Is there any difference between Airbus and Boeing on yaw damper technology?

The difference is more between conventional flight controls vs. fly-by-wire rather than Airbus vs. Boeing.
Both Airbus and Boeing used separate yaw dampers in their non fly-by-wire aircraft, which applied their commands with separate actuators to the rudder control servos, without feedback to the mechanically linked rudder pedals in the flight deck. The exact details will differ for every aircraft of course, but the general design is quite similar. Consider e.g. the Airbus A310 and the Boeing 747:
Airbus A310:

The rudder is controlled by 3 actuators, which are commanded by a
single cable run from the rudder pedals. Rudder artificial feel is
provided by a spring-loaded rod.
Additional inputs to the rudder come from:

the rudder trim, and
the 2 yaw dampers.

[...]
The yaw damper ensures the following functions:

dutch roll damping
turn coordination which becomes active if sufficient control wheel deflection is applied [...]
yaw compensation in case of engine failure, provided the AP is engaged in CMD with SRS (takeoff) or GO AROUND mode annunciated on
FMA.

Yaw damper command is transmitted to the rudder but there is no
deflection feedback sent to the rudder pedals.

(Airbus A310 FCOM - Flight Controls - Rudder Control)
Boeing 747-400:

Yaw Dampers
Two independent yaw damper systems operate continously in flight to
improve airplane directional stability and provide turn coordination.
The upper and lower yaw damper actuators are powered by hydraulic
systems 3 and 2 respectively. Yaw damper inputs do not result in
rudder pedal motion.

(Boeing 747-400 FCOMv2 9.20.9 - Flight Controls - System Description)

On the other hand, fly-by-wire aircraft usually integrate the yaw damper logic into the fly-by-wire commands sent to the actuators. Consider e.g. the Airbus A320 and the Boeing 777:
Airbus A320:

Electrical Rudder Control
The yaw damping and turn coordination functions are automatic. The
ELACs compute yaw orders for coordinating turns and damping yaw
oscillations, and transmit them to the FACs.
Rudder Actuation
Three independent hydraulic servojacks, operating in parallel, actuate
the rudder. In automatic operation (yaw damping, turn coordination) a
green servo actuator drives all three servojacks. A yellow servo
actuator remains synchronized and takes over if there is a failure.
There is no feedback to the rudder pedals from the yaw damping and
turn coordination functions.

(Airbus A320 FCOM - Flight Controls - Description)
Boeing 777:

The yaw control law contains the usual functionality employed on other Boeing jetliners, such as the yaw
damper and rudder ratio changer (which compensates a rudder command as a function of airspeed).
However, the 777 FBW rudder control system has no separate actuators, linkages, and wiring for these
functions, as have been used in previous airplane models. Rather, the command for these functions are
calculated in the PFCs and included as part of the normal rudder command to the main rudder actuators.
This reduces weight, complexity, maintenance, and spares required to be stocked.

(Boeing B-777: Fly-By-Wire Flight Controls 11.8.2 - Yaw Control)


Why is the gradient line for climb in a one-engine-inoperative condition during take off under CS-25 described in percentage?

Gradient as percentage is defined as the ratio between vertical displacement and horizontal displacement. For example, a 5% climb gradient means than for every 1 000 meter of horizontal travel, the aircraft climbs 50 meters. If you want to convert it to an angle you have to use the inverse tangent:
$\textrm{atan}(0.05) \approx 0.05 \ \text{rad} \approx 2.86 ^\circ $

In take-off configuration with the landing gear retracted, the steady gradient of climb for one-engine-inoperative (OEI) is not less than:

2.4 % for two-engined aeroplanes,
2.7 % for three-engined aeroplanes and
3.0 % for four-engined aeroplanes,

(source EASA CS-25, Amendment 20, CS 25.121)

The reason for the difference is that two-engined aircraft loose 50% of their thrust, while four-engined aircraft loose only 25% of their thrust in a OEI situation.

In other configurations there are similar differences.


Do autopilots have an 'activate' switch/button, in addition to 'on' and 'off'?

It is complicated (-;

First thing first, autopilot systems come with a variety of forms and complexity. The autopilot on a Cessna 172 or Piper Cherokee is going to be very different than the one on a Airbus or Boeing. Different manufacturer will also design their autopilot interface differently; while there are certainly similarities, there is no set standard in the aviation industry for autopilot interface and behavior.

  (...) is simply changing the altitude and heading numbers, or (...)
  there is some sort of button that commands the autopilot to execute
  the new commands?

Both.

Here I will discuss using the Boeing 777 autopilot, which I am familiar with. On the autopilot panel, there is a heading window, which displays a 3-digit number. Directly below the window is a knob, turning the knob counterclockwise will decrease the heading, while turning clockwise will increase it. The knob can also be pushed.

The heading section of the Model Control Panel on a Boeing 777. The knob is a 5-in-1 control: the outer ring can be rotated, the knob can be rotated, and the center of the knob is a push button.

If the autopilot is in "HDL SEL" mode, it will fly the heading specified in the window. In this mode, if the pilot rotates the knob to adjust the heading, the new heading will take effect immediately. However, if it is in "HDL HOLD" mode, turning the knob will adjust the number, but it will have no effect - the plane will hold the current heading. Pushing the knob will change the mode from "HDL HOLD" to "HDL SEL", and the plane will then turn towards the selected heading.

A similar logic exists when climbing using the "VNAV" mode. Let's say the plane is initially cleared to 5000 feet after takeoff. The pilot set the altitude window to 5000 prior to the takeoff roll. Once autopilot is engaged, it will climb to 5000 feet, after which it will hold that altitude. At any point, the altitude knob can be rotated to set a new altitude, say 7000 feet, however it will not take effect - the plane will still climb to 5000 feet, then stop the climb there. To climb to a higher altitude, the pilots have to push the knob, after which the plane will initiate a new climb from 5000 feet to 7000 feet. This is because ATC often clear planes to "step climb", i.e. climbing then holding at certain altitudes until reaching the cruise altitude. The autopilot function is designed around that.

  where is that switch (or button)?

It depends on the model. Other manufacturers call them differently, but the logic is similar. Also, advanced autopilots have a lot of functions; the ones I discussed are just selected examples about heading and altitude selections. Pilots can also "arm" an autopilot mode, i.e. mark it so that it will become engaged when certain conditions meet. The specifics of that are too long to describe in this post.

Like I said, it gets complicated.


Can a commercial pilot with MPL , obtain PPL for general aviation in the UAE?

Note: You did not specify a country. I will answer for EASA.

You don't need to obtain a PPL, you may use the MPL (Multi-Crew Pilot Licence) if you fulfill the requirements. From EASA Part-FCL (emphasis mine):

  FCL.405.A MPL — Privileges

  (a) The privileges of the holder of an MPL are to act as co-pilot in an aeroplane required to be operated
  with a co-pilot.

  (b) The holder of an MPL may obtain the extra privileges of:

  (1) the holder of a PPL(A), provided that the requirements for the PPL(A) specified in Subpart C are met;

  (2) a CPL(A), provided that the requirements specified in FCL.325.A are met.

  (c) The holder of an MPL shall have the privileges of his/her IR(A) limited to aeroplanes required to be
  operated with a co-pilot. The privileges of the IR(A) may be extended to single-pilot operations in
  aeroplanes, provided that the licence holder has completed the training necessary to act as PIC in
  single-pilot operations exercised solely by reference to instruments and passed the skill test of the
  IR(A) as a single-pilot.

You can find the requirements of Subpart C in the same PDF on page 145f.


What is the shiny stripe around the aft wing–body fairing on an A380?

I chanced upon the answer in a 2005 Flight Global article when I was looking into a hypothesis that the fairing is made of different materials.
The stripe is a flexible joint:

Although the fairing is not theoretically designed to carry fuselage loads, Airbus has built the fairing to absorb the inevitable bending loads that will be transferred from the primary structure of the fuselage to the skin fairing through the belly substructure. The metallic strip visible in the aft section of the fairing on MSN001 is a flexible joint designed to help further absorption of these loads.

My conclusion is that if painted, the paint would crack with flexing, which isn't a good look. Knowing that it's a joint, finding the relevant 2005 Airbus patent was much easier:

An object of the invention is to provide a fairing capable of absorbing compression and bending forces due to the movements of the aircraft and transmitted to the fairing, regardless of the size of said fairing. More specifically, the object of the invention is therefore to reduce the frontal loads received by the fairing. A further object of the invention is to reduce a total weight of an aircraft equipped with a fairing.


Does having permission waive the 500 foot distance requirement in 91.119(c)?

See 91.905, List of rules subject to waivers:

91.119   Minimum safe altitudes: General.

In other words, 91.905 allows the FAA to issue a waiver to 91.119, e.g. for airshows. The form for requesting a waiver is here, and the FAA can set more or less whatever conditions they like for granting one.


Is there a way to calculate thrust without running experiments?

For the case of an electric motor driving a propeller, you can get a rough approximation for the thrust t from the power p consumed by the motor when the plane is flying with a constant velocity v, since $p = t \times v$. Of course, you should use coherent units, as newtons, m/s and watts, and include a large correction factor, as the real efficiency will be very far from 100%...

For the stationary case, in Wikipedia (entry 'disk loading') you can find an expression for the stationary case, derived from momentum considerations.


Why was F-18 chosen to escort Space Shuttle Endeavour?

Because it's what they had at the time and it's what the astronauts were flying. Only one of the NASA F/A18's is actually a two seater:

  The aircraft were obtained from the U.S. Navy between 1984 and 1991.
  One has a two-seat cockpit while the others are single-seat aircraft.
  NASA research support aircraft are commonly called chase planes and
  fill the role of escort aircraft during research missions.

Their main purpose is to keep an eye out for external issues with the carrier. This podcast covers most of the nitty gritty details of that operation.

  Chase pilots are in constant radio contact with research pilots and
  serve as an "extra set of eyes" to help maintain total flight safety
  during specific tests and maneuvers. They monitor certain events for
  the research pilot and are an important safety feature on all research
  missions.

