Have there been any attempts to standardise pay across different aircraft types? TL;DR: I'm hypothesising the cockpit experience and pay in small aircraft vs larger aircraft is disproportionate to the risk and responsibility.

I read the argument that pilot pay should be equal across all types, irrespective of it is an A320, A330 or A340 for instance.

The smaller aircraft have greater number of rotations every day, leading to greater workload and hence the pilot responsibility could be argued to be equal to that of a larger aircraft which spends (a lot) more time just cruising.
The bigger aircraft often operate to better, more equipped airports than their smaller counterparts. This again may put more load and responsibility on the pilots.
The bigger aircraft, while in some cases more complicated, may have more automated systems and assistance rather than greater pilot workload, although this of course depends on the aircraft.

I'm wondering if anybody (airline, union etc.) has done this or is trying to do implement it. It would be mighty unpopular among some pilots but I think there is a little bit of sense to the argument.

Why is this a problem?

It also causes the least (or less) experienced pilots to command the small aircraft, rather than perhaps a more sensible mix of age and experience.

Paying additionally by the years of experience makes sense to me, but not this push to constantly advance to get a higher salary.

As it stands, the only way to make more (aside from years of experience) is to upgrade to a larger aircraft type. Aside from requiring retraining (expensive type rating), it puts the pilot into a new aircraft (and possibly airports) he is unfamiliar with.

Jumbos and widebodies are harder to fly than amateur pilots often think. The larger and more complex the aircraft, the further ahead in time the pilot has to be thinking in order to get the plane where he wants it in the configuration he wants it. This increases pilot workload even when "just cruising".

There is a natural progression from smaller to larger planes and fewer to more passengers through all levels of pilot training and certification. You don't put a student pilot in an A380 with 500 passengers aboard for their first solo flight. You give them an LSA costing $25-50k and don't allow them to carry any passenger besides their instructor. As the pilot gains skill and experience, they transition to larger and more powerful aircraft and gain more privileges and responsibilities. Parallels abound; knowing how to drive a 19-foot speedboat on a lake is no qualification to be at the helm of the Carnival Magic, or the U.S.S. Ronald Reagan. Knowing how to drive a go-cart in a parking lot is no qualification to be behind the wheel of a road train.
Case in point, someone who just got their ATPL/AMEL likely has had zero stick time in an actual airliner; they're simply too big and expensive to run for flight schools to be using them for flight training. What they've probably logged actual flight time in for an AMEL category rating is closer to a Gulfstream or Learjet; a high-performance narrow-body T-tail. That's not really going to cut it for a 737, but that experience transfers pretty seamlessly to a regional narrow-body airliner like a Bombardier or Embraer; slightly bigger and heavier but not totally out of the ballpark in configuration and performance envelope. Establish a good track record in those and you work your way to the 100-seaters like the E195, and from there you can continue to work your way up as your experience and mentality allow you to handle the longer lead times and increased complexities of bigger jets.

Smaller flights tend to bring in less revenue and lower margins, primarily due to increased competition on those routes. Yes, the seats turn over more quickly as the smaller jets make shorter hops, but do the math; DFW-PHX costs about \$320 a seat, you have about 130 seats on a 737 or A320 running that route, and you can run it about 3 flights between those two airports gate-to-gate in 10 hours. Total revenue, somewhere in the ballpark of \$125,000 a day. A ticket from DFW to LHR costs about \$2200, it's run with a 777-300ER which carries about 320 people per flight, and you run that once in the same 10 hours. Total revenue for a full flight, about \$700,000. Even given that the DFW-LHR flight will probably require two crews and the fuel costs per passenger mile will be higher, you're still making more money off the larger plane, provided you can fill it.
Now, this very same cost breakdown doesn't seem to hold at first for a smaller regional jet. Waco to DFW is an hour gate-to-gate and costs ~\$325 a ticket. In the same ten-hour flight time of the DFW-LHR flight, a 50-seat ERJ145 could theoretically bank up to \$162,500, more than the DFW-PHX flight. But, nobody's paying \$325 to fly from Waco to DFW unless they're "stupid rich"; you can drive between those cities in an hour and a half for the cost of about 5 gallons of gas, and people know that. They also know that if they're going to DFW to catch another flight for a week or ten-day trip, the parking fees will eat them alive. A more realistic scenario is ACT-PHX via DFW. That ticket costs \$360. Recall that DFW-PHX by itself was \$320, so your passengers are paying more like \$40/seat for the regional connector, and at that rate 10 trips with a 50-place jet would bring in a scant \$20,000/day, which might barely break even on fuel and fees to operate that route.

Older pilots want more pay. As in many industries, workers expect their experience to count for something on their pay stubs. From simple cost-of-living raises to the money needed to raise a family and eventually retire, airline senior captains often end their careers in the low to mid six figures. They also start lower on the pay scale and so get less of a head start building a home and retirement; regional jet operators start around $22,000 which is just above poverty line (which is part of the complaint; a pilot's starting salary is not a living wage especially considering what it takes just to be able to apply for the job). Given the costs of flight training, and the starting salaries of other skilled trades, learning to fly for a living is not the obvious choice. So the ones that stick with it and work their way to senior captain expect to make up the difference at the end of their careers.

Given all of these points, it makes sense to put junior pilots making less money in smaller airliners, and increase the airframe size and the pay as a pilot advances through their career.


Do all the engines/propellers of a VTOL UAV need to be identical? I would like to know if VTOL UAVs necessarily need $n$ identical propellers and motors. Have any design solutions considered the possibility of using non identical props? For instance 2 motors of type A, 2 motors of type B, and 2 motors of type C. If so, where can I find some studies or some designs that have attempted this kind of solution?

MIT CSAIL has researched apparently quite thoroughly on this and the end result it would seem is that you can have ANY different size of motors be able to fly given that it satisfies certain conditions such as thrustep etc. For this I would recommend following developments in two areas (as far as my knowledge goes)
First of ckurse MIT CSAIL's video demo of their custom software to simulate all combos of motor thrust and positioning

The next 'guy' you want to follow is ofcourse
Raffaello D'Andrea  and his research team. His TED talk explores drones at mechanical disadvantages, here you go

https://www.ted.com/talks/raffaello_d_andrea_meet_the_dazzling_flying_machines_of_the_future

That's as far as my enthusiast knowledge could help


Does the air resistance of a jet engine air intake count towards the jet's total air resistance? Does a jet engines air intake count to the overall jets geometry dependent air resistance?

Taking the Saab Tunnan as an example it's overall air resistance has to be quite low because of it's huge air inlet.

Or does the inlet create air resistance as well but just not so much?

This is a very tricky topic. Basically, you need a clear definition of what is part of the engine-related drag and what is counted towards airframe drag. Splitting up drag between components will always produce an arbitrary result.

An intake can generate thrust as well as drag. In case of the SR-71 at Mach 3, the intake contributed 54% of overall thrust, if you just look at the pressure distribution over the intake walls.

Generally, the job of the intake is to feed air to the engine compressor at just the right speed and as homogeneously as possible. This requires it to suck in air when at rest from all around the aircraft, and to ingest only the core of the stream of air flowing towards it at high speed. What is not ingested needs to flow around the intake lips and the engine fairing, and the drag caused by this process even has its own name, being called spill drag.

At high speed the intake needs to slow down the oncoming air and does so already ahead of the intake lip at subsonic speed. At supersonic speed this slowing down is accomplished by shocks, which adds another source of drag. This can be easily counted as part of the airframe drag, but done right this slowing down will compress the air, which in turn will increase thrust. Apart from viscous losses, this compression pays for itself and is almost drag-neutral in a good design and at ideal conditions.

Next is the flow around the intake. Just throttling the engine will change the airflow through it and thus the flow pattern around the intake. You can imagine that some configurations show marked changes in aerodynamic drag depending on the throttle setting. In case of the Eurofighter, the vicinity of the canard will even cause throttle changes to result in notable pitch moment changes.

To answer your question: Yes, the intake will produce drag, and how much it does depends on how you define drag. It is certainly wrong to think that the stream tube hitting the intake will just disappear into it and not add any drag.


For a coaxial rotorcraft, Do horizontal and vertical stabilizers in the tail really make much of a difference? Can we design a coaxial rotorcraft without tail and tail boom completely? What might be the consequences?

The horizontal tail surface is there to generate downforce that holds the nose up in cruise. Without it, the faster you go, the more the nose points down as the machine's body wants to "trail" so to speak behind the tilted rotor disc.
The vertical tails are to provide a weathervaning effect to make the machine passively point ahead in forward flight without requiring a torque control input.
Take those surfaces away, and you are stuck with pointing down at an unpleasant angle while cruising, and having to constantly work the pedals to keep it pointed straight. A fly by wire yaw stabilization system working the pedals in the background could probably take care of the yaw part, but I don't know how you could deal with the pitch part without a stabilizer surface.


When must a pilot report Negative Contact? Which regulation or AIM section specifically states when a pilot is required to report negative contact?  It appears that different airports in the US have slightly different expectations.
My understanding is that ATC is only relieved of separation requirements when the pilot says "Traffic in Sight". However, if ATC says "Cessna 123, Number 2 following traffic on downwind", if pilot says "Number 2 following traffic on Downwind, Cessna 123", that is the same as saying Negative Contact. In other words, unless the pilot says the magic words "Traffic in Sight", ATC treats anything else as negative contact. Until the pilot reports traffic in sight, or ATC says "Report Traffic in Sight", it seems like unecessary radio traffic for pilot to report negative contact, unless specifically asked by ATC.
Also, what do tower controllers think about this?

