How do I become a flight test engineer? I would like to know the whole process to become a flight test engineer (FTE).

Do I need to take any FAA specific flight test engineer certificate, apart from an aerospace engineering degree? Do I also need a minimum of flight hours with a commercial pilot certificate?

Go to college and get an engineering degree, preferably in Aerospace Engineering or Mechanical Engineering with an Aerospace emphasis.  You may also want to pursue a Masters in this area after your Bachelors.
Seek out the best schools you can get into; the biggies in aero are Georgia Tech, University of Michigan, Purdue, University of Washington, and MIT.  But there are a lot of other schools out there with fine aero programs as well and attending there doesn't limit your job opportunities.
Study hard and get good grades.  If you can't hack your freshman and sophomore years, engineering may be a bad choice as a field for you; seek a different major.  On top of that, I strongly suggest you get involved with extra curricular engineering projects or clubs in your department.  This will give you a lot of hands on engineering experience and help build up a knowledge base for you of how to be a good engineer.
I'd also get involved with either a RC airplane club, or better yet, join a chapter of the Experimental Aircraft Association and participate in the flight testing of home-built aircraft.  Again, it starts giving you a lot of hands-on experience in the flight testing of airplanes, which looks really good on a resume.  You can also get a couple of good references this way.
In your junior or senior undergraduate years, start looking for co-op or internship opportunities at the major OEMs, i.e., Boeing, Northrop-Grumman, Cessna, Beechcraft, Airbus, Piper, HondaJet, Gulfstream, etc.  Preferably seek out internships in flight testing.
Finished with school?  Time to land your first job in flight test.  Attend your school's career fair and chat with the reps from the OEMs.  File applications for jobs on OEM careers sections.  Get your LinkedIn page set up.  It's now time to get references from your internship supervisors.  This first job is always a challenge as many OEMs are looking for experienced people and there are very few entry level roles.  You first job may not be in flight testing and will probably not be very glamorous.
Aerospace is a limited and challenging field to get into.  Work is also limited to certain areas of the nation, primarily Seattle, LA Area/SoCal high desert, Dallas, Wichita, Greensboro, or Savannah.  In addition, the business is quite fugitive, having many boom and bust cycles.  Layoffs happen very frequently and you may have to move from place to place to find work.  Starting salaries and benefits packages can be spartan.
Flight testing may or may not be as glamorous as you think it is.  Your primary duties will be generally doing test plans and reports for flight test operations, rigging aircraft of a/c systems with sensors, wiring and data collection boxes, possibly some 3-D CAD modelling.  Even the engineers on the aircraft are sitting at crew stations monitoring data collection.  After that you'll be doing data analyst activities, crunching numbers, etc.
A PPL or CPL is not necessary to be a flight test engineer.  Test pilot is another story altogether and is considerably more difficult.  There are civilian routes into this business but are extremely difficult to obtain.  Most test pilots get their education by becoming military pilots and attending either the USAFTPS at Edwards AFB or the USNTPS at NAS Pax River.  In addition, most of the test pilots at the major OEMs come from the military route and, IMO favor the military guys over non-military, particularly in flight testing high performance military aircraft.  Near as I know there are no requirements from the FAA to become a test pilot but the only private test pilot school in the nation is the National Test Pilot School in Muroc, CA.  They require applicants to possess an engineering degree, a CPL and at least 750 hours total time in fixed/rotor wing.  With an 11 month course to obtain a Masters of Science in Test Piloting costing an astounding $960,000.00, you will definitely need corporate sponsorship.  There are a few firms which will do this.  Burt Rutan's Scaled Composites in Muroc promotes test pilots from its engineering corps.  So does Cirrus in Duluth, MN.  But the competition to obtain these slots is fierce.  All in all, if you want to be a test pilot, go the military route, either in the Military Academies or a ROTC unit at your college.


When were checkerboard fiducial markers first used in aircraft development? It's common to put checkerboard fiducial markers on aircraft to help analyze rates and orientations from photographs and film.

When was this practice first used on aircraft?

Flight test versions of the German V-2 rocket were painted in a checkerboard scheme, presumably for fiducial-marking purposes (or just high visibility); was this already established practice at the time of the first V-2 test flights in 1942?

This was not already established practice in 1942.  Not even in the early 1950's NACA crash tests, which were so methodical that you'd have expected these checkerboards:  seven movie cameras along the runway, a flashing light atop the fuselage for movie synchronization, a pile of onboard cameras and instruments, white paint and red-dyed fuel for better imaging.  The closest this came to checkerboards was dotted lines on the fuselage sides.

This long NASA video includes many images of these aircraft.  No checkerboards anywhere.

A skim through online photos suggests that even automotive crash tests didn't start using checkerboard fiducials until the 1960's.


What would escorting an airliner which is the subject of a security threat by fighter jets achieve? Today, a Ryanair flight from Lithuania to London Luton was the subject of a presumed hoax, and had some form of security alert. In the incident, it was diverted to Stansted, while being escorted by fighter jets.

What would the escort actually achieve? My best guess is to shoot it down in case of a hijacking, but this seems like a particularly grim prospect and perhaps maybe not one that would be so publicly on display. Is there another reason why it might have been escorted?

There are multiple reasons:

Yes, they'll shoot the aircraft down if deemed necessary. It is a grim prospect, but sometimes grim events require grim responses - there is little doubt that many lives would have been saved had the 9/11 aircraft been shot down prior to them hitting the WTC's.
It's a threat and a show of force. Not all hijackers / criminals / terrorists are keen on the idea of losing their life. For those, the threat of being shot down can contain the situation and give some control. If nothing else, being shot down would prevent their political message from getting out - you can't really negotiate the release of prisoners (Or whatever) if you're dead on a blown up plane.
The fighters can get up close and personal with the aircraft. This allows them to see it, see inside and potentially communicate with people on board. During an event where the situation is unclear they may be able to report whether the aircraft is under control and/or command of the pilots.
There are internationally recognised intercept signals that a fighter jet can perform - using lights and motion (Rocking wings etc). This will allow them to pass instructions to an aircraft with no radio capability.


Do I need to get a new AFSP approval if I change to a new part 61 instructor? I have an AFSP (TSA) approval to train with a part 61 provider. I want to transfer from them to an individual part 61 CFI. Do I need to go through the AFSP approval process again?

Yes, you will need to apply for AFSP approval for your new training provider. The Application Guide for the AFSP says in step 7:

  It is possible for a Candidate to have several active training requests at a given time. These requests may be for the same or different flight training providers. Each training request form will be processed separately; AFSP approval is valid only for the Provider listed in the application.

So if you needed AFSP approval for your original training provider, you will need a new approval to switch to a different provider.


What does the isolation valve do in the landing gear extension/retraction system? The isolation valve is an integral part of the landing gear extension/retraction systems on most larger aircraft.

What is its funtion?

As a general answer: the purpose of an isolation valve is to separate two (or more) parts of a hydraulic system from each other. The separation is necessary to prevent a failure of one part of the system affecting the rest of the system. For example, a leak in landing gear actuator system might bleed the entire system - rendering the whole hydraulic system inoperable.

An isolation valve may also be used to prevent system operation if certain conditions are not fulfilled. Say, if airspeed is above a predetermined limit, the isolation valve would remain closed preventing the operation of the gear.


What causes lift in aerofoil: Bernouli's principle or Newton's third law? Reading materials online, I observe some explanation based on Bernoulli's principle, quoting the difference of air pressure above/below and difference of speed of airflow. While some say's Bernoulli's principle works only in special conditions (laminar, non-viscous, non-compressible, streamline, etc.) and the correct working is based on Newton's third law whereby, the airfoil pushes the stream downward and experiences lift upwards.

Bernouli's principle is also said to not work for symmetrical aerofoils and inverted aerofoil.

What is the final verdict of what causes lift?

I think it is erroneous to say that lift is solely produced just because of the Bernoulli's or Newton's law.

Sixty Symbols(A youtube channel) explains the best how a wing actually produces lift.

Instead of explaining I thought this video might help you more than my explanation. And also look at "Incorrect Lift theory" by NASA


What is in 'stealth paint' to reflect radar, and how does it work? What is in the 'stealth paint' that coats stealth bombers e.g B-2 spirit, and stealth aircraft, e.g SR-71, and how does it absorb/deflect radar?

Stealth paint doesn't reflect radar, it absorbs it by converting it to heat which is absorbed and dissipated by the airplane's structure. Paint is only one of several Radar Absorbing Materials available.

The paints known have suspended balls of ferrous material which absorb radar energy at certain frequencies and convert the radio energy into mechanical vibration, which causes friction and in turn heat.


Why high air speed at flat surface don't always produce low pressure? Can you explain why flat surfaces don't produce low pressure in both experiments ,even air has high speed in both case?
1)experiment where flat surface DON'T produce low pressure:
http://www.cabrillo.edu/~jwelch/bernoulli/bernoulli.wmv
2)experiment where flat surface produce low pressure:
video 2

3)experiment where curved surface produce low pressure(as always):
video3
If car has flat floor 5cm from ground,will this floor produce low pressure or we must add some curvature to lower pressure?
What is key for low pressure,airflow  speed or airflow curvature?

TL;DR
Speed does not produce low pressure, increasing it does!
Bernoulli's equation says that for a fluid flow, in absence of elements doing work on the stream or having work done on them by the stream¹, $p + \frac12\rho v^2$ stays constant². But it does not tell us what constant. You can have flows with any combination of $p$ and $v$, and all will still satisfy the Bernoulli's equation.
Straight box in the first video
The blowers increase the energy of the air inside the hose. The energy can manifest as either static pressure, $p$, or dynamic pressure, $\frac12\rho v^2$. How it chooses? Well, if the static pressure $p$ inside the hose is higher than outside, the pressure gradient accelerates the air out, converting the static pressure to dynamic, and if the static pressure in the hose is lower, the gradient decelerates the air. So the flow will quickly stabilize so that the static pressure matches the ambient³, and the added energy determines the velocity.
Therefore there is the same static pressure everywhere and the box between the two nozzles has no reason to move.
Curved surfaces in the first video
The other experiments in the first video are not explainable by Bernoulli's equation alone, but instead effects of inertia and viscosity have to be taken into account. As the air flows around the curvature:

If the curvature is towards the stream, the air starts hitting the wall and needs to turn around it. Since it has mass, it requires force to turn it, which creates additional pressure on the wall, and a pressure gradient forms near it turning the air molecules further away. Bernoulli's principle tells us the velocity will, somewhat counter-intuitively⁴, decrease, but that is just a side-effect; we already know the pressure is increased.

