What free online educational resources should pilots in training be aware of?

The Airplane Owners And Pilots Association has lots of great articles on both training and new legislation that comes down the pike. The new information is really nice as it lets you know what may be or has changed since the last time you studied the books. Its worth it to join as well (its less than $50 a year and if you have just started to train they are about to bombard you with mailers anyway).

Along with publishing the FAR's the FAA publishes lots of Briefings and really great supplemental material and handbooks like this one on airspaces.

There are lots of Youtube channels out there with great information like FlightChops and the like, but note they are not in any way approved training and may not cover every aspect of what they are doing.

I like to read GeneralAviationNews.com. Its a nice source of informative articles on what is happening around general aviation, airplane ownership advice, mechanical engine advice, and all all manners of legislation that are being discussed as well as fun fly ins and things like that.


Why are gate numbers also marked on the outside of a terminal?

Of course it's helpful to the pilots, and the ground crew, too. Even if you know the airport layout in detail, it's easier to go to the gate signposted as C27, say, than to go to the seventh gate on the left. Now remember that everybody who has ever flown a plane into that airport has flown there for a first time, and everybody who's ever worked ground crew there had a first week.

Just imagine the corresponding situation of a car park with somewhere between a few tens and a couple of hundred bays, where it's very important that you park in your assigned bay. Would each bay's number be written on it? Of course it would.


Does a strobe light shield act like a wingtip fence [or better sort of vortex generator]?

It is a long structured question, I will try to keep up with the answer. As first I would like to cite some sources.

I compared both CS23 and CS25 looking for the certification requirements on the lights. They are almost identical. If you want to check them yourself the keyword you want to use is "Lights". The paragraphs of main interest are from 1285, to 1401, three pages in total. That said.

  I was told it is a shield to avoid to blind the pilot during night flights when the strobes are on.

The light on the wingtip in this case are both position lights and anti-collision lights, the strobing light you refer to.

The Position lights: are described in paragraph from 1285 to 1391. There are requirements on the shielding of them and the intensity of the lights from different radians as well as the colors of them, but none from an aerodynamic point of view.

From a design point of view the shield should define some dihedral angles:

  (b) Dihedral angle L (left) is formed by two
  intersecting vertical planes, the first parallel to the
  longitudinal axis of the aeroplane, and the other at110º to the left of the first, as viewed when looking
  forward along the longitudinal axis.

  (c) Dihedral angle R (right) is formed by two
  intersecting vertical planes, the first parallel to the
  longitudinal axis of the aeroplane, and the other at
  110º to the right of the first, as viewed when looking
  forward along the longitudinal axis.

  (d) Dihedral angle A (aft) is formed by two
  intersecting vertical planes making angles of 70º to
  the right and to the left, respectively, to a vertical
  plane passing through the longitudinal axis, as
  viewed when looking aft along the longitudinal axis.

For the anti collision lights:

  The aeroplane must have an anticollision
  light system that

  (1) Consists of one or more approved
  anti-collision lights located so that their light will
  not impair the crew’s vision or detract from the
  conspicuity of the position lights

  (b) Field of coverage. The system must consist
  of enough light to illuminate the vital areas around
  the aeroplane considering the physical configuration
  and flight characteristics of the aeroplane. The field
  of coverage must extend in each direction within at
  least 75º above and 75º below the horizontal plane of
  the aeroplane, except that a solid angle or angles of
  obstructed visibility totalling not more than 0·03
  steradians is allowable within a solid angle equal to
  0·15 steradians centred about the longitudinal axis in
  the rearward direction.

On the aerodynamics effect of vortex generator at the wingtip, I found a paper: Modification of a wing tip vortex by vortex generators. It is an experimental study without a wingtip device though. Citing from the conclusion paragraph:

  The results show that substantial redistribution of wake circulation can
  be achieved by the merger of the tip vortex with a co-rotating
  vortex from the VG with the core radius of the merged vortex increased by a factor of up to five relative to the undisturbed
  tip vortex.

The VG was though positioned at a $\eta =\frac{y}{b} = 0.2$ inboard of the wingboard.

Summing up the comments above, and merging this interesting explanation from @Peter Kämpf on winglets, what I think (yes, it is an "opinion", but otherwise I would need to run some 3D CFD and/or a wind-tunnel test) is that:

There would be a vortex generated in certain conditions for sure.
To design this device in order to avoid winglet stall separation would be quite a challenging task. The device its positioned very outboard which would make it be more or less aligned with the winglet. I think that the close span-wise coupling between he device and the winglet would make it hard for the vortex to develop in such a way to re-attach the flow on the winglet.


Do fighter jets have an auxiliary power unit?

Most current fighters do.
A notable exception is the F-16. The F-16 has a "jet fuel starter" (JFS), which is a small jet turbine started by a bottle of compressed air. Unlike a normal APU, the JFS is linked directly to the main engine, so the only thing you can use it for is to start the main engine.
An APU can also provide backup electrical power. Since the F-16 doesn't have one (and is a fly-by-wire aircraft, which requires electrical power for normal operation), it has an emergency power unit (EPU) instead. The EPU uses a special fuel called H-70, which is 70% hydrazine, and 30% water. Hydrazine (normally used as rocket propellant) doesn't require an external source of oxygen. It carries its own oxygen supply; expose it to iridium and it immediately (well, within a couple milliseconds, anyway) starts to release large quantities of hot gas. This runs a small turbine, which can restore power within about 2 seconds. The main shortcoming is that hydrazine is quite hazardous (though the 30% water content of H-70 renders it substantially less hazardous than pure, anhydrous hydrazine).


How can I calculate the C.G height for an Aircraft?

No that's a really good question. Weight and balance calculations for flying are only concerned with the location and value of the variable weights - fuel, pax, baggage. The C of G of the "empty" aircraft with all those components is considered a single unchanging value for the purpose of establishing a starting point before loading, and is established just by weighing the airplane while empty.  The weight of the components is a factor in where the empty C of G will end up, but once established the entire "empty" machine is treated as a whole unit for W&B purposes.

Vertical C of G is not normally provided with A/C weight and balance data.  Although the location is not too big deal with most types of aircraft, for gyrocopters it's quite important because they need to have the engine's thrust line as close as possible to the center of mass for safety reasons (if you unload the rotor on a gyro by pushing over or from turbulence, and the thrust line is significantly above the vertical C of G, the thrust line tries to spin the mass of the machine like a pinwheel about the C of G and the gyro tumbles, with fatal results; the tall and spindly look you see on small gyros today is driven by the need to mount the engine/prop lower to get the thrust line close to the vertical C of G).

On a gyro, the procedure is to hang the loaded ready-to-fly-state machine by the rotor hub, establish a vertical reference line below the hang point, then hang it by another point at the same level that is offset forward or aft using a special "2 hang point" rotor hub fixture, or alternatively if it'll work you can tip the machine back on its main gear until it balances, and establish a vertical reference line extending up from the gear contact point.  Where the line extending down from the rotor hub, and the line extending up from the gear balance point (or the two downward lines if you had a dual hang point fixture) intersect, is the vertical C of G.

To figure this out on a 172 you would need to do something pretty radical, like support it along the fuselage at one point using a suitable resting fixture just forward of the main gear but aft of the C of G, and tip it back until it balances and establish a vertical reference through the balance point, then move the fuselage rest forward to ahead of the C of G and tip the plane nose down to establish a new balance point and vertical reference.  Then where the vertical axis of each balance point intersects, there's your vertical C of G.  Of course you'd want to do this while loaded with people, fuel and bags and you'd probably have to pay them or offer them free beer, after you've found an airplane to use, built a fixture, and all that.  You could also probably do it by weighing on scales with the nose propped up high on a block so it points up at an angle, then reweighing with the mains propped up high on two blocks so it points down on an angle, then do some clever geometric calculations to work it out.

Or, if you are as lazy and cheap as me you could call Cessna's Tech Support organization and tell them what you are doing and what you need, and somebody somewhere should be able to find the data (hopefully) without charging you for it.


What is the minimum set of registers to fill for ADS-B?

ADS-B IN systems do require information from the 0x6X registers. These registers hold information like ADS-B protocol version, system design assurance level and position source quality indicators.

Without this information the ADS-B IN system can establish a track, but the quality will be flagged as too low to display.

For reference, get a copy of RTCA DO-260B or EUROCAE ED-102A, whichever is cheaper. They are the Minimum  Operational  Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance – Broadcast  (ADS-B).


Why does the Boeing 787 use Cabin Air Compressors (CACs)?

Boeing is moving to a bleedless system with the 787 Dreamliner and GEnx engines. In the past engine bleed air has been used extensively for ECS (Environment Control System) e.g. cabin pressurization and air conditioning, as well as anti and de-icing, ozone removal and many other uses.

Nevertheless Boeing firmly believes that the overall efficiencies and improved reliability are a large step forward. Moving to a bleedless system will reduce the mechanical complexity of the 787 by more than 50% compared to the 767. The new system is expected to be save about 3% in fuel and 15% more efficient overall.

The move to a bleedless architecture has been the subject of much enthusiast debate e.g. here because while many advantages are apparent it's not immediately obvious that the increased electrical loads to replace the pneumatic functions isn't a moot point. Additionally, Rolls Royce wasn't willing to commit to developing a bleedless system, so the Airbus A350 XWB will continue to use bleed air pneumatic systems.

Boeing summarizes the primary bleedless architecture benefits as being:

  Improved fuel consumption, due to a more efficient secondary power extraction, transfer, and usage.
  Reduced maintenance costs, due to elimination of the maintenance-intensive bleed system.
  Improved reliability due to the use of modern power electronics and fewer components in the engine installation.
  Expanded range and reduced fuel consumption due to lower overall weight.
  Reduced maintenance costs and improved reliability because the architecture uses fewer parts than previous systems.

