Why could these aircraft not prevent total hydraulic failure? Then have been three incidents (UAL232, JAL123 and DHL 00-DLL) where damage to an aircraft resulted in complete loss of hydraulics. Only one of them managed to make it down safely using differential thrust. Each plane was from a different manufacturer. All three aircraft had triple or quadruple redundant systems. This seems like plenty of redundancy, but the Achilles heel is that, regardless of how the systems are piped around the aircraft they all have to converge at each of the control surfaces.

My question is why did damage in one part of each hydraulic system result in loss of the entire system? For example in the DHL incident the left wingtip was damaged with all three lines. Is there not a way to isolate that wing and cut off hydraulic flow to protect the integrity of the rest of the system? You would lose control surfaces on that wing, but still have elevator, rudder and the other wing. Do any aircraft have a way of preventing this?

There is no general answer, but the closest one would be bad luck.

In the DC-10-10, the aircraft of UAL232, all three hydraulic lines went through one hole in the tail bulkhead which was damaged when the engine failed. This is no real redundancy, and it was bad luck that this particular spot had been hit.

In the Lockheed Tristar, the same accident would had damaged probably just one line. There, the three hydraulic lines are separated as much as possible in the tail section (one on top, one on the left side and one of the right side of the fuselage). Lockheed has more experience with military design than Douglas, so the Lockheed engineers were more aware of redundancy issues.

In the A300 involved in the Baghdad attempted shoot-down incident, the same bad luck happened. Normally, each aileron is powered by two out of three hydraulic circuits, so one would remain if the other two are hit. Note that the green and yellow circuits were lost immediately, while the blue circuit failed only 20 seconds later. But in this case the damage extended to the spoilers, so all three circuits were affected in the end. There is only so much damage you can protect against.

Also for JAL123 the damage was just too extensive to not involve all four hydraulic circuits. The rear pressure bulkhead burst and blew most of the vertical tail surface away. It can be argued that the real culprit, however, was the chief pilot's refusal to don his oxygen mask. But the aircraft was only marginally stable and very hard to control.

Hydraulic fluid needs to be pumped continuously to heat it so its viscosity stays low. If you add shutoff valves, this pumping could be impeded. Note that the spoilers of the Baghdad A300 were not sucked open because non-return valves blocked the lines to the spoilers, helping in keeping the aircraft flyable.

If you put shutoff valves along the hydraulic lines, you open up a new avenue for failures. The idea of hydraulics is that you use just one pump and can create the maximum force at any point, since rarely would you need to move several actuators at maximum load in parallel. Splitting up the circuitry to enable the pilot to isolate parts of the airplane will take away this advantage, resulting in a more complex, heavier hydraulic system. And who knows if those added parts would not malfunction or be mishandled, resulting in a new class of entirely avoidable accidents.


What gas is used in airships to give them buoyancy? In the 1930s Zeppelins used hydrogen as the lighter-than-air gas to fill the envelope and give the ship buoyancy.

What kind of gas is used today?
Is it still as flammable as the Hydrogen from airships of the 1930s?

There are only 2 lifting gasses that are lighter than air and practical for airships;

helium: this is a noble gas which means it's inert. Safest option all around except that US has a monopoly on the production of it. That is also the reason Zeppelins didn't use it because of an embargo the US had at the time against the Germans.
hydrogen: even more buoyant than helium but flammable. It is however easier to get by running a current through water or dropping some metals in an acid.

The third option for providing buoyancy is heated air, this requires a heat source to counteract the loss of heat over time.


Does the lead in fuel harm passengers? In view of this other question, does the lead first enter or infiltrate the cabin air dispersion system, and then pervade or permeate the entire passenger cabin, thus every breathing passenger?

I believe that the recirculated cabin air passes through HEPA filters, but does this resolve all the menaces and perils of lead? If not, how can passengers protect themselves?

Your question is based on an incomplete picture of how aircraft work, I'm afraid.

As a passenger on any major airline, the only lead you are likely to encounter is in the shielding on the security x-ray machines.

Fact: leaded fuel is relatively uncommon

First, tetraethyl lead is only present in what we call avgas - it is most often sold these days as 100LL (100-octane low-lead) aviation gasoline. Avgas is only used in piston (reciprocating piston) engines. Very few commercial aircraft in this day and age are powered by piston engines; most airliners are turbine-powered (jet or turboprop engines) and are fueled by Jet A, a variation on kerosene.

Fact: exhaust shouldn't be in the cabin anyway

Second, even if you happen to be flying in a light aircraft with a piston engine (or old, think Douglas DC-3 in Alaska), you shouldn't actually encounter any lead. The cabin air intakes are positioned intentionally to avoid any exhaust entering the cabin.

If exhaust carrying lead particles was reaching the cabin, you've probably got bigger and more immediate problems with carbon monoxide poisoning anyway.


Why does the Dassault Falcon 7X's cockpit have ashtrays? While looking at the first implementation of Fly-By-Wire systems, I see that the 7X was the first business jet with fly-by-wire controls.
Looking at the Wikipedia page for the aircraft, I saw this photo of the cockpit and noticed the ashtray in front of the sidestick on either side:

Why would there be an ashtray in the cockpit?
The Telegraph states that they are still present in aircraft toilets due to a FAA regulation that requires an ashtray be present in the event that a cigarette is lit and needs to be extinguished. However this doesn't cover the flight deck, surely the FAA doesn't assume a pilot will light-up during flight?

In the United States, the smoking ban only applies universally to air carriers on scheduled flights. The regulation states

  §252.4   Smoking ban: air carriers. Air carriers shall prohibit smoking on the following flights:

  (a) Scheduled passenger flights.

  (b) Nonscheduled passenger flights, except for the following flights
  where a flight attendant is not a required crewmember on the aircraft
  as determined by the Administrator of the Federal Aviation
  Administration:

    (1) Single entity charters.

    (2) On-demand services of air taxi operators.

  (c) Nothing in this section shall be deemed to require air carriers to
  permit smoking aboard aircraft.

So for essentially all of the Falcon's missions, such as Part 91 corporate flights, and private charters under 10 passengers, smoking is permitted, as long as an "adequate number of self-contained, removable ashtrays" are provided (14 CFR 23.853(c)(1) or 25.853(f)).

Said ashtrays from a factory photo:

Closeup found here:


What are the considerations to make when altering logbook entries? As I move through my flying experience, the possibilities for logging time change.

Before I was rated, my time with an instructor is logged as dual only.  Most of these entries were made by various CFI's.
After being rated, FBO checkouts and flight reviews were logged as PIC and dual. These entries were often made by myself and signed by the CFI.
When receiving instruction for endorsements (tailwheel, high performance, complex), entries were usually made by the CFI.  It appears that most of the time, the instructors making the entries favored the idea that until I had the endorsements, they would leave the PIC column blank and log only dual received.

The answers to This Question lead me to believe that I could indeed have logged this as both dual and PIC, since I was rated for the category and class for which I was flying.

Here is the question:  What consideration should be given before I go back and add PIC to the flights which were filled in by someone else?  Should I leave it alone or claim the time as PIC since I was both "sole manipulator" and rated in that category and class.

I couldn't find any specific regulations, but correcting errors or omissions in the time logged is fine. You should never change an actual endorsement, though.

To take a simple example, if your CFI wrote in 1.1 hours instead of 2.1 hours then I assume you wouldn't hesitate to correct it later because it's an obvious error of fact. In the same way, if your CFI didn't write in PIC time when he should have then there's no problem if you add it yourself. Provided of course that you're sure that you were indeed allowed to log PIC time for that particular flight.

If you want, you could add a comment or keep a record of changes on a separate sheet in case someone ever questions it. But for what it's worth, I have a number of corrections in my logbook and no one has ever asked about them. There's a high degree of trust involved in logging time because it's so difficult to verify the information independently.

But you shouldn't touch an endorsement. Only an instructor can make an endorsement and even if there's some mistake then you should ask the instructor to enter a new one rather than change it yourself. Endorsements get much more attention that time logged, and I think it would be tough to explain why you changed one yourself.

Finally, you mentioned your instructors' "thinking" or "ideas" about logging PIC time. I don't want to make assumptions about their intentions (and I'm not a CFI), but logging PIC time is defined by regulation, not by what individual instructors think. If you're still in contact with those CFIs, it would be a great topic to discuss with them. Perhaps they didn't deliberately omit the PIC time, they just forgot or overlooked it. Or maybe they did have a reason, but in that case they should at least be able to explain it to you with reference to the regulations.


What type of helicopter is shown in Star Wars The Last Jedi featurette? There is a nice featurette about the new Star Wars movie The Last Jedi.

One shot (1:32) shows a helicopter in the background. What is it?

The helicopter is a Eurocopter EC135 T2+. The registration (just above the tail rotor) EI-ILS shows that this aircraft was registered in Ireland, this helicopter was sold to PDG HELICOPTERS in early 2016 and is now on the U.K. register as G-GLAB. Based on this information it suggests that this particular segment was filmed before or shortly after early 2016 when the aircraft would likely have been re-painted with the new registration.

Eurocopter are a popular "household name" in aviation with numerous models operated throughout Europe and beyond in various roles such as airborne policing, medical flights (air ambulance), and some military operations. Eurocopter now operate as "Airbus Helicopters" and subsequently the EC135 has been re-designated as the H135.


Why does the Sukhoi-30 MKI have both canards and conventional horizontal stabilizers? I believe the Sukhoi-30 actually has 3 means of altering pitch and roll - canards, horizontal stabilizers, and thrust vectoring.

Image courtesy www.mars.slupsk.pl

What is the point of this?
Does it lead to an increase in maneuverability?

Yes, increased maneuverability and redundancy are the main benefits of this layout. Another is a wider possible range of the center of gravity.

