What is an Enhanced Vision System? I was reading something at work that said the company has installed an Enhanced Vision System on the aircraft.

  FedEx Express added Enhanced Vision Systems to 250 aircraft, which will greatly improve pilots’ ability to land in low visibility conditions and mitigate potential weather delays. The company is the only U.S. carrier with FAA approval to use the technology as low as 100 feet on instrument approaches.

What is this system and how does it work? Is it some type of FLIR?

It's essentially a combination of the capabilities of FLIR and NVG's, although built into the aircraft and displayed in the HUD rather than worn as goggles.

FLIR, short for Forward Looking Infra Red, uses far-IR to see the heat of terrain, and essentially turns the night into day, at least for the field of view that it "sees."  NVG's (Night Vision Goggles) use near-IR and magnified visible light.  The near-IR doesn't see terrain except to the extent that it's illuminated by something (moonlight, starlight), and far-IR can't do much to tell the difference between a light that's on or off (so you can see the runway edge, but you can't see the runway edge lights using FLIR, for instance).

The two systems have slightly differing capabilities and limitations; when both are fused together onto a display (i.e. a Heads-Up Display, or HUD), the pilot gets a pretty amazing picture of what is directly in front of him: the far IR shows him the runway edges, taxiways, pavement, buildings, obstructions,etc, and the near-IR makes lights (i.e. runway and taxiway lights) stand out.  In low visibility, the far-IR tends to be somewhat more degraded than the near-IR, whereas at night & with no moon, the near-IR shows the lights but not much else.  Together, the pilot can get the runway lights in sight sooner than he could unaided, and by the time he's landing he can see "everything" in the field of view of the HUD, not just the lights themselves.

Example videos:

YouTube
Vendor's Website
AvWeek

When the videos get down close to the runway, you can pick out the glowing approach and runway lights, which is where EVS differs from pure FLIR.  With FLIR, you'd see (sort-of) the lights and any framework that they're mounted on, but they wouldn't particularly glow -- they might as well be off.  With EVS, the crew can fly a CAT I ILS and actually have the approach lights, runway, runway lights, etc all in sight at the 200' DH, even when the visibility is such that with the naked eye, none of that would be visible.  At major US airports, CAT III approaches are often available, but elsewhere they may be rare or nonexistent.  The EVS capability has the promise of allowing crews to successfully land out of a Cat I approach (i.e. at an airfield that only has Cat I ground equipment) even in Cat III weather; from the OP's quote, it looks like they already have approval to use it for Cat II conditions. And for a global carrier like FedEx, that's a capability worth having.


Why does the MiG 21 use anti surge shutters? Does the MiG-21 use anti surge shutters at high speed? What is the adverse effect on the airflow at such high speeds (despite using a conical spike ahead) which warrants the use of antisurge shutters?

Look just ahead of the wing root, below the two red triangles. The anti surge shutter is covered with a red coloured blanking secured with a yellow cord.

Normally, such shutters are close to the intake and covered by simple spring-operated doors which open when the pressure in the intake duct drops below ambient pressure at low speed.

AV-8B air intake (picture source). The upper two doors are open while the lower three are closed.

The ideal area of a jet intake varies with flight speed. At low speed, it needs to be large so the amount of air swallowed by the engine can be supplied without too much resistance. At high speed, a big intake captures and compresses more air than the engine needs, which increases drag. Especially fighter jets need an intake which is optimized for high speed flight, and in consequence much of the engine power is needed to suck in enough air during the take-off run. Adding doors which are sucked open in that condition will add another pathway for air to reach the engine, which in turn will help to avoid a compressor surge and improves thrust at low speed.

I have no detail knowledge of the MiG-21 intake, but it appears to me that the anti-surge doors are only open at low speed and high engine RPM. At high speed, the pressure in the intake duct is a lot higher than the ambient pressure and the doors are firmly closed.


What's the difference between hover power and maximum continuous power? What's the difference between hover power and maximum continuous power? Is a pilot clear to fly when its hover power is more than maximum continuous power calculated by table?

As the name implies, hover power is the amount of power required to hover.  If you add people/cargo to the aircraft the hover power increases.  Secondarily, it depends on air density as the rotor is more efficient in denser air - less power is required to lift the same weight.
Maximum continuous power (MCP) is the maximum power the engine/drive system should be exposed to for long periods of time.  This power level may be exceeded for shorter time periods.  For example, there may be 30 second or 2 minute power limits that are substantially higher than MCP (120%+).
I am not sure about takeoff clearances, but as you may already know power decreases substantially in forward flight and hence a helicopter that needs 1.1*MCP to lift off could still cruise around well below MCP.  Also, fuel burned in flight reduces weight and power so that less power is typically needed in the final hover/landing.  This can be upwards of a 10% reduction in weight.


Are GPS airspeed meter and altimeter going to replace traditional measurements? When are GPS Airspeed meters and Altimeters going to be introduced if they not already in use?  Are they more accurate than systems in use now and? How much would it cost to install them to older aircraft?

In general, even very low cost GPS units can provide very very accurate velocity vectors.

Altimetry is not quite as good, and is different in that it is referenced to a geoid, rather than a barometric pressure.

Similarly, airspeed is referenced to the ambient air, and while a GPS is a very accurate velocity vector source, there is no easy way to know the ambient air velocity.

So in short, by necessity to maintain safe flight, airspeed utilizes relative wind on an aircraft, and there is not an easy way to determine that with GPS alone.  And altimeter settings by convention, are based upon ambient air.  However, by utilizing a geoid model reasonably accurate absolute altitudes or MSL altitudes could be determined. Cost? As much as an old cellphone and some software.  Pretty cheap.


How does the squat switch work? This question came up after reading the following comments on the answer to this question: Raising the flaps right after touchdown. Good or bad?

Philippe Leybaert Jan 20 at 23:00

  The retractable landing gear is an excellent point. It makes perfect
  sense. I've read a few stories of pilots retracting the gear after
  landing instead of raising the flaps.

voretaq7♦ Jan 20 at 23:02

  The retractable landing gear issue is particularly notable because
  early in the landing roll you may not have enough weight on the wheels
  to activate the squat switch that prevents you from retracting your
  gear while you're using them to hold the plane up. Your perfect
  landing could turn into a nice, smooth gear-up incident if you bump
  the wrong switch.

Questions

How does the squat switch work (which prevent gear up on the ground)?
Are there any reports on issues of gear up on ground? (I only know
one)?

The squat switch is essentially a sensor that senses if the weight of the aircraft is resting on the gear. This can be done by load cell or displacement sensor on the suspension arm.

Like any other component these sensors can fail though engineers prefer them to fail in an always on (weight on the gear) state; that way the pilot just needs to deal with gear failing to retract after takeoff and needing to land again to check it out rather than dropping the plane on the ground and causing more damage.


Leading edge or trailing edge stall? As I wrote last week, I'm doing a Benedek 10355 airfoil performance report.
At this point of the work I am analyzing the stall of the profile at a fixed Reynolds number.
The Benedek 10355 airfoil has a thickness of 10.1%.
Figure 1 shows the polar calculated with Xfoil at Reynolds number = 500,000.

Analyzing this curve, and comparing it with this answer, it seems to me that we are in the presence of a trailing edge stall.
In fact, after the stall angle ($\alpha_{stall} = 11.0°$), the decrease in the lift coefficient is gradual ... Then, however, at about $\alpha = 21°$ there is a sudden change in lift, as if a laminar bubble had "burst".
Figure 2 shows the separated zone for $\alpha = \alpha_{stall} = 11.0°$.

Looking at the pressure coefficient, there appears to be a laminar bubble very close to the leading edge (Figure 3).
As the incidence increases, this bubble moves more and more towards the leading edge and, when $\alpha = 22°$, it "disappears".
This makes me think that maybe at low incidences there is a trailing edge stall and then, going further, a leading edge stall ...
Maybe I'm missing something?
Thanks.

The Benedek 10355 clearly stalls with separation starting from the trailing edge.
There is some discussion whether a laminar-to-turbulent transition due to rising pressure will always involve a laminar separation bubble, however short. But that bubble does not constitute a leading edge stall.
XFOIL is not meant for flow simulation deep into the stall region. It does amazingly well to capture the initial stall but cannot possibly model the extended separated flow well into the stall accurately. I am actually surprised that it still produces converging results at such high angles of attack.
In short, I would not expect the XFOIL results above 12° AoA to model accurately what is happening in reality. Especially that sharp drop in lift at around 21° looks very much like the code settling on another solution but not something that can be expected in real life.


What is the possibility of technology like Google's balloon reducing overall connectivity and internet costs? What is the possibility of technology like Google's balloon and its Facebook equivalent replacing satelite technology for internet access in aircraft given the rise in the next tech revolution on the internet of things, aircraft will soon be among those always connected devices and possibly autonomous? Are there regulatory impediments and would one such project on a massive scale congest air traffic?

I doubt it, let's take a look at Project Loon from X (previously Google X):

The balloon is to be flown at an altitude of 20km (about 65,000 feet). The balloons cover 5000 square kilometers (about 1930 sq miles). So the balloon and its service (red triangle) kind of looks like this:

The full length of (a) is 60,000 meters (60km). Assuming the ground area is circular, the area of service at the ground (b) is about 94.9km in diameter. So you need to figure out what the service area is at 40,000 feet (about 12.2km). I won't go over the math here (that is for a different section of SE), but you get:

Beam Angle at the balloon: 134.2°
Length of (d): 37km
Service area at 12.2km: 1075 sq km

So the problem here is that the balloons would have to be spaced much closer since the (d) of the balloons would have to overlap to give continuous service. At the speed of an airliner (550mph, or about 885kmh) the airliner would speed through the 37km wide circle (assuming it takes a straight line through the center) in about 2.5 minutes. This is either hindered or helped by the balloon speeding through the stratosphere (Project Loon's record is currently 162kmh). So you would pass through anywhere from 3.1 minutes (with the balloon) to 2.1 minutes (against the balloon) worst case.