787 No-Bleed Systems: Saving Fuel and Enhancing Operational Efficiencies

Pneumatics have long been one of the weakest links in modern airliners. Moving to an electric system should increase reliability of a number of sub-systems, portend reduced long term maintenance costs and aids in the improvement of many components.

One example of the advantage of moving to an "electric aircraft" is to facilitate the move from hydraulic to electric braking. I think it's a great move.


How do you encode a radial to radial turn in ARINC-424?

So you fly the KEA transition like this:

Track RDL 265 KEA until crossing RDL 187 ATV or 27 DME KEA
Start a right turn and intercept RDL 192 ATV
Fly RDL192 until intercepting final

As egid said, RDL 187 ATV is a lead radial which is a radial used only to tell you when to start a turn.

As far as 20 DME ATV, it is actually only used for the PELAG transition so you can ignore it in this case.


Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)?

No, cg ahead of neutral point is stable, cg behind it is unstable.

Neutral point is the centre of pressure when both main wing and horizontal stabilizer fly at the same $C_L$. If the cg is just a bit ahead of neutral point, the tail still flies with positive lift. If it is ahead of the centre of pressure of the main wing, tail flies with negative lift.

The further forward the centre of gravity is, the bigger the difference between the coefficients of lift and therefore the stronger restoring force. But note that since it is second order negative feedback, the restoring force creates phugoid oscillation and if that is too fast, it will be difficult for the pilot to stop. So the stability has to be just right.

This is still true in canard configuration, but there the forward surface that flies at higher $C_L$ in stable configuration is the canard.


Does the FAA intend to discontinue domestic flight plans and switch to ICAO-only on October 1, 2015?

The link from your question paraphrases and links to “Flight Service Program Changes,” of which the April 13, 2015 7:49:56 AM EDT revision is reproduced below with added emphasis in boldface.

  Flight Service Program Changes

  Recognizing a shift in users’ preferences for automated services, the
  FAA is changing its Flight Service operation to make it more efficient
  and reduce costs. The agency will continue to maintain the highest
  level of safety.

  The changes come as general aviation pilots are transitioning from
  traditional Flight Service assistance to more automated and web-based
  tools to obtain services. Through the use of updated technology Flight
  Service is taking the opportunity to eliminate redundancies and
  underutilized services.

  “None of these changes will affect core flight service safety
  functions such as search and rescue, emergency services, Notice to
  Airmen (NOTAM) entry and dissemination, and pilot weather reports,”
  said Steven Villanueva, Deputy Director Flight Service, in the ATO's
  System Operations Service Unit. “We are phasing in the changes to ease
  the transition for users,” he added.

  On October 1, 2015, the FAA will consolidate Flight Watch services
  into routine flight services inflight frequencies to eliminate
  unnecessary duplication of service and provide greater convenience for
  pilots. These services provide inflight weather information to pilots.
  After that date, these services will be available on the same
  frequencies that pilots use to open and close flight plans and to
  receive updates on NOTAMs or Temporary Flight Restrictions (TFRs).
  Dedicated Flight Watch frequencies will be decommissioned.

  The FAA is proposing to phase out legacy Remote Airport Advisory
  Service. Seven of the airports do not meet the Agency's criteria for
  receiving advisory service. Flight Service is collaborating with our
  user groups on possible impacts and will be posting the proposed
  change in the Federal Register for public comment.

  The FAA is also proposing to implement flight plan filing for civil
  aircraft exclusively under the format used by the International Civil
  Aviation Organization (ICAO). Flight plans contain specific
  information relating to the proposed flight of an aircraft and
  controllers use them to provide air traffic services. Today pilots
  file flight plans in the U.S. under either the domestic or ICAO
  format. The use of one format will simplify the process and align U.S.
  flight plans within ICAO standards.

  General aviation pilots increasingly have turned to automation in
  recent years to file flight plans and receive pre-flight briefings.
  New technology such as ADS-B is providing more inflight options to
  pilots. Flight Service will incorporate the industry's newest
  technologies and reduce or eliminate other functions to create
  efficiencies and value. The changes to Flight Watch and RAA are the
  first in what is anticipated to be a series of right-sizing
  initiatives surrounding flight services provided to pilots.

  Send your comments, questions, and suggestions to us and share your
  thoughts and ideas.

  Flight Service National Airspace Initiative (FSNI) Frequently Asked
  Questions (FAQ)

The linked FAQ list contains a question similar to yours, quoted below including the boldface from the original. Note the first sub-bullet under Flight Plan Filing that states the intent but does not provide a specific date.

  What changes will be made in the Long Term?

  Long term changes are being worked with stakeholders. Increased usage
  of technology and automation and less reliance on human delivered
  services will be considered. These changes include:

  Emergency Frequencies — Pilots, when in distress and using an emergency frequency (121.5MHz), will be responded to by an Air Traffic
  Control (ATC) specialist with surveillance capability, improving the
  timeliness of emergency handling.
  Flight Plan Filing

  Increased automation that will provide a pilot numerous alternatives for flight plan filing, modification, activation and closure
  Transition to exclusive International Civil Aviation Organization (ICAO) format for civil aircraft

  Search And Rescue (SAR) — Increase automation to include Automated Activation and Closure of flight plans
  Relaying Instrument Flight Rules (IFR) Clearances — Flight Service is working with Air Traffic Control (ATC) to develop alternative solutions to enable pilots to talk directly to an air traffic control facility when requesting an IFR clearance.

See also the related Aviation.SE question Will the frequency 122.000 MHz no longer be used for Flight Watch after October 1st 2015?


What is the most accurate geometric plane for referencing terrestrial flight attitude?

There are a few definitions because there are a few ways to look at it.

The most general thing to say would be something along the lines of,

  An object's angles in pitch, roll and yaw from a set of chosen zero planes

It boils down to what you want to use as your point(s) of reference. Attitude indicators use the horizon and if you really want to talk about it even space ships have gimbals that measure attitude relative to a plane. Your definition assumes only attitudes of aircraft flying over the earth (using the surface as a reference plane) which is valid for most cases but not all.

Furthermore, attitude indicators are referenced to the horizon because it is an easy to find and generally a universal level plane with respect to the Earth's surface. In aircraft flight we care about the Earth's surface because that is where we want to get back to and subsequently not smack into in the process, so its convenient to reference to its plane. Furthermore at high altitudes the horizon is still the horizon; it simply becomes a space/Earth horizon as opposed to a sky/ground horizon.

Here is the view out of the SR-71's cockpit (@73,000ft, this is also the highest-flying non-experimental plane ever made) to illustrate the point.

(source)

To extend the frame of reference explanation, we can look at the Apollo rockets FDAI, which is a nice example of a 3-axis attitude indicator that was referenced to multiple planes instead of a single horizon. In this case the flight attitude was simply the angle from a chosen set of zero planes.

  The first one is called "Inertial Stellar". That means that the reference is a 3D system of coordinates which is fix relative to the stars. That does not mean that there's only one system of reference. In fact, there is a system of reference for almost every phase of the mission. Some are reused during the flight and. in order to be accessed multiple times, they are stored in the computer as "REFSMMAT", which stands for REference For Stable Member Matrix.

-- EDIT --

Since the root question has changed somewhat ill add to my answer. Generally speaking airplanes are oriented to the gravitational force below which is taken to be the "vertical". The vertical is often taken to be the tangent of the ground beneath it but since the ground may also not be level this is a generalization. There is a bit of real world discussion involved here since attitude indicators, the point of reference for an aircrafts attitude may not always be perfectly level to the ground. A pilot is mainly concerned with setting his 0 degree point with a pitch that will yield flight that does not increase or decrease in altitude. This is called "straight and level flight". In reality, due to the design of aircrafts this may in fact be a few degrees of pitch away from true 0 on the reference plane. Power settings will also effect this angle.


What is the purpose of the little eyebrow windows on the Boeing 737?

From Wikipedia: Boeing 737:

  Most 737 cockpits are equipped with "eyebrow windows" positioned above the main glareshield. Eyebrow windows were a feature of the original 707 and 727. They allowed for greater visibility in turns, and offered better sky views if navigating by stars. With modern avionics, they became redundant, and many pilots actually placed newspapers or other objects in them to block out sun glare. They were eliminated from the 737 cockpit design in 2004, although they are still installed at customer request. These windows are sometimes removed and plugged, usually during maintenance overhauls, and can be distinguished by the metal plug which differs from the smooth metal in later aircraft that were not originally fitted with the windows.

Boeing has also determined that removing the windows reduced the cockpit noise by about 3 decibels. The newer models have 10 vortex generators above the cockpit where the windows used to be. It also reduces maintenance costs.


Constant speed propeller blade pitch compared to fixed-pitch

I believe an Arrow operates the same as my Cardinal.

With low manifold pressure, there is not enough oil pressure to drive the prop into anything other than flattest configuration no matter where the prop control is at.
If you were take off with prop control not all the way in, then in theory the prop would start getting more twist as engine speed came up, and it would be like taking off in 4th gear or similar, when you want to start your acceleration in 1st gear and then shift up (think of how the RPMs bog down as you cycle the prop at 1700 to ensure the prop can twist).

So you start with the prop and controls all the way in to take off (balls to the wall - RPM, prop,  mixture, carb heat - or levers if so equipped).

Once you get to altitude, you can start bringing them back, equivalant to shifting up and taking your foot off the gas (and leaning the mixture for increased). RPMs are dialed down, prop control is dialed out to increase pitch.

Then when you come from altitude and prep for landing, you play the game of reducing RPM, decreasing pitch, reducing RPM, decreasing pitch, until eventually you are in the pattern, prop is all the way to support a go-around, and RPM controls all the power, same as taking off.


