to U.S 4th and 4.5-generation fighters, including the MiG-29 and Su-27/30/33.  Many variants of these were specifically designed to out-maneuver the U.S. F-15, F-16 and F-18 (which would likely be the first jets in-theater), and which would still be a match for the F-22 in a visual-range "furball".


How does the control-board on small drones govern their flight?

Maneuverability with this kind of drone comes with their small size and high thrust-to-weight ratio. When you scale such a dynamic system, the time scales with the square root of size, so the smaller the drone, the higher its agility.

Can it do automatic stall recovery? No, because it never stalls.

Can it do aerobatics? Only figures which are possible with quadcopters if the right control sequence has been pre-programmed. Regular model airplanes can fly pre-programmed aerobatic figures on the press of a button for at least a quarter of a century now.

Does it sense gravity? Yes, by filtering the signals from the accelerometers through a low-pass filter. The dominant vector of the filtered acceleration points down.

Can it sense the wind? Only sudden wind changes, but not constant wind. If you add an absolute reference like a GPS receiver, then yes.

A gyro is a sensor for rotary movements, so it will only sense rotations. It does this with three tiny oscillating structures, orthogonally mounted to each other. A rotation will cause the resulting Coriolis force to move them, affecting their capacitive characteristics, which can be used to measure the turn rate in all three axes.

Accelerometers are needed to measure acceleration, and they also measure gravity. Here again, you will find three tiny, orthogonal structures which are deflected by an acceleration, much like a horizontal beam bends down in Earth's gravity field. Again, capacitive changes are measured and converted to acceleration values in the three axes.

Modern sensors combine both acceleration and turn rate in so-called six-axis sensors (PDF!). They can be run at several Kilohertz, so they take measurements a couple of thousand times per second. This fast rate is needed for the almost instantaneous reaction of virtual reality glasses; the sensors in drones should be happy with a few hundred measurements per second.

A big problem of such miniaturized sensors is their drift: If you only rely on a gyro, you will never be able to tell if the system is truly at rest. If the system is standing still, you can use a 6-axis sensor and take the direction of gravity to remove this drift. But you will still not be able to avoid drift in the rotation around an axis parallel to the gravity vector, plus in a moving system you cannot measure the true direction of gravity, so another sensor is needed which is not affected by movements: A compass. It measures the magnetic field, again in three axes, and can be combined with the first two in 9-axis sensors.

The latest generation of sensors can include an ASIC to run sensor fusion software, so the output is not only digital, but already corrected for drift and presented as quaternions, which are superior for calculating rotations. Also, they include only the two inertial sensors and can act as a sensor hub, so the compass can be placed freely where it is least affected by soft iron effects (PDF!) and is connected via a databus (I²C or SPI).


What is the regular stall speed of an F-16 flying in a straight-and-level path?

I don't have exact data either, but we can easily make an estimate with data from Wikipedia. Note that I explicitly neglect high-lift devices which are present on the F-16 (notably the strakes which generate significant vortex lift), the variable angle of incidence (washout), as well as body lift which probably plays a significant role as well.

I estimate the aircraft weight to be around $150\,\mathrm{kN}$. The wing area is approximately $28\,\mathrm{m}^2$, and the NACA 64A204 airfoil used has a max $C_L$ of 0.8. Fill that in the lift equation $$L=\frac{1}{2}C_L\rho v^2$$

we get a speed of about $100\,\mathrm{m/s}$ which is about 200kts.

At the angle of attack of max $C_L$, the L/D is about 10, so the engine has to deliver $15\,\mathrm{kN}$ of thrust to maintain level flight. This angle of attack is approximately 6 degrees, and due to the probably positive angle of incidence between the fuselage and wings, the thrust angle is likely less than 6 degrees w.r.t. the horizon. This means that the vertical component of thrust is about $\sin(6°)\approx 0.1$ times smaller, or $1.5\,kN$. Given the error bars on my estimates (especially the aircraft weight), this does not warrant a recalculation because most of the weight is indeed carried by the wings.


Actually, the Boeing design for the SST (Super Sonic Transport), the Boeing 2707 had a swing wing (swing wing was the hot thing during that time). It is instructive to look at what happened to that design.

Image from up-ship.com

The 2707 concept was a larger aircraft, with a 174' wingspan for TO/landing and low speed regime (In comparison, Concorde had a 84' wingspan and carried half the passengers). While the wings were swept back, the wing area increased from 1828 $m^{2}$ to 2743 $m^{2}$.

The engines were carried on the stabilizers. During the development Boeing had significant problems associated with the swing wing mechanism. Concerns about stability and payload capability lead the design to be lengthened and added canard to the front to meet rotation requirements, though the new design 2707-200 kept the swing wings.

Image from up-ship.com

However, these changes did little to solve the problems. When kept near the centerline, the hinge mechanism interfered with the undercarriage. The hinge mechanism was so heavy that it negated the advantages of swing wing, leading to a very poor payload to weight ratio. Studies indicated that the aircraft would run out of fuel halfway across the Atlantic, essentially killing the project.

Due to this, Boeing decided to use a fixed wing platform in its next iteration, the 2707-300, which looked quite similar to Concorde.

"Boeing 2707-300 3-view" by Nubifer - Own work. Licensed under CC BY-SA 3.0 via Commons.


Commercial aircraft have fuel dump nozzles located under the wings for dumping fuel. Fuel is gravity jettisoned from the tanks located in the wings and the fuselage. Large commercial aircraft such as the 747 can dump upto 6000 lbs of fuel per minute with all fuel jettison pumps open.

Interestingly, the fuel dump nozzles are in different places on different aircraft:

On the 747, the nozzles seem to be located near the wingtips:

Whereas on the A380, the nozzles seem to be much closer to the fuselage:

It is triggered by activating the FUEL JETTISON switches in the cockpit on the overhead panel. Below is a picture of the same in an Airbus A380-800. Both the ARM and ACTIVE switches must be pressed to start a fuel dump.


When and why is the SELCAL (Selective Calling Radio System) check being performed?

If you are unsure whether your SELCAL equipment is properly working, you can request a SELCAL check on a HF-enabled radio frequency. The ATC unit will send out your SELCAL code for testing purposes.

I'm not sure my selcal unit is working, is there anywhere I can test it?
Yes, several places.  If you are located in:

Europe or east of 30W on the North Atlantic, call Stockholm Radio
Eastern USA/Canada or west of 30W on the North Atlantic, call New York ARINC
Western USA/Canada or Pacific, call San Francisco ARINC
Asia - daylight hours, call Singapore or Manila Radio on 8942 KHz USB

[...] All selcal checks are free of charge - just call the station with the aircraft registration mark and when you receive a reply ask for a selcal check and give your selcal code.  The operator will send out your selcal tone and - assuming the selcal unit is working - you will hear a chime in the cockpit.  Acknowledge by replying 'selcal check okay' and you're done!   If your selcal check doesn't work the first time it is worthwhile asking for a 'double' which means you get 2 sets of tones sent out back-to-back and often cures any issues.
(Source: www.selcalweb.co.uk)


Can anyone identify this tri wing twin engine WW1 plane

Gianni Caprioni's 1918 Ca 4 triplane bomber, shown here with Caprioni and three companion aircraft. It has a twin boom tail and a pusher prop in the center pod, so it is a triengine triplane.

source


To answer the list in order:

Do they fly hop to hop?

An F-18 can go 3,000km unloaded (ordnance adds a lot of drag) and that's plenty from Australia. You can get to the Middle East in 4 hops, no air-to-air refuelling required. Although if you do an in-flight top-up you can add more than the regular range because you don't need to burn any fuel getting back up to cruise altitude.

Where would these stops be? Surely not at civilian airports?

Military aircraft often use civilian airports - they run on Jet-A just like everything else. Fighters were not an uncommon sight at the general-aviation terminal where I'm from. Loaded warplanes are a political issue, unloaded ones are a good day's business for the airport's fuel merchants. (note that fighters are almost never completely unloaded - the ammunition for the gun has a significant effect on the airplane's trim. But you can't see it.)

An external fuel tank can be added in place of weapons.

Are they actually shipped via regular military cargo ship?

Fixed-wing aircraft are typically not boxed for surface shipping. Helicopters are - no wings to remove and rotors can be folded up very easily.
Carrier-based aircraft will depart from land - the only way you get a Hornet off of a docked carrier is with a crane, so they typically launch everything when still offshore, and load the aircraft the same way on departure.

The USA has enough carriers that they would not deploy a carrier-based fleet like this. If they are adding / replacing aircraft there are probably enough already in Europe - send those where needed and replace them in the normal course of events.

Would they fly supersonic to reduce travel time?

No, they won't go supersonic to reduce the flight time. > Mach 1 consumes so much fuel you'd never get there. Fighters have supersonic capability so they can get there or run away as needed, but it's never used for very long. The Concorde was designed from the beginning for Mach 2 cruise, so it didn't use afterburners once at Mach 2. Subsonic flight suffered as a result, fighters would not accept the subsonic flight handling issues as that's rather important to them :-)

Bombers are often deployed directly from home bases. All modern strategic bombers have global range with in-flight refuelling. You don't leave North Dakota, blow up something in Longwayfromherestan and get home for dinner, but you don't land anywhere else either. Yes, it's many hours in the same seat but the military crews are used to it.


Is it possible that commercial airliners use an EMALS or other form of electric launch?

As a start I suppose the reason of that is a particular configuration where only a short runway could be build, for instance in a highly touristic island!

EMALS uses an electromagnetic linear motor operating on a track, some studies in the 50’s have been done in Europe using the same principle not to launch the train but as a continuous powering system however they were abandoned because of compatibility between countries.

