What is the speed above which painting an aircraft black (or dark) results in lower skin temperature? The Concorde is painted white for thermal reasons. If I understand correctly, it is done to reflect as much radiation as possible in order to lower tempereture. But when the skin is really hot (SR-71, reentry vehicle such as the Space shuttle,...), it is better to have a black/dark skin so that it radiate internal heat.

In short: when flying supersonic the aircraft should be painted white to reflect incoming radiation, and when flying hypersonic the aircraft should be painted black to radiate internal heat.

My question is: what is the speed threshold below which painting white results in lower temperature and above which painting black results in lower temperature?

This might be more of a question of altitude and the ability to remove heat as a function of air density. The SR-71 has a cruising altitude of ~26km and the Concorde has a crusing altitude of ~18km. Of course a black coating makes it absorb more solar irradiance, but the designers would have taken this into account and ensured that energy balance favors a desirable temperature. The SR-71 had at most two passengers, so comfort cooling can be focused on the cockpit compared to a passenger plane. The Concorde on the other hand had a much larger passenger cabin which would lead to much higher parasitic cooling losses with a black skin.

This chart shows the decrease in density, but whether it's enough to justify a different approach to passive thermal control is hard to say:

The implication is that at high altitudes, heat generated inside the vehicle needs to be removed by a combination of convection and radiation. This is especially important for the Space Shuttle as it needs to maintain temperatures in the lack of atmosphere although the black tiles did not serve as the orbiter's primary radiators. They only needed to insulate the orbiter from aerodynamic forces and the high temperatures. While reentering the atmosphere, the heat comes from air compression in the atmosphere and the black tiles would have helped to re-radiate some of the heat away from the orbiter.


How does the X-59 acheive quiet supersonic flight? By what mechanism or combination of mechanisms does the X-59 reduce the noise produced by a sonic boom?  What are the design choices that lead to quiet supersonic flight?  I really want to understand the issue from an aerodynamic perspective as well as a design perspective.

The X-59 has not yet flown, but according to the designers...

  The ground noise is expected to be around 60 dB(A), about 1/1000 as
  loud as current supersonic aircraft. This is achieved by using a long,
  narrow airframe and canards to keep the shock waves from coalescing.
  It should create a 75 Perceived Level decibel (PLdB) thump on ground,
  as loud as closing a car door, compared with 105-110 PLdB for the
  Concorde. The central engine has a top-mounted intake for low boom,
  but inlet flow distortion due to vortices is a concern.

Source: Wikipedia

Additional information on this NASA project is available here.


Does a rotating control surface change the wing area? It might seem like a silly question, but bear with me.

I am using the standard formula to calculate the lift force of a wing.

L = (1/2) d v2 s CL

My question related to the s part of the formula. According to what I can find this area is calculated as if the wing was completely flat. This would be from the leading to trailing edges.

Wing Area = Wing Span x Average Chord

I am currently doing exactly this, to calculate the lift force of the wing on a computer model while model is flying. It is done at regular intervals along the wing span and added up. The sections that include control surfaces have different chord lines when the control surfaces rotate as expected causing the desired affect on lift.

BUT I noticed that the wing area also changes with the change of control surface angle. As the control surface moves closer to its upper or lower limits the surface area changes too.

Control surface at 0 deflection and at max

Red part is a control surface.

Obviously on a simple wing, the rotating control surface cannot affect the (3d) surface area of a wing. Or should not. Yet using the formula to calculate the s part the wing now has a smaller area.

To get to the point, I would like to know if this makes sense. Does the ...effective wing area change during operation of a control surface ?

TL; DR: It depends on the specific assumptions of the aerodynamic model, but normally wing area is considered constant.

Wing area normally is defined as fixed, and the CL is given so that any control surface deflection results in a change in lift curve, i.e. CL, only. One could equally attribute the change in total lift following a control surface deflection to an associated change in surface area, and then the resultant new CL to go with the deflection would accordingly be of different magnitude, so as to yield the same total lift.

It is a convenient general convention as far as I know, to consider wing area (also very appropriately dubbed as reference area) as constant, because it essentially eliminates one variable from the model.


How thoroughly can WINGS phases substitute for a flight review? If a pilot is unable to act as PIC because they need a flight review, can they instead earn WINGS credits and complete a phase? Everything I have read, including §61.56 and AC 61-91J, suggests that the answer is "yes", but apparently nobody else is discussing it because I can't seem to find the answer on the internet.

All Wings courses that qualify for a Flight Review include both a online knowledge activites as well as required flight activities that are verified by a CFI. These flight requirements generally end up being longer than the 1 hr minimum flight time required by §61.56.  But if you are completing a Wings course for the purpose of insurance discounts, a BRF would be a side benefit.

  To accomplish each Phase of WINGS, you can attend safety seminars,
  take online courses, and demonstrate flight proficiency.

Also, section 1.3 of the Wing Manual states:

  Note that pilots need not accomplish the flight review requirements of
  14 CFR part
  61.56, if he or she has satisfactorily completed any Phase in the WINGS - Pilot  Proficiency Program

So, the answer is yes, you can use Wings phases as a substitute for a flight review, but it might not be the most efficient way.


Will I be denied or fail a first class medical if I have a recent history of depression? I plan on going to get a medical exam for a first class medical certificate to attend flight school. Now, a couple months ago I met with a primary physician to get my blood and urine checked as a pretest, and everything checked out okay.

However when the physician asked me about my mentality, I mentioned I feel depressed from time to time (due to my current job which was at a call center) and he put this on my medical history "Depression, major, single episode, mild"... I currently am not and won't consider antidepressants because the depression is not bad at all, it more or less was just the job I was in.

Will having that on my medical history without being prescribed any medication cause me to be denied the certificate?

According to 67.107 which is the guideline for AME's for mental issues, depression is not listed as a reason for automatic denial for first-class medical certificates.

The issue with depression comes in when you are on medication to control it, as many antidepressants are considered "do not fly". It isn't impossible though even with medication and treatment, the FAA has published a guideline for AME's about antidepressants and treatement, and the granting of special issuance medicals.

What you do not want to do is to lie about it on your application, as falsifying your medical history is grounds for revocation. Getting your medical reinstated after lying about your prior conditions is very difficult (and expensive).


What would happen if pilots were locked out of the flight deck? Imagine in a hypothetical situation that a commercial airliner was left on autopilot at some cruising altitude. Next imagine that all pilots left the flight deck and the door slammed shut behind them (presumably locked). Would they all be completely trapped outside the flight deck or would there be a way to enter should there be a situation like this?

The flight deck door lock system is normally turned on while on the ground and remains on for the entire flight. Once the system is turned on, the door DOES lock automatically when closed and can only be opened by the pilots in the flight deck.

If both pilots left the flight deck, the door would lock automatically behind them unless the door lock system was first turned off. There are procedures in place which can allow the door to be unlocked from the cabin in case no one in the flight deck can open it due to incapacitation.

The pilots can also over-ride this unlocking procedure in case they suspect criminal interference or un-authorized entry attempt.  This over-ride feature prevented the Captain of Germanwings 9525 from re-entering the flight deck as the suicidal First Officer deliberately crashed the aircraft killing all on board.


ETOPS 330 vs ETOPS 370 etc; how are the exact minute ratings derived? I am curious how the ETOPS minutes categories are arrived at. i.e. Say, ETOPS 330 vs 370.

What extra abilities would the 370 type have demonstrated that the 330 didn't have? Was it just that the test flight on single engine was longer? Or something else in the design limits the type to a 330 minute rating while on the drawing board? Is it the MTBF of a component? Or is the 330 minute time back calculated based on actual empirical testing and the associated failures?

On ground and maintainance procedures is there any substantial difference when dealing with a ETOPS 330 aircraft vs and ETOPS 370 aircraft?

Could there be (say) an ETOPS 430 in the future?

If there is a formula currently used to generate an ETOPS time, no regulatory agency has made it public that I can tell. ETOPS originally grew out of a "60-minute rule" that was the result of earlier distance-based rules (see this report for a thorough history). My guess is that 60 minutes was a case of  "well, that sounds good" rather than the result of any rigorous analysis. From there, it's mostly been manufacturers slowly pushing and getting approved for longer and longer extensions (usually just a percentage increase that results in round-ish numbers—again, this is policymaking, not engineering).

However, take a look at the formulas on pages 9–11 of this report. I hesitate to declare them the way to generate an ETOPS number because the report is thirty years old and a lot of it is critical of the methodology, so the regulators may have changed their approach between then and now. In fact, the report does not explicitly state that the formulas within were in use at the time, either. Nevertheless, if there is a formula in modern use, it likely uses a similar approach of quantifying the probabilities of failure and using a factor to convert them into a do-not-exceed diversion time.

The difference between certification at 330 minutes (777) and 370 minutes (A350) may be due to FAA vs. EASA methodologies and/or decreases in inherent failure probabilities as technology matures. With that, I doubt there is much difference in maintenance of a 777 vs. an A350 (at least as far as ETOPS goes—obviously their construction necessitates very different maintenance programs), as that 40-minute increase for the A350 was approved without any in-service maintenance data. It may be different for the 787, which was not initially certified at 330 minutes.

Going forward, I have no doubt we'll see ETOPS 430 and above. Airbus already toyed with 420 minutes for the A350, and perhaps we'll see that after some time in service. At 420 minutes, there are only a few non-compliant routes; I expect that, eventually, any new twin will be able to fly unrestricted between any two points within its endurance.


Do planes use autopilot when flying in formation? I've attempted to fly formation over a multiplayer flight simulator, and it was incredibly hard. It felt really hard to have the two planes go exactly the same heading, so it was always necessary to sway the yoke back and forth, constantly overshooting and undershooting.

When real planes fly formation, do they use some sort of short-range radar system and an autopilot to maintain the formation shape? I could imagine that manually flying formation would cause extreme fatigue due to the need to constantly fiddle with trim and yoke, and would be very dangerous.