In which direction does air flow over a wing as observed from ground?

particle A moves up and left

No, up and right

  particle B moves down and left

Yes

  particle C moves down and left

Yes

The low pressure region first pulls in the particle A, so it initially moves in the opposite direction of the wing, and then the adverse gradient over the rear part of the wing will gradually accelerate it so it ends up moving a little in the direction of the aircraft. It also ends up moving down much faster than forward, which is how the lift can be much more than the drag.

On highly cambered wings the pressure may be higher than ambient everywhere on the underside, but most wings are not cambered that much and there is some reduction of pressure under them as well, so even the particles moving under the wing accelerate a bit to the right, opposite of the wing, and only then change to moving left along the flight direction.


Can anyone identify this biplane from a TV show?

My vote would be a Waco UPF-7 without the engine cowl:

Source: American Power Museum
Looks like the one in the OP post has a different engine in it or at least a relocated cooler, but the struts and windscreen design look very similar. (Struts on the right side are distorted by perspective, look at the ones on the left). Gear bracing is also pretty close.


Would a roll cage plus a safety harness make a small GA aircraft accident more survivable?

Short answer: Yes, a better harness would help in some cases, but will achieve little all by itself. A roll cage might help in a few cases, too, but would be much less effective per pound.

Your observation is correct: Race car drivers enjoy a lot more protection than GA pilots. This is a rather recent development; 30 years ago the chances of surviving a crash were equally low for both.

There are two factors which stand out in a GA crash:

Peak accelerations which will lead to internal lesions like the rupture of the coronary blood vessels, and
Physical injury by sharp objects entering the occupant's space, including flailing limbs being hurt by sharp objects in the cabin.

Both factors were also killers for race car drivers, and they were defused with improved design methods. A safety cage is not just a frame around the driver: Any structural failure must occur in such a way that the structure fails away from the driver. In addition, the whole car body is designed to crumple at roughly the same force over its length in order to use the available crumpling zone as well as possible. This allows to decelerate the safety cage at a constant rate, so that peak loads can be avoided. A third factor is a much better restraint system, of which the HANS device is only one aspect. Using five-point harnesses would already help to avoid "submarining", a process where the pelvis slides under the lower restraint, causing the harness to press against the intestines. The image below shows the regular pelvis location and body contours as a dashed outline and the displaced pelvis and spine as a solid line. The displaced body is the shaded area.

None of this was considered at the time when today's GA aircraft were developed and the regulations were set up. At that time the prevailing opinion was that nothing could be done in a crash, so all efforts focussed on avoiding crashes in the first place. Adding only parts of the whole protection packet will be of little help - only their combination can effectively protect the race car driver. Even the 5-point harness needs an adequate seat and properly located mounting points to be effective.

The US army has collected all research up to the early Eighties in a five volume work called the Army Aircraft Crash Survival Guide (pdf!). It's intention is to improve the crash survivability for helicopter occupants and Army aviators, and much can be transferred to General Aviation. If you look for hard stats, you will find at least lots of citations there. I remember in particular a study from the Fifties where statistics showed that J-3 pilots would mostly be killed in a crash, while the rear occupant would show only minor injuries. In the study, Pipers filled with test dummies were put on rails and crashed into an earth wall. Funny stuff.

A J-3 in Army livery. Then it was called O-59 or L-4.


How would an Inverted Delta Wing planform behave, Straight part forward? Are there wings shaped this way?

To answer your second question first: Yes, these are mainly used for ground-effect craft. The idea is to have positive angle of attack, a tapered wing and a trailing edge with constant, low distance from the ground. This question covers their sweep angle.

The RFB X-114 ground-effect craft. Forward is right.

The wing would behave just like an ordinary, forward swept, low aspect ratio wing. It would have a small lift curve slope and a lousy stall behavior, because the low tip chord will make the tips stall first, causing the whole wing to roll. Unfortunately, the low leading edge sweep will prevent vortex lift, which is a typical feature of regular delta wings, to develop, so the stall angle will be only little higher than that of a straight wing with the same aspect ratio.

Manta rays are more diamond-shaped, so their "wing" has positive leading edge sweep and almost zero quarter-chord sweep. Their stall behavior would also be nasty if they were rigid, but the wings are actually fins and used for propulsion, not lift, so stall does not mean that the fish plunges to the ground. The picture above shows an Atlantic manta and was shamelessly copied from Wikipedia.


When can I log PIC time in a new category of aircraft?

Short answer: you can log ASEL PIC time only while flying solo, not while flying with an instructor.

You seem to be mixing up a few regulations: you quoted 61.63 but the text is actually from 61.31. However, since neither of those regulations are about logging time anyway, they aren't directly relevant.

The rules on logging PIC time are in 61.51:

  (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;

  (ii) When the pilot is the sole occupant in the aircraft;

  [...]

(e)(i) says that you can log PIC time as sole manipulator only if you're rated in the aircraft. In your case, you aren't rated for ASEL (yet!) so you can't log ASEL PIC time while flying with an instructor, even if only you touch the controls. This is probably what the school is referring to.

However, (e)(ii) says that a private pilot who is the only person on board the aircraft may log PIC time, without any additional requirements (although other regulations do require an endorsement in order to solo, as you said).

That means, you can log solo ASEL time as PIC, but not training time with an instructor.


How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate?

TLDR

How will ADS-B actually reliably replace primary radar?

It will not.

Will GPS have become mandatory by 2020?

Strictly speaking no, effectively yes.

There are so many problems with that quote from FR24 that I don't know where to start.
Let's take it apart and analyse it sentence by sentence.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though

Totally irrelevant. An ADS-B receiver allows reception of ADS-B signals from other aircraft. These can be used for example to create a Cockpit Display of Traffic Information (CDTI) or augment TCAS. The number of aircraft with this so called ADS-B IN is increasing, but the equipage percentage is probably below 2% at the moment.
If you are concerned with using ADS-B operationally for ATC then airborne receivers are not of interest. Instead you should be looking at the transmitter side.

[ADS-B receivers ...] will become mandatory for most aircraft around the world by
2020.

ADS-B receivers are not mandatory in any foreseen regulation yet. ADS-B transmitters are / will become mandatory in many parts of the world. The EU and USA rules mandate ADS-B in 2020.

When mandatory, ADS-B will replace primary radar

ADS-B is not intended to replace primary radar. As a new addition to the surveillance technology mix, it will not replace any other technology in the short term.
ADS-B is a form of cooperative surveillance, meaning that the aircraft or vehicle that is supposed to be tracked needs to cooperate by transmitting the ADS-B signal. Primary Surveillance Radar (PSR) on the other hand is a non-cooperative surveillance technology; it works by detecting signals reflecting off the target of interest. PSR can detect aircraft that do not transmit any signals. Since PSR and ADS-B are fundamentally different in their working, they have fundamentally different use cases. ADS-B will not replace primary radar and never was intended to.

will replace primary radar as the primary surveillance method used by ATC

Primary Surveillance Radar is not the primary surveillance method used by ATC in many parts of the world. Since PSR only gives azimuth and range measurements essential information like aircraft identification and altitude is missing. Therefore Secondary Surveillance Radar (SSR) is the primary surveillance method used for ATC since it provides altitude and identity in addition to azimuth and range measurements.
Secondary radar, like ADS-B is a form of cooperative surveillance; it relies on the aircraft replying to an interrogation from the radar. ADS-B can and will replace a number of secondary radars, but not all of them.

Now for the rest of your question.

it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar.

Indeed a large number of aircraft do transmit their position on ADS-B that is based on a non-GPS source, usually from the INS. INS can not provide the accuracy and integrity that GPS can provide and therefore you see large position errors on FR24. The fact that the position is not to be trusted is reported in the ADS-B message in the quality indicators, but unfortunately FR24 doesn't use this information.
All the ADS-B mandates require that the system is able to provide high integrity data, which effectively means that GPS is required as position source, although theoretically other means are possible as well. Therefore in 2020 all aircraft subject to the various ADS-B regulations will have to be GPS equipped.

Of course, the ATC doesn't need millimeter precision, or even an
accurate position within a kilometer,

ATC does not need millimeter precision (GPS wouldn't provide that anyway) but they do need position accuracy within a kilometre and sometimes within about 10 meters (e.g. runway entry stop bar monitoring). ADS-B  with appropriate GPS source will be able to provide that.


Can airplane performance be increased by cruising "on step"?

The Pilot Training Manual For The C-54 Skymaster has this to say about it:

The manual is available at Archive.org


Has an airplane ever been launched from a submarine?

Yes,

The HMS M2 had a single seaplane aboard that it was capable of launching.

  Her 12-inch gun was removed, replaced by a small aircraft hangar, the
  work being completed in 1927. This could carry a small Parnall Peto
  seaplane, specially designed for the M2, which, once its wings had
  been unfolded, could be lowered onto the sea alongside by a derrick
  for take off. On landing, the aircraft was hoisted back onto the deck
  and replaced into the hangar.

...

  In October 1928, a hydraulic aircraft catapult was fitted, to enable the seaplane to take off directly from the deck.

The I-400 class submarine was also capable of aircraft launches.

  The type name was shortened to Toku-gata Sensuikan (特型潜水艦 Special Type
  Submarine). They were submarine aircraft carriers able to carry three
  Aichi M6A Seiran aircraft underwater to their destinations. They were
  designed to surface, launch their planes, then quickly dive again
  before they were discovered.

There are some more info blurbs here worth checking out about various submarine/aircraft creations.


Am I likely to get permission at a Private Airport?

Can't hurt to ask!
I don't know the specifics of the situation, but I would be willing to bet that your chances are better than you think.  If it was one farmer with a grass strip and no accommodations for visiting aircraft you might be out of luck, but from what you describe it doesn't sound like one extra airplane on the ground for a few hours is going to make that much of a difference.
In fact, it might be that Fred (or whoever) will also be visiting friends for Labor Day and might be willing to let you use his ramp.
They can't say yes if you don't ask.