As far as power is concerned, what was theoretically possible for a train could be possible to push or to pull an aircraft, but coming to the practical application, on which part of the aircraft the pushing or the pulling force shall be applied for launching; the landing gear presently available are not designed to accept such shear forces bringing the aircraft at least to V1.

If we consider the EMALS is just to reduce the runway length and is used just to bring an additional takeoff acceleration power, and supposing the location on the aircraft of the applied EMALS forces is solved, we get into another problem, that is the decision of the pilot at any moment before V1 to continue the takeoff or not? Should the pilot decide to cancel the flight, how to coordinate pilots decision with the EMALS disconnection, should it fail to disconnect the consequences could be terrific.

From my point of view this system requires specially designed compatible aircrafts, the economical feasibility can’t be general but specific to particular geographical configurations.

As a project why not? But economically it is hard to justify.


Propeller rings (ducted propeller) have been used un aircraft, notably in some early VTOL designs, like the Bell X-22, XV-11 Marvel and also the RFB Fantrainer.

Bell X-22. Image from diseno-art.com

They do have some advantages like,

Better noise reduction and safety compared to 'open' propeller.
For the same thrust, the propeller can be made smaller.
The duct can be designed such that it provides a limited amount of thrust vectoring, useful if we are using it for STOL.

However, they are not widely used because of various other reasons like,

For effecient operation, the tip clearance should be very small. Any increase in tip clearance leads to rapid degradation in effeciency.
The duct and its support structures increase the weight.
The duct increases the drag at cruise speeds (as drag increases as square of velocity).
If we decide to use a lower sized propeller as a result of using the duct, the effeciency suffers- as we are actually accelerating a lower mass of air more (rather than accelerating a higher mass of air less, which is more effecient).
The design of the duct is difficult- the aiflow into the duct comes not only from the front, but is also sucked from the sides from the sides (i.e. the capture area is greater than the duct frontal area). If not designed properly, the flow may seperate, leading to loss of effeciency.

However, ducted fans are used in a number of small UAVs (in RC model jet aircraft, for example, in lieu of miniature jets), electric aircraft like the Airbus E-fan and also some dirigibles.

In case you are asking about connecting the propeller tips like in the maritime propller in the link, the resulting stresses at high speed would tear the whole assembly apart, causing more problems than it solves.


Why do Dassault Falcons only have one thrust reverser?

As always, it is a trade-off.
On the positive side, thrust reversers allow you to slow down faster and reduce brake wear.
On the negative side, there are costs to install and maintain them. They add weight, which increases the required take-off length reduces the maximum payload and increases fuel consumption.
Dassault found the optimum for their design in one thrust reverser, others go with TR on all engines, none or optional.


What maintenance actions are required after a bird strike?

Depends on the nature of the strike. If a glancing strike—e.g., off the top of the windscreen, or the upper surface of a wing—maybe none is needed. Other strikes could take out the windscreen (some have, in fact).


Which airlines allow you to gain hours with them by paying them

There are pay to fly services, however regulations require a minimum number of hours before you can do any kind of part 121 or part 135 work. So while you can get some extra hours in Jets by paying them that way or turbo props or whatever else you’re looking for, it’s not recommended. Probably the best starting gate once you have a commercial pilot certificate is just doing flight instruction work until you gain the required 1500 hours.  After that you should have offers for most of the regional carriers to work for them as a first officer.


Will control surfaces on a plane be less efficient at a higher altitude?

Please let me assume that you wonder about control surface effectiveness rather than their efficiency. Both are closely related, but I prefer to address their effectiveness - doing what the pilot demands them to do.

The aerodynamic forces are proportional to the dynamic pressure $q$ of the flow, which is density times velocity squared, as in
$$q = \frac{\rho}{2}\cdot v^2$$

To create the same amount of lift at a higher altitude, the airplane has to fly faster, so in effect the forces can stay the same. Only the true air speed will be different, which helps to get where you want faster. The control surface effectivity remains unchanged.

If we dig a little deeper, the atmosphere cools down with increasing altitude until you reach the Tropopause. This cooling makes the engines more efficient but also reduces the Reynolds number of the flow around the aircraft. A lower Reynolds number translates into a thicker boundary layer, which will reduce the effectiveness of control surfaces a tiny bit and will reduce the range of their best effectivity. But this effect is so small that it has no practical consequence.

Airliners fly at transsonic speeds at altitude, so now sonic effects become more pronounced. For the control surfaces this means that they will lose their effectiveness at higher deflection angles. For small corrections all is very similar to low level, slow flight, but if you need full deflections the control surfaces are indeed less effective at altitude.

Another effect will be rather more pronounced, and this is aerodynamic damping. Damping is the tendency of a system to create additional forces from movements which run against this movement. Take a wing: If the aircraft rolls, the wingtips will move up on one side and down on the other. Both movements will add a small angle of attack, lowering the total angle of attack on the up moving tip and increasing it on the down moving tip. Both tips will see a change in lift which will counteract the rolling motion. If the rolling speed is $\omega_x$, the resulting change in angle of attack $\alpha$ at a wing station $y$ is
$$\Delta\alpha = arctan \left(\frac{\omega_x \cdot y}{v}\right)$$
As you can see, the flight speed is in the denominator, so a higher flight speed will create a smaller angle change for the same roll rate. The same holds true for the other axes of movement, and in consequence the higher-flying aircraft needs either more pilot attention or an artificial damper.

I guess it is the lower damping which made you think that the control surfaces are less effective. They aren't.


What is (theoretically) the most efficient shape for an aircraft, assuming you don't have to carry any cargo?

The best shape for an airplane or glider would look like the ASH-30mi, an open class glider with an 86.9ft wingspan, and a 41:1 aspect ratio (wing length to wing chord). It has a glide ratio exceeding 60:1, and is considered the state of the art for gliders.


How is the due date for landing gear replacement determined?

To my knowledge, all lifed components are tracked from installed date. That is from personal experience, but I can certainly have a look through the regs to find an official source, if you require.


Is it normal for visibility to be missing from the METAR?

My questions are:

  Is it really normal to see reports without viz?

Yes!  Well, maybe not "normal", but it does happen.  Automated weather stations are like another piece of equipment and components fail from time to time, including the components which measure the visibility.  The other components also fail from time to time (I've see ASOS/AWOS reports without temperature, barometric pressure, etc.)

  Why does this happen so often for AWOS when ATIS doesn't have problems
  with it?

You shouldn't see this at all with ATIS, because a human is involved, and if the equipment which measures visibility is inoperative, a trained observer (often the same person that is manually recording the ATIS) can look out the window and record the visibility themselves.

  Should this be reported to some entity (NOAA/FAA) to get these systems
  repaired or exchanged?

I would let the airport manager know just in case they weren't aware.  They are responsible for notifying the FAA.

  The fact that it is missing visibility information does not mean that
  visibility is unobstructed, right? I have heard people assume this.

If the visibility is missing, it means just that.  It is missing.  It might be 1/4 SM or it might be 20 SM. Some people may make assumptions that if there is no precipitation, that the temperature / dew point spread is high, etc. that the visibility is good, but that is a potentially dangerous assumption because visibility can be influenced by many different things.  There could even be a dust storm coming through when everything else looks good!

  If the recording says "VISIBILITY MISSING", is it indicated in the metar
  somewhere?

Sort of.  It's just "missing" (i.e. the entire wind block from the METAR will not be there), so if you don't see it then, well, it's missing.


How do Airbus planes flare in normal operation?

You don't do full stall landings in airliners.  You arrive over the threshold at reference speed (the full flaps approach speed), ease the thrust off (how soon depends on the airplane, but you need to be at idle before touchdown) and start to flare but only enough to reduce the sink rate to close to zero and then you tease the pitch to maintain a gentle sink until you touch.   If you wanted to do a full stall type landing, there is sufficient energy still available at reference speed that you would have to hold it off and float halfway down the runway, and you'd touch down with the stick shaker going.  Not a desirable outcome in a 150,000 lb soda can.

If you arrived over the threshold too much below reference speed, there is no energy reserve to save yourself if you are sinking too fast and there goes your tires, if you're lucky.

Water landings often don't go so well if the tail is too low (if the tail touches in too nose high an attitude, it tends to bounce upward and drive the nose over and things can go south).  So it's possible the FBW system actually protected them by limiting Sully's ability to pitch up.  They seemed to have touched down at an ideal attitude, getting a nice ski effect from the nacelles without the tail hitting too early.

In the famous Airbus airshow incident, the airplane descended gently into the trees as it struggled along way on the back side of the power curve (in that regime, the only option is to descend).  Had the crew more pitch up authority available to them, it is possible they would have semi-stalled into the trees at a much higher sink rate. That was Airbus's response to the criticism over the FBW's pitch limiting behaviour.


VFR Navigation, rerouting timing corrections due to wind - rule of thumb?

If you know or have a reference to Ground Speed, estimating reasonably accurate flight timings is straightforward. Whatever your GS, you will cover 1/10th of the distance in 6 minutes. So at 90kts, 9 miles in 6 mins. At 70kts, 7 miles in 6mins and at 120kts, 12 miles in 6 mins etc. If you mark your pen/pencil with 5 & 10nm distance marks measured in the same scale as your map, it’s easy to interpolate say, 7nm (for say 70kts GS) and mark your map with 6minute marks. If needed dissect these markings in half, for 3 minute markers.


Does the main helicopter rotor blade shape affect noise?

