What material are High Pressure Compressors made of? What material are GE engine High Pressure Compressor (spools) made of ? i.e. aluminum, titanium ?

Various alloys. In general , I believe even the same stage may have different alloys in various engines. As the air progresses through the compressor it heats and alloys are chosen accordingly. Over the years new alloy modifications are developed ; some more subtle changes are longitudinal grain orientation  and single crystal blades. I have done failure analysis on gas turbines, steam turbines and gas compressors and don't recall which blade went where. However, I never saw an aluminum blade ( or vane). GE , and others, have various 13% Cr steel blades they often used. As far as I know Ti and graphite are only used in the first stage fan. I only worked on older design engines, I expect Ti is used in earlier compression stages. The final stages of compression  are so hot that nickel alloys are used. I mean you need a very specific question to get a specific alloy.


Why are W, S, N, E highlighted on a flight management computer? Why are W, S, N, E highlighted on a flight management computer (FMC)? My assumption that they represent West, South, North, and East.

Here is a Boeing FMC:

And here is one from Airbus:

N, E, S, W is definitely used for entering lat/long coordinates. On the Airbus graphic in the question, you can see the pilot has entered their starting latitude and longitude with N and W.

But why are they highlighted? I'm going to guess and say it has to do with the transition between old-style numeric nav computers and "modern" FMS. Old style IRS only had a numeric keypad like this Concorde unit:

(Source)

and you can see that N, E, S, W is entered on the 2, 6, 8, 4 keys. So instead of typing "3253.9N" on the CDU you'd hit "325392". You'll still find these numeric keypads today on some forgotten corner of the overhead panel for who knows why, even on a 777.

I think the squares are a cue for the old time pilots that you can't enter a N coordinate by hitting 2, you have to hit the N key, and it also helps them find it.


How can I get historic flight data by N-number? I'd like to get flight data for the last 10-20 years for an aircraft for which I have an N-number. Basic information-locations and times of arrival and departure.

Is there a free database which will produce these records?

Some websites offer historic flight records, but they are not free. And they don't go back for decades and are not complete.

If you really must, try the FAA Registry, see if the owner is listed, and get in touch if their contact information is listed.

The FAA only keeps broad records, as this answer explains.

Don't get me wrong, but it's like asking where a car has been to for the past 20 years. But I could be mistaken.


What do aircraft accident investigators do in between incidents? Consider aircraft accident investigators in your average first- or second-world nation. Something like the US NTSB, the French BEA, the Canadian TSB, the Australian TSB, ... (whichever you happen to be most familiar with is fine)

Now, most of the time, aviation ticks along nicely. Aircraft take off, aircraft cruise, aircraft land. Aircraft are checked up and maintained. It's not like you are going to call in an accident investigation team for a hard landing or a broken light bulb.

Yet when something does happen (especially in a known location) that warrants an investigation, investigators are on the scene quickly, not uncommonly within hours of an incident.

Considering how long investigations can take to complete, it seems likely that multiple teams would be needed, so that one can be brought in quickly without unnecessarily disrupting any ongoing investigation. (Aircraft incidents do, unfortunately, happen more often than once every several years, especially in large countries with significant amounts of air traffic.) On the other hand, incidents happen rarely enough that it seems to me, most of the time, a large number of such investigators would basically be sitting around just waiting for something to go wrong. That doesn't seem reasonable, so...

What do these people actually do while aviation pretty much just works?

NTSB investigators handle General Aviation incidents that you never hear about, as well as the Commercial incidents that make the nightly news.

There are new incidents every single day.
Try reading AVHerald for a sense of what happens, or searching the NTSB Aviation Database.

They have plenty of work.


Why are tip tanks used? The answer to this question identifies the "pods" as tip tanks.

Why do manufacturers sometimes use tip tanks rather than wing tanks? Or is it only when the wing isn't large enough to hold the desired fuel quantity? It seems like filling the wing with fuel first would be more efficient due to less drag.

Tip tanks are only used when the wing volume is not sufficient to carry the needed fuel. They can mostly be found on the early jets which had thin wings for a higher Mach number of drag rise onset and very thirsty engines. They offer two advantages over conventional external tanks:

Their wing tip location allows the highest wing bending relief for a given fuel mass.
They widen the wing by their diameter, so reducing induced drag a bit.