Or is this simply a case of actual pilots having actual training, instead of my amateur "training" of fiddling around in the simulator overusing instruments?

Nope, pilots do not use autopilot to fly in formation. An autopilot cannot achieve the precision required for flying formation (e.g. keep the wingtip aligned to that door). Usage of autopilot would also hinder responses when an immediate correction is necessary.

You may refer to the TV episode Jet Stream and Blue Angle training videos on the internet. From the pilot's perspective, the planes are always moving around, though in a confined space. Continuous small adjustment is necessary. From the ground, the small floating cannot be noticed visually because the observer is too far away.

P.S. Based on my 17 years of experience with PC flight simulator, flying in formation in multiplayer is definitely doable, and can become second nature with practice.


Who is the owner of the world's only private Boeing 787 Dreamliner? I have just stumbled across this video and I was wondering, if anyone knows who is the owner of this particular airplane (world's only private Boeing 787 Dreamliner)? I mean a man or a company -- an actual user or property owner. Not an airline / operator, because it seems to be Deer Jet.

The video itself does not reveal plane's registration or serial number, so I cannot look it up in any aviation database.

I've tried to locate it in Flightradar 24 database, based on airline / operator -- Deer Jet -- but it lists Gulfstreams and Dassault Falcons only for this operator. Neither Boeing 787 nor any other Boeing.

The registration number of this aircraft is well visible in the video: 2-DEER.
According to Planespotters, this B787 was first leased by Jet Aviation Flight Services from 2014 to 2016. During this time it was registered as N28MS.
In 2016, the plane was bought by Deer Jet, a company owned by HNA and received the registration number 2-DEER. The video was published in 2017. That means 2-DEER was not privately owned, but available and furnished for private charter clients. The video title is a tad misleading.
Deer Jet was trying to sell it for 300 million US$ in 2018 (source).
In February 2020, 2-DEER was spotted still in Deer Jet livery. It appears that they couldn't find a buyer.
The registration number starting with 2- might look unfamiliar to most of us. A company called 2-REG offers these personalised registration marks. Here is an advertising text from www.weareguernsey.com that summarizes it:

2-REG is an ICAO (International Civil Aviation Organisation) registry,
well-recognised around the world for accommodating all aircraft
categories, including commercial aircraft, business jets, helicopters,
and general aviation. In 2015, 2-REG began to issue Aircraft Operating
Certificates (AOCs), and six have been issued so far.
Run by the Dutch company SGI Aviation, 2-REG pays royalties to the
Guernsey government on operating income. The registry has built its
reputation by being extremely responsive and customer-focused,
offering its worldwide coverage coupled with competitive fees, making
it the most successful registry within the aircraft leasing sector.


Is it ok to use an iPad (or similar consumer hardware) and electronic charts for GA in Europe? I'm wondering if it is it ok to use a consumer tablet and electronic charts (e.g. within the AirNav Pro app) instead of the paper version for recreational VFR single-piston flights?

Edit: to clarify, my question is indeed about official, up-to-date charts, accessed with consumer hardware (I mention AirNav Pro, but it could well be any pdf reader for that purpose) as opposed to paper medium.

No, these kind of apps (AirNav, VFRNAV etc.) are not meant to replace paper charts. You'll find phrases like this on most of the websites which offer those apps you will find paragraphs like this:

  Please note that this software is not intended to replace a certified navigation device. You should always use official aeronautical documentation when preparing and performing a flight. You should always use certified navigation devices when performing a flight. [Source: https://itunes.apple.com/us/app/air-navigation-pro/id301046057?mt=8 ]

There is certified software like for example Jeppesen Mobile TC available.


Declaring an emergency: How should you report fuel remaining? When a flight has to declare an emergency, they are asked for souls on board and fuel remaining. Sometimes the ATC will want fuel in minutes (or hours) and sometimes in pounds.

Could we get a description of why there is no standard for the fuel remaining description? In other words, why sometimes time remaining and other times weight?

(NOTE: I've tagged this FAA-regulations, because I mostly listen to RealATC, or variants, of US ATC, but this question shouldn't be limited to FAA Regs)

There is no standard because there's no standard emergency. The response to "Say fuel onboard" (or the way the question is phrased) depends on who cares most about the answer.

Fuel Remaining (in minutes)
This is how you report it when you declare a minimum fuel emergency, and it's most useful for approach and tower controllers to know exactly how long they have to get you on the ground before your engine gets quiet (and if they can get you to an airport in that timeframe).
Fuel Remaining (in hours)
This is what you'll give for most general emergencies, and also what's entered on flight plans. It's the most generally useful number to a pilot or controller.
If you're VFR-only pilot stuck on top of an overcast and call ATC with a pan (urgency) message because can't see a way through the controller will want to know how far they can vector you to find a hole before it becomes a fuel emergency too.
Fuel Onboard (in Pounds/Kilograms or Gallons/Liters)
The folks who care about this number drive around in ugly fire trucks wearing silver jumpsuits (the ARFF crews): If the worst happens they want to know how big a fire they might have to deal with, and how much foam they're going to need to handle it.


What is the commercial passenger aircraft top speed record? Aircraft manufacturers claim that commercial passenger aircrafts cruise at the speed of 400-500 knots.
But, is the top speed of a commercial passenger aircraft recorded (while in air)? If so, what is the top speed?

"Speed" is not a singular term in aviation. There are many different ways to measure speed. See for example Why is there a difference between GPS Speed and Indicator speed? Most commercial jets cruise with a true airspeed in the range 400-500 knots and an indicated airspeed in the range 200-300 knots.

For the purpose of passenger transportation, ground speed is interesting since it is a determining factor in how long it takes to get from A to B. But do note that ground speed has relatively little to do with aircraft performance, since two identical aircraft could have vastly different ground speeds if one was flying in a headwind and the other in a tailwind.

According to groundspeedrecords.com, the fastest ground speed ever recorded for a commercial aircraft is the Concorde with a record of 1205 knots. This website relies on pilots snapping a picture and uploading it to the site as proof, so it is possible that that record has been broken without it being recorded.

Excluding the Concorde, which is no longer in service, the fastest record is a Boeing 747-400 recorded at 752 knots in 2010. At the time of reaching that speed, the aircraft had a 213 knot tailwind (probably flying in a jetstream).


Is Ground Direction Finding commonly used? I've recently come across the concept of Ground Direction Finding. My (limited) understanding of it is that it is a system that allows you to see where a VHF transmission is coming from. I gather it was used in the Earhart-era of aviation for navigating, and could be a rudimentary air-traffic control system where there is no radar.

I think it's still used these days in boats but what about aviation? And is my understanding of it essentially correct?

Yes, it is commonly used, especially in large complex airspaces.

If two VHF direction finders are installed at sufficient distance from each other, they allow the ATC system to triangulate the position of a VHF transmission. This position is then indicated on the air taffic situation display, which helps the air traffic controller to quickly correlate the transmission to the transmitting aircraft, often before the callsign has been transmitted. The controller still has to crosscheck the callsigns, but it helps to reduces workload.

Example of a VHF direction finder


Does the AIM really require anti-collision lights to be used during the day? AIM 4-3-23. Use of Aircraft Lights
  a. Aircraft position lights are required to be lighted on aircraft operated on the surface and in flight from sunset to sunrise. In addition, aircraft equipped with an anti-collision light system are required to operate that light system during all types of operations (day and night).

Doesn't this imply that anti-collision lights are required to be on during the day? This is obviously not the practice?

Anti-collision lights are required for day and night as specified in the AIM. There's an exception to turn them off if it interferes with the pilot's visibility (e.g. while in the clouds the strobes can be distracting).

  This is obviously not the practice?

This is the practice.


Does a 100 Hour Inspection Start Over Every Annual? If an airplane receives a 100 hour at 350 hours and has an annual at 400 hours, does the next 100 hour inspection need to take place at 450 hours or 500 hours?

The annual will reset the 100 hours.

See 91.409

  (b) Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection and been approved for return to service in accordance with part 43 of this chapter or has received an inspection for the issuance of an airworthiness certificate in accordance with part 21 of this chapter. The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.


Does ATC have to maintain separation on diverging aircraft? Say you have an eastbound flight at FL210 that wants to climb and a westbound flight on a converging track at FL220 that wants to descend. There's the minimum required 1000 ft vertical separation. Once the planes have passed each other can ATC clear them to climb/descend right away or do they have to wait until they are 5nm apart before they can break the vertical separation? And what about TCAS? Would it consider that a conflict?

Yes, this is allowed, subject to certain conditions. See FAA Order 7110.65 (the ATC "bible"), section 5-5-7, for the full details.

  ...all other approved separation may be
  discontinued and passing or diverging separation applied when ... Aircraft are on opposite/reciprocal courses and you have observed that they have passed each other; or aircraft are on same or crossing courses/assigned radar vectors and one aircraft has crossed the projected course of the other, and the angular difference between their courses/assigned radar vectors is at least 15 degrees.

That's in the terminal environment; it's still possible in the enroute environment but a bit more restrictive due to the greater aircraft speeds and poorer radar accuracy.


What does it mean "two plane thrust vectoring nozzle"? It has been said that only Sukhoi 35 has the "two plane TVN" others like F 22 and Su 30MKI (produced by India under license at Hindustan Aeronautics Limited)  have "one plane TVN".So what the difference ?

This statement is using "plane" in the mathematical-geometric sense, which is of course very confusing when we talk about a part of a "plane" in the "flying machine" sense.

So, a "one-plane" thrust vectoring nozzle can only move up and down or left and right (typically up and down). So, this gives you pitch control using the thrust, without requiring airflow over the elevators. This is sometimes called "1D" thrust-vectoring, but it is also (confusingly) sometimes called "2D" thrust-vectoring (see the last paragraph).