Yes. Especially tip shape, which is optimized in most modern helicopters (and upgraded older ones) to reduce noise.
Basically, the blade tip shapes are optimized to reduce the noise caused due to blade-vortex interaction (BVI). From Parametric Study for the low BVI Noise BERP Blade:

The BVI (Blade Vortex Interaction) phenomenon,especially parallel interactions between the helicopter main rotor blade and the tip vortex, is the cause of significant noise and vibration problems in helicopter operation. In particular, the descent  and banked  turn  flight  conditions  are known to produce significant BVI.

This phenomenon is explained in this article in rotorandwing:

... higher-pressure air will spill out from under a rotor blade at the tip, get sucked toward its upper surface, and create a strong corkscrewing swirl of air, or tip vortex. ... an approaching blade will often come into the vicinity of a tip vortex left by a preceding blade. When the high-energy swirling air strikes the rotor blade, it causes a sudden change in angle of attack and an associated change in pressure on the surface of the blade. This is what creates the loud impulsive noise that can be annoying to those on the ground.

If the blades are designed in such away that the pressure change due to the BVI is reduced, the noise can be reduced. This is the reason for the differing shapes of modern helicopter blade tips. A good example is the Eurocopter Blue Edge concept, which Airbus claims reduces noise by 3-4 dB:

Blue Edge™ main rotor blade has a double-swept shape to change the blade-vortex interaction (BVI) that occurs at the tip of any helicopter blade – significantly reducing noise levels created during a typical descent profile.
This ... noise reduction technology has been validated ... flight trials using a five-blade Blue Edge™ main rotor on an EC155, showing a 3-4 dB noise reduction.

Eurocopter Blue Edge Wing; image from wired.com
That varying the tip shape will affect the BVI and consequently noise, was well known (for example, see the NASA paper Evaluation of Helicopter Noise Due to Blade-vortex  Interaction for Five Tip Configurations by Danny R. Hoad, which compares the sound produced by various tip shapes). However, with the advances in computer modeling and CFD, a number of modern helicopters incorporate blade tips that have been designed with the express purpose of reducing BVI and noise.


How can I avoid flight simulator software reinforcing bad habits?

I posted this answer on FB a while back:
I’m a commercial pilot as well as a flight sim fan with somewhere in the neighborhood of 7000 hours logged on Flight Simulator and Falcon 4.0 games.  I disagree with the assertion that the sims are so different from the real thing.  While it is true that the kinesthetics of being in a real airplane are different from the sim, the handling and behavior of the sim is so close to the real thing that the same mistakes I make in the sim I make in the real thing.  They are also very effective platforms for practicing instrument flying and approaches.  It’s not surprising that Redbird makes use of a proprietary version of FSX for its flight simulators.
That being said there are several notable areas where the entertainment sims can be off from the real thing:

Avionics may be simulated poorly or unrealistically.  Alabeo payware is a major offender here.

Games do not adequately simulate weather hazards.  Flying into a thunderstorm in FSX is a benign experience.  This can kill you in real life.

ATC interaction is appallingly unrealistic, akin to having an adult conversation with a talking Barbie doll.  Probably the worst offender is XPlane’s default “ATC” which is absolutely nothing like the real thing.

Airport and navigation databases are terribly out of date and cannot be updated.  This appears to be changing with the advent of Flight Simulator 2020 which periodically updates its airport and nav databases, but FSX and XPlane are horrible.  This dramatically affects realism when flight planning IFR flight and shooting practice approaches, as, at least in the case of FSX, the default approaches are now 14 years out of date!

Terminal departure and arrival procedures are often not modeled properly or absent altogether, making operations from major airports unrealistic, especially for jet flying.

Considerable errors exist in the default terrain as well as airports. Some of the worst instances I’ve found were in FSX and include an airport in the middle of a 1000ft deep pit.  Ft Worth Meecham Intl. has the peculiar obstacle hazard of floating houses along the glidepath to runway 35.  Gustaf III / St Bart’s and Courchevel are wretched as well.

Some sims (FSX) suffer from control lag ie a delay time between control input and movement of the control surfaces, creating an unrealistic control response which makes it difficult to fly demanding maneuvers eg aerobatics accurately.

Some sims are developed around certain types of aircraft and do not replicate other aircraft or systems well.  FSX turbine and jet engine simulation is a great example of this.

Edge of the envelop flight characteristics such as spins and aerobatics are often modeled poorly or completely absent in these games.

Ground handling and and takeoff and landing characteristics are unrealistic compared to the real thing.  This is really apparent when flying tailwheel airplanes, particularly in XPlane.

There are fixes to some of these problems.  Some payware aircraft are extremely realistic - VRS’s F-18E and RealAir’s BE60 Duke and Lancair Legacy quickly come to mind as does PDMG’s 737NG and 777 aircraft.  Additional addons for weather as well as cockpit avionics like Flight1’s GNS430/530 and GTN650/750 GNSS heads are great examples as they use the Garmin trainers as their core. ATC add-ons and Live ATC in multiplayer games alleviate the controller flaws in the default software.
If  you do intend to use a desktop flight simulator software to train on, I suggest that it be used as a means to augment actual training in an aircraft following the instructions of a CFI.   This allows you to accomplish a lot of training on basic airmanship and a fraction of the total cost compared to actually going out flying in the real airplane. While it diminishes the principle of effect during learning, It can accomplish a great deal of goals in terms of rote procedures and psychomotor practice.   Again a good flight training syllabus should be followed under the direct supervision of a certified flight instructor combined with actual flight training in an airplane.  When that’s done desktop flight simulators can be extremely useful to a budding airman. I learned how to do crosswind landings using FSX  and have always been very good at them ever since.   If training is done solely by means of a desktop simulator and self-didiaction,  this can lead to negative outcomes, such as incorrect use of the flight controls.  I remember one point early on in my flight training, there was another student working with a flight instructor who had logged in a lot of hours in desktop flight simulators by himself prior  to beginning flight training.   He had developed the undesirable habit of using elevator trim in order to maintain altitude during cruise flight wile playing sims and it took a great deal of work from his flight instructors to break him of this habit and install the correct use of trim in order to neutralize control pressures when in cruise.


All the treadmill does is increase the RPM of the tires at rotation.  Airliner tires are generally spinning at somewhere around 800-1000 rpm at, say, 140 kt depending on the diameter.  If the treadmill is running at 140 kt in the opposite direction, the tire would have to be spinning at double whatever it is normally when the airplane achieves liftoff speed.

The question becomes:

Can the airplane's thrust overcome the tire's increased rolling, bearing and brake drag (from casual brushing contact of the brake pads) friction when spinning at double its normal RPM?
Can the tire survive running at double RPM, well above its speed rating, without blowing up?

For:

Probably yes, but it would extend the takeoff distance quite a bit, but...
I would say probably not.  An airliner tire will have a speed rating that has a good margin above the normal operating speed, in the low 200 kt range, with probably some fudge factor above that. So it could probably survive up to, say 230 kt.  Taking it to double its normal operating speed, an RPM equivalent to 280-300 kt, is probably too much and there is a pretty good chance at least one of them would blow up.  Almost certainly if they were recaps, which most airliner tires are.


Do ETOPS rated airplanes ALWAYS fly with ETOPS requirements?

The short answer is No (as per the regulations)

The ETOPS regulations state that an aircraft conducting ETOPS operations must be compliant to ETOPS regulations regarding its configuration (based on engine count). But aircraft capable of operating ETOPS flights only need to comply with the Advisory Circular when actually flying ETOPS. In other words the ETOPS AC only applies when the planed route falls under the guidance of the AC.

  All two-engine airplanes and three- and four-engine passenger-carrying
  airplanes operated under part 121 are required to comply with §
  121.161. This regulation imposes special requirements for ETOPS for these airplanes. These operations are defined as:

  (1) Two-Engine Airplanes. These are flights whose planned routing
  contains a point farther than 60 minutes flying time from an adequate
  airport at an approved one-engine inoperative cruise speed under
  standard conditions in still air.

  (2) Passenger-Carrying Airplanes with More Than Two Engines. These are
  flights whose planned routing contains a point farther than 180
  minutes flying time from an adequate airport at an approved one-engine
  inoperative cruise speed under standard conditions in still air.

However, how an airline choses to route and use its fleet is up to them. Since ETOPS airframes may require different maintenance or other related things it may not be effective for a given airline to use ETOPS maintained aircraft on Non-ETOPS routes.


Can drones/UAVs fly over international waters without being subject to the FAA's regulations?

Short answer: the FAA's UAV regulations probably stop applying at 3 nautical miles, but this is more about maritime law than aviation regulations and you might get a better answer on law.SE.

The FAA's UAV regulations do state where they apply. 14 CFR 107.1(a) (emphasis mine):

  Except as provided in paragraph (b) of this section, this part applies
  to the registration, airman certification, and operation of civil
  small unmanned aircraft systems within the United States

14 CFR 1.1:

  United States, in a geographical sense, means (1) the States, the District of Columbia, Puerto Rico, and the possessions, including the
  territorial waters, and (2) the airspace of those areas

Defining different types of marine jurisdiction is apparently so complicated that it requires a diagram (33 CFR 2.1), and the definition of "territorial sea" is in 33 CFR 2.22. Unfortunately, there are two possible definitions: 12 nautical miles or 3 nautical miles from shore. My reading of it is that for treaties, criminal law and other 'big stuff', the waters extend 12 miles. For more general purposes, it's 3 miles per 2.22(a)(2):

  Unless otherwise specified in paragraph (a)(1) of this section,
  territorial sea means the waters, 3 nautical miles wide, adjacent to
  the coast of the United States and seaward of the territorial sea
  baseline.

Having said all that, I don't know for sure what the definition of "US territorial waters" really is. The 3NM distance seems very likely (to me) for part 107, and you might get a better answer on law.SE.


Where can I find old (historical) FIR map?