Is the Dassault "ministop" related to the Dunlop Maxaret ABS?

This 1953 issue of Flight International mentions the Messier Ministop, it seems it was shown publicly for the first time at the Salon de Paris, July 1953:

  A particularly interesting development was the new anti-skid device which is being marketed under the name of Ministop. The device consists of an accelerometer—a free mass rotating with the wheel—together with a centrifugal governor and an associated electrical circuit. The general principle of operation is very similar to that of both the Dunlop Maxaret and the Westinghouse Decelostat ...

From that wording I'd say Ministop is unlikely to be a license-built version of Maxaret.


How to reproduce NASA/Langley airfoils for general aviation with long chord lengths?

You are right, such a thick trailing edge is not right. It will add pressure drag unnecessarily – quick fix: Just taper the last 10 or 20 percent of the airfoil such that a thickness remains which is sufficient for structural reasons. You don't want a trailing edge that is as sharp as a knife edge. It has no stiffness left, so it will warp and will easily be damaged.

The more elaborate way: Do as if there would be a hinge at the nose and you close the two sides, just like scissors. That will leave the contour unchanged. The "hinge" location is ideally the center of the nose radius.

Tapering the last 10 or 20% will steepen the pressure rise there and will let the airfoil stall a tiny fraction of a degree earlier than with the unchanged surface contour.


Why does the attitude indicator gyro not precess the same way that a directional gyro does?

The self erection device in the attitude gyro that levels it when you start up on the ground also keeps it erect (its axis pointed at the center of the earth) when precession tends to send it off kilter during flight.
Air driven gyros use a pendulous vane arrangement that controls the exhaust ports for the air being sucked through, and which always work to keep the gyro aligned with vertical when stabilized in non-accelerated flight.  This system is imperfect, because the pendulous control system is influenced by accelerations and causes its own errors, but works well enough for light aircraft.


It is not part of the standard phraseology by any means, but it is fairly common, as is a very quick pleasantry when changing frequency.

  [station name], Good morning, G-ABCD, [request]

and

  G-ABCD, changing to [other station] thanks for your service.

The thing to remember is to not choke up a busy station with overuse of non-standard comms, and therefore to remember when it might be inappropriate to stray from the standards.


Which kind of identification and authentication is used to secure an airplane?

Most smaller propeller planes have nothing. When you're in you can fly away. This seems to be the same for those private jets and older Boeings. I don't fly planes, but I've seen that Airplane Repo program on Discovery, and they just get in and fly away. How this works with the newest planes - no idea.


Was this bumpy Boeing 777-300 landing caused by windshear?

Some comments state this was a wind-shear, is that correct?

Looking at the windsock which appears in the right bottom corner at 0:12 and remains visible for about 4 seconds, you can tell there is little wind, not what you would associate with wind shear.

  I want to ask here, what caused this very bumpy landing?

It is always hard to judge the actions of the pilots without knowing what was going on in the cockpit, and what control inputs were given. But by looking at the position of the control surfaces I am fairly sure that this is a case of Pilot Induced Oscillations. I.e. the pilot tries to correct a disturbance but his efforts cause the aircraft to overshoot to the other side of the desired attitude, leading to an increasing series of corrections in opposite directions.

It seems to me that the flare is a tad early, causing the aircraft to float over the runway. At the same time several cycles of alternating roll inputs are given, causing increasing oscillations. I could distinguish at least 3 full cycles before touchdown. Every cycle is a bit stronger than the previous one.

The last right-wind-down command before touchdown is strong enough for all the starboard spoiler to deflect. This, combined with the low speed due to early flare causes the right wing to loose most of its lift. The aircraft touches down hard on its right gear first. At the moment of touchdown, the roll command is strongly left wing down as can be seen from the spoiler extension on the left wing. With almost no lift on the left side and standing on the right gear, the aircraft rapidly rolls to the left.

The left roll is then counteracted again with right ailerons up but too late; the right gear lifts of again. When then right gear touches for the second time, still a right wing down command is given, causing the right roll to continue.

This process of chasing the roll is continued and mixed with similar behaviour in pitch.

Command right wing down, slight spoiler deflection

Command left wing down, slight spoiler deflection

Command right wing down, slight spoiler deflection

Command left wing down, slight spoiler deflection

Command right wing down, right spoiler deflection.

Touch right gear down,  left wing down, left spoiler deflection. This causes a rapid left roll.

Touch left gear down, right gear looses contact, right side spoiler deflection. This causes a rapid right roll.

Halfway the roll to the right, all main wheels are on the ground. Instead of damping the roll by left deflection, a right roll is still commanded.

This continues down the runway, also combined with pitch oscillations. Every time the pilot corrects, he is essentially too late to damp the oscillations.

  Is required by rule to make a maintenance check after such a landing?

If a hard landing is made, the aircraft will detect that by itself and will send a report to maintenance if a hard landing check is required. See this published patent application.


What minimum radio communication equipment is required in a homebuilt aircraft in India?

According to DGCA Rule 63 you need a radio onboard to operate both VFR and IFR

  63. Aircraft for which radio apparatus is obligatory-

  (1). All aircraft registered in India and required to be operated in accordance with VFR/IFR condition shall be provided with communication equipment which
  is capable of conducting two-way communication at all times with those
  aeronautical stations and on those frequencies as prescribed by the
  appropriate authority. The communication equipment so provided shall
  be of a type approved by the airworthiness authority of the country of
  manufacture of aircraft and acceptable to the Director General.

  (2).   An aircraft shall be provided with navigation equipment which will enable it to proceed :

(a)   in accordance with the flight plan; and

(b)   in accordance with the requirement of air traffic services, except
      when, if not so precluded by the appropriate authority, navigation for
      flights under the visual flight rules is accomplished by visual
      reference to landmarks at least every 110 kms (60 Nautical Miles).

DGCA Rule 9 also adds some stipulations to the operation of the radio its self

  9. Radio-telegraph apparatus -

  (1) No person shall operate radio transmitting apparatus in any aircraft registered in India unless he
  holds a licence of the type required by the provisions of Part V in
  respect of aircraft required by these rules to carry radio-telegraph
  or radio-telephone apparatus, as the case may be, and issued in
  accordance with those provisions.

  (2) Radio-transmitting apparatus carried in aircraft shall be operated
  under the conditions defined by the International Tele-communication
  Convention (Madrid, 1932) and the general radio-communication
  regulations annexed thereto as far as these apply and shall be
  operated only during the hours in which such operation is permitted by
  the Central Government.

  (3) Aircraft registered in India and required by these rules to carry
  radio-telegraph apparatus shall be fitted with such apparatus in
  accordance with the provisions of Part VII.

  (4) Nothing in this rule shall exempt any person from those provisions
  of the Indian Telegraph Act, 1885, and the rules made thereunder which
  require that radio-telegraph apparatus shall be licensed.

You can find the rest of the rules here. I went through them briefly and cant find a whole lot on certifying home built aircraft. Or what rules they would actually fly under. There is a short list of things related to actually getting an airworthiness cert here but it does not touch on VFR/IFR classifications related to that.

As for the safety aspect of it, that depends on your countries aviation infrastructure and if carrying a radio benefits you. If no one is listening on the other end then its not going to add much but weight to the aircraft. From a regulatory standpoint you may need it to enter certain airspaces (you may also need a transponder for that as well) I cant find a whole lot on the DGCA website about airspace regulations so its hard to say exactly when you would need one. I can only compare it to places I have flown and here in the US while you don't legally need a radio in uncontrolled airspace its still a good idea to have one and to use it as you should (CTAF announcements, flight following if you like, 121.5 if you need it etc.)


Why do aircraft (namely military) need/use formation lights?

Formation lights are used so that aircraft can fly close formation at night.  They are typically luminescent strips positioned so that an aircraft can be in the desired position (i.e. fingertip formation) and have references fore-aft, up-down, and near-far similar to references as used for daytime formation flying.  Something along the lines of, "keep the position light just below the formation light (strip) under the cockpit and the aft fuselage light (strip) just touching the wing hardpoint, and you're in the right position.  If the position light goes farther below the strip, you're getting high; if the hardpoint is sliding back into the aft light you're getting close, etc."

Obviously these references are very specific to each individual aircraft.

You can see the formation lights in this picture of an F-14; the dots are actual position lights, while the strips are the formation lights.  Source

As to why only military aircraft have them... how often have you seen or heard of civil aircraft flying formation at night?  Yeah, me neither.  There are very, very few non-military reasons for formation flight, and essentially none that require it to be done at night.


What types of aircraft perform military airlift operations?

There is no single "key player". It depends on the task. You need big, long-haul transporters like the C-5 or the An-124 to move large amounts of troop and equipment quickly. This is called strategic airlifting and is done with turbofan-powered aircraft which are similar to airliners. But they need prepared and secured air strips of considerable length (the NATO standard airstrip is 8000ft long).And then you need tactical transporters which can fly as close to the action as possible, and they must have STOL capabilities and defensive equipment to survive their missions. This requires mostly turboprop powered planes like the C-130, the C-160 or the An-12 for big equipment or the CASA 235 or the An-26 for smaller units or parts. The C-17 was meant to be a strategic airlifter with some tactical capabilities, but it cannot replace the smaller turboprops entirely. The A-400 and the An-70 are large tactical transports with some strategic capabilities, but this comes at the expense of usability on unprepared or improvised airstrips.
And then you need helicopters for the missions which require the highest amount of flexibility and closeness to the action.
All three depend on the other, and only together they become effective. That is why I cannot give you a key player.

Key characteristics of strategic transporters:

turbofan engines
swept wings, high flight Mach number and range
high wing, so the fuselage is close to the ground for easy loading and unloading
large nose doors and/or ramp in rear fuselage.