So looking at it from purely an "is it possible to provide internet service by balloon to airliners" perspective, I would say no, not economically anyway. You would need roughly 3 times the number of balloons to cover the same area at altitude as you would be on the ground. The balloons are designed for ground service, so if airplanes did use them, they would have to switch back and forth from satellites and balloons every 2-3 minutes.


What is the rationale behind the Embraer 190's control column design? As per this picture of the Embraer 190, but also on other Aircraft, the Control column stick has a very awkward position for hands. Why is this design chosen? To me it seems that the position of the hands would be really uncomfortable as the pilots hands would be tilted at 45 degrees and not stay vertical, thus putting a lot of stress on them.

I think it's quite similar to a steering wheel with your hands a bit to the upper half. You rest your hands onto the yoke with this variant, as compared to the other ones where they are vertical and inclined to slide downwards.

While I'm certainly not sure, it could be a sensory thing as well, perhaps allowing better feel for the position as you are putting less energy into your arm and tiring it out less.

I think it's a minor design detail and had it been uncomfortable/ problematic they would have got rid of it pretty quickly.

Embraer Angled Yoke

Bombadier Vertical Yoke


Would USAF ATC experience help in getting a civilian ATC job? Can I join the US air force as an air traffic controller, and then use my ATC skills from my service to become a civilian ATCO?

Yes, military air traffic controllers often transition to the civilian system after their military retirement. But if your end goal is the commercial world it might not be for you. The military want people fully committed to the job - committed enough to serve in a war zone where people would love to drop a bomb on your head. It is not something you can do for just a year or two and then move on, the minimum service commitment is, I think, 8 years. If you are motivated enough to accept all that then you can sign up after high school.

In the US the civilian ATC is run by the Federal Aviation Administration. Their requirements are on their website, and it looks like you'll need a degree or three years of suitable work experience. Once you meet those requirements you just wait for an advertisement and apply.


Why are these 3 planes so close? I spotted these three planes flying east then turning south.  They were over the Hopi reservation and heading toward Winslow, AZ at 10:42 AM local time (MST), today February 5 2021.  They were at 15k to 20k feet or so.  Can anyone explain?

The primary reasons large airliner-sized aircraft would stay in formation is for a photo flight, or for reasons related to military operations.
Very few civilian airliner-sized aircraft are grey.
The two grey aircraft are surely military tankers, both of the same type.  The white aircraft is in position to receive fuel from the front aircraft.  Based on color and large size (relative to the grey aircraft), the white aircraft is a 747-based military aircraft, such as a VC-125 (the type used as Air Force One) or an E-4.  With this in mind, the size of the tankers suggests that they are KC-135's.  A close look at the grey aircraft shows that they appear to have engines located well outboard on the wings, i.e. have four engines on the wings rather than two, further supporting this identification.
It is very common for military tankers to fly in a loose "trail" formation like this.  I've seen it dozens of times.  Whenever I see airliner-appearing aircraft flying in this type of formation I always reach for my binoculars and they usually turn out to be KC-135's, KC-46's, or KC-10's.
As to the "why" part of your question, a possible answer might be that two tankers operating together gives both crews a chance to practice refueling another aircraft of some particular type-- in thus case the E-4 or VC-125-- without having to put two different receiver aircraft in the air.  There are many other reasons why the military often finds it advantageous to move several aircraft through the airspace together as a single formation rather than as completely independent flights.
FlightRadar24 is a good resource for answering a question like this-- it allows one to zoom in on a given area and play back tracks from a given time in the past.  Much military traffic is not captured, but one could at least verify the presence or absence of civilian traffic at the time and place of interest.  ASE user DeepSpace has now provided this link, showing that in this case the ADSB transponder of a KC-135R was in fact captured at the time and place of interest.  The flight started and ended in Phoenix, and included two passes over the Hopi reservation.  The reported altitude over the Hopi reservation at the time of interest was 22,075'.


Is there a good source for estimating the per unit cost breakdown for a classic GA plane like the Cessna 172? For instance, on a Cessna 172 what percent is spent on:

Raw materials
Labor actually manufacturing the plane
Testing the completed individual planes
R&D Making the plane (at this point, very little for the 172 I suspect
Certification testing
Insurance
Avionics
Anything I forgot...

You can find an estimate in the Roskam book "Airplane Design Part VIII"

Which is partial on google books:
https://books.google.nl/books?id=GIHHFkd829cC&printsec=frontcover&dq=roskam+part+VIII&hl=en&sa=X&redir_esc=y#v=onepage&q=roskam%20part%20VIII&f=false

However, most Engineering Universities may have a copy of this series in their library. Most of the data you want to know, companies try their best to keep them private as it is information the competition would like to know.
In general most aircraft programs are based on an fixed set of sold units. If you know the cost of the individual units (aircraft) and the amount of units needed to sell to reach the break even point then you have a fairly good estimate over the program cost.


Under what circumstances is the Aircraft 'Stability-Axis' system used in either design or analysis of aircraft? I understand the definition of aircraft 'Stability-Axis', but I am unsure as to the circumstances in which forces or moments would be expressed in this axis system. Is there a convention?

What I do know:

It is useful for wind tunnel analysis, where forces and moments are
often calculated from a force balance fixed to the aircraft that is
invariant with model Alpha and Yaws with the aircraft for variations in Beta. (Obviously depending on windtunnel/model set-up).
It can be used as an approximation for Wind-Axis is Alpha is known and Beta is assumed to be small.

What I don't know:

Many things
Why Stability-Axis is used instead of Body-Axis or Wind-Axis?

Body-axis is used to construct the equations of motion and express the aircraft angular velocity and Euler attitudes. By conventional, it is defined such that the x-axis is aligned with the nose and the x-z plane is the plane of symmetry.

Stability-axis, whose x-axis is aligned with the longitudinal incidence of the flow against the body-axis, is used to express aerodynamic data, which are derived from semi-analytical methods (e.g. DATCOM, ESDU), computational methods (e.g. vortex-lattice methods, Navier-Stokes), wind tunnel testing, or full flight data.

Stability-axis is used for aerodynamics because lift and drag are conveniently aligned with the axes. And as the OP alluded to, it is much easier to rotate the aircraft against the wind tunnel balances, than to rotate the flow against the test model.

Wind-axis has its x-axis aligned exactly with the flow velocity vector. It's not really used anywhere. We don't use wind axis because it breaks the x-z symmetry and makes things more complicated than need be. It does not offer any additional benefit than stability-axis.

Image from: https://www.mathematica-journal.com/issue/v8i2/features/codegeneration/contents/html/Links/index_lnk_3.html


What is the second "door" on the DC-3? Is under the cockpit used to access avionics on the ground or something like that ?

It seems to be an access door for the pilots, judging from this photo. Not a super easy entrance, the door in the back is a bit lower down.


What materials are used for aircraft windshields? Is hard plastic or some form of plastic replacing glass in aircraft windshields? Any idea/companies/players in this field??

I have heard there is recent trend as plastics are light in weight and have properties comparable to that of glass.

On light aircraft, acrylic windshields have been the norm.  On higher speed aircraft, glass and laminates are used, particularly with heated windshields. The laminates may use polyvinyl butyral (PVB) is tough and ductile which enhances mechanical properties and impedes shattering of glass layers and the transmission of cracks from one layer to another.

Layered products may have various coatings which affect transmission of light and RF, reduce reflectivity, include elements for heating. While I do not know of an aircraft application, it is common in automotive for layers to include radio antennas.

Glass is an amorphous, non-crystalline material and acrylic plastics and glass have very different properties.  Glass tends to shatter with sufficient impact. However, glass tends to have high resistance to temperatures that subsonic aircraft will normally encounter. Glass also retains good clarity properties over the operational domain of subsonic aircraft. Acrylic tends to be pretty shatter resistant, although I know a turkey vulture which proved otherwise (grin). Acrylic is more suspectable to abrasion and scratches, and therefore requires cleaning and maintenance considerations which glass does not.

It really depends upon the type of aircraft and the design criteria, which include the operations, temperature, heat requirements (ie hot windshield for de ice), weight considerations, EMI considerations (coatings, etc.) and so on.

Both materials have been in use for a long time.

So what kind of aircraft are you talking about?


Do captains and first officers always fly together or are they mixed? I was wondering if the same captain and first officers always fly together for some time as some sort of a team or are they mixed around in every flight? I suppose if you have one plane that is the case but what happens in huge airlines such as emirates? Wouldn't it also be a drawback always flying with someone that you do not actually know?

In airlines where pilots bid on monthly lines it is conceivable that a pair of pilots fly all month together. However, trip trading and reassignments rarely let this happen. It is far more common that a crew only stays together for a single "pairing" that lasts between 1 and 4 days (domestic 121 USA).

It's not a drawback to fly with new people because training emphasizes standard flying according to the FOM. Ideally, every pilot is exactly the same.
  Practically it is close enough that you can drop any two pilots from the same airline into an airplane and they more or less operate it exactly the same way.


What is the origin of the term "Tyro" for inexperienced pilots? Over the years I have been aware of the term "Tyro" to mean a new or inexperienced pilot. I'm pretty sure it is/was used more in a military setting than a GA one, but none the less I was interested to know if anyone knows the origin of this term, and the reason why it is used? Is it still used commonly?

This may be a term only used in the RAF, I don't know.

It's just a synonym of novice, it is used outside of aviation. Like many other aviation terms (e.g. pilot) its origins precede the first manned flight and it was and is used for non-aviation novices. Aviators borrowed many existing terms.

tyro (n.)
1610s, from Medieval Latin tyro, variant of Latin tiro (plural tirones)
"young soldier, recruit, beginner," of unknown origin.

http://www.etymonline.com/index.php?term=tyro


Can passenger distribution cause a parked airliner to tip? This question on Travel.SE recounts a situation where a passenger, about to disembark from a commercial airline flight, was asked by a flight attendant to wait for other passengers to "catch up".

This answer suggests that if too many passengers were to disembark from the front, while passengers at the back remained at the back, the plane might tip backward; and that the flight attendant was acting to avoid this possibility.

Is this really a risk on modern airliners, or even regional jets or turboprops?  (The aircraft in the question at hand was a 737.)  Are flight attendants trained to redirect or hold passengers in order to avoid it?