The best I can find is a ATC radar screen used in a dated report (appears to be a report by the Hong Kong Civil Aviation Department in 2000). It shows the FIR regions and waypoints of near Hong Kong, Hainan island and Vietnam:

Here is a dated enroute chart of Hong Kong in 1996. I'm not sure if the waypoint names will be useful for your further search:

How I end up with these: for the first one I used keyword "Sanya AOR". Given the dates you stated in the question, Kai Tak airport was still operational during that time. So I searched for charts of Kai Tak. A full set of charts of the airport would contain an "enroute chart" which lists waypoints. Search engine is Bing.


How heavy is too heavy to land on a grass strip?

Your second question is really easy to answer:
Q: How do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?
A: Big Fat Low-Pressure Tires.

A wide, low-pressure tire has a larger contact patch with the ground, spreading the aircraft's weight over that larger area helps prevent the tire from digging into the ground, and lets the aircraft roll more easily on rough or soft surfaces.

Your first question - How much weight can a grass strip support? is a much tougher one.  The bearing strength of an unimproved surface depends on a number of factors (some big ones: The composition of the runway bed, its drainage, and what kind of surface cover - turf, rocks, etc. - it has).

If you look at aerial photos of Edwards Air Force Base you'll notice part of the runway complex includes a dry lake bed:

Some pretty heavy stuff has taken off from and landed on those runways - space shuttles have landed there.
Similarly TACA Airlines Flight 110, a Boeing 737, landed on a levee at NASA's Michoud Assembly Facility (and was subsequently repaired and took off again, though they used an access road for the takeoff rather than the grass levee).

Conversely if you tried to land a Cub on a grass strip that's saturated with water after a rainy month there's a chance your tires might sink into the mud.

CBR Technologies has a nice presentation on unpaved runways, which includes some discussion of how their bearing strength (and thus maximum landing weights, tire pressure limits, etc.) can be calculated, but each unimproved runway will be different, and the values will likely change over time.


Looks like a Boeing Stearman to me.

I couldn't find anything based on the registration (NC55383) of the aircraft.

Source: wikimedia / scan from Robert L. Lawson (ed.): The History of US Naval Air Power. The Military Press, New York (USA), 1985. ISBN 0-517-414813, p. 72. US Navy cited as source.


What is the relationship between "automatic flight control system AFCS", "autopilot", "flight director" and "flight management system FMS"?

Since you are studying maintenance, I would suggest you obtain a copy of ATA iSpec 2200, Information Standards for Aviation Maintenance ATA Standard Numbering System.  Most large aircraft manufacturers and airlines use the ATA Standard Numbering System in organizing their maintenance documentation.

To your specific question, the ATA spec lists the following:

The FMS is the primary tool for managing the navigation.  It is part of the larger Navigation system which includes the aircraft attitude, air data, and positioning sensors. The FMS supports flight planning (path definition) and integrated position determination.

  Chapter 34 NAVIGATION, section 60 Flight Management Computing.

  That portion of the system which combines navigational data to compute
  or manage the aircraft's geographical position or theoretical flight
  path. Includes items such as course computers, flight management
  computers, performance data computers, and associated control display
  units, warning annunciators, etc.

The FM function outputs a desired path and guidance data to the displays and AFCS. The FMS is one source of path and target (altitude and or speed) data for the AFCS. The other input source is by crew input via the Mode Control Panel (MCP).

The AFCS is functionally independent from the FMS and primary flight control system (FCS), though it is sometimes physically integrated with one or the other or both.

The AFCS has two major components, autopilot and auto throttle. Autopilot includes the Flight Director.

  Chapter 22 AUTO FLIGHT, section 10 Autopilot

  That portion of the system that uses radio/radar signals, directional
  and vertical references, air data pitotstatic, computed flight path
  data, or manually induced inputs to the system to automatically
  control the flight path of the aircraft through adjustment to the
  pitch/roll/yaw axis or wing lift characteristics and provide visual
  cues for flight path guidance, i.e.: Integrated Flight Director. This
  includes power source devices, interlocking devices and amplifying,
  computing, integrating, controlling, actuating, indicating and warning
  devices such as computers, servos, control panels, indicators, warning
  lights, etc.

and

  Chapter 22 AUTO FLIGHT, section 30 Auto Throttle

  That portion of the system that automatically controls the position of
  the throttles to properly manage engine power during all phases of
  flight/attitude. This includes engaging, sensing, computing,
  amplifying, controlling, actuating and warning devices such as
  amplifiers, computers, servos, limit switches, clutches, gear boxes,
  warning lights, etc.

The AFSC computes the necessary control inputs to fly the aircraft along the defined path and meet targets.  It provides control inputs into the flight controls (ATA Chapter 27) or in the case of the Flight Director, it generates a display that directs the pilot to make the needed control inputs.

This steering will steer the aircraft to the path defined by the FMC, or it can steer to a simple path entered by the pilot directly into the AFCS via the MCP.  In older aircraft, the autopilot steers by using a series of special actuators to manipulate the controls. In modern fly-by-wire aircraft, it provides input directly to the FCS as the FCS already has the necessary actuators.

Auto Throttle provides throttle adjustments to meet speed targets from the FMC or MCP.

By the ATA standard, the FMS, AFCS, and FCS are three distinct systems. As noted above, modern fly-by-wire systems are beginning to blur the boundaries.  Without the need for separate actuators for the AFCS, it just becomes software much of which is getting merged with the primary FCS.


Why has the rest of the world gone after delta wings while not the USA?

The Gripen and Eurofighter are canards with a delta wing (as is the J-37 Viggen). This, coupled with relaxed stability, gives the first two unmatched maneuverability, and good transsonic and short-field performance. While the Viggen is a stable design, it too has very good short-field performance.

The Mirage III (and the F-102, F-106, B-58, J-35 Draken and Concorde) were first-generation Deltas from the mid-1950s which were optimized for supersonic speed. By using a highly swept delta wing, low supersonic drag could be combined with short take-off and landing distances.

So it is not true that the US "never went after delta wings"; they just outgrew that configuration when they shifted their priorities away from high supersonic performance.

Note that the more modern Mirage F1 is an unstable conventional configuration while the last Dassault design is also an unstable canard.

Regarding the Tejas: It is designed for the specific requirements of the Indian Air Force. One of the unusual design requirements was the low vulnerability to bird strikes, because Indian airfields have a high birdstrike rate. Therefore, no leading edge devices were allowed, and to give the Tejas still a respectable short-field performance, a delta wing was the best choice. Full disclosure: This was actually the requirement which led to the LCA having a delta wing, and the Tejas, which resulted from the LCA, does have leading edge flaps after all.


What Is this on the fuselage of The Fokker F100?

Yes it's the apu exhaust ... there is a post about it:

Where is the inlet for the APU on a Fokker 70/100?


Are an aircraft's nuts and bolts generally either all Imperial or all Metric, or can they be mixed?

Why not mixed? If the airframe is manufactured in a "metric" country but the engine is from the US, you will find metric nuts and bolts behind the firewall (and on the engine mount) but Imperial on the engine.

If I remember correctly, the Robin DR-300 and DR-400 that towed me many times had just that mixture. Lycoming engine and French airframe.


Would random sitting arrangement be the best way of boarding for passengers?

The Mythbusters tested various boarding patterns in their previous season (see the digest here, full episode here) and came to the conclusion that "random-random" (close to Southwest-style) was the fastest way to board (though the least liked), and that the current most common method among traditional airlines (back to front) was the slowest. Alternate styles including two forms of "WILMA" boarding (Window-Middle-Aisle, United does this) and "Reverse Pyramid" style (which U.S. Airways used to use) were both fast and very well-liked by the volunteers. I have some issues with their testing methods though:

They only allowed for a single test of each boarding plan, using the same group of volunteers each time. That means two things:

Order of testing matters, because your group will generally become accustomed to getting on and off a plane, which will speed things up as the day progresses.
Any "noise" inherent in a variable of the test that you are neither controlling nor testing cannot be cancelled out and can skew the results.

They tested the control (back to front) first, meaning their volunteers were totally unfamiliar with the "plane" and thus would be expected to be slowed by inefficiencies born of this unfamiliarity. If no other test were repeated, it should have been this one (at the beginning and end of the day) to compensate for this.
First/business class were always boarded first (this is common but not universal among airlines), while every other ticketed passenger regardless of age was boarded according to their individual seat, with only "lap children" boarding with their parent (airlines nowadays require a ticket and seat for anyone over 2yo, so boarding families with pre-adolescents was not really tested). This is opposed to other variables such as overloaded passengers, guys heading "upstream" to the lavatory etc which they sprinkled in to spice up each boarding experience.

One final thing; boarding faster only means you're sitting in a cramped airliner seat longer. Getting passengers aboard an airplane is usually the easiest and quickest part of turning an airliner around. Unloading/loading baggage, refueling, preflight checks etc all take longer in the aggregate than anything going on inside the cabin.


Why should lower limit of a Control Area (above 3000 ft) coincide with a VFR cruising level?

VFR cruising levels are always located between two IFR cruising levels. With half of the required radar separation minimum to each the IFR level above and below it.
For example:
required separation minimum: 1000ft
IFR lvl: 200
VFR lvl: 195
IFR lvl: 190
The sectors are split at VFR cruising levels to ensure, that both the upper and lower sector can use ALL of their IFR cruising levels while maintaining separation to aircraft that may fly at the highest/lowest level of the sector below/above.
Example:
Sector A is working from FL145-FL245
Sector B is working from FL245-FL355
Sector A may use every IFR cruising level within the sector (FL150, FL160, ..., FL240).
Sector B as well (FL250, FL260, ..., FL350).
As the difference between the highest and lowest level is always 1000ft, separation is ensured also between aircraft flying on top of each other in different sectors!