A "two-plane" thrust vectoring nozzle can move up and down and left and right. So, this gives you yaw control without using the rudder in addition to pitch control. This is sometimes called "2D" thrust-vectoring, but it is also (confusingly) sometimes called "3D" thrust-vectoring (see the last paragraph).

In a two-engine aircraft, in both cases, you can add another "virtual" axis by having independent vectoring of the two nozzles, e.g. one up and one down, which gives you roll control. That is why, for two-engined aircraft, we sometimes talk about "2D" and "3D" even if the individual nozzles only can move on one or two dimensions.


How does Pitot Heat influence airspeed indication (assuming no ice or moisture)? Should Pitot Heat ON (in dry weather conditions) decrease airspeed indication because warmer (so less dense) air flows through the tube?

No, there is no decrease or change with the use of pitot heat.

The pitot system measures pressure. There is no airflow through the system in normal operation. Any airflow through the system would be due to a system failure which led to leakage.


Acceleration of a supersonic aircraft after breaking the sound barrier People on the physics stackexchange recommended I ask here, so I am pasting my question regarding supersonic aircraft.

I stumbled upon an interesting plot; in particular, the dependence of wave drag on the Mach number:

It is curious to see that the drag coefficient drops so abruptly in the supersonic regime, but I am even more curious if the total drag force acting on the airplane also drops, i.e. if the plane starts accelerating in the supersonic regime.

I did some quick analysis of the problem. The drag force is defined as
$$F_{\text{drag}} = C(v) v^2 ,$$
where $C(v) = A C_D(v) \rho(v)/2$ is the newly introduced drag constant with the drag coefficient $C_D(v)$, flight medium density $\rho(v)$, and cross-sectional area $A$.

Assuming these relations, one can taylor expand the equation for the drag force to find that

$$\Delta F_{\text{drag}} = C'(v)v^2 \Delta v + 2Cv \Delta v.$$

As $C'(v)$ is clearly negative in the supersonic regime, this means that the total drag force drops if
$$C'(v) < -\frac{2C(v)}{v}.$$

It seems that this criterion could indeed be satisfied in air, as the right hand term in the inequality is a fairly small number.

Is there anyone that could elaborate further on what actually happens to the drag force as the plane breaks the sound barrier? Do the plane engines lower their power in order to maintain reasonable supersonic cruising speed, and if they didn't, would they be under too much heat load?

Your Taylor expansion only uses the first two terms, so it is only a crude approximation. But still, your observation is correct, however, whether it applies depends on the aerodynamic quality of the particular aircraft.

Practical supersonic aircraft have been designed to minimize the Mach drag peak. The ways to do this should be familiar:

Stretch the aircraft lengthwise so it becomes long and thin.
Smooth out the cross sectional area distribution over length ("area ruling")
Use swept wings with thin airfoils

If this is properly done, the drag peak will be small enough to have overall drag increase with Mach number above Mach 1. In case of the F-16 the drag coefficient rises from 0.02 (subsonic) to 0.045 (Mach 1.1) and stays roughly constant with increasing Mach number, so the absolute drag still grows with speed squared. No significant decrease in the drag coefficient occurs because of the complex flow
around the entire aircraft. Only when you have a poor design will drag actually become lower at low supersonic Mach numbers.

Another factor is thrust: Since ram pressure at the intake increases, so does thrust of the same engine as speed increases. This is the main reason why Concorde could already super-cruise 20 years prior to Lockheed marketing inventing this term.


Is it bad for the engine to perform a touch & go? I just watched a video about the traffic pattern. In the comment section below, the uploader spoke about touch & go's:

I don't like to do touch and go's, as a student pilot I used to do them cause you saved money, but its not worth it. Always better to get off the runway, let the engine stabilize**, and then set up properly for the next flight/takeoff.
** by stabilize I mean let the engine warm up under its own power on the ground, because the engine was just at idle or low power for a while during a landing, with the air flow turning the prop and cooling it, and abrupt power-up for a takeoff can "shock" it, that is when usually engines fail.
Dont get me wrong, I'll do a go-around in a second and engines are tough and this does not happen often, but the bigger the engine, the less you want to do that. Some flight schools even forbid touch & go's now.

By GVad The Pilot. Video Link.

Is it really bad to make touch & go's for the engine?
Do some flight schools really forbid touch & go's?
Are his concerns reasonable?

As an automotive engine mechanic with 35 years of performance oriented experience and 29 years of that with ASE Master Certification and about a dozen other engine specific certifications.  Twenty or  so years ago I trained to be a piston engine aviation tech.
So I think I can say with some certainty that I know a thing or three about how engines work and what sort of things can have a detrimental effect on them.
"Thermal shock" is definitely bad for an internal combustion engine.  That said, touch & go's will not cause thermal shock to occur in a piston engine, here's why.
Thermal shock is what significantly rapid and drastically uneven temperature change in an engine is called.  This means if one or the other isn't occurring then there's no thermal shock occurring.

(To be accurate, it's more a degree of severity, as these conditions happen during normal operation and always will.)
The minimum operating temperature called for by the engine manufacturer is to avoid thermal shock.  It is important enough for the manufacturer to stress because it is the only reasonable condition the pilot can produce enough thermal shock to damage the engine.  When the engine is below the minimum, applying full power causes the pistons and rings to heat up and expand much more rapidly than the cylinders. This in turn reduces the clearance around the pistons and can dramatically increase the friction.  The increased friction compounds the problem by adding to the heat and expansion of the piston.  More expansion -> more friction -> more heat... This quickly becomes a deadly self reinforcing loop.  Because the pistons can't shed heat to the cylinders fast enough the cylinders don't expand and very quickly there's no clearance and the piston seizes up inside the cylinders.
Rapid temperature change in part of the engine without a matching change in the rest. This is the part of thermal expansion that breaks things.  The piston to cylinder heat transfer is very inefficient. But the transfer of heat, to the air from the engine, is even slower. So, once the engine is warmed up, it stays that way till it's parked in the hangar and you're headed out. That's just physics 101.

The alternative situation is not likely to happen and if it did, your last worry would be the thermal shock to your engine as the engine wouldn't be running long enough to get damaged, from heat shock that is.  What I'm talking about is full submersion of the engine in water. Given this scenario I am doubtful "thermal shock" would really be on your mind.  To be honest, I'm not sure if even the coldest water you could keep liquid is cold enough to chill the cylinders to the point of interference.

The belief that touch & go's cause too much "thermal shock" probably comes from mechanics understanding the cumulative effect of dissimilar thermal expansion caused by repeated full throttle application during the touch & go's, but being unwilling or unable to explain that it is really only an issue during the run-in period of time when the new rings are seating in freshly honed cylinders.

The only other way I can see touch & go's being an issue is if a freshly built engine is put together with too little clearance between the pistons and the cylinders.  In the extreme, repeatedly heat stressing the engine could scuff its way to a seized piston.  None the less, that wouldn't be thermal shock anyway.  That would be builder error and the probable source of stories that say "Touch & go's will blow your engine up!"


Where can I find speed and thrust data for airplanes during the approach? While trying to gather information on fuel consumption during the approach I have found this equation :

$$f=C_{f1}\cdot (1+V_{TAS}/C_{f2})\cdot T$$  (on this FAA paper)

$f$ is the fuel flow
$T$ is the thrust acting parallel to the aircraft velocity vector
$C_{f1}$ and $C_{f2}$ are the first and second thrust-specific fuel consumption coefficient
$V_{TAS}$ is the true air speed

From what I have understood Cf1 and Cf2 are constant and their values can be found (all the documents regarding these issues are 100 pages papers so I have not read everything yet).

I would like to plot the fuel flow for different airspeeds and thrusts. What are the typical values during the approach ?
I would like to have these data for several flights during the last 20 or 25 minutes of the flight.

What I am looking for is exactly what flightaware.com provides except that they use ground speed (an exemple here). How can I convert to TAS ?
Also they don't provide anything on thrust. Do you know where I can find thrust data ?

All you need is collected, though not published, in airlines' Flight Data Monitoring (FDM).
Airlines collect a load of parameters for training and statistical purposes. If you can contact an airline, tell them what you're doing and make it sound like you could improve fuel economy, there's a good chance you can get some of the data.
Another place to seek would be aircraft manufacturers like Boeing or Airbus.


How are pitch angle and pitch rate measured in an airplane? What equipment or method is used to obtain measurements of the pitch angle (θ) and pitch rate (q) during flight?

Edit: I am asking about how the aircraft systems sense these quantities

The pitch of an aircraft relative to the horizon is observed on the Attitude Indicator which also displays roll angle. These are required for IFR flight but not VFR flight here in the US. As such they are found on most but not strictly all aircraft. These are typically gyroscopically based.

(source)

Here is a basic cutaway of the interior showing where the gyro is mounted. They are often vacuum (or positive pressure) driven but can be optical or electrical as well.

(source)

Angle to the relative wing (angle of attack) is shown on an angle of attack indicator. While not required these are becoming more popular. These are controlled by a sensor on the outside of the aircraft.

(source)

The sensor that measures this on the exterior looks like this:

(source)

As far as I know there is no pitch rate info on your standard panel.


What advantages are provided by an unstable canard configuration for fighters? Why are most canard fighter aircraft unstable?  A stable design would let the canard provide lift.  My guesses are:

Increase pitch rate
Decrease stability while supersonic (supersonic, the aircraft are stable)

You want your control surface to create only little lift, so it has enough margin for maneuvering. This confirms your first guess.

Your second guess is also correct: Flying supersonically shifts the center of pressure back and increases stability. If the aircraft is unstable subsonically, it will suffer far lower trim losses supersonically.

Please read all about unstable designs in this answer. For conventional, stable configurations it makes most sense to put the control and damping surfaces at the back, and a canard is the better choice for unstable, highly maneuverable aircraft.


How is the central hub / shaft casing of a two-spool jet engine assembled? This is the usual view of a two-spool turbine engine:

(From source)

I don't know if shaft casing or hub are correct names, this is the external skin of the volume around the N1/N2 shafts.