I saw Is passenger weight distribution important on a commercial flight? but it is about the aerodynamic effects of passenger distribution, which are not relevant when the airplane is parked on the ground.

It's possible.

When FedEX used 727s a lot they had regular occurrences of cargo being loaded too far back while the front was empty. The airplane would sit on it's tail. Not usually damaging but having the front door quite a bit higher than usual was an issue. If no one was on board it was awkward to get it back to normal, and FedEX really doesn't like delays. At my airport in the 90s you often saw a post attached to the tail while they ere loading.

Highly unlikely a 747 will rise up like a bucking horse, but it is possible that you can take enough weight off the nose gear of something smaller that the strut can extend. This would make the cabin door rise by about a foot, probably hit the jetway, and upset most everyone onboard. What would the passengers probably do? That's right - run downhill.


Is the descent performance of the PMDG 737-800 accurate? I often fly the PMDG 737-800 (with winglets) in my home simulator and most of the time, I find it hard to slow the aircraft down for landing when I descend. Most of the time, I need to deploy the speedbrakes to keep descending without increasing speed. I usually start my descent earlier than the marked top of descent point to make the flight a little smoother.

So I was wondering if this is a bug or if the real aircraft actually behaves like this. I have wondered about this for some time. I can only dream about becoming a pilot and find out myself cause of my bad eyesight.

Here is a video on YouTube of me landing the 737 where I had to deploy speedbrakes.

Since this video is getting a little outdated: I can no longer recall exactly the payload or weight of this aircraft. But I normally fly with a heavy payload.

You haven't provided everything possible to really ascertain the issue noted, but based on what you have I've noticed the following things:

You'll find most 737s at 240-250 KIAS passing through 10000 feet, rather than the 220 knots that you were going at (while 220 KIAS is above minimum clean speed at normal landing weights, it's still slower than most)
Being heavy actually makes minimum drag and max L/D speeds higher, which means that for the same speed, you'll actually descend at less of a gradient than if you were lighter
The Boeing 737-800W has a idle descent ratio of about 1:20, which means that at 250 knots you're only going to be descending about 1200fpm -- the 737-800 (especially with winglets) is a very slick bird.

For your reference, you can see the distances required for an idle descent (.78/280/250), plus allowances for a straight-in approach:

|=====================================================|
|          |       |      |       DISTANCE (NM)       |
| PRESSURE |       |      |---------------------------|
| ALTITUDE | TIME  | FUEL |  LANDING WEIGHT (1000 KG) |
|   (FT)   | (MIN) | (KG) |  40  |  50  |  60  |  70  |
|----------|-------|------|------|------|------|------|
|   41000  |   27  |  340 |  102 |  119 | 133  |  142 |
|   39000  |   26  |  340 |   97 |  114 | 127  |  136 |
|   37000  |   25  |  330 |   92 |  108 | 121  |  130 |
|   35000  |   24  |  330 |   88 |  103 | 116  |  125 |
|   33000  |   24  |  320 |   84 |   99 | 111  |  120 |
|   31000  |   23  |  320 |   80 |   94 | 105  |  113 |
|   29000  |   22  |  310 |   75 |   88 |  98  |  106 |
|   27000  |   21  |  300 |   70 |   82 |  92  |   99 |
|   25000  |   20  |  300 |   66 |   77 |  86  |   92 |
|   23000  |   19  |  290 |   61 |   71 |  79  |   85 |
|   21000  |   18  |  280 |   57 |   66 |  73  |   78 |
|   19000  |   17  |  270 |   52 |   61 |  67  |   72 |
|   17000  |   15  |  250 |   48 |   55 |  61  |   65 |
|   15000  |   14  |  240 |   44 |   50 |  55  |   58 |
|   10000  |   11  |  200 |   30 |   34 |  37  |   39 |
|    5000  |    7  |  150 |   18 |   19 |  20  |   21 |
|    1500  |    4  |  110 |    9 |    9 |   9  |    9 |
|=====================================================|


How are military jets given official names? How is a military jet given a nickname? Does the manufacturer name the plane? For example, did Lockheed Martin name the Raptor (F-22) and Lightning II (F-35)?

Typically for modern US aircraft, since the military pays the contractor for the aircraft's development, so they are paying for it, and get to name it. This article says the USAF named the F-35 as the "Lightning II". However, the reality is much more likely, the F-35 Joint Program Office (the JPO), named the aircraft, given it international status and being a joint aircraft, and the announcement was made by the USAF. The JPO, being a joint office, always has a Program Executive Officer (PEO), and a Deputy, from both the USAF and USN (alternating positions). The F-22 was likely named by the USAF F-22 project office, which was based at Wright-Patterson Air Force Base, Dayton, Ohio. See here.

However, this isn't always the case, at least historically. For example, the Lockheed Orion & Hercules,  (and the Constellation) follow a tradition of Lockheed of naming their aircraft after celestial objects, stated here and here. The A-12 was also named by flight crews as the Cygnus (swan), another constellation, to follow this pattern. So, some US military aircraft, in the past, have been named by the manufacturer.

As another example, the F-111 is believed to be the only US aircraft to go through its service career without being officially named. On its retirement, the USAF designated it the "Aardvark". While the name was made official by the USAF, it came from the term the crews used for it.

  Its American crew called the F-111 the Aardvark. It was a good name. Like the South African aardvark, the F-111 was a solitary night hunter. Like the aardvark, it rooted in the dirt, guided by excellent senses. And like the aardvark, it had very long range. For much the same reasons, Australians called it the Pig.

So, while in the past, sometimes the manufacturer named the aircraft, sometimes the crews, it would seem, today, the US government, through their military force, chose the name.


Do airline pilots have to wear face masks in the cockpit because of COVID? Do the flight crew with a commercial airline have to wear a face mask while on the flight deck, as both crew are in close proximity to each other and in a tight confined area? If not, why not?

I can answer this question accurately - not as an airline pilot directly, but as the son of two.
To keep it short, no. Airline pilots are free to make their own decision about their safety in the cockpit. In the airport, yes. In the taxi, bus, or ground transportation, yes. But, when they enter the cockpit, they usually sanitize the entire touch area, use some germ-x, and settle in for the 12-14 hour flight. When going back into the cabin, they put a mask back on (while on break).
There have been some cases reported in the pilot body but not enough to cause any large issue. Thanks for your question.


What to do when moisture forms on the helicopter's canopy in a weather front? Moisture has condensed on the canopy of your helicopter as you fly through a weather front – what should you do?

That won't happen "flying through a front" where the temperature of the plexiglass is at at or warmer than the ambient air - you just get drops forming and streaming back.  What has happened to me though, is condensation when climbing into a warm humid inversion on a sunrise clear sky departure where the surface temperature was much cooler than in the inversion.  On my airplane which has a sliding bubble canopy, the outside of the canopy fogged up suddenly at about 200 ft agl as the cool plexiglass hit warmer humid air and it was like the canopy was covered in wax paper.  A bit alarming initially.  I just used the attitude indicator to stay right side up, helped by the opaque blue of the sky above, until it started to stream and clear up on its own after a minute or so.


What exactly are the "sealed" leading-edge slats in B737 short field performance aircraft? "The 737 design enhancements allow operators to fly increased payload in and out of airports with runways less than 5,000 feet long. The design enhancements include a two-position tail skid that enables reduced approach speeds, sealed leading-edge slats that provide increased lift during takeoff, and increased flight spoiler deflection on the ground that improves takeoff and landing performance"

Source: https://boeing.mediaroom.com/2006-07-31-Boeing-Delivers-First-737-with-Enhanced-Short-Runway-Package-to-GOL

What are the sealed leading-edge slats in B737 short field performance, and how are they different from the normal leading slats in typical B737?

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

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

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


How does a weak shock-wave boundary-layer interaction create wave drag (other than through direct shock losses)? I'm trying to understand the physical cause of wave drag, beyond the simple statement "the presence of shockwaves increases the drag".

As far as I understand, in the case of a weak BLSWI (so without separation), the drag increase is due to:

Direct shock losses (momentum deficiency of freestream flow through shockwave)
A change in the state of the boundary layer due to the compression waves in side the BL

But how exactly does the second point work? If the BL becomes thicker after the SW, wouldn't that mean that the drag decreases because the velocity gradient and therefore the shear stress decrease?

I feel like I'm missing some tiny aspect...

Edit: I've found the following statement in this report, which maybe explains my problem better:

  The
  increase
  in
  drag
  occurs
  directly
  because
  of
  the
  wave
  drag
  associated
  with
  the
  presence
  of
  shock
  waves.
  However,
  the
  drag
  also
  increases
  because
  the
  boundary
  layer
  thickness
  increases
  due
  to
  the
  sudden
  pressure
  rise
  on
  the
  surface
  due
  to
  the
  shock
  wave,
  which
  leads
  to
  increased
  profile
  drag.
  Lynch
  has
  estimated
  that
  at
  drag
  divergence
  the
  additional
  transonic
  drag
  is
  approximately
  evenly
  divided
  between
  the
  explicit
  shock
  drag
  and
  the
  shock
  induced
  additional
  profile
  drag

Why does the increase in BL thickness cause more drag? Shouldn't the velocity gradient at the wall reduce, leading in less shear stress? Or should I see it in a way that the already turbulent BL "takes up more energy in its vortical structures" from the flow, due to the increase in thickness?

As you know, wall shear stress $\tau_w$ (and thus friction drag) can be described as:
$$ \tau_w = \mu \frac{\partial u}{\partial y} \vert_{y=0} $$
where $\mu$ is the dynamic viscosity of the fluid and $\frac{\partial u}{\partial y} \vert_{y=0}$ is the fluid velocity gradient at the wall.