Key characteristics of tactical transporters:

turboprop engines (mostly)
straight wings with extensive high-lift devices for STOL capabilities
moderate flight Mach number and range
high wing, so the fuselage is close to the ground for easy loading and unloading, low loading of landing gear for soft ground capabilities
ramp in rear fuselage.

Key characteristics of helicopters

that whirly thing on top


What is this airport outside of Cairo, Egypt?

The two runways to the east form an existing air force base called Wadi Al Jandali AFB. The new western runway and terminal form a new civilian airport called Katameya/Al-Qatameya Airport. It is also known as Capital International Airport.

(Source)

The airport is set to serve a new administrative capital that is yet to be completed.

Another new airport, Sphinx Airport, is west of Cairo. It is also adjacent to an existing AFB called Cairo West.

  Al-Qatameya will serve the new administrative capital project and the Cairo West airport will receive charter flights, along with Cairo International Airport to ease pressure.

They are not operational in the sense of scheduled air traffic. Google Earth shows what can only be the new Sphinx Airport (west of Cairo West AFB):

30°06'15.42" N  30°54'03.38" E


Why did the propellers not fail to the feathered position in the Atlantic Southeast Airlines Flight 2311 Crash?

You can find the full text of the NTSB investigation here and according to that report (bolded for emphasis),

  Examinations of the left propeller components indicated a propeller
  blade angle of about 3 degrees at impact. This position was based upon
  the position of the pitchlock acme screw. The left PCU ballscrew
  position indicated that the PCU had commanded a blade angle of 79.2
  degrees. The discrepancy between the ballscrew position and the
  position of the pitchlock acme screw is a strong indication that a
  disconnect between these two components occurred prior to impact and
  that the left propeller had achieved an uncommanded blade angle below
  the normal flight range.

And later in the report

  Using measurements and the inspection procedures for the quill and
  transfer tube of the Hamilton Standard Alert Service Bulletin, it was
  determined that the left PCU quill spline was worn to the extent that
  its gear teeth did not engage the transfer tube spline. In addition,
  the test cell and flight tests showed that the propeller blade angle
  could not be controlled by the PCU with a disengaged transfer tube. In
  the test cell, the blade angle moved toward high pitch; however, the
  propeller was operating at zero airspeed and did not experience normal
  flight loads. In contrast, the flight tests showed that the blade
  angle would move toward low pitch with a disengaged transfer tube. The
  blade characteristics indicate that centrifugal and aerodynamic
  twisting moments tend to move the blades toward low pitch.

  The Safety
  Board believes that the worn quill on the left engine PCU became
  disengaged from the transfer tube prior to the loss of control of the
  airplane during the approach to Brunswick. Moreover, the propeller
  blades moved to a low angle, resulting in an asymmetric lift and drag
  condition that exceeded the capability of the pilots to counteract
  with the airplane controls available.

In simple terms the unit that controls the propellor pitch experienced a failure from worn out components that lead to a case where the propellor did not completely feather. The specific part seems to be the PCU quill (to answer your question directly).

Even later in the report they discuss why the failure was not really covered by the testing originally. The testing was not necessarily incorrect as much as it may have been incomplete and not foreseen this issue with the quill.

  The investigation found that wear of the quill was not considered
  during the certification of the propeller system because of the very
  light torque loading on the quill during flight. Service history of
  the PCU quill prior to the introduction of the titanium-nitrided
  transfer tube indicted that quill spline wear was not a problem.
  Additionally, the manufacturer provided an analysis during
  certification indicating that even in the event of a failure, the
  propeller would either drift into the feathered position or maintain
  the blade angle present when the failure occurred. However, the
  accident involving flight 2311 and the subsequent investigation
  have determined that these assumptions, though originally supported by
  numerous engineering evaluations and manufacturing experience, are
  invalid and that there are single failure modes that could result in
  an uncommanded propeller blade angles below flight idle.


How much freight can a Boeing 737-800 NG carry if the majority of seats are removed?

At 160 seats, a -800 carries 16 tons of people in the cabin, plus freight/luggage in the belly, plus fuel. At 180 seats, that becomes 18 tons of pax. If you assume one 50# suitcase per person, add a quarter of that again, so 20 to 22.5 tons.  Much more than that you're probably starting to run up against the Max Zero Fuel Weight limit, although I don't have the right charts in front of me to give an exact answer there.

If the flight is long enough, max takeoff weight becomes an issue, as your fuel load starts to cut into your lift. Obviously lots of factors go into how much fuel a flight requires; "today's" winds & temperature & destination weather (alternate required? How far away?) will change how much weight today's flight can take.

Ballparking it with back-of-the-envelope math, anything up to about 3-4 hours, you can probably get those weights I mentioned above (i.e. 22 tons or so). Much more than that, I'd expect max takeoff weight to limit you, so more distance = less payload.

If the legs were 5000km between refueling stops, then you're well into the limit being max takeoff weight, rather than what the jet could carry on a short-haul.

Depending on the nature of the cargo, the limits on floor loading in the cabin + the cargo compartments may be limiting, or volume may be limiting. If you're hauling pingpong balls, it's all about volume. If hauling gold, floor weight capacity & total weight will be more limiting. If it's all rucksacks, then you can probably leave the seats in & load around them, and only total weight will play.

Disclaimer, doing this with no references in front of me, and I may have a number off in there. When there is a specific need to haul "this" from A to B, the guys with the charts & load plans come up with a far more specific answer. The above is simply a rough ballpark.


Do turbine engines on multi-engine aircraft rotate in opposite directions to offset torque?

Torque is less of a problem due to the effective roll damping of a wing, but gyro effects and prop wash are important. Gyro effects first became an issue with rotary engines in WW I. A rotary engine has its crankshaft fixed to the airplane, and both the cylinder block and the propeller rotate. This gives better cooling at low speed and produces a flywheel effect, so the engine runs more smoothly. But when you yaw, the gyro effect pitches the aircraft up or down, so any precise maneuvering becomes very hard.

With the increasing engine power in 1916 and 1917, this effect became so severe that geared engines were developed where the cylinders rotate in one direction and the propeller in the opposite direction. As a consequence, the propeller had only half the RPM in air as it had with the cylinder block. This gave great propeller efficiency, but also big propeller diameters, so airplanes with those engines needed a high landing gear. Below is a picture of a Roland D XVI with a Siemens & Halske III counter-rotating rotary engine from 1918 (source). This was an excellent fighter aircraft for its time with almost no gyro coupling.

Today, high power propeller aircraft tend to use identical engines but left- and right-handed gearboxes so the propellers run in both directions. This is less due to gyro effects and mostly to produce benign stall characteristics. The prop wash of a propeller increases the local angle of attack on the wing on one side and decreases it on the other side, so the wing will stall first on the side with bigger angle of attack. If this side is always to the right of the propellers, the aircraft will roll right in a stall. In the WW II period, a number of multi-engined aircraft used left-hand and right-hand turning engines in order to have the prop wash cancel out.

With jets, the rotating inertias are much smaller because the diameters are smaller. But there is one exception: The Bristol-Siddeley Pegasus engine of the Kestrel, Harrier and AV-8B jump jets needs to have the low pressure spool run in opposite direction of the high pressure spool to balance its gyro effects. If that would not be the case, a yawing motion would produce a pitching motion, and vice versa. When you sit on a jet, the thrust of which is equivalent to your weight, tilting this jet fore or aft only slightly will produce a quick shift of your location, so any maneuvering in hover will become extremely hard.

USMC AV-8B Harrier in hover (picture source)


What terminologies are used when ATC instructs a pilot to stop or proceed with his transmission?

If a pilot forgets to release his mic button, you can't talk to him. VHF radio is simplex, so only one station can transmit at the time. If more than one station transmits at the same time, the signal will sound garbled. As a result, there is no standard phraseology to use in such a situation. All you can do is wait for the pilot to discover the issue. See this question for additional info.

CPDLC (text based communication between ATC and the cockpit) does have a "CHECK STUCK MIC" message that can be sent to aircraft on a frequency with a stuck mic. But CPDLC is not globally available, and only used in the en-route environment.

When someone is waiting to transmit, ATC will use the phrase "Go ahead" to indicate that they can start their call. This will not be very useful in the case of a stuck mic, for the reason described above, but the "Go ahead" phrase is often used in other cases where ATC has told a pilot to wait ("Standby"), typically because ATC is busy on the phone or another, more important radio call has to be made first.


Yes, at least for flights that are not in the "vicinity" of the airport.
FAA CFR 91.103

Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include--
(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;

You are not required to call Flight Services for a weather brief, but you are required to get the weather reports (which can be called a personal brief). If you do all flight services and happen to have an accident, then your accident will contain the details about your call.


What you probably are looking for is the Biot-Savart law.

This is originally describing the magnetic field generated by an electric current; however, it was soon found that it equally well describes the induced speeds caused by a vortex. The vortex is the bound vortex of the wing plus the free vortices trailing behind it. Integration over their length will give the induced speed at a given point. This speed is proportional to the inverse of the distance to the vortex squared. The vortex strength depends on air speed and span loading; the proportionality being with the inverse of flight speed, aircraft mass and the inverse of wingspan squared.

There is also a dependency of downwash speed with the inverse of air density at the altitude where the glider flies, but this will be canceled out when the effect at ground level is calculated. What is left is an inverse dependency on the local air density at ground level.

Note that you will only get the induced speed, not the pressure from the Biot-Savart law. In order to calculate the pressure increase, you need to add the equation for the pressure rise of the downwash interfering with the ground. This pressure is proportional to the downwash speed squared and air density, so now all density effects are canceled out and you will be left with those factors for the pressure rise from an aircraft flying overhead:

the square of the induced downwash speed

which depends in turn on:

the inverse of height squared
aircraft mass
the inverse of wingspan squared
the inverse of flight speed

Note also that this does not consider viscous effects.