In the video linked in Federico's comment you can see that the canards reach high negative deflection angles in high-alfa attitudes: This means they will still operate in attached flow when wing and tail are already fully stalled. A canard-less airplane would only have thrust vectoring for control in this situation. Also noticeable in the video is the negative static stability which becomes evident in the lively elevator movements during the landing approach towards the end of the video. When you need agile pitch controls for artificial stability, it helps to have a second pair of them in highly dynamic maneuvers. The small size of the canards might be misleading: Since they will affect the flow over the inner wing, they are quite effective and powerful control surfaces.

A multirole, long-range aircraft will have to deal with a wide variety of ordnance and fuel loads, so the addition of canards will allow to trim a wider range of c.g. positions. Without them, some load configurations, and consequently some missions, might not be possible.


Has a micro-burst or wind shear caused fatalities on takeoff? Delta Flight 191 at Dallas-Fort Worth (1985), TANS Perú Flight 204 in Peru (2005), FedEx Express Flight 80 in Japan (2009), and Eastern Air Lines Flight 66 at New York's JFK, are all examples of aircraft fatalities due to wind shear or micro-bursts during approach.

Have there been any airliner fatalities due to micro-bursts or wind shear on takeoff?

 source

UPDATE: As a hobby, I have spent my life following aircraft accidents. At the outset of this question, I was unaware and doubted any such occurrences had happened. The answers have certainly been an eye opener and shows the breath of knowledge this group has.

So far:

Canadair C-4 Argonaut three crew and 29 passengers killed
Pan Am Flight 759 all 145 on board and 8 on the ground killed
Spantax CV-990 crash of EC-BNM off Arlanda (1970)
Continental Flight 426, Denver, (1975)
Continental Flight 63, Tucson, (1977)
United Flight 663, Denver, (1984)
(possibly - AeroméxicoFlight 2431) (2018)

One (of several) incidents that I remember was Pan Am Flight 759. It encountered windshear after takeoff from New Orleans' Moisant Airport July 9, 1982 and crashed, killing all aboard and several on the ground.


Can I log simulated instrument time in a complex plane without having a complex endorsement? My safety pilot is endorsed for complex, and will be PIC for the entire flight. I would also log PIC for the time I spend under the hood.

I will eventually get my complex endorsement, but insurance is much cheaper if I have time in type, so in the long run it will be cheaper for me to build time in type and work towards my instrument rating at the same time.

Yes. You can, as long as you have the appropriate rating and are the sole manipulator of controls. From a FAA response to a similar question:

  ... a private pilot may log pilot-in-command time, in a complex or high performance airplane, for those portions of the flight when he or she is
  the sole manipulator of the controls because the aircraft being operated is single-engine land and the private pilot holds a single-engine land rating.

  ... 14 CFR section 61.31 requires a person to have an endorsement from an authorized instructor before he or she may act as pilot in command of certain aircraft (a complex airplane, a high performance airplane, a pressurized airplane capable of operating at high altitudes, or a tailwheel airplane). These endorsements are not required to log pilot-in-command time under 14 CFR
  section 61.51(e). (emphasis mine)

Excerpt from §61.51 Pilot logbooks:

  (e) Logging pilot-in-command flight time.

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

  (iv) When the pilot performs the duties of pilot in command while under the supervision of a qualified pilot in command provided—

  (A) The pilot performing the duties of pilot in command holds a commercial or airline transport pilot certificate and aircraft rating that is appropriate to the category and class of aircraft being flown, if a class rating is appropriate


What is the wire running from the CG to the tail? I was looking at airplanes a while back and noticed that there is sometimes a wire running from about the CG near the wind to about halfway up the tail.  I see this more on 737s but I have also seen it on C130s.  What is this wire?  To hold up the tail?  I have seen: What is this wire going from mid-fuselage to the tail on this 737? but I am more interested in all of the other planes that have it too.  Are there planes that actually use guy-wires for support or are all of the wires I see antennas?

Sounds like an HF antenna. It absolutely isn't structural, as neither of those aircraft needs guy-wires for structural stability. (You'd also expect to see it on every example, not just some, if it were required that way.)

This article provides a lengthy discussion of HF antennae, for those interested in going deeper into the subject.

This thread has a related discussion of why not all HF antennae are of the long-wire type. Essentially, newer designs often replace the long wires.


Time of flying from Toronto to Philippines Please I would
Like  to know how many hours I will be flying
Across the ocean . I will be flying on an aircraft 773 boing 777.

A direct flight from Toronto to Manila is about 16 hours. It is classified as a "Polar" flight and very little of it would be over the open ocean.

Depending on the weather and upper wind conditions, you would be initially flying either North East, or North West, from Toronto.

Most of the flight will be over land, with some of it over the frozen Arctic Ocean or Sea of Okhotsk near Russia.

FlightRadar24 PR119/PAL119


Why does wing bending cause wingtip twisting? (semanticscholar.org)

When aft swept wings bend/flex, the outer wing is twisted leading edge (LE) down (this reduces wave drag), the reverse (LE up) is true for forward swept wings.

I've searched the site and googled it, but I wasn't able to arrive at an explanation (Wikipedia also remarks on the fact, but does not explain the causes).

I would say it's because the twist in question relates to Mean Aerodynamic Chord, which is always aligned with the longitudinal axis and airflow, and diverges from the airfoil chord line when the wing is swept (it becomes a truncated slice you might say).

Assume a wing that can bend along its spar axis but that has perfect torsional stiffness for our purposes.  When a straight wing is bent up, the incidence of the MAC at any given point, being perpendicular to the bending axis and the same as the airfoil chord, remains the same from root to tip.

On a swept wing that is bent up, the airfoil chord line, perpendicular to the span wise center line (B), also remains untwisted along its bending axis (in our perfectly torsionally stiff wing), but the MAC, which is now different from the airfoil chord line because it's aligned with the longitudinal axis (A), sees a geometric reduction in incidence in proportion to the bend.


Airplane gently rocking its wings during whole flight I was on a flight with a regional airline a while ago in a Bombardier airplane (if I recall correctly) where the airplane would slowly and gently rock its wings just a little bit on the whole flight. I wasn't a long flight but wasn't very short either at roughly 2 hours.

It was very comfortable and the rocking made for a very gentle cradle feeling

My question is, why would that be? Is it economical to do so for any reason? Is it possible that the airplane had no autopilot and the pilot was flying it by hand? What could cause such a behaviour?

I can give you two theories.  One is it's a very very mild case of "overcontrolling" of the autopilot roll servo as it chases a perfectly wings level condition and continuously applies a teeny bit too much left, ooops, then a teeny bit too much right, oops, and back and forth.

The aileron control circuit is cables running to hydraulics at the ailerons, with a spring loaded roller in a curvy V shaped cam device that centers each side of the aileron cable circuit. The breakout force to move the control circuit out of neutral (forcing the roller from the center V of the cam) is fairly high and this can actually be a problem for pilots hand flying at higher speeds where the ailerons are quite snappy - applying enough force to break out the circuit and move the wheel left gives too much, and back the other way... too much... and back and forth you go.  It can be a pain.

The autopilot servo is basically a little electric winch connected into the aileron cable circuit, doing the same thing the pilot does. It could be that something in the electronic control loop, or mechanically within the servo, can cause this back and forth oscillation on some airplanes, sometimes.  It may be related to wear and backlash within the autopilot servo.

The other theory is it's more or less something similar, but in the yaw mode with the yaw damper not able to fully dampen out a dutch roll mode because the amplitude was within the yaw damper's sensitivity band. Dutch roll is rolling due to yaw, which in swept wing jets is very strong, such that it can set up continuous self energizing oscillation.  It could also be related to wear and backlash in the yaw damper actuator, which is a pair of electric linear actuators (little electric screw jacks), in the push rods going to the rudder hydraulics.

If you saw the wing tip just going straight up and down, it's mostly likely the first theory.  If you could see the winglet moving in a little circle (indicating rolling and yawing), it's more likely the second theory.

I can really only theorize because the issue was never fully explored on the RJ program as this phenomenon was random, very minor and not dangerous, and was never complained about by operators because only someone sitting near the wings that can look straight out at the winglet ever notices it.  It was never bad enough to get anyone sick for example, at least on the RJ fleet.


Do multi-engine jets need all engines with equal age to reduce asymmetry in thrust and fuel consumption arising out of deterioration? Let's assume, one of the engines on a twin-jet needs replacement and the jet is is fitted with an old engine and a newer engine after the due replacement, the older engine would tend to produce less thrust at higher fuel consumption due to deterioration on account of aging compared to the new one. Though the aircraft are designed to handle such asymmetries and modern FBW FCS should be able to handle it without bothering the pilot for any manual corrections, logically it makes sense to think one would like to do some basic engine matching to make sure the asymmetry is as small as possible. Do they follow this in real life? Are there guidelines in MRO procedures specified (both Civil and Military Jets) on how to choose a replacement engine, so as to maintain symmetry in the thrust profile?

It doesn't quite work that way.  When an engine wears out it's rated thrust doesn't decline; its "ITT Margin" (or some similar phrase - basically its thermodynamic margin at takeoff) declines.

When setting takeoff thrust on a turbofan there is a target N1, or target torque on a turboprop or turboshaft, or Engine Pressure Ratio on a pure jet, and the engine has to achieve that target, so it's making rated thrust or power, more or less, no matter how old it is.  What happens when it ages is the temperature required to get there goes up.

There is a maximum hot section temperature  (Inter-Turbine Temperature - ITT or Turbine Inlet Temperature - TIT, or Turbine Outlet Temperature - TOT, or something else, depending on where the probe is) for the engine when producing takeoff thrust or power.  On a new engine, the actual maximum temperature that you will see will be well below the maximum allowable (the temperature margin).

As the engine wears (turbine and compressor blade erosion mostly) and its efficiency declines, what happens is the hot section temperature when the engine is at takeoff rating goes up.  It creeps higher and higher as the engine ages until eventually the margin is gone (or down to some minimum), temperature exceedences on takeoff may start to occur, and it's time to come off.

So there isn't really a thrust asymmetry problem with mixing new and old engines, although there will be a small fuel flow difference, but you have an automatic balancing system to account for this.