As for why they have them:

NASA has a long history of using high performance jet trainers to keep the astronauts sharp. Since many of the older generation of shuttle pilots came out of the Navy or were test pilots, choosing former Navy aircraft is only logical. For much of the Apollo era as well as the shuttle era they actually used T-38's for training and keeping the astronauts current. NASA likes high performance, safe, simple jets for astronaut training as there are things you encounter actually flying that you just don't in the simulator.

  "It's actually our most important training that we do as astronauts,"
  said Terry Virts, who flew as the pilot of STS-130 aboard shuttle
  Endeavour. "It’s the one place where we're not in a simulator. It's
  real flying and if you make a mistake, you can get hurt or break
  something or run out of gas. There are a lot of things that happen
  real-world in a T-38 that don't happen in the simulator."

The actual shuttle trainer is a GII that flies with the gear down, full flaps, and thrust reversers to simulate the descent of the shuttle.


Why does the DHC-6 have only one set of elevator cables?

The Twin Otter isn't certified in a category that requires duplicate control runs with split surfaces.  It's more or less a single engine Otter with two PT-6s and a redesigned tail.  Those requirements apply to heavier aircraft than the DHC-6.


What does the area around the fan blades in the engine nacelle contain?

Yes a bit of the engine called the nacelle is missing. This is an aerodynamic structure that slows the air down before it enters the compressor, the pointy bit in your photo. In the drawing underneath it states Inlet Cowl. It has some thickness, because it requires a certain nose shape for its function. Mainly a hollow casing indeed, but with a high tech shape.


the Chord is the line stretching from the leading edge of the wing to the trailing edge parallel to the centerline.

There are different variations to describe the chord of the whole wing.

The Standard Mean Chord
Defined as $\frac S b$ with $S$ the surface area and $b$ the span of the wing. This is not used in aerodynamics.
The Mean Aerodynamic Chord
Defined as $\frac2 S \int_0^{\frac b 2} c(y)^2 dy$ with $y$ the coordinate along the wing and $c(y)$ the chord at $y$.
For swept or delta wings this gives the equivalent chord for a rectangular wing, and is important for placing the Center of Gravity for stability.


Why is Iceland an efficient connection point between London and Boston/NYC?

One of the benefits of having a stop halfway is that the aircraft can refuel.

It therefor doesn't have to carry the fuel for the second half of the flight during the first half, which saves fuel.

The uplift of fuel costs approximately 4% of its weight per hour of flight. That means the fuel you consume in the 4th hour of flight has burned approximately 11.5% of its original weight, just to carry itself. The fuel burned in the 7th hour of flight has burned 25% of its original weight , just to be carried for the previous 6 hours. (which means you have to load 133% of the fuel used in the 7th hour)

So a 10% detour in distance, with an extra stop, does not increase the total fuel consumption much. It may even reduce the total fuel consumption of the flight.

And it also means that the aircraft is lighter, so it is more efficient to use a smaller aircraft. Typically you see mostly Boeing 777, 747, 767 and Airbus A380 and A330 across the Atlantic, but if you look at the traffic in Keflavik, you'll note that most aircraft are B757 and A321.

These aircraft perform well at mid-ranges, but on longer ranges they have to reduce their payload for fuel uptake for the long flight. For longer flights the bigger (and more expensive) aircraft are more efficient.

So the effect of the extra stop is that you reduce the fuel consumption, so that you can use smaller aircraft with lower fuel capacity which in turn means lower capital expenses. This makes the Iceland option economically attractive.


In what circumstances could a 787 stay powered on continuously for 248 days?

Aircrews seldom have to power off an aircraft completely (also known as a cold and dark cockpit). Airliners usually stay powered on at the gate. This is known as a "short turn-around": engines are stopped, the APU is stopped, but electrical power and air conditioning is still supplied by ground equipment.

This has the advantage of minimizing turn-around time (after all, airliners are meant to stay in the air to make money). The cabin also has to be cleaned and prepared for the next flight at this time, so something must power the lights.

It is theoretically possible to stay on this way for 248 days. However, airliners have scheduled maintenance, which completely powers off the aircraft. They are also powered off if the expected time at the gate is long. So, in reality, it's very unlikely to have an airliner powered continuously for 248 days.


When should I start turning in this circling approach?

The FAA instrument flying handbook discusses techniques and the procedures by which circling approaches must be conducted.1

You must remain within the protected area or circling radii, but as soon as you enter this area, you may begin the circling maneuver.  Usually, this means you will begin the maneuver well before the missed approach point.  In particular:

a) "The criteria for determining the pattern to be flown are based on personal flying capabilities and knowledge of the performance characteristics of the aircraft. In each instance, the pilot must consider all factors: airport design, ceiling and visibility, wind direction and velocity, final approach course alignment, distance from the final approach fix to the runway, and ATC instructions."
b) "Fly no lower than the circling minimums until positioned to make a final descent for a landing." (page 10-20 of Ref 1).


How can I determine takeoff and landing from an ADS-B feed?

It depends on the type of ADS-B feed you have.

On the lowest level, the messages transmitted from the aircraft, usually contain information about the air/ground status of the aircraft1. When the aircraft is airborne, Airborne Position Message are transmitted (types 9 - 18 or 20 - 22) for position and altitude. Airborne velocity is in message type 19. On the surface, message types 5-8 are used to transmit both surface position and movement. So by looking at the raw messages, the transition between airborne and surface type indicates landing, vice versa in indicates take-off.

This is accurate for most larger aircraft, smaller aircraft and helicopters don't always have a weight-on-wheel switch or similar device that is used to toggle the air/ground status. In these case speed may be used as trigger which is a less accurate indicator.

If you don't have access to the raw messages, you have to find an other way to do it.

Typically ADS-B feeds for air traffic control consist of ASTERIX 021 reports. If you have an ASTERIX category 021 data stream, the air/ground status of the aircraft is included in Item I021/040, first extend, bit 7 - Ground Bit Setting (GBS).

If you have a stream with only latitude, longitude and altitude, then I would try to use the altitude field. On the surface there is no altitude transmitted (it is not in message type 5-8, the bits are used for surface movement), so if the altitude is no longer there, you can expect the aircraft to be on the ground. However, this depends of course on the implementation of the ADS-B decoder; it may just give stale altitude data instead of blanking the altitude on the ground.

If you don't have good detection near the landing zone, you can try to extrapolate the altitude graph from the final approach section and based on that estimate when / where the landing would be. You may want to adjust for the flare if you need to.

1I am assuming 1090ES ADS-B here which is the defacto global standard; UAT and VDLm4 technology varieties of ADS-B also contain air/ground status but in a different manner.


Why is the Boeing 747-300 not in service anymore?

The 747-300 was somewhat of a cross between -200 and SP. The -300 was almost same as the -200 except for the increased length of the hump. The -300 was never a success and was soon overshadowed by the -400.

The 747SP was the result of a requirement for longer range aircraft. Boeing engineers basically cut around 47' from the 747 fuselage in SP. However, by the time it flew for the first time, commercial air travel had changed (in no small part due to the high oil prices) and the model never sold as much as Boeing had hoped. However, when it was observed that there were no adverse effects when the rear of the cockpit lined up with the front of the wing, Boeing introduced the 747-300 with increased length of the hump with little else in terms of major change from -200.

However, just two years after -300 was introduced, Boeing introduced the -400, which was a significant improvement- it eliminated the flight engineer, increased range among others, while retaining the -300's hump (which meant more economy seats). 747-400 is the most successful model of the series, with more than 690 orders. Most airlines jumped straight to the -400, effectively killing the -300.

Actually, the 747-200 production (with over 390 made) ended after the -300 version (of which only 81 were made). So they are more of contemporaries.

Also, it'll be a stretch to call that the older 747 versions are still in passenger service. 747-100 is not flown with passengers anymore, current operations of 747SP hardly qualify as passenger service and the most successful of these, the 747-200s are also being slowly phased out, if you discount the VC-25s flying around with US President (which are slated to be replaced by 747-8s).


How long before departure can an IFR clearance be copied?

You can generally get your clearance 30 minutes prior to the departure time you listed on your flight plan.  If you call clearance delivery prior to that they likely won't have it and will tell you to call back 30 minutes prior to departure.


Can a GA magneto-spark plug ignition survive an EMP?

Can the coil and capacitor (condenser) magneto/spark plug ignition
  system as exists on most general aviation aircraft survive an EMP?

Like anything it depends on the EMP's strength, the angle of incident to the device and the device in question. The various things in an aircraft will break down for different reasons. But for the rest of this lets assume an EMP that is sufficient to fry stuff. A magneto will suffer toasted coils as the induced current could potentially generate enough heat to degrade the resin coating on the wire they use on coils. It should however be noted that the magneto is more than likely shielded to prevent interference with avionics which will play out in your favor here.

  How about most turbine engines? I'll use a PT-6 as an example. I
  assume the FADEC would be toast?

Most likely yes. Again the unit is generally housed at least in some capacity in a shielded case but modern PCB components don't play nice with large induced currents. Turbines do have an Igniter system that may see some damage depending on the severity.

  How about other parts of the electrical system such as the battery?

Things like interior lights will most likely blow out, batteries (depending on type and casing) could go. Overheating is a concern if enough current is forced over the line.

  Is there any data or research on this?

There is some Mil-Specs (MIL-STD-1541 (USAF)) out there that cover it for the military. This question covers it for airliners. You can find at least one book on the topic here. There was a lot of research on this that pertains to missile systems as one anti-missile defense was to detonate a nuke in front of the oncoming unit. If the blast did not take it out the resulting EM field would fry what ever guidance and control systems were on board (assuming it was not ballistic). As such, work went into making the units EMP hardened. I'm not sure that work is very public though. The soviets were big users of vacuum tubes as they believed them to be far less susceptible to EMP's. It looks like this company does testing and research on EM hardened aircraft.