It the curvature is away from the stream, the air would want to just continue straight. However there can't remain a wedge of a stagnant air behind the receding surface, because it would have higher pressure that forces it to move, and viscosity prevents forming another flow in a wedge shape to fill in, so the original stream sticks to the surface. And this means a pressure gradient again establishes that overcomes its inertia, this time with low pressure near the surface.
Bernoulli's principle tells us the velocity, again somewhat counter-intuitively⁵, increases, but that is again just a side-effect; we already know the pressure is reduced.

Second video
Last, the experiment in the second video relies on the fact that the air is expanding in multiple directions, so the cross-section available to it increases⁶.
According to the above, the velocity on the outer edge will be such that the static pressure matches ambient. But before it gets there, it has to squeeze through the still much smaller area near where the hose is connected. Since the same volume⁷ of air needs to get through both per second (due to conservation of mass), and to do that, it must move faster. And since it moves faster, its static pressure is lower, and this creates the pressure differential that pulls the plate towards the nozzle.
This is the only of the experiments that is actually demonstrating Bernoulli's principle.
Curvature v.s. speed
You see some of the cases are governed by one and some by the other. Fluid dynamics is extremely complex and difficult to calculate – the “easy solvability” or Navier-Stokes equations (that describe fluid dynamics; Bernoulli's equation is included in the set) is famous.
Generally, the curvature works in most situations, but watch out for aerodynamic stall (when the angle is too steep for viscosity to keep the fluid flowing ‘orderly’), while velocity only works when you have control over it, which is generally just in closed pipes.

¹ Bernoulli's equation simply expresses the internal energy of a fluid. If you do work on it you are adding energy, so that energy has to be added in the equation.
² Potential energy term omitted for brevity.
³ Dynamics of jet and rocket nozzles is much more complicated, but at the low velocities involved it holds pretty well.
⁴ It is counter-intuitive because it looks like the air has to move through less space, but since there is no opposite wall, it is free to expand as it suits it.
⁵ This is counter-intuitive because it looks like the air needs to expand to more space, but again there is no other wall, so it can pull more air from far and wide to fill that space.
⁶ I am not completely sure this case does not involve some additional effects.
⁷ Actually the same mass, and to be accurate we should include the adiabatic expansion of air, but for the speeds involved air can be approximated as incompressible.


Can a bird strike destroy the stealth capability of F-35? There is a video according to which a F-35 aircraft has been struck by a bird in Israel. This, allegedly, disabled its stealth capability.

More than a year earlier, two storks allegedly damaged another F-35 and made "maintenance work" necessary.

Can a bird that lives in Israel and surroundings actually make a F-35 lose its stealth abilities?

The F-35 is a plane that relies on safety via stealth, rather than armor. The F-35 fuselage can be dented much like any other plane. The F-35 uses a variety of methods to reduce the its radar signature, most of which revolve around the "skin" of the fuselage - the way the fuselage is physically shaped is meant to reduce the reflection of radar, and there are various paint and material considerations as well.

When at any considerable speed, a bird strike will tear right into the fuselage. Take for example the following picture of a bird strike on a separate military fighter.

When the fuselage is damaged in this way, the radar waves are no longer being strategically reflected or absorbed, and the aircraft becomes suddenly very visible on radar.


How would a hole in the side of a plane affect its flight? Let's say an explosion blew a hole in a plane's fuselage without ripping the aircraft apart or damaging the engine and wings. How would the hole affect its flight?

Most planes fly best without any extra holes, of course! The specific effect of a hole is modifying local airflow, increasing turbulence, and increasing the aircraft's wetted area on the side of the breach.

As long as there isn't a matching hole in the back, however, the airflow inside separates (similar to a pickup truck), so most of the airflow is still normal around the fuselage. The turbulence will also disturb flow over the wing root, if the hole is located forward of the wing, slightly reducing lift on that side. All in all, range will be reduced and more control inputs will be required.

Other than that, a hole in the side does not prevent a plane from operating normally. One can think of a conventional aircraft as the wing, pushed forward by the engine, and stabilized by the tail. The fuselage just connects them together and houses the payload (and controls, but we're not thinking about that at the moment).

Most fuselages are streamlined for minimum drag, but a plane can fly with a non-streamlined fuselage as well, just not as fast and not as far. Fixed-wing gunships, for instance, fly operationally with large holes in their side for the armament.


Can private flights between the US and Canada avoid using a port of entry? I'm considering getting back into flying private after a 15 year absence. I'm a dual US/Canadian passport holder. I'd like to fly between homes on Nantucket, MA and rural Nova Scotia regularly. Is there any type of permit that would provide an exemption from landing at a port of entry every time?

CANPASS gives a streamlined entry into Canada, allowing you to land at a port of entry and just report by phone.  To go into the US, you need an eAPIS account.  I think that with eAPIS you still need to meet in person with a customs person.  A friend in Montreal used eAPIS to travel to Vermont regularly.  He said it was a lot of hassle to get set up and go through the process the first time, but after that it was not too bad.  On the way back he just had to land at a convenient POE and call in.
Not quite an avoidance of POE requirements, but otherwise pretty convenient.


How much training does an A320 pilot need to fly an A321? From flight simulation and from pictures, it appears that the cockpit of the  A318, A319, A320, and A321 have few if any differences.

Could an A320 pilot fly an A321 without further training?
If not, how much training would he need? Maybe he has to practice landing, because of the different length of the aircraft.
What big differences in the cockpit do those planes have?

From Flight Training International:

  Airbus A320 Type Rating

  The Airbus A320 family of jet airliners consists of five aircraft:       the A318, A319, A320, A321 jet airliners and ACJ business jet.
  Only one type rating is required to fly these Airbus aircraft, as       they have similar flight decks.
  To earn a type rating for these Airbus aircraft requires the    completion of a technical course that lasts anywhere from 8 to 12 days depending on your pace.

So the answer from a regulatory point would be no additional training.  Some airlines might have policies that require a minimal check, probably in a sim.


What radio frequency bands are associated with the long wire antennas, why do some aircraft have them and others don't? On many older aircraft, and in vintage aircraft photos, you can often see long wire antennas strung from some point on the top of the fuselage to the top front of the vertical fin. On newer aircraft, it's either a rare find or not seen at all. The fact that some aircraft even today still have them leads me to wonder about their function - which radio frequency bands they are used for, whether those bands are no longer used, or whether the frequencies are still in use but the long wire has been superseded for most installations by a more compact antenna design.

Knowing that modern aviation radio operates in the 108-137 MHz band, and that most modern aircraft use much shorter rigid antennas, the long wire would seem to be inappropriate or unnecessary for that purpose. So, why would aircraft of a more modern design still be equipped with one? What other radio communication (presumably much lower frequency) would call for such an antenna?

Depending on the aircraft you could be looking at one of a few things.

(source)

On some older aircraft it may be the communications antenna for older style radios (pre UHF/VHF days). Most of these radios have been phased out for some time but you will see it in pictures.

The more common thing you are seeing (based on your top of the fuselage to front of the tail description), as it is still in place on many small GA planes and any bigger planes that have not had the unit removed is the ADF Sense antenna. This antenna helps to deal with the directional ambiguity problem in an ADF unit. Basically the loop antenna is capable of knowing where the station is but only on a 0-180 degree plane. It does not know if it is in front or behind the antenna. The sense antenna allows it to solve this ambiguity. These units operate in the 190-535kHz range. For what its worth the FAA is pulling a lot of ADF's and their associated approaches as GPS takes over but they seem to still be common elsewhere in the world.

Here is one,

(source)

  Why do some aircraft have them and others don't?

Since it seems like we have settled on the ADF sense antenna as the one in question it boils down to weather or not your airplane has an operational ADF installed. ADF's are still popular elsewhere in the world but here in the US much of the ADF system is being pulled as it was in part replaced by the VOR system much of which is to an extent being supplanted by GPS today. Since a large chunk of the GA fleet and to an extent the commercial fleet has airframes dating from the ADF days many of the planes still have the systems installed. Depending on where you are located, if you routinely fly one of the remaining ADF approaches or otherwise use an ADF regularly you may elect to keep it in your aircraft. I can tell you that none of the aircraft I trained in had operational ADF units although two of them had units with antennas but were non operational and simply had yet to be removed.


How does ATC locate a stealth aircraft if its transponder fails? Stealth aircraft are built to have near-zero radar visibility, so that, in a warzone, it'll be harder for the enemy to see it on their radars to shoot it down.

However, air traffic control also use radars.

Normally, this wouldn't be a problem, since the aircraft is carrying a transponder that continually broadcasts positional and movement data (so-called "secondary" radar returns), not just to ATC, but also to other nearby aircraft (so that they can avoid running into it).

If an aircraft's transponder fails, though, then ATC is reduced to tracking it via "primary" radar returns (literally bouncing radio waves off of it and seeing where they're reflecting from)... but stealth aircraft are specifically built so that they can't be picked up by most primary radars (at least without a lot of trouble).

So how does ATC - and, for that matter, all the other planes in the air who would rather not run into it - know the position of a stealth aircraft if its transponder fails?  Do they have the flight crew of the stealth aircraft continually read off map position, height, speed, direction, etc. data and try to use this to steer other planes away from it?