Staggering engine times is common.  On an older, heavily depreciated jet especially, the engines may represent a huge part of the value of the airframe and it's a common practice to stagger the age of the engines avoid having to change out two at once when they wear out.


Do major passenger airline jets have airbrakes/speedbrakes? Now I've heard that passenger airliners do not generally have airbrakes , and simply use their spoilers as airbrakes since they can perform the same function.

Are there any major jets that use speedbrakes (by major I mean widely used by countries or airlines, and it must be a passenger jet)

Yes (for some values of major)

Adrian Pingstone

(c) Harm Rutten


How is a horizontal stabilizer attached to the fuselage and works as a trimmable Horizontal Tail as well? I am trying to figure out how does the horizontal stab attached itself to the plane and moves up and down? Is the HS attached by the rear spar and to the fuselage?

Fighter aircraft usually have their tailplane connected to the fuselage with a single pivot joint, as visible in this F-15 drawing:

Passenger aircraft often have a horizontal tailplane which is a single part, and as such, the entire stabilizer is being trimmed by an actuator around a pivot, as can be seen in this 727 drawing. The actuating system is different on newer aircraft, but the idea is the same.


Are there any heavy lift jets (like Stargazer) with zoom climb (like Ye-66A MIG-21)? Are there any intercept jets (that can zoom climb like 1961 April 28 113,891ft Ye-66A Mig-21 Turbojet) built for heavy lift?  I see the Stargazer and Stratolaunch as really slow heavy lift vehicles that cannot zoom climb.  I saw one article about using an F15E as a rocket lifter.  Is it silly to dream of a purpose-built large jet that could carry a large payload (like a Pegasus rocket) and zoom climb to a high altitude (100,000ft) before releasing its heavy rocket payload?

Zoom climb of the carrier is useless. The rocket can do it alone after release with the same effect and is slightly more aerodynamic as it does not have the large wings of the carrier.

The rocket starts with certain amount of energy upon release. Part of this energy is kinetic, given by speed of the carrier, and part of this energy is potential, given by altitude. Now a zoom climb means exchanging kinetic energy from going fast for potential energy to climb higher. But the total amount of energy does not change. And since the rocket still needs to increase both it's kinetic and it's potential energy (accelerate and climb) it does not really care which form of energy it gets.

Now the orbital vehicle needs energy corresponding to 7.9 km/s and a launch aircraft like the Stargazer can give it perhaps 0.57 km/s (of which only ~0.24 km/s will be speed, the rest is the ~12 km altitude). This is a tiny fraction of what it needs, but:

It helps significantly that the rocket does not have to penetrate the thick low atmosphere. At 12 km the pressure is 19% of what it is at sea level. Going higher is not going to help all that much.
Due to the rocket equation (the amount of fuel needed is exponential) it can still make significant difference to payload fraction.

What we are likely to see is carriers that can give the orbital vehicle more initial energy. By going faster (like the F-15), by going higher (like the Stratolaunch) or both (using scramjets it to gain some higher hypersonic speed at altitude perhaps 30 km). But they won't do zoom climb, because that does not add any energy and adds complication.


How do I get a copy of a Pilot Operating Handbook? I need the POH for a Cassutt Plus or any variation for a University Project but I can't seem to find anything on the plane.
Any other information (flight capabilities etc) would be helpful - with references if possible!
Thanks

Generally you won't find published POH documents for plans built homebuilts, as each aircraft is unique, the individual builder being the "manufacturer".  Possibly, a builder has made his own POH. Best bet is to write to the guy currently marketing the plans and selling parts, Creighton King.


Question about CDI/OBS and HI Could somebody please explain me the differences between using the OBS and the Heading Indicator? As much as I could understand, once you tune into the VOR, the OBS starts to show how much deflection/ deviation is there. You then turn the OBS to align the needle which shows the direction (in terms of angles) that you need to fly to reach that VOR.

Does this mean that the OBS is the instrument to derive the direction, and serves no direct purpose in terms of heading? I.e. the Heading Indicator is the one that then should align be flying in that new direction, right?

Could somebody please explain me the differences between using the OBS and the Heading Indicator

They are two very different instruments. The OBS is used to track inbound/outbound from a VOR station, whereas the HI (or, DI - Direction Indicator) shows the current direction of travel (assuming it has been aligned correctly with a compass).

"Turning the OBS to align the needle" is not really the usual way to use that instrument * - you set the OBS to the radial you intend to fly, and use the needle to align yourself with that radial.

If OBS and DI agree with the needle centred, you're on the radial selected by the OBS.

* There is of course a use-case where you just want to go direct to a VOR, and then you may just turn the OBS until it shows "TO" with a centred needle. But if you're doing this regularly you might be using it wrong.


Can I do a CAT III ILS approach if minimums are above CAT I? I was going to an airport with weather at minimum for CAT I approach. Not wanting to know whether or not I could make it I thought about asking ATC if I could do a CAT III approach even though the weather was still above minimum for CAT I. Eventually the weather got better but my question is: Could I have requested from ATC and be granted a clearance for a CAT III approach (abiding by the CAT III weather mins) even thought the current weather was above CAT I minimums?

Yes, you can request it. We (ATC) may or may not grant it. It can be quite complicated for us to establish CAT II/III procedures, including having to switch over to backup power, turning on stopbars and other protective measures, limiting aircraft and vehicle movements on the maneuvering area, closing certain taxiways etc.

Doing a CAT III approach is not just looking at a different chart, it can involve a lot of complicated procedures for the airport!


How much effect does the ground have on flight control when flying low? In this awesome video, shot at the "Mach Loop" in Wales, F-15Cs fly down the valley, then cross a ridge very low to the ground. While it's difficult to tell from the video just how low they really are, they seem to be fairly close. How much effect on flight characteristics do the pilots feel as the ground comes up to meet them, then falls away?

To clarify a couple of points from the comments, I'm interested in any effects on flight control that are caused by flying fairly low over that ridge and through the valley (to grandmother's house we go).

At the high speed of the F-15s in the video ground effect is almost negligible. Ground effect is about the restriction of downwash by the ground, and at high speed and air density downwash is rather small. Also, the duration of close proximity to the ground is too short for ground effect to fully develop.

The biggest effect will be air turbulence. Since air will neither flow into the ground nor out of it, the vertical speed of atmospheric turbulence is greatly reduced when flying close to the ground. Also, the scale of turbulence will get smaller with proximity to the ground. While gusts may rock your wings when flying high, at very low altitude (less than one wingspan) the turbulence will average out over the wing.

When flying over a ridge, any wind speed component perpendicular to the ridge line will mean updrafts on the windward side of the ridge and downdrafts on the leeward side. This will be the strongest effect in this particular case of F-15s flying over a ridge at Mach 0.7 to 0.9.

Now back to that ground effect meme which seems to find wide agreement here: Ground effect is a stationary process which is strongest at low speed, when the downwash behind the wing is strong. In the case shown here, the aircraft is only momentarily in close proximity to the ground. The pressure field around the aircraft will never reflect that proximity because it is gone before the aerodynamics could start to react. This is completely instationary aerodynamics, and air, having mass, will take some time to respond. However, it is never given this time.


Does FAA simulator (AATD/BATD) count toward experimental rating 1,000 hours requirement? In the USA, a pilot can obtain an AEA (authorized experimental aircraft) rating to fly demilled surplus military turbojet-powered aircraft under FAA 5-1580(A).  Part of its requirements are to have logged a minimum of 1,000 hours pilot flight time, including 500 hours as pilot in charge.

A student pilot can NOT log hours in a FAA-aproved AATD or BATD simulator, unless an instructor is present.

But, AFTER someone has their private pilot's license, are they then able to log some hours in an FAA-approved AATD or BATD solo (without instructor) toward their 1,000 and/or 500 hours required for the experimental rating?  (Perhaps even using a simulation of the surplus military aircraft they want to have authorization for?)  Or, is that level of distrust from un-verifiability still there after someone has their private pilot's license?

The short answer is no. As you said in your question, the regulations you quoted require you to have logged flight time, which is defined in 14 CFR 1.1:

  Flight time means:

  (1) Pilot time that commences when an aircraft moves under its own
  power for the purpose of flight and ends when the aircraft comes to
  rest after landing; or

In other words, you have to go fly a real aircraft. And 1.1 defines "aircraft" clearly too, so you can't argue that it includes simulators.

But even if you could use a simulator, you would still need an instructor present. 14 CFR 61.51 talks about logging simulator time for various purposes, but in every case it also says that an "authorized instructor" must be present and endorse your logbook. That makes sense: if you're playing around by yourself in a simulator, you could be doing anything at all, including reinforcing your own bad habits. The instructor is there to ensure that what you're doing is useful and relevant, and you're learning from it.


Why do (almost) all military transports have high wings and civilian transports low wings? I've observed that almost all military transports have high wings (with a pronounced anhedral) and almost all civilian airliners have low wings (with a pronounced dihedral) — why is this?

The high-wing configuration gives the military the advantage of easier loading and unloading (especially when operating from irregular airfields), and lower overall height for the aircraft, but then why do civilian aeroplanes hate this design?

Except in the erstwhile USSR, I haven't really seen high-winged military aircraft being used as civilian airliners (Boeing 737 size and beyond), though I can't think of any reasons why not.

For military transport aircraft:

Wings further off the ground will mean that the engines will also be further off the ground, reducing the risk of foreign object damage.
You can move vehicles under the wing, and taxi across smaller taxiways.
The fuselage can be closer to ground, facilitating transport.
No wing spar crossing the cargo floor.

Reasons why most commercial transport jets don't have it:

Many are converted passenger jets.
They are mainly pallet loaded, less vehicles and bulky objects.
They normally use prepared airports, which military jets cannot assume they will.
They often loaded from a side door, and it makes little difference I think if you go up one meter or three meters.

  but then why do civilian aeroplanes hate this design?

Airlines don't 'hate' this design, look at the BAe146 Quiet Trader for instance. It's just not that many commercial aircraft from which cargo jets are derived are built this way.