The shock-wave boundary-layer interaction (SBLI) can change these two factors through several mechanisms:

A change in boundary-layer thickness: As you noted correctly, an increase in boundary-layer (BL) thickness will decrease the velocity gradient, compared to an otherwise similar BL velocity profile.
However, my conclusion after reading several publications is that the increase in BL thickness doesn't always occur. BL thickness seams to scale with shock strength and very weak shocks actually produce a thinner BL.
A decrease in fluid velocity: The shock-wave will obviously result in a decreased velocity outside of the boundary layer, which decreases the velocity gradient. This will also decrease wall shear stress, so it cannot be the dominant mechanism either, since it contradicts our experience of increased friction.
An increase in turbulence: The turbulence of the BL is significantly amplified by the shock-wave through different mechanisms.
The image below visualises the distribution of reynolds shear stresses, which are a measure for turbulence. A high magnitude (positive or negative) of reynolds stresses means high turbulence:

 Reynolds shear stress distribution of a SBLI. Notice the strong negative values in the lower BL behind the SBLI. Negative values indicate slow fluid moving upwards and fast fluid moving downwards. Also notice the value is zero very close to the wall, because the wall prevents vertical fluctuations. Image Source (I added the white markings)
The increased turbulence causes an increased momentum exchange between the lower and slower part of the BL with the higher and faster part. This momentum exchange changes the velocity profile of the BL. There are higher velocities closer to the wall. This results in a higher velocity gradient at the wall.
I believe this must be the main cause of increased wall shear stress. The stronger turbulence will probably decay downstream, so the velocity profile will slowly change back to the normal turbulent flat plate type.
Increase in pressure and temperature by the shock-wave: The rising temperature and rising pressure (not so much) increase the dynamic viscosity of the fluid and thus increase shear stress.

Most likely there are other factors that also play a role. I am no expert in SBLIs, I only read through a fair amount of literature to come to my conclusions.

Literature that I found especially helpful:

Anderson, J.: Fundamentals of Aerodynamics : My standard book for aerodynamics. Fairly easy to read. Section 9.10 briefly covers SBLI.
Schülein, E.: Skin-Friction and Heat Flux Measurements in Shock/Boundary-Layer Interaction Flows This paper shows the skin friction along an SBLI for different shock intensities. It provides schlieren images and figures that show the thickness of the boundary layer. It also discusses the causes for the observations.
Schülein, E.: Documentation of Two-Dimensional Impinging Shock/Turbulent Boundary Layer interaction Flow Here the velocity profile of the BL is measured at different locations for the same test setup as above. All the interesting data is presented in the appendix. You can see how the SBLI changes the velocity profile and BL thickness. It also explains how to calculate the skin friction from the velocity profile data (quite complicated)

Humble, R. Et al.: PIV Measurements of a Shock Wave/Turbulent Boundary Layer Interaction This is also the source of the image I used. They used Particle Image Velocimetry to measure the flow field of an SBLI. The part about turbulence is very interesting.
Anyiwo, J. and Bushnel, D.: Turbulence amplification in shock-wave boundary-layer interaction This paper investigates the mechanisms that amplify turbulences in an SBLI. Also very interesting.
Schlichting, H. and Gersten, K.: Boundary-Layer Theory Great book, every aspect of BLs. Detailed, long, harder to read than Anderson.


Can I take position behind a plane for V formation legally? First, I must find a plane with the same capabilities to fly in its wake. As a pilot, what would I need to locate other planes near me? Or find and weight on a plane going in the direction with equal cruising speeds to follow? Then would there be and issue of just taking position without asking. Should I ask for permission to do so, from the pilot which I'm shadowing or no I need more?

Here are the regulations in 14 CFR 91.111:

§91.111   Operating near other aircraft.
(a) No person may operate an aircraft so close to another aircraft as
to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by
arrangement with the pilot in command of each aircraft in the
formation.
(c) No person may operate an aircraft, carrying passengers for hire,
in formation flight.

"Formation flight" is defined in the Pilot/Controller Glossary:

FORMATION FLIGHT− More than one aircraft which, by prior arrangement
between the pilots, operate as a single aircraft with regard to
navigation and position reporting. Separation between aircraft within
the formation is the responsibility of the flight leader and the
pilots of the other aircraft in the flight. This includes transition
periods when aircraft within the formation are maneuvering to attain
separation from each other to effect individual control and during
join-up and breakaway.
a. A standard formation is one in which a proximity of no more than 1
mile laterally or longitudinally and within 100 feet vertically from
the flight leader is maintained by each wingman.

And there are ATC instructions for handling formation flights but they're too long to quote here; see the ATC Orders 2-1-13.
So, let's assume that the distances between the aircraft in your proposed flight would make it a formation flight, per the FAA's definition in the PC/G. Putting all that together:
As a pilot, what would I need to locate other planes near me? Or find and [wait] on a plane going in the direction with equal cruising speeds to follow?
No idea. This depends very much on the type of aircraft and operation. Two friends in C172s flying half an hour for lunch somewhere? Two Boeing 777s flying internationally? But practically speaking, this would be near impossible without prior coordination on the ground. How would you know what route the other aircraft is following, or what cruising speed they plan to use?
Then would there be [an] issue of just taking position without asking. Should I ask for permission to do so, from the pilot which I'm shadowing or no I need more?
Per 91.111(b) you must have an arrangement with the other pilot first. In theory you could do that in the air but people who do formation flying seriously - and safely! - spend more time on planning than on flying. I don't know many pilots who would form up in formation with a stranger who just asked.
If you're thinking of using the V formation for commercial passenger flights then see this question. Although as you can see from 91.111(c), it isn't legal today, even if the pilots coordinate and work out all the necessary procedures first. And that's without even considering what their employers and insurers would say about it.


What is the icing probability required for an AIRMET? For an icing AIRMET to be issued, moderate icing must be forecast over an area of at least 3,000 square miles. What probability of moderate icing is required before that AIRMET will be issued?

For any AIRMET, the event must have a probability of 50% at least (that is more than playing playing heads or tails).

Related US NOAA definition for AIRMET Z:

  AIRMETs are issued every 6 hours, where there is a 50% probability
  that any of the following en-route weather phenomena will cover an
  area of 3000 square miles or greater.

Phenomena for AIRMET Z are:

  Icing: Areas of moderate airframe icing, other than convectively
  induced, including the areal extent. Freezing Level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected.

Freezing level is defined as:

  The lowest freezing level above the ground or at the SFC as
  appropriate. Freezing levels are delineated using high altitude VOR
  locations describing the location of the lowest freezing level above
  the ground or SFC as appropriate.

  Freezing levels above the ground are delineated at 4000 feet intervals
  above mean sea level (AMSL). Multiple freezing levels above the ground
  are delineated using "BOUNDED BY" followed by high altitude VOR
  locations. The range of freezing levels across the forecast area is
  also included.

G-AIRMETs, the graphical version, are issued every 3 hours (but still describe the next 6 hours).


Can I derive the wind speed and direction? If I know the IAS and GS (with directional components) of an aircraft, can I derive the wind speed and direction?
I'm working on an application that simulates the flight of an aircraft.  Another application that takes as input, the computed trajectory of my aircraft, which includes the IAS and GS of the aircraft (at points along the trajectory) needs to know the wind speed and direction at those points.
I think it is possible, but am not sure, to isolate the wind speed and direction using the IAS/GS (or both).
Can someone confirm this?  Is this a reliable way to do this?  Do I need other information to complete the calculation?

No, you need the True AirSpeed (TAS).
The Indicated AirSpeed is not a realiable measure of it, since it is not even calibrated against the instrument mounting position and is not intended to measure the speed relative to the airmass (what you actually need to derive the wind components).


Is there a regularized width of yellow taxiway ground bands? What is the width of the yellow stripes that are painted on the ground of the taxiway?

Is it the same on the whole taxiway or does it vary from an area to another? Is it mentioned in the aerodrome design manual issued by the international organization of civil aviation?

In the FAA's document about standards for airport markings, it says:

  e. Characteristics.

  (1)

  Width. The taxiway centerline marking width, which is either 6 inches (15
  cm) or 12 inches (30 cm), is based on the type of taxiing operation as described below. Uniform
  width must be maintained for the entire length of the taxiway except under the following
  conditions.

  (i)

  The taxiway or part of the taxiway is designated as a SMGCS taxi
  route. Under this designation, the width of the taxiway centerline must be 12 inches (30 cm)
  wide and, on light-colored pavement, further outlined in black. The taxiway centerline width of
  any remaining section of the taxiway that is not part of the designated SMGCS taxi route may
  change abruptly at that point or at the intersection with other taxiway centerline markings, for
  example, reduced from 12 inches (30 cm) to 6 inches (15 cm).

Source, chapter Taxiway Center Lines, page 43

For the taxiway edge lines, following rules apply:

  e. Characteristics.

  (1)
  The outermost edge of both marking schemes must be painted along the
  edge of the usable pavement.

  (2)
  The continuous taxiway edge marking consists of dual, continuous lines
  with each line being at least 6 inches (15 cm) in width and spaced 6 inches (15 cm) apart (edge to edge) [...]

Source, chapter Taxiway Edge Lines, page 48

This really is a great question, I have never thought about airport markings being such a complex thing.


Where do I find charts like this for Europe or Global I am playing Xplane11 and I am trying to find charts like this

But for Europe or USA and shows every ILS approach, detalied maps etc. For europe I have found one site such as http://www.uvairlines.com/admin/resources/charts-europe.htm but it does not show every airport. Is there a site that shows for any airport ?

Note: I am looking for free sites, not sites like navigraph.

The search term you're looking for is "instrument approach procedure" where sometimes "procedure" will be replaced with "chart" or "plate". Jeppesen provides these commercially for the world in a convenient standardized format for a hefty fee, but the aviation authority for each country also publishes their own versions as part of the ICAO-mandated Aeronautical Information Publication (AIP). In most cases these can be found for free online though it may take some digging.

For the US, the third-party (free) website airnav.com provides a convenient index for the FAA information - search for an airport from their homepage and scroll down for the links under "Instrument Procedures".

Picking a European country at random, Austria's charts are here. Generally you'll want to search for the country's AIP first, then dig through that to look for the instrument procedure charts

I flew myself from California to the UK and back (via Canada, Greenland, Iceland & the Faero Islands) without paying anything for charts this way - paid a lot for fuel though!


What's the point of the Total In-Flight Simulator? Picture of the contraption:

Source: wikimedia.org

After a quick research, and Simon's link, it turns out to be a simulator for how different aircraft would handle. And was useful is simulating crosswind with the fins installed mid-wing.