The short answer is that in your example there's no reason to assume that the controller was asking the pilot to maintain visual separation. If the controller had wanted the pilot to do that, they would have asked if the pilot had traffic in sight and the pilot would be expected to answer the question.
The ATC orders have an entire section on visual separation (section 7-2-1) with lots of details. There are two types: ATC-applied and pilot-applied, depending on who's actually responsible for maintaining the separation. I think you're asking about pilot-applied separation, and there's really nothing in the dialog you quoted that suggests the controller intended the pilot to maintain their own separation. In fact, it's possible that the controller was maintaining visual separation themselves based on seeing both aircraft, and that doesn't require the pilot to have the other aircraft in sight.
If ATC assigns pilot-applied separation, they need to hear two things from the pilot: they have the traffic in sight, and they will maintain visual separation. See 7-2-1(a)(2)(c):

If the pilot reports the traffic in sight and will maintain visual
separation from it (the pilot must state both), the controller may
“approve” the operation

The Orders have an example and it's laid out as a three-step process: first the controller tells the pilot where the other aircraft is, then they ask the pilot to confirm traffic in sight, and finally if the answer is positive they ask the pilot to maintain visual separation. Since ATC is asking a question, the pilot has to answer one way or another, whether that's with "traffic in sight" or "negative contact".
Coming back to the example you gave, the controller simply told the pilot that they were number 2 behind other traffic. The controller didn't ask if the pilot had traffic in sight and while the pilot might volunteer that information, the controller didn't need it at that point in time (they might need it later if the aircraft are getting too close, or something else changes).
Long story short, if ATC needs to know if you have the traffic in sight then they'll ask you.


Can an instructor sign off for my check ride if I didn't fly with him since a year? I am at the end of my training as a student pilot with an overkill amount of flying hours, but I had a couple of complications in the way.

I flew with my old instructor (instructor A) for about 3 years, and he signed me off for everything but the check ride.
When it was time for me to go to college, I had to find a new instructor (instructor B) and we trained for about a year until he decided it was about time I go for the check ride. Sadly our plane had engine issues and I had to cancel my check ride.
So I had to go back home to my old instructor (instructor A) who I haven't flown with for about a year. The last time we flew, he also said I was ready for the check ride, but that was about one year ago.

My question is this: Can my hometown instructor (instructor A) sign my off for my check ride even though I haven't flown with him in a year?

I stayed current with my flying and I even flew the same type of plane (One was a Cessna 172n and the other was a Cessna 172m).

No.  Instructor A needs to provide at least three hours of training in preparation of the practical test within two calendar months of the checkride.

  §61.39   Prerequisites for practical tests.

  (6) Have an endorsement, if required by this part, in the applicant's logbook or training record that has been signed by an authorized instructor who certifies that the applicant—

    (i) Has received and logged training time within 2 calendar months preceding the month of application in preparation for the practical test;

    (ii) Is prepared for the required practical test; and

    (iii) Has demonstrated satisfactory knowledge of the subject areas in which the applicant was deficient on the airman knowledge test;

  §61.109   Aeronautical experience.

  (a)  For an airplane single-engine rating...

    (4) 3 hours of flight training with an authorized instructor in a single-engine airplane in preparation for the practical test, which must have been performed within the preceding 2 calendar months from the month of the test

  §61.103   Eligibility requirements: General.

    (f) Receive flight training and a logbook endorsement from an authorized instructor who:

    (1) Conducted the training in the areas of operation listed in §61.107(b) of this part that apply to the aircraft rating sought; and

    (2) Certified that the person is prepared for the required practical test.

Instructor A has to provide the required training to the applicant before signing him off for the practical test.  I would agree it doesn't have to be within two calendar months... but three hours are required from an authorized instructor with two calendar months.  However, I would not, under any circumstance, sign off an applicant without at least three hours of training within the preceeding two calendar months.  How else would I know they are prepared for the practical test?


What are the dynamic stability properties of an aircraft with wings exactly at the vertical CoG? Please ignore for a moment the practical issues with building and operating an aircraft like this.

Suppose a fixed-wing aircraft which, when viewed directly along the roll axis, has the wings centered exactly at the vertical center of gravity. Suppose that this aircraft is in stable, level flight in otherwise still air. (Yes, this is a little like the spherical cow in a vacuum from physics.)

If there is a disturbance, such as a localized updraft or downdraft affecting one of the wings more than the other, then broadly speaking and as I understand it so far (feel free to address this if I've got it completely backwards), after exiting the disturbance and with no further control inputs, a high-winged aircraft will tend to return to neutral roll (dynamically stable) and a low-winged aircraft will tend to increase its roll angle further (dynamically unstable), as a result of the differences in localized airflow above and below the respective wings.

However, what would be the behavior in such a situation of our hypothetical mid-winged aircraft that I described above?

My hypothesis is that the aircraft would maintain its roll or bank angle, which (unless compensated for, say by rudder input) in turn will result in a rotation along the yaw axis and a subsequent turn.

Am I right or wrong? Why?

It is a common misconception that the vertical CG position (relative to the wing) plays a major role in roll stability. It doesn't; it's mostly about aerodynamics. (You mention it yourself in "as a result of the differences in localized airflow...")

There are other important factors at play as well: dihedral angle, wing sweep, design of the vertical stabiliser.

Presumably, you are interested in the behaviour of an aircraft with the perfectly neutral roll stability. If so, it needs at least these conditions:

General symmetry about the horizontal plane, which includes:

Zero wing dihedral;
Symmetric (e.g. round) fuselage;
Wing at the centre of fuselage (vertically);
Symmetric vertical stabiliser (e.g. with dorsal fin);
Absolute rigidity so that the symmetry remains under load.

Zero wing sweep (straight wing);
Lack of any artificial stability augmentation.

(Technically speaking, the geometric symmetry is not necessarily required: we can compensate one effect with another, say, wing sweep with anhedral. But in the spirit of the original question, let's assume the full symmetry).

We should also assume a reasonable stability in pure yaw.

So, if you induce a pure roll disturbance and then remove it,

the aircraft will start rolling.

Because it has some inertia about the roll axis, it will not simply stop when you remove the disturbance. The only force that will counteract the roll now is roll damping. It naturally occurs because the descending wing has higher angle of attack (and vice versa). The strength of the effect depends on the square of the wingspan, but in theory the spherical cow will keep turning forever, albeit with ever-diminishing speed.

So far, nothing would induce a yaw, and the airflow would still be symmetric.

However, as roll develops, and the aircraft keeps the same pitch balance, the lift will become insufficient to keep it level. The usual thing happens:

the aircraft enters a spiral dive.

Now, this is not a symmetric situation anymore. There is a side force from gravity; sideslip will develop; the aircraft will yaw due to its yaw stability that we assumed. Things start to get complicated (that is, 'normal'), with many variables involved. Even in the vertical plane, despite postulated symmetry, the shadowing and other cross-flow effects on the top and bottom of the wing will be different (the wing, by definition, cannot be aerodynamically symmetric when it generates lift), and thus some roll effect may be observed.

So it's hard to predict further behaviour in general. At one extreme, with stubby wings and slender body and very strong initial disturbance, the aircraft may barrel roll many times and may even enter a peculiar stable roll motion due to inertia coupling. At the other, the aircraft will just roll a little and will enter a gentle (nearly) coordinated turn with very slight descent.


Why is the pilot in this video retracting the spoilers so shortly after landing? I watched this video (link with time) and wondered why the pilot has retracted the spoilers so shortly after landing.

Because the runway is wet and it looks like the plane is still quite fast even in slow motion. Wouldn't it be better to have more braking grip?

The pilot doesn't retract the spoilers until the airplane has slowed down considerably, at the speed it's traveling at that point the spoilers would be adding no benefits so there's no point keeping them deployed.


Did this aircraft illegally exceed 250kts below 10,000ft? As far as my knowledge goes: There is a 250 kt speed limit under the altitude of 10.000 feet. This screenshot seem to show an aircraft below 10.000 feet and traveling at 285 kts at the time i captured the screenshot.

I don't think this aircraft posed any threats to other aviation traffic as there is no other traffic in the airspace. I was just curious.

EDIT:

Shortly after: I see this aircraft taking off from the same airport. Doing the same thing as the first plane.

First of all, this aircraft was flying in Europe, not in FAA land. In Europe there is no general limitation to 250 knots below 10000 ft. There is no EASA airspace restrictions, every country has their own set of rules. There are airspaces that have speed restrictions below 10000 ft, others don't and sometimes aircraft are allowed to go faster than 250 knots with ATC permission.

Secondly, the 250 knots is an indicated airspeed limitation. At 10000 ft, 250 KIAS (Knots Indicated Air Speed) will be approximately 289 KTAS (Knots True Air Speed). And then there is the additional effect of wind. With a strong tailwind you can exceed 350 knots ground speed while still flying below 250 KIAS.

Thirdly, the data in the image you linked is provided by ADS-B. ADS-B can transmit either ground speed or air speed. Under the European ADS-B mandate, aircraft must transmit ground speed, not air speed. But since this mandate will not affect this particular aircraft until December 7, 2017 it might very well be transmitting True Air Speed.

Note that ADS-B data is not purely derived from GPS. The Mode S transponder which provides the ADS-B out functionality on this aircraft is attached to various data channels and is getting Indicated Airspeed, barometric vertical rate. In fact ADS-B vertical rate is preferably from a barometric source.


Does the headwind component change noticeably when transitioning from a crab to a slip on short final? I don't know if I'm overthinking this, but it has been causing a bit of confusion in my head.

Knowing that the headwind component affects the angle of climb/descent, I was wondering how it affects landing when transitioning from a crab to slip technique?

During a crab the nose is facing more into the wind, but when kicking the nose out to align with the runway, I am trying to imagine if that causes a slight loss of lift or a sudden decrease in descent angle? As there would be more of a crosswind component in relation to the direction the wings are facing?

I might be missing something here, but it seems important to know the physics of this when you're only 100 or 200ft above the runway!

Let's start by looking at a pure "kick out the crab" technique, where we don't lower the upwind wingtip at all.

Even though we don't usually use the word "sideslip" when we talk about "kicking out the crab", we do in fact cause a sideslip when we press that downwind rudder pedal, even if we don't lower the upwind wing.

When you kick out the crab you force the aircraft to slip (or we could say skid) sideways through the air.  A yaw string on the front of the windscreen would blow sideways, in the downwind direction (if there were no propwash).  This is not an efficient way to fly.  Initially, the slip angle is fully as large as the crab angle you just kicked out.  As the flight path starts to curve (because you didn't lower one wing to stabilize the situation)-- i.e. as the aircraft starts to "accumulate drift" (which in reality is actually a turn -- a curve in the flight path--a rudder-commanded "flat turn" or skidding turn)-- things change, but that's the initial situation.  So the more crab you kick out, the more the aircraft is forced to fly sideways.