In order to know the limit of detectability, you also need to know the sensitivity of your measuring equipment.


I am fairly sure that these are the fuel hatches. They are placed right over the wing and main fuselage fuel tanks. (The hatches are where the fuel is put into the craft, not drawn out for sampling). During ground fueling, the hatches are used to refill the tanks.


The Blackburn Ripon -- a British carrier-based torpedo bomber -- is such example (first flight 1926):

Source: aviadejavu.ru via Flight

Source: Imperial War Museums -- seen here landing on HMS Furious

Earliest mentions I found -- not necessarily carrier-based, and not necessarily real(?):

Source: Flight -- 1911

Source: Flight -- folding wings on the aerial-saw concept from 1915 (click image to view)


What are advantages and disadvantages of Rotax and Lycoming engine designs?

The biggest difference between these two engine designs is that the rotax contains a reduction gearbox which allows it to turn faster and hence develop more power per cubic inch of displacement, and the lycoming is a direct drive engine limited to a rotating speed that maintains the  prop tips subsonic. This gives the rotax better power-to-weight ratio  but causes it to run hotter and shortens its time-between-overhauls. In addition, the gearbox itself requires overhauling which adds to the overhaul costs. In practical terms, air-cooled & geared engines for GA aircraft have not proven as popular as direct-drive engines in similar applications; in fact it is common to replace geared engines with direct drive engines with larger displacement to improve reliability and reduce rebuild costs.


Why can't I build an entire jet engine out of carbon fiber?

No, you can not. Carbon fibre composites always contain a matrix component which serves different functionalities, but (in a micro nutshell) bonds and keeps the fibres in place. What you get is a very complex material with lots of lightweight potential, but also very different property profiles compared to metals.

Most matrix materials (thermoset, thermoplastics) will not work up to engine (combustion chamber) temperatures and either degrade/burn or melt/vaporize/burn long before reaching even a meagre few hundred Kelvin (say, 500K). In fact, carbon fibres will not work as well in an oxygenated high-temperature environment, and degrade/burn up as well.

One composite material that works and is used already in engines are ceramic matrix composites, like (amongst others) e.g. C/C-SiC structures, but those are very special animals (and even there the carbon fibre parts are susceptible to burn). Especially in terms of mechanical properties, these cannot compete with superalloys.

But also for many other, ambient temperature parts, metals are much better suited. Think of example of a ball bearing - doing that completely in composite will not work really well (if at all).


What is the reason for this route while flying from USA over Cuba?

There are two large warning areas there, W-465 A and B. Both are active "intermittently" up to 70,000ft. It seems likely that the route was taken to avoid those areas.


What does the Airspeed indicator show when your pitot tube is blocked?

Depends on how much blockage. If fully blocked, as in air tight, the pitot side becomes like the sealed aneroid side of an altimeter and it only reacts to changes in static pressure, indicating the difference between ambient static and the pressure trapped in the pitot side.

If the blockage is not air tight, like you usually get with bugs, somewhere between zero and actual, in proportion to the blockage.  If the blockage is something that can move around, like a slug of water in the pitot line, you get wild and crazy fluctuations.


What does it mean when there is no ceiling or "CLR" in an airport METAR?

Vertical Visibility with a number means that cloud layers cannot be determined because of haze, fog or mist, instead it's a measure of how far up the weather station can see. Because its sensors cannot penetrate further the station cannot tell you whether there's clouds above that level or not, but it does say in this case that there are no clouds below 1000 feet.

You may see VV/// sometimes, this means cloud cannot be measured. If fog, mist or haze and poor visibility is reported as well then it's likely that the fog is so thick the sensors cannot penetrate it, if on the other hand visibility is good with no fog or haze it's likely a sensor malfunction.


Why did the Boeing Max operated by Lion Air disintegrate upon crashing in the sea from a low altitude?

Because of a high speed impact

The official accident report published by the Komite Nasional Keselamatan Transportasi (KNKT) shows the data from the FDR (flight data recorder) for the accident flight. This is the altitude and airspeed it recorded:

As you can see, the aircraft reached an airspeed of around 450 kt before the FDR stopped recording (the Vmo of the 737 MAX is 340 kt according to b737.org). The altitude was around 6000 ft, which I would not really call "low altitude". Also note how fast the altitude dropped towards the end: the aircraft went from about 5000 ft to impact in only around 22 seconds resulting in descent rates in excess of 10,000 ft/min.

The report further says:

  The recovered wreckage identified consisted of, but was not limited to, parts from
  the left and right engines, both main landing gears, parts of the empennage, parts of
  the forward and aft left passenger doors, parts of the wings, a flight crew oxygen
  bottle, and parts of a circuit breaker panel. The damage to the aircraft suggested a
  high energy impact.

So the primary reason for the disintegration was the high speed at the time of the impact. No commercial airliner could possibly survive this. Have a look at this car crash test at only 200 km/h:

(HIGHSPEED CRASH TEST - 200 KM/H Ford Focus vs. Concrete Block on YouTube)

The car is completely destroyed by the impact on a concrete block. As Zeiss Ikon pointed out in the comments, "at even 100 kt, water is harder than concrete". And 450 kt is over 800 km/h, meaning 4 times faster (i.e. $ 4^2 = 16 $ times higher kinetic energy). Nothing that can still fly could possibly be built to survive this.


Can I log time in a Cri-Cri twin-engine aircraft?

What a positively delightful looking aircraft!

I see no reason you couldn't log time in this as multi-engine time - it clearly has two engines, with two separate sets of controls (at least throttles, from what I can see).

Similarly I see no reason you couldn't fly it IFR as an experimental aircraft, provided it's "properly equipped" (consult the FARs for all the details on what you need), passes an IFR pitot/static test, and has appropriate radios (Transponder which you'd likely need anyway, Com, and Nav).
Whether you'd want to take it up in real weather is another matter though :-)

For that matter if you want GPS you could certainly have it without much weight penalty: throw in a Garmin GNS 430W and cover the Nav/Com and GPS in one ~7-8lb box.  Whether or not that kind of investment in what's basically a single-seat super-ultralight is "worth it" is a judgment call only you could make, but it definitely seems possible.


What are the pros and cons of scimitar propellers?

The biggest benefit of a scimitar propeller is the higher operating Mach number that can be achieved. Tip sweep delays the onset of shocks on the propeller and reduces drag at high subsonic speed. The noise reduction is achieved by reducing shocks on the blade tips at the same tip Mach number - supersonic props are known to be extremely loud.

The major disadvantage is the torsional moment created by the swept back tip. This can be somewhat mitigated by sweeping the inner blade forward which results in the typical scimitar shape. Still, in-blade torsion moments mean that the blade has to be built such that the deformation in flight will result in the desired blade twist. The use of stiff carbon fiber allows to keep this pre-twisting low.

That scimitar propellers have a high blade count is a result of their high operating speed: To keep the tip Mach number low, the diameter must be limited which increases the load on the single blade. Flying fast needs a lot of thrust, so the activity factor (power absorbed by the propeller per blade area) is high. To reduce the local Mach number on the blade, both chord and blade count are increased while the relative thickness of the blade's airfoil is reduced when compared to a conventional propeller. This results in a lower operating lift coefficient on the blades and lower supervelocities which again helps to increase the operating Mach number.


To determine wing loading, do you need to subtract the weight of the wing from the gross weight?

No, you don't subtract any weight from gross when determining wing loading.

In airliners wings can be 40% of empty weight which is often about half of gross, so it would make a 20% difference in the loading calculation.


Based on PANS-OPS, what is the earliest turning point that can be used in a missed approach?

I can not figure out the earliest point where turning is allowed to be initiated, during a missed approach.

It is up to the designer. Nothing in PANS-OPS prohibits turns before MAPt but in that case an additional area extending from the earliest FAF needs to be protected as shown in figure I-4-6-14 PANS-OPS Vol. II.
Because early turns are not always feasible, advisable or convenient, we often disallow turns before MAPt, in which case a note to the effect will be added on the profile view of the approach chart. Any turns >15° will then be executed in the final missed approach area (Vol. II I-4-6-10 6.4.6.3.1).


Will a diagnosis of psychosis ban me from getting my recreational pilots license or my private pilots license?

Well, according to the CAA:

Class 1
(a)  Psychotic disorder
A history of, or the occurrence of, a functional psychotic disorder is disqualifying unless a cause can be unequivocally identified as one which is transient, has ceased and will not recur.
Class 2
Psychotic disorder
A history of, or the occurrence of, a functional psychotic disorder is disqualifying unless in certain rare cases a cause can be unequivocally identified as one which is transient, has ceased and will not recur.

Discussion on another forum suggests that generally speaking they will want the pilot to be off all psychotropic medication for a period of time (1-2 years) without any recurrences but they will generally allow a pilot to be on medication for depression.
I would recommend you work with an appropriate mental health expert in resolving your condition, so that you can be sure future episodes will not recur, you can then hopefully ascertain to yourself and to your medical examiner that your episode was transient, so that you can continue your pursuit of your PPL.
Remember in the worst case scenario (your condition does not pass), even if you cannot get a medical, you can still fly a plane. You just can’t solo. You can fly with a flight instructor or a certificated pilot. And you can fly a single seat ultralight with no certification. So you will still get to fly, don't worry.
Good luck, I'm sure you can do it.


Can anyone identify the Make and Model of this nosed-over plane?