Later designs used the area rule to shape the tip tanks, so they could help to reduce wave drag a bit, especially on designs which predated the knowledge of efficient transsonic shaping.

On fighter aircraft, tip tanks should be jettisoned with the start of air combat because their high roll inertia contribution puts the aircraft at a clear disadvantage.

The XF-104, the prototype of the Lockheed Starfighter. Where else would it allow to store all the fuel?


Does the EASA LAPL(A) have a FAA equivalent? I have an EASA LAPL (A) fixed wing pilots license and would like to know what the equivalent FAA license would be.

I'm not sure about their equivalence (meaning one cannot fly in US with a LAPL right way), but the closest that comes is the Recreational Pilot license.

The requirements are roughly similar:

30 hours flying experience; however, the FAA requires 3 hours of solo flying and cross country requirements are slightly different.

The limitations are also roughly similar:

Recreational pilot license doesn't allow the pilot to have more than one passenger, while the LAPL does (upto three).


How do airlines handle their non-standard passengers for the load control process? How are non-standard passenger weights maintained in the systems?
How they do they handle these passenger weights between check in and load control system and etc..

Airlines are responsible for weight and balance control programs. In the US, FAA AC120-27 describes how these work, and it specifically describes the treatment of special passenger groups like sports teams:

What are the standard average weights for special passenger groups that do not fit an operator's standard average weight profile?

a. Sports Teams.
(1) Actual passenger weights should be used for nonstandard weight groups (sports teams, etc.) unless average weights have been established for such groups by conducting a survey in accordance with the procedures established in Section 3 of this chapter. When such groups form only a part of the total passenger load, actual weights, or established average weights for the nonstandard group, may be used for such exception groups and average weights used for the balance of the passenger load. In such instances, a notation should be made in the load manifest indicating the number of persons in the special group and identifying the group; e.g., football squad, etc.
(2) Roster weights may be used for determining the actual passenger weight.
(3) A standard allowance of 16 pounds per person may be used to account for carry-on and personal items as provided in the operator's approved carry-on bag program.
(4) If the carry-on bags are representative of the operator's profile but do not meet the number of bags authorized per person, the operator may count bags and use a 16 pounds per bag allocation.
(5) Actual weights must be used in cases where the carry-on bags are not representative of the operator's profile.
b. Groups that are predominantly male or female should use the standard average weights for males or females provided in Table 3-1.
c. Military Groups. The Department of Defense (DOD) requires actual passenger and cargo weights be used in computing the aircraft weight and balance for all DOD charter missions. This requirement is specified in DOD Commercial Air Carrier Quality and Safety requirements (reference 32 CFR part 861, section 861.4(e)(3)(ix), as revised). FAA-approved air carrier weight and balance control programs may be used to account for carry-on/personal items for mixed loads of military and their dependents (such as channel missions). For combat-equipped troop charters, the Air Mobility Command (AMC) will provide guidance to account for the additional weight. If aircraft operators perceive that the weights provided are understated, they should seek confirmation of the actual weights and should make reasonable upward estimations and adjustments to those passenger and/or bag weights.

In other words, use actual weights or come up with an appropriate method to determine appropriate average weights for special groups.


What are the reasons for an installed engine to produce less thrust than an uninstalled engine? Is this only due to the various flight conditions encoutered during operation, such as side winds and turbulence, or are there other reasons? Also, is interference between fuselage and power plant involved?

No, these losses are well known and are called installation losses.

Their reasons are:

Intake losses. In a ground test the engine will be fitted with a screen to avoid foreign object damage, but none of the long intake tubes which are common especially in supersonic aircraft.
Generator loads: The engine drives generators to supply the aircraft with electricity. This can be simulated in a ground run, but often the generator load is higher in flight.
Bleed air: Some of the compressed air inside the engine is piped to a heat exchanger and then into the cabin for pressurization and heating, or used for leading edge de-icing. Again, this can be simulated in the static test but sometimes bleed air losses are higher in flight.

Interference between fuselage and power plant is only to be expected with fuselage-mounted engines. This can be both beneficial (as in the precompression at high angle of attack and speed on the Rafale or the F-16), or it can distort the flow when side-mounted intakes are operated at a high sideslip angle.