In a blown wing, such as blown flaps, air is driven by the engines through ducts in the wing and out through various orifices to boost the airflow over the wing. This helps increase lift, especially to give good STOL performance at low forward speeds.
The augmentor wing takes the sophistication a stage further by arranging the air outlets so that the flow draws in or entrains ambient air into the wing flow, further increasing or augmenting mass flow and the efficiency of the system.
A typical example was the DHC-5 modified with an augmentor wing; a little bit about it is given on Wikipedia. The Rockwell XFV-12 was a supersonic VTOL fighter prototype, which blew the air directly downwards through vertical slots in the wing, however it did not entrain as much additional air as expected and lift augmentation levels were disappointing.
The system has never reached operational use, mainly because the weight, complexity and cost have never been justified by the performance gains.


Are modern fighter jets bulletproof and armored?

With very select exceptions like the Su-34, and the A-10 if you consider it modern, they are not. Even in these cases only the pilot and a few critical parts are armored. Most of the aircraft's skin offers no meaningful resistance even to small arms.

Extensive armor coverage that could withstand modern air-to-air or surface-to-air weapons simply isn't viable for a fighter for weight reasons. Redundancy is used to resist near misses and light hits instead.

The reasons are different depending on the weapon. MANPADS like the Stinger go for the nearest heat source (engine exhaust). Their capabilities against a fighter are limited to fringe cases (flying too low, caught on landing/takeoff, and a very lucky shot), and armoring the engine nozzle isn't viable anyway.

The lightest effective anti-aircraft weapon, the Sidewinder, uses a continuous rod warhead, a ring of steel expanding at almost 1 km/s. Due to high inertia, it will cut through anything except for the engine, major bulkheads and longerons, and hard elements like the A-10's "titanium bathtub". Aramid lining, which is the only thing a modern fighter could afford weight-wise for full-airframe protection, will be cut.

The heaviest is the S-400 SAM, which intercepts the target at 2 km/s with a 180 kg fragmentation warhead. The resulting hypervelocity impact (3+ km/s) has solids behave like liquids, with impact results determined by mass rather than strength. The low odds of surviving a warhead this size aside, when it's mass that determines these odds, it's always better invested in something functional rather than just armor.

Most mid-size AAM and SAM also fly an intercept course at high velocity and use high-brisance explosives for blast fragmentation. Against resulting high impact velocities, mass is better invested in more redundant hardware, not armor. That is very different from low velocities, common on the ground, where thin layers of high-strength materials can prevent projectile penetration.

Larger fighters are indeed more resistant to damage than smaller lighter ones (see the one-wing F-15). Armoring a few critical elements can help improve survivability, but this armor has to be very localized. The Su-27 family has an armor plate between the engines, so that one wouldn't damage the other if it disintegrates after a hit. Close air support (CAS) aircraft that attack tanks and other ground vehicles carry some protection for the pilot - notoriously vulnerable to punctures, unlike most of the plane - against ground fire.

A combination of these factors is why armor has largely become limited to CAS aircraft: high-strength armor is very weight-effective against the low velocities of land warfare weapons, but much less so against hypervelocity impactors and fragments used in air combat.

See a related question for a few more bits of info on aircraft armor and protection:
Why do modern fighter jets use 20mm guns?


Why do some airports have class E airspace to the surface only at certain times?

The class E airspace protects IFR traffic departing and approaching the airport from incursions by VFR aircraft.

visibility and cloud clearance requirements for Class E are more strict compared to Class G (which is what the airspace reverts to after-hours).

3 statue mile visibility, 500 below, 1000 above, and 2000' horizontal from clouds for Class E

Compare that to 1 sm / clear of clouds for Class G

You could see that someone pushing the limits of VFR, scud-running to/from the airport might pose a danger to an IFR aircraft on approach to the airport and might pop out of the clouds and get a surprise.

As to the choice of hours, that's likely an FAA decision based on traffic volumes.

Normally you see this at airports that serve rural locations with scheduled passenger service throughout the day (and possibly later at night as well).  Pellston, MI is another airport that comes to mind with a Class E surface area, but it is 24 hours a day.  I think Delta serves Pellston with 1 or 2 flights a day from Detroit.


How is the pushback tug connected to the airliner?

You're correct that there are two ways to connect the tug with an aircraft.

In case a tow bar is used, it is fixed laterally at the nose landing gear, while allowing slight movement for height adjustment. Usually, the tow bar may pivot freely laterally and vertically at the end that attaches to the tug rather than the aircraft end, which is fixed. The following image shows the tow bar attachment point in Airbus A380.

Image from alamy.com.

The attachment points differ from aircraft to aircraft, so the tow bar basically operates as an adapter between the aircraft and the tug.

"Pins for pushback". Licensed under Public Domain via Commons.

When using a towbarless tug, the nose wheel is simply scooped off the ground using hydraulic (for large aircraft) or electric power. The tug has a movable arm, which is opened and the tug is moved and placed so that the nose wheel fits inside the fixed arms of the tug. The movable arm is then closed. The movable arm and the opposite side has sloping edges, over which the wheels slide and the nose is lifted off as the arm closes due to hydraulic power.

This video shows how its done in case of Airbus A380.

Video from Youtube: How to lift up the world's largest commercial aircraft the Airbus A380. by user Kasavubu

The nose wheel dimensions and aircraft weight do matter and tugs are certified for different ramp weights.


How far ahead will ATC issue runway crossing clearances at controlled FAA airport?

I suspect that you're thinking of this procedural change that took effect from June 30th, 2010. Before the change, a taxi clearance could indeed allow you to cross multiple runways, but now you can only be cleared to cross one runway at a time:

  This change establishes the requirement that an explicit runway
  crossing clearance be issued for each runway (active/inactive or
  closed) crossing and requires an aircraft/vehicle to have crossed the
  previous runway before another runway crossing clearance may be
  issued. At airports where the taxi route between runway centerlines is
  less than 1,000 feet apart, multiple runway crossings may be issued
  after receiving approval by the Terminal Services Director of
  Operations.

It was a big change at the time (according to what I've read) and it was widely publicized by the FAA and other organizations. The relevant ATC instructions are in the ATC orders 3-7-2:

  Aircraft/vehicles must receive a clearance for each runway their route
  crosses. An aircraft/vehicle must have crossed a previous runway
  before another runway crossing clearance may be issued.

The orders have several examples and the corresponding phraseology.


What are CP-like codes or designations after SAS or Scandinavian flight attendant names?

I emailed the SAS museum in Oslo to inquire about the acronyms in this photo description. It took awhile, but I finally got the following reply:

I'm a retired SAS-captain volunteering at our museum. These codes are
IATA communication codes. CPH is Copenhagen airport, CV is the name of
a SAS-sub group, the cabin attendant's group there. So if I were to
send a old fashioned telex to that group, I would include a NAME with
address CPHCV. Similarly, my former address would be my NAME, OSLOA.
These addresses were used within almost all airline companies, an
external message to me would be addressed NAME, OSLOASK, where SK
denotes SAS.
CPHKA is the address of the administrative SAS-department in charge of
all SAS stations within Denmark. CPHZC-S is unknown to me, but -S
indicates a further sub-division.
We used to have mail departments at all SAS stations, and this type of
addressing was extensively used on envelopes and in phone-directories.
These days, we still have mail departments, but these are greatly
reduced since e-mail has taken over communication.
You'll find more info on internet:
https://en.wikipedia.org/wiki/IATA_airport_code and:
https://www.iata.org/en/services/codes/
Best of luck with your studying.
Leif Leren, at the SAS-museum at OSL.


How much fuel is used for the different phases of the flight of a typical airliner?

From this site by Boeing for the B737-800: fuel used for TO and climb = 2,300 kg.
From this site: fuel burn in cruise for B737-800 is 2,500 kg/hr
From this site: fuel burn at idle of a CFM56 engine = 300 kg/hr, so for the half hour descent two engines burn through 300 kg.

Total fuel for a 4 hour cruise flight is therefore 2,300 + 4 * 2,500 + 300 = 12,600 kg. Percentages:

TO & Climb =  19%
4-hour cruise = 79%
30-minute descent at idle: 2%


Does Part 135 allow single-engine night IFR operations in a turboprop?

Do the Part 135 rules allow single-engine turboprop night IFR Part-135 operations?

Yes, there is nothing preventing it provided the pilot is night current and the airports are suitable for night operation.

You can find the FAA's 135 general requirements here.


How can I tag along on an Atlantic crossing?

Contact companies that do ferry/delivery services.  You'd probably have to pay for your own immersion suit, IF someone was willing to let you come along.  Yes insurance will also probably be an issue.


What are the consequences of attaching an unused extra engine under Boeing 747 wing?

Operationally you would be more restricted in terms of trimming. Back when we had manual loadsheets there would be a demarcated area '5th pod operations limit'. Only the gods of the loadsheet deptartment would be allowed to do the loadsheet for these flights. I think Qantas would put a loadsheet man on board for these flights but I could be mistaken.

The flight would be re-timed as the usual 90 minute turnaround time would not be sufficient. I believe 2 hours on top of the regular turn was the norm.

An additional fuel stop was usually required as the fuel-burn was atrocious with that big thing hanging off. The weight penalty was compensated by reducing cargo.

I was working in Singapore in the 80s and we usually had Qantas doing this a few times a year. The engines were ex-SIN .. I don't know if they were overhauled in SIN or if they came into SIN on another freighter. At the time there were not many B747 freighters operating into Australia. I believe Flying Tigers had one but that was from US to Australia to SIN.. It did not do SIN-Australia.


Is Part 91 Instrument currency for SIC similar to Part 135 requirements?

Only if you fall under the Fractional rules (Subpart K).  Otherwise SIC currency requirements for turbojet airplanes are specified in 14 CFR 91.531, which requires you to be current via 14 CFR 61.55 and has no instrument currency requirement (although you do need your instrument rating to fly under IFR).


Is it feasible to circumnavigate the Earth in a sailplane?