Do a turboprop’s compressor and turbine rotors windmill when the engine is shut down and feathered?

A feathered propeller is designed not to windmill in flight, however in the case of a power turbine or even a propeller connected to both the compressor and turbine, it's far easier to turn than an old radial when in the feathered position.

Nothing physically stops the propeller from turning when feathered beyond the force required to turn the power turbine/gearing or whatever the propeller is connected to and the air hitting it at the right angle. Straight airflow shouldn't turn it, however any flow from the sides would turn a feathered turboprop. Unlike a turbofan, the propeller changes pitch so the air affects it differently.


You can find the amount of unusable fuel in the EASA Type Certificate Data Sheet. Here is an example table for the A320-200 from that document:

In general, the cockpit displays will always show the amount of usable fuel only. This is mandated e.g. by EASA CS 25.1337:

  (b) Fuel quantity indicator. There must be means to indicate to the flight-crew members, the
  quantity, in litres, (gallons), or equivalent units, of usable fuel in each tank during flight. In
  addition –

  Each fuel quantity indicator must be calibrated to read ‘zero’ during level flight when the
  quantity of fuel remaining in the tank is equal to the unusable fuel supply determined
  under CS 25.959;
  Tanks with interconnected outlets and airspaces may be treated as one tank and need
  not have separate indicators; and
  Each exposed sight gauge, used as a fuel quantity indicator, must be protected against
  damage.

(EASA Easy Access Rules for Large Aeroplanes)


How are Airbus pilots trained for using the mechanical backup control systems?

According to AIRBUS but more specifically Airlines, have training syllabus.
Airbus Company states, clearly how, and how often to train Mechanical Backup Flying.

Each airline, which operates airbus A/Cs has to adhere with these recommendations and/or dictations.

Several failures of systems, or combination of failures can lead to the mechanical backup "state".

Every six months airline pilots are going to the Simulator for checking and training (LPC/OPC/SIM REC).

  Of course there is training regarding mechanical backup.

How often?

It depends each time on the scenario, the seasonal systems to be trained, the systems failures, and many other factors.

Mechanical backup is a possible scenario by the Airbus, and well defined (but as always not exhaustive) in books, how can be happened. For this reason IMO all airliners provide this training to their pilots, and of course all the time (every six months) there is the possibility to fly a scenario leading to mechanical backup.

  Generally the aircraft can fly and land with mechanical backup as long
  as the situation permits, the failures, the pilot skills, and plenty
  of other factors.

How difficult is?

1) Pilot Skills

2) Environment (Weather)

3) e.t.c (The list is non exhaustive at all.)

  Aircraft manufacturers and airlines want to give proper and sufficient training to their pilots, and professional pilots want to
  receive proper and adequate training, and fly remote and difficult
  scenarios in the simulators, in order to be ready for "anything" that would be happened.
  So to summarise, and once again to answer your question, in general
  terms, YES airline pilots get training regularly, on the Mechanical Backup
  Scenario to FLY and to LAND the A/C  and the difficulty is
  something generic, for some people would be less difficult than some
  other.


Why is the first turbine in a turboprop (or turboshaft) connected to the compressor?

You can only use for shaft horse power what is available after the energy to drive the compressor is subtracted. This subtraction is done by the high pressure turbine. Any energy left in the exit flow after that can be extracted to drive a propeller, a fan or a generator.

What happens in your arrangement if the power turbine extracts too much energy to keep the core engine running? No, clearly the priority has to go to keeping the core engine alive and to only use what is left.


Do ICAO specifications have the force of regulation?

According to About ICAO from their own site:

  ICAO works with the Convention’s 191 Signatory States and global industry and aviation organizations to develop international Standards and Recommended Practices (SARPs) which are then used by States when they develop their legally-binding national civil aviation regulations.

So ICAO continues to develop standards and recommendations without the force of regulation.


What is an Air Defense Identification Zone (ADIZ)?

What is an ADIZ?
FAR 99.3 says:

Air defense identification zone (ADIZ) means an area of airspace over land or water in which the ready identification, location, and control of all aircraft (except for Department of Defense and law enforcement aircraft) is required in the interest of national security.

To put it simply, if you're going to fly in the United States ADIZ, the United States government wants to know:

Who you are
Where you are
What you're doing

What do I need to have before I fly through one?
A few things, actually.  They're listed in AIM 5-6-1

You generally need to file a flight plan (but there are exceptions). (FAR 99.11)
You need a two-way radio. (FAR 99.9(a))
You need to have a Mode C transponder, and it has to be on. (FAR 99.13)
If you're going DVFR (rather than IFR), you need to give an estimated time of ADIZ penetration at least 15 minutes before entering it. (FAR 99.15, also reference AIM 5-6-1.4b)

What do I need to do while I'm in one?

Remain no more than 5 minutes off the proposed ETA to your various waypoints
Remain no more than:

10 miles from the centerline of your route, if over land
20 miles from the centerline of your route, if over water


According to Aeronautical Information Circular 26/11 VFR Navigation Charts—Clarification of the Maximum Elevation Figure the

  The MEF is calculated by taking the higher value of:

  the top elevation of the highest obstacle plus the vertical accuracy (variable) of the terrain
  source data; or
  the elevation of the highest terrain plus 328 feet plus vertical accuracy of the terrain source
  data.

So the 2700' could come from 2290' plus 328' rounded up to next hundred feet.


Are you cleared to next altitude when you get "direct" while flying SID?

As an example, say you receive an IFR clearance from clearance delivery prior to departure such as: "N1234 cleared to Los Angeles Airport, ATOME TWO DEPARTURE, ARRIE Transition, Q-One Forty-five, climb via SID, Squawk 2252, etc." Then, after passing 2000 ft. if the tower controller instructs you to "proceed direct ZUMBI" then he/she (controller) should issue you an altitude assignment because you are being taken off of the lateral path of the SID.  In the alternative the controller could assign an altitude by using the phraseology (or similar) "N1234 proceed direct to ZUMBI, climb via the ATOME TWO departure." The "climb via" verbiage in this sequence is the equivalent of assigning an altitude.

After switching frequencies to the radar controller he/she should instruct you (for example) "after ZUMBI climb via SID" (assuming that ATC wants you to meet the KIMSH restriction and climb to the published Top Altitude of 15000 ft).

Here is some information from the ATC Controller's Handbook 7110.65W, para. 2-6-4, g. 2 and the associated Note: (Air Traffic Controller's handbook, 7110.65W)

(Although the reference paragraph and note, as it is published in the controller's handbook, is an example when a deviation around weather is necessary, the principle underlying my explanation is the same (there is just not an example similar to the circumstances you describe in your question).

  When approving a weather deviation for an
  aircraft that had previously been issued a crossing
  altitude, including Climb Via or Descend Via
  clearances, issue an altitude to maintain along with
  the clearance to deviate. If you intend on clearing the
  aircraft to resume the procedure, advise the pilot.

  NOTE−

  After a Climb Via or Descend Via clearance has been
  issued, a vector/deviation off of a SID/STAR cancels the
  altitude restrictions on the procedure. The aircraft’s Flight
  Management System (FMS) may be unable to process
  crossing altitude restrictions once the aircraft leaves the
  SID/STAR lateral path. Without an assigned altitude, the
  aircraft’s FMS may revert to leveling off at the altitude set
  by the pilot, which may be the SID/STAR’s published top or
  bottom altitude.

Also, from the FAA's Pilot/Controllers Procedures & Systems Integration Workgroup FAQs regarding "Climb Via" procedures, here is an applicable Question and Answer (FAQs Climb Via):

  Q.
  I’m on a climb via clearance and the controller vectors me off the procedure.  Do I continue to “Climb Via?”

  A.  NO.  You are laterally and vertically off of the
  procedure.  The controller will issue an altitude to
  maintain and provide a further “expect” clearance.
  Note: Request an altitude to maintain if the
  controller does not provide one

The main point is that a clearance to fly a SID that contains a Top Altitude and verbiage that includes "Climb Via" assumes that you will fly the lateral route as published, thereby committing you to meet the altitude restrictions.  If you are taken off of the published lateral routing ("cleared direct" to a fix somewhere down-range on the SID) the controller needs to issue you an altitude assignment since you (while proceeding direct to ZUMBI) are no longer on the published SID lateral route. In the alternative the controller could assign an altitude by using the phraseology (or similar) "N1234 proceed direct to ZUMBI, climb via the ATOME TWO departure." The "climb via" verbiage in this sequence is the equivalent of assigning an altitude.

From the NBAA Access Committee briefing regarding "Climb Via" procedures:
(NBAA Climb Via Briefing) showing ATC Phraseology when being cleared direct to a fix on the SID.

Pictorial display from the NBAA briefing (see link above) of pilot/controller responsibilities and phraseology when being cleared direct to a (down-range) fix on the SID after departure:

If the controller did not issue me an altitude instruction ("maintain," "climb via," etc.) upon taking me off of the SID I would have asked for one.

Here is a link to the ATC Controller's Handbook (7110.65W) so you can review the reference noted above. Also, here is a link to a FAQ's publication regarding Climb Via Climb Via FAQs


Like transponders, can flight recorders be turned off?

There is no switch for the FDR, but there is a circuit breaker and by pulling this you are removing aircraft power from the FDR.  However, this does not mean you are necessarily turning it off, as some FDR can be equipped with an internal power supply as a backup to aircraft power.