  Except in the erstwhile USSR, I haven't really seen high-winged military aircraft being used as civilian airliners (Boeing 737 size and beyond), though I can't think of any reasons why not.

Military jets are very expensive to operate and the east block Antonov 124 probably satisfy the most demand. There are a few commercial C130 (L100) around though. It's also not like the US airforce will give us C-5 Galaxy to a commercial company to do whatever it wants with it.


Can I log flight time while I am a student? If I were to obtain a private pilot certificate, would I acquire flight hours during my training, or would I have to wait until I am certificated to obtain flight hours?

If I can log hours during training, then about how many do you think I could obtain while training to earn my private pilot certificate?

Yes, your time you acquire during training is loggable time and counts toward your total flight time.

You will only log pilot-in-command during solo flight and during the checkride.  After passing your checkride, you will log pilot-in-command during further training in the same category and class of aircraft as well as flights during which you act as pilot-in-command.

The minimum legal requirement under part 61 for an ASEL private pilot is 40 hours.  I would shoot for efficiency and earn your certificate with the least amount of training you can, and any time you would like to pay out of pocket beyond that, fly without an instructor on board.


Are there typical values of pitot-static and TAT probe measurement uncertainties? I'm working on flight sensors for a project and I'm supposed to model noise in the Static Port, Pitot tube, and TAT probe measurements. I looked a lot for it online and could not get a convincing answer.

I need the uncertainties of Static port, Pitot tubes and TAT measurements. By uncertainty, I mean + or - certain pascals for Static port and Pitot tubes and + or - certain degree Kelvin for the TAT probe. Is there a typical uncertainty value for these, say for example a Boeing 747 and I guess a digital sensor as the worse case? I don't know much about the companies supplying these sensors or the installation method on the aircraft.

It would be great if someone could help me! Cheers!

The pressure and temperature measurement devices are analog instruments, extracting parameters that are of value to pilots from physical phenomena. Unfortunately, the physical world throws in a variety of effects, linear and non-linear, that make it difficult to extract the parameter of value. Noise is one of them, but in a way that is exactly what we want to measure since pressure is just the average value of noise in the speed of gas molecules in air.

In the pitot tube there are a couple of effects that make the measurements non-trivial:

Friction between the air and the tube causes a velocity distribution that varies with flow radius relative to tube radius.
Fluid mechanics causes different behaviour at different conditions, such as laminar or turbulent flow, or supersonic shock waves.
Inertia of the particles creates a time delay in pressure measurements as a function of airspeed and could can cause back pressure surges.
Old fashioned thermodynamics such as the ideal gas law impose non-linear functions on the relationship between pressure, volume and temperature.

From Wiki

All these effects need to be considered in order to measure just the total pressure and the static pressure, and then to extract the airspeed, altitude etc from these.

If it is calibration data of the sensors that you are after, that science is fortunately over 100 years old.


Where to download NATs (North Atlantic Tracks) information? I am reading about NATs (North Atlantic Tracks) and I found out they are created and regulated by the following entities:

westbound flow: Shanwick Oceanic Control
eastbound flow: Gander Oceanic Control Centre

Since I cannot find their information but from other sources (non official!), my questions are:

What are their websites?
Where can I download the NATs information they generate?

Note: the information should be official and possible machine-readable.

Your question

  I cannot find information from [Shanwick Oceanic Control and Gander
  Oceanic Control] What are their websites?

Direct answer

ATC centers have no public websites in general. Aeronautical information is distributed by a fixed dedicated network, the aeronautical fixed service (AFS) and its data network AFTN.

From ICAO North Atlantic Operations and Airspace Manual (NAT Doc 007):

  The agreed OTS is promulgated by means of the NAT Track Message
  via the AFTN to all interested addressees. A typical time of
  publication of the day-time OTS is 2200 UTC and of the night-time OTS
  is 1400 UTC.

FAA, other agencies and aviation blogs relay the NAT-OTS notam on the web.

You can read below if you are interested in jet streams and how the track system is rebuilt twice a day to take advantage of current winds.

Polar jet stream

The jet streams are extremely strong west winds blowing at cruise (tropopause) altitude. The polar jet streams blow at about 50/60° latitude, and the northern polar jet stream is the one blowing over the North Atlantic ocean, the busy route between North America and Europe.

According to the theory, still in discussion, the prevalent wind in altitude in this area is due to the interaction between interleaved Ferrel cells (high pressure) and polar cells (low pressure):

Atmospheric circulation (source)

(Note the tropopause is lower at the pole than at the equator.)

This results in a wind blowing from a south cell to the closest north cell, speed is often between 50 and 150 kt, but 200 kt has been seen. However, for an observer on the ground, due to Earth rotation (Coriolis force) the flow is meandering eastwards from Canada to northern Europe at an altitude of about 8-10 km, as visible on this image of today's polar jet at 250 hPa isobar:

Source: NullSchool

Colored curves are isotachs, areas of equal air speed. Blue is 0 and purple is more than 200 km/h.

The thickness of the polar jet streams is usually less than 5 km and their width a few hundreds of km. To take advantage of jet streams when flying to Europe, aircraft must be packed into this area.

Daily updated organized track system

Aircraft fuel consumption and ground speed can be significantly improved by using tailwinds when flying eastwards, and minimizing headwinds when flying westwards, rather than just following the shortest path (great circle) between two locations. The North Atlantic Organized Track System (NAT-OTS) is a set of eastbound and westbound routes over North Atlantic, computed twice a day to this effect.

Tracks to North America are named starting at letter A, tracks to Europe are named starting at letter Z. Routes for Europe are in the middle of the polar jet, routes to North America are the northernmost, where the jet is the weakest.

Today's polar jet stream and corresponding NAT-OTS (look at eastwards tracks S/T/U tightly packed in the channel where the wind is most beneficial):

NAT-OTS vs. polar jet stream at 250 hPa isobar on 17 Feb 2020. On top, blue is 0 and purple is more than 200 km/h.
Sources: NullSchool (top), SkyVector (bottom)

North Atlantic track system

ICAO guidance for planning and using this airspace between FL285 and FL420, where almost all commercial traffic occurs, is described in NAT Doc 007.

Crossing the ocean is currently done without radar surveillance and coordinated by five oceanic area control centers (OAC): Gander, Shanwick, Reykjavik, Santa-Maria and New York East. The tracks use a set of fixed entry and exist points at their extremities:

Overall structure, with entry and exist points (source)

Those points change over time when the NAT airspace is reorganized, e.g. with the adoption of HLA in replacement of MNPS.

Track system construction

The process is collaborative between airlines and ATC.

From Wikipedia:

  The day prior to the tracks being published, airlines that fly the
  North Atlantic regularly send a preferred route message (PRM) to
  Gander and Shanwick. This allows the ATC agency to know what the route
  preferences are of the bulk of the North Atlantic traffic

From SKYbrary:

  The night-time OTS (eastbound) is produced by Gander OAC and the
  daytime OTS (westbound) by Shanwick OAC, each incorporating any
  requirement for tracks within the New York, Reykjavik, Bodø and Santa
  Maria Oceanic Control Areas (OCAs). The OTS planners co-ordinate with
  adjacent OACs and with domestic ATC agencies to ensure that the
  proposed system is viable. They also take into account the
  requirements of opposite direction traffic and ensure that sufficient
  track/flight level profiles are provided to satisfy anticipated
  traffic demand.

and:

  Following the initial construction of the NAT tracks by the
  appropriate OCA, the proposed tracks are published on an NAV Canada
  operated internet site for review and discussion by interested
  parties. One hour is allocated for this process during which any
  comments will be considered by the publishing agency and any agreed
  changes are then incorporated into the final track design.

OTS Changeover Period

From SKYbrary:

  To provide a smooth transition between the night-time and the daytime
  OTS, a period of several hours is allocated between the termination of
  one system and the commencement of the next. These periods are from
  0801 to 1129 UTC and from 1901 to 0059 UTC.

  During the changeover periods, some restrictions to flight planned
  routes and levels are imposed and opposite direction traffic must
  remain clear of the inbound (terminating) OTS.

Suggested (good) reading

Flying the North Atlantic Tracks, from AeroSavvy.
North Atlantic MNPS Airspace, from Airline Pilots Forum and Resource


What were the 20 airspace classes in the US before 1993? ICAO adopted the current airspace classification in 1990. The US followed suit in 1993.

But, I found this:

  In case you didn't know, the U.S. had 20 different types of airspace designations prior to 1993. (VATSIM)

Were there really 20 classes?
What were they and their functions?
And if possible, how did they compare with the rest of the world pre-1990 in a general sense?

What I could find was the Oct 1993 issue of Flying Magazine. From which, I couldn't find the 20 classifications, just 7 old names and some changes.

The FAA order 7400.2D from 1993 includes the change, but I can't find the similar order that precedes it. The preceding document is here, but unhelpful.

Also via here we find: Transition Areas, Continental Control Areas, and Control Areas. All three are now under Class E. That brings the total to 10 out of 20.

Thanks to @Pondlife for the FAA document and for checking the AOPA magazine article.

I don't claim to have a definitive answer, but started listing the airspace I recall memorizing or seeing on charts.  So here's my current list:

Continental control
positive control area
Air defense identification zone
Oceanic control area (may not be USA)
terminal control area
airport surveillance area
tower control area
transition area
control zone
uncontrolled airspace
Military operational area
prohibited
restricted
alert area
warning area
caution area (1943 aeronautical chart)
danger area (1943 aeronautical chart)
airways
airport traffic area
national park
wildlife refuge
fire control area (became restricted as I recall)
special use area
laser demonstrations
blast area (later warning area, now TFR)
controlled airspace


What are those trucks towing around, and why? I was in Prague airport and saw those things going around:

As for title, what are they towing and why? I can imagine it could be to alter airport layout, but I don't see a use for it.

Sub question for and additional answer's virtual bonus: is it funny as it seems? I could imagine drifting those things around :-D

They are called "Jersey barriers" and are temporary barriers used to alter the layout and to introduce defined lanes for vehicles, passengers etc.