It depends on the aircraft.  One example was on the F-117, which had removable radar reflectors on the sides of the aircraft, allowing the jet to be detected by primary and secondary ATC radars.

As to more modern stealth aircraft, it’s possible they use similar systems to ensure ATC tracking in peacetime.


Do the CAA's recent changes to medical requirements mean I wasted my time getting a medical? I'm studying ab initio towards an EASA PPL. To be ready for my first solo, I obtained a class 2 medical certificate earlier this year. I had to take a day off work to do it and spend some cash that could have paid for flight time! Recently, the CAA has announced changes to the medical certificate requirements, aimed at reducing the need for private pilots to see an AME, but the announcement is not entirely clear to me.

Assuming these changes come into force before I first fly solo, could I have obtained an EASA PPL (and used all the privileges of that licence) without having obtained the class 2 medical certificate? If so, and assuming I don't intend to progress to a CPL or ATPL, is there anything else I need my certificate for, or any reason to renew it when that comes up in five years?

No, I don't think so.  I believe there is a distinction between a UK PPL and an EASA PPL.  UK PPLs are defined purely by the CAA and only permit flying of aircraft outside the scope of the EASA licensing regime, such as some vintage and home-built aircraft.

The CAA proposal applies to the UK NPPL and PPL licenses not to the EASA PPL (with the exception that it may apply to pilots flying non-EASA aircraft in the UK only).

As it stands it is still necessary to have a class 2 medical to use the full privileges of an EASA PPL, although as footnote 1 says the CAA "aim to influence EASA in considering reviewing the medical requirements for EASA Private Pilot licence holders flying EASA aircraft".


How are fuel tanks tested and certified? What ground tests are carried out for testing and certifying wing fuel tanks for use in passenger aircraft? Is there any chamber to simulate flight conditions? If so, how is the chamber constructed?

The exact regulations governing an aircraft's fuel tanks depends on many factors such as type and number of engines, construction material, mounting method, number of passengers, aircraft gross weight, etc.

But a good overall set of requirements is given in 14 CFR 23.965 - Fuel tank tests. It includes such items as pressure testing, vibration, sloshing, inertia loads, venting, duration and number of cycles of each test, etc.

14 CFR 23.967 thru 23.101 - Fuel tank tests provides additional criteria but also includes requirements for components such as access panels, fuel sensors, filters and strainers, lines, pumps, drains, valves, jettisoning, etc.

Other than a visual inspection for leaks and corrosion, the primary field test for an installed tank is to plug all the orifices and pressurize the tank to 2-3lbs for about 10 minutes. Large rubber stoppers that look similar to corks (with a bolt running through them that expands the rubber) are often used to plug the larger opening such as fuel filler openings.


Can I take the FAA CFI knowledge exam(s) based on my EASA ATPL knowledge? One of the requirements to be a flight instructor in the USA is FAA theoretical exams.

Can I sit those exams with EASA ATPL theory basis?  Also, do I have the right to sit those exams without completing an official FAA theory exam preparation process?

There is no official theory exam preparation process that is required to be able to take the test, you can do all preparation on your own. If you are asking if having passed the EASA ATP theory exam means you don't have to take the FAA CFI then the answer is no, you definitely would have to take the exam, although I don't think that's your question.

I think what you're asking is if the material in the EASA ATP is the same in the FAA CFI and you don't need to study. While the material is very similar and there will be a lot of overlap the two are not the same, there will be questions which ask specifically about FAA regulations, testing standards and student training which are not obvious and you are not likely to pass without knowing the material.


Do airplanes dump fuel before landing? Presumably planes aren't completely empty of fuel when the land, so they don't start refueling from zero, is that right? Is it true that they must dump excess fuel before they land? How much? Presumably not all, in case there's a problem with the landing and they need to abort?

Is it true that they must dump excess fuel before they land?

Nope. You don't siphon your car just before your destination. Fuel is money.

Except in emergency in big planes, fuel then is a potentially lethal burden (weight).

  ... in case there's a problem with the landing and they need to abort?

Contingency fuel just for aborted landings, weather deviations, etc., is added during the fuel loading for such things. You get to keep it at the destination.

Further reading:

When are aircraft required to dump fuel for emergency landings?
Can a wide body airliner land safely with a full fuel tank?
Fuel planning—fuel calculation


How far away should the vertical stabilizer be from the leading edge of the main wings? I am wondering what the advantages and disadvantages are of placing the vertical tail nearer to, or farther from, the main wing. For instance, when the vertical tail is just behind the wing, do the wing's effects on the airflow around the aircraft decrease the effectiveness of the vertical tail?

The reference point is not the leading edge but the center of gravity (CG), which is a little further back.
The ability of a fin to generate a yawing moment is proportional to both its area $A$ and its distance $l$ behind the CG. It is therefore also proportional to their product. This has units of $L^3$ so it is known as the tail volume $V$:
$V = A.l$
So the nearer the fin is to the CG, the larger it needs to be. On the other hand, the smaller a fin is, the farther back you have to build it out.
In practice everything in front of the fin, usually the fuselage and/or wing, can affect the airflow over it and reduce its effectiveness. This is especially significant at high angles of attack, such as when taking off or landing, and extra height is often added for this reason.
However a very tall fin also exerts a strong rolling moment, which can cause awkward stability problems such as Dutch roll.
At supersonic speeds, the pressure above the rear of the wing and fuselage falls, reducing the effectiveness of the fin. However the pressure under them increases, so underside strakes become very effective for their size.
Placing a very large fin above and close to the main wing is about the worst thing you can possibly do.
The optimal distance back depends on many design factors but is usually around a half- to three-quarter span, though it can be further for supersonic aircraft.


What is this open panel at the rear of this airplane? I saw this photo at another interesting question:

There's an open square panel. What is it?

P.S. Any tag suggestions are welcome. All I could think of was "tail".

EDIT: are there any other tags for this kind of thing? The APU tag seems to be a crystal ball because I had no idea what it was at the time.

It is the Auxiliary Power Unit (APU) inlet door.

Image from Boeing Aero Magazine


What will happen if an X shape wings mounted on biplane? The question is What will happen  if an X shape wings mounted on a light biplane? What will happen in the place which upper wings come closer to lower wing?

Drawing from the hypothetical of "could a starwars X-Wing fly" which is brilliantly answered by James Gull here it is possible to achieve flight in this configuration

  ...It's not a very aerodynamic design. All those intersections at the
  fuselage will create a ton of drag due to interfering aerodynamics.
  The engines and wingtip pylons are not helping in that regard either,
  as they are not well blended in. The wings are essentially a biplane
  configuration, which definitely works. The designer would need to take
  extra care near the fuselage due to the surfaces being so close
  together; their wakes would interfere with each other. Careful tuning
  could help turn this into somewhat of an advantage in slow flight.

The Strop SP-7 was an early proposed design for such an aircraft. But it only
remained in this configuration for takeoff and slower flight segments, the wings folded into a mono plane configuration for high speed cruise. I cant find any evidence that it ever actually took flight.

Some aerodynamic "trials" have been done on the "design".

If you are asking about an "X" looking from the top down, it appears that Sikorsky dreamed up something like that and went as far as running tests on the design as well as building some of the major components but the project was ultimately canceled and it never flew.


Why would a pilot use ailerons for countering asymmetric thrust in mid-flight? A inexperienced 747-400 pilot used ailerons for counteracting asymmetric thrust in 1998 aboard United Airlines Flight 863 (YouTube recreation).

Why would an airline pilot do such a mistake? Why would a pilot disregard the counter yaw training he got from the Baron 58 to the 737?

Is he the only pilot that did this or are there other pilots that do the same mistake too?

Perhaps he simply forgot rudder.

This AOPA article "PROFICIENT PILOT LAZY-RUDDER SYNDROME" written about this incident, presents an interesting point of view on the matter, that basically good design and complacency leads to little regular rudder use so in emergencies its initially over looked.

  This pilot might have been affected by "lazy-rudder syndrome," the
  reluctance to use a rudder to the extent dictated by given
  circumstances. Some believe that this reflects a general decline in
  basic stick-and-rudder skills. A commonly observed symptom of this is
  lowering the right wing during initial climb after takeoff to
  incorrectly compensate for a single-engine airplane's left-turning
  tendency.


How is the drift corrected in INS-driven attitude/heading indicators? (wikimedia.org) EFIS instruments during a test flight.

How is the drift (accumulated error) nullified in INS-driven attitude/heading indicators?

The related post 'How are attitude indicators kept accurate?' does not cover the INS-driven flight instruments, whether electro-mechanical or pure-EFIS installations. In a purely mechanical system one of the solutions is the ingenious 'pendulous vanes' as the other post shows.

While there are two major types of INS systems, the strap-down and the stabilized platform, I don't know if the drift correction is the same (computational?) or different.

I'm asking out of curiosity and thus I'm looking for a simple (as opposed to inner workings) answer. For example, if computational, what inputs/sensors are used to correct for an unknown amount of drift? If mechanical, what is the equivalent to the 'pendulous vanes' when it comes to INS installations?

I've tried searching for an answer, but I keep arriving at very recent [and very technical] innovations/papers, and not the core principle.

Update:

I found this blog post, which is not about aircraft-grade INS systems, but if I understand it correctly, sensor fusion makes such instruments not prone to long-period drift. Is the same principle used in the INS-driven flight instruments?

Inertial navigation systems do not need to correct for drift.

Modern optical gyroscopes drift about 0.001°/h and even the mechanical ones used aboard the apollo spacecraft back in 1969 achieved 0.03°/h (see Apollo 8 Mission Report, page 119). After a 20h-flight with those mechanical gyros, accumulated drift would be 0.6° and therefore less than the error of a self-erecting, ordinary attitude indicator during a turn.

The reason ordinary attitude indicators need an erection mechanism is not that they are imprecise, it is because earth turns and the aircraft moves.