How does inlet spillage drag produce a lip suction effect? From this page:

  Spillage drag, as the name implies, occurs when an inlet "spills" air around the outside instead of conducting the air to the compressor face. The amount of air that goes through the inlet is set by the engine and can change with altitude and throttle setting. The inlet is usually sized to pass the maximum airflow that the engine can ever demand and, for all other conditions, the inlet will spill the difference between the actual engine airflow and the maximum air demanded. As the air is spilled over the external cowl lip, the air is accelerated and the pressure decreases. This produces a lip suction effect, which partially cancels out the drag due to spilling.

I understand most of the above, except starting from "This produces a lip suction effect..."

If I'm getting this right, the unwanted air, stuck in the inlet, leaves the inlet moving forward, spills out, sucking the airframe with it, thus negating some of the drag caused by the spillage?

Yes, your last sentence gets it about right. Just one detail: Air does not move "forward" to flow out again once it has entered the intake - which air goes where is all settled ahead of the intake. Never forget, though, that this effect is small and spillage leaves a net drag component.

When compared to a sharp-lipped intake as used on supersonic planes, subsonic intakes are more forgiving for a mismatch between capture area (area of the stream tube which goes into the engine) and the actual intake lip area. Below I tried to fake the flow around an intake lip using Inkscape and added blue and red shading to indicate pressure. The core flow gets compressed ahead of the intake (blue shade, diverging streamlines) while the outer flow is pushed to the outside of the intake lip by the increasing pressure and now has to follow the contour of the intake. Like with air flowing around the upper side of an airfoil, the air is accelerated (converging flow lines) and a suction peak forms on the intake lip (red shading). This suction peak sits on the forward-facing part of the intake lip and provides some suction.

The sharp intake lip of a surpersonic intake would produce separated flow under the same conditions with a considerable increase in drag.


How can the Cirrus Vision jet be cheaper than its competition? Cirrus Vision SF50 costs USD 2 million, compared to USD 5 million for comparable private jets.

How is Cirrus able to offer a jet at such a substantially lower price? What compromises are customers making when buying this plane versus a competing one (Eclipse Canada, Cessna Citation M2 and Embraer Phenom 100 as per https://www.usatoday.com/story/money/2017/07/06/price-cutting-private-jet-shakes-up-aircraft-market/102607220/).

I read an article on the SF-50 recently published in AOPA Pilot and, to be honest the Cirrus Jet is not even in the same league as the other aircraft mentioned.  People think 'oh it's a jet for $2 mil - I want one' but remain fairly ignorant of exactly what that means compared with other jet aircraft.

The primary differences in performance between the Cirrus and other light jet aircraft is that it's much slower and suffers from 'short legs'.  Depending on payload, the Vision has about at 600 NM range at its max cruising speed of 300 KTAS, burning a stiff 84 gallons/hr.  It sounds enticing to an SR-22T owner looking to upgrade, but consider that high end turboprops like the TBM-9XX are leaving you in the dust by at least 20 knots and using 40% less fuel to do so.  Ranges of approx 1200 NM are possible, depending on payload, but at an economy cruise of 240 KTAS, making it faster than the owner's old SR-22T but slower that nearly all turbobprops and probably more expensive to operate.  The Vision is also not RVSM certified, limiting operations to 28,000 ft.  As a yardstick for comparison, an Embraeer Phenom 100 with a similar sized cabin, at roughly twice the cost of a Vision is capable of over 400 KTAS (Mach 0.70) at FL410 with an 1100 NM range with alternate and IFR fuel reserves.

Ther are other items that aren't immediately visible but play an important factor in the cost difference, chiefly cabin environment.  The Vision's pressurization isn't on par with similar jets, probably being limited to a 5-6 psi cabin differential, yielding around an 8,000 ft cabin altitude at 28000 ft.  The Cirrus is spacious compared with an SR-22 but it's nothing impressive when it comes to light jets and it's not even in the same league as larger jets.  Many of the systems do not have the creature comforts of the big boys e.g. The Cirrus still uses a castering nosewheel as opposed to NWS equippped on most jets, etc.

The Cirrus represents a solid, if