I can't find a description of how this volume is made. It seems descriptions omit a number of elements for simplicity, for instance, this drawing from this answer:

(Source)

My interest is about the green area. It's not clear to me:

How airtightness is obtained overall on both sides of the core air duct?
How airtightness is obtained in locations like 1, 4 or 5 when rotating elements meet stationary (or N1) elements?
How torque is transmitted from the HP turbine to blade 2? or how elements (like 3) between the turbine and the compressor discs are assembled together to form a rigid wall/pipe?
How stators (like in 3) are fixed?
Why the central hole size varies for the rotor discs (like 2)?

Cross-sections would help but I can't find ones.

Bottom line: Without necessarily entering into details (but they are welcome), could you provide an overall description of how this "hub" is assembled, particularly the HP part?

The first thing to know when designing jet engines, is everything leaks. But the OD of the engine core is generally pretty tight. This is achieved using large solid structural pieces, for example the entire LPT outter casing is generally one piece. These large pieces are bolted together with an excessive number of bolts to make them very tight. Any penetrations will have various types of seals and gaskets, as well as high clamping forces. Although the outer case is pretty tight, the cavity between the core case and the bypass duct is still actively purged with air from the low pressure compressor (labelled as undercowl cooling on the image below).

GE90 Engine Airflow Diagram (This is actually a widely available training poster but I stole it from here, you can find similar for just about every engine)

As for the inner side of the core, this is where literally everything leaks. As you can see in the picture above, Stage 1 (S1) High Pressure Compressor (HPC) bleed air is used to actively purge the cavity under the Low Pressure Compressor (LPC) and early stages of the HPC. It is then plumbed though a small duct around the low pressure shaft to the Low Pressure Turbine (LPT) (zoom in on the image and flow the arrows in the yellow fill).

Your location number 1 is the split between the LPC rotors (spinning) and the Fan Frame (stationary). Your location number 5 is the split between the LPT (spinning) and the turbine rear frame (stationary). In these locations (and in all gaps in the ID of the core) the air is allowed to flow back into the flow path. The purge air must be at a higher pressure the flow path air, and allowing it to flow back into the main flow path actively purges these gaps and in the turbine prevents hotter gas path air from backflowing into the rotor cavity.

Your location 4 is at the end of the compressor and beginning of the combustor and is known as Compressor Discharge Pressure (CDP) air. The image below (source) shows what is going on here. A faction of the air is directed directly into the combustor through the fuel nozzle assembly, while the rest bypasses and is used as dilution air, as well as combustor liner cooling. Another faction is taken off the OD to cool the S1 HPT Vane and Shroud, while a larger fraction is taken off the ID to cool the S1 HPT Vane, Blade and rotor, as well as actively purge the gap between the S1 blade and vane. If this cooling air was ever lost, the HPT would literally oxidize/melt/creep away in a matter of seconds. The gap at location 4 is sized so the correct about of flow goes into the combustor dome and the right amount bypasses the fuel nozzles. Note that in ultra modern combustors, the diluation holes no longer exist and a much larger fraction of air enters at the fuel nozzle location.

Torque is transferred from the HPT to the HPC through the spool arms (basically a shaft), which is what connects each rotor to each other. I have highlighted this in red in the image below. The full spool is created in multiple ways. Some of the HPC rotors are forged together into sub-spools. Some of these sub-spools are welded together (becoming more common) and then some joints are done with bolts. In the second image below, is the HPC spool for the CFM56, I have circled the bolted connections. Bolts are generally used when there is a change in materials so welding can't be done. The first couple of stages will be Ti, while the later stages are a Nickle based super(ish) alloy. The HPT rotor is a Nickle based super alloy but is generally being pushed to the limit too much to be welded. When it is all put together, you have a very stiff shaft.

 (Source PDF)

The stators are attached to the outer casing. Generally there is a circumferential slot in the case, the stators slide into this slot. Generally the stators are brazed together in multi-vane assembles to reduce air leakage. There is also a labyrinth seal on the ID of each stator to reduce air leakage.

The S1 and S2 HPC rotors (your location 2) have a larger ID than the other rotors so they can fit over the bearing race when they are unbolted from the spool and to allow tool access to the bolts. The S3 rotor is the largest because it support the arm going to the bearing, a still largish blade and the bolted joint of the spool join. As you go aft the rotors get a little smaller because they support smaller blades, but that is counter acted by raising temperatures that reduce the material capabilities.

Overall engine assembly (Youtube Video of GEnx-2B being assembled). Depending on the engine it'll be assembled from Fan back or LPT forward. So from LPT forward (Very simplified). First take the built up LPT module and attach the LP shaft (aka fan shaft/fan mid shaft). Next, you would build up the HP section including outer casing seperately, then slide this onto the LPT, attach the HPT outer casing to the LPT case with an excessive number of bolts, if there is a mid turbine frame it would be installed between the two turbines with bolted joints on both sides. Then you install the fan frame to the HPC case and then the fan spool to in the LP shaft and the fan case to the fan frame. With the right tools, it all slides together pretty easily. One way the LP shaft is connected to the LPT and Fan Spool through splined connections and a large coupling nut is installed on each end of the shaft to prevent the splines from backing off. Sometimes the shaft is bolted directly to the LPT or Fan spool directly.

Sorry for being so long winded, but I hope that answers your questions.

Here is one more flow diagram for reference (source):


Why is the F-117A the only plane to use the fully faceted design? One of the keys to the stealth capability was its faceted design. The concept is that curved surfaces have a larger radar cross-section. The idea of making all surfaces flat to reflect radar away from the receiver seems a pretty sound concept. It must have been pretty effective, since they had to treat the windows because the pilot's helmet had a major effect on cross-section.

But all of the low-observable aircraft made since, such as the B-2 Spirit and even Lockheed's own F-22 and F-35 have a significant amount of curved surface area.

Why has the fully-faceted concept not carried over into newer low-observable designs? Does it just have too much negative affect on handling characteristics?

The F-117 was designed at a critical point in aeronautical history where we didn't have enough computing power to analyze the radar cross section of more complicated geometry, but did have the computing power to provide artificial stability to unstable designs.

The faceted design is not ideal for stealth or aerodynamics. Obviously the corners are less aerodynamic than a smooth surface. But it also means that if the facets align with a radar source they will reflect a strong return. The compromise is that it's easy to calculate the reflecting angles of radar energy off of flat surfaces based on various locations of radar sources relative to the aircraft.

A curved surface means that only a small section of the surface is really reflecting directly back to a radar source, while the rest is scattered. Together with radar absorbing materials, this can provide effective stealth. It just takes more analysis to determine how different curves and features will reflect radar energy.

You can see the evolution of stealth designs from the Have Blue (prototype for the F-117) in the mid 70's, to the Tacit Blue prototype in the late 70's, to the B-2 in the early 80's.


Which GPU (Graphics Processing Unit) is used to generate the graphics for avionics? I guess it isn't the GTX 1080Ti.

But which GPU(s) has the Boeing 737-NG for example? How does it differ from a GTX 1080Ti (cost: around €800), or Quadro P6000 (cost: around €7100)? How many GPU's are required? And what is the cost of one single GPU-Module for a 737?

Real Aircraft displays do not use 'off-the-shelf' display adaptors. They will have some graphics processor (often developed by one of the major companies ATI/Nvidia) but this will often be an older/mature chipset. Each display also might have multiple graphic processors. The cost of an aircraft display is more closely related to the cost of developing/certifying the hardware/software than the physical cost of the components. You can't realistically compare an Aircraft display to your desktop PC.

Aircraft displays aren't just dumb monitors, each is a computing device in their own right that processes the data from the aircraft's systems. Each display will continue to work if another fails. Modern standards such as ARINC661 moves some of the processing/behaviour to other computing units, but the display unit itself is still responsible for drawing the graphics.


What is the nominal frequency of the satellite communications terminal on MH370? What is the nominal frequency of the aircraft terminal on flt 370? (The terminal used to provide the handshake with the satellite). The raw data provided by the accident team does not specify this frequency.  This frequency info would help in checking their BFO calculations.

I looked through the report about the flight path analysis that is available from this page.

In Appendix G they explain in more detail how the analysis was done. On page 56, they provide the details about the frequencies (emphasis added):

  The uplink and downlink Doppler may be calculated from the relative movement of the aircraft, satellite and GES using the signal frequencies of 1646.6525 MHz (uplink) and 3615.1525 MHz (downlink).

This means the aircraft terminal transmits (uplink) on 1646.6525 MHz and receives (downlink) on 3615.1525 MHz. However, it is not that simple.

  The aircraft terminal adjusts its’ transmit frequency to compensate for the Doppler induced on the uplink signals by the aircraft velocity. Aircraft heading and ground speed are used to calculate the Doppler shift the signal would experience if the satellite was at its nominal location over the equator.

This is called Automatic Frequency Control (AFC) and Appendix G goes into more detail about all the factors involved, and provides tables with example calculations.


What is the airplane type in this formation seen above Eugene, Oregon? This flight of 2 flew by my house in Eugene, Oregon recently. I assume they're military pilot trainers but I am having trouble coming up with an ID.

They are most likely Embraer EMB 312 Tucanos. Military Tech, Inc. has at least 10 registered in Eugene. Here is one in a similar paint scheme:

Source


Do landing planes brake with wheel-brakes or air-brakes or both? I suppose they do both, but what percentage of the kinetic energy is absorbed by the wheel brakes and what percentage by air-friction (in typical cases)?

Depends on how long the runway is.

If you have a 12,000' runway and your terminal is at the far end of it, you can roll out with no wheel braking and as little (or no) reverse thrust as you're allowed, and you'll be close to taxi speed by the time you're ready to turn off.  In that case, the brakes only absorb enough energy to get you from maybe 60-80 knots down to 20-30, since the drag devices lose most of their effectiveness by that point.