Earth turns at a rate of 15°/h at the equator and 0°/h at the poles. The attitude indicator must adapt to the unknown drift unless it knows its latitude, in which case the drift is also known.
Aircraft travel considerable distances. The local horizon in Cape Town (34°S) differs from the local horizon in Berlin (52°N) by 86°. An attitude indicator must correct for these changes as well, unless it knows it just travelled from Cape Town to Berlin.

Inertial navigation systems do not suffer from these issues. They know their position, the route they travelled and how long they've been flying. They just leave the gyroscopes as is and apply corrections to the display only.


What methods are used to perform head tracking in the cockpit? In this question, I asked about a helmet with built-in HUD, and have a follow-up question:

What methods are used (or have been used) to perform head-tracking in the cockpit?

The website for that particular helmet (which I'm surprised exists like it does ... could I simply order one? doubt it ...) has the following (edit: here is additional information for that helmet):

  High-speed tracker with a full-spherical Field of Regard(FOR); choice of Hybrid-Inertial or Magnetic tracker

Accelerometers are used, however for lower latency and higher definition, tracking infrared emitters is the better choice1.

The A-10 as you wrote uses inertial or magnetic trackers. The Eurofighter helmet uses infrared tracking.

  ... the Typhoon’s HMSS features lower latency, higher definition, improved symbology and night vision.

It's the one on the left (1source).

You can already see it has an organic brain of its own; those bumps are the IR LEDs for tracking.


Are pilot YouTubers breaking any rules? Recently, I've been watching a few YouTubers (mostly FOs) who make great videos like this:

Then I realized... They must spend a lot of time setting up cameras, playing with them, hitting record and stop etc. This could potentially distract them or make them miss something (a warning, a pre flight check etc)..

Are they breaking any formal rules doing this? If an accident occurred during filming would they be under investigation due to negligence?

If the FAA considered making videos itself (i.e. separate from any unrelated rule violations within those videos) to be a problem, they would have put a stop to it long ago, either by explicit rulemaking or the catch-all "careless or reckless" when no other rule applies. They haven't, at least with respect to GA pilots--who are allowed to do all sorts or things that charter and airline pilots are not.

Several YouTubers I follow have said they mount the cameras and start recording before preflight, don't touch them again until after the plane is down and secured, and edit out the boring parts later that show this.  That seems to mitigate the physical safety issues.

The other issue is pilot distraction if they're talking to the camera, but I don't see that as any worse than talking to passengers, which is commonly accepted within reason, and is perhaps less of a risk since the camera doesn't talk back.


How does the Guardian counter-MANPADS system work? I came across this picture of a FedEx McDonnell Douglas DC-10 equipped with an anti-missile defense system.

(wikimedia.org)

According to the picture description:

  FedEx became the first U.S. carrier to equip its aircraft with an anti-missile defense system in 2006. The gray oval Northrop Grumman Guardian pod can be seen on the belly of this FedEx MD-10 between and just aft of the main landing gear.

How does this system work? Does this successfully work in countering all types of missiles? Can it be reused over multiple missile strike incidents? Are all commercial airplanes compatible with this system? Is it economical for a major airline to fit all of its fleet with this system?

I have worked on and designed DIRCM systems.  Most of the technical specifications on these systems is classified.  Also, the U.S. is not the only country with DIRCM systems.  In particular, Israel is equipping their airliners with the C-MUSIC system.

How does this system work?
Basically, a laser at the wavelengths of the Infrared Missile seeker is directed at the incoming missile.  It is pulsed in such a way as to cause the missile to think that the hot spot that it is tracking (generally the engines) is not in the right spot for proportional steering. Without proportional steering, the missile avionics thinks that the missile will miss the target, so it corrects for this.  As it corrects, the laser signal continues to misinform the missile as to where the target is in its reticle, eventually causing the missile to "break lock" and fly off into a random direction. Before the laser can be directed at the missile though, there are several other steps, which is why you have a complete pod.
First a missile warning system needs to pick up the fact that a missile has been launched at the aircraft (generally the job of a series of IR cameras that give a lower hemispheric view of the aircraft's environment). Then the system tells the turret to turn to a specific point in space. The turret has its own IR camera that is responsible for tracking the missile, and once it has achieved this tracking, only then does the laser start to fire.

Does this successfully work in countering all types of missiles?
This is only effective against purely infrared guided missiles (mainly MANPADs and some varieties of air to air missiles, such as the AIM-9).  If the missile has any sort of radar assist, or is radar guided, a laser will do no good.  There are some classes of laser riding missiles that could potentially be confused by these type of systems, but no anti-air types of laser guided missiles are deployed.

Many IR guided missiles do try to counter these countermeasures, so the more sophisticated DIRCM systems need to be employed. IR missiles come in many different varieties of seekers and wavelengths they use, so the DIRCM system needs to have a laser on those wavelengths.  Not only that (and this is where the secret sauce comes in) each seeker operates on different frequencies of their reticles (and reticle patterns). So to truly confuse a missile, one needs what is known as a Jam Code. These are generally classified.

Can it be reused over multiple missile strike incidents?
As long as the laser is operational, the system can be used over and over again.  Some lasers have limits to how long they can fire, and how often they can fire (the Northrop Viper laser is notoriously power limited and has a horrible MTBF).  However, as long as there is power, it doesn't run out of ammo.  This is the main advantage of DIRCM systems over using infrared countermeasures such as flares, which are limited by the number of flares one can carry.

Are all commercial airplanes compatible with this system?
The Guardian, as well as BAE's JetEye, and Elbit's MUSIC system is designed as to be integrated onto any large airframe.  Israel has already flown it on 707 and 737 airframes, and plan to integrate it to the A-320, 747, 757, 767, and even some Embraer airfrmaes.

Is it economical for a major airline to fit all of its fleet with this system?
Not really, unless one gets shot down by a MANPAD.  The DIRCM system will cost well over a million $US (probably closer to two).  And then there is the aerodynamic penalty of hauling a 350-500lbs pod on your belly.  The concept Elbit is employing is to install aircraft with a B-kit (an interface plate and the necessary wiring).  Then the airline can buy a limited number of A-kits, and install them only on the aircraft that are going to an area that may be dangerous. Notionally, this operation can be done in an hour.

As to the danger, all I can say is that over 500,000 MANPADS have been manufactured, and many thousands are in the hands of people that wouldn't mind causing a "problem" for anyone they can inconvenience. Thankfully, the training required is a bit more than firing an AK-47, so most attempts have been unsuccessful.


Under what conditions can a flight continue beyond the point of in-flight re-planning? I'm studying ICAO Annex 6, but I am confused a little bit about the paragraph below:

6-2.2.3.4.2(b)
A flight to be conducted in accordance with the instrument flight
rules shall not take off or continue beyond the point of in-flight
re-planning unless at the aerodrome of intended landing or at each
alternate aerodrome to be selected in compliance with 2.2.3.5, current
meteorological reports or a combination of current reports and
forecasts indicate that the meteorological conditions will be, at the
estimated time of use, at or above the aerodrome operating minima for
that operation.

Here I have simple question about following situations.

Can I continue flight at the point of in-flight re-planning if:
"destination airport WX > minima" & "alternate airport WX < minima"

Can I continue flight at the point of in-flight re-planning if:
"destination airport WX < minima" & "alternate airport WX > minima"

it’s just a complicated way of saying: you are not allowed to begin an IFR flight either from the ground (“shall not take off”) nor can you file a new one in the air (“continue beyond the point of in-flight re-planning”) unless some weather conditions are met at the destination and nominated alternate(s).

I other words the “point of in-flight re-planning” is not a decision point, it’s just: the point where we would like to file (re-plan) a new flight plan (in-flight).

And the conditions are the same wether you are on the ground or already flying: either (destination WX > operational minima) or (ALL nominated and required alternates > planning minima)

note that (depending on jurisdiction) most of the times a minimum 2 alternates are required if destination WX < operational minima.

  Can I continue flight at the point of in-flight re-planning if:
  "destination APO WX > minima" & "alternate APO WX < minima"

if destination > minima, under specific circumstances you might not require an alternate. if still required, it must be nominated and above the planning minima. So answer : probably no.

  Can I continue flight at the point of in-flight re-planning if:
  "destination APO WX < minima" & "alternate APO WX > minima"

if destination < minima, you will most likely require 2 nominated alternates and the must BOTH be above planning minima. Shory answer: yes


Why are different distances listed for the ILS and the LOC on this approach chart? Maybe someone has an idea why are there two separate distance values (marked in blue) for ILS and LOC approaches?

A very weird case, but each country can do things their own way.
The ILS FAF - GS intercept at 2700' - is 0.2 NM inside the LOC FAF. In the FMC database, the former is coded as FI01, and the latter is VI311. The LOC FAF is also 6.5 DME off of the LOC DME, identifier IAV, which corresponds to 7.4 DME off of VNO (the Vilnius VOR).
The key to seeing this is to consider how you get different distances from the VI312 IF point to the depicted FAF(s). Since VI312 is a single point in space, you can see where the two FAF's are relative to it... the ILS FAF is 0.2 NM farther away (which corresponds to it being 0.2 NM closer to the runway in the other set of blue-circled numbers.
Why the approach designer made the LOC FAF not quite coincident with the ILS glideslope intercept point, is its own question, and I have no insight there. (Totally mystified, in fact.)


Mach Factor - Factor increase of thrust due to the ram effect In the book of Corke, Chapter 7 about engine selection, he speaks about the Mach factor which is a factor larger than 1 in supersonic to take into account the ram effect.

In his case study he takes 2,39 for a cruise speed of Mach 2.1.

He mentions that there exists algorithms and books that allow to compute this Mach factor.

I am looking for a figure showing this Mach factor in function of the Mach number. I have been looking like crazy.

If anyone has sources or this type of chart, it would be great.

Thanks

It sounds like what you are describing are the isentropic compressible flow equations. See equation 6 & 7.

At Mach 2.1, these give a stagnation pressure rise of 9.14, and a stagnation temperature rise of 1.88, since air has a value of 1.4 for gamma.