For any given approach airspeed and any given crosswind component, as headwind increases, so will the required crab angle, so when you "kick out the crab", you'll have more sideways flow over the aircraft, and the sink rate will increase.  You may also notice an increased tendency to roll toward the downwind wingtip, due to slip-roll coupling.

It may help clarify things to understand that "kick" is a little bit of a misnomer.  After the "kick", if touchdown is delayed, to keep the plane aligned with the runway, you have to keep HOLDING some rudder pressure.  If you released it, the plane would tend to swing back into line with the original flight path-- the nose would swing back toward the original heading.  After a few seconds though, the plane has "accumulated enough drift"-- i.e. the flight path has TURNED or curved enough-- that the aircraft heading will be aligned with the runway even with no rudder pressure. You do not want to be in this situation, because it means you have TURNED your flight path through the airmass so much that it is fully aligned with the runway heading, and you have TURNED your ground track toward the downwind edge of the runway.  You'll soon be in the weeds.  That's the trouble with the pure "kick out the crab" technique-- if touchdown is delayed, you have a problem.

Up till now this answer addressed a pure "kick out the crab" technique, but you did in fact mention sideslip in your question.  If at the same time you give the downwind rudder input, you also lower the upwind wing, you'll have the same initial slip angle as described above, and a headwind will affect the required slip angle-- and the resulting sink rate-- in the same way as described above.  But if you lower the upwind wing to stop the rudder-commanded turn, then the situation will be stabilized and you'll see that same slip angle (and that same sink rate) all the way to the ground -- assuming of course that the crosswind component and the headwind component are both constant all the way to the ground, which may very well not be true in reality.

(We're ignoring here the effect that banking has on sink rate, which is real but but probably quite a bit smaller than the effect of flying sideways through the air (slipping/skidding) due to the rudder deflection.)

At any rate, whichever technique you use, for a given crosswind component and a given approach speed, more headwind = a larger crosswind component = a larger slip angle = a higher sink rate.

If you are crabbing along on final approach with NO wing-down correction, and you haven't yet "kicked out" any of the crab angle, then the plane doesn't "know" anything about the wind at all, for a given power setting and angle-of-attack.  It's only after you "kick out the crab", or after you initiate a wing-down cross-controlled correction, that the strength of the headwind matters, because of the way you must adjust your control inputs to match the required slip angle, which is affected by the strength of the headwind.

In actual practice, with the cross-controlled wing-down technique, it seems to work well to imagine that the lowered upwind wing is somehow "opposing the wind" and "stopping the downwind drift", while the rudder is controlling the heading.  The real truth though is that what the lowered upwind wing is really doing, is stopping the rudder-commanded flat (skidding) turn that would otherwise take place if we "kicked out the crab" and didn't touch down immediately after.


How are the in-flight meals cooked? When I traveled on a commercial aircraft, I had a meal which came in a small box, I was wondering if the meal is precooked and then re-heated or if it is cooked on the plane?

In general, food for commercial coach passengers is made on the ground and then stored on the plane in special refridgerators, until it is time to eat.
The food is then heated in convection ovens and served. Sometimes there are special occasions for higher class seats/airliners.

Safety standards and space constraints mean your food’s made on the ground, near the airport. Big boys like LSG Sky Chefs produce 15,000 bread rolls every hour (24hrs a day, 365 days/yr), and 30,000 sandwiches a day.
As a rule of thumb, food is prepared 10hrs before it gets eaten. Those “Made Fresh!” stickers are there to mock you. It’s not ready-to-eat by any means at this stage; generally, chicken is cooked 60% of the way, and steak 30% to done, with the final phase occurring onboard. Once cooked, it gets blast-chilled in special fridges -- in a not-quite-frozen-but-not-edible state, your food awaits transfer to the tarmac.
Flight on time? Food on the plane? Time to finish the prep-work. Contrary to what most people think, planes generally don’t have microwaves. Your little tray makes its way into a convection oven for about 20min. Convection ovens have a fan (which pushes hot air onto the food) that's both faster and means a lower cooking temp.

https://www.thrillist.com/travel/nation/airline-meals-how-your-airplane-food-is-made-thrillist-nation


How to obtain true altitude from indicated altitude, temperature and QNH? maybe it's a little impolite to ask but I got stuck with knowing bunch of formulas and not understanding how to use it:
Can you help me on understanding how the correct answer 7300ft came out of the following question:
Given the following information, what is the true altitude? (rounded to the nearest 50 ft) QNH: 983 hPa;  Altitude: FL 85; Outside Air Temperature: ISA - 10° (1,00 P.)

My attempt based on the fomula from here:

$A_T$ is our estimated true altitude, in feet
$\Delta_{ISA}$ is the difference above ISA, in °C
$A_I$ is our indicated altitude, in feet
$A_{Ik}$ is our indicated altitude, in thousands of feet

$A_T = (4\times A_{Ik} \times \Delta_{ISA}) + A_I$

$A_{Ik} = 8.5$
$\Delta_{ISA} = -10° - ( 15° - (8.5 \times 2) ) = -10° - ( 15° - 17°) = -10° - (-2°) = ISA-8$
$A_i = 8,500$

Therefore,
$A_t = ( 4 \times 8.5 \times (-8) ) + 8,500 = ( 34 \times (-8) ) + 8,500 = -272 + 8500 = 8288$
So my incorrect answer is 8288'. The correct is 7300' :(
I also realize I did not use QNH in my calculations.

You need to correct your pressure altitude (FL85) for the barometer setting QNH first before you apply the temperature correction.
I assume a 30ft / hPa offset here:
$\textrm{barometric altitude} = \textrm{pressure altitude} + (\textrm{QNH} - 1013.25)\cdot 30 = 8500 -907.5 = 7592.5 \textrm{ft}$
Based on the pressure difference between sea level and your altimeter's measurement, the estimated altitude is 7592.15, assuming an ISA temperature profile.
Now, due to the fact that the actual temperature is lower, the atmosphere shrinks, and you end up flying lower.
Note that I will use -10° C for $\Delta_{ISA}$, because that was given in the question:

Outside Air Temperature: ISA - 10°

Now applying your correction formula yields:
$A_T = (4\times A_{Ik}\times\Delta_{ISA})+ A_I = (4 \times \frac{7592.5}{1000}\times -10)  + 7592.5 = -303.7 + 7592.5 = 7288.8 $
Rounding that to the nearest 50 ft results ins 7300 ft.
This is an approximation which is a bit on the low side. But underestimating true altitude is usually safer than overestimating.


What are the experience requirements and process for becoming an FAA Designated Pilot Examiner? When it comes time to hire a new Designated Pilot Examiner (DPE), what does the FAA look for? What triggers new DPE job openings? How do the application and onboarding processes work? Are there more written tests? Is there a “checkride checkride”?

You can start with the FAA's reference page for Designated Pilot Examiner.

It will point you to the FAA Order 8900.2 General Aviation Airman Designee Handbook.  It's all there.  You'll need to talk to your local Flight Standards District Office (FSDO) as they are responsible for administering the program. The general eligibility requirements are:

  General Eligibility Requirements. The managing office must carefully consider all of the requirements before designating an
  applicant or renewing a designee. The general requirements for
  appointment, which are reviewed by the NEB, are:

  a.    Minimum Age. Minimum age of 23 years for all designees.

  b.    Personality Traits. Must exhibit integrity, professionalism, and
  sound judgment.

  c.    Training Requirement. Must have satisfactorily completed the
  applicable training requirement before appointment/renewal as a
  designee.

  d.    Knowledge of Guidance Materials. Must have access to, and
  adequate knowledge of, the current regulations, policies, directives,
  and related guidance material appropriate to authorized functions.

  e.    Command of English Language. Must be able to fluently read,
  write, speak, and understand the English language.

  f.    Objectivity. Must maintain the highest degree of objectivity
  while performing authorized functions.

  g.    Good Record. Must have a good record in the aviation industry.

  h.    Technology Access. Must have internet access, as well as access
  to a computer and printer with hardware/software sufficient to support
  the required activities.

  Note:  A designee is not required to be a U.S. citizen.

  Note:  Additional eligibility requirements for specific designee types
  are included in the applicable designee type specific chapters
  (Chapters 6 and 7) of this order.

As to what the FAA is looking for (this from a DPE, I used to work with), the FAA is looking for someone they can trust to perform the job to standards. They expect a DPE candidate to be an experienced instructor.  They'll look at the the history of students signed off for check rides by that person. If any significant number of recommended students aren't passing check rides or 'just squeak by', it won't reflect favorably on the applicant.  OTOH, if the recommended students are well prepared and almost always pass, that's a good indication. That shows the instructor's standards are the same as the FAA's.

'Openings' for DPE positions are driven by supply and demand. The purpose of any Designee is to help manage the workload imposed on the FAA's office. If the office can handle the number of applicant's, they won't add a DPE. And if an existing DPE isn't performing a minimum of examinations, they'll lose the Designation. Most of the DPEs I've known worked for flight schools where there was a steady stream of students.


How to find black boxes underwater absent any pinger signals? If CVR/FDR cannot be seen visually on the seafloor (because covered by a thick layer of mud/sediments) and no pinger signals are detected, how can they be found? Metal detectors? Any other methods?

case in point: The FDR of crashed Lion Air flight 610 was recovered on 1 Nov 2018, but 2 months later now the CVR still has not been found (while the pinger batteries are rated for 30 days only):

  « The search for the CVR is hampered by thick mud. The signal source is difficult to ascertain its position considering the sea floor is mud with a depth of more than 1 meter. The ping signal from the CVR has not been received for 2 days now (5 Nov). There are other means to find the CVR however. » (source)

Which "other means" are this ?

update 18 Dec 2018: Lion Air contracts MCS to locate and recover the CVR of the 737MAX.

update 21 Dec 2018: an expert answered the question for me. I'll paraphrase his explanation as an answer below.