On the other hand, on a really short runway, you may be getting on the brakes immediately after touchdown, which means that the amount of time that you're dissipating energy due to drag (spoilers) and reverse thrust is reduced, and much less energy is lost that way.  So in that case the brakes get a great deal of your kinetic energy.

The case beyond that would be if you didn't use reverse thrust, your brakes would get nearly all of your energy, with only a small amount dissipated due to aerodynamic drag.


What was the speed limitations for droop nose on the Concorde? Lowering gears or extending flaps comes with limitations. Given the answer of this question I suppose the limit speed for full nose down is about 325 kt. My question is: what were the limit for all positions (visor down but nose full up, nose in take off position, nose in landing position, any other position I don't know)?

After checking the systems chapter on the nose, it turns out a dual uplock failure would limit the drop to 5° (see updated answer). If it is just one uplock that failed, the nose/visor should hold, but reducing the speed is out of caution.

As for the speed limits, here they are from the aircraft limitations section of the manual:


How much extra weight is added by strengthening a piston-prop fighter for carrier landings? To land on a carrier, an aircraft must be strong enough to withstand sudden deceleration by the tail hook catching the arresting wire. I think it also requires strengthening the undercarriage to survive a harder "plop" on landing.

So I would like to know, how much extra weight is added due to both of these things?

I'm interested in the WW2-era piston props. The modern jet fighters are much heaver, so I'd rather save that for a later question.

Ideally, the greatest example would be a piston-prop from this era made in two versions: land and naval. Then we could just compare loaded weights. But I can't think of any. Don't think I've ever heard of that. All of the carrier-based aircraft from WW2 that I know of, were purpose built for carriers and never had a land version. Hopefully someone knows where to find airframe specs for these aircraft and has the engineering know-how to say how much steel would be unnecessary for a land version.

The British adopted the Spitfire and Hurricane for carrier use.
Using the Spitfire as an example, here's data from Wikipedia.

Spitfire Mk.VB is 5065lbs clean.

Seafire Mk.III is 6204lbs clean.

They're not exactly equivalent otherwise, but close enough.
So the Seafire is roughly 1140lbs heavier than the land based equivalent, or over 20%.

Of course this will vary wildly for different variants compared, but it's a ballpark figure


Who would be allowed to operate with no navigation lights over a populated area? I know little about aircraft, FAA or FARS, but I've tried to research something I saw one evening without much success. I was hoping an expert could chime in.

One evening, while watching for satellites, I saw a black blob cross the sky, and thought that was really strange. A few minutes later, it happened again, in nearly the exact same position, and in the exact same direction, but this time I realized it was accompanied by the sound of a prop plane. This went on a few times, each time the sound of the prop plane and then a black blob going south to north with no lights. Eventually, the 4th or 5th time, an SWA 737 was coming in for a landing at a nearby airport, MAF if it's relevant, perpendicular and in the path of the dark prop aircraft, when suddenly the dark aircraft lit up it's navigation lights. I could only assume it did that because of its vicinity to the commercial flight

From what I've read, all aircraft abide by FAA regulations, even the military in general. Does anyone have any idea of who would be allowed to operate an aircraft, with no navigation lights, over a populated area? My only guess is that maybe federal law enforcement would be able to do so?

Military aircraft actually aren't bound by FAA rules. They have their own set of rules to follow. Having said that, though, their rules are basically "Follow the FAA rules, with a few exceptions." And, yes, one of those exceptions is for lights. Military aircraft can turn off their lights after dark, but only if they're actually flying a mission in an active war zone, or if they're doing training in a restricted area that's been cleared of other aircraft.

According to FAR §91.209(b), "The anticollision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off." The "operating conditions" mentioned generally refer to a situation where the glare from the bright, flashing anticollison light reflecting off a cloud would blind the pilot. Note also that this only applies to the anticollision light, the position lights (the steady red, green, and white lights on the wings and tail) must be turned on from sundown to sunrise.

Since you didn't mention the sound of gunfire or bombs dropping, I think it's safe to rule out the "military plane in active war zone" explanation. And since there was a civilian 737 in the area, we can also rule out the "military plane on training exercise" explanation. Thus, I think its safe to assume that this was just some guy (probably practicing his night takeoffs and landings) who forgot to turn his lights on.


What is the difference between a Mechanic and a Repairman? In reviewing some of the data published by the FAA, I noticed that they make a distinction between a mechanic certificate and a repairman certificate.  What is the difference?

The basic difference is that a mechanic can perform maintenance on any aircraft; a repairman can perform maintenance only on aircraft for their employer, or that they own.

The full details are in 14 CFR 65 Subpart D (mechanics) and Subpart E (repairmen). Here are the basic privileges for each one (emphasis mine):

  A certificated mechanic may perform or supervise the maintenance,
  preventive maintenance or alteration of an aircraft or appliance, or a
  part thereof, for which he is rated (but excluding major repairs to,
  and major alterations of, propellers, and any repair to, or alteration
  of, instruments), and may perform additional duties in accordance with
  §§65.85, 65.87, and 65.95.

And:

  A certificated repairman may perform or supervise the maintenance,
  preventive maintenance, or alteration of aircraft or aircraft
  components appropriate to the job for which the repairman was employed
  and certificated, but only in connection with duties for the
  certificate holder by whom the repairman was employed and recommended.

"Repairman" also applies to experimental and light sport aircraft owners. If you build your own aircraft you have repairman privileges to maintain it, but you can't use those privileges to work on other aircraft, per 65.104(b) (emphasis mine):

  The holder of a repairman certificate (experimental aircraft builder)
  may perform condition inspections on the aircraft constructed by the
  holder in accordance with the operating limitations of that aircraft.


In what conditions are visible Wingtip vortices created? If I would want to create a visible wingtip vortices on a given plane, what are the conditions that I would need to meet?

Wikipedia gives the following explanation:

  The air cools adiabatically in the center of the turbulence at the end of the wing, as this is an area of particularly low pressure. The air often reaches temperatures below the dew point temperature, which leads to the condensation of the water in the air to water vapor / mist and a vortex is visible. When approaching with moisture-saturated air, you can see such vortex heads even at several points on each wing, in addition to the outer edge often, for example, at the respective end of the buoyancy aids (flaps).

Now to the question:
What conditions would be needed the have them built on a oltimer plane like the Douglas SBD Dauntless or the Douglas TBD Devastator?

I would assume Ideal conditions would be a tropical climate near the Ocean with a high humidity near ground level.

You're definitely right about humidity, but from experience I'd say a colder environment works better than a warmer one. Case in point: unstable/rainy autumn weather (as common in Northern Europe) shows (tip) vortices very well, whereas I haven't seen them so much in hotter places (Southern USA, Caribbean). Having ensured visiblity, a larger pressure difference between lower and upper side of the wing (caused by higher speed and higher lift force) increases the tip vortices in size/strength. This explains the fact that (wake vortex) separation minima are largely dependent on aircraft mass (which scales with lift).

Additional (or stronger) vortices can be created artificially using vortex generators. As turbulent flow separates less easily than laminar flow, these are used (on wing leading edges, engine cowlings, tailplanes) to trigger turbulent flow and thereby prevent/postpone separation. This is especially important if there are control surfaces (such as ailerons or a rudder) downstream, which lose their effectiveness in separated flow. In theory, you could use this same principle to increase vortex visibility.

Whether this also works in theory of course depends on safety standards and regulations concerning modification and the size of the vortex generator required. Examples from the world of sports aerodynamics, where surface roughness or 'transition strips' are integrated in suits worn by speed skaters or cyclists. As you can see on the images in the linked article, these are fairly small.


What's the starting envelope of APU? I was reading the APS-3200 training manual (pg 268) and faced this part:

What's the starting envelope of APU?
What does mean "Normal start throughout the operating envelope: minus 300 m to 11900 m (minus 1000 ft to 39000 ft)"?

The starting envelope determines the range of altitude (and possibly temperature) at which the APU can be started in flight. In your case, the manufacturer specifies that the APU can be started at altitudes between 1000 ft below MSL and 39,000 ft above MSL, which is also the operating envelope of the APU.
Here is an APU operation and restart envelope of an Airbus A320:

(source: Airbus A320 Limitations - APU)
The cyan range is the operation and restart envelope (as a function of altitude and temperature). This APU model can only be used for bleed air and electrical power up to 22,500 ft altitude. Above, it can only provide electrical power. It also says that the battery is capable of starting the APU below 25,000 ft.


Why are Russian cockpit panels painted in turquoise? Almost all cockpit panels of Russian aircraft types are painted in turquoise, whereas elsewhere the cockpit panels usually exhibit a dark grey color.
As this "rule" seems to be quite prevalent, I am curious as to the reason/origin.

This can be seen in the cockpit of Mikoyan MiG-31:

Image Source

According to Cold War Air Museum:

  ...the color chosen by Soviet designers helps to reduce stress and
  maintain a pilot's effectiveness on long missions.

  ... the scientists found out that this color keeps pilots awake and
  not getting tired by the black or grey of a cockpit panel, especially
  under terms and condition of long range flights or under heavy work
  load.

This URL contains a color guide for cockpits.


Are aircraft wings airtight? Is there any no air proof design for the wings? Or are  some of the parts are equivalent to outside pressure and some have gauge pressures?

Are some parts airtight and others not?

No part of my Cessna Cardinal wings are airtight. Even the fuel tanks are vented to allow air in to replace the fuel this is used.
The area around the flaps, flap motor aileron control cables, etc., none are airtight.


What are the three sources that are relayed to the brakes and steering control unit of an A320? There is the sentence:

  Electrical signals from the three sources are relayed to the brakes and steering control unit (BSCU) and are computed into the nose steering angle.

The question: What are these sources and where they located?

I don't know where your text is from, but the 3 sources are:

  The BSCU receives orders from:

  ‐ The Captain's and the First Officer’s steering hand wheels (orders added algebraically),
  ‐ The rudder pedals,
  ‐ The autopilot.