For the bonus, they are not fun to tow.  If that line starts snaking, you have to stop until they all settle down before moving off again. Try drifting with those, and the weight would get so far outside your turn radius in no time flat that you'll spin the vehicle and jack-knife the lot.

Border Barriers UK©

RentPlasticJerseyBarriers.com©


What are the forces acting on the doors of the main landing gear? When opening and closing the landing gear doors, what forces act upon them and how much power would the actuators need to exert upon the doors to open/close them mid-flight (assuming the landing gear cavity is not pressurised)? I haven't found anything on the actual doors in my research, only the landing gear itself, so I would appreciate some sources if you can provide them.

Aircraft have a maximum gear extension speed (V$_{LO}$, as in maximum landing gear operating speed), which is normally 1.6 times stall speed, so when the doors move, the dynamic pressure is rather low. Therefore, the forces during gear extension are not the only ones to check.

In cruise, suction on the gear doors might be the limiting factor for both stiffness and strength of the door itself, its latching mechanism or the hydraulic cylinder. I had the misfortune once to live through a flight test where part of one main door broke off in flight: It had no latch and was held shut by the hydraulic cylinder, which was mounted near the rear edge of the door. Suction and elasticity caused the forward edge to be pulled out into the airstream, so it soon became an air scoop, which only increased the pressure difference between the gear bay and the outside. Consequently, the door failed and needed to be redesigned, now with the actuation mechanism near the forward edge and an added latching mechanism.

If the door is aligned with the flight direction, make sure you look at the forces at maximum operating speed and maximum sideslip angle and treat the door like a regular fin. If the door opens into the airstream, use the proper drag coefficients for a flat plate and add some margin to account for oscillating flow separation. Make sure that the eigenfrequencies of the door or its operating mechanism are nowhere near the frequency of such flow patterns.

Here is a short note on the T-38 strut door loads which was written on occasion of the qualification of the T-38 as a chase plane for shuttle re-etries. The maximum gear extended speed had to be raised so the T-38 could follow the shuttle on the way down. Unfortunately, only measurements are given.


Why are commercial aircraft windows typically oval instead of circular? Pretty much what the title says. Surely a circular window would allow more window area while not compromising on strength?
Some planes(Gulfstream jets) do have circular windows. Is there a reason for this? does it have something to do with fitting windows without in between structural components?

It's done mainly for the passengers' viewing benefit in making structural trade offs over total opening area.  For a given area, a round window has less "meat" of fuselage structure between them.   Aesthetically, a round window looks too much like a ship's porthole unless you make it really large, and then it looks like a fish bowl.
So for a given area, a window that is elongated vertically allows for more variation in eye height than a circular one where the viewing area is equally distributed vertically and laterally, and provides more fuselage structure between the windows (important as an inflated tube in circular tension, or under "hoop stress").
The 787 went with very large passenger windows to take advantage of the carbon structure and bring as much light inside as possible.  But they still stayed with the round-corner vertical rectangular shape so that short and tall people both get a good view.
The CRJ200, as a corporate jet derivative, suffered from very small windows that were set low, and if you're ridden in one and are tall, you have to bend over to see out.  When the follow-on model, the 700, was in development, a lot of work went into moving the windows up and enlarging them so that most people got a decent view (actually it was solved by moving the floor down a bit).
And finally, actually the Gulfstream's windows aren't round, but ovalized horizontally.  This is likely to accommodate lateral seating variations as a priority over vertical variation because of the corporate seating arrangement and is disadvantageous structurally in terms of the fuselage tube's hoop strength.


What crash does FAA SAIB CE-10-11 refer to? The Federal Aviation Administration released Special Airworthiness Information Bulletin CE-10-11 on December 23, 2009 advising pilots of what procedures to use when resetting tripped circuit breakers. The background section refers to a crash where the airplane's weather radar system wiring caused a burning smell and the pilot pulled the breaker. When the breaker was reset for the next flight that flight subsequently crashed. I'm curious what flight this bulletin refers to because I'd like to read its associated NTSB report.

From the details it sounds like the accident was N501N, a Cessna 310R that crashed on 10 July 2007 (NTSB Summary Report (PDF)).  This airplane had been previously written up for an anomaly with the weather radar and subsequently improperly released back into service.  The accident crew reported smoke in the cockpit and post-crash analysis revealed substantial electrical component damage, burned wire insulation, etc.


What is formula for induced drag in stalling regime? I'm wondering what is the formula for induced drag in a stalled regime, i.e. in a regime where the $C_L$ (coef. of. lift) has started to decrease but is still nonzero.

I've a feeling that the conventional formula for induced drag
$$D_i = \frac{L^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$$ (taken e.g. from this answer) which in essence depends on lift $L$, does not explain induced drag in stalling regime (if it would, it would erroneously imply that the induced drag is equal for a pair of AoAs $\alpha_{\text{before-stall}} < \alpha_{\text{after-stall}}$ which both correspond to the the same $C_L$).

I've come across the evidence that induced drag continues to grow as square of AoA even in stalled regime (e.g. schematic Fig. 4.14 from av8n.com)

but can't find more.

Thanks.

That formula describes the "lift-induced drag" for a planar wing. Lift-induced drag

As the definition implies, this drag is induced by the generated lift, the effect of stall (flow separation) is not contributing to this drag component.

Therefore, there is no other formula for the stalled regime.


Are taxiing aircraft to hold short of taxiway hold short markings if not instructed by ATC to do so? At some airports, some taxiways have taxiway hold short markings (single dashed yellow lines against a black background). Do taxiing aircraft by default need to hold short of these markings if they're not instructed by ATC to do so?

This is one biggest pet-peeves of mine that even old, experienced pilots have missed after years of flying.

Taxi instructions are just that: Instructions, not clearances. Mind you there are some clearances when on the ground aswell, crossing a runway, entering a runway, backtracking, maybe some others.. mostly involving the runway. But a general text like ‘Taxi to holding point Charlie via Golf’ is an instruction.

Ok, so the difference is just pedantic but there is one big caveat: when you are cleared to land, cross, enter on a runway you are guaranteed the runway is, well, Clear and will continue to be so for the ammount of time you need it to land/take-off.

This is not true for taxi instructions. You are instructed on what path to take on the ground, but the taxiways and route to your runway is most definitelly not cleared of other aircraft (unless we are talking about low visibiliy ops, but just bear with me for now). When taxing you are still supposed to look out for other trafic and try to avoid hitting other airplanes taxing around as much as you can.

You might be instructed to go via one taxi path and then later told to give way to another aircraft comming from your left. See how the path has not been ‘cleared’? It is not a clearance.

All this talk to sum up: entering/crossing a runway is a clearance. Crossing a (non-mandatory) taxiway holding point is not. Is the answer slowly building up ?

Crossing a runway hold short makrking (two dased lines and two solid lines) you need clearance.

Crossing a taxiway holding marking (or intermediate holding position, or cat 2/3 holding when not in low-vis) does not require one.


How hot would pressurized air get if the air conditioning failed? I understand that aircraft are pressurized by pushing air into the cabin using the engines, and using a valve to let the air escape at a controllable rate thus achieving the desired pressure differential. However, when air is compressed, it heats up and if I understand correctly, all pressurized aircraft have air conditioning units to keep the temperatures at a reasonable level.

If all air conditioning units were to fail, how hot would the air being shoved into the cabin actually be? If this depends on aircraft and or engine type, any example as a ballpark figure would be appreciated.

The air conditioning is in a so called "air cycle machine" (ACM). If it fails, the air will not be compressed (done in the same machine).

Temperature goes up and down as the air passes through the machine's heat exchangers, compressors and turbines, reaching 250$^\circ$C at peak.

Wikipedia article

Image from ECS


Do ramjets use variable geometry supersonic inlets? Ramjets are very often portrayed as super simple, I've read on few occasions that the only moving parts in a ramjet are the fuel pumps. However, turbojets operating at similiar speeds (mach 2 - mach 3) use very complicated supersonic intakes with variable vanes to maximize the number of shockwave reflections and their angle and to adjust for the refraction of the flow on these shockwaves. Why is that the ramjets do not require such advanced measures to maximize pressure recovery?

Why is that the ramjets do not require [variable geometry] to maximize pressure recovery?

Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide the about the same mass flow to the engine, regardless of the forward speed of the aircraft. And, at high speed, it doesn’t want to spill excess air, because that creates spillage drag, especially when the intake has sharp lips, need to reduce form drag at high speed. But, at low forward speed, it needs a bigger intake area, as the forward motion of the aircraft isn’t providing a lot of air. So, the intake has conflicting constraints. It needs a small area at high speed, but a large area at low speed. It also needs to form an oblique shock at high speed, as an oblique shock has less total pressure loss than a normal shock wave. The oblique shock wave needs to be kept out of the intake, hence typically a spike protrudes forward of the intake. The spike moves forward as the aircraft speed increases, and the geometry is made such that this spike reduces the intake area as it translates forward.

Now, consider a RAM jet. It typically operates from Mach 3 to 6, say. So, it’s going from a speed at which shocks exist, to a speed at which shocks still exist. It might still be an operating range of 6-3=3 Mach, compared with 2.5-0=2.5 Mach, so not much different. But, in terms of the change in fluid behaviour of the air, 0 to Mach 2.5 is a much much larger change in behaviour than Mach 3 to 6.

For example, the angle of an oblique shock at Mach 2.5 is 23.6 degrees. Mach 3 is 19.5 degrees and Mach 6 is 9.6. (See formula here at NASA). So, between zero speed an Mach 2.5, the shock changes from not being there at all, to starting as a normal shock of 90 degrees, to having an inside angle of 23.6 degrees (a change of 90-23.6=66.4 degrees). In comparison, for the RAM jet, the shock only changes angle by 19.5-9.6=9.9 degrees. It’s pretty easy to accommodate a change in shock angle of only 9.9 degrees with the same geometry.


Is airflow deflected inwards or outwards by wing sweep? We are often told that wing sweep causes the airflow to be deflected outwards. I can understand this for the underside, but for the upper surface it seems inconsistent with other effects.