The quote in the question (bolded emphasis mine):

  You also cannot turn off the black box, as it runs throughout the flight, recording every 30 to 60 seconds.

is a case of an "expert" or journalist confusing the lack of an on/off switch as an inability to turn something off.  As you'll find many examples of in the media, they don't always get it right.  The circuit breaker may not be a true on/off switch, but pulling it does accomplish de-powering (assuming no internal power supply) and thus turning off the FDR.


Is a license required to transmit in the designated aviation band?

The FCC does not presently require a license for Aviation Radio installations in aircraft, where those are VHF and UHF (as I recall).  These are covered in Part 87 of the FCC regulations.

Travel outside the US requires a station license, and I believe HF transmissions require a license.

Similarly the operator (pilot) needs a permit when traveling outside the US, but does not need the permit within the US, to answer your question about the operator.


The aero-dynamics of a flight in reduced gravity are different than a flight operating in normal conditions. When an aircraft is experiencing zero gravity, it does not stall because the wings are not generating any lift, because the aircraft is in a state of weightlessness.

In the paper for the requirements for zero gravity flights, it is mentioned (PDF):

  ... zero G flights stalling conditions are different as compared to
  conventional flight maneuvers, because the plane is weightless and is,
  therefore, in no need of aerodynamical support. On the other hand, a
  minimum speed is required to execute the delicate control through the
  parabola. For this reason the stalling speed is taken as the minimum
  aircraft velocity required for effective control.


On cars I think it's because static charges don't build up as much but theoretically it's still possible for a spark to occur when the nozzle touches the tank inlet and provides a ground path.

On airplanes there are two issues:

Static charges built up in the airframe in flight.  For this it is essential to ground the aircraft before the fuel nozzle is brought to the tank inlet.  The risk is of a spark just before the nozzle touches the filler neck, the nozzle itself being grounded, which is right at the location where the air fuel ratio is favourable to ignition. It's also a good idea to touch the nozzle to the fuel cap before you remove it in case the tank isn't adequately bonded to the rest of the airframe, but I don't think anybody does this.

The other one is static charges built up within the fuel tank by the shearing action of the incoming fuel column when adding fuel.  In a large refueling operation, the column of fuel builds huge static charges within the mass of fuel itself, which result in static discharges within the fuel (there are films that show this; a freakin' light show).

Most dangerous is fueling from a plastic can.  When I used to fuel my airplane from cans I made a grounding wire arrangement that bonded the fuel in the can, the fuel tank flange and the ground (using a nail on the end of the wire pushed into the dirt) together.


What is the best design for a small flying wing with special flight envelope?

I know, flying wings are cool. But once you have built and flown one, you get a more nuanced view. To find the right configuration, start with a look at your objectives. You want:

excellent performance and
an observation platform with good stability.

Now let's see what a flying wing will do for you. Apologies for the heavy use of technical terms; please follow the links for more verbose explanations:

It will have little pitch damping, so your airframe will wobble through the air with every gust or control command. You will need wing sweep to get at least a little aerodynamic pitch damping.
Depending on wing sweep, wing spar stiffness and pitch inertia, the frequencies of the wing bending oscillations and the fast period mode might converge within the desired speed range, resulting in flutter.
With a span of 2m and a root chord of 20cm the aspect ratio is above 10. If you sweep the wing, your low speed characteristics become outright nasty. I know, you will use engine power for take-off and landing, but once you fly a tight circle, you will bring the wing close to stall even at higher speed, and then a high aspect ratio swept wing will start to roll on its own and pitch up. In other words: It will become uncontrollable.
A reflex airfoil only produces lift in its forward part. The rear part produces a little downforce at high speed and proportionally much less lift than the forward part at low speed, so the whole wing will become less efficient. It will be better to reduce wing area and to use an airfoil with an excellent ratio of lift to drag.

My advice: Put part of the saved area at the end of the fuselage as a horizontal tail surface. This will give you ample pitch damping (goes up with the square of the tail surface's lever arm) and enables you to use a high-performance airfoil on a very small wing which produces much less drag than a flying wing of the same minimum speed. If you want good high-speed performance it is important to keep the wetted area at a minimum to minimize friction drag.

Now that you have expanded the question and added portability, let me propose something that would combine all three characteristics: The Leading Systems Amber. It was designed to fold to the size of a torpedo to be carried onboard Navy ships by pivoting the wing by 90° and folding the two tail surfaces forward. Combine this with a folding propeller and hand-launch it to avoid conventional take-offs.

Leading Systems Amber in flight (picture source)

Landing was done by deep-stalling the aircraft. To quote from an article about Abe Karem in Air&Space magazine:

  Most ingenious of all, Amber could be recovered by putting it into a
  “deep stall”—a feature used in free flight glider models to escape
  thermal updrafts and stay within flight time limits. The stall would
  bring Amber to “a near-vertical landing so it could be used from small
  ships, submarines or trucks/trailers,”

At least, this is what I would do: Build a scale model of Amber.


How effective are modern aircraft at clearing smoke?

The most relevant data I could find was located here, in a study on "In-flight cabin smoke control". In the abstract, they stated that

  Tests on a Boeing 757, modified to allow smoke venting out through the top of the cabin, showed that the buoyant smoke front moved at 0.46m/s (1.5ft/sec) with and 0.27m/sec (0.9ft/sec) against, the axial ventilation airflow. Flight tests in a modified Boeing 727 showed that a ceiling level counterflow of about 0.55m/sec (1.8ft/sec) was required to arrest the forward movement of buoyant smoke. A design goal of 0.61m/s (2ft/sec) axial cabin flow would require a flow rate of 99m3/min (3500ft^3/min) in a furnished Boeing 757. The current maximum fresh air cabin ventilation flow is 78m^3/min (2756 ft^3/min).

In short, during continuous generation of smoke, cabin air will not be recirculated quickly enough to stop smoke from filling the cabin; however, if the smoke production is temporary, then it should dissipate quickly (time of course depends on the size of the cabin and the amount of smoke produced).

On page 2 of this article, a discussion of airflow occurs and contained this image, showing circulation patterns on wide-body airliners. A linked study on that same page noted that an under-floor displacement air distribution system [like the commonly used type picture below] can provide a much better air quality than a mixing air distribution system. The paper goes on to provide more diagrams of airflow and comparisons of various effectiveness if you're interested.

Boeing has a fairly detailed summary of their cabin airflow here. It says:

  Inside the cabin, the air flows in a generally circular patterns and exits through floor grilles on either side of the cabin and on airplanes with overhead recirculation, the air may exit through the overhead. The exiting air goes below the cabin floor into the lower lobe of the fuselage.

So at least on most Boeing planes, air exits at your feet, though it can exit at the ceiling. Airbus also has a page here, but with much less detail. Airbus claims a complete recirculation every 2 to 3 minutes, while Boeing merely states airflow is continuous. The article linked below says in general, airplane recirculation occurs aproximately 20 circulations per hour.

An NBC article on cabin air states that to pick up a sickness, you need to be close, usually within two rows. The article goes on to say that:

  The aircraft is divided into ventilation segments of three to seven rows; you share air only with passengers in your segment.

So presumably if there were smoke, it would be captured in a small area were it not too pervasive.


Why do most military aircraft use different engines than commercial aircraft?

When military and civilian aircraft of similar speeds are compared, their engines differ mostly by their names. It is simply too expensive to develop a civilian type in parallel to a military type when later both are used in the same way. Examples:

The B-52 engine is called J57 and was made by Pratt & Whitney. The same company called this design JT-3C when it was mounted to the Boeing 707 and the DC-8. Later models used a turbofan development of the J57, the Pratt & Whitney TF33, which was called JT3D in civilian use.
The C-5A Galaxy pioneered large bypass ratio turbofans with the General Electric TF39, which was used on the Boeing 747, DC-10 and A300B as the CF6 later.
Even the same aircraft will sometimes use a different name for basically the same engine, depending on the use: The T-39 Sabreliner used the J60 engine while the civilian Sabreliner called the same engine JT12.

But even when the flight regime is different, component reuse is common between civilian and military engines:

The core of the General Electric F101 (installed in the B-1A bomber, the supersonic first version of the B-1) became the CFM-56 turbofan which is used in the Boeing 737 or the Airbus A320.

Supersonic flight requires a high nozzle velocity, therefore high bypass ratios are unsuitable. However, the differences between sub- and supersonic engines disappear the closer you move towards the gas generator core. High-pressure compressor, combustion chamber and high pressure turbine look and work the same. Civilian engines add a large fan in front while supersonic military engines sometimes add an afterburner. The biggest difference, however, is in the intakes (large pitot intake with blunt lips for subsonic aircraft versus adjustable spike or ramp intakes for supersonic flight) and the nozzle (fixed for subsonic flight versus a complex, adjustable convergent-divergent nozzle for supersonic flight).


What do the terms 'holding out' and 'common carriage' mean?

Oddly enough, "common carriage" is not defined in the FAR's, but that is because it is a "common law term" and not specific to aviation.

AC120-12A - Private Carriage Versus Common Carriage of Persons or Property contains guidance that can be used to help determine whether or not an operation falls under common carriage.  Take a look at it for specific examples, but the "plain language" version is:

Common Carriage: A carrier becomes a common carrier when it "holds itself out" to the public, or to a segment of the public.

Holding Out:  A carrier is holding out when they represent themselves as willing to furnish transportation within the limits of its facilities to any person who wants it.

There are lots of ways to "hold out", but the basic idea is if someone in the general public comes to you and asks you to fly them and you agree, you are probably holding out.  Any form of advertising demonstrates a clear sign that you are holding out.  There are also other activities that qualify, so look over the AC if you have further questions, and if that doesn't answer it your best bet is to contact an aviation lawyer.  You could also contact your local FSDO with questions.