The CVR on a Boeing 737 is located in the aft cargo compartment. The CVR itself is not built of the same materials as the aircraft, which is mostly aluminium and composites. By mapping the location of other components of the aircraft found in the sea bottom it should be possible to make an educated guess of the general location of the CVR (assuming the aircraft did not disintegrate, sending everything in different directions). One common way of tracking down the CVR is by using an underwater magnetometer to detect ferrous metal objects. The problem is that something as small as a CVR will only be detectable up to about 20' under the ocean floor. If it is buried deeper than that it will be difficult to find. That's why they have pingers, but those stop working when the battery runs out.


Is it possible to ping ACARS repeatedly and track an aircraft? This is a followup to What data does ACARS send back to base? Can it be used to track a plane?

I had asked this as a part of the earlier question, but it seems to be a large enough issue to be separate (plus no answers covered this aspect).

All electronic communication systems have low level diagnostic and channel check packages (commands). By pinging, I mean to send a system specific query message, and estimate the response time at base station or first receiver. Repeat the procedure multiple times, and calculate the response time differences. (Assumption) If the response time is decreasing, then the aircraft is moving towards the receiver, if it is increasing, then the aircraft would be moving away. Couple the time difference obtained from multiple responses, with the location of the receivers (which may be satellites or ground stations). Then, approximate the plane heading, location or speed, with reference to the receiver, and then with reference to the ground.

Would a process like this be possible with a high speed aircraft's ACARS? Are there any aspects specific to an aircraft that would make this impractical?

When ACARS communicates via INMARSAT satellite (other options are VHF or HF to ground stations), the satellite can ping the receiver on the aircraft and determine the distance from the delay of the reply. The satellite is geostationary, meaning that it is a fixed point 35786 km above the earth. Based on the distance from the satellite you can calculate a circle where the aircraft must be on. This has been done for MH370.

Since the geostationary orbit is so high, a single satellite covers a large portion of the earth. Only a single satellite covered the MH370 flight.


Is it currently possible to build a jet engine with 1,050 kN of thrust? (236,049 lbf) Do we currently have the engineering know-how to build a 1,050 kN jet engine?

This question originated from the idea of changing a 4-engine aircraft into 2-engines. I chose the A380 since it seems to have the highest takeoff weight.

One A380 engine produces a max of 350 kN. If one engine goes out, the A380 still has 1050 kN available, and per regulations that should be enough even for takeoff. So if we switch to a two-engine aircraft, per regulation we need enough thrust for takeoff even if one engine dies, which means each engine would need 1,050 kN max.

I read this and this. They're in the same context of reducing a 4-engine plane to 2-engines, but do not answer the question if 1050 kN is achievable for a single engine.

The limit to the size of turbomachinery is considerably larger than the size of current aero engines.

To estimate how large cores can get, we can look at power plant SCGT. You can get heavy frame turbines as large as 576 MWe for the SGT5 (this is half of a typical nuclear reactor at 1000 MWe). The aeroderivative LM6000 delivers 40 MWe out of a 270 kN turbofan core, and the largest aeroderivative, Industrial Trent 60, provides 60-70 MWe out of a 415 kN thrust turbofan core.

This would suggest an thrust equivalent of 3.5-4 MN or 800,000-900,000 lbf for the SGT5. You can't just fly a frame turbine like this, except in the hold: it's slow to spin up, its speed is on the low side, and it's not certified to aviation standards. But that's just a matter of design. It simply shows the amount of power that can be crammed into a single turbine core.

So, could we make a 1 MN engine? Yes, we are making cores with several times the power. Could we make an airworthy 1 MN engine? With effort, perhaps, of course it would cost money to develop. Would it be as good as the engines we have? That's the million dollar question. Or it would be, and more of a billion dollar one, if there were a market for it.


Why does my flightplan generator come up with a very non-direct route between these two airports? I'm currently obtaining my private pilot license and I had a random thought pass through my head so I wanted to see how long it would take to get from my airport (KGBR) to an airport in VA (KCHO). The flight plan provided is not as I expected which I assume its due to the busy airspace surrounding my flight. I was wondering why the flight path generator didn't give me a more direct route heading west and then south instead of bringing me down the coastline. Any insight would be appreciated!

I assume you used some kind of "use airways" option to vector you on an airway flight and generate this path. First and foremost a quick note, this path, for a small single engine trainer is outright dangerous and you should not fly over that much open water when you can easily avoid it.

Its more than likely that the algorithm that generated this is not the best one. For a quick comparison if we throw this into foreflight and have it select an airway path we come up with the following path.

It looks like in some way your software went east first then hit some of the restricted airspace southeast of DC, got confused, did some crazy routing around it. This put you out in the Maryland/Delaware area then took you north again. A turn east to avoid the Philly Bravo, then north again along the coast but staying out far enough to avoid the NYC Bravo. A strange path indeed and it looks as if you may have set it to avoid Bravo airspaces if possible. If you simply set it to generate an airway course from point A to point B then this routing is just the result of poor computing, in my opinion.

  I assume its due to the busy airspace surrounding my flight.

I don't know what flight planner you used (you may want to let us know) but with a PPL you are more than allowed to fly right through all airspaces (Class A even, if you have an instrument rating and your plane can make it up there). The planners I use have never routed me due to "busy" airspace. The only airspace on your route that would be of concern is the DC SFRA which requires a quick online course prior to your flight I order to be allowed to enter.

On any note, if you are going VFR, you can fly direct, weather and airspace clearance permitting. For example, you only need to avoid the TFR over DC right now. You could fly the following course and only ever need a clearance through the Allentown Class C and the DC Bravo.

Side Note: I fly out of KPNE or DYL in the Philly area, I never have an issue getting a clearance right through the Philly Bravo VFR when I request it.

I also see in your comments that you are flying this in a Cherokee 140. Im a bit curious about the 5000 ft. ceiling you list. Is this a limit on your airframe? That plane has a quoted celling of just over 10K. I did my training in Archers and Warriors and have taken them VFR up to 7500 ft. with out issue and plenty of power left to climb. On any note, the highest minimum obstacle avoidance altitude I see on the route is down by the LDN VOR coming in at 4400 ft.


How do planes receive their flight plan? When planes land, do they receive a hardline connection to ATC in order to receive the next flight plan, or do they receive their flight planes wirelessly from ATC?

The airline files the flight plan to the air traffic control (ATC) on behalf of the crew. The crew receive the print out before the flight.

The airplane "receiving" the flight plan can then mean one of two things:

Flight plan programming

The crew will use this printed flight plan to setup/program the FMS in the cockpit.

Alternatively, the airline can send the flight plan to the crew wirelessly via ACARS, the crew then prints it out in the cockpit.

If the route flown is frequent and rarely changes, the airline can have it saved on the plane, the crew will then load the saved plan, and revise it—making sure it matches the print out.

It's time to get on with the flight

The crew will call ATC on radio requesting clearance to their destination as filed.

If everything matches, they get the go-ahead. If not, amendments will be relayed, and changed accordingly in the FMS.

Alternatively, the clearance can be requested digitally using the ACARS, CPDLC-DCL, or in the near future, SWIM.

Any digital communication is subject to airport, airline, and airplane equipment.


Can a bullet hit make the cockpit door open? If there is a hijack situation and the terrorist guy shoots at the cockpit door to go inside, will that work?

Are the cockpit door locks in commercial planes like 737s etc good enough to handle a bullet shot?

The FAA standards seem to anticipate the use of firearms by people attempting to gain unlawful access to the cockpit.

Opening locks with firearms is probably not as successful as TV would have you believe.

Cockpit Door Standards

  Press Release – FAA Sets New Standards for Cockpit Doors
  Release No. APA 01-02
  January 11, 2002
  Contact: Alison Duquette  Phone: 202-267-3462 WASHINGTON, DC
  In response to President Bush's call to strengthen aircraft security, the
  U.S. Department of Transportation's Federal Aviation Administration
  (FAA) today published new standards to protect cockpits from intrusion
  and small arms fire or fragmentation devices, such as grenades. The
  Aviation and Transportation Security Act authorizes the FAA to issue
  today's final rule that requires operators of more than 6,000
  airplanes to install reinforced doors by April 9, 2003.

From FAA Press Release

This press release doesn't actually say that the locks on the doors must resist being fired on using handguns but it is apparent that the scenarios envisaged included those where handguns were present. There are obviously other measures to prevent passengers carrying combat shotguns into the cabin surreptitiously.

Guns and Locks

  In movies and television, a locked door, or a padlock on a cage is never an impediment so long as the Hero has bullets to spare. One or two shots is generally enough to destroy the lock, allowing the door to open.
  Unfortunately, in real life — as shown by the MythBusters — this requires a high powered gun at close range, which causes lots of very dangerous shrapnel. Lesser firearms, especially handguns may harmlessly bury rounds in even standard padlocks to little effect. Only SWAT teams and soldiers ever do this in real life, and it involves a shotgun, Kevlar body armor, specialized ammunition (a powdered metal breaching round, often jokingly referred to as "Avon Calling"), and full face protection. Even then, the goal is not specifically to destroy the lock, but to destroy the surrounding door or the hinges.

From TvTropes Shoot out the lock

Other references

Mythbusters - shooting locks


What is the difference between a vertical stabilizer and a rudder? I saw two questions on locating these surfaces ahead of CG:

Vertical Stabilizer and Rudder

How are these the same or different control surfaces?

The vertical stabilizer is immobile (if it starts to move, you're in deep shit), and provides stability in yaw (it keeps the aircraft pointed more or less in the direction that it's moving).

The rudder is mobile, and typically attached to the aft edge of the vertical stabilizer. It provides controllability in yaw (it allows the aircraft, if necessary, to point in a different direction than the one it's moving).

The rudder allows the aircraft to slip sideways when you want it to; the vertical stabilizer keeps it from slipping sideways when you don't want it to.

(Original via flickr.com)


What is a normal EGT range of a jet engine? Out of curiosity at what temperatures are the exhaust gasses are expelled from jet engines? A normal reciprocating engine from what I recall should be in the 700 - 1100 degrees Celsius range. Does this apply to turbofans/turboprops/turbojet engines also since the fuel properties should be similar? Or are the temperatures much lower as the engine is being used mostly at high altitudes?