No. Both night and oceans would be major obstacles.
Night time can be survived by parking the glider in ridge lift but it requires a steady wind during the night. This is possible and has been demonstrated decades ago, but is only possible with the right weather conditions. To have those lined up between thermal-powered daytime flying is rather unlikely.
Oceans are too homogenous to create thermals. You will notice that when crossing a large body of water in a glider: Over water the air is calm while over land it is easy to find thermals. Why else would albatrosses resort to dynamic soaring if the much less stressful thermalling would be possible? Dynamic soaring works much less well for human-sized gliders because of the limited height of the wind shear layer above the ocean's surface. Besides, it is beyond normal human capability to fly intermittently at 3g over many hours without sleep.
If you start high enough, a large lake can be crossed since the lack of thermals means you also will not hit downdrafts and your glide ratio will be easy to predict. But no thermal will carry you high enough to hop between Pacific islands or from Scotland to Iceland, which is a nearly 1000 km overwater trip.
For a true circumnavigation you would need to touch the southern hemisphere and there are too many long stretches of water to plan any feasible course. Only flying across Siberia and on to Alaska, Canada and Greenland does not make it a circumnavigation. Also, I very much doubt you will find thermals over large bodies of ice and snow, too.


If a plane had a laser/infrared ruler its four extremities pointing downward, could that work as an altimeter device?

Yes, you could create an altimeter using a laser.  And they exist; See Laser altimetry for aircraft and UAVs by Renishaw as one example.

Laser altimeters, however, have a significant disadvantage over radar altimeters: They can be obscured by weather, like clouds and rain.


When Airplane is reducing altitude from flight level (cruise), which one is more efficient

There are (mostly) two ways to reduce altitude:

Reduce speed until your wings don't generate enough lift in level flight. Typically the nose will drop by itself since you are no longer at your trimmed airspeed. You might be able to re-trim so that you are in a level descent.
Drop the nose manually without reducing (or even increasing) speed

Airliners at altitude operate very close to what is called the coffin corner where an increase in speed means you overspeed the aircraft, and a reduction in speed means you stall. So preferably the pilot will simultaneously reduce the throttles while lowering the nose to maintain a specific speed.

Tilting the aircraft up (nose up) will reduce your speed and eventually you will fall to the earth (as you stall). This is a very inefficient way to get down (not to mention it scares the hell out of the passengers). The aircraft buffets, a wing may drop if the plane is not kept coordinated, and is overall a very dangerous thing to do (see the coffin corner link above).

The reason the nose is raised while landing is so that the aircraft can bleed off extra speed and reduce the descent rate for touch down (as well as reducing the stress on the nose gear). This is also a high-drag situation and is, in that effect, inefficient.

  What is the allowed maximum angle tilt down nose?

90° because after that you're tilting up...

In all seriousness you can nose-down until you hit the maximum airframe speed without causing damage. For airliners this is usually a lot more than passengers are comfortable with (especially if the stewards have the rolling carts out), so pilots make a concerted effort to keep the floor as level as possible. For smaller aircraft like the 172, you can configure it such that you won't exceed this speed and point the thing almost straight down, it's actually pretty fun.


What is the difference between a go-around and a missed approach?

Approach, missed approach, departure & holding
Approach, missed approach, departure and holding are published instrument procedures. They are characterised by waypoints, alitudes, headings, climb-, and descend profiles.
An approach procedure tells you how to get from cruise to short final, from a holding to short final, or from cruise into a holding.
A missed approach procedure tells you how to get from climb out into a holding or to another point from where you can commence a new approach.
A departure procedure tells you how to get from climb out to cruise flight.
Takeoff, landing, go around & touch and go
Landing is the process of getting an aircraft from short final to a full stop on the runway.
Takeoff is the process of getting an aircraft from a full stop to climb out.
A go around takes an aircraft from short final to climb out directly, not touching the runway.
A touch and go takes the aircraft from short final to climb out, touching the runway in the process.
Relationships
The procedures listed above can be combined arbitrarily. Even though a missed approach is often flown after a go around, they are not synonymous. Missed approaches can also be (and are) flown after a touch and go, or, for training purposes, after a takeoff. Also, a missed approach is not the only thing you can follow after a go around: Sometimes, you want to perform a departure and continue toward your alternate destination.


If you really want to become a helo pilot, just begin a search for a flight school which does helicopter training.  I do recommend Precision Helicopters out in McMinville, OR, but there are plenty of good schools, all with fine instructors, out there.

As with any kind of flight training, it requires three things:

Commitment:  Once you start your training, be prepared to commit to finishing it.  Flying will teach you a lot about your capabilities and limitations as a human being.  Some areas are going to come easily to you, others you are going to struggle with.  But you can persevere if you work hard at it.  It is imperative for success here that once you start, you do not terminate your training or have extended delays or absences from your flying.  Flying aircraft is a perishable skill and lessons learned can erode away if not regularly practiced.

Time:  be prepared to budget at least three months, maybe more depending on how easily you progress, to flight training.  This will encompass 2-3 flights a week with classroom prep on the side.  This is an ideal amount of time as it keeps your flying regular to maintain proficiency in skills learned while allowing you to fully digest and ruminate on previous lessons.  It also gives extra time should you encounter learning plateaus or other difficulties in your training.

Money:  Unfortunately, no bucks, no Buck Rogers.  Flying is expensive and that goes doubly so for helicopters.  For a private pilot certificate with a helicopter class, I would anticipate spending an estimated \$20,000-\$25,000 on your training.  That will encompass all of your costs, including dual flight time, supplies, ground school, simulator work, FAA written exams and the practical exam (checkride) in the helicopter.

The military is another good option for flight training.  However most service branches require that you having been commissioned as an officer before your 29th birthday in order to be eligible for their flight training programs.  You will also need good eyesight and be in excellent physical condition.


Why does an aircraft have to be flying straight and level to facilitate a jump?

One of the reasons why many WW2 pilots did not make it out were the G-forces and acceleration in directions other than the door when they had to get out. The bailing out survival rate appears to be correlated to the ease of exit.

If you're going to have a smooth and safe departure, you're happiest doing it during level flight when there's no interference from plane motion.

(source: lonesentry.com)


Do aircraft automatically send a message to the ground if there is a catastrophic failure?

There is nothing formally standardized as far as I know however newer Rolls Royce jet engines (and presumably those from other major manufacturers) do have a continuous inflight data link that reports engine data in real time to their reporting centers. You can read up on it here as well as this discussion with lots more info.

  Aerospace Operations Centres are able to track the health of thousands
  of engines operating worldwide by using onboard sensors and live
  satellite feeds. Rolls-Royce are world leaders at equipment health
  management (EHM)

The centers are potentially able to detect certain emergencies remotely based on the parameters reported.

Since you note the pilot making contact presumably with the radio: Pilots can report an emergency without the use of a radio by Squawking 7700 on the transponder as well.


How can I calculate stall speed using foilsim?

All you can do with any airfoil analysis software is to calculate the stall angle of attack and lift coefficient. To get to the stall speed, you need to have knowledge about the wing loading and wing shape of the particular aircraft you need to calculate the stall speed of. For a single airfoil no stall speed can be defined - this is only possible for a specific airfoil on a specific airplane.

Foilsim allows you to add this information via the "size" button. Next, click on the "shape" button and increase the angle to the maximum where the flow on the upper side does not yet separate from the airfoil contour. If you now change to the "Flight" button, you can enter the speed and altitude. Select "Lift" on the right side and see how the lift changes with speed.

If you divide the lift by the gravitational acceleration of the selected planet (yes, you can run Foilsim on Venus or Mars, too - on earth that would be 9.806 m/s²), you get the mass of the aircraft that will stall at the selected speed.

However, since this is a very simple simulation that is not meant for actual aircraft design, and the airfoil shape cannot really be selected beyond camber and angle of attack, I advise you to not build an aircraft with those results. The fact that the stall angle of attack is always 10°, regardless of the airfoil contour used, should be warning enough.


Is induced drag not caused by tip vortices?

Oh, but they are all* correct!

First, we have to say that we are not talking about cause here. There is a flow around the wing and it has certain properties and neither is cause of the other; they are both effects of the flow. What we are talking about here is explanations why every flow that produces lift also produces induced drag.

And as far as explanations go, there can be more than one, because the physical theories have a lot of redundancy in them. You can usually arrive at the result by multiple ways.

You can use the orientations of the normals (the Peter's answer), you can use laws of conservation of energy and momentum (my answer) or you can use the potential flow theory and that explanation will indeed involve the wake vortices. Because the potential flow theory is a mathematical apparatus for analysing fluid flow as a combination of vortices and from that angle of view, the energy is indeed carried away by the wake vortices.

However, the wake vortex explanation has one problem: it is often associated with different misconception, the one that the vortices are caused by transverse flow around the tip and the resulting idea that it can be eliminated by doing something with the tips specifically.

But that's not what the potential flow theory says. According to the potential flow theory, there is a vortex originating from every point along the span and they combine to form the two tip vortices. So like the other explanations, it also clearly says the induced drag is related to any generation of lift. And if you understand that, it is correct. I still consider it unnecessarily more complicated.

* All the three you listed that is. The ones that claim the wingtip vortex is caused by transverse flow around the tip are not, because it isn't. The wingtip vortex is just the vortex line of the overall wake (and should preferably be called wake vortex).


What is the aviation term for what the navy calls "fire control system"?

It is still called the (Airborne) Fire Control System; found about it here from a Google search.

Well, the acronym for (Airborne) Fire Control System is FCS (As well as Flight Control System) according to a military acronym website


How to choose the right ILS when the runway has more than one?

When an airport has multiple ILS approaches to the same runway (usually labelled ILS-Z, ILS-Y, ILS-X, ...), they all end up using the same ILS for that runway, but they differ in how you fly the approach until final.

Consider e.g. KTUP, which has an ILS-Y and ILS-Z approach to runway 36. Here is the chart for the ILS-Y:

(source)

This ILS approach starts at an RNAV waypoint GAGYE and therefore lists "RNAV 1-GSP required" as a requirement. The ILS-Z approach looks completely different:

(source)

This ILS approach has two IAFs: CEVSI on the OTB 12-DME arc and the VERON NDB. It therefore lists "ADF or DME required" as a requirement.