If you already have the preliminary aerodynamic data (which won't change mid-testing) to simulate different planes, what's the point of taking that data in the air vs. a ground based simulator?

There's a detailed and very interesting Air Force paper on this: CAPABILITY OF THE TOTAL IN-FLIGHT SIMULATOR (TIFS). The main point it makes is that TIFS wasn't just about gathering aerodynamic data, it was about testing aircraft configurations (including cockpit layouts) in actual flight, and training pilots on them. Here are some extracts, but you can read the paper yourself for the full details.

Test different configurations:

  It also surpasses the utility of past variable stability aircraft
  through the realism possible in its separate, new evaluation cockpit.
  The capabilities and features of this in-flight simulator considerably
  broaden the ability of the aircraft designer to deal with difficult
  trade-offs in flying qualities problems. A base configuration can be
  set up and then its stability and control characteristics can be
  systematically varied for investigations to gain research knowledge
  pertinent to flight vehicle and flight control system design.

Simulate a number of very different aircraft:

  It has been designed to reproduce in actual flight the flying
  qualities of a wide range of large airplanes.

Test different cockpit configurations:

  The simulation cockpit is removable, so that it can be replaced by
  other cockpits of different configurations.

Simulate a new design in flight, before the new design has even been built:

  Flight evaluations of the flying qualities of new airplane
  configurations can be accomplished, such as investigation of the
  behavior of extremely large airplanes. This can be extended to
  investigating the flight control characteristics of entirely new types
  of designs, as for example, reproducing the landing approach flight
  characteristics of a large re-entry vehicle.
  [...]
  The flight characteristics of specific aircraft can be simulated in
  great detail, both to evaluate flying qualities in advance of the
  first flight of the actual airplane and to investigate difficulties
  that may arise during the airplane's flight test program.

Train pilots on new aircraft:

  Finally, a well established and highly valuable use of in-flight
  simulation is pilot training. In-flight simulation of emergency
  conditions can be conducted safely because, if the evaluation pilot is
  having control problems, the safety pilot can switch the system off
  and resume control of the normal base airplane


Can a GA headset be used for listening to normal audio or with a cell phone? I own an older but still nice noise-cancelling general aviation headset from Denali, with the two-plug adapter.

I wonder if I can/shall repurpose it.  For example, I now have to take a number of international flights, 14 hours as a sardine...  not fun.  Does a GA headset make for a good entertainment and silence headset?  Or, I could use the GA headset for phone calls from my Android cell phone.

Good idea / bad idea?

I actually know a guy that does this. He bought a nice noise canceling headset but then stopped flying and now he uses it on long flights and quite enjoys it. For what its worth noise canceling headphones have come down in price over the years and you may simply be able to sell the headset and buy a less bulky set of headphones for the money you make. But if you dont want to go through the hassle then there is nothing stopping you from using aside from having a boom mic dangling off it.

For what its worth a lot of noise canceling headphones now have mic inputs as well so you can answer calls from them as well.


What's the precision of a modern Inertial Measurement Unit, and how is it obtained? This recent comment reports that:

  the IMU on new (plane) would localize the aircraft to within 3 feet
  after a cross-country flight, without any GPS input other than the
  starting location.

I somewhat doubt about this statement, at least for an IMU based solely on inertial measurement: over the duration of a flight, I fear much more error accumulates.

So what's the precision of a modern Inertial Measurement Unit over say the duration of a flight, and from what sources is that obtained? If some source (in particular, GPS) becomes unavailable, how does it degrade that accuracy?

Let's have a look at documentation of actual modern IRS. It says (on page 6) that if GPS data is lost, it will maintain RNP0.1 for 8 minutes, RNP0.3 for 20 minutes and RNP1 for 2 hours. That means that in 8 minutes the error is no more than 0.1 nautical mile with 95% confidence, in 20 minutes it is no more than 0.3 nm with 95% confidence and in 2 hours the error is no more than 1 nm again with 95% confidence.

For general description how it works see What is an Inertial Navigation System? How does it work? and of course the appropriate Wikipedia article. Basically it measures acceleration and rotation (with gyroscopes) and integrates it over time. Modern systems do the calculations digitally with sophisticated filters to smooth over some errors.


How do holding patterns work? How does a pilot follow a holding pattern? Do pilots have to manually change heading and altitude (through the autopilot) each time they are making a turn? Or is it something the Flight Management Computer (FMC) can handle?

Generally speaking the Flight Management Computer supports holding patterns. The pilot inputs the holding fix (usually a NDB/VOR), the direction of the hold (Left/Right), the inbound course, and the time/distance that one holding leg shall take to execute. These information can be found on aeronautical charts, as IFR holding patterns are published for each airport. As far as I know pilots are also trained to fly holding patterns manually, but you can understand that flying the pattern manually is not as precise as flying the pattern via the autopilot.


How long does a non-ETOPS flight need to be able to fly in an one-engine-out situation? Let's say that you're in a 737, mid-cruise in a non-ETOPS situation, and you lose one engine. Are there any FARs governing how long you need to be able to stay airborne? Or is all that's required the ability to continue safely and land at nearest appropriate airport? Are you required by any regulation to be able to fly for an X number of hours before landing?

I couldn't find anything in the FARs about this, only regulations regarding ETOPS and engine out climb. Just want to make sure I'm not missing anything. Thanks!

No, nothing addresses that case.

Let's say you're 20 minutes out from your destination, and because it's a clear VFR day you have minimum normal reserves -- roughly an hour of flying (on 2 engines, at medium-to-low altitude).  Those reserves are already prescribed for you by FAR's.  Now you lose the engine.  How long can you fly?  Until you run out of fuel or the other engine quits -- so in all probability, about that same hour, plus or minus.

Different case, you're over at Atlantic heading to Bermuda or the Azores or Iceland or NYC to the Caribbean, or over the Gulf of Mexico -- not ETOPS but a long ways from anywhere.  The fuel on board will be lots more, because you were planning for a much longer flight.  (And again, those fuel requirements are defined elsewhere.)  You now have the added twist of drifting down on one engine to a single-engine ceiling -- you set max continuous thrust on the good motor, and descend at a speed that gives you max range, which probably comes out to a few hundred feet per minute of descent.  Back in the flight planning, the check had to be performed that at the worst case, you could lose the motor, drift down, and continue to reach a suitable airfield without needing more gas than you'd have.  But in something like a 737, the gas you'll have on-board anyway is almost certainly more than sufficient.

But in this latter case, the time you can stay aloft is going to be hours -- way beyond what you had in the first case.

So no, the regulations don't prescribe a time that you have to be able to keep flying after losing one engine.  You DO have to be able to keep flying -- that's important -- and you have to have enough fuel to get to a destination with the one motor shut down.  But in terms of a prescribed time, that isn't given in the FAR's.


Why does Google's Wing drone have dimples? The question What operational aspects of Alphabet's Wing drone delivery service will be the same as that of an airline? includes a cropped photo of a google drone from FAA Certifies Google's Wing Drone Delivery Company To Operate As An Airline.

It is a winged aircraft with two long rows of vertical electric motors and propellors, and two more forward pointing motors for thrust, one on each wing.

The nose of the spacecraft is large and hemispherical and appears to be covered in small hemispherical dimples.

Question: Why the dimples?

Images below have been cropped, zoomed and substantially sharpened to enhance the visibility of the dimples.

Google's Wing Drone

  Customers who took part in Wing's drone delivery test program in Virginia approach their package after it was dropped on their lawn. Credit: Wing Source

These are not actually dimples but shallow bulges. I found a higher-res image online and checked it out. You have to expand the image to full-size and look closely; the angle of sunlight and shadow gives them away:

Similarly, this side view shows that the bumps continue down the side:

The drone is not very large, less than 2 m (6 ft) long. They are not large enough to have significant aerodynamic effect at the speeds the drone flies. Lots of much faster planes have rows of rivet heads which stand out more than these do. They are about the size, shape and spacing one might expect from internal fixings holding a light supporting framework together, maybe there are pop-rivets under there. Another possibility is anchor points for internal equipment or wiring.
To be honest I am disappointed. Dimples are used on golf balls to stabilise flight and have in the past been studied as a way to reduce drag on aircraft. I had hoped at first that work might have been brought back to life. Then again, some cetaceans (whales etc) do have knobbles on their fins, we think to control the boundary layer and reduce drag, but these are just too small and shallow and not organised in the expected way for that.


Are there any rules regarding construction near a private strip, such as stand off distance? I would like to construct a facility next to a private strip.  What are the FAA rules concerning this such as stand off or height?

Best is to contact your FAA regional office as the details of the procedures seem to vary a bit depending on your region, and the use of the airport.

AC70/7460 gives guidance the lighting and lighting of obstructions that may affect the airspace. The first paragraph reads:

  A sponsor proposing any type of construction or alteration of a
  structure that may affect the National Airspace System (NAS) is
  required under the provisions of Title 14 Code of Federal Regulations
  (14 CFR part 77) to notify the FAA by completing the Notice of
  Proposed Construction or Alteration form (FAA Form 7460-1). The form
  should be sent to the Obstruction Evaluation service (OES). Copies of
  FAA Form 7460-1 may be obtained from OES, Airports District Office or
  FAA Website at http://oeaaa.faa.gov.

More information can be found here   here.

The reporting requirements are laid down in CFR 14 part 77 §77.9


How do I find the registration or serial number for the aircraft I flew on specific trips in the past? I understand that there are various paid subscription services that allow me to search back up to a year in the past for the IDs of any flight and the histories of any aircraft.

Is there a free (or one-time fee) service or API for identifying the aircraft I've flown on specific dates and flights in the (up to several years) past?

The US (if that's your area of flying) is very kind in publicly releasing stats compared to other territories, but the answer is still you can't without a paid service.

See:

Where can I get information on ATC facility activity, like the number of flights handled in a month?
How can I get historic flight data by N-number?

In short: The FAA only keeps broad records that do not contain information such as tail numbers.