Turbofan without afterburner

In non-afterburning engines the exit temperature inside the nozzle is around 600°C at full continuous thrust. If the ambient temperature is low, less heating is needed for the same thrust, and exit temperatures are lower. The maximum temperature for a given engine is, however, independent of altitude and given by the materials inside the engine. In flight the continuous maximum can be higher than on the ground, when the cooling by the external flow is less effective.

Temperature profile at a distance

This report gives results of ground measurements on the Pratt & Whitney JT8D of the Douglas DC-9 and shows how quickly the temperature drops behind the aircraft due to mixing and radiation.

Jet temperature behind a DC-9 at take-off power. Picture source.

Jet temperature behind a DC-9 at idle power. Picture source.

With afterburner

With full afterburner the engine uses approximately five times as much fuel as when running dry, and it shows in the exhaust gas temperature. This report details measurements of nozzle pressures and temperatures on one of the General Electric F-404 engines used in the X-29 program. The plot below is taken from the report and shows exhaust-jet temperature summaries in the horizontal centerline for various power settings (PLA = power level angle; 120° is full afterburner and 87° is military thrust).

Nozzle temperature survey of the GE F-404 afterburning engine. Source.

The test conditions were 30.000 ft altitude and Mach 0.3, simulated in an altitude chamber. The temperatures were measured with a rake just a few inches aft of the engine nozzle. Note that the temperatures are given in Rankine; the maximum of 3800°R is 1838°C. The wake of the engine's center body causes lower temperatures in the center of the jet at XSRL = 0. The nozzle edge is at 12.7 inches from the center; due to symmetry only one half of the jet temperature profile is shown.


How do big planes like 737 manage fresh air for passengers during flight? How do the big planes (e.g. A380 or 737) flying on high altitudes manage to get fresh air for passengers and crew during the flight?

I know the inner space of the plane used for passengers and crew is sealed and the pressure is maintained within some range.

However in standard flight altitude of about 10-14 km there is not enough oxygen to breathe.  I am not sure whether the standard oxygen ratio is still about 21% and you only have less air (lower pressure).

We talked two possible solutions and maybe I am still wrong:

using some pumps and small pressure tanks to suck in air from outside till we get higher pressure and then use some valves to "replace" the breathed up air with the fresh one
use some pressure tanks loaded with oxygen + some cleaning devices to clean the air and add oxygen to keep the same oxygen level - but this would need to get the oxygen before take-off and such tanks would be really dangerous

So how is it in reality?

They bleed hot compressed air from inside the jet engines, cool it down and pass it into the cabin. There are outlet valves in the fuselage that allow stale air out and which control cabin pressure and air-refresh rates.

A few aircraft types, such as the 787, now use a compressor to take in air from under the aircraft. They do this because it can be more fuel-efficient (not for any other reason). It is arguable whether it creates an additional fire hazard.

See also

Why is the compressed air (from Engine Compressors) taken from High Pressure (HP) stage instead of Low Pressure (LP) stage
Can an aircraft re-pressurize at high altitude?
Is constant air pressure maintained inside the fuselage?


Why is it more difficult for ATC to manage a larger airliner? After playing a few ATC simulation games, and reading at a few places (don't remember where), I am certain that it is more difficult to manage a larger airliner for the ATC.

Suppose that one A320 and one A380 is trying to land at a certain airport. In another case, if two A320s are queuing up for landing, what extra precautions would the ATC have to take in first case, mainly in separating the aircraft and ensuring that they are on time?

They must leave a greater amount of time in between landings/take-offs as the size of the aircraft increases due to wake turbulance

There were historically 3 classes of aircraft for the purpose of wake turbulance classification

H (Heavy) aircraft types of 136 000 kg (300 000 lb) or more;
M (Medium) aircraft types less than 136 000 kg (300 000 lb) and more than 7 000 kg (15 500 lb); and
L (Light) aircraft types of 7 000 kg (15 500 lb) or less.

With the introduction of the A380-800, where it is near MTOW, a fourth category has been added

Super Heavy for Airbus A380-800 with a maximum take-off mass in the order of 560 000 kg

How much time must be left? Well that depends, for an A380:

Arriving Aircraft

  MEDIUM aircraft behind an A380-800 aircraft — 3 minutes;
  LIGHT aircraft behind an A380-800 aircraft — 4 minutes.

Departing Aircraft

  3 minutes should be applied for a LIGHT or MEDIUM aircraft and 2 minutes for a non-A380-800 HEAVY aircraft taking off behind an A380-800 aircraft when the aircraft are using:

  the same runway;
  parallel runways separated by less than 760 m (2 500 ft);
  crossing runways if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1000 ft) below;
  parallel runways separated by 760 m (2 500 ft) or more, if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1 000 ft) below.

Source for timings: http://www.skybrary.aero/index.php/Airbus_A380_Wake_Vortex_Guidance


Does the F-35B use its articulating rear jet nozzle for thrust vectoring? Does the F-35B use its articulating rear jet nozzle for thrust vectoring?

In hovering or STOL/STOVL/VTOL flight yes it can vector its thrust but the system is not operable during conventional flight modes.  No version of the F-135 is publicly known to be capable of this.  Unlike the Harrier, F-35Bs cannot use the vertical lift system in the conventional flight regime.


Can carrier-based aircraft takeoff while the carrier is stationary? Reading about aircraft carrier operation turns out that carrier operation relies a lot on catapult and carrier itself turns into wind with high rev on propellers. This generate as high wind speed as possible to make a takeoff easier for fixed wing aircraft.

My question is in the absense of carrier's own speed and turning into the wind, would fixed-wing carrier-based aircraft be able to takeoff using catapult and its power alone?

Aircraft carriers using STOBAR (the skyramp) always need to maintain a speed of about 55kmph in order for the aircraft on them to take off. This is done to generate enough windspeed on the deck to assist the aircraft to take off. And STOBAR system already hampers the carrying capacity of aircraft, so for takeoff from a STOBAR carrier at rest, a further reduction in takeoff weight of the aircraft would be required to generate enough lift for the aircraft to take off at such low speeds. This reduction in weight would depend on the aircraft carrier and the aircraft as such and might create scenarios where this is entirely not possible.
For Aircraft Carriers equipped with CATOBAR, the conventional steam piston drive which is used to catapult the aircraft provides approximately 95 MegaJoules of energy to accelerate the aircraft to speeds where it can takeoff safely. Hence the arguments stated about the STOBAR carriers can be sequentially applied to this class as well. But recent Electromagnetic Aircraft Launch System provides around 122 MegaJoules of energy to accelerate the aircraft. Given the extra amount of energy available to accelerate the aircraft at higher speeds, it is quite possible that it is going to be easier for the aircraft to take off from a static aircraft carrier equipped with electromagnetic catapults when compared to the aircraft carriers equipped with convention steam piston catapults. But the exact details for the feasibility of this idea would be a matter of national interest and might not be available online.
Also, one should not ignore the extra amount of stresses that an aircraft would have to face while taking off from a stationary aircraft carrier as it would have to accelerate to a higher speed at the same available length of the runway and hence this might prove detrimental to the lifetime of the aircraft.
Edit:
Under research technologies such as Ground Carriers, if implemented would also assist further in reducing the weight of the aircraft and hence make it easier for the aircraft to take off from carriers at much lower speeds.


Is there a system where plane/heli auto adjusts prop pitch/mixture? This one is simple: is there a system that automatically adjusts things such as:

Mixture (I Am bored of listening for engine sounds)
Prop Pitch (I Hate auto adjusting "gears")

So I can only spend time adjusting throttle ?

There sure is but it is generally independent of what is traditionally considered a "glass cockpit" and is often wrapped up under the FADEC system. One of the earlier production designs for this found its way into the Mooney M20PFM which was a Porsche Powered Mooney that had an electronically inject 3.2L Porsche car motor in it. Most importantly it had "single lever operation" where the throttle, prop, and mixture were all wrapped into one lever. For a variety of reasons the plane was ultimately a failure and I do not belive any are still flying with the Porsche motor.

As Ron mentions in the comments some of the Diamond's like the DA42 are set up this way.

  So I can only spend time adjusting throttle?

This is perhaps the more important part of the question. Perhaps one of the main reasons that planes are set up with the throttle, prop and mixture adjusts is to allow for fine control. Automatic control systems generally operate in the most "efficient" range but as a pilot you may wish to operate in other ranges, run lean of peak or cruise at a very slow but efficient burn. Your POH will give you all the relevant information to operate your plane in all sorts of ways.


How do I include a STAR in an ICAO flight plan? Would it be in accordance with ICAO norms to include a STAR identification right after the airway name in the field 15 of the flight plan? Or should the connecting point between the airway and the STAR be included before the STAR?

Example without CP:

  R460 LKN R594 JAL5E

Example with CP:

  R460 LKN R594 JAL JAL5E

ICAO Doc 4444 Appendix 3 has the clearest text (and an example):

Standard arrival route
The designator for the standard arrival route from the point of leaving the defined route to the point at which the approach procedure is initiated.
Note.— Standard arrival route need only be included where appropriate.

Let's break down the language:
"The designator for the standard arrival route from the point of leaving the defined route to the point at which the approach procedure is initiated."
Which means:
(defined route – airway)
(last applicable point on airway)
(first point on STAR)
(STAR designator)
In the example given (below), both points are shared (FTM), similarly for the SID – emphasis mine:

N0460F290 LEK2B LEK UA6 FNE UA6 XMM/M078F330 UA6N PON UR10N CHW UA5 NTS DCT 4611N00412W DCT STG UA5 FTM FATIM1A

(Highlight mine.)


How to correct CG on a not leveled aircraft? I am tasked with weighing a fixed wing aircraft. the aircraft can not be leveled because the leveling points are damaged. therefor it is going to be weighed not leveled, an inclinometer will read a pitch angle and three scales will read the reaction from the LH and RH mains and nose. after the reactions are known and the pitch angle is know, how can I correct the cg for a leveled aircraft.

You have a problem because you would have to know where the vertical center of gravity is to be able to mathematically calculate a correction factor to apply to the weighing points.   The higher the vertical C of G is, the more rear weight bias you will get if the plane is nose-high from level, or front weight bias if nose-low.  How much to allow for?  You don't know.