So, which one should you choose?

During flight planning, you should plan to use the approach that best fits your aircraft's equipment. If your aircraft does not have ADF or DME installed, ILS-Z in the example above would not be possible. On the other hand, if your aircraft is not RNAV approved, you cannot fly the ILS-Y. If you can do both, then file a route that ends up at whatever IAF works best for you.
During the flight, it should be clear which approach you should fly based on which waypoint you end up at. In the example above, if had filed a route to the VERON NDB, then you would fly the ILS-Z. If you filed a route to a beginning of a STAR, then you fly the matching approach from the last waypoint of the STAR (KTUP in my example does not have STARs, but other airports with multiple ILS approaches might). This would usually be the RNAV variant, since most STARs are RNAV anyway.
If still multiple approaches are possible, ATC should clear you for one in particular, e.g. via "Cessna 12345, cleared ILS Yankee approach runway 36".


How can a pilot anticipate cloud formation?

Clouds can be formed in many different ways. Before we go through them, we need to establish some basic facts.

Clouds are made up of liquid water, or solid ice. Clouds are not water vapour; vapour is a gas and is invisible. Once more, because this is important: clouds are liquid water, not water vapour.

Water droplets in clouds are extremely small. Clouds are visible because light is scattered in these droplets. The water droplets can stay suspended in the sky due to friction and air currents.

In order for water vapour to condense into liquid water droplets, to form clouds, condensation nuclei must be present. This can be tiny particles of dust or other impurities around which water droplets can form. This is where the idea of human cloud generation comes form - by adding condensation nuclei to the atmosphere, clouds are more likely to form.

A mass of air can contain a certain amount of water vapour (=gas). The maximum amount depends on several factors, of which the most important one is temperature. Cold air can contain less water vapour than warm air. If more water vapour is present than the air can contain, the water has to condense (the opposite of evaporate), to form water droplets - clouds.

So, how are clouds formed then?

By adding moisture

Air can only contain a certain amount of water vapour. If we try to add more water vapour to the air, it "won't fit". As a result, the water vapour will condense, forming visible water droplets - clouds. This can happen over a surface of water during a warm day, when surface water evaporates. The air won't be able to absorb the evaporated water, so instead it has to become liquid.

Cooling from below (advection)

Warm air can contain more water vapour than cold air. When a warm, moist airmass moves over a cold surface, the temperature of the air will drop. When reaching a certain temperature, the air becomes saturated - it contains exactly as much water vapour as it possible can. We call this temperature the dew point. If the air is cooled further, some of the water vapour in the air has to condense into water droplets. Imagine squeezing a saturated sponge - the water has to come out.

Adiabatic cooling (convection)

On a sunny day, the sun will heat up the surface of the earth. Because the surface is not uniform, it will heat up at different rates. A sandy field will get much warmer than a wet area, for example. The air directly above such hot areas will also be heated up, by radiation from the surface. Hot air has a lower density than cold air, so at some point it will start to rise ("heat rises"). As this warm bubble of air rises, it will start to expand. This is because pressure drops with altitude. As the air expands, it will cool off. Now we're back at the basic principle that cold air can hold less water vapour than hot air. As our bubble of air rises, and cools down, at some point, it will reach the dew point, and a cloud will form.

The adiabatic lapse rate is the rate at which a bubble of rising air will cool off with altitude. Rising air will cool with approximately 3 degrees C per 1,000 ft of altitude gain (dry adiabatic lapse rate - DALR), simply because it expands as a result of the lower pressure. When air starts to condense, and a cloud forms, this process releases latent heat. Simply put, it took energy to make the water evaporate, and this energy is released again when the water condenses. This means that once our bubble of rising air reaches the dew point, and a cloud starts to form, it will actually cool off slower. The saturated adiabatic lapse rate (SALR) is approximately 1.5 degrees C per 1,000 ft.

Now, for how long will our bubble of air keep rising? It will keep rising as long as the air surrounding it is cooler than itself. The general temperature of the atmosphere at a given location is known as the enviromental lapse rate - ELR or ELT. As long as the DALR is less than the ELR up to the dew point, rising air will form clouds.

Frontal lifting

A warm front is when a mass of warm air moves forwards, displacing a mass of colder air. Since warm air has a lower density than cold air, when the two air masses meet, the warm air will be forced upwards, above the cold air. This causes the air to expand, and cool. As this happens, the air might reach the dew point, and clouds will be formed.

Orographic lift

Orographic lift is quite similar to frontal lift. Instead of being forced up by cold air below, an air mass is forced upwards by terrain (i.e. wind blowing towards a mountain). Then it's the same old story - the air rises, expands, cools and water vapour condenses.

Turbulence and convergence

Mechanical turbulence, for example due to rough terrain, can cause vertical movements in the atmosphere. If these are large enough, air can be forced upwards, which, as we now know, will form a cloud.

Convergence is when two air masses move towards each other, creating a vertical stream of air in the middle. This column of air moves upwards, and clouds are formed.

These are the basics of cloud formation. As you can see, they all depend on the same basic principles that I covered in the beginning of this answer.


What are the services that MROs provide for corporate jets (private jets)?

In general, the MRO services for corporate jets includes, but is not limited to:

Aircraft major and minor inspections and overhaul
Aircraft defect rectification and trouble shooting
On-site defect rectification (away from base).
Compliance of regulatory directives.
Replacement of avionics units.
Aircraft engine maintenance.

The costs associated with these activities will vary depending on the model, location and aircraft condition. If you want actual costs, it would be best to contact the service provider directly.


Why aren’t any JP-1-type (extreme-narrow-cut) fuels in use today?

JP-1 is not in use because it's expensive and there's cheaper ways to deal with flash point and freezing point.

The navy developed JP-5 for carrier use where a higher flash point is required, and it is easier and cheaper to produce than jp-1
A higher flash point isn't that big a gain in most use cases, it's not as if the fuel is going to instantly burst into flame if it reaches 40°C - with good equipment and safety practices Jet-A(1) is still pretty safe to handle
Anti-gelling additives, tank heaters and recirculators work very well to keep fuel from gelling or freezing. This has proven very effective, BA-38 is the only case I can think of in recent memory where ice in fuel has caused an issue, and that was due to water ice crystals rather than the fuel itself, which didn't freeze. JP-1 wouldn't have helped in that case


What is the area between the taxiway and runway shown in the picture below?

Additional information on the airport would have been nice, but from the markings it looks like it simply is a pad where two aircraft can be positioned at once.

This can be useful where you have a case of eg. one aircraft needing additional time to be ready for take-off while the aircraft behind is all set, or a larger aircraft in front being held back to allow a smaller aircraft to pass it and take off first.

Copenhagen Airport (CPH) has a similar layout at the end of one of the runways, as shown below, although this is using two taxiway stubs rather than one solid pad.


Can a class 1 medical certificate be obtained when glasses are needed to achieve 20/20?

Yes, you can (provided you don't have any other disqualifying conditions).

14 CFR 67.103 Eye

  (a) Distant visual acuity of 20/20 or better in each eye separately, with or without corrective lenses. If corrective lenses (spectacles or contact lenses) are necessary for 20/20 vision, the person may be eligible only on the condition that corrective lenses are worn while exercising the privileges of an airman certificate.

  (b) Near vision of 20/40 or better, Snellen equivalent, at 16 inches in each eye separately, with or without corrective lenses. If age 50 or older, near vision of 20/40 or better, Snellen equivalent, at both 16 inches and 32 inches in each eye separately, with or without corrective lenses.

In your case, you will have a limitation placed on your certificate that says that you must wear corrective lenses while performing the duties under your Class-1 medical certificate.


Which models of the Boeing 737 are still in production?

The variants in production are:

700
800
900ER
BBJ
C-40
737 AEW&C
P-8
MAX

In bold are the passenger variants. The BBJ is a business jet variant. The first three are 737 Next Generation. As of February 28, 2019, there is a total of 38 unfilled orders for that family (discounting the military P-8). Whereas there are 4,636 unfilled MAX orders as of the same date.


What does the lift coefficient take into consideration for flaps?

The lift equation is:

$$ L = C_L \frac{\rho \times v^2}{2} A , $$

where $C_L$ is the lift coefficient, $\rho$ the density of the air, $v$ the relative velocity and $A$ the wing area. The idea behind the lift coefficient is to combine all aerodynamic effects related to the shape of the aircraft into one dimensionless number, which can be experimentally derived as a function of the angle of attack:

(example lift curve from Wikimedia)

When extending the flaps, two things can happen:

The shape of the wing changes, i.e. wing camber is increased. This will modify the lift curve:

(image taken from this answer)
The total area may increase (e.g. for Fowler flaps, see also this question: Does deploying flap change the aspect ratio of the wing?).

So the $C_L$ is taking the change in camber into account, but not the change in area, which will also contribute to increased lift.


When do the lift and drag vectors contribute a force component along a glider's path of travel as seen from the ground?

In thermals.

More precisely: Every time when the updraft strength increases. The glider will continue on its original path due to inertia while the airflow will have a positive angle when referenced to the path of travel. Now the aerodynamic force will point slightly forward and accelerate the glider.

Experienced pilots use this by pulling more than 1 g when flying into a thermal and less when updraft strength decreases. This helps them to gain some more energy from the rising air.

Drag is per definition the part of aerodynamic forces parallel to the flow direction, so for drag the answer is: Always. With changes in vertical airspeed, the drag component parallel to the direction of movement will merely change with the cosine of the angle between the aerodynamic and the kinetic coordinate systems.

If you insist on excluding changes in vertical air speed, your question will have no answer regarding lift.


What records should a CFI keep to prove they took due care in the quality of their instruction?

Have a well defined syllabus, and then a daily activity sheet where you write down the lesson you did, the time it took, and how the student performed. If anything peculiar happens, make a note.