But, the air does not come to complete rest (stagnation) in the intake, hence these values are not obtained.

What you need to know, is the actual Mach number at the engine inlet, which is I assume where you are suggesting is the location of a total pressure 2.39 times higher than the freestream total pressure. Typically, this is around Mach 0.4. But, since the freestream flow is supersonic, that means it will have had to go through some shock waves, to get to a subsonic speed. This means there will be pressure losses, associated with those shock waves. Normal shock waves (a shock wave perpendicular to the flow) result in higher total pressure losses than oblique shock waves. Hence, supersonic aircraft have pointed cones in their intakes to create oblique shock waves, to minimise the pressure losses. To calculate the total pressure losses from these, you need to know the geometry of these cones to know if they cause 1, 2, or maybe even 3 oblique shock waves, as the pressure loss of each shock depends on the strength of each shock wave.

Jack Mattingly, in his text book "Aircraft Engine Design" gives a good description of how to calculate the losses associated with oblique intake shock waves. It's fairly complex. See here. The figure you want is 10.35, but that wasn't shown in the free preview when I tried it, but maybe the pages they hide are random, and you will have better luck.

So, in summary, to verify the 2.39 factor, you need to know the Mach number at the engine fan inlet, and be able to calculate the total pressure losses of the shock waves, which requires info of the intake geometry.


Is it theoretically possible to create an aircraft like this? I was browsing through some funny airplane pictures when I came across this. An airplane with the engines replaced with the cockpit and the cockpit replaced with a huge turbofan.

I was intrigued by this idea, but then it also brought up the question: Is it theoretically possible to create an aircraft like this and be possible for the aircraft to take flight? (as in fly in the air for prolonged time)

Source: YouTube

The idea has been tried. This WWII aircraft is built around the idea very much like yours: engine in the main fuselage, pilots/passengers in the nacelle.

It must be reasons to choose this design. The design requirements were:

One engine.
Crew one or two
Best possible view into to all sides
(reconnaissance aircraft).

For the passenger aircraft as pictured it is probably just not optimal.


Why in the Seneca 2 the Rated Horsepower are higher at 12000 feet than at sea level? (215Hp at 12000feet, 200Hp at sea level) Reading the POH I found that the Rated Horsepower at 12000 Feet are higher compare to sea level. I have in the past studied the back pressure of the exhaust gasses but I cannot completely understand how it works.

Back pressure affects power because the more pressure opposes the exhaust, the more exhaust gas is left in the cylinder when the exhaust valve closes.  This trapped exhaust gas, effectively inert for purposes of the engine cycle, limits the amount of intake charge that can be drawn from the manifold at a given manifold pressure.

The more intake charge you have, the more power you get (within limits), so the less unscavenged exhaust you have diluting the intake charge, the more power you can generate for a given manifold pressure.

This effect can be mitigated to some extent with valve overlap -- where the exhaust valve remains open for a short time after the intake valve opens; this allows the incoming intake charge to push some additional gas out through the exhaust valve -- providing there isn't too much "back pressure" on the exhaust side.


How to become a helicopter pilot? I once had a dream of becoming a helicopter pilot. Then life happened: married young, kids, school, career, etc. I'm working in a good career while finishing a Ph.D. in engineering. But still, I find myself wondering "what if?"

My dad was in a similar situation when he was my age (33) when he started flying (and selling to pay for the habit of flying) powered parachutes. I soloed for the first time in a PPC at 10-years-old and have hundreds of unlogged flight hours. Not a helicopter, but still, some flying experience and lots of good memories.

So my question that isn't career advice: what are the paths for getting into a helicopter flight career? And, are there any routes for people like me?

Military? Probably too late for me. I've heard that there are age restrictions but I'm not certain regarding the specifics. Others may be interested, though.
Local flight school? I'm hesitant. I know people who have gotten burned by the flight schools at local airports when the school closed and the owners disappeared with their money. These seem sort of like a Ponzi scheme anyway, IMO, since once you become an instructor, you train others, who are also becoming instructors, who are also... turtles all the way down...
Commercial Flight School?

Are there any grey-beards out there who can offer some perspective on the three options? Or offer other alternatives?

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

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

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

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

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

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


Has an airplane ever been launched from a submarine? Airplanes are routinely launched from ocean going vessels, namely carriers.

Missiles are routinely launched from submarines.

Has an airplane ever been launched from a submarine?

From questions about what qualifies:

Manned or unmanned is ok, but payload mass has to approximate a least a small person
One way trip is ok
Airboat dropped in the water is ok
Airbreathing primary propulsion required, rocket assist for takeoff is ok
Wings for lift are required, control fins alone do not qualify

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.


What are the random, differently-coloured rectangular patches on the top of this Boeing 737's fuselage? The top section of this green-painted Boeing 737's fuselage was slightly faded and I noticed a few dark-green, rectangular patches on its top. They look like patch kits on a bicycle tyre's inner tube. What are they? My understanding is that hairline cracks/weaker spots in the fuselage can't simply be patched like this?

They aren't "repair patches" themselves, they are repainted areas where some sort inspection and/or repair was done to the underlying structure, that required removal of the paint in the area. What the inspection or repairs were done?  Well, you'd have to have been there to know, but the 737 does have a lot of structural inspection requirements related to fatigue cracking in fuselage frames and skins.
There may be actually be skin outside doubler patches riveted on in those dark spots to reinforce areas where crack repairs were done, but the borders of the dark spots are just paint masking lines and it's hard to tell in the photo what was done.
Another possibility is that surface corrosion was found at those spots and the paint in the area was stripped, the corrosion was blended out and the zone repainted.
They should probably have their paint guy do some mixing to match the fresh paint to the faded paint, and apply the paint with an edge feathering technique to avoid the sharp masking lines (you have the edges of the mask supported above the surface to allow some paint to spread below, to create a soft edge), so that folks in gate areas looking down at their incoming flights would be less alarmed by the Quilting Bee look.


Why do Czech Airlines have the airline code (IATA airline designator) "OK"? Is there any specific reason why Czech Airlines have OK as the IATA airline designator?

According to an urban legend:

  Urban legend says that when airline names were awarded -- I believe in 1946/46 -- the Czechoslovak delegation, not versed too much in English, just said OK to the questions asked. So the name stuck.

I do not much believe in the legend, but still the question remains.

I would expect that at the time when the (then) Czechoslovak Airlines were assigned the code, there were not so many airline operators as nowadays and that CS (or something similar) must have been available.

It starts with the International Telecommunications Union, which assigned radio call sign prefixes for each country in 1927. The OK prefix was assigned to Czechoslovakia. Other than the largest countries (US, USSR, UK, France, etc.) there seems to be no specific pattern to the assignments. When ICAO assigned aircraft registration prefixes for each country it usually based them on the ITU prefixes. In 1947, when ICAO assigned 2-digit airline identifiers they chose to use the country’s registration prefix for Czechoslovakia’s flag carrier. IATA originally based their airline ID’s on ICAO’s 2-letter ID’s. In 1982 ICAO changed to their current 3-letter format.


Do professional pilots have to show their passports every single time when flying back & forth between the same airports? What process do pro pilots have to go through with their passports when on the clock?  Are they afforded any shortcuts or do they have to go through security EVERY SINGLE TIME?

Pilots (and aircrew) usually go through an expedited security screening, usually separate from passengers, in order to reduce wait times.

For example, US TSA has a program called the Known Crewmember Program that allows pilots and crew members to bypass normal (passenger) security line.

Pilots (and aircrew) have to show the documents as required by the local authorities. Usually, this will be the passport/visa or General Deceleration. This varies from country to country.


How does the efficiency of a piston aircraft change with altitude? Turbine aircraft are more fuel-efficient, per mile, the higher they fly and this is for two reasons:

drag depends mostly on indicated speed, but the same indicated speed corresponds to higher true speed, and
the lower temperature increases efficiency of the engine.

Now both effects should also apply to piston-engined aircraft. Plus

spark-ignited engines are more efficient with open throttle, which again occurs near operating ceiling.

So I would expect piston aircraft to also fly as high as possible, and most piston aircraft should have no problem to get to at least 10,000 ft (where supplemental oxygen is not needed yet). But in various stories and documents I notice even long flights are often made much lower. Below 5,000 ft.

So is there actually some reason why piston-powered aircraft would be more efficient at lower altitudes? Or any other reason to prefer flying low?

You are right, flying higher will be more efficient. However, the inverse relation between thrust and speed in propeller aircraft means that the available speed range is much smaller, and thus the possible altitudes are more restricted. Simply put, the possible gains are smaller.

If you look at the thermodynamic cycle of a turbo-compound engine, it looks almost like that of a jet. Still, the high bypass ratio translates into a lower optimum flight speed and the lower power-to-weight ratio usual for piston aircraft limits the maximum altitude and makes them more susceptible to adverse winds. Also, turbo-supercharging, which is much more efficient that supercharging, was developed to a practical solution only a few years before gas turbines were introduced, so pre-WW II piston engines relied on the less efficient supercharger for boosting power, reducing the benefit from flying higher yet again.

Why historic flights were performed at lower altitudes has probably more to do with the low ceiling of the initially heavy airplane and the lack of supercharging than with a deliberate choice by the crew. Maybe the lack of cabin heating or de-icing equipment, or even an open cockpit, made lower, warmer altitudes also preferable. Another reason might be the need for terrestrial navigation in the absence of today's infrastructure of radio beacons and GPS satellites. That flying higher translates into better range was known as early as WW I.


What are the difficulties of landing on an upslope runway? On runway like the one in Courchevel Altiport, how difficult is it to land? what is diffirent in the aircraft landing configuration when landing on an upslope runway vs normal runway?

EDIT:

Thanks for the answers guys, I didn't know that the airport had a level "touch down" point. still I am curious what should a pilot do (regarding air speed, pitch, flaps ...) if the runway itself was 10% upward (no level parts)... would the plane be in a "climbing-like" configuration? what about speed?