When the plane is level, it doesn't matter where the vertical CG is.  You would be better off finding some other part of the structure that is aligned with the longitudinal axis from drawings or the maintenance manual, typically something like an upper fuselage longeron, to level the airplane.


What is the procedure for getting permission to operate an aircraft within a pop-up VIP TFR? Consider a situation where there is a Temporary Flight Restriction (TFR) due to VIP movement (i.e. the President of the United States is in Los Angeles), and I am based at an airport that is within the TFR.  I'm a VFR pilot and I want to go for a little flight from my airfield that doesn't have a control tower, exit the TFR, and then return later in the afternoon (re-entering the TFR) and land at the same airport.

What would be the procedure to get permission to:

Take off and exit the TFR-covered area (non-towered airport), and
Return to the same field later on a different flight (same
non-towered airfield)?

Usually when the President visits LA or San Francisco there are one or more inner rings and at least one outer ring. The inner rings are usually 10 nm in diameter and you can’t enter them under any conditions. The outer rings, on the other hand, are able to be flown into if you are on a VFR or IFR flight plan and in continuous contact with ATC. Here’s a portion of the TFR at Martha’s Vinyard.

  FOR OPERATIONS WITHIN THE AIRSPACE BETWEEN THE 10 NMR AND 30 NMR
  AREA(S) LISTED ABOVE, KNOWN AS THE OUTER RING(S): ALL AIRCRAFT
  OPERATING WITHIN THE OUTER RING(S) LISTED ABOVE ARE LIMITED TO
  AIRCRAFT ARRIVING OR DEPARTING LOCAL AIRFIELDS, AND WORKLOAD
  PERMITTING, ATC MAY AUTHORIZE TRANSIT OPERATIONS. AIRCRAFT MAY NOT
  LOITER. ALL AIRCRAFT MUST BE ON AN ACTIVE IFR OR VFR FLIGHT PLAN WITH
  A DISCRETE CODE ASSIGNED BY AN AIR TRAFFIC CONTROL (ATC) FACILITY.
  AIRCRAFT MUST BE SQUAWKING THE DISCRETE CODE PRIOR TO DEPARTURE AND AT
  ALL TIMES WHILE IN THE TFR AND MUST REMAIN IN TWO-WAY RADIO
  COMMUNICATIONS WITH ATC.

If you are departing or landing at an non-towered field, you need two radios (or an Garmin SL30 radio) to monitor ATC and communicate on the CTAF.

I have done it lots of times VFR into LA and it really isn’t much different than getting flight following.

I don’t have any experience with departing from a non-towered airport into the TFR, since the last time I entered the TFR the approach end of the runway at Catalina Island was in the 30 mile ring, but the departure end was not. So I don’t know for sure but from talking to the people at the FBO and reading the TFR, I think you need to open you flight plan either by phone or on the radio (Hawthorne radio would be who you call and the frequency depends on where you are in the LA basin), contact ATC (in this case LA Center, again frequency varies) on the ground before you take off, then make your normal calls on the CTAF. Then contact ATC again when you are airborne.


Is there a standard for the frequency/interval between flashes of anti collision strobe light? As far as I've spotted on lots of aircraft, I've noticed that the anti collision flashes are in a similar interval, no matter if it's a Boeing 737, Airbus or an F-16 Fighting Falcon. Is there any international standard or any sort of aviation safety rules that standardizes the frequency of the anti collision strobe light? If there any such rules, does it regulate whether the lights should have a single or double flashes at a time?

For the FAA, this can be found in 14 CFR Part 25 - AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES, Subpart F - Equipment, Lights (§§ 25.1381 - 25.1403):

  § 25.1401 Anticollision light system.
  (a) General. The airplane must have an anticollision light system that -

  (1) Consists of one or more approved anticollision lights located so that their light will not impair the crew's vision or detract from the conspicuity of the position lights; and

  (2) Meets the requirements of paragraphs (b) through (f) of this section.

  ...

  (c) Flashing characteristics. The arrangement of the system, that is, the number of light sources, beam width, speed of rotation, and other characteristics, must give an effective flash frequency of not less than 40, nor more than 100 cycles per minute. The effective flash frequency is the frequency at which the airplane's complete anticollision light system is observed from a distance, and applies to each sector of light including any overlaps that exist when the system consists of more than one light source. In overlaps, flash frequencies may exceed 100, but not 180 cycles per minute.

Similar sections can be found for other types of aircraft in other sections; the specified flash rate is identical for all of them.

US military aircraft are not bound by FAA rules, of course, but they tend to voluntarily comply with them anyway unless they have a good reason not to.


Where can I find the full report of this controlled F-4 fighter jet impact? I found the following video on YouTube

They used an F-4 Phantom jet as a test article in a crash into a concrete wall at 500 mph. It wasn't actually a crash because the plane was not flying.

I would like to know the disposition of the components of the aircraft if possible. Since this was actually a materials science test and not an aviation test, I am unsure if that even exists. Where can I find the complete report of this controlled impact?

The test was carried out by the Sandia National Laboratories under terms of a contract with the Muto Institute of Structural Mechanics, Inc., of Tokyo. The purpose of the test:

... was to determine the impact force, versus time, due to the impact, of a complete F-4 Phantom — including both engines — onto a massive, essentially rigid reinforced concrete target (3.66 meters thick).

The reports on the tests can be found in these places:

Full-scale aircraft impact test for evaluation of impact forces: Part 1, Test plan, test method, and test results by von Rieseman, W. A. et al.and

Full-scale aircraft impact test for evaluatioin of impact force: Part 2: Analysis of results by K. Muto et. al.

Interestingly, the aircraft used in the tests were flyable. The report notes:

A flyable F4D aircraft was acquired by Sandia. The aircraft was modified slightly in order to support the aircraft at four locations with a special carriage structure. ... the aircraft weighed 12.7 tonnes (with some avionics removed). The impact weight was 19 tonnes, which included ... 4.8 tonnes of water that was added to simulate the fuel weight...
The target consisted of a block of reinforced concrete 7m square and 3.66m thick ... with a combined weight of 469 tonnes.

The aircraft was crashed with the engines and only a few parts removed:

A fully functional F4  was used in the impact test.Only the gears and flaps at the main wings were partly removed and replaced by a carriage structure and sleds.

This can also been in the following mass distribution diagram for the aircraft used in the test.

Image from Full-scale aircraft impact test for evaluation of impact forces: Part 1, Test plan, test method, and test results by von Rieseman, W. A. et al.
Finally, the test was for measuring the impact forces, not the strength of the wall, as stressed by Sandia:

At Sandia, a spokesman, John German, said the point of the test was to move the wall, as a way to measure the impact forces. ... as a preliminary step in building a computer model of such impacts, ...
Mr. German said: "That test was designed to measure the impact force of a fighter jet. But the wall was not being tested. No structure was being tested."


Should I wait for Tower to provide runway exit instructions? This is somewhat already answered here (Is it a flight violation to mishear ATC instructions for exiting the runway?), but this question is more geared toward taking action before hearing from Tower.

I fly out of a towered airport somewhat often. 99% of the time when they provide me with a runway exit (Taxiway B in this case), it's the same one.

Sometimes however, they do not provide me an exit instruction quickly enough despite the fact that I'm rolling up to where I should be exiting for Taxiway B. They might either be busy with other aircraft or simply don't call out an exit instruction. Sometimes I've just rolled past Taxiway B and other times I start heading my aircraft in that direction just to finally hear Tower give me instructions to exit via Bravo.

So, the question is, if I know I'm almost always going to be assigned a given taxiway, should I take it or should I always be waiting for Tower to assign one and thus just keep idling down the runway until I get an assignment from them?

This becomes even more important when flying into unfamiliar airports. Do you just keep rolling or do you take a taxiway when you know you can?

I will like to clarify, 100% of the time, I have received a taxiway instruction AFTER I've started to point the nose of my aircraft at a taxiway. I have never actually exited the runway without finally receiving the instruction.

Note: I am assuming you are asking about US regulations.
If the tower does not give you an instruction, you should not wait for one. Leave the runway at the next available taxiway and then wait for further instructions:

4−3−20. Exiting the Runway After Landing
The following procedures must be followed after landing and reaching taxi speed.
a. Exit the runway without delay at the first available taxiway or on a taxiway as instructed by ATC. Pilots must not exit the landing runway onto another runway unless authorized by ATC. At airports with an operating control tower, pilots should not stop or reverse course on the runway without first obtaining ATC approval.
b. Taxi clear of the runway unless otherwise directed by ATC. An aircraft is considered clear of the runway when all parts of the aircraft are past the runway edge and there are no restrictions to its continued movement beyond the runway holding position markings. In the absence of ATC instructions, the pilot is expected to taxi clear of the landing runway by taxiing beyond the runway holding position markings associated with the landing runway, even if that requires the aircraft to protrude into or cross another taxiway or ramp area. Once all parts of the aircraft have crossed the runway holding position markings, the pilot must hold unless further instructions have been issued by ATC.

(Aeronautical Information Manual, emphasis mine)


What is the significance of hover performance in helicopters (HIGE, HOGE)? I was looking at a helicopter performance chart and it listed two values in feet, identified as "Hover in ground effect (HIGE)" and "Hover outside of ground effect (HOGE)."

What do these numbers mean, and why are they significant?

Ground effect is a condition where the performance of the aircraft increases while operating near a firm surface. In general, a helicopter is considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground and the effects become negligible when the helicopter is more than a rotor diameter above the ground.

While hovering near the ground, the induced velocity at the rotor is reduced, which reduces the power requirement for a given rotor thrust (lift); equivalently, the rotor thrust is increased for given power due to ground proximity. Also, near ground, the rotor tip vortex is reduced.

Due to this, the helicopter can hover at higher gross weight (equivalently, at higher altitude for given weight) in ground effect (HIGE) compared to out of ground effect (HOGE).

Image from copters.com

Usually, the altitude at which a helicopter can hover at a given weight (and atmospheric conditions) is given for both HIGE and HOGE conditions. HOGE is much more restrictive compared to the HIGE. For example, the following image compares the hoer performance of R44 helicopter in and out of ground effect.