I believe this is a Stinson HW-75 (Also called the 105 Voyager).
I know for sure this is a Stinson aircraft due to the logo which was often rendered on the side of aircraft with a double underline and certainly has the long tail on the upper part of the S as you described:

Logo from the orignal picture, rotated and de-skewed
The identifying features which make me think this is an HW-75/105 Voyager are:

Similar decals down the side of the fueselage as shown in image below
Twin Supports meeting at the main gear roots
Shape of the step for access to the cockpit
Grill both side and below the spinner

source: Wikimedia commons
However there is a slight chance that this is actually another Stinson model such as a 108 or L-5 Sentinal which both share some similarities


The short answer is no.
All the glider feels is its flight relative to the air. What the ground is doing beneath it is not relevant, the wind could be blowing a hundred kilometers an hour and the glider would just be carried along without feeling a thing. This is why unpowered balloons always drift with the wind.
To detect and respond to ground speed requires a sophisticated control system.


How does one specialize in a certain sub-field of aerospace engineering?

I'm going to speak for a North American post-secondary experience. The simple answer is, you don't get to specialize in any particular domain within aerospace engineering as an undergraduate, even if your major is aerospace engineering. Why? Because you don't know anything yet for you to specialize. How can you know what you like, if you don't know anything yet?
The first two years of college are foundational studies, during which the courses and materials are more or less the same across any engineering field. You mentioned aerodynamics. Well, you can't do aerodynamics if you don't know linear algebra, vector calculus or basic fluid dynamics. The last two years would be more tailored to your major, but these will only give you a general gist of all the major domains that aerospace touches.
You get to specialize once you enter industry, or if you choose to pursue even higher educations (Master's, PhD, professorship). Whatever it is, what you ended up specializing will probably be very different than what you thought you'd be doing at the start of college, either because of change of interest (now that you know what the fields actually are), or monetary reasons (field A is more lucrative than field B), or pragmatic reasons (because field A has N openings and field B has zero).


What priority if any is given to Emergency Services in flight clearance?

I work as a fixed-wing pilot for a hospital-based Air Ambulance program. Let me add my experience to the ATC orders accurately cited by Pondlife's answer: the MEDEVAC call sign (the old "Lifeguard" is still used when filing the flight plan) indicates a need for priority handling.

ATC is often helpful in providing us with priority routing. This includes giving us direct clearances, moving us up in the landing que, allowing us an unrestricted approach speed, and otherwise expediting our flight. However, ATC does not always provide priority handling, and I have often experienced ground delays of 5-10+ minutes as ATC apparently prioritizes other traffic, including air cargo and the like. This is rarely a big problem as patients are typically stable, but it can be frustrating when priority handling is expected.

However, if medical staff determine that a patient is in critical condition, we will announce this to ATC—typically accompanied by a request for a specific clearance or change to a previous clearance. This announcement re-iterates our need for priority handling and indicates the urgency of the situation. In this way we can work with ATC to best care for the patient. We will often make such an announcement when checking on with approach control for their information with no specific accompanying request. Occasionally, however, we will need to request such priority handling because ATC has issued us speed restrictions or delaying vectors to put us behind other traffic. ATC is very good at working with us when we do need the priority handling.

In other words, ATC usually automatically gives us priority handling, but when they don't—and we need it—we ask for it and they are very good at working with us to provide the priority handling.


Is extreme wind shear an official classification? Is it rare to land in 65kts headwind, gusting 95kts?

Wind shear is also gusts, but the difference is that wind shear involves changes in speed and direction, where gusts are generally in the same direction. 65kt gusting 95kt is a very rare situation that no airline captain would want to tackle unless the situation demands it (low fuel, some other emergency) and the winds are straight as an arrow down the runway. You could land the airplane that way, but a 95 kt gust can certainly lift a wing or cause weathervaning when the aircraft is taxiing off the runway.

Now, if the report is for wind shear gusting that high, it would be crazy to try to land an A340 (or any other aircraft for that matter) unless the pilot had zero other choices (or just didn't know that there was low-altitude wind shear). The max crosswind component for landing on an A340 is around 37 kts.

From what you are describing, it seems they experienced wind shear, and not just gusts.


Why can't I ever get my right ear to pop on descent in a plane?

The eustachian tube is a vent line that connects your inner ear to the back of your throat. Like the sinuses, higher pressure air flows outbound from the inner ear (when cabin pressure is dropping) with pressure changes easier than inbound (when cabin pressure is rising).

So the air trying to get back in on descent has a harder time than when it was leaking out unnoticeably on the climb, and a pressure differential builds up before it equalizes the pressure - the pop.  Part of the tube is through cartilage and it's helped to open more by mechanically "massaging" it so to speak by moving your jaw.

It sounds like the tube for your right ear, which is partly in cartilage and partly in bone, is misshapen enough that it takes an abnormally high pressure differential to equalize pressure when the pressure change happens quickly like on an airplane.  This is putting a lot of stress on your eardrum (which is what the pain is from).

Decongestants often help but if it is a severe chronic problem, which yours sounds like, as a last resort there is a procedure that is usually done temporarily on babies (where the tube is blocked until they grow more - one of my kids had one put in, which falls out on its own later), called a shunt, where a doctor implants a tube across your ear drum to allow the inner ear to vent that way.

Anyway, you should see a doc about it if it's that bad.


How do I get certified to fly under FAA 333?

The FAA's FAQ says:

  Is an FAA-issued pilot certificate required to operate a civil UAS under an experimental airworthiness certificate or a grant of
  exemption under Section 333?

  If the aircraft is issued an
  airworthiness certificate, a pilot certificate is required.5

  Pilot
  certification requirements for petitions for exemption under Section
  333 are evaluated on a case-by-case basis.

The footnote links to this FAA order called Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft, which says:

  b. UA PIC Certification and Ratings Requirements.

  (1) The PIC must hold and be in possession of, at a minimum, an FAA private pilot
  certificate, with either an airplane, rotorcraft, or powered-lift
  category; with single- or multiengine class ratings, appropriate to
  the type of UA being operated.

  (2) The PIC must have and be in
  possession of a valid second-class (or higher) airman medical
  certificate issued under 14 CFR part 67, Medical Standards and
  Certification.

So, it looks like there are two possibilities:

Your UAS has an airworthiness certificate, therefore you need a pilot's license per the Order (private at least, not sport or recreational)
Your UAS doesn't have an airworthiness certificate, therefore your section 333 petition will be evaluated individually and the FAA will tell you if you need a license or not


What are these "lines" on the A319 flaps, are they damaged?

You are seeing the steel reinforcing sheet metal at the line where the spoilers rest on the flaps when both are retracted.  It is there as a bearing surface since there is some inevitable contact between the spoilers and the top surface of the flap when they are both retracted during flight.


How long does AFSP processing take the second time?

Assuming that you're a category 3 applicant, you can start training as soon as they confirm receipt of your completed application but there's no specific processing time. This is from the AFSP FAQ (direct links to FAQ items don't work):

  Category 3 Candidates may commence training after receiving the "Permission to Initiate Training/Fingerprint Receipt" e-mail, which AFSP issues upon receiving all required information (including fingerprints and the fee).

There is a guaranteed processing time for categories 1 and 2 (aircraft greater than 12,500lbs MTOW), however:

  In accordance with Section 612 of Vision 100 and the IFR, the AFSP conducts a threat assessment on Category 1 Candidates within 30 calendar days of receiving all of the required information, including fingerprints and the fee. The AFSP conducts a threat assessment on Category 2 Candidates within 5 business days of receiving all of the required information, including fingerprints and the fee.


How do airline pilots maintain night currency?

The night currency requirements are in 14 CFR 61.57, which does provide exceptions for pilots flying for an air carrier:

  Except as provided in paragraph (e) of this section, no person may act
  as pilot in command of an aircraft carrying passengers [at night]

61.57(e)(2) has this exception for part 121:

  (2) This section does not apply to a pilot in command who is employed
  by a part 119 certificate holder authorized to conduct operations
  under part 121 when the pilot is engaged in a flight operation under
  part 91 or 121 for that certificate holder if the pilot in command
  complies with §§121.436 and 121.439 of this chapter.

121.436 has general PIC requirements for part 121, and 121.439 has recency requirements, which are 3 takeoffs and landings in the last 90 days in the same aircraft type, or an equivalent simulator.

So part 121 pilots need three takeoffs and landings in type in the last 90 days, with no specific requirement for them to be at night. That's all based on just reading the regulations, someone who's actually been there and done that could probably add some more information.


Which types of aircraft are required to carry ELTs?

Everything you need to know (at least in the USA) is here.

Regarding airliners, it appears that they are carved out of the general requirement to have an ELT in 91.207.f.2.   I guess that makes some sense.  If you're flying a scheduled flight, presumably we should be able to find the plane, since we know the takeoff time and approximate airspeed, and we have radar tracking by ATC/ARTCC.  Unscheduled (especially VFR) flights are where things get dicey.


What kind of test is going on in this image?

This is most likely some kind of a test where high speed photography (as in 5,000 - 10,000 frames/second) will be used to capture the detailed motion of the fan blades.
Types of tests that might require this type of video include fan blade out testing, bird strike testing, and icing / hail testing.  There may be others but those are the most common.

Why the color of blades are white

In order to get 10,000 frames per second you need a lot of light. There will be many high powered lights shining on the engine.  You don't want harsh reflections from the lights, so you need a matte finish, the flatter the better.  The normal blade finish is way too shiny. So they are painted.

with red on its borders?

just for contrast, so you can very clearly tell in video where the blade stops and the fan case starts.  The exact color is arbitrary, and different OEMs have different traditions.  You are just looking for a set of colors are very different from one another.

why are they numbered?

So in the high speed video, you can see what happens to what blade.  For example, in a hail test, you might observe that a giant chunk of hail hit blade #15.  You could then go inspect blade #15 afterwards and see if there is any damage.

why is the color of spinner yellow?

Same as the red border. Just for contrast, so you can see where the blades stop and the spinner starts.

What the operator is doing?

I don't know exactly. Obviously some kind of assembly or disassembly procedure.  You can tell in that photo that the inlet is not attached, he's right up against the plane of the fan blades.  So it's not a fully assembled engine on the airplane or on a test stand.  It's clearly in the factory somewhere.  But what exactly he's doing I don't know.  Honestly could just have been "hey we need some publicity photos for Aviation Week, pretend to do something interesting with the engine".