Start a personal flight journal for future flights, it's a hobby some enjoy. I did at one time, and when I couldn't catch the registration, I asked the flight crew.


What prevents a passenger from opening the emergency door on his own will, mid-flight? I have never sat on the emergency seats near the wings, with the emergency exit. I have seen cabin crew instruct the passengers seated near the door on how to operate the door.

My question, what prevents a 'rogue' passenger from opening them mid-flight?

At cruising altitude there is between 4 and 8 tons of pressure acting on the inside of the door. There aren't too many passengers capable of exerting that much force on the handle (and even fewer handles that won't just snap off). Latch type doors have interlocks or over-center latches that prevent operation with a pressurized cabin.

It's theoretically possible to open the over-wing exits at approach altitudes when there is no pressure differential. Other than being windy/noisy it would not affect the flight in any way. Cabin doors usually open forward, good luck pushing them open against the airflow. If the person who pulled the window plug wants out, it's their (and only their) funeral. I've sat in the open doors of light aircraft in flight hundreds of times, legs in the breeze, no seatbelt, while the plane makes fairly steep turns toward the open door side. It's not a problem.


I have really intense ear pain during descent in commercial flights. Would it be better or worse if I was flying a small non-pressurized plane? Usually — but not always — after descending in a commercial jet airliner, my ears really hurt for several days, accompanied by minor hearing loss.

Nonetheless, I am still in love with aviation. I am thinking of starting my lessons to become an ultralight (UL) pilot. Could flying a non-pressurized plane actually make these problems worse?

Commercial airliners are generally pressurized to a cabin altitude of around 8000 feet when at high altitudes. So it depends a bit on where you live. If you live near sea level, you're feeling around 8000 feet of pressure change. If you live somewhere higher like Denver, you're only feeling around 3000.

Small unpressurized planes, especially ultralights, will tend to fly at lower altitudes, usually within a few thousand feet from the ground. If you live at a lower elevation, this will be much less pressure change than in an airliner. If you live at a higher elevation, it would be more comparable.

That being said, it may only take a few hundred feet of elevation change for the pressure to noticeably change, and each person may react differently. If you're worried about making commitments to flight training before you know how you'll handle it, you can go on a discovery flight, even just in a small Cessna.


What is the difference between turn rate and yaw rate in aircraft? Attitude heading computers generate both a turn rate and a yaw rate. How are these different from each other? Is Euler yaw rate the same as turn rate?

Rate of turn is easy to define: it is the rate at which the direction of aircraft motion changes with respect to the ground (rate of change of track). If the aircraft is not slipping and there is no change in wind, then it is equivalent to the rate of change of heading.

Yaw rate, on the other hand, is slightly messier. Technically, yaw rate of the rate of change of the z-axis Euler angle. As an industry standard, the Euler angle in the aviation industry is defined by the z-y-x rotation, which means that the yaw occurs first, followed by pitch, followed by bank. Therefore, yaw rate is technically exactly equal to the rate of change of heading.

In many applications and loose technical speaks, however, yaw rate may also refer to the third component of the angular velocity. Angular velocity is the instantaneous rate of rotation about its axis of rotation. If the aircraft is rotating about the vertical axis only, then the third component of the angular velocity is, again, exactly equal to the rate of change of heading.

In a coordinated turn, however, the aircraft is pitched and banked, and therefore it is not rotating purely about the vertical axis. In this definition, then, yaw rate is not equal to turn rate.

As an aside, it's mathematically incorrect to call any component of the angular velocity a "rate", since angular velocity is not a derivative of any quantity, even though angular velocity can be transformed into Euler rates if you know the current Euler attitudes.


Is the increased humidity on modern jets artificial? Aircraft manufacturer’s including Boeing and Airbus have emphasised on greater passenger comfort owing to an increase in humidity on newer jets such as the 787 and A350 artificial? Or is it a direct result of a greater cabin pressure?

A Reuters article confirms the humidity just comes from the passengers, on the 787 at anyway:

  (...) the air is dry and moisture comes mostly from passengers.

And this Flight Global article:

  (...) the system does not add moisture through active humidifiers.

This APEX article has no discussion of higher cabin pressure being an influence on humidity.

Lastly, this Business Insider article says Boeing indicates the higher pressure alleviates the feeling of mountain sickness that can come from being at 8,000 ft vs 6,000 ft.

  [S]ince there isn't a perfect one-to-one correlation between altitude and jet lag, Boeing has taken additional measures to mitigate the symptoms. These measures include an increase in cabin humidity as well as a new air-filtration system.

So, it's real humidity, and not really an effect of cabin pressure.


What are the standards on displaying Crew Awareness Messages? Are there any standards on where in display Crew awareness system messages needs to be shown and how many messages at a time needs to be displayed. I checked SAE ARP4102-7 but I didn't find any guidance.

Yes, certainly there are standards for crew awareness messages. The number of messages displayed is normally limited by screen space allotted, which is may vary from around 5 to 15 messages  and when there is a 'spillover' the messages continue on the next page. The pages are selectable via a simple cancel/recall push button and may also automatically change as corrective actions are taken by the crew or the cause/fault ceases for whatever reason. There is Colour Coding, eg red for Warnings and amber for Cautions. Green and white are also typically used for normal but temporarily used systems eg 'Anti-Ice' or Passenger signs.
All the above describe some of the features of modern crew alerting systems, for details refer to the following document:
(You might find it useful to go to Appendix 5 (Definitions), right at the end of this Advisory Circular, before reading the main body of the document. Appendix 5 makes mention using diagrams as to where in the pilots field vision the visual alert should ideally be.)
Click here for the FAA AC 25.1322-1 'CREW ALERTING'
The above linked AC shows us acceptable means of complying with FAA Rule which is - 14 CFR Part 25.1322 reproduced below.

§ 25.1322 Flightcrew alerting. (a) Flightcrew alerts must:
(1) Provide the flightcrew with the information needed to:
(i) Identify non-normal operation or airplane system conditions, and
(ii) Determine the appropriate actions, if any.
(2) Be readily and easily detectable and intelligible by the
flightcrew under all foreseeable operating conditions, including
conditions where multiple alerts are provided.
(3) Be removed when the alerting condition no longer exists.
(b) Alerts must conform to the following prioritization hierarchy
based on the urgency of flightcrew awareness and response.
(1) Warning: For conditions that require immediate flightcrew
awareness and immediate flightcrew response.
(2) Caution: For conditions that require immediate flightcrew
awareness and subsequent flightcrew response.
(3) Advisory: For conditions that require flightcrew awareness and may
require subsequent flightcrew response.
(c) Warning and caution alerts must:
(1) Be prioritized within each category, when necessary.
(2) Provide timely attention-getting cues through at least two
different senses by a combination of aural, visual, or tactile
indications.
(3) Permit each occurrence of the attention-getting cues required by
paragraph (c)(2) of this section to be acknowledged and suppressed,
unless they are required to be continuous.
(d) The alert function must be designed to minimize the effects of
false and nuisance alerts. In particular, it must be designed to:
(1) Prevent the presentation of an alert that is inappropriate or
unnecessary.
(2) Provide a means to suppress an attention-getting component of an
alert caused by a failure of the alerting function that interferes
with the flightcrew's ability to safely operate the airplane. This
means must not be readily available to the flightcrew so that it could
be operated inadvertently or by habitual reflexive action. When an
alert is suppressed, there must be a clear and unmistakable
annunciation to the flightcrew that the alert has been suppressed.
(e) Visual alert indications must:
(1) Conform to the following color convention:
(i) Red for warning alert indications.
(ii) Amber or yellow for caution alert indications.
(iii) Any color except red or green for advisory alert indications.
(2) Use visual coding techniques, together with other alerting
function elements on the flight deck, to distinguish between warning,
caution, and advisory alert indications, if they are presented on
monochromatic displays that are not capable of conforming to the color
convention in paragraph (e)(1) of this section.
(f) Use of the colors red, amber, and yellow on the flight deck for
functions other than flightcrew alerting must be limited and must not
adversely affect flightcrew alerting.


Were WW2 American fighters designed to operate from dirt airstrips? Fighter or fighter-bomber planes built in America during WW2, such as the P-36, P-39, P-47, P-51. Were these designed to takeoff and land on dirt airstrips?

My impression, and correct me if I'm wrong, is the following: Most airbases inside USA/Canada at this time had paved runways. At least half the fighters were designed to takeoff and land on carriers. Also, I think every single photo in wikipedia I've seen of these planes on the ground, is on a paved runway.

Now there is of course the Western Front in Europe, and some American aircraft did get sent to the USSR in lend-lease. I do not know if Britain and France had the luxury of paved runways at the time, but in Russia, most certainly did not. So you would think at least some of them had to had the strong undercarriage required for dirt airstrips. But I am not sure if they came that way or had to be modified after arriving in Europe.

One way to investigate this is to start looking at the airfields America had during WW2. Wikipedia has a giant category page for this, but even tho most of those pages have a history section, sadly I could find nothing about paved/unpaved. However, this photo from Midway shows paved runways, and I'm pretty sure Wheeler Airfield during Pearl Harbor had a paved runway too. If they were doing that in the middle of the Pacific, my guess is that most airbases in the 48 states and east/west coast mainland also had paved runways.

The reason I ask is because a stronger undercarriage requires more weight. Dirt landings require bigger wheels which take up room in the fuselage that could otherwise be used for fuel or bombs. Therefore, if paved surfaces are available, it makes sense to take advantage of it in your design. But as I said above, I can't be sure if paved surfaces would be available everywhere they intended to use them.

(Source)

Opening theaters in the Pacific, for example, using fighters/bombers fitted with bush tires would not have been a good choice indeed. Maybe one pilot can pull it off. But not all, and certainly not the damaged planes.

As planes got heavier before WWII, and especially during WWII due to the massive engines and the all-metal construction, came the need for inflated tires, paved runways, hydraulic brakes...

... and the need for the Marston Mat. By using perforated steel planks, a runway can be built almost anywhere in a relatively short time (source).

(Source)

Above shows a North American P-51 taking off from Iwo Jima. Note the pattern on the ground. From far away in other B&W images the pattern does not become clear and the planes look as if they're using pure dirt runways.