  The BSCU transforms these orders into nose wheel steering angle.

(A320 FCOM DSC-32-20-10)


Why is only a single underwater robotic submarine Bluefin-21 being deployed in the search for MH370? As my question states, why is only a single Bluefin-21 being deployed underwater to scan the seabed with an area of around 40 square miles?

Why aren't multiple Bluefin-21's getting deployed when deploying multiple machines would make the search faster when everything here is a very time consuming process?

Bluefin-21

  Why not multiple Bluefin-21's

There is only one Bluefin-21.

The other Bluefins 12S, 12D, 9M etc are mostly for shallow water work.

Other AUVs

The Bluefin-21 is not the only commercial Autonomous Underwater Vehicle (AUV) equipped with side-scan sonar.

The US Navy has spent $20M on an order for eight Knifefish (a Bluefin-21 derivative) but these are not due to enter active service until 2017. They are for mine-detection and so (I surmise) may not be well suited to deep-ocean usage.

The Royal Navy has a number of REMUS 600 AUVs but these are not designed for use deeper than 600M

It may be that most existing AUVs equipped with side-scan-sonar are not capable of deep-ocean use.


Do some aircraft lower the landing gear mid-flight? Are there situations in which a pilot could/has to lower the landing gear during flight for reasons not directly connected to landing? (i.e., not part of the landing checklist)

Originally I thought I had seen some around the location indicated by the pin on the map below (17° 25' 30.093" N,  78° 32' 27.4734" E), in the evening time around 4:00  and the weather was clear.

That's nowhere near midflight. You are seeing aircrafts approaching Begumpet Airport (VOHY)'s runway 27. This airport has no airline service, but it is used for some general aviation including VIP flights and as military airbase.

The point directly north (about 3 km) from the coordinates you gave (N17° 25' 30", E78° 32' 27") is about 6 km, or 3.2 nm, from threshold of VOHY runway 27. Given usual 3° glide slope the aircraft would be just about 1000 ft above runway elevation, which happens to be the latest point where landing checklist should be complete. Yes, that means gear down. It also means it will be touching down in about a minute and a half.


Covid-19 - Why are there still a lot of international flights? I hope this question is relevent to Aviation. If not, please let me know. Below is a snapshot from Flightradar24. While there are far less flights than usual, there are still a lot of flights en-route.  I thought there were controls put in place last week in several countries. Do these flights have special permission?

The simple answer is that no one has shut down any airspaces yet and that is what stops planes from moving.

While many governments have put travel bans in place for people airplanes often carry other things like mail and cargo and it may still be viable and profitable to fly them. There is also the compounded fact that many people are trying to get home to quarantine them selves and in the short term there is still high demand for flights to move people back to their home countries.

Airlines are starting to cut flights due to reduced demand and over the next few months you will see a drop in flight traffic but if the airspace is open planes will keep moving.


What is the difference between axial compressor blades and centrifugal compressor blades? Is there a particular difference?  The way they're overhauled is the same ? (I mean do we use the same overhauling methods ?)

The blades are shaped completely differently. Typically a centrifugal compressor is one solid piece that looks like a turbocharger and an axial compressor is a flat disk that looks like a fan with individually manufactured blades inserted, though axial compressor disks with integral blades "blisk" is starting to be used. Here are compressor pics that show how they are used:

Two stage radial/centrifugal compressor circled (with a three stage axial turbine following)

Fifteen stage axial compressor

A radial compressor can usually generate higher pressure in a single stage so it's usually simpler, while an axial compressor is easier to stack in multiple stages to get much higher pressure ratios.


Should 3 takeoffs and landings to stay day current be solo? Let's assume  that I fly every weekend with a friend which is not a pilot. Can I log that time as PIC and count the takeoffs and landings toward the currency requirement as stated below:

  Sec. 61.57: Except as provided in paragraph (e) of this section, no person may act
  as a pilot in command of an aircraft carrying passengers or of an
  aircraft certificated for more than one pilot flight crewmember unless
  that person has made at least three takeoffs and three landings within
  the preceding 90 days, and -- The person acted as the sole manipulator
  of the flight controls; and The required takeoffs and landings were
  performed in an aircraft of the same category, class, and type (if a
  type rating is required), and, if the aircraft to be flown is an
  airplane with a tailwheel, the takeoffs and landings must have been
  made to a full stop in an airplane with a tailwheel.

Does sole manipulator of the flight controls means solo and by yourself (alone) in the airplane such as Cessna-172 or does it mean you can have a non-pilot passengers?

Yes you can count that time as PIC and towards this currency requirement.

The regulation requires that you are the only one who moved the flight controls during takeoff and landing. This means you could fly with anyone else in the cockpit (including a flight instructor, another pilot, or non-pilot friends) but you alone must be the person who performs those takeoffs and landings in order to remain qualified.

This eliminates a few scenarios from counting towards the currency requirement, including:

You flew with another pilot, counted yourself as PIC (because you were more qualified or by prior arrangement) but didn’t actually do the work yourself
You flew with an instructor, and they helped you with the landings


What are the design considerations for variable-pitch folding-propellers? Folding props are increasingly used on self-launch gliders in order to reduce drag. Here is an example of 1m dia. props from LZ Design's FES system:

For self-launch gliders, due to minimally-sized power plants the props are optimized for the safety-critical climb phase, leaving cruise performance as an afterthought.

At full-scale power levels, i.e. 10-30kW, how are design considerations different for folding props vs. standard fixed-root props?

In specific, could an existing folding propeller be retrofit by simply having a root which pivots, or would the already-structurally optimized root of a folding propeller make this unworkable?

[*] Examples at What are the advantages/disadvantages of a forward-folding propeller?

Yes, although contest rules to date haven't yet motivated doing both at once.  In F1D international competition, propellers are commonly variable pitch (Indoor Model Airplanes: The Best of Indoor News and Views, ed. Tim Goldstein, pp. 138-144) to better exploit the gradually diminishing power during a flight, and sometimes variable diameter (ibid., pp. 119-126) to reduce drag like a folding propeller would.  Although not a contest winner, R/N Models' "Maverick" kit included a folding propeller purely for drag reduction.

No: if you need variable pitch for efficiency from a variable-power engine, and folding for drag reduction, it's simpler to reduce drag by feathering (changing pitch so far that the propeller won't windmill while unpowered) instead of by folding.

No, for a special case:  For propellers of the size you show, 8 to 16 inches diameter, 300 to 2500 watts electric power, a typical flight profile alternates full power climb with zero power glide (hopefully finding thermals to stay aloft).  Partial throttle only wastes energy.  Proving that mathematically is hard, but it has been verified with numerical simulation.  "Cruising" is best done with a burst of climb (what the propeller is best at) followed by a long glide (what the wing is best at).  At any rate, for a full on / full off power source, a single pitch is optimal.


Why do I hear airplanes' engine sounds in the sky? I've been wondering all my life about an issue related to aviation. I see airplanes, large airplanes they look commercial in nature but I'm not sure, flying in the northern side of where I live and where the sea is located. I don't live next or near an airport, there's only one airport located far away from where I reside, about 20 or more Km. But I always hear airplanes flying, the sound of the engine is like the sound of thunder. Sometimes I hear the sound at night, sometimes I hear it at evening and I always ask myself, why do they fly here and at a very low altitude and the airport is just so far from my location? Though, I see the airplanes themselves, so I'm sure they're flying over the sea I guess.

20 km is not that far for an airliner. At the speeds they move over ground, even in approach phase where they move slower than in cruise, they are few minutes from landing or after taking off.

Depending on prevailing winds, the airport might be using an approach path or another. That means some days you'll see airliners overflying you non-stop, while other days you might not see a single aircraft.


What factors determine the direction of the resultant force vector? The overall aerodynamic force acting on a wing that is creating lift is not straight upward but is inclined backward. We usually consider the vertical and horizontal axes of the force separately. Lift is defined as the portion of the resultant force that is perpendicular to the relative wind, and induced drag is the portion parallel with it. The slope of the angle of the resultant force is the L/D ratio. This ratio changes with angle of attack as shown in the following diagram. As the AoA increases this vector leans further back creating a different L/D ratio (more drag for the same amount of lift).

Then looking from the wing's point of view, the portion of the RF perpendicular to the chord is the normal force (N) and the part parallel to it is the axial force (A). We can see that the the angle of the RF with respect to the wing chord is less affected by changes in AoA.

I've never seen a chart that graphs out the N/A relationship. Does this ratio change at all with AoA or is it a fixed property of the wing design? If it is not fixed, what dynamic factors (speed, AoA, etc.) affect the direction of the resultant force?

Yes, the resulting force is not perpendicular to the wing's chord but can point either way, slightly forward and backward. The very first polar charts which were published show this nicely, because those plots were true polar plots and used the wing chord as the reference orientation for 0°.

Illustration on page 196 of Otto Lilienthal's "Der Vogelflug als Grundlage der Fliegekunst" showing measurements on model wings as polar plots (source). Note the left part: Here the forces are plotted in the wing section's coordinate system.

The N/A relationship was covered by the work of Edward C. Polhamus, who did a lot of research on this at NACA Langley and published several papers with equations for calculating the leading edge thrust, the force which helps to tilt the resultant force vector forward.


If a beginner paraglider / paramotor wing fully stalls, how much vertical distance is needed for recovery? In paramotoring, a beginner wing is a wing with A, B, C, D and F (brake) lines.

When a full stall is induced, a pilot may fall a long, long ways before the wing will recover, and most will need to plan for it.

On a calm day, what is the vertical recovery distance for a beginner wing?

As an example, one could use the Gin Bolero 6

Any extra second in recovery adds up to 60 meters of height. For this reason there is no way to tell how much height the next stall will require.

From the manual of Gin Bolero 6:

Page28, Dynamic Stall: the sink rate in a controlled stall induced by pilot is approx 8m/s.