For example on a sharply-swept delta, sideways flow out from below, up around the leading edge and back in again over the upper surface becomes dominant. It feeds the vortices employed by Concorde to enhance lift at low speeds. The sharper the sweep, the more the air over the upper wing flows inwards. The same effect is seen in miniature with tip vortices on a straight wing. This makes sense to me because the pressure above the wing is low, so that would naturally attract air in towards that zone.

So, does sweeping a straight wing really reverse such effects, or do say wing fences on the upper surface actually stop the air flowing inwards?

As a partial answer following up on those to date, here is an image from Where is the spanwise flow? How does the span wise flow point the air towards the wingtip?, showing a snaking in-then-out flow over the wing. Note the inward flow over the leading edge and wing section, which is just the point where wing fences are located.

On sharply-swept deltas which sit inside the bow shock cone at supersonic speeds, 1980s studies of supercritical attached flows show the air flowing upwards round the leading edge and forming a crossflow to meet an inboard "crossflow shock". See for example W.H. Mason, “A Wing Concept for Supersonic Maneuvering,” NASA CR 3763, December 1983. But it is less clear to me whether this inbound crossflow is physical, or a theoretical notion along the wing profile which, due to the sharply tapering chord with span, physically might still flow outwards:

This contrasts with the low-speed vortex flow over such wings (e.g. Concorde) at high angles of attack, where the vortex inflow is detached and the boundary layer flows outwards.

So it seems the full the answer must be complicated, with the pressure distribution along the chord, and hence the in-out flow changes, depending on the angle of sweep, the speed of flight, the angle of attack and how far above the wing you are considering.


How are cloud bases and tops reported on PIREPS? Let's say I read a PIREP as follows...

KATL UA/OV KATL/TM 1522/FL080/TP C210/SK SCT013-TOP079/WX FV05SM
 HZ/TA M04/WV 24540KT/TB LGT/RM IN CLR.

I'm in KATL where the field elevation is 1,026, so if I were reading a METAR that reported bases at 013 I'd expect to enter the clouds at 2,300' MSL. I'm reading a PIREP here though, so based on this am I expecting to enter the bases as 1,300 MSL or 1,300 AGL? I have the same question with the tops.

It depends. For a PIREP cloud Heights are reported as they are read by the pilot.

FAR AIM 7-1-16(b)

  ... In reports disseminated as PIREPs, height references are given the same as received from pilots, that is, above MSL.

This means some conditions may be reported MSL as read of the altimeter at local barometer settings while others may be set at standard pressure and read from there. I would assume that altitude reports less than 18,000ft are MSL.


What is aerodynamic imbalance? What is aerodynamic imbalance on a propeller? The most amount of hint I got was the shape of the airfoil. All I know is that it is the different thrust each prop blade produces due to different angles on each blade station. I am looking for a more in detail explanation and some examples.

I think he wants to know that you have an understanding of P Factor, which is the main "imbalance" or thrust asymmetry issue that has a direct impact on you as the pilot when you fly the plane.


Under what circumstances can the Initial Climb Area extend more than 2 nautical miles? While studying of FAA's Instrument Procedures Handbook, I noticed that on page 1-16 (Chapter 1: Departure Procedures) it defines the Initial Climb Area (ICA) as follows:

  The ICA is the segment of the departure procedure that starts at the DER and proceeds along the runway centerline extended to allow the aircraft sufficient distance to reach an altitude of 400 feet above DER elevation and to allow the establishment of positive course guidance by all navigation systems. A typical straight departure ICA extends 2-5 NM from the DER along the runway centerline extended. It is 500 feet wide each side of the runway centerline at DER, then spreads out at 15°

However, I can not understand how it is possible for the ICA to extend more than 2 nautical miles, since the maximum horizontal distance covered by an aircraft until it reaches an altitude of 400 feet above DER elevation can be found by using the default minimum climb gradient of 200 feet/NM, which corresponds to 2 nautical miles. For an ICA to extend more than 2 nautical miles, a climb gradient of less than 200 feet/NM would be required, something that violates the rules imposed by FAA.

Could someone point me in the right direction?

The definitive reference for this stuff is the TERPS, and it says that the ICA can extend to 10NM(!). See 14-1-6(b):

  Length. The ICA length is normally 2 NM, measured from the ICAB to the
  ICAE along RCL extended. It may be less than 2 NM in length for early
  turns by publishing a climb gradient. The ICA may be extended beyond 2
  NM to maximum length of 10 NM. A specified altitude (typically 400
  feet above DER) or the interception of PCG route must identify the
  ICAE.

I may have missed it, but I didn't see anything in the TERPS that explains how to set the length. However, the ICA definition from the IPH says (emphasis mine):

  Initial Climb Area (ICA). An area beginning at the departure end of
  runway (DER) to provide unrestricted climb to at least 400 feet above
  DER elevation.

I couldn't find a formal definition of "unrestricted" but in ATC-speak it usually means "go for it" (e.g. ATC Orders 4-3-2(e)(1)). In this context, though, I think it means "no lower limit" rather than "no upper limit". In other words, the ICA will be extended to allow for 'worst case' scenarios where an aircraft's climb rate is extremely limited.

That suggests hot and high airports, and if you look at the ODPs for a famously high airport like Lake County (Leadville), CO (KLXV) at 9,934' MSL you'll see that the departure procedures are indeed almost straight, e.g.
DAVVY ONE, which is more or less straight out for 20NM from runway 34.


Who does ATC call when a plane disappears with no response? Do they just call 911? Or is there some special unit that’s specifically trained around disasters and rescue?

Most countries have established some sort of Rescue Coordination Centre responsible for search and rescue operations in case an aircraft or ship goes missing. The RCC typically has direct access to various SAR resources such as helicopters. They can also call in local emergency teams (police, firefighting, ambulances etc.)

How exactly the RCC is notified of an incident can be different. It may be through a normal emergency telephone number (such as 911) or via some direct communication channel.


The ATC is unable to provide radar service to pilots, what's the standard way to inform this? Nowadays, ATC provides radar services to pilots, such as: vectors and surveillance, however, depending on the radar status, it can be out of service; on initial call the controller will inform the pilots about this issue, please, what's the right way to say this, using standard phraseology?

Without checking the Controller's Handbook, what I hear most often are two things:
a. Standby and remain clear of class [b, c] airspace, for VFR operations.
b. (surveillance) radar services not available, squawk xxxx, report yyyy
c. Radar out of service, N1234, radar service terminated.
An example of b was when going to an airport near DCA, radar was out of service.  The controller handled my flight but relied on talking with a controller from Baltimore. This was a few years ago, and I now understand that the remote radar can be transmitted to other controllers taking a remote controller out of the loop.
The ATC Handbook is FAA JO 7110.65W, https://www.faa.gov/documentlibrary/media/order/atc.pdf
Radar can be limited due to equipment outages, and also the display available to the controller.  For example tower displays are for orientation and not certified for separation and other services.
The order states that controllers may advise that:
PRIMARY RADAR UNAVAILABLE (location)  or
RADAR SERVICES AVAILABLE ON TRANSPONDER EQUIPPED AIRCRAFT ONLY
Since there are many conditions for radar not being available and since an aircraft can be in various states (VFR/IFR, approach, visual approach, etc.) it would be best to consult the Order for the recommended phraseology.  It is noteworthy that CONUS radar coverage has improved since the Order, and as a result there is often backup coverage for many locations which could influence services which can be offered.
Finally, operationally ATC still has local and regional variations, which may influence the options and corresponding phraseology utilized.


Are there different symbols or graphics to identify different types of GA fuels? Are there different symbols to identify the different types of fuels used for general aviation aircraft?

As Walker notes in his answer AvGas is typically noted by color when talking about telling it apart. In terms of aircraft markings for fuel this is governed by FAA FAR 23.1557 Miscellaneous markings and placards. and some more notes in this AC,

  (c)Fuel, oil, and coolant filler openings. The following apply:

(1) Fuel filler openings must be marked at or near the filler cover with-

  (i) For reciprocating engine-powered airplanes -

    (A) The word “Avgas”; and

    (B) The minimum fuel grade.

These are fairly simple regulations to comply with so there is a wide variety of markings available, but the end results tends to be a decal looking something like this:

(source)

Above ground storage tanks will also have to comply with local state regulations on petroleum storage, here are the New Hampshire regulations as an example. Generally this will require an NFPA style decal but pilots dont really use those to identify anything. The typical signage for 100LL is a red back, white fount, all caps AVGAS 100LL this is common on tanks,

and trucks alike,

(source)

(source)

Its also important that you as the pilot are present for fueling both to verify type and quantity. If there is any doubt about what is in a truck or at a pump some should be pumped into a canister and verified by color before fueling an aircraft.


Which one is considered the departure time of a commercial flight? I was wondering, the publicly announced departure time of a commercial flight, is it the ETOT (estimated takeoff time) or is it the EOBT (estimated off-block time)? Or maybe some other time?

The scheduled time is the EOBT. Flights that depart within 15 minutes of it are considered on-time.
Standard taxi times for each airport are included in the scheduled flight time.


How much do the fan blades of a GE90-115b stretch when the engine is spinning at maximum RPM? I've always wondered what kind of stresses and resulting deformations jet engines are subjected to when running.

I don't know the exact answer, but it's on the order of 0.100" (i.e. it's more than 0.010" and less than 1.00").  The more interesting part is that it's not just a simple linear stretching. The blades have a twist to them, and at speed they untwist, which is a strong contributor to the total change in length from 0 rpm to redline.