  Marston Mats were among the most important inventions of World War II (airspacemag.com).

As seen in your other question, How are modern jets modified to takeoff/land on a dirt runway?, Russian fighters are built with unpaved runways in mind. As are modern military transports. Transports have the luxury of space for many tires. But for jet fighters, it was the advances in tire manufacturing, which itself was due to the jet age that brought the swept wings and consequently the faster takeoff and landing speeds.

(Source)

Of course a humid tropical rainy island is different from a bone dry desert. The different soils and the water content affect their bearing and shear strengths. In North Africa, you may not need special reinforcements for the soil where its strength can be 4 times as high (think dry lakebeds).


What does 1FD2 mean? On the EFIS of Airbus you can see 1FD2 on the autopilot section of the FMA. what does it mean?

It means that both Flight Directors are engaged:

Flight Director (FD) Engagement
The FDs are engaged automatically whenever the FMGC powers up.
Ground Engagement

The symbol "1FD2" appears on both PFDs.
No FD bars appear on the PFDs. The PFD displays FD orders when a mode is active on the corresponding axis.
The FCU windows display dashes.

(A320 FCOM Autoflight - Flight Guidance)
It will look something like this:

The pilots can disengage one or both FDs. In this case -FD2, 1FD- or nothing will be displayed.

Manual Flight Director Disengagement
One FD off:

The FD bars no longer appear on the associated PFD.
The corresponding FD is disengaged.

Both FDs off:

The FD bars disappear on both PFDs.
If no AP was engaged, lateral and vertical modes disengage. The A/THR, if active, automatically reverts to (or remains in) SPEED/MACH mode.
If one AP was engaged when the FDs are switched OFF, this AP remains engaged in the active modes but the FDs are no longer displayed.

If one FD fails, the other one will be used to provide guidance on both PFDs. In this case 1FD1 or 2FD2 will be displayed.

Automatic Flight Director Disengagement
If one FD fails or one FMGC is not valid, both PFDs display the remaining FD.


Could I receive flight training in my company's jet, while on company business with passengers? On occasion I will travel on my company's corporate jet. From a legal/FAA standpoint, are there any regulations against receiving flight training from a CFI with the proper ratings and currency during those flights? I'm a private ASEL pilot, so this would be working on an instrument/multiengine/type rating. And does the answer change if the jet is certified for a single crew member vs requiring 2 pilots? Assume that my employer is ok with this if it's legal.

It is an interesting question and needs quite a few FAR's to figure out. Let's start with having other people on board:

If the aircraft is not certified for single-pilot operations, then you cannot based on FAR 61.89(b)

  (b) A student pilot may not act as a required pilot flight crewmember on any aircraft for which more than one pilot is required by the type certificate of the aircraft or regulations under which the flight is conducted, except when receiving flight training from an authorized instructor on board an airship, and no person other than a required flight crewmember is carried on the aircraft.

If it is certified for single-pilot operations, you may be able to receive training as long as you are not PIC and you are not sitting in the left seat (as almost all single-pilot jets that are certified single-pilot usually have a restriction to left seat only).

However the real rub comes in when you consider that you are a private pilot, not a commercial pilot, and the FAA's extremely broad definition of "compensation". On more than one occasion, the FAA has held (and won in court) that getting flight time in furtherance of a certificate is compensation. Technically just receiving your normal compensation (hourly wages) is enough to meet the definition of being compensated for flying.

The only way to get a real good answer to this, because it is a very broad subject and open to legal interpretation, is to engage your local FSDO.

I'm guessing though that the ultimate answer will be no. Aside from the issues with the FAA and FAR's depending on what operations your employer is conducting under (121, 135, 91, etc), your employer will also have to consider aircraft insurance. Many corporate jets are insured only for certain pilots or pilots who meet minimum qualifications and the insurance policy may completely bar primary training of any kind in the aircraft.

So talk to your FSDO, if you get an OK from them, the next step is to talk to the corporate attorney or an aviation lawyer and have them review the insurance documentation to see if you are allowed to sit in one of the seats and manipulate the controls.


What gas turbines have the following configuration? Is there any jet engines--with published data on the dimension of the turbines and compressors--that are of the configuration of 2 Low Pressure compressor, 8 High Pressure compressor, 2 High Pressure turbine, 5 Low Pressure Turbine? Thank you very much!

I couldn't find any that exactly match. The closest ones I found where
PW1000G-56 : 2-8 and 2-3
PD-14 : 3-8 and 2-6
What is the reason your looking for such specific architecture?


Where are the first structural damages after maximum speed is passed? What 2 parts of the airframe are the first to have structural failure after exceeding maximum speed?

Sorry. I just thought there might be a part that usually failed first.

Q: Which link of a chain will fail first when you exceed the maximum design tension?

A: The weakest link.

It could be any link since none is designed as the weakest link.


When a typical commercial airliner flies overhead, what is the actual noise we are hearing? We're all familiar of the sound planes flying quite high overhead and you hear a faint drone, even when planes are lower to the ground you hear them scream at a slightly higher pitch, but what exactly are you hearing?

I would guess it's either the sound of the fan blades spinning extremely fast or the noise of the air (thrust) being forced out the rear of the engine?

It depends on the type of operation:

Approach (close to airport):

Much aerodynamic broadband noise from the air-frame
Modern turbofan engines are less noisy than the air frame during approach. But engines emit most of the tonal components in the sound.
Depending on the type of aircraft and angle of descent the pilot might need to use air-brakes to reduce speed. That will cause turbulence and noise.
On some Airbuses you hear a typical high pitch annoying sound that is caused by air flowing across cavities under the wing (Fuel Over Pressure Protector to be exact, not from the engine).

Departure (close to airport):

Pretty much all the noise you hear comes from the engines, such as the buzzsaw noise generated when the tips of the fan blades reach supersonic speeds. Hot high speed gases causes turbulence and that creates low frequency noise, especially behind the aircraft.

Overflight at 35.000-40.000 ft:

What you hear is low-frequency broad band noise that originates from turbulence behind the engines. All the high and medium frequencies are gone because the air between aircraft and ground absorbs much of the sound. The sound will also fluctuate due to differences in wind speed, humidity and temperature, along with the flight path and along the sound propagation path.

Further reading / sources

1. Conference lecture by Prof. Dr. Ing. J. Delfs,
Head of Technical Acoustics, German Aerospace Center

2. ECAC doc 29 (Technical documentation)

3. An Overview of Aircraft Noise
Reduction Technologies


How can a pilot become established on the KBIV, ILS or LOC Rwy 26 Approach? Consider KBIV (Holland) in west Michigan:

Now consider the ILS / LOC to Rwy 26 at KBIV:

Questions:

There is a note on the IAP: "Procedure NA for arrival on PMM VOR/DME airway radials 004 CW 091" Does this mean only V55, V193, V274 or V170 or does it mean any radial between 004 to 091 off of PMM?
There is no feeder route on the plate with NoPT. Does this mean the course reversal at JAVPO is mandatory?

From experience, most of the time, it is vectors to final without a course reversal. My question is more about the charting, not the real world result, and what to do if the radar was broken that day...
How do I get to KBIV?

To answer question #1, you are not allowed to use this approach when arriving to the PMM VOR from radial degree numbers 004 ascending to 091. This means that you can not fly in to PMM from the Northish, Northeast, or East. This is probably to avoid conflict with the major congested area of Grand Rapids and its associated airport’s Class B airspace. Also, V170 is no longer an active victor airway. And, PMM R-29 is unusable for VOR only navigation. However, this does not preclude using this approach for local flights and radar vectoring where an arrival would not be necessary.
To answer question #2, the course reversal at JAVPO is not mandatory. It is a Holding pattern in lieu of procedure turns. JAVPO is the Intermediate Fix for the approach. You can use JAVPO or its associated intermediate leg as a gate for a straight-in approach on the IAP. Just state your intentions and get clearance to maintain spacing and separation.
Wbeard52 has given a great acronym to remember when a procedure turn is not required. Another one to remember is the one derived from the FAA‘s list of situations not requiring a procedure turn included in the Instrument Flying Handbook. It is the acronym taught to prepare for the FAA written knowledge test and the practical checkride.
S.H.A.R.P.T.T.

Straight in approaches
Hold in lieu of procedure turns
Arc approaches
Radar vectoring
No Pt noted on the approach
Teardrop course reversal approaches
Timed (start) approaches

One point to note is that a procedure turn should be planned anytime the angle inbound to join the approach will result in a turn of greater than 90°. This means that flying from PMM on the 033 radial direct to JAVPO without vectoring would result in a course reversal. However, you could hold on the published hold at PMM to get set up for the approach. This would necessitate holding at a traditionally very busy (aircraft dense) fix, though. Probably something both you and ATC want to avoid for a GA aircraft if possible.


What are these strange blades behind the N1 fan in a Boeing 747-8 engine? It looks like a second fan or something.

Those are Outlet Guide Vanes / Fan Exit Guide Vanes. They prevent the flow (bypass flow in this case) from rotating. The these vanes do not rotate, they are fixed to the housing. They also provide structural support.

Because there is no energy lost on the rotation of the flow the efficiency is increased.


Why does Thai Lion Air use loads of 737 ERs for 1-2hr routes? Thai Lion Air primarily flies short-haul domestic routes and most of the time I find myself on an ER version of a relatively new 737. Isn't ER for extended range? And thus costs a lot more per plane?

Incidentally their sister company Malindo also has what seems to be an unnecessary amount of ERs (I just flew on one this morning from KUL to BKK - a 110min flight - and one the week before from KUL to HKT, again under 2hrs).

Operationally speaking, this may come as a surprise, but it's not the range of the ER, rather the additional exits and flat bulkhead, which allow more seats (note: Lion Air was the launch customer of the -900ER).

Exits: even if there is room for more passengers, a limiting feature is typically the exits. More exits, more passengers can evacuate in the same duration in an emergency.
Bulkhead: the typical bulkhead is spherical; a flat one would allow more floor area.