As far as transporting a third part for hire, both common and private carriage require an air carrier certificate issued under 125 or 135 depending on the size of the airplane1. This means that your trip for a friend would be in violation because you don't have the required air carrier operating certificate.  (Thank you @p1l0t for clarifying this for me!)

119.23 says:

  Operators engaged in passenger-carrying operations, cargo operations,
  or both with airplanes when common carriage is not involved.

  (a) Each person who conducts operations when common carriage is not
  involved with airplanes having a passenger-seat configuration of 20
  seats or more, excluding each crewmember seat, or a payload capacity
  of 6,000 pounds or more, shall, unless deviation authority is issued--

  (1) Comply with the certification and operations specifications
  requirements of part 125 of this chapter;

  (2) Conduct its operations with those airplanes in accordance with the
  requirements of part 125 of this chapter; and

  (3) Be issued operations specifications in accordance with those
  requirements.

  (b) Each person who conducts noncommon carriage (except as provided in
  Sec. 91.501 (b) of this chapter) or private carriage operations for
  compensation or hire with airplanes having a passenger-seat
  configuration of less than 20 seats, excluding each crewmember seat,
  and a payload capacity of less than 6,000 pounds shall--

  (1) Comply with the certification and operations specifications
  requirements in subpart C of this part;

  (2) Conduct those operations in accordance with the requirements of
  part 135 of this chapter, except for those requirements applicable
  only to commuter operations; and

  (3) Be issued operations specifications in accordance with those
  requirements.

The FAA takes a very dim view of people who start competing with actual air carriers and are unlicensed to do so.  It's a quick way to have your license suspended and end up with some hefty fines and other legal troubles.

1 There are of course the regulations that allow operations "in furtherance of a business" which is why I specified a third party.


How do large airport refuelling systems work?

To complement the other answers with pictures:

(Source)

Both systems shown above work well for big airports. The system depicted at the bottom is called a Hydrant Fueling System.

(Source)


What are the legal requirements to be a "crewmember"?

I'll take a stab at this one, though I can't guarantee the correctness of my interpretation (the regs don't seem exactly clear on the subject - at least not to me, so there's a lot more "inference" here than I'd normally be comfortable with).

In most cases under Part 91 the only requirement to be a "crewmember" is that the pilot in command says you're a crewmember -- i.e. the PIC determines that a person is qualified to perform a specific duty and designates them to perform it.
In the absence of anything more specific this is the only requirement I can turn up (and it's an inferred requirement - I didn't find anything specifically stating this, but 91.533 regarding flight attendants strongly implies that it's the correct interpretation. There's also a counsel opinion from 2008 - the Schehr letter which talks a little about who is a "crewmember" for purposes of riding in jumpseats that are placarded "For Crew Use Only". That interpretation is based on Part 135, but could be logically extended to Part 91 (and possibly some 121) operations too).

There are frequently more specific requirements though, particularly for positions that are important to flight operations. For example if you buy yourself a nice shiny Lockheed Electra to duplicate Amelia Earhart's trip under Part 91 rules you still cannot simply designate someone off the street to be the Flight Navigator: the FAA has specific requirements for Flight Navigator certification, and someone acting as the flight navigator would need to meet those requirements.

There are also more stringent requirements that are imposed for Part 121 and 135 operators. For example they can't simply designate a person as a flight attendant based on the PIC's determination that they are qualified: There are requirements for filling that crew position which must be met (FAR 121.421 & FAR 135.295 respectively, and possibly other regulations).


How do you compare private jet and commercial jet efficiency?

This has nothing to do with private vs. commercial. The size difference is what matters.

If you compare the absolute cost of moving the aircraft from NYC to LA, the 737 will be way more expensive, regardless of version. Since you asked only which one is more expensive, I don't see a need for detailed reasoning. Just use the data provided in this answer and it becomes obvious that fuel consumption will scale with the mass of the aircraft.

If all passengers have to share the cost, the cost per passenger will be much higher in case of the Phenom 300. Or almost any other Bizjet. The Phenom will even need to land somewhere enroute to refuel. The distance from NYC to LA is bigger than the maximum range with full payload.

But that kind of comparison does not make sense - both airplanes have been designed for different purposes, so their strengths are in their unique field of use. They do not transfer well to that of different aircraft types.


Does EASA allow using a portable GPS for RNAV capability?

The answer, broadly is no, handheld GPS units can not be used for IFR RNAV routes (or approaches for that matter). To know if a specific GPS unit is legal for IFR navigation you need to check up on a variety of paperwork, thats covered in this answer. Strictly speaking if you could find a certified unit it would be legal but there are no current units (to my knowledge or searching) that are.

If we look at the regulations and bounce around a bit we can see why. AC 90-108 tells us about the use of RNAV systems and states that one bust be compliant with installation instructions in AC 10-138D of which there are many different regulations. The AC does not actually specifically state that the unit must be panel mounted but in a round about way it does:

The definition of display which is used widely in the document does state it must be mounted to the panel,

  Display. Within the context of this AC, a display is a device mounted
  in the aircraft instrument panel to convey course guidance navigation
  information to the aircrew for controlling the aircraft along the
  intended route. Displays may also be used to convey other information
  as well.

There are also lots of regulations about wire mounting to avoid issues and failures as well as other mounting regs. If you are interested its worth going through the document.

See Pondlife's comment bellow for more info on panel mounted GPS'

  just normal en-route navigation over fixes defined by coordinates
  only. That way, the GPS is not critical—if it fails, I can always
  request vectors or rerouting over a navaid.

In order for the GPS to be non critical you must fly at altitudes serviced by other nav-aids. The reason GPS routes are coming into use is that they, in some cases, allow flying at lower altitudes that may be beneath ATC radar or out of range of nav-aid coverage. In the case of a failure of a GPS unit you would then be with no means of navigation and terrain avoidance.


The first one is the holding position marking. It denotes the entrance from the taxiway into the runway. The dashed lines are in the side of the runway.

The second one is the non movement area boundary mainly for vehicles, which divides the movement and non-movement areas of boundary- the movement area is on the dashed line, for moving into which you'll need ATC clearance.

Images from FAA Guide to Ground Vehicle Operations


Can a civilian purchase a new (Not Retired) fighter jet?

Certain jets, yes.

But not the F22.

See current listings:

https://www.controller.com/listings/aircraft/for-sale/list/category/10072/turbine-military-aircraft


What FAA rules govern tethered, outdoor UAV flight?

The FAA's rulemaking document for UAVs says that tethered UAVs are not considered balloons or kites (thanks to Steve V. for finding it):

  [...] a small unmanned aircraft that uses powered systems
  for actions such as propulsion or steering is not a balloon or kite
  subject to part 101

And:

  [...] the FAA notes that the definition of small UAS in this rule includes
  tethered powered small UAS

So a UAV is not a kite and it is subject to all the general UAV regulations (I think they'll be published as 14 CFR 107 at some point). There are also a couple of FAA sources that say that if a tethered aircraft of any type is capable of free flight then all the usual regulations apply. This is from the interpretation you quoted:

  Since 1957, the FAA has repeatedly affirmed its position that any
  vehicle designed for tethered operations only, and not for "free
  flight" should be considered a kite.In contrast, if an ultralight vehicle is designed to be tethered only
  until it achieves a predetermined altitude and is released for free
  flight, that ultralight is an aircraft regulated under part 103
  during the entire operation.

There's also a very clear statement on tethered balloons:

  Free and Tethered Flight Considerations. If a balloon is considered an aircraft under one circumstance (such as free flight),
  it must still be considered an aircraft under other operating
  circumstances, including tethered flight. Tethered balloons are
  considered aircraft, and must be operated in compliance with all
  construction, certification, airworthiness, registration, and
  operating regulations applicable to aircraft.

That comment about "operating circumstances" seems to be the general FAA position on tethered aircraft.


Short answer

If a single range should be stated for ACAS II, it would be the reliable range of 14 NM, even if targets can displayed by current TCAS II up to 30/40 NM.

Reliable means here: a track can be established with a probability of at least 90 per cent for aircraft within the surveillance range. The reliable range of 14 NM is achievable with a ground plane antenna, a power of 250 W and a Mode S sensitivity of –74 dBm.

The mean number of aircraft in a spherical volume is proportional to the cubed radius. Each aircraft tracked by a ACAS system must be interrogated, and its position/altitude data stored. This creates stress both on the ACAS equipment and on the radio spectrum used for ATC (SSR and ADS-B in addition of ACAS). The current balance between stress and benefit is a range of 14NM which can be reduced to 4.5 NM if the number of aircraft or the number of ground interrogators prevent managing the nominal range. The spectrum stress should be relieved with the forthcoming ACAS-X.

In addition, while the nominal monitoring range is 14 NM and the guaranteed range is 4.5 NM, RA/TA are issued based on estimated time before separation failure (more accurately before the closest point of approach):

From Eurocontrol ACAS guide

(In the answer, ACAS is related the principle, TCAS is the technical implementation of the principle.)

Diversity of ranges

There are different range values that can be considered:

Overall pulse detection range is 30 NM.
Reliable surveillance range is 14 NM.
Recommended RA range is 12 NM.
Guaranteed range is 4.5 NM.

The range must be limited because it impacts the number of transponder addresses to be stored for a given transponder density as well as the number of interrogations within a volume, hence radio spectrum use and reliability.