Have a well-written lesson binder too. Even if you don't need to use the binder to teach a lesson (like steep turns, that's not really much of a ground lesson), have it in there anyway to show that you know what you're talking about. If you have a page that only says "Teach steep turns" and a lawyer gets a hold of it, there's nothing to prove that you actually know how to teach steep turns.

AS for legally required records:

You must sign the logbook of anyone you have given flight or ground training.
You must maintain a record of the following:

The date and name of each person whose logbook/student pilot certificate you endorse for solo flight privileges (I keep a record of every endorsement).
The date and name of each person you endorse for a knowledge/practical test, and the kind, date, and result of the test.

You must keep the records for at least 3 years.


Why are airplanes parked at the gate with max rudder deflection?

The control surfaces of airliners are not connected to the pilots' control via cables; they are operated by a hydraulic pump. When the engines are shut down, there is no hydraulic pressure in the pipes, and the control surface is free to move.

The rudder is moved from the center position by wind. If you observe the gates, you should note that all airplanes have their rudder deflected to the same direction. In a perfect no-wind scenario, the rudder would stay neutral.

There are no gust locks which the pilot must arm before leaving the cockpit on Boeing aircraft (and I presume Airbus too). Gust locks are, however, common in general aviation aircraft. They can also be found on some small business jets.

The reason being, control surfaces on small aircraft work "both ways" - if you move the elevator by hand, the yoke in the cockpit will move as well. These are called "reversible flight control systems". Therefore, when the control surfaces are moved by wind, the entire system - yoke, cables, wheels, bearings etc. are all moved back and forth. This brings a significant amount of wear and stress to the system. Strong wind may also cause the surfaces to deflect past their designed maximum, causing damage.

The control surfaces on airliners are much heavier and much stronger. Also, they are not "directly" connected to the cockpit's controls - when there is no hydraulic power, pretty much every component is disconnected. Unless you're facing gale force winds, it is safe to just let them swing free with wind and gravity.


How useful are simulators in pilot training?

Flight training devices (FTD) are the non-moving, no-visuals cockpit replicas you seem to be reference as "computer simulators".  These provide a real cockpit layout (real avionics, switches, buttons, etc) that are used as procedure trainers and to learn cockpit flows.  The main purpose of these is so you are aren't wasting time in the expensive full flight simulator and can get right to work once you get in there.

Once FTD training is finished, you move into the full flight simulator (FFS, typically a level-D sim per FAA certification) which are simulators with exterior visuals and a 6-axis hydraulic motion platform.  These are the devices used for the actual flight training in airline training.  You are expected in your first sim session to already know your way around the cockpit and not be too distracted by where all the buttons you need are so you can focus on the flight training.  Being at that comfort level for the first sim session is the purpose of spending time in the FTD to start.


How can two private pilots flying together log time and split costs legally?

Short answer: if your neighbor acts as PIC, then he can pay all the costs and you can both log PIC time for however long you were sole manipulator of the controls. But you have to avoid becoming his personal pilot, because then you're getting into commercial territory.

As background, remember that acting as PIC and logging PIC time are different things. And I'm assuming that you're going to fly VFR only, i.e. there are no IFR and/or safety pilot issues (we have a few questions on that already), and that you're both fully qualified to act as PIC (medical, currency etc.).

First, logging PIC time. 14 CFR 61.51 has the rules, and the key one for your scenario is:

  (e) Logging pilot-in-command flight time. (1) A sport, recreational,
  private, commercial, or airline transport pilot may log pilot in
  command flight time for flights-

  (i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;

That means, when you're actually controlling the aircraft then you can log PIC time, regardless of who's acting as PIC. For example, if your neighbor has the controls for 0.5 and then you take them for 0.5, you can each log 0.5 PIC time. But in a single-pilot operation, only one person can log PIC at a time.

Second, sharing costs. Because a C172 is a single-pilot aircraft, if your neighbor is acting as PIC then you're a passenger. Note the wording in 61.113 (emphasis mine):

  (a) Except as provided in paragraphs (b) through (h) of this section,
  no person who holds a private pilot certificate may act as pilot in
  command of an aircraft that is carrying passengers or property for
  compensation or hire

  [...]

  (c) A private pilot may not pay less than the pro rata share of
  the operating expenses of a flight with passengers, provided the
  expenses involve only fuel, oil, airport expenditures, or rental fees.

As you can see, if your neighbor is acting as PIC he must pay at least 50% of those specific costs so he can always pay 100% if he wants; that just comes down to what you agree with him. If you act as PIC then the reverse applies: you must pay at least 50%.

Third, whatever arrangement you have with your neighbor, make sure you don't become a de facto personal pilot. If he starts asking you to fly him places he needs to get to, or even to just accompany him on flights because he isn't comfortable alone, then you should be very cautious. In theory, if he always acts as PIC then it's OK because you're just a passenger. But if there's an accident and the FAA has a reason to believe that he wasn't genuinely acting as PIC then you'll both be in trouble. To take an 'extreme' example, let's say he breaks his leg and asks you to fly him somewhere, with him paying all the costs. He couldn't be acting PIC because of his injury (61.53) therefore you would be acting PIC without paying any costs. That's receiving compensation for carrying a passenger and it requires a commercial certificate.

Finally, a couple of practical points to think about:

Two pilots in a single-pilot aircraft can lead to confusion and even accidents. Make sure you have a good pre-flight briefing that includes who is acting as PIC, and in flight always do a clear exchange of controls. And decide up front what happens in case of an emergency, like engine failure: does the person flying keep flying, or does your neighbor always take the controls?
You should have appropriate insurance. Even if he puts you on his policy as a named pilot you might still have to pay thousands of dollars as a deductible in case of an accident. If you don't already have it, it might be worth getting your own non-owned insurance to cover that, but it all depends on the details of the policies. If you're an AOPA member they can probably help you out with that.


What determines the positioning of PAPI lights from the runway threshold?

US DoT Order JO 6850.2B contains installation criteria for visual guidance lighting systems. The following is taken from that order.

For siting PAPI along the runway, in case an (ILS) electronic glide slope is present, the lights are located such that the PAPI visual approach path coincides with the one produced electronically as much as possible.

In case of runways without an electronic glide slope, the following criteria are considered for locating the PAPI:

Threshold Crossing Height

  The TCH is the height of the lowest on-course signal at a point directly above the intersection of the runway centerline and the threshold. The minimum allowable TCH varies according to the height group of aircraft that use the runway, ... The PAPI approach path must provide the proper TCH for the most
  demanding height group that uses the runway.

Glide Path Angle

  The visual glide path angle is the center of the on-course zone, and shall normally be 3 degrees when measured from the horizontal.

Obstacle Clearance Surface

  The PAPI obstacle clearance surface is established to provide the pilot with a minimum clearance over obstacles during approach. The PAPI must be positioned and aimed so that no obstacles penetrate this surface.

Image from US DoT JO 6850.2B Visual Guidance Lighting Systems

Basically, in case ILS is there, align the PAPI with that; else locate the PAPI such that it satisfies TCH and OCS.


What happens when a takeoff clearance is cancelled by the ATC?

What happens in the control room?

Aborting a take-off at high speed is dangerous, even if the aircraft is travelling below V1. If the aircraft is already rolling, the controller will make a quick decision whether the situation justifies an aborted take-off. For example, if there is traffic infringing the runway and a crash is imminent the controller would attempt to stop the aircraft. If an aircraft is attempting to take-off from a taxiway in snowy conditions it might be safer to just let him take-off unless there is other traffic on the taxiway.

If the aircraft hasn't started rolling the decision is of course easily made.

  What does the ATC do and how does he/she make sure pilots understand
  the cancellation?

Using standard phraseology is the best way of making sure the message comes across, especially in international aviation.

If the aircraft is not yet rolling, the proper ICAO phraseology is:

  Tower: Big Jet 345 hold position, Cancel take
  - off, I say again cancel take
  - off (due to vehicle on the runway)

  Big Jet: Holding, Big Jet 345

If the aircraft already started rolling the controller would say:

  Tower: Big Jet 345 stop immediately, (Big Jet 345 stop immediately )!

  Big Jet: Stopping, Big Jet 345

No immediate verbal reply is to be expected as the flight crew have other things to care about at that moment. The effect of their actions should be clearly visible from the tower.

  How do the pilots in the aircraft taking off respond? Are they trained for this scenario?

The crew will assess whether they can stop safely. Below V1, they will probably abort the take-off, above probably not. The final judgement is made by the captain. He might reason that it is safer to get airborne and pass the infringing traffic overhead than running into it at full brakes. They are trained for aborted take-offs, but since there are so many non-standard situations the final decision is made on the basis of a mix of training and expert judgement.

  Is a takeoff cancelled even after V1?

Since ATC does not know when the aircraft passes V1, a controller might attempt to cancel the takeoff after V1. The flight crew will only attempt to cancel the take-off after V1 if it is clear that there is no way the aircraft can become - and stay - airborne safely. In such a case it becomes damage control rather than damage prevention.


The sharp pods are called anti-shock bodies or flap track fairings. See this question for further information. Simply put, htey are there to reduce drag- transonic drag by smoothly varying the cross sectional area (Whitcomb Area rule) and by covering the flap actuators.

The straight line connecting the wing leading and trailing edges is called chord line; the distance between them is called chord length (both are usually called chord).

The upper and lower wing sufraces are called, well, upper surface and lower surface of wing.


Why weren't the XB-70's wingtips fixed in the down position?

Well, for starters, the drooped wing is not producing any lift in subsonic speeds. So, the flat area (the fixed part) should be significantly larger for producing the lift. This increases weight, which leads to a vicious cycle.

Another thing is the wing would panels would not clear the ground while the aircraft was on its landing gear. For testing the wing fold in ground, the aircraft had to be put on tall jackstands.

Image from keypublishing.com

As you noted, lengthening the landing gear enough to allow the wing droop would have been a big issue not the least because the aircraft weight was already increasing alarmingly (the weight had shot up from 483,000 lb to 537,000 lb).