As far as landing in a up-sloped runway is considered, the pilot will make a lower approach if not careful as he/she may consider the ground before to be sloping up as well. From flytime.ca:

  When a runway is upsloping, the pilot thinks that the runway continues on an upslope from the terrain before it, hence thinking the terrain in front of the runway is upsloping as well.   The pilot will judge their altitude as too high, because they perceive the terrain continues on an upsloping, positive angle towards the runway  and will consequently plan a low approach that can cause landing short of the runway.

Image from flytime.ca

Another thing to note is that the landing roll will be reduced (on the other hand takeoff roll would be increased, if takeoff is on uphill direction; else, it's reduced). For example, in this airport, the actual landing point is level- shortly after the aircraft touches down, it will start traveling uphill, and the landing roll will be shorter. One (passenger) describes the landing as:

  During the approach we were below the runway and climbed up to it to land. After touchdown, we slowed down very fast due to the upslope. Very different than the normal runout after a landing.

Of course the location of this particular runway means that the landing has to be done correctly the first time around.


Is it feasible to add an aftermarket hydrofoil to a light aircraft? Could long, aftermarket hydrofoils be added to the water-skids of a plane (like this Cessna with pontoons or smaller)

Cessna 208 Caravan
not designed to have a hydro foil, to increase its ability to land on choppier water?
In theory, the pilot would first enter the water with the skids, clear of the chop, then stall the hydro foil similar to stall landing a plane thus giving the plane a soft or possibly vertical landing.

Grumman JRF-5 with hydrofoils

It's been done.  David Thurston designed a system for the Lake LA-4 that was purported to allow it to land on 5ft waves.  It may have been a bit of a handful (the Bucaneer is a demanding airplane on its normal hull) and was crashed by someone who took it for a spin without Thurston's authorization, and that was the end of that.


How do I calculate the tail size of my rc plane? I'm in the conceptual design phase and my plane is supposed to be of 1.7kg. Wing dimensions are 1m*0.3m and AoA that it will fly at will give a pitch down moment of 0.03. The moment arm between CG and tail is 1m. How do i calculate the size of the tail. It's supposed to be a conventional tail.

There's a formula for calculating minimum tail volume coefficient (tail area X distance from the wing) for light aircraft.  Not sure if the same value applies to RC but it's a good starting point. This is from David B Thurston's book Design for Flying (Thurston was an engineer on numerous Grumman programs going back to the Hellcat).

(Tail Arm ÷ main wing MAC) X (Tail surface area ÷ wing area) = .55 or greater

Tail Arm is the distance from the C of G to the 30% chord point of the horizontal tail.

.55 minimum is for a symmetrical airfoil section tail typical of light planes like Cessna 150s.  I would use a higher number for a flat sheet tail surface, and being RC, so maybe .6 to .8.

While you're at it, Google "tail volume RC aircraft" and you'll find some more information specific to RC that'll be helpful.


Have any aircraft ever been attacked by chemical, biological, radiological or nuclear weapons? Information security compromises? Have any aircraft, military or civilian, ever been attacked by chemical, biological, radiological or nuclear weapons?
Terrorist attacks and test firings included. Attacks on persons on-board the aircraft (rather than the aircraft itself) included.
Information security attacks on the aircraft itself (such as computer viruses) would also be of interest, although I'll exclude jamming of communications and sensors such as radar.
Sources I've already consulted:

Airport and Aviation Security, Elias
USACBRN School, Wikipedia
The Jihadist CBRN Threat, Stratfor
Flying High, Flying Safe, Wolff
Practical Aviation Security, Price & Forrest
Counterterrorism: A Reference Handbook, Steven & Gunaratna

Definition of 'chemical weapon': sure, explosives are made out of chemicals (as is everything else) but I'm talking about chemical weapons as defined by the Organisation for the Prohibition of Chemical Weapons:

The term chemical weapon is applied to any toxic chemical or its precursor that can cause death, injury, temporary incapacitation or sensory irritation through its chemical action. Munitions or other delivery devices designed to deliver chemical weapons, whether filled or unfilled, are also considered weapons themselves.
The toxic chemicals that have been used as chemical weapons, or have been developed for use as chemical weapons, can be categorised as choking, blister, blood, or nerve agents. The most well known agents are as follows: choking agents—chlorine and phosgene, blister agents (or vesicants)—mustard and lewisite, blood agents—hydrogen cyanide, nerve agents—sarin, soman, VX.

For 'radiological weapons', I'm not talking about visible electromagnetic radiation (light) or invisible electromagnetic radiation (radio etc) but rather radiological dispersion devices, defined by the US Department of Defence as:

any device, including any
weapon or equipment, other than a nuclear explosive device, specifically designed to employ
radioactive material by disseminating it to cause destruction, damage, or injury by means of the
radiation produced by the decay of such material

Ford, J. (March 1998). Radiological Dispersion Devices: Assessing the transnational threat. National Defense University - Institute for National Strategic Studies - Strategic Forum, No. 136. as cited in Rickert, Paul R., "The Likely Effect of a Radiological Dispersion Device" (2005). Faculty Publications and Presentations. Paper 47.
'Biological weapons' as defined by the United Nations:

Biological weapons are complex systems that disseminate disease-causing organisms or toxins to harm or kill humans, animals or plants. They generally consist of two parts – a weaponized agent and a delivery mechanism. In addition to strategic or tactical military applications, biological weapons can be used for political assassinations, the infection of livestock or agricultural produce to cause food shortages and economic loss, the creation of environmental catastrophes, and the introduction of widespread illness, fear and mistrust among the public.
Almost any disease-causing organism (such as bacteria, viruses, fungi, prions or rickettsiae) or toxin (poisons derived from animals, plants or microorganisms, or similar substances produced synthetically) can be used in biological weapons. The agents can be enhanced from their natural state to make them more suitable for mass production, storage, and dissemination as weapons. Historical biological weapons programs have included efforts to produce: aflatoxin; anthrax; botulinum toxin; foot-and-mouth disease; glanders; plague; Q fever; rice blast; ricin; Rocky Mountain spotted fever; smallpox; and tularaemia, among others.

What immediately comes to mind is the Lockheed XF-90, an early US jet fighter, which had a very strong and heavy airframe. For that reason, the second prototype (of only two ever built) was subjected in 1952 to three nuclear bomb tests in the Nevada desert (not in flight, however. It was sitting on the ground).

At this time, an unguided nuclear-armed air-to-air missile, the Douglas AIR-2 Genie, was developed and tested once in 1957. Several USAF officers were standing below the test spot, but no aircraft were used as targets. The missile was fired by an F-89 Scorpion and could be carried by the F-101, the F-104 and the F-106.

The purpose of the AIR-2 was to attack tight formations of Tupolev Tu-4 bombers in the days before guided SAM missiles were available.

I am sure the Russians had their own score of nuclear tests on aircraft, but I do not know anything specific.


What is the relationship between EGT, Fuel Flow, N1, N2 and Air Temperature? I initially wanted some sample data for EGT, Fuel Flow, N1 and N2 to build a prognosis system that predicts engine failure (ECTM).
I searched for some sample data online but got to know that there's virtually no way of acquiring that. (Where can I find sample data required for Engine Condition Monitoring?)

Now, I have to generate my own data for this. It is crucial for me to understand the relationship between the 5 important parameters that I'm taking into consideration.

Parameters are EGT, Fuel Flow, N1, N2 and Air Temperature.

For now, I've understood this:

Temperature increases, EGT increases.
Temperature increases, Fuel Flow increases.
N2 decreases, Fuel Flow increases. (src - What does it mean for N1/N2 to float?)

Can you guys please give me more insight like the ones above? Also, please correct my mistakes. I'm a computer science student and this is all very new to me.

I am not really sure what sort of engine failures you want to predict, but ECTM is usually used to detect things such as;

Worn air seals
Blade tip wear (compressors and turbines)
Compressor fouling
Eroded blades and turbine nozzle guides

These hardware issues cause changes in the “state variables”;

N1 (LPC) compressor efficiency
N2 (HPC) compressor efficiency
N2 (HPT) turbine efficiency
N1 (LPT) turbine efficiency

These changes in component efficiency then result in changes of the engine parameters;

N1 rpm
N2 rpm
Fuel flow
Exhaust gas temp
etc.

By detecting changes in the measurements, ECTM aims to tell you which module (LPC, HPC, HPT, or LPT) has degraded. Usually, it’s not possible to say why the efficiency has decreased (i.e. you can’t tell if it’s erosion of the blade leading edge, or worn blade tips), you can only tell which module is affected.

But the relationship you want between changes of efficiency and the measured parameters, is very complex. For that step, you need a performance simulation program, to give sufficient accuracy if you are actually going to apply the approach to data from a real engine. Otherwise, all one can say is, when an engine degrades, fuel flow, temperature, and rpms will go up. And they go up for most (or all) of the faults above. So, unless you know how much they change for each fault, you can’t tell which module has degraded. Then, your method is just saying “the engine has degraded” (not so useful), rather than why (and, only the LPC needs a repair, for example, which is cheaper and quicker than an HPC repair, because it requires a lot more disassembly).

But, if all you want to do is test an approach in theory, you could just make up the increase in each parameter, it doesn’t have to be accurate, because it’s just a mathematical example, you are not going to apply it to an actual engine, so the predicted effects of degradation don’t need to match reality closely. Just make up 4 different patterns of degradation;

degraded LPC: 50 rpm N1 increase, 200 rpm N2 increase, 5% inc in fuel flow, 200 deg inc in EGT
degraded HPC: 100 rpm N1 inc., 200 rpm N2 inc, 7% in in fuel flow, 150 deg inc in EGT

etc...

Then, create multiple examples of each fault by add a random amount of noise to each parameter, say < or up to 1% of full scale value (and you can make full scale up as well, let’s say, N1 = 8,500 rpm, N2 = 12,250 rpm, fuel flow = 3.2 lb/s, EGT = 650 deg C.)