ACAS II recommendations by ICAO

ACAS II ICAO recommendations are found in Doc 9863, Airborne Collision Avoidance System (ACAS) Manual:

Interrogation pulse detection range is 14 NM for Mode A/C and 30 NM for Mode S.
The equipment should have the capacity for surveillance of any mix of Mode A/C or Mode S targets  up  to  at  least  30  aircraft.
Surveillance must be reliable within 14 NM, with an aircraft density of 0.3 aircraft per square NM within 5 NM (and up to 142 aircraft within 30 NM).
The equipment should assess as possible collision threats only those targets within a maximum range of 12 NM. No target outside this range should be eligible to generate an RA.
As density increases, reliability decreases.
If the overall target count ever exceeds the surveillance capacity at any range up to 14 NM, the long-range targets may be dropped.
The system reliability is guaranteed within the minimum range of 4.5 NM. This radius is deemed adequate for providing protection for up to 500 kt encounter.

TCAS version 7.1

TCAS II is the sole implementation of ACAS II which fully comply with ACAS II. The reference (paying) documents are: DO-185B (RTCA) and ED-143 (EUROCAE).

From the EASA document ACAS Guide

  TCAS II can simultaneously track up to 30 aircraft, within a nominal
  range of 14 NM for Mode A/C targets and 30 NM for Mode S targets. In
  implementations that allow for the use of the Mode S extended
  squitter, the normal surveillance range may be increased beyond the
  nominal 14 NM. However, this information is not used for collision
  avoidance purposes.

This is confirmed in FAA document Introduction to TCAS II V7.1

  TCAS has a requirement to provide reliable surveillance out to a range
  of 14 nmi and in traffic densities of up to 0.3 aircraft per square
  nautical mile. [...] TCAS can simultaneously track up to 30
  transponder-equipped aircraft within a nominal range of 30 nmi.


How can I compute the minimum possible bank angle for a turn with a given radius?

Given a radius $R$ and a velocity $V$, the bank angle can be computed by:

$\phi = \textrm{atan}\left(\frac{V^2}{R\cdot g }\right)$,

where $g$ is the acceleration by gravity. This shows that the lower the speed becomes, the lower the bank angle becomes. The lowest attainable speed is the stall speed, however we have to take into account the load factor during the turn.

The load factor $n$ during the turn is:

$n=\frac{1}{\cos \phi}$

Due to the increased load factor, the stall speed increases in the turn:

$V_{stall,turn} = V_{stall} \cdot \sqrt n $

The minimum bank angle for a turn with radius R will occur when the speed is equal to the stall speed for that angle.

Solving this mathematically for $\phi$ results in:

$\phi_{min} = \textrm{atan}\left(\frac{V_{stall}^2}{\sqrt{R^2g^2-V_{stall}^4}}\right)$

Note that is the theoretical approach; in practice you would not fly at the stall speed, especially not during turns.

Units are all in SI, you have to apply conversion factors if you want to use aviation units.

Example:

$V_{stall}$ = 50 m/s (approx. 97 knots)
$R$ = 400 m
$\phi_{min}$ = ~39.6 degrees
$V$ = ~57 m/s (approx. 111 knots)


Does navigation database provided by the database providers like Aeronavdata or Jeppesen consists of company routes?

The company routes are maintained by your company.  Some FMS installations have a computer program where you can create the company routes and upload them to each FMS.  Other FMS requires each FMS to be programmed.
All FMS have the ability to see if the routes have been changed due to a database change.   If the pilots wanted to use those company routes the FMS would require them to verify the routes are stored correctly.


Why is there a difference between aileron upwards and downwards deflection angles?

You're correct that the ailerons are deflected differently when they are deflected up or down. This is done in order to counter the adverse yaw effect which occurs when ailerons are deployed. Consider the situation when the ailerons are deployed.

Image from aerospaceweb.org

One aileron is deflected downward while the other is deflected upward. One the side with the downward-deflected aileron, lift increases as the deflection effectively increases the camber of that portion of the wing. The opposite happens in the other side.

However, the drag is also affected by aileron deflection- both induced and profile drags. For same deflection, the profile drag increase is same in both the wings. However, the induced drag on both sides are not equal, with a larger amount the wing with the down aileron (as the lift is more and induced drag is proportional to the square of lift).

One way of overcoming this is to deflect the ailerons differentially, i.e. deflect the down aileron by a lesser amount than the up aileron. The following table shows the aileron deflection in a number of aircraft, which shows the differential aileron deflection ($\delta_{A}$ is the aileron deflection, in degrees).

Table from Aircraft Design: A Systems Engineering Approach by Mohammad Sadraey

This adverse yaw can also be prevented by the use of,

Frise ailerons
Use of spoilers (in the wing up aileron)
Cross coupled controls (rudder and ailerons).

There is another reason to use a differential aileron- to prevent tip stall at high angles of attack (stall speeds). The rolling couple on the aircraft is always the difference in lift between the two wings. At or near stalling speeds, it is better to reduce the lift in up-going aileron than increase it in down moving one and risk stalling it.


In Antonov An-225, why there are radioactive / nuclear signs?

"Radiation" is a broad term and does not always imply radio active isotope material. High power radar units can have adverse effects on the human body.

  The power that radar systems emit varies from a few milliwatts (police
  traffic control radar) to many kilowatts (large space tracking
  radars). However, a number of factors significantly reduce human
  exposure to RF generated by radar systems, often by a factor of at
  least 100:

  Radar systems send electromagnetic waves in pulses and not continuously. This makes the average power emitted much lower than the
  peak pulse power.
  Radars are directional and the RF energy they generate is contained in beams that are very narrow and resemble the beam of a spotlight. RF
  levels away from the main beam fall off rapidly. In most cases, these
  levels are thousands of times lower than in the main beam.
  Many radars have antennas which are continuously rotating or varying their elevation by a nodding motion, thus constantly changing the
  direction of the beam.
  Areas, where dangerous human exposure may occur are normally inaccessible to unauthorized personnel.

The warnings are likely due to the emissions of the radar units in the dome.


Why are airplanes described as the safest method of transportation?

Good news

The good news is: you really are safe in an airliner, so safe that you could spend every minute of every day of the the rest of your life flying in one, and still not face any greater meaningful risk of a disaster.

It's difficult to answer your question why it's so safe (i.e. what makes it safe), but the record is there, plainly, that it is so safe.

Bad news

The bad news is: if one has an irrational fear of flying, then no objective facts or understanding will help lessen it.

If the fear is based on misunderstanding or lack of knowledge, then of course learning more will help overcome it. However, that's not the basis of a phobia, or the kind of fear you seem to be describing.

Advice to understand flight and aircraft is well-meaning, but not actually that useful. Someone can have an excellent understanding of aviation
engineering and aerodynamics and all manner of subjects, and know that it is exceptionally safe, and still suffer from crippling fear of actually flying.

Everyone is different, but such fear is often brought on by an event or change in life that has nothing whatsoever to do with flying, but somehow gets expressed in a fear of flying.

The comfort and support of another person on board could be the most valuable thing you seek before your flight. I'd suggest expressing your fear, without embarrassment or shame, to the staff at the gate. They and the crew on board will have helped people in such cases many times before, and they are good at it.

Simply getting used to an object of fear is one of the best ways of dealing with it.

I hope you make your flight, and I hope to hear that having done it once, the fear diminished by being stared down, and that subsequent flights render it lesser still. Please tell us about it here.


Why do most aerobatic aircraft have conventional landing gear?

The tailwheel is much lighter and smaller than a nose wheel. That means reducing airplane overall weight and drag (because the landing gear is usually fixed). And it's probably cheaper, too.


Why is a centrifugal compressor in aircraft engine less susceptible to stall and surge?

The reason comes down to the principle of operation of the compressors - a centrifugal compressor, as it's name implies, just spins around really fast, and the centrifugal force compresses the air. As you can see in the image below, the vanes of the impeller of a centrifugal compressor just help push the air around in a circle (unlike an airfoil) and centrifugal force does the compressing:
The centrifugal compressor is circled in blue. As you can see, when the impeller rotates, it pushes air to the side.

Image source - https://upload.wikimedia.org/wikipedia/commons/6/6f/Saturn_MD-120_cutaway.jpg

An axial compressor has blades that work like little rotors (which are really just rotating wings) - the rotors accelerate the air, and the stators slow the air down again, using that kinetic energy to compress the gas. Just like wings, these blades can stall and lose function whenever the input flow is disturbed enough (such that the local angle of attack of a portion of the blade is too high for the given local flow conditions). This stall can then potentially spread to other blades, or to the entire engine.

As you said, the vanes of an impeller in a centrifugal compressor don't work like airfoils (instead, they kinda just rotate and push air around, kinda like a turnstile), so if they encounter airflow disturbances, they won't stall like an airfoil would. Of course, if said airflow disturbance is something like severely restricting airflow, the compressor will surge, but this is considered rare in practice.

Therefore, it's much easier to get an axial compressor to surge or stall (just by disturbing the airflow a lot) in comparison to a centrifugal compressor, which requires airflow to be restricted.


How is smoke detected in the cabin when flying without passengers?

The threat for a cabin compartment smoke or fire comes from paying passengers & baggage.
On a ferry flight, you have neither, so the threat is greatly reduced or eliminated.
The FAA authorizes ferry permits for only 1 or a few  segments to get an aircraft to a facility where maintenance can be performed, these ferry permits take into account the risks involved and thus why no revenue cargo / passengers can be carried.


What are the differences between third-party base maintenance, line maintenance and component maintainance?

Base maintenance: The maintenance work is done in a hangar, with special tools and equipment. Third-party just means that someone else than the operator of the aircraft is running the shop.

Line maintenance: The work is done on the flight line, basically on the apron out in the open.

Component maintenance: The work is done on some component that had been removed from the aircraft before. Typically, this item had been swapped against a good one in line maintenance and is now looked at in detail in a specialist shop. Items like this are called LRU (line-replaceable unit). Components could also be as large as a whole engine which needs to be serviced in a special shop and is removed during base maintenance.