Also, you lose the outboard elevons; now for getting the same control response the inboard control surfaces have to be increased in size, which has to be come either from wing or increase the wing area- it is again a mess.


Are there design differences between propellers powered by air (wind-power) and propellers for aviation?

Blades of wind turbines, airplanes and helicopters are essentially the same. They are all wings, roughly to do the same job. However, each one is designed or enhanced to do their special job. The differences can be:

Number of revolutions
Wing span
Rate of flow
Tip speed
Density of medium
Material
Load

Turbine blades are designed to move in large volumes of rather slow moving air (usually 10-30 mph, 16-50 km/h) and not create turbulence that fouls the next blade. Aircraft propellers are designed to move high velocity air and avoid the turbulence by moving out of the way.

Wind turbines use blade pitch to adjust the rotation speed and the generated power. Wind turbine's control system adjusts the blade pitch to keep the rotor speed within operating limits as the wind speed changes. In aircraft, blade pitch is usually described as "coarse" for a coarser angle, and "fine" for a finer.


What was the first aircraft designed with a plug-system fuselage, and why?

This should be a Zeppelin. You said "aircraft", not "airplane", so lighter-than-air systems qualify also.

Already the model submitted with the patent application in 1895 (yes, back then the inventor had to demonstrate that the invention is actually implementable!) showed several cylindrical sections, akin to a train of cylindrical balloons. The final patent (Reichspatent No. 98580) was for a "Lenkbarer Luftzug" (dirigible aerial train). Adding more sections would increase length and payload.

Later, when the maximum diameter of Zeppelin airships was limited by the size of available hangars, their mid section was of constant size and allowed to add plugs to increase payload and range. This was, among others, the case with LZ 104 which turned out 30 m longer than initially planned.

The last big airliner not designed for stretching was the Lockheed Constellation. It had a continuously varying cross section from tip to tail.


When is flotation gear legally required for pilots and passengers on a non-commercial flight in a small airplane over water?

Apparently, the answer is never.

After several days of searching, I was unable to find anything in the FARs that requires the pilot of a small aircraft that is NOT being flown "for-hire", to have flotation devices on board, for a flight over water, regardless of the distance from shore.

Section 91.205 requires flotation devices, but only for flights "for hire" over water. So this section doesn't apply to this question.

Then, there is section 91.509(12) which requires flotation and other survival gear for flights 50 nm, or 30 min out from shore, but this entire section is under Subpart F - Large and Turbine-Powered Muti-engine Airplanes and Factional Ownership Program Aircraft. So this entire section doesn't apply to this question either.


Why do we have to check the speed brakes during GPWS memory items on an A320?

Because that is how American Airlines Flight 965 went down.

Long story short, the pilots deployed the speed brakes to increase descent rate during approach, and did not notice they were off course. The GPWS warning sounded. The pilots reacted within one second. They did everything they could - except they did not retract the speed brakes. They crashed.

Why is it a memory item? Because GPWS means imminent terrain contact. You don't want to wait around for some automatic mechanism to retract the speed brakes for you. Let alone take the chance of that mechanism failed. You do it. NOW.


What do airliner manufacturers do with aircraft for which have no buyers?

New airliners without a buyer will be handled on a case-by-case basis. Of course manufacturers prefer to not be in such a situation, and there is no standard system for handling this. However, there are some typical ways that this is handled.

One option it to store the aircraft at the manufacturing site. This is the same (and pretty much only) option that is used for aircraft without engines. This has the advantage of being easier to work in and deliver, but will of course take up valuable space.

To save space at the factory, the aircraft can be moved somewhere else. Storage areas already exist primarily for older aircraft being stored or scrapped, but new aircraft can go there as well. Boeing has done this with some 747-8 aircraft, as well as C-17's.

Of course the manufacturer has spent money assembling the aircraft, and it is costing money to have it sit somewhere, so they will be eager to cut their loss as much as possible. These aircraft will probably be offered at a significant discount compared to placing a new order. However, buyers may prefer to wait and receive an aircraft straight from the factory, rather than one that has been sitting in storage for a while. There may also be issues with customized options applied for an original buyer that would need to be refurbished or accepted by a new buyer. Looking for new buyers and rearranging deliveries is not unheard of and is part of the business of making aircraft.

It may be that no one is willing to buy an aircraft, or the aircraft would otherwise not be economical to put in service. The aircraft may in this case be sent to a museum, as with the RJX. In a similar case, the first three 787 aircraft were not deemed economical to put into commercial service after the test program, and were sent to museums.

The last resort would be to scrap the aircraft. This is certainly disappointing for a new aircraft, but in some cases it is the only option left to regain any value.


How do I get permission to fly when diagnosed with Sleep Apnea?

When an Aviation Medical Examiner, AME, issues you a medical certificate, that is the relevant "permission" you need. Having sleep apnea complicates that issuance process, but doesn't necessarily make it impossible. Along with what you read on the AOPA forums, I would suggest making an appointment to talk with a local AME, preferably one who has accomplished this process with other pilots before. He can give you a picture of what the process is likely to look like in your case, which is a better answer than assuming that your journey will look like that of somebody you read about on the internet.


Why is it appropriate to announce ‘stable’ on final approach?

The FAA offers some advice on this:

  An unstable approach is simply an approach that does not meet the
  criteria for a stable approach established by the aircraft operator.
  As an illustration, Flight Safety Foundation defines a stable approach
  in the following terms:

  On the correct flight path:

  ILS Approach - ILS within 1 dot of the localizer and glide slope.
  Visual Approach - Wings level at 500 feet AGL.
  Circling Approach - Wings level at 300 feet AGL.
  Only small heading and pitch changes required.
  Speed within +20/-0 kts of reference speed.
  Aircraft must be in proper landing configuration.
  Maximum sink rate of 1,000' per minute.
  Appropriate power settings applied.
  Briefing and checklists complete.
  During IMC - Stable by 1,000 feet AGL.
  During VMC - Stable by 500 feet AGL.

  If the approach is not stable by 1,000 feet AGL or 500 feet AGL
  (depending on weather conditions), or if the approach becomes unstable
  below these altitudes, the pilot should initiate a missed approach/go
  around. The pilot may initiate a go around at any time above or below
  these altitudes if deemed necessary. It is possible for a pilot to
  initiate a go around even after touchdown on the runway, but not after
  the thrust reversers have been deployed.

They are simply verbally verifying the approach is stable and they don't feel they need to go around. I would think Various OpSpecs dictate how go-arounds are initiated but in reality either pilot can initiate a go-around.  If an approach is unstable a go-around should be initiated.

IATA advises that callouts are used which may be why carriers are adopting them.

  Furthermore, the adoption of calls of “STABILIZED”, “UNSTABLE” or
  “GO-AROUND” at a given point on the approach (stabilization
  altitude/height for example) may improve decision making and
  compliance to ensure a timely go-around is carried out. While a
  “STABILIZED” callout might be required at either 1,000 feet or 500
  feet above touchdown, the “GO-AROUND” command can and must be made at
  any time prior to deployment of thrust reversers. Once again, if such
  callouts are adopted it is essential that an acknowledgement is made
  by the other pilot in every case


Did this aircraft illegally exceed 250kts below 10,000ft?

First of all, this aircraft was flying in Europe, not in FAA land. In Europe there is no general limitation to 250 knots below 10000 ft. There is no EASA airspace restrictions, every country has their own set of rules. There are airspaces that have speed restrictions below 10000 ft, others don't and sometimes aircraft are allowed to go faster than 250 knots with ATC permission.

Secondly, the 250 knots is an indicated airspeed limitation. At 10000 ft, 250 KIAS (Knots Indicated Air Speed) will be approximately 289 KTAS (Knots True Air Speed). And then there is the additional effect of wind. With a strong tailwind you can exceed 350 knots ground speed while still flying below 250 KIAS.

Thirdly, the data in the image you linked is provided by ADS-B. ADS-B can transmit either ground speed or air speed. Under the European ADS-B mandate, aircraft must transmit ground speed, not air speed. But since this mandate will not affect this particular aircraft until December 7, 2017 it might very well be transmitting True Air Speed.

Note that ADS-B data is not purely derived from GPS. The Mode S transponder which provides the ADS-B out functionality on this aircraft is attached to various data channels and is getting Indicated Airspeed, barometric vertical rate. In fact ADS-B vertical rate is preferably from a barometric source.


Do airplanes need wings?

Wait a minute! Airplanes have to have wings by FAA regulation, so you won't find any airplane without wings in the US.

FAA definition:

  Airplane. An engine-driven fixed-wing aircraft heavier than air, that
  is supported in flight by the dynamic reaction of the air against its
  wings.

My advice here: If you encounter an airplane without wings (an unapproved case) do only your duty, call FAA immediately at 866-TELL-FAA (866-835-5322):

(Source)

Aircraft on the other hand has benefited from FAA's leniency, and don't have to have wings:

  Aircraft. A device that is used or intended to be used for flight in
  the air.

As you can see, they are not required to have an engine, nor large rocks in the cargo hold to ensure they are still heavier than air. No, the only requirements for an aircraft are that:

At the beginning you intend to make the thing fly. It's perfectly legal to try to see if you could save wings from the bill, as long as you are honestly expecting it to fly.
You want it to fly in the air, not in the water, certainly not in the vacuum of the space. That's the part requiring all your attention during the design. The lift must be carefully monitored, you need to remain in this tiny layer called atmosphere, else you leave the aircraft category, and maybe the aviation field too.

This is a wingless aircraft:

Full scale Lippisch's Aerodyne, a VTOL aircraft built at Collins (Source)

This one is fully in the law, don't call FAA...

It's certainly a stupid answer, I did my best for, but it's correct :-)


What are the advantages and disadvantages of level flight with a nose-down attitude?

Generally, a slight nose-up attitude gives the best performance. This allows the fuselage to create some lift without producing too much drag, thus filling the drop in the spanwise lift distribution created by the interruption of the wing by the fuselage. However, in passenger aircraft this will make the job of attendants much harder because their trolleys need to be pushed uphill and will roll away when not secured.