Is this the first single-engine helicopter clean-sheet design since 1976?

Most modern helicopters use two engines for added safety. This reduces the field of competing designs. Also, a good engineer will always leave proven details unchanged and focus on improving flawed designs. So it is hard to prove that a new design from an established helicopter company will truly be a clean-sheet design.

The possible new designs from 1976 could be:

Aerospatiale AS-350 Écureuil: First flight 1974, introduced 1975. Note that the AS-355 is a two-turbine variant of the single-turbine AS-350.
Robinson R-22: First flight 1975, introduced 1979. It has a single reciprocating engine and was indeed a clean-sheet design by Frank Robinson who started the company in 1973. He had worked previously at Bell Helicopter and Hughes Helicopters.


Do Air Passenger Number statistics on a route refer to both directions of flight?

By looking through the BTS website you linked (and is cited in the Wikipedia listing), you can find the answer.
The first 3 sections on the infographic; Summary Data, Carrier Shares, and Total Passengers include both departure and arrival passengers totaling about 14 million passengers.
The fourth section Top 10 Destination Airports (US Only) is less clear and you can't cross-check to the other totals because it is limited to only the top 10 airports. But it is a clue that the title says Destination Airports which implies departures only.
But there is a potential check. At the top, you can select the airport. By selecting Orlando, FL: Orlando International from the pulldown, you can see the data from there. That shows in the Top 10 Destination Airports that PHL is number 3 and the total number of passengers is 539,900. Since this is different than the 548,320 PHL-MCO number, it would appear that section references departures only.


The engine wouldn't fail. Avgas powered engines use 2 magneto systems which are independent of the battery and alternator, so if your alternator dies or you lose the electrical system it will run as if nothing happened. Starting it does require the battery and electrical system though to run the starter motor. Aviation engines with computerized ignition systems have backup batteries so they will also not fail due to an electrical system malfunction.

As for the "6 pack" cluster of instruments 3 are not powered at all: Airspeed Indicator, Vertical Speed Indicator, and Altimeter. 3 are powered, although there is no hard and fast rule the Attitude Indicator (AI) and the Heading Indicator (HI) are usually vacuum powered from the engine and the Turn Coordinator is usually electrically powered. There are many exceptions to this.

So keeping the electrical system off doesn't affect the primary instruments much, provided the airplane stays in VFR. What captain cheapskate won't have for sure is any radios, transponder, GPS, ADF, VOR, or any other communication or navigation aids.

All these things you could probably do without on a completely VFR flight, provided you don't need communications. However, the engine temperature and oil pressure gauges, EGT gauge, and fuel gauges all work using electricity as well, and I'd really hate to lose those. A typical steam gauge Cessna isn't going to have an electrical fuel pump, but many piston singles do. Running without the electrical system means you'd lose that to, and that's considered an important safety system to back up the mechanical one in case of failure.


What is exemption 17347 in the context of dispatch for FAA domestic carriers?

Exemption 17347 was requested by the Regional Airline Association to extend a pre-existing alternate weather regulatory exemption to apply to regional airlines operating under part 121 because the FAA has changed the language referring to other carriers used in grants of exemption.

  As a separate issue, when A4A previously petitioned for this exemption, the FAA granted relief to the petitioner’s member airlines, as well as to all other “similarly situated” certificate holders, which included any air carrier operating under the provisions of part 121 (Exemption No. 3585). However, in an effort to manage exemptions granted by the FAA and to facilitate operator compliance with the conditions and limitations of a grant of exemption, the FAA is no longer including “similarly situated certificate holders” in grants of exemption.

Regulatory Filing Source


What are these things hanging off the trailing edge of the wing?

Those are static wicks -- Basically these are little wires screwed to the airframe.
Their purpose is to discharge the static electricity that an aircraft picks up moving through the air - especially in clouds.

The static discharge tends to happen at "pointy" protrusions from the aircraft - if this happened through antennas it could cause radio and navigation interference, so to prevent that static wicks are installed, providing a low-resistance protrusion for the charge to dissipate through (and as an additional benefit, dissipating the charge helps ensure your aircraft doesn't attract lightning strikes).

Here's a couple of closer pictures:


Will an axial compressor work with an ideal fluid?

First of all, incompressible flow is subsonic, however subsonic flows are not all incompressible. An ideal gas flow may be "incompressible" until it meets the compressor. Even if the vehicle is travelling at supersonic speed, the flow is reduced to subsonic ("incompressible") flow by the intake system before reaching the compressor. At this point the subsonic flow gets compressed. Presumably this is what is meant here.

The compressor stages each reduce the cross-sectional area, while the fans draw the gas backwards to maintain velocity. Thus the gas, ideal or not, must compress to fit the reduced volume available to it. This compression creates heat which raises the gas temperature, further raising its pressure.

Viscosity is necessary to establish a circulatory flow component around each airfoil-shaped blade (the Kutta condition). It is this circulatory component which creates the velocity differentials needed for the Bernoulli principle to create the dynamic pressure differentials which make the blades do their work. Superfluid helium (close to absolute zero) has no viscosity and has been found to exert neither lift nor drag on an immersed body.


Is the passenger capacity of the A350 really only 250 pax?

Airbus themselves give max passenger seating 440, and typical seating 3-class 300-350, for the A350-900. https://www.airbus.com/aircraft/passenger-aircraft/a350xwb-family/a350-900.html

The undeveloped A350-800 is listed with 276 passengers. Maybe that's where you got your 250 number.


Does adding more engines to commercial aircraft increase safety?

In the modern day and age, it is very difficult to put a numerical value to this type of question, simply because the chance of something happening is so incredibly remote. This is not a clear-cut equation and is dependant on several factors.

A few thoughts:

The impact of loosing one engine will be less significant on a four-engined aircraft than a two-engined aircraft. You only loose 25% of thrust as compared to 50%. However, two engined jets are built with greater safety margin and are able to handle an engine failure at any phase in the flight. Four engined jets are certified to be able to fly on two engines on one side of the wing.

In theory, you are twice as likely to suffer an engine failure with four engines, since you have twice as many engines. While the lost thrust would be less significant, very rarely damage can propagate to cause damage to other parts (See QF32 and ELY1862). In general, it is easier, cheaper and faster to maintain two engines than four. Should a complete engine swap be necessary for instance, you only need to do it twice instead of four times. Every extra step is an step where something could theoretically could go wrong.

The GE90 (Boeing 777) is certified to handle 'large' birds (4kg) as compared to the CFM56 (Airbus A340) only being able to handle 'medium' birds. Hence, the Boeing 777 engines are likely to be a bit more robust.

From a human perspective, pilot workload will be lower with two engines rather than four. Should something happen in a time-critical emergency, there will be more systems you need to keep an overview of. See CAL006.

Furthermore, not every engine-related incident is spared simply cause you have four engines. BA Flight 9 lost all four engines due to volcanic ash. On BA Flight 38, the engines failed in unison due to ice in the heat exchanger on the engines that were operating in identical conditions.

Despite ETOPS being around for several decades and applying to thousands of aircraft I have not heard about a single accident involving commercial jets operating in that domain.


Is an FAA Pilot License valid in the U.S. if held by a foreigner?

Yes. Tens of thousands of foreigners - both inside and outside the US - hold FAA certificates, including me. Your nationality is only relevant in these situations:

Foreigners must have TSA approval before starting flight training (US citizens need the approval before soloing). See this question.
Getting the TSA approval might be more difficult for people from certain countries (this is mainly a security/political issue, not an aviation one)
An FAA certificate isn't an immigration document: it doesn't give you any right to enter the US, or to work here. That means, even if you hold an FAA commercial or ATP certificate, you still need a work visa or green card in order to work as a pilot in the US. See this question.

But once you get an FAA certificate, your nationality is irrelevant. The FAA only cares about your license being "valid" in the strictly aviation sense: is your medical current, is your flight review current, is your instrument rating current etc.?

Even if you surrender your green card and leave the US permanently, that won't change anything about your FAA certificate (unless maybe you get expelled and the DHS requires the FAA to revoke your certificate on security grounds). You just send them your new address in whatever country you're in, they update their airman database, and that's it.


What should be the minimum time spent in cruise (for e.g. a B737)?

Above is a simple model to break down the problem into its simplest form. Of course the model can be refined. Assuming for a short flight that the ground speed doesn't change much from after takeoff to before final approach, it becomes apparent that reaching the top and descending right away gives the shortest time burning fuel (blue line) and longest time with the engines idle during descent.
Operationally you need a few minutes in cruise for ATC purposes, approach briefing, etc. So the 5 minutes mentioned by @CptReynolds in a comment makes sense.
The same model should hold true for long haul flights, only if aerospace materials and propulsion would efficiently allow a sub-orbital flight from say London to LA, but alas that's not the case.
Refining the model

* Half-climb and half-descent.
Here the flight is divided into equal time segments (maintaining the 'same ground speed' assumption). I've taken into account the shallower descent rate, and the decreasing climb rate / thrust / fuel flow the higher the jet climbs. The numbers indicate the fuel flow (FF) ratios per time-segment.
With real FF figures, further refinements can be made.
Real Boeing 737 FPPM manual (flights <500 NM)

(Click to view)
Here it is very clear that the longer the short flight (even by a little), the higher the cruise altitude. There is not a point where there is a cut-off. And from the right-hand side: the lighter the plane, the higher it should cruise.

Note regarding the cruise FF ratio being fixed at 1.25: looking at a 737 AFM, the FF doesn't change by much from FL250 to FL410 at the same weight (2374-2458 lb/hr/eng; the Mach number rises though). So, for short hops with lower cruise levels, I've maintained the cruise FF for that simple model.