What is the purpose of downthrust? Many light general aviation aircraft-- and perhaps many other aircraft as well-- are designed with some of amount of downthrust in the engine mounting, relative to the centerline of the fuselage.  Presumably the wing incidence angle is chosen to streamline the fuselage in some particular flight regime.  So why is any downthrust designed into the engine mount?  Exactly what is the designer trying to optimize, by including downthrust in the design?  In what undesirable way would the aircraft's flight characteristics be different if the downthrust were not there?
For example, is the intention that adding power (without re-trimming the elevator) should not make the aircraft climb?  Or that adding power should not cause a change in trim airspeed?  Or that adding power should not cause a change in trim angle-of-attack?  Would there be a "wrong-way" change in trim airspeed when power was added, if the downthrust were not present?  If so, why?
Note that one reason for some reduction in trim airspeed when power is added to climb but angle-of-attack is held constant, is given in this related answer.  (See the tables-- and note that a similar reduction in trim airspeed would also occur when power was reduced to descend.)  The present question anticipates that this effect may be minor enough not to be the main reason for including downthrust in the design of any particular airplane.
This question is aimed primarily at aircraft with nose-mounted propellers and conventional tailed configuration, so please be sure to address this configuration, but feel free to discuss other configurations as well.

With regards to John's comment, we should remember the other player applying torque to the Center of Gravity, the horizontal stabilizer/ elevator/ stabilator/ trim tab.  Ideally both the thrust line and the horizontal stabilizer will be 0 degrees to the horizon with the wing at optimal cruise AOA.
Under power, relaxing static stability with a little competing downthrust will lessen the need for excessive trimming any time thrust is changed, and also leaves static stability full on when the plane powers down to land.
This is a very safe and widely used application of downthrust for GA aircraft.  Because the cosine of the small downthrust angle is very small (cos 3 degrees = 0.9986), effect on the horizontal thrust component is minimal.


Does an RMI show true or magnetic north? Does the compass card of a Radio Magnetic Indicator (RMI) point to true north or magnetic north ?

In a typical installation, the compass card of an RMI indicates magnetic north.

While it is possible to switch that reference on an EFIS display to true north by applying the local variation, that is not the common practice. If you see a "steam gauge" RMI (i.e. not a digital display, but a mechanical compass card) in a panel, chances are excellent that the compass card is slaved to the same heading source as everything else -- magnetic north. In the exception cases, that source can be switched between true & magnetic, but that's uncommon.


How does speed affect lift? I understand the lift coefficient shown above. But does lift increase with speed, and how?

I understand I can't apply this to the graph as it is a coefficient, but if I had an angle of attack of 10 degrees and a speed of 0 knots, we would obviously have no lift assuming there isn't any wind, but if we increased our thrust and speed, we would gain more lift, and if we increase AOA even more, we get even more lift and the same with thrust.

But what about 0 degrees with symmetrical airfoils, does lift also increase with speed, and does it work similar?

The lift equation is $$L = \frac{1}{2} C_L · v^2 · S · \rho$$

where $S$ is wing area, $C_L$ coefficient of lift, $\rho$ air density, and $v$ airspeed. Using units of measure $\text{m}^2$ for $S$, $\text{m}/\text{s}$ for $\text{v}$, and $\text{kg}/\text{m}^3$ for $\rho$, the result is lift in $\text{N}$ (newtons).

Concerning symmetrical airfoils, they work quite well too, and the equation given above is perfectly valid for them. If it’s true that at zero degrees their $C_L$ is zero, they give zero lift regardless of the speed (zero times anything is zero).


Why did Airbus skip a few numbers when developing the A380? So at the time the A350 didn't exist so I will list the aircraft not including the A350. So it goes A300, A310, A318, A319, A320, A320, A321, A330, A340 then it skips to the A380. So why did Airbus skip a few numbers?

The question of aircraft name variants have already been discussed, but I just want to add that other reasons have also been proposed:

The '8' in the 380 resembles the double decker configuration of A380.
It was named A380 because it is twice the size of A340
And of course, it is the lucky number for Chinese.


Why is there a relationship between maneuvering speed and weight? Could you explain the relationship between maneuvering speed and weight?
Why are they linked?

stall speed goes up as weight increases. Since maneuvering speed is set a certain margin above stall speed, it goes up too as weight increases.


Why do aircraft manufacturers produce families of aircraft? Let's take the case of the Airbus A320 family of aircraft. Airbus manufactures A318, A319, A320 and A321 planes, which all belong to the same family.

What is the need to manufacture multiple aircraft from the same family having different ranges and capacity, when one could just invest in making just two or three different planes from three different families providing a wide range of range (distance) and capacity?

There are several advantages, here are a few:

Design costs are lower for a family of aircraft; major parts of the aircraft are the same in the family.
A family of aircraft share the same type certificate. This means it is cheaper for the manufacturer to certify a family of aircraft.
Within a family of aircraft, pilots can operate all aircraft with no
or very little additional training required. This saves the airline a
lot of money.
Maintenance on a family of aircraft is cheaper since the majority of
parts are common to all family members. This means having less spare
parts in stock and less training for maintenance personnel.
For an airline there is often a benefit in having some capacity
diversity in a fleet. A new route often starts at low capacity. When
the volume of passengers rises replacing the flight by a  larger
aircraft from the same family is not disruptive to crew schedules.


How do propeller planes sync their propellers? While reading this question, I found that the answers did not address the point I had come for.  On planes with electronic propeller synchronization, how is it actually accomplished?  I know why propeller synchronization is useful but I am curious about the mechanisms behind how it works.

I fly a large 4-engine turboprop.  It uses constant speed hydraulic props, which automatically change blade angle to maintain 1020 rpm.  The sync box (which I was unable to find a picture of) takes inputs from each engine's tachometer generator (where it gets the RPM signal) and a pulse generator, which is a magnetic pickup right behind the prop.  There is a magnet on the number 1 blade of each prop.  With this information, the solid state logic components inside the sync box can slightly change blade angle by sending power to the speed derivative servomotor.  This is different from the answer that @davidinnes provided, but I suspect that he's correct for the type of propellers he's familiar with.  The sync system on the P-3 also takes inputs through a potentiometer attached to the power levers, and can start increasing or decreasing blade angles if you change your power setting quickly.  It works faster than the hydromechanical system and lessens RPM overshoots and undershoots as the prop works to get back to 100% rpm.

In this picture you can see syncrophasing in action.  The props are all slightly out of phase with each other, which reduces noise and vibration. #2 and 4 look pretty close, but it's far from a perfect system.  You can definitely tell the difference from the cockpit when it's working well.  Again as @davidinnes mentioned, you can adjust the system to the change where the point of minimum (or maximum) noise is in the cabin.  We call it "running the buzz" up and down the tube, and it's a great way to annoy a navigator that you don't like


How to calculate W/V from true HDG/TRK/GS using a CRP-5 flight computer? I am currently studying for my EASA ATPL (airline transport pilot license) and one of my subjects is General Navigation.

I have stumbled across this question (picture) that has completely stumped me, I have not been able to find help anywhere else. I know it can be solved by drawing wind triangles but I am solely interested in solving it using the CRP-5 flight computer.

I am very much used to calculating desired headings for a certain track when given wind, but not the other way around. Any help would be greatly appreciated.

To work this problem in reverse:

Set the True Course (Track) at the Index and slide the grommet over the Ground Speed (GS) of 242 knots:

Draw a dot with an arrow behind it, marking the wind vector at the intersection of:

a)  8° right of center because the wind correction angle or heading is right of course (74° TH minus 66° TC); and

b)  230 True Airspeed (TAS)

Now, above, you are able to visualize the flight.  You could even draw your airplane in the middle and see the wind is a right quartering tailwind.

Finally, rotate the wind to the Index to read the answer.  Slide the center grommet over any line that's easy to read.  Read the wind speed up from the grommet (35 knots) and wind direction (from 180°) at the index:


Why do people put $1 & OVC on aircraft bills of sale? When purchasing an aircraft a bill of sale is created to show transfer of ownership from the seller to the buyer. Bills of sale include what the seller was paid in exchange for the plane. But apparently bills of sale for aircraft are commonly done differently.

  The first item to fill in is the price of the aircraft, where it says "For and in consideration of $___." To preserve the confidentiality of the transaction, the FAA will accept the phrase “$1 & OVC” (Other Valuable Considerations) in this spot, and most Bills of Sale are filled out in this fashion.
  source

At first I thought maybe it was some way to get out of sales or use tax, but the FAA would certainly not approve tax evasion. The state will calculate taxes either from the lien amount or the usual market value. This would definitely not pass muster for the sale of a car or a house. The only time the state treasurer ever had me put $1 was when I was giving a car to a family member.

Obviously the price is being withheld for some reason, but I can't really figure out what that reason is. The price is easily found with a records search. The only person this hides the sale price from is the FAA. And since the FAA approves that wording they obviously don't care. Why do people not want the FAA to know how much they paid for their plane?

This practice is really more of an accounting/finance question than an aviation one, but it's common enough that it's probably worth addressing here.

When reviewing an aircraft's registration record you will frequently see the price on the FAA Aircraft Bill of Sale form given as "\$1.00 & OVC" (One dollar and Other Valuable Consideration) as shown on the example below (click to enlarge).

The "Other Valuable Consideration" can be anything of value: Personal favors, other property (a car, a different airplane), blackmail information on someone you don't like, or (most commonly) money in excess of \$1.

In a strict cash sale recording the amount on the bill of sale in this manner protects the sale price from prying eyes: Anyone with \$10 to spend can request an aircraft records dump from the FAA which will contain every bill of sale filed, but if the price is listed as some small dollar amount plus "Other Valuable Consideration" it's impossible to determine the exact price paid using only the bill of sale.
Note that in cases other than a strict cash sale other documents filed with the FAA (for example recording a lien against the aircraft to secure a loan financing the purchase) may disclose the purchase price depending on how those documents are filed: Some include the amount of the debt the aircraft is securing, others simply state "The debt identified by loan number 12345." which would still protect the confidentiality of the sale price.

As a means to avoid taxation recording the sale price as "\$1 & OVC" almost never works: The tax authority is going to ask you the value of the "Other Valuable Consideration" and base the tax due on that amount.
If you tell the tax authority the aircraft was really only worth $1, and record it as such, they're going to want to know why: Generally even an unairworthy aircraft is worth more than that for the value of its parts.