Now, you have examples with noise, of known faults. See how well your method can correctly find the faults....

Sure, it’s a little crude, but if you can’t get hold of a performance simulation program that lets you change the efficiency of the modules to generate the degraded fault behaviour, at least it lets you do something. You can play around with the numbers, and see what size enables you to detect a fault, and what size does it start to get confused/ wrong.


Why was the Boeing 747SP unsuccessful? Why was the 747SP not very successful with sales and longevity in service with the airlines that did purchase it?

It just was not a great fit for the use case it was built for at the time it was released:

The Boeing 747SP was hit by the same problem that many airlines have
suffered from throughout history… rising fuel costs. The aircraft was
heavy, didn’t carry as many passengers as a normal 747 and was
expensive to run (four engines after all). Boeing had expected to
build up to 200 of these aircraft, but their dreams were never
realized.
Additionally, by the early 1980’s, other aircraft started to arrive
that could easily perform the same route and provide a better
passenger experience. The last commercial flight of the Boeing 747SP
flew in 2006 for Iran Air. As the country is currently under embargo
and spare parts cannot be flown in from Boeing, it is likely that they
retired them out of necessity, rather than just replacing them with
something better.

It was a clunky 4 engine fuel hog, which was not great with rising oil prices

The aircraft, despite its long range, was not popular with airlines,
and only 45 were built. This was mainly due to its four-engine
configuration, which meant high fuel usage and higher costs for
airlines, making it difficult to break even on ultra-long-haul
flights. Shorter flights weren’t profitable either, as the 747SP’s
high fuel capacity added weight and increased fuel burn on short hops.
The aircraft’s failure was not due to a lack of effort on the part of
airlines.


Did Boeing really use ferrets to run wires through aircraft during construction? I've seen many articles on the internet (here, here & here) claiming that Boeing used a team of ferrets to run cables through hard to reach areas when building aeroplanes, but I've never seen any official sources to back these claims.

Is there any truth to this, or are they just baseless internet rumours?

No, Boeing didn't do that.
I asked their assembly plant tour
to confirm or deny this, when I found
a report that around 2010 some visitors heard this from a tour guide.  On 2020 Nov 3 they replied:

Hello,
Thank you for contacting us with your inquiry. This is a popular urban myth, there are no records or any other evidence that this was done at Boeing.
Thank you again for your interest.


What is the usage ratio of the An-225 Mriya? Does anyone have data on how much time the An-225 (only one was produced; it is currently the largest aircraft in the world) spends flying/on missions and how much it waits for customers?

This is a very unique aircraft within the scope of contemporary aviation:

Also, there is a copy of the plane - partially built.  Does that suggest there might be need for the completion of second aircraft, to meet demand?

As a privately-operated aircraft, this is probably not going to be a question which is completely answerable. All that is available is best guesses and incomplete information!

That said, the An-225's last tracked flight on FlightAware – as of today – was March 15, 2012 between Bangor, Maine (KBGR) and Port Harcourt, Nigeria (DNPO). That suggests that the An-225 is either being excluded from flight tracks, or simply hasn't visited the US since early 2012.

Aside from that, the only other likely place for flight information is photo sharing websites like airliners.net and flickr, or on youtube. Browsing through flickr, I tried to track down dates of recent photos in unique locations (which is a guess at best):

May 21 2013
June 24 2013
September 25 2013
September 26 2013
November 12 2013
November 17 2013
December 10 2013

I'm sure there are more, but I stopped searching. It looks like it's probably doing charter cargo trips at most a couple of times a month, which is honestly more frequently than I expected! If you're interested, I'd encourage you to track down more on the sources I mentioned.


How are these parameters derived from readings of sensors in an aircraft: angle of attack, slip angle, and airspeed? From what I've learned, the reading comes from

an angle of attack sensor and a slip angle sensor. Let's call the readings $\hat\alpha$ and $\hat\beta$ respectively. These are of either the weathervane type or the differential pressure type.
airspeed sensor, i.e. a pitot tube. This probe measures the total and static pressure $\hat p_\text{total}$ and $\hat p_\text{static}$, and the airspeed $\hat u$ is obtained from them.

Here's the catch: in real flight, the measured angles $\hat\alpha$ and $\hat\beta$ may depend on not only the true values $\alpha$ and $\beta$ but also other factors which include true airspeed $u$, and the measured airspeed $\hat u$ may depend on $\alpha$ and $\beta$ too. How is this problem solved? How are these sensors calibrated? Are they tested in a wind tunnel where every possible combination of $(\alpha,\beta,u)$ is given to produce $(\hat\alpha,\hat\beta,\hat u)$ and form a lookup table?

They use test flights.

They attach more instruments to the plane and move them outside the influence of the planes own turbulence.

From the known good values of these test instruments they can calibrate the actual instruments.


How does a towed banner maintain a steady tilt? Today a small airplane is towing an advertising banner, about 20 by 30 feet, in a circular path around my town's university campus.  The banner stays tilted about 45 degrees to make it easier to read by those whom the airplane is circling.  Winds are 10 knots gusting to 20, if that matters.
How can it maintain that tilt?  Surely the tether can't exert a torque on the banner.
Websites of companies such as Van Wagner Aerial Media and Airplane Banners Nationwide say they can do this, but without explanation:

Each advertisement will be tilted, with your choice between zero and 90 degrees, towards your audience.

Banners have a lead pole to keep the leading edge straight. This pole is weighted at the bottom, which ensures that the top and bottom of the banner remain in the correct place.
By itself, this would ensure the banner remain perfectly vertical. To achieve the proper angle for viewing on the ground, a tab is attached to the top of the lead pole, adjusted to the required angle to push the top of the pole in the desired direction to achieve the right viewing angle.
Here's a quick sketch to try to illustrate the setup, with the banner, pole, weight, trim tab, and harness:

Here, we're looking at about a 45 degree angle from ahead of the banner, with the trim tab adjusted to the left of the banner's direction of flight, causing the top of the banner to tilt to its right:

In other words, the tab at the top of the pole acts similarly to an airplane's rudder, where deflection to one side results in a force applied in the opposite direction.
More reading here:
Aviation Stack Exchange: How are banners kept upright?
Pilots of America: Banner aerodynamics
Quora: How are flying banners stabilized?


Is the true altitude always lower than the indicated altitude as long as a correct local altimeter (QNH) is set on the altimeter? Given the fact that the altimeter setting is corrected for the temperature of the airport, on the condition that a correct local altimeter setting (QNH) has been made to the altimeter, is the true altitude always lower than the indicated altitude as the altitude increases above the airport elevation regardless of the temperature of the airport, since the temperature generally drops as the altitude goes up?
For example, even when the temperature in the airport is, say, above 30 degree Celsius, do we still fly lower than the indicated altitude of, say, 5000ft, just like when we do when the temperature is below 15 degree Celsius, standard ISA temperature?

No, the true altitude can also be higher than the indicated altitude, if the temperature is higher than expected from the International Standard Atmosphere (ISA). This case is usually not as important as the opposite case because flying at higher true altitude does not create a risk due to terrain clearance.

In a standard atmosphere, the indicated altitude
reflects the true altitude above the mean sea level
(MSL) and therefore provides a reliable indication of
terrain clearance.
Whenever, the temperature deviates significantly
from the standard temperature, the indicated altitude
correspondingly deviates from the true altitude, as
follows:

Extreme high temperature: the true altitude is higher than the indicated
altitude,
Extreme low temperature: the true altitude is lower than the indicated
altitude, thus creating a lower than anticipated
terrain separation and a potential obstacle clearance hazard.

(Airbus - Approach-and-Landing Briefing Note - 3.1 - Altimeter Setting - Use of Radio Altimeter)


Would it help airlines, if they knew passenger weight before hand? Its known that pilots and technicians calculate the weight of the plane once everything is loaded, then they can decide on how much fuelling etc. needs to be done.

Would it help airlines, if they knew the weight of all the passengers before hand, say at the time of ticket booking. Having that information, would it help them in optimizing the amount of cargo/fuel etc for the flight?

Despite of the "delay" during the check in procedures, it would be safer than the average companies usually use.
For instance, the latest FAA guidelines on aircraft weight and balance control set individual passenger weight averages at 190-195 pounds for adults and 82 pounds for kids, depending on whether it's summer or winter. (Based on the assumption people wear heavier clothing in winter.)
I have read an article, and there are more pieces of information.

Airline begins weighing passengers for 'safety'


Helicopter flight: Does the rotor disc plane and hub plane rotate along axis perpendicular to rotational axis? In many pieces of literature, diagrams as below are shown to explain the directional flight of Helicopter. I have the understanding that the cyclic input will only change the pitch angle of the blades and hence the amount of lift forces acting will vary around the disc. BUT the disc-plane (more specifically, the hub plane), will remain parallel (to the reference plane, for example, the skids, or the top surface of the helicopter). I mean, the disc plane shouldn't pitch down or up as shown in the diagram.

Am I correct? Also, I am finding hard to give a suitable name to the "reference frame", I tried to define above. Is there any standard name for that?

The shaft does not tilt, but the disk does.

You understand the function of cyclic control correctly. It indeed changes the pitch of the individual blades, shifting the centre of lift.

However, the blades are not strong enough to take that much bending load. If the rotor was completely stiff, they'd quickly break off. So instead the blades are allowed to flap up and down. That allows the plane of rotation of the blades to change first when you apply cyclic input and the body attitude follows.

The body attitude follows. When the rotor tilts, the action like of lift gets tilted with it, so it no longer passes through the centre of gravity, which will create a moment that will try to pull the body into the attitude corresponding to the rotor. With individual flapping hinges or flexing there is additional, more direct torque on the rotor hub trying to do the same, while on teetering rotor this is the only effect and the way cyclic is rigged prevents the rotor from trying to tilt too far relative to the body.

See also How are pitching and rolling moments transferred from rotor to the body?