Data from helicopterforum.verticalreference.com and aneclecticmind.com

In a sense, HOGE represents the absolute limit of the helicopter performance and it should be considered for operational calculations where there is no availability of rigid surfaces for vertical takeoff and landing or their presence is not confirmed.


Which aircraft did I photograph at ZRH? I took a couple of pictures of various aircraft and I would like to know which aircraft I photographed. I have no idea, expect that all of them took off from ZRH and should be commercial airliners:

One of the simplest things you can do is to type in the registration into Google which will normally give you results of the type and history of the aircraft.

Otherwise, many airlines don't operate too many types of aircraft so a quick look on Wikipedia and looking through 'fleet' should show you which types it may be, from which elimination should be pretty quick.

The registration is normally visible on the back of the fuselage and under the wing (D-ABEP on the rear fuselage below), and the last two letters are (often) printed on the aircraft 'forehead' above the cockpit windows, the top of the tail and the nose gear doors (EP on the nose gear doors below).

Aircraft from your photos:

Boeing 737
Airbus A320
RJ85
Airbus A320
Airbus A321
Fokker 100


Do laser Christmas lights have the same dangers as laser pointers aimed at planes? With new laser Christmas decorations like this one, a laser appears to shine in patterns on a house for a decorative effect.  If this is aimed so that some of the beams miss the house and go over it into the sky, could they cause problems to planes?  Would the effect be similar to a laser pointer, and could the person with the decoration be prosecuted as shining a laser at a plane?

A fine beam is needed do draw patterns. A regular led would require a moving focusing mirror, something expensive.

There is a non-standard warning: "Laser aperture, Max output < 1 mW" near the leds:

As the leds are in the visible spectrum, this is equivalent in power to a Class 2 laser.

  A Class 2 laser is considered to be safe because the blink reflex
  (glare aversion response to bright lights) will limit the exposure to
  no more than 0.25 seconds. (Wikipedia).

This image shows the effect of a 5 mW laser light. You may use it for a 1 mW laser light by dividing the distances by 5:

Adapted from original on Wikipedia

or from the original site:

  A 0.99 mW Class 2 laser beam can temporarily flashblind a pilot or driver, causing afterimages, within 240 ft (73 m) of the laser.
  It can cause glare, blocking a pilot or driver's vision, within 1050 ft (320 m) of the laser.
  It can cause distraction, being brighter than surrounding lights, within 2 miles (3.2 km) of the laser.

The above distances are for a tightly focused beam, which is probably not the case of an inexpensive laser device. The effects of a more diverged beam are weaker.

More info: This hazard chart.

To sum up:

Most countries have regulations for dazzling a pilot, regardless of the mean and power used, it's the effect that is taken into account.
A class 2 laser led couldn't blind someone unless the person is at a few meters (15 m for temporary blindness and 3 m for eye damages, if the eye is forced to stay open).
As the class is not mentioned on the label, I would be careful when seeing this item.
There are actually 2 laser leds, a red and a green, is it 2x1 mW or 2x0.5 mW? In practical the lasers probably can't illuminate the same eye, due to the divergence of their beams.
To draw a pattern a laser light must move, so the time of illumination of a point is reduced when the device is working correctly.


What is the cause of the noise heard when the landing gear is deployed? Why is it that on most airliners, when the landing gear gets deployed, you always hear a loud noise like a bang. Is this due to the equipment locking in place or is there some other reason? As an external person I would think that there should be a mechanism that pushes it down slowly, avoiding this noise but it doesn't seem the case.

The locks release and typically the gear falls free into place.  It is often aided by hydraulic motors (i.e. to overcome anything that might be trying to hold the gear up, such as slush from the takeoff runway that froze after the gear was in the well), but most landing gear will freefall into place within a couple of seconds even without hydraulic assistance after the uplocks are released.

What you're probably hearing is the sudden onset of the wind noise -- when retracted the landing gear is streamlined, but when extended there is now an open cavity where the gear had been, and this open area in the fuselage creates a lot of wind noise from the suddenly turbulent airflow.  Going from a relatively quite state to a loud one suddenly gives the impression of a "bang."  This is possibly enhanced by whatever noise is made by the uplocks that hold the landing gear in place releasing, and perhaps some louder noise as the initial moments of the gear extension may be noisier than the steady-state noise of being fully extended.

Also, the noise due to the turbulence will subside as the airspeed slows, and extending the landing gear often comes during a phase of flight when the airspeed is being reduced in preparation for the final approach segment and the subsequent landing.


How does trim work on an A320? In the very interesting answer from Jan Hudec to a question about the various flight control computers on Airbus planes, two different means to trim the aircraft are mentioned: The stabiliser and the elevator.

How does this work in practice? How is the aircraft trimmed in flight, both as seen from the flight deck (i.e., the user interface to the pilots) and as actual input to the control surfaces?

The faster an aircraft is, the wider range of elevator deflections it needs. And so do aircraft with large change of weight (amount of fuel used) during flight. However larger elevator deflection causes higher drag. It is more efficient to move the whole horizontal stabilizer instead. Therefore jet aircraft generally have the forward part of the horizontal stabilizer movable.

Now it would be possible to simply have the whole stabilizer move. And supersonic aircraft generally have just that. It is called stabilators. However having the stabilizer split lends itself to elegant solution for trim.

This takes advantage of the fact, that the aerodynamic forces tend to put the elevator in the position of lowest drag almost straight (slightly bent in direction of less lift) behind the forward part. So when the forward part is moved, the lift changes without changing the force on the elevator. So the elevator is used for the small adjustments where the elevator is intended to return to the neutral position, which the aerodynamic forces help with, and the stabilizer is used to trim for current airspeed.

In aircraft with mechanical controls, the elevator force (or a fraction of it via power drive) acts directly back on the control column and the pilot can feel it, while the horizontal stabilizer is actuated via mechanism that holds selected position (may be electrical with jack-screw like on DC-9 or hydraulic, but with no feedback) and connected to the trim, which is usually pair of large wheels on the sides of the centre console.

Now in Airbus A320 and all newer models the side-stick does not have mechanical link and the pilot does not feel the force on the elevator anyway. In normal law, the ELACs automaically adjust the elevator and stabilizer so that with side-stick in neutral position, the aircraft maintains 1 G vertical acceleration and therefore straight flight (at any flight path angle). Since the trim wheel does have direct mechanical link, it turns as the ELACs (or SECs) adjust trim.

However the distinction between elevator and trim is still useful in case of failure. When the system degrades into direct law, the side-stick position corresponds to the elevator deflection and the trim wheel position corresponds to the stabilizer position. Since the side-stick still returns to neutral position, the distinction of using the elevator for momentary adjustments and trim to balance the aircraft to fly (approximately) straight at current speed holds (just because the side-stick is spring-loaded, the force needed to deflect it does not increase with speed as it does for mechanical controls).


Why is the A380 a double-decker rather than a longer airplane? It seems like making the A380 a double-decker increases the cross-sectional area of the fuselage, thereby increasing drag on the airplane. Wouldn't it be more efficient to fit so many seats in a longer airplane rather than a wider airplane?

So, is the A380 a double-decker because there is a limit to how long an airplane can be, would it be impractical to make a longer airplane, am I wrong about the cross-sectional area/length trade off when it comes to drag, or is there some other reason?

That's right, having a longer plane would reduce drag by lowering the cross section (altho there is also drag linked to the surface area of the plane).

There is a few reasons why you don't want to do it:

Cylinders don't scale this way:

Take a cylinder with a radius of 1 and a height of 10.

Volume is ≈31.41

Double the height, volume is ≈62.83

Double the radius, volume is ≈125.66

Increasing the length scale linearly and increasing the volume scales at π * n * n

You will need a stronger structure to support the additional length and surface area of your pressure vessel, oscillations during turbulences, takeoff rotation, ...
All your cables, hydraulics, fuselage, will have to be stretched
Passengers will take forever to reach the back of the plane when boarding
Maneuvering in the airport will be impossible, you will need new airports  (credit apple maps)


Before a flight, what are the most important things to check in an aircraft (walkaround)? During the pre flight inspection, usually the copilot checks many parts of the aircraft. Which are the parts and Why do they check them?

Many things are checked on a walkaround. The pilot-in-command should do the walkaround, not the co-pilot. The main classes of items are as follows:

structural integrity; the aircraft must be sound and undamaged and ready to fly

sensors; sensors such as antennas and gauges should be undamaged and not have anything blocking them or adhering to them

clean flight surfaces; the control surfaces and flight surfaces must be smooth and clean; for example, the wings must not have ice on them

no leaks; hydraulic fluid, oil, fuel, and other critical liquids should not be leaking out of the aircraft

chocks, control locks and covers removed; all chocks, protective coverings and locks should be removed before flight and stowed

correct tire inflation; wear on the tires should not be excessive and the tires should be inflated properly

fasteners present and correct; all bolts, shims, nuts and other fasteners should be present, undamaged, and correctly installed

hatches and fairings secure; removable inspection plates and any other small doors, fairings and cowlings should be in place and secure.

lights ok; the lights on the aircraft should operate correctly and their lenses should have no cracks or other defects

blades ok; the propeller or turbine blades (as the case may be) should be perfect with no cracks or chips

Overlooking any of these sorts of items can cause (and has caused) serious accidents or incidents in flight or during takeoff and landing.


What's the red light in the left wing of a N.A. T-6 for? There's a yearly flight show near my location, and there's always this North American T-6 (D-FUKK):

Today, I noticed that there's a single white light in the leading edge of the right wing, but a white and a red light in the left wing.

What's that red light for?

It's the passing light. They're no longer in use.

In an era of virtually zero radar air traffic control, two planes headed for each other at high speed at night needed those [very bright] lights. They would use the right-of-way rule to stay on the 'right lane' and safely pass each other.

(jetphotos.com) Clearer view for D-FUKK.

(Code of Federal Regulations) Points (a) and (c) match the T-6.

  a strong red light to warn passing airplanes displayed usually in the leading edge on an airplane flying after dark (merriam-webster).