Page20, Spiral Dive: you need 120-150m clearance to attempt a spiral Dive.

If you took an SIV course and trained in Dynamic Stall, you will find the answer. This is as far as I can go at answering your question. Any further appreciation or direct answering would be a mistake and huge liability.


Why does the 1090 ES use the same frequency for both uplink and downlink? The 1090 ES is the transponder system used in ADS-B.
Why does the 1090 ES use the same frequency (1090MHz) for uplink as well as downlink? 1090 ES is an extension of Mode S. The Mode S used 1030MHz for uplink and 1090MHz for downlink. Why did the 1090 ES change that? Having two frequency channels would be better (More space less noise due to message collisions) for communication right?

It's a broadcast protocol - devices spontaneously announce themselves for listeners to hear. There are no interrogations to put on a different channel.


Why do we calculate the pitching moment coefficient about quarter chord point? Why do we calculate the pitching moment coefficient about quarter chord point, when force and moment system can be transferred to any convenient point?

Because the quarter chord point is the most meaningful point: the wing or surface lift vector can be found here. Like taking the CoG as reference point for gravity forces.

Theoretical and practical analysis have shown that if the moment and force reference point is taken at 25% of Mean Aerodynamic Cord (MAC), the magnitude of the aerodynamic moment remains nearly constant when the angle of attack changes. This location is called the wing's Aerodynamic Centre (AC).

The lift and moment system can be transferred to any convenient point, but that is the point of the quarter chord point :)


What factors should be considered when taking private pilot renter's insurance? I'm just about to start on my first solo and was informed by my CFI that I need to have renter's insurance at a cost of about $180/year.  I looked on AOPA for this insurance and confirmed that the prices is correct if I choose no coverage for "liability for damage to non-owned aircraft".  What are the factors I should consider in making the decision to add/not add this other level of coverage?

Ignoring what you should and should not have in terms of coverage for your self there are some facts to note about getting gap insurance.

You need to speak with your flight school and ask them what they require you to have as every flight school could be insured differently. For example, when I got my renters insurance I had to have 10K of aircraft damage coverage as the flight schools insurance did not kick in until then. Had anything happened I would have been responsible for the 0-10K worth of damage. In this case you would want coverage for damage to the aircraft. If the flight school has insurance on any and all damage you may not need it on your insurance.


Why would a pilot pick this military airport as an emergency landing airport? Looking at this old question, a comment suggested that Skyway Airlines flight 1267 was a possible answer to that question. Out of curiosity, I looked at the NTSB link of the Nov. 1996 incident report and was very surprised to find that this flight was on its way from Milwaukee, WI (KMKE) to Indianapolis, IN (KIND) (a 206 mile flight) but diverted to Fort Campbell, KY (KHOP) which is 191 miles south-southwest of KIND.

On its way from the KIND area to KHOP, the flight missed Cincinnati KCVG (85 miles from KIND) and Louisville KSDF (96 miles from KIND) as well as numerous other smaller airports that should have been able to accommodate a Beech 1900D.

I could understand this choice of destinations if KHOP were a civilian airport that happened to be a hub/maintenance facility for Skyway Airlines, but it's a military base.

Why would a pilot go so far out of his way to land at a military airport when there were numerous other suitable airports that were significantly closer, including, presumably, KIND itself, since the flight had to pass by there to reach KHOP?

Emergency landings often dictate landing at the safest airport, not necessarily the closest one. As I said in the comments, the flight was operating IFR and had turned the window heater on presumably for anti-ice. It could be that a line of weather was preventing VFR operation at closer airports and they diverted to the closest possible VFR airport.

When encountering an emergency there are things you don't want to do. You don't want to panic to get the aircraft down if the aircraft is perfectly flyable. You don't want to make major configuration changes, especially multiple ones, quickly. Pilots are taught to "Aviate, Navigate, Communicate" in that order. As a pilot you should prioritize flying the plane, work the problem, and determine the safest place to put down.

It is possible in this case that they could have continued IFR, but if they encountered icing below the missed approach point they would have been put in a situation with icing and no visual references.

It is also possible that the pilots recognized that some electrical problem was at fault, and even without icing conditions they would want to remain VFR because they didn't trust the electrical system would continue to function throughout the IFR approach and landing.


Is there a system of automated sharing of current (experienced) weather between aircraft in flight? For example, if say - Aircraft 1 is flying a certain route, being followed by Aircraft 2 at an interval of say, 5 minutes (over the same position). Is there a system by which Aircraft 1 can transmit the weather it is experiencing at that moment to Aircraft 2? Something like crowd-sourcing weather from all on-board sensors of aircraft in the air? Akin to ADS-B for weather?

Aircraft to aircraft, there is not.
Ground or satellite to aircraft exists in the form of FIS-B, and other commercial providers.

However, aircraft to ground in an automated fashion exists.
It's called Aircraft Meteorological Data Relay (AMDAR).

[It's] a program initiated by the World Meteorological Organization. AMDAR is used to collect meteorological data worldwide by using commercial aircraft.
Data is collected by the aircraft navigation systems and the onboard standard temperature and static pressure probes. The data is then preprocessed before linking them down to the ground either via VHF communication (ACARS) or via the [Aircraft to Satellite Data Relay].—Wikipedia

So, if it becomes widespread, FIS-B, et al., would benefit from AMDAR if they share the data. Thus realizing the aircraft to aircraft weather relay, in a sense.


Is the turning motion of a banked airplane caused by true centripetal force? Virtually every explanation I have read of the aerodynamics of a turning airplane ascribe the turning motion simply to centripetal force, without further comment.  I'm hoping someone can check my understandings and conclusions whether this is true or not.
As I understand it, centripetal force is defined as the net of the forces on an object that keep it on a circular path.  Put another way, it's the resultant of the force vectors on the object.  Further, centripetal force always acts in a direction orthogonal to the object's direction of motion.
Now, with a banked airplane, the horizontal component of lift, in and of itself, will simply create a lateral force that introduces sideslip. The heading of the airplane must be continuously changed in order to change the direction of that lateral force, and thereby change the force vector from a lateral one to a radial (centripetal) one. This is done independently with the addition of yaw, either coordinated via rudder input, or uncoordinated via weathervaning.
Assuming all this is correct (again please check me), is it mathematically correct to state that true centripetal force is created by the resultant of the force vectors here (the lateral forces adding sideslip and the rotational forces adding yaw)?
Thank you very much for your help.
(EDIT: Through the answers below, I understand now that my third paragraph above is incorrect.  The horizontal lift vector remains perpendicular to the direction of movement (relative wind), not the airplane's heading.  Therefore, while yaw is necessary to maintain coordinated flight, the rotational force vectors behind it are not addends in the centripetal force calculation.)

Centripetal force is defined as the component of the total force acting on the object that is causing it to follow a circular path. It is the force exactly perpendicular to the velocity vector.
To create a circular path, the force needs to turn with the velocity. If you have a force that keeps its spatial orientation, less of it will be centripetal and more of it will just accelerate the object as it moves and the resulting path will be parabolic (e.g. the ballistic curve under gravity).
The horizontal component of lift always contributes wholly to the centripetal force, but this is again quirk of the definitions: lift is the component of the aerodynamic force on the wing that is perpendicular to the relative wind and drag is the component that is parallel to it. So analysing the situation in the reference frame of the air mass, relative wind is the velocity vector.
Now if the plane didn't turn with its velocity vector, it would just gain side-slip, the lift would decrease—banking into side-slip reduces the angle of attack—and the aircraft would stop turning—it's flight path does turn initially, even if the body does not—and settle in sideways flight. So we need to make the aircraft rotate with the turn.
Now just like linear motion, rotating motion stays constant¹ unless acted upon by a moment of force (a.k.a torque). So when initiating the turn, a yawing moment of force needs to be created to also initiate the rotation.
A moment of force may be created by a pair of force of opposite direction and different action lines, so it does not have to cause any linear acceleration, but in case of aircraft usually an initially unbalanced force is created on the tail that starts the rotation, but also accelerates the aircraft sideways until balanced by drag on the fuselage. Note that this force is out of the turn (and up), so it reduces the centripetal force a bit.
But in a steady turn the torque is zero. Or oscillating around zero as the correct angular velocity is maintained via feedback loop. So the horizontal component of lift is the only centripetal force in a coordinated turn. In not coordinated turn, some side force on the fuselage is added or subtracted depending on whether it is a slip or skid.

¹ Angular momentum is conserved but the angular velocity does not have to be. Depending on the axis of rotation and mass distribution (moment of inertia) you may get strange phenomena like the Dzhanibekov effect.


How does the landing gear damp vibrations? How does a landing gear on an aircraft reduce the vibrations transferred to the airplane structure, while landing?

By having a spring element and a damping element - some small planes use use leaf or tubular legs bending to act as springs, other use rubber bungy cords, others use, like bigger aircraft commonly do, a gas strut damped by liquid viscosity. The damping in the puddle jumpers is the scrub of the tyres moving laterally on the runway as the suspension moves up and down. Shimmy damping in the nose gears can be either by elements providing viscous damping or coulomb/friction damping.


Is ATC communication subject to FCC profanity regulations? I've never had instance to swear or use profane terminology on ATC frequencies but I've always wondered if the same FCC restrictions which apply to terrestrial entertainment radio stations are in effect for air traffic as well.  I'm guessing that yes, they do and you can run afoul with the FCC over the use of bad language over aviation VHF frequencies?  Does anyone else know about this?

18 USC 1464 seems to spell out a criminal act, but 'any obscene, indecent, or profane thing' is fairly vague language and subject to quite a bit of interpretation.  Is this to be cross referenced against, say, the indecency lines in the Supreme Court case FCC vs Pacifica Foundation in respect to George Carlin's 'Seven Dirty Words'?

Some examples: in the film Threshold: The Blue Angels Experience, an opposing Solo makes the following radio comment while performing the Dirty Loop:

"That's very interesting, boss.  I just got down to 40 knots on the top of that son of a bitch."