How can I calculate the rolling moment of an aileron for a given plane based on its performance? Let's say I have an already-built airplane with known basic characteristics like weight, wing span and wing surface, and I can measure the time of all possible manoeuvres at different speeds.

How can I calculate the roll moment of an aileron in its maximal deflection? The precision of hundreds of [kg*m] is sufficient.

If you know the rolling speed at a given flight speed, you can calculate the aileron effectiveness and use that to calculate the forces. The final rolling speed is reached when roll damping and the aileron-induced rolling moment reach an equilibrium: $$c_{l\xi} \cdot \frac{\xi_l - \xi_r}{2} = -c_{lp} \cdot \frac{\omega_x \cdot b}{2\cdot v_\infty} =  -c_{lp} \cdot p$$
Thus, your aileron effectiveness is $$c_{l\xi} = -c_{lp}\cdot\frac{\omega_x \cdot b}{v_\infty\cdot(\xi_l - \xi_r)}$$
The roll damping term is for unswept wings $$c_{lp} = -\frac{1}{4} \cdot \frac{\pi \cdot AR}{\sqrt{\frac{AR^2}{4}+4}+2}$$
and the moment per aileron now is $$M = c_{l\xi} \cdot \xi \cdot S_{ref} \cdot b \cdot q_\infty$$
Calculate the moment for each aileron separately; normally the left and right deflection angles are not exact opposites, which helps to reduce stick forces.

If you only need an approximation, maybe do it like this:
You first need to have all dimensions and the deflection angles. I expect you don't have lift polars of the wing section, so you need to approximate the lift increase due to aileron deflection with general formulas. This is $$c_{l\xi} = c_{l\alpha} \cdot \sqrt{\lambda} \cdot \frac{S_{aileron}}{S_{ref}} \cdot \frac{y_{aileron}}{b}$$
and the moment per aileron now is $$M = c_{l\xi} \cdot \xi \cdot S_{ref} \cdot b \cdot q_\infty = c_{l\alpha} \cdot \sqrt{\lambda} \cdot \xi \cdot S_{aileron} \cdot y_{aileron} \cdot q_\infty$$
Nomenclature:
    $p \:\:\:\:\:\:\:\:$  dimensionless rolling speed (= $\omega_x\cdot\frac{b}{2\cdot v_\infty}$). $\omega_x$ is the roll rate in radians per second.
    $b \:\:\:\:\:\:\:\:\;$  wing span
    $c_{l\xi} \:\:\:\:\:\:\;$  aileron lift increase with deflection angles $\xi$
    $\xi_{l,r} \:\:\:\:\:\:$  left and right aileron deflection angles (in radians)
    $c_{lp} \:\:\:\:\:\:\:$  roll damping
    $c_{l\alpha} \:\:\:\:\:\;$ the wing's lift coefficient gradient over angle of attack. See this answer on how to calculate it.
    $\pi \:\:\:\:\:\:\:\:$  3.14159$\dots$
    $AR \:\:\:\:\:$  aspect ratio of the wing
    $\lambda \:\:\:\:\:\:\:\:$  relative aileron chord
    $S_{aileron} \:$  Surface area of the aileron-equipped part of the wing
    $S_{ref} \:\:\:\:\:$  Reference area (normally the wings's area)
    $y_{aileron} \:$  spanwise center of the aileron-equipped part of the wing
    $v_\infty \:\:\:\:\:\:$ true flight speed
    $q_\infty \:\:\:\:\:\:$ dynamic pressure

Depending on the relative chord length of the aileron, this formula is good for maximum deflections of 20° of a 20% chord aileron or 15° deflection of a 30% chord aileron. Remember: This is a rough estimate for straight wings.


How is engine fuel kept from freezing? The following table shows the operating temperature of Jet fuels (Source http://www.skybrary.aero/)

I know a lot of flights will be cancelled during bad weather conditions including the cold. How is the Engine fuel kept from freezing when the flight is operated at the mentioned fuel freezing temperature?

In regions and weather conditions where this is likely to be a problem, other fuels are used instead of Jet-A.

For example, Jet-B has a freezing point of -60C and is used during the winter in some parts of Russia and other very cold places.

For information about fuel freezing in flight, see this previous question/answer.


What improvements did Airbus make as a result of the Air France 447 accident? As far as I can tell, the following factors all contributed to this crash:

Pitot tubes susceptible to ice crystal blockage
Crew not aware that they can stall the airplane due to alternate law
No return to normal law after reliable speed information was available to the flight computers
"Unlinked" sidesticks which allow the pilots to command different inputs, unaware of what the other pilot is commanding
Stall warning goes quiet when the airspeed is too low

Of these, the first factor (pitot tubes) was already in the process of being mitigated. What about the other factors though? Were the onboard systems modified in any way to fix these shortfalls?

I'm no expert but it seems like at the very least that the following would help:

If the stall warning didn't stop, but instead changed into an even more ominous "you're now tumbling down at less than 60 KIAS" warning.
Also, given how extremely rare alternate law appears to be, it might not be out of order for the airplane to announce that it is now very much possible to stall the plane by pulling up (in actual words).
Finally, it sounds like the unlinked nature of the sidesticks was a big part of the problem; are there good reasons to leave them unlinked in future designs?

Some context:

Wikipedia article about the crash
CVR transcript and comments

What improvements did Airbus make as a result of the Air France 447 accident?

The official report has a section headed

5 - CHANGES MADE FOLLOWING THE ACCIDENT
...
5.2 Airbus
Review of the “Unreliable speed indication” procedure
Flight Operations Telex (FOT) of 9 September 2009 recommending, at the next recurrent
training course, a session on the simulator at high altitude in normal and alternate law
including:

Manual aeroplane handling,
Carrying out the UNRELIABLE SPEED INDICATION / ADR CHECK PROC procedure.

Most of the changes were carried out by other parties, Air France, EASA, etc.

If the stall warning didn't stop, but instead changed into an even more ominous "you're now tumbling down at less than 60 KIAS" warning.

Opinion: If the computers have decided they don't know what the heck is happening, it's probably a bad idea for them to make something up at random to shout at the crew. If the computers always know best, they should just probably shut-up, activate the cockpit wrist-restraints and fly the plane.

it might not be out of order for the airplane to announce that it is now very much possible to stall the plane by pulling up (in actual words).

Wild guess: Maybe this would help but not when the PF is already ignoring stall warnings and determinedly pulling up almost continuously. Generally less is more when the crew are likely to be confused and overwhelmed. There's probably a hundred things you can more easily accidentally do when you are flying in alternate law and when you are in alternate law you may not have time to patiently wait and listen whilst the computers enumerate them all to you.

the unlinked nature of the sidesticks was a big part of the problem; are there good reasons to leave them unlinked in future designs?

So that the pilots can engage in wrestling matches? Yes, it seems there needs to be an even clearer indication of conflicting inputs but I'm not sure if mechanical feedback is necessarily the best method. Pilots are already supposed to be trained and tested on clear handover of control.


Is the A350 more efficent than the B787? I have wondered this because the Trent 1000 engines are very similar to the Trent XWB engines on the 787. And if the A350 has better efficiency then why do so many airlines purchase the 787?

The answer as to why companies seem to be favoring the 787 over the A350 is in several parts, none having very much to do with efficiency.

First, a grand total of just 6 A350 XWBs have been built. The A350 is just ramping up production this year, while the 787 has been in production about 5 years now. The 787's teething problems have spurred some interest in its competitor, but as the first composite airliner to start accepting orders, the Dreamliner has a distinct first-mover advantage.

Second, Airbus has seen some production problems in its larger airliners, which tends to make airlines nervous. The A380 saw cancellations totaling over $10 billion in lost revenue primarily due to its inability to make them fast enough, causing Airbus to indefinitely delay production of freight variants that major shipping companies were waiting for. Smaller, older production lines have been doing better but the A380 line is still taking up a lot of Airbus's resources that could be spent making 350s.

Third is simple cost. A 787-8's base price is about \$225 million. An A350 starts at about \$270 million. \$45 million a plane is nothing to sneeze at; it means you can have 6 Dreamliners for the cost of 5 A350s, and the major airlines are buying 40-50 of these planes each so the savings is considerable.


Reason for the multiple sidelobes in an ILS glideslope signal Why are there multiple side lobes in the glide slope signal from an ILS?

All directional antennas have sidelobes (and all real antennas are at least somewhat directional). I won't get into detail of the fundamental physics reasons behind that, as that's beyond the scope of the aviation stack exchange, but briefly it's related to diffraction and the Fourier relationship between the aperture distribution and the far-field radiation pattern.

Various design techniques can reduce the amplitude of the sidelobes, sometimes to the point where they're undetectable or at least insignificant. However, these all involve tradeoffs that compromise other performance parameters, size or cost, so if the sidelobes aren't problematic or can be easily avoided operationally, it's not worth trying to suppress them. Also, most glideslope antennas use passive phased-array techniques, and phased arrays are especially prone to sidelobe generation (called grating lobes in that case).


What year did the seat belt sign come in to use? What year did commercial airlines start using a lighted seat belt sign?

The earliest reference to seat belt signs I can find is Scheduled Air Carrier Rules, 14 C.F.R. (1941)

§ 61.342 Seat belt sign. An aircraft shall not be operated in
scheduled air transportation unless a suitable means for warning
passengers to fasten seat belts is provided. [As added by Amdt. 129,
CAR, Sept. 5, 1941, effective Oct. 1, 1941, and Amdt. 130, CAR, Sept.
12, 1941; 6 P.R. 4691, 4753]

https://www.loc.gov/item/cfr1943001-T14CIP61/
The amendment appears in 6 FR 4753
https://www.loc.gov/item/fr006181/
It's unclear where the motivation for this regulation came from, but it is likely mentioned in the Civil Aeronautics Board's meeting minutes for September 5, 1941. Unfortunately it does not appear that these minutes have been digitized. The relevant records should be available in the National Archives RG 197.2.