Therefore, a neutral attitude is preferred in cruise. When the aircraft slows down, it needs to increase its angle of attack, so the nose-up attitude results. When flaps are deflected, the zero-lift angle of attack is reduced, so a smaller angle of attack will again be possible. The precise attitude is a side effect of speed, air density and flap settings, and is not deliberately controlled when flying a holding pattern. By this time the flight attendants have returned to their jump seats, anyway.

Note that some aircraft will fly a nose-down attitude by design. The Boeing B-52 has powerful flaps which are designed to allow it to lift off without rotating, and when the aircraft accelerates during the initial climb after take-off or flies its approach, the fuselage will have a nose-down attitude. See the picture below of the B-2, the B-1B and the B-52 all flying at the same speed, each with its own fuselage attitude.

B-2, B-1B and B-52 flying at the same speed (picture source).


What size/engine/payload aircraft would be needed for carrying two passengers and a motorcycle?

The question is incomplete without information about the range this aircraft should have. Pilatus in Switzerland is always fond to tell of one of their customers who bought not one, but three PC-12s for exactly this purpose, and has one plus a Harley-Davison motorcycle stationed on each continent of interest. The size of a PC-12 allows the installation of a crane, so loading and unloading of the motorcycle are simplified.

PC-12 side view, with open cargo door (picture source)

Obviously, even a PC-12 does not have the intercontinental range to go anywhere, but if you plan to stay within one continent, range is not an issue. Alternatives in this class are the Czech Aero 270 or the Cessna 208 Caravan. All use a PT-6 turboprop engine, and you will need professional maintenance to keep them flying, so using them does not come cheap.

A more reasonable alternative would be piston-engined, and here the Cessna 206 or 207 (which @Terry already suggested) would be a candidate. However, fitting the bike in will be a challenge.

If you want more internal space and are not afraid of antique taildragger airplanes, the Antonov An-2 or the Canadian DHC-3 Otter would also be a good and reasonably priced choice - however, maintaining one (and especially its radial engine) will also burn a hole in your pocket.

DHC-3 in Pensacola (picture source)


How do landing gear retracting into the wing affect performance when deployed?

There are two main geometrical characteristics that affect the airflow in the situation mentioned. From one side you have a cylinder with wheels at the end; on the other side during the transition you have an open cavity.

Let's talk about the first one. You actually have a cylinder flying perpendicular to the air speed. This is kind of canonical problem in fluid dynamics, there is a lot of literature about this problem, but to explain the effect I would like to take a simple example that I took from the website, the infinite 2D cylinder. For that, I include this picture of flow over a cylinder:

Note for fluid dynamics experts: This is not relevant if Reynolds Number (Re) is around 10,000, but for understanding the concept is good enough.

So, imagine that we are cutting landing gear perpendicular to its length and we visualize the flow.

What do we see?

Two main things....

Looking at the flow lines, they are almost not affected at 3-4 diameters of distance from the cylinder perpendicular to the flow direction there is a very local influence of the existence of the landing gear.

However we see that down the flow there is a big influence with the creation of chaotic structures. That is creating a big increase in drag downstream of the landing gear and any surface behind.

Basically... low influence on the red direction and significant influence on blue. Answering your question, influences the wing in a local area.

The influence anyhow is negative on the lift, as the flow is accelerated locally, reducing the static pressure below the wing, having a lift reduction, but is a local effect. This also have an influence on the wing lift distribution and a small effect on drag... lower than the one directly produced by the landing gear itself.

Now.... the cavities, they have a bigger influence, firstly because they have bigger extension parallel to the airplane surface (the landing gear goes perpendicular to the airplane the cavity goes along the surface). Secondly, their flow behaviour is different, goes in and outside the cavity. I found this web page very good on this, but basically the flow behaviour is like these image:

I found several flows similar to the second one, but depends on the airplane. You can see that flow will enter and go outside and will have a big influence.

Finally, the biggest influence is found when the airplane is deploying or retracting the landing gears when you have changing conditions, difficult to predict and affecting the airplane dynamics. Usually you can notice it when landing or taking off, when landing gear is deployed you fell significant vibration on the airplane.

Concerning your questions:

How does the landing gear affect aerodynamics when deployed and when deploying?

There is a significant drag increase, slightly the lift, but also creates some unsteady behaviour affecting overall airplane mechanics creating vibrations (anyhow, nothing not already taken into account).

Is it different for aircraft whose landing gear do not retract into the wing?

I understand that you refer to airplane with non retractable landing gear. The effect is similar but usually the configuration tries to avoid  having the landing gear close to the wing, as finally they don't need to retract them back.

Does this modify performance critical when taking off or landing such as stall speed, optimal climb speed?

Well, stall speed is usually defined by wing lift curve, and not so much affected by the landing gear. Usually the limitation is on the upper side due to separation.
Climb speed is basically engine thrust minus drag, the drag increases the climb speed is lower. That's why airplanes retract the landing gear as soon as possible.


Yes, sometimes control towers or other ATC facilities are constructed to support air traffic control. If, for example an ATC tower is needed (there was none previously or it was destroyed), a mobile replacement is used.

For mission control Airborne Warning and Control Systems (AWACS) are used:

But these are quite expensive to fly. If possible, mobile surface based radars are brought in to provide a surveillance picture.

Mobile towers, radars and navigation beacon are not solely used in war situations. In natural disasters they have proven to be useful as well:

FAA sends temporary Air Traffic Control tower to Haiti


What is this racetrack-shaped Special Use Airspace, and why isn’t it on the VFR sectional?

This is indeed a designated air refueling track used by military tanker aircraft (all AR-nnn SUA areas are, and they all share that same general racetrack shape).

The reason why it isn't on a sectional is because it's simply not a concern to anyone who isn't in a high-performance, high-altitude airplane -- air refueling (of fixed-wing birds at least) typically takes place well into the FLs (FL280-FL330 is typical, although it may be lower or higher depending on what the tanker is gassing up), and doesn't bother anybody below.


Why is a Cessna C560 circling Washington DC during the protests?

According to the Washington Post, an unidentified C560 was also seen circling protests in Baltimore in 2015. The FBI stated the aircraft were theirs and "specifically used to assist in providing high-altitude observation of potential criminal activity to enable rapid response by police officers on the ground."

FlightAware shows the callsign of the flight over DC tonight as JENA120, consistent with the callsigns used by FBI flights during the Baltimore flights.


Is there a specific formula or way to calculate flex temperature for an Airbus A320?

The RTOW (regulatory takeoff weight) charts are runway and configuration specific, so the length and slope (and permanent obstacles) and all taken care of (see red box below).

There is no manual formula, either the charts or a performance calculator such as the Airbus FOVE or the company's proprietary software.

This is what an RTOW chart looks like:

The information needed (example):

  Takeoff from Paris-Orly, Runway 08
  Slat/Flap configuration: 1+F
  Actual TOW = 66 tons
  OAT = 24ºC
  Wind = +20 Kt headwind
  QNH = 1013 hPa
  Air conditioning: Off
  Runway state: Dry

Source: Getting to grips with aircraft performance (Airbus via skybrary.aero)

Here's why there is no equation, from the Boeing book Jet Transport Performance Methods:

  Here’s the complication: for the very first increment of time after engine failure, one second, we
  need to calculate the average acceleration in order to find the airplane speed after one second.

  But
  we can’t find the average acceleration because we don’t know the thrust after one second. And we
  don’t know the thrust after one second because we don’t know the speed after one second. And
  we don’t know the speed after one second because we don’t know the average acceleration.

What they end up doing is iterative calculations and flight tests to come up with the graphs / tables.


What are the various fees associated with flying?

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

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

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

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

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

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


Why paint a British roundel on a non-British Lynx?

Best guess; the photos are from the delivery flights in the 90s. The helicopter in the photo has the same registration as when it was first delivered (ZH583) and has not been active since.
Update: Although I'm having problems finding an authoritative source, it looks like the registration was resurrected for test flights post-upgrade. Mark Youd has taken a few photos of ZH583 recently(flickr), it was listed in the April update from a site called "Military Aircraft Markings" and looking at various planespotter forums and various flight tracking sites(1)(2) you can find multiple reports of flights being undertaken using the original ZH583 registration.
The roundel and registration is temporary and easy to remove, but was required for the test flights in the 90s as well as prior to delivery post-upgrade. Looking at other photos you'll see the Portuguese registration on the tail; on your photo this area appears to be covered up to avoid having an aircraft with two different visible registrations during test flights.
As for your hypothesis; the second is totally off target. There are loads of barriers in place to avoid having any situation involving a trigger-happy fighter pilot, such as predefined rules of engagement and procedures civilian and military control centers follow.
My guess is that the first one is closer to the truth; although I haven't found a source for it, I can imagine that it would be easier for the factory and the test pilots undertaking the upgrade and test flights to schedule and log flights using the UK registrations.


What happens when a propeller is placed in beta while feathered?

Depends on they type of turboprop engine.  With a straight shaft turboprop engine like a Garrett TPE331, this is not possible as feathering the props are controlled by the conditioning levers and also shut down the engines in the process.  A free turbine turboprop would allow a person to feather the engines and then pull the power levers to the reverse position.  Note that the propeller lever control does not set beta here, rather the power levers do, moving from idle to beta to reverse.


Is it possible for me to work at NASA or other research companies as aircraft mechanic?

Probably not, unless you are a U.S. citizen (dual-citizenship should be ok.) According to NASA,

  Other than under extremely rare exceptions, you must be a U.S. citizen in order to work for NASA as a civil service employee.

It's extremely unusual for non-citizens to be able to work in the U.S. in agencies like NASA or the Department of Defense, where there is necessarily lots of equipment and/or information subject to ITAR controls and/or which is classified. This will generally also be true for contractor positions.

At my present job, we are not allowed to disclose anything more than high-level marketing material regarding products subject to ITAR to anyone who is not a U.S. citizen, for example, even though we don't make weapons or anything like weapons.