Conversely if you get a ridiculously good deal on an airplane it can be to your advantage to fill out the bill of sale accurately & present that along with other documents pertinent to the sale to your tax authority: You may be able to get the sales/use tax based on the ridiculously good deal rather than an appraised market value.


Why isn't dumped fuel burned? Fuel dumping seems to have a lot of restrictions, like the need to do it above a certain altitude and over water. It also has to end up somewhere, even if it vaporizes, which is less than desirable. It seems like burning off fuel as it's dumped could solve these problems. Obviously there's the risk of rapid unplanned incineration, but what are the other factors?

There's no benefit to burning dumped fuel, it actually introduces risk to the airplane which is dumping fuel and any other aircraft in the vicinity and possibly people on the ground depending on the altitude at which is done. Building a system to burn it is not worth the time, effort and money given there's no good reason to do it.

Dumped fuel will vaporize and mix in with the air, and then break down over time from exposure to air, heat and UV light. This is not ideal for sure, but burning it isn't environmentally friendly either as it produces CO2, CO and soot. Evaporation of dumped fuel is a miniscule problem when looked at in context as it's such a rare occurrence. Canada measured 58 million liters of gasoline and diesel lost to evaporation from retail outlets and pump in 2009, if you extrapolate that to the US it's got to be over 100 million US gallons of fuel evaporating into the atmosphere every year. An airbus A380 contains 320,000 liters of fuel, if you dumped all that at once it's not going to make a statistical difference.


How dangerous would it be to take off at night with no ATC, no runway lighting system and have a single engine working? I was wondering how dangerous it would be to  have an engine failure (with a twin-engine) during take off at night when you have no runway lights (and no city close-by) and no ATC?
Would using the aircraft flight instruments alone be considered IFR?
Thanks

I was wondering how dangerous it would be

When you preface a question with the above, you're asking for opinion, and when it comes to the details of what you ask, there will be widely differing opinions. That said, and in the context of the U.S., and if I understand your question (apologies if I'm misreading), the details of your question break down into three separate areas. One can come up with a number of possible scenarios which might combine those details, but let's use a concrete instance to better explain. I at one time was the company pilot for a firm that manufactured and maintained computerized sawmill equipment at mills throughout the west coast of the U.S. If one of our computerized systems broke down, we would fly to the mill as soon as possible. Some of these mills had private airfields alongside the mills or nearby. We could not reach ATC on the ground from any of them, and most had no runway lighting. Typically we would go into an airfield during the day, and often it would be dark by the time we completed repairs and left.

  no ATC

All night flights were done IFR. We would get an IFR clearance over the phone with a "void if not off by" time. So you takeoff, abiding by the clearance, and contact ATC on the assigned frequency as soon as you have enough altitude to reach them.

  take off at night when you have no runway lights

As long as you have landing lights and are familiar with an airfield, the lack of taxiway and runway lights is not a significant problem. What is a problem at remote runways is the possibility of animals (usually deer) on the runway, which is why we typically had at a car precede us down the runway while flashing their lights and honking.

  an engine failure (with a twin-engine) during take off at night

The twin engine aircraft we had available were a Cessna 310 and later a King Air A90. Unlike air carrier airplanes, continuing the takeoff if an engine failed during the takeoff roll was not an option, but the probability of that happening wasn't affected by time of day. Day or night, the only option was to abort. We were no more exposed during the night than during the day in that respect.

If an engine failure happened after takeoff, day or night, you would handle it in the same way. Fortunately, we were operating out of airfields where our departure path wouldn't need to be altered due to the reduced single-engine climb rate.

We had a single-engine Cessna Turbo 210 available as well. During the time that was our primary aircraft, we had no qualms about single-engine night or IFR operations. Many may disagree with such. However, although as I remember Part 135 of the FAA regs does not allow single-engine night or IFR, Part 91 does (or at least did) and I never personally knew anyone who had problems while doing so. Obviously there is some additional risk, but we considered that acceptable.


How long has the term "situational awareness" been used in aviation? I was puzzled to read (Wikipedia article on situational awareness) that:

  The term can be traced to World War I, where it was recognized as a crucial skill for crews in military aircraft.

but in the next paragraph:

  There is evidence that the term Situational Awareness was first employed at the Douglas Aircraft Company during Human Factors Engineering research while developing vertical and horizontal situation displays and evaluating digital-control placement for the next generation of commercial aircraft.

and then the next:

  Situation awareness appears in the technical literature as early as 1983

I imagine that the concept found work in understanding piloting very early on; what is the history of the term in aviation?

According to M. Press, in his 1986 unpublished manuscript Situation Awareness, let's get serious about the clue-bird which is cited by many authors, the concept of SA can be traced back to aces of WW1, in particular Oswald Boelcke.

According to M. Press, his success was due to him being aware of the enemy before the enemy sees him.

From Situation Awareness Analysis and Measurement, Mica R. Endsley, Daniel J. Garland, p7:

  The earliest discussions of SA undoubtedly derive from the pilot
  community going back as far as World War I (Press, 1986)


White light illuminating the engine coming on at intervals in flight; Curious what's the protocol? I was recently flying Air China from Beijing to Bombay & happened to seat on the Window seat close to the wings.

I could see the engines and there was a light that could illuminate the engines. As I recall it was switched on at some point at the end of taxi and pre takeoff but I don't remember exactly.

What I do remember is that at some points during flight I saw the engine illuminated by this light and at other points (pretty much most of the flight duration) the light was off.

Just curious what the protocol was.

The aircraft type seems to be Airbus A330-300.

https://uk.flightaware.com/live/flight/CCA889

This was a neutral color steady light and not one of the red / blue ones nor the strobe. From the color it looked more like LED illumination.

PS. From the related question on SE Aviation the responsible light seems to be what they call a "Wing light". But my question is different than that one because why would the flight crew turn on the wing light during cruise altitude flight periodically?

There was no crew in the cabin for a damage / ice inspection (I've seen them use wing lights for this purpose post a deicing spray but then the flight crew comes into the cabin to look )

The answer is in this post at the very bottom.  In the US these wing lights are usually switched on to make the planes more visible during takeoff and landings despite the fact that the lights are designed for ice inspection.  I don't know if China has the same protocols or not but from what you described it sounds like they do.


What trade-offs does an airline make when selecting an engine type for their aircraft? So it is my understanding that on most commercial jets, the airline has a choice of multiple engine types, such as GE or P&W, etc. My question would be why an airline would choose one type over the other. Also if there is a "better" choice, why would the airlines choose one of the others?

Power

Some models are different in power, a consideration if you intend to operate hot and/or high, or heavy. Otherwise that extra power is mostly fuel-inefficient.

Competition

In situations where two or more alternatives are available, there's healthy competition. Engines are around 25% of the cost of a new aircraft, so there would be a big money to gain when airlines bulk order from a certain company.

Boeing took this a step further on the B787 where it's easier than ever to switch the type of engine to prevent this type of 'vendor lock-in'. Historically,  it was very difficult to switch due to the technical complexity in these systems.

Customer Relationship

If an airline has good contacts with a manufacturer and know how they work, they may prefer to opt for that comfort when considerable money is on the line.

Part Commonality

The more type of the same engine you operate, the better economies of scale you achieve. This is especially important as substantial maintenance work is concentrated around the engines.

This made for instance American Airlines store Boeing 757 aircraft that it got through a company acquisition that did not have the same engine type as the remaining fleet.

This thinking is not limited to your company, but for instance within a group of companies, as heavy maintenance is often centralized.
It can even apply across types. For instance, Korean Air ordered 787s and 747-8 both with GEnx engines. To quote GE Aviation on the two slightly different engine types:

  The -2B contains an identical core and similar materials as the -1B, delivering synergies for mixed GEnx fleets


Why can APU supply both bleed air and electrical power up to 10.000 feet but bleed air alone up to 17000? I am reading my company's FCOM and I saw there the following information.

APU Bleed + Electrical Load: Max Alt 10000 ft.

APU Bleed: Max Alt 17000 ft.

APU Electrical Load: Max Alt 41000 ft.

If APU can supply bleed air up to 17000 ft, why supplying electrical load at the same time is not possible?

Does bleed air usage on the APU affect the generation of electrical power?

Thank you all for your answers.

It's a function of the relative demand each service puts on the APU.  You have to remember that bleed is power consumed; you're just taking it in a different form.

As a turboshaft jet engine, the primary limitation on total power is the amount of heat the turbine can take.  When you take bleed from the compressor, the turbine is forced to generate a lot of torque to drive the compressor but it's not getting the mass flow downstream through the burner can and turbine that it would get without any bleed (it's like you were cranking a fan by hand to cool yourself, but some joker put a piece of cardboard between you and the fan to divert most of the air - you're getting some air but you'd be pretty hot and bothered and you just can't do the total work you could do if you were getting all the air).

Since one of its primary functions is to provide hot air to the cabin as opposed to making thrust, a much higher proportion of its mass flow is bled off an APU's compressor compared to a main engine, and is a significant portion of the engine's energy potential. The generator, on the other hand, puts much less demand on the engine.

The result is for the same shaft HP available to drive the generator, the turbine temperatures are much higher when bleed is on, basically because it's doing the job of providing shaft HP and providing compressed air. Hence the 10000 ft limitation when doing both at the same time.

The other altitude limits are based on the effect each service individually has on the APU's temperature limits.  The compressed air service, the bleed, uses most of the APU's power potential and maxes out at 17000 ft, with the generator a distant second and when it's the only service the APU has the HP available to run the generator all the way to 41000 ft.


Can a flow which is initially supersonic, become subsonic without a shock wave? I am doing my college project in which air is first accelerated to supersonic  conditions in a nozzle. Then it is added into slow moving subsonic air. I see that my flow becomes supersonic and then subsonic but I don't see a shockwave. All this is done on Fluent using various 2 equation models.

Yes, it is possible . The shockless transition from supersonic to subsonic. It has been tested both experimentally and numerical solution. Read this AIAA Paper.

But in your case I would suggest you to be a bit cautious and recheck your mesh and boundary condition in CFD solver.