(Source) Early Connie's had them in the nose (b).


Why can’t level D simulators produce a G-force effect? From what I know so far, simulators can’t simulate this feeling, I’ve heard some say it will never be possible. Is that true? There’s really no way of producing this in a simulation?

They can produce sustained G-forces, by tilting the sim without the occupants noticing. Of course, this tilting of the 1G gravity vector is limited to practical angles of about 45°, producing 0.7G of simulated sustained horizontal acceleration.

Vertical sustained acceleration is a different matter altogether: the 1G is always already there and can only be reduced by tilting the sim, while in manoeuvres with increased load factor we're looking to increase beyond 1G.

Fortunately the inner ear sensors can be tricked to an extent: they are very good at detecting acceleration onsets but get flooded by sustained acceleration inputs. Our brain seems to rely on the inner ear for detecting the first onset of acceleration, then on our peripheral vision to read the motion velocity. So if we heave the sim rapidly and show the movement on the visual, we cannot detect very readily if we're not pressed down by g-forces anymore after some time.

Image source

Level D simulators are for civil aviation training, which teaches how to limit load factor. Military sims are a different kettle of fish, as mentioned in this answer. G-seats like depicted above provide cues to our body skin sensors, as do G-suits.

Centrifuges can of course produce sustained g-forces, and they do exist, used by the military to train sustained g flying and by medical facilities to check how long a person can remain conscious. A combination of a centrifuge with a civil 6-DoF system is not really possible for realistic acceleration simulation.


What is the teardrop-shaped appendage on the T-6 Texan? Can anyone explain what is the teardrop-shaped item behind the cockpit?

It is the RDF antenna 'bullet' (or teardrop, actually) fairing. This image shows the (loop) antenna itself.

Source: mission4today.com

Initailly, RDF antennas were circular loops mounted above or below the fuselage.

"Earhart-electra 10" by Originally uploaded by w:User:Logawi - Transferred from en.wikipedia, description page is/was here.. Licensed under Public Domain via Commons.

Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing.

Source: airpages.ru


Why is a clock required to be mounted on the instrument panel for IFR flights? A clock is required for IFR flights in the US but any clock doesn't do; the clock should be the one mounted on the instrument panel. What's the idea behind this requirement? Is it because random clocks the pilots might carry with them for their flights cannot be trusted?

The clock needs to be positioned such that you can see it easily as part of your instrument scan, and the sweep second hand needs to be there to make it easy to time intervals that are not whole minutes.

The regulation has been there for a long time, so the requirement should be considered in the light of how things used to be as well as what they are now.

A very practical example from 1969 follows. The regulation predates that by many years, I am sure, but the 1969 environment shows the need.

You're in a light single engine airplane (think Cessna 150 and later a Cessna 172). You're working on your instrument rating in actual conditions in the Pacific Northwest in a non-radar environment. You're in moderate turbulence and having to keep on eye on the icing. And, though it seems ridiculous now, we didn't use headsets but instead picked up a mic for each transmission, of which there were many because in the non-radar environment there were lots of reports.

You're inbound to the VOR on the airport, after which you'll proceed outbound to the NDB that is the final approach fix. Once at the NDB, you will fly 30 seconds farther outbound on the Rwy 16 ILS, which by the way since you have no HSI you're having to navigate by reverse sensing on the old VOR/ILS CDI with its swinging left/right needle. At the 30 second point, you're to turn right to the procedure turn outbound course and fly for 45 seconds (but maybe if that wind is from the east you should go for 60 seconds - better watch that CDI needle to see how fast you get full deflection).

Oh, damn, the instructor just covered up the directional gyro to simulate its failure. Now your only heading reference is the wet compass, which is bouncing around in the turbulence, and, besides, you can't remember whether it's going to precede or lag (and by how much) in the turn you have to make to reverse course for the inbound leg of the procedure turn. Ah, but wait, all you have have to do is fly a standard-rate turn for 60 seconds and you'll be in good shape.

So, thus far, we've had three separate needs to time things to seconds in a situation where we have our hands full - left hand on the yoke, right hand for throttle, carb heat, microphone.

Given all that, do you really want to have to keep pulling back the sleeve on your left arm to monitor a relatively small clock face? Much better to have a larger clock face right in front of you on the instrument panel.

Do we really need such a clock now? Perhaps not, but it doesn't hurt, and regulations are notoriously resistant to change.

And does anybody actually fly procedure turns anymore?


Why does the UK CAA medical questionnaire need to know hair colour? The CAA (UK Civil Aviation Authority) application for a medical certificate asks for hair colour. Why? Are there any diseases / conditions more prevalent to particular colours? Most of the questions are seem to be rooted in common sense, but is hair colour really relevant?

Update in response to comments. Height, weight, sex, eye colour and race ALL have a relevance to medical conditions. Height and weight can indicate BMI and possible eating disorders, many conditions are gender specific, and some medical conditions are more common in different races (Cystic fibrosis is more common amongst northern Europeans, Sickle-cell anaemia is mostly a problem for those with African ancestors). Even eye-colour can (according to google) indicate a increased likeliness of a particular condition.
If hair colour really is there just for general identification purposes, then it's the only question on the form (other than name and CAA reference number) that has no medical purpose.

The same reason they take down your eye color, height, weight, sex, race, etc.  They’re just general stats for identification.


How useful would the space shuttle OMS actually be in atmosphere? When speaking of space shuttle landings, they always mention that it is "very poor glider, with one shot, no go-arounds."

Of course this is true; however, if the Shuttle found itself in a situation where it was a little short on delta-v to reach land/a runway (perhaps during an RTLS abort), would the OMS make any meaningful difference in extending gliding distance?

Not all that useful - the OMS engines had 6,000 lbf of thrust each.  For comparison, the Allison J35 (used in the F-84) produced 5,600 lbf of thrust.

Comparing weights, the shuttle weighted 165,000 pounds empty (reference) while the F-84G weighed 11,470 pounds empty.  Since we have two OMS engines on the shuttle, we're double that to 22,940 pounds. So we have to push approximately 8 times the weight.

There are a few other considerations - The OMS engine only had enough fuel for 1,000 ft/sec of delta-V, fuel would be needed for entering orbit, and later 200 ft/sec for reentry, along with whatever else was used for maneuvering during the mission. Therefore there probably wasn't a whole lot of fuel remaining. Also in my comparison don't forget that the shuttle is not known for being very aerodynamic, so you might be able to manage small changes, but nothing of significance.


Have there ever been accidents due to a vacuum failure that hasn't been noticed? When the vacuum system fails, the artificial horizon slowly goes back into it's non-vacuumed position. I found no picture on that but it's a quite high angle of bank that is indicated.

Has there ever been an accident because of the failure of the vacuum system? In flightsim, I always needed some time to realize the AH is no more working and adjusted my bank to "fly straight" again.

There have been many accidents because the vacuum system failed; AOPA has a good article that discusses some of them, and see this NTSB preliminary report for an example.

As for accidents caused because the pilot didn't notice, that's a more difficult question: if the pilot didn't survive then there's no definitive way to know what happened and what he experienced. It's always possible that the pilot was fully aware that the vacuum system failed, he just wasn't good enough at partial panel flying to keep the aircraft under control.


Why is the numerically determined location of the center of pressure inaccurate near Cl = 0? I am working on an assignment and I need to calculate the center of pressure of an airfoil, through a numerical method (the panel method). I am getting a weird result because it gives its position on a point outside the chord for a determined angle of attack.

When I asked my professor he said that under an angle of attack near the angle of attack that makes Cl=0, it may come out that way, because the center of pressure is calculated through an expression that involves a quotient with the Cl=0 on its denominator.

I am confused because I don't know if it is just that the method fails in that case, or if it has some physical explanation.

If some parts of the airfoil create lift and others a downforce, the center of pressure can be outside of the airfoil's chord at low lift coefficients. This condition is fulfilled for cambered airfoils, airfoils with a deflected flap and especially for rear loading airfoils, which have low camber in the front and high camber in the rear part. Supercritical airfoils meet this last condition.

In airfoil theory, an airfoil creates lift and a moment, because the lift is assumed to attack at the quarter chord point. This point is special because here the moment coefficient does not change with the angle of attack (at least in the inviscid, linear potential theory, which is sufficiently close to reality at large Reynolds numbers to be useful). In reality, positive camber will cause the resulting lift force to act aft of the quarter chord point, and the pitching moment is negative. When lift becomes small and the pitching moment stays constant, the lever arm of that small lift needs to become large to achieve the same moment, and this is when the center of pressure can slip out of the airfoil's chord.

Below you see the result from XFRL5 V6.0.5. I plotted the local center of pressure as a green line. It's elevation over the plane of the wing shows the amount of lift locally created, and the location in streamwise direction shows that it leaves the local chord when lift becomes low. Note that when moving out in spanwise direction the location jumps from far aft of the airfoil to far ahead when the local lift turns negative. At the point of no local lift you have a division by zero error, patched over here by a straight connection between the results from the single panels.

Location of the local center of pressure on a swept wing (own work).

When angle of attack is increased, all additional lift has the Birnbaum distribution, so the local center of pressure moves towards the quarter-chord location.


If pitching moment creates drag, how can high and low moment airfoils have almost identical drag polars? If pitching moment creates drag, how can high and low moment airfoils have similar drag polars?

Let's call a symmetrical airfoil like NACA0015 a low moment airfoil having no moment at an AOA of 0.

Let's call NACA 2415 a medium moment airfoil. Let's say it has a medium moment at an AOA of 0.

Let's call Eppler399 a high moment airfoil, and it has a high moment at an AOA of 0.

yet....

I understand pitching moment creates drag. While the above airfoils have vastly different moment polars at an AOA from -10 to + 10, they almost identical drag polars,at an AOA from -10 to +10.

How can this be?

See pics below

A high Cm airfoil does not mean it will be draggy by itself. However, it does mean that when combined with trim devices (e.g. tailplane, elevator, canard, elevon), the overall drag may be higher due to the trim drag component.

Since 25% MAC, about which pitching moments are generally cited, generally gives around 20% static margin for low speed aircraft, it's a rough first indication of how much trim would be required.