The 737-900ER is the same size as the 737-900, but can carry an 26 additional passengers in a one-class configuration. It incorporates an additional pair of exit doors and a flat rear-pressure bulkhead that boosts maximum capacity to 215 passengers in a single-class layout.
— Boeing 737-900ER roll-out in Renton; flightglobal.com; 10 August, 2006

(planespotters.net) One additional exit per side.

(airliners.net) An example of a flat bulkhead on the Boeing 727.


What happened to the Ju 52 (“Aunt Ju“)? I remember when I was a child, my parents and me often saw the Aunt Ju flying. But about 5 years later it disappeared. We never saw it again.

Does anybody know what has been the use of the Ju 52 after the 70s?

According to Wikipedia there are still 8 JU-52's flying today. Which one do you remember seeing flying?

List of airworthy Ju 52s


Why were the windows on the Concorde about the size of a hand? I just saw a comparison between the 787 and Concorde windows. I could be wrong but it almost seems like a hand could cover the majority of the window.

The Concorde flew above 15,000m. At this altitude a sudden reduction in cabin pressure would prove hazardous to crew and passengers with most falling unconscious within a few seconds. The low air-pressure would also render the oxygen supply system inefficient and most passengers would suffer from hypoxia.

Thus we have the reason for the smaller windows, should a breach occur then the size of the hole being small would reduce the rate if loss of air-pressure inside the cabin. This, combined with a reserve air-supply to augment the cabin-pressure as well as a rapid rate of descent manoeuvre to bring the aircraft to a safe altitude would reduce the the risk of hypoxia.
So basically the small windows were designed to reduce the rate of air escape from the cabin should a hull breach occur.


Why aren't passengers trained and evaluated for operating emergency exits in commercial aircraft? The passengers sitting on the overwing exit doors are expected to operate the doors in times of emergency, with only a verbal instruction. Is that enough?
Q. Why can't we have a designated area in the airport where a setup of emergency doors are kept and the passengers are trained and evaluated there before being alloted the seats on the emergency row. It can be short 2 min course, where passenger actually opens the door and puts it aside.

There have been instances which highlight that proper training is required for passengers Soource:

Passengers will frequently open an exit as soon as evacuation begins which may result in them evacuating into danger. This occurred in the Ryanair engine fire in Stansted Airport in 2002, where passengers evacuated themselves onto a burning wing, despite airport fire services personnel shouting at them to return inside the aircraft and evacuate via a usable exit. Source

Another hazard in the use of overwing exits is them being improperly opened (usually a result of passengers in these seats not paying attention to the verbal briefing provided pre-departure, or not observing the opening instructions on the safety card and on the exit). The majority of overwing exits involve the passenger physically removing the hatch from its frame and disposing of it outside on the wing without blocking the exit. Research conducted at the Cranfield Institute in the UK Source

Above situations taken from Wikipedia
https://en.wikipedia.org/wiki/Overwing_exits#Hazards

Former TWA here.

Emergency exit seats used to only be given to passengers who are ready, willing and able to assist in case of an evacuation. They could not be assigned in advance, the passenger had to be asked and evaluated in person by the boarding agents, who would have been held responsible if a not-able passenger was to be seated in any of those rows/seats. This used to be the rule, it was deemed that safety was more important than selling seats.

Maybe the rules have changed since?


What is a demonstrated crosswind component? What is a demonstrated crosswind component? Is it possible to land in a crosswind greater than the demonstrated crosswind component? Is it safe to?

A demonstrated crosswind component is highest crosswind (corrected to make it 90°) which has been shown to be possible to safely land by a test pilot. It shall not require exceptional skill by an ordinary pilot, however it does not mean every pilot will be able to do so. It is also NOT a limit (contrary to what some say) - if the pilot does decide to land higher crosswinds, they can do so (but please take caution).

This number may not necessarily be the highest crosswind component that it's possible to land in -- it just has to be above the minimum capabilities required in the regulations. For example, say an aircraft, flown perfectly (yes, we're dreaming), was capable of handling a 30 knot crosswind component (no gust, ISA conditions), the manufacturer only needs to demonstrate safe landing with a crosswind component of one-fifth of the reference stall speed in the landing configuration. This would be:

10 knots @ 50KIAS VS0
12 knots @ 60KIAS VS0
14 knots @ 70KIAS VS0
15 knots @ 75KIAS VS0

Reference:

  (a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VS0.


What would be the advantages and drawbacks of a single-propeller tiltrotor? When looking at existing tiltrotors, most of the designs use two propellers, sometimes four. helicopters (and light planes), on the other hand, generally use only one propeller.

Have there ever been designs or proposals using a single rotor (or contra-rotative rotors on the same axis) instead? If so, what advantages and drawbacks did they have? If not, are there specific reasons why this configuration is always ignored?

The most obvious one would be that the engine would sit right on the cockpit, taking its place or at least blocking visibility. But we can imagine a tiltrotor UAV where this would not be an issue.

For the needs of this question, tail-sitters like the Convair XFY Pogo are not considered tiltrotors, as the entire craft changed orientation instead of only the engines.

Such an aircraft can't exist.

Helicopters don't work with a single rotor, they need at least two of them to counter torque. There are several popular designs, including conventional, coaxial, intermeshing and tandem rotors.

All tiltrotors operate as tandems in helicopter mode. It's the only setup where both rotors can be placed at the nose, tail or wings. In all other designs, at least one of the rotors needs to be mounted in the middle, above the center of gravity, where it can't be tilted without crashing it into something.


Is an eclipse considered 'night' for regulatory purposes? A partial or total eclipse is as dark as twilight, but only for a short period. Does it count as night flying for regulatory purposes?

I'm looking for a UK answer, but information about the US is interesting too.

No: that does not count as night flying.

The CAA defines "Night" (Air Navigation Order, Article 129) as:

‘Night’ means the time from half an hour after sunset until half an hour before sunrise (both times inclusive), sunset and sunrise being determined at surface level;

Similarly, the FAA's definition (FAR 1.1) is:

The time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.

Since an eclipse is neither a sunset or sunrise, nor does it happen between evenings and mornings, it does not fit the definition of "Night" for legal purposes.


Is there a landing gear system that extends automatically to prevent belly landing? To my knowledge, most aircraft with retractable gear are equipped with a landing gear warning system that makes repetitive beeping sounds in case the pilots try to land without the gear down.

I imagine big jetliners like Airbus and Boeing would have a far more advanced landing gear warning system, but does any of them have a landing gear system that extends automatically if the pilots fail to extend the gear and continue their descent for landing despite the warning?

The simple fact is that an unintended gear deployment could be catastrophic, as it has a profound impact on the flight dynamics of the aircraft (one of the reasons there are warnings for both retraction and extension).

The benefits would be marginal as checking gear down is part of the bread-and-butter procedures for all retractable gear aircraft.

Keep in mind adding some automation to an aircraft has far ranging impact, just off the top of my head adding automatic gear mechanism would require:

Standardization of the automation
Modification of procedures
Training of pilots, and then re-certification of pilots
Training of maintenance crews, and then re-certification of the crews
Possible change to the maintenance schedule of aircraft
Certification of the new system by the regulator

and you would still need a backup / redundant system; and now you are back to the manual controls.

I can imagine this being implemented in remote piloted or autonomous aircraft though; as an additional workload relief.


What is it called when you roll without yawing or pitching? My instructor made me do an exercise that consists of banking the sailplane left and right around 30 degrees without moving from the axis. I need to aim for a specific point, and start with the exercise. This is a coordination exercise but does it have a name? It's like a dutch roll but I need to stay in the roll axis without moving.

In the aerobatic community we would call that the beginning of an Aileron Roll.

It's harder than it looks.


What are the advantages and disadvantages of a delta wing compared to a swept wing? What advantage does a delta wing have over a swept wing with high AR for supersonic flight? And what disadvantage does a delta wing have over a swept wing with high AR for transonic flight?

The Concorde is made to cruise at supersonic speed--delta wings.

Most commercial airliner--swept wing with high AR.

Would you design a supersonic airplane with the conventional airliner swept wings? On the other hand, would you design a transonic flight with delta wings?

Benefits of a delta wing:

Due to the large root chord, a delta wing combines low relative wing thickness with a sufficiently thick wing spar for a lightweight structure. Since a low relative thickness keeps wave drag down (a drag component which occurs only in supersonic flow), this makes delta wings especially attractive for supersonic aircraft.
The large root chord gives the delta wing a high internal fuel volume even at a low relative thickness.
The large root chord also provides it with a large surface area which helps to bring the minimum speed of the aircraft down.
With sufficient leading edge sweep, a delta wing produces vortex lift, so flow separation can be turned into a means of increasing lift.
A delta wing is naturally stable in pitch, therefore it does not require a separate tail surface.

Drawbacks of a delta wing:

The large wing area causes more viscous drag for the same amount of lift compared to a high aspect ratio wing. Swept wings have a better lift-to-drag ratio (L/D) than delta wings.
High-lift devices like fowler flaps are hard to integrate into delta wings. The higher relative thickness of regular airliner wings allows to integrate large flaps more easily, and the rearward location of the trailing edge of a delta wing will produce intolerable pitching moments if such flaps would be deflected.

Generally, the moderately swept high aspect ratio wings of subsonic airliners are ideal for flight at high subsonic Mach numbers. Delta wings are superior only in supersonic flight, and due to their good low speed characteristics they offer the best overall compromise for supersonic aircraft.

North American XB-70 in flight, a classical delta wing design (picture source). Note the canard wing, which has little sweep and a low aspect ratio: This kind of wing is ideal for supersonic flight, provided it has a low relative thickness. The main wing of the XB-70 is a delta wing to keep the take-off and landing speeds relatively low and offers the best compromise of low and high speed characteristics. Also, the wing tips could be deflected downwards to improve directional stability at supersonic speed and to improve L/D by using the waverider effect.

The wings of both the Boeing 787 (top, picture source) and the Airbus A350 (bottom, picture source) show their powerful high-lift devices in these pictures, which allow them to fly with less than half the wing area of a comparable delta wing aircraft. Also, the lower surface area translates into lower drag at high flight speed, and the higher possible wing span helps to reduce induced drag, making those wings far superior for efficient flight at high subsonic speed.