And then there are these two potty mouths who work (or used to work) for Southwest Airlines making some less than flattering comments about their coworkers on a hot mike.

Both seem to violate the terms of 18 USC 1464.  Is one more deserving of prosecution than the other?

I'm not sure if FCC broadcast regulations specifically apply to airband VHF: It's not really a "broadcast station" in the sense of FCC regulations, but rather a wireless service (RT) on spectrum delegated to another agency. The FAA handles regulation of the service, the FCC just gave them a particular chunk of the VHF spectrum to use for it.

Obscene or profane transmissions are generally frowned upon by both pilots and controllers though, and the FAA specifically prohibits "obscene, indecent,or profane language" on their communication services (JO 72103Z) - Controllers can face disciplinary action over it, and I imagine they could find something in the regulations to charge a pilot with if they wanted to.


How high can manned subsonic planes go? What is the maximum altitude a manned plane that flies subsonically can reach? Subsonic airplanes can't go as high as super- and hypersonic ones of course, so record holders are the latter ones. But ironically, the altitude record for leveled flight is set by the subsonic NASA Helios propeller aircraft at ~97,000 ft. However that's an uncrewed and weird-looking one. Can manned airplanes that fly subsonically go above the Armstrong line (60-63,000 ft) and higher (either in leveled or in parabolic flight), considering realistic fuel limitations? How high can subsonic airplanes go compared to supersonic ones?

76,000 feet.  Airbus Perlan Mission II glider, Sept 2, 2018.
The U-2 has a higher service ceiling, 80,000 feet, but it hasn't reached that.
(Perlan II's service ceiling is 90,000 feet.)


What are the pros and cons of high-wing compared to low-wing design? What are the advantages and disadvantages of a high-wing vs. low-wing aircraft design? When might one prefer one to the other?

Is the answer the same for large and small aircraft?

This is actually a really hard question to answer, as there are a ton of factors. I'll try to cover a few.

Visibility is probably the reason you most hear when it comes to GA pilots.

High-wing aircraft offer better visibility below the aircraft, especially for passengers in 4-seat or larger aircraft, as the wing doesn't block it.
Low-wing aircraft can offer better visibility above the aircraft, as the wing remains mostly out of the field of view.
Note that a lot of midair collisions involve mixed types; low-wing aircraft descend into high-wing, or high-wing aircraft climb into low-wing.

Landing characteristics are different, as low-wing aircraft can incur more ground effect than high-wing.
Aerodynamic stability of the two designs differs a bit; the center of lift can be higher in a high-wing design, often offset with dihedral in low-wing aircraft.
Ground clearance (and debris protection), specifically in multi-engine aircraft, is going to be greater when engines are mounted on a high wing rather than a low wing. This is true of both propeller and jet engines, although propellers tend to have a larger radius for a given aircraft size.

There are a bunch more, but that ought to get the argument discussion going.


Why "Throttle...Open 1/2 travel" on engine troubleshoot checklist? Why do some engine troubleshoot checklists (on small single engine piston aircraft) call for throttle to only be open 1/2 travel? I suspect it has something to do with modern fuel injected engines, but haven't found anything explaining why half throttle, instead of full throttle.
Below is an example, however, I saw at least one other checklist online for another single engine piston (non-Cessna) that had similar "1/2 open" throttle language.

Revised following update to OP:
If the engine quits and you're gliding along with it windmilling and the throttle pulled back to idle, if it comes to life at some point, you may not even be aware of it because you can barely tell the difference between off and idling when it's windmilling in a glide.  On the other hand, pushing it full forward isn't such a good idea either.
You want it set to a power setting that makes it obvious it's come back to life, but you don't want to set it to full throttle because you don't want it going full bore instantly, which is generally not helpful to the engine because the oil pump may only be making 20 or 30 psi while windmilling, or none at all if it stops windmilling, and full power before the oil pressure can catch up is murder on the plain bearings of the crank journals that depend on oil pressure to keep crank journal and bearing separated from physical contact.
So half throttle makes a nice compromise.


Which are the requirements to fly a N-registered aircraft operated by a non-US airline? There are several non-US airlines which operate N-registered aircraft for domestic and international flights.

Let's say for instance Avianca, which has N-registered aircraft but is a non-US airline (flag carrier of Colombia with headquarters in Bogotá and main hub at El Dorado International Airport). What are the pilot certificates required to fly those aircraft? FAA or Colombian pilot certificates?

There are a few aspects to this. First off all N-Registered aircraft must be owned and operated by a US based entity. As has been mentioned this could be any US based company or US resident. In reality foreign companies can set up US based branches or entities fairly easily under which to own the aircraft. The aircraft must also comply with all FAA regulations for maintaining the N-Number (annual inspections etc.) This means that the aircraft must be inspected at an FAA approved/licensed facility (they do exist outside the US). The planes can (if practical) be flown back into the US for inspections if no foreign facility is available.

When it comes to flying the plane the pilot must carry what ever is locally required to fly in and out of that country/airspace both from an equipment standpoint and a pilot standpoint. Many countries will accept/recognize different pilots certificates for inbound international traffic, that is the subject of its own question as there are many combinations out there. For what its worth the US FAA certificate and the ICAO/EASA certificate are pretty highly recognized around the world.


Is inverted flight possible under these conditions? My understanding of flight is that the primary aspect is the angle of deflection, and the purposes of the shape of the wing is to maximise efficiency. Could a commercial airliner with asymmetric aerofoil sustain inverted flight by keeping its nose up, or something similar?

For the purposes of the question, ignore issues with the fuel or engine, and assume that the wing shape is fixed and not specifically built to accommodate inverted flight, including not having flaps or anything in the aileron department.

Yes, based purely on the wing shape an airliner could fly upside down and maintain a straight inverted flight. The lift of the wing is a function of the vertical angle of the airflow, the angle of attack.

Image source

The picture shows an airfoil pointed upwards, but the same forces apply when the wing nose point downwards. Then turn the picture upside down, and you have lift pointing upwards again.

The situation is valid for an asymmetric airfoil as well, the only difference being that it provides lift at AoA zero. Zero lift is provided at a slight negative angle of attack, so the aircraft nose will be further up when inverted, still below the stall angle though.


How could I find  lift coefficient slope? I'm trying to find  lift coefficient slope through the below diagram, could anyone explain for me please, how could I find it?

by the way, I depend on this equation  =  (−0)


Where can I find the turn radius on the ground for the Piper Seminole? I meant to be doing a quiz for a class and one of the questions is asking for the turn radius of the Piper Seminole. However I cant find it anywhere in the POH.

Do you know where I can look?

33.0 ft From pivot point to wingtip, it is in the POH page 1-2


Why is autolanding ILS a thing, but not autotakeoffing ITS? In this answer to the question of why zero-visibility landings are legal and routine but not zero-visibility takeoffs, the poster makes the following point:

When an aircraft lands in near zero visibility, it is a fully automated process that is actually done without input from the pilot. The pilots are only needed to taxi off the runway and park the aircraft. It is called "Autoland" and many of today's aircraft have it.
As of today no manufacturer has designed an aircraft with "Auto Takeoff" but I suppose it is not impossible.

(Emphasis added.)
Which seems odd, given that autolanding with an ILS is a mature technique using mature technology, and autotakeoffing with an ITS (instrument takeoff system) wouldn’t need much more on top of that (just some additional antennae on the aircraft’s tail and a couple new routines for the autopilot) - it’d mostly be pretty much an ILS running in reverse, something like this:

What’s the dealbreaker for an autotakeoff-supporting instrument takeoff system?

Autoland exists at least in part because a pilot can't safely hit (the right part of) the runway (at the right speed and attitude) without visual references, so a lot of precision electronic guidance equipment is needed, and then even more equipment to ensure the first equipment is working correctly.

In contrast, the sky is much bigger than the runway. It's comparatively easy for the pilot to hit the sky even without visual references (provided vis is good enough to taxi out). So none of that fancy stuff is necessary.


Why do some aircraft carriers have an upward starting runway? I noticed that some aircraft carriers have an upward starting runway while others don't. What's the reason for this? What are the advantages of both designs and how come not one of them has prevailed over time?

The upward slope on the runway is also known as a "ski jump", and it is to give VSTOL (Vertical or Short Take Off and Landing) aircraft get aloft on the short carrier deck. Carriers without this use aircraft catapults to quickly get airplanes up to flying speed.

The advantages of the ski-jump are its simplicity, which means the carrier has lower build and operating costs. The drawbacks are that VSTOL airplanes are generally more limited in their capabilities, the airplanes that are launched by catapults pack a lot more punch.


Why were these Hawkeyes flying in a circular pattern? A couple of months ago I was near the Wallops Flight Facility in Virginia, when I saw 3 or 4 planes flying in a tight circular pattern. The planes were on the larger side, prop driven and had large radar dishes mounted to the top of the fuselage. Based on that, I'm guessing they were E-2 Hawkeyes of some variant. I don't remember the livery, unfortunately.

The planes were flying roughly 200ft above ground level. They were all on the same rough flight path, and were circling. The circle was probably 0.5 to 1 miles in diameter. The flight was right over the eastern edge of the base. I didn't see the planes take off or land, but in the time I was watching, they probably flew 4 revolutions.

At the time I was guessing that the planes were flying like this because they were doing some sort of weather detection, but I was wondering if any of you could provide me with a more definite answer. So my question is:

What was the purpose of having those planes circle the area?

They were most probably training touch-and-go passes. USN conducts carrier landing practice at NASA's Wallops Flight Facility for E-2 Hawkeyes and C-2 Greyhounds.

As Lt. Commander Mike Ferrara, an operations officer with the Navy's Carrier Airborne Early Warning Squadron 120 (VAW-120) explains in this article:

  These two aircraft are very difficult aircraft to fly — that's why, when we're up here, you'll probably see us going around for three hours at a time, take a break, and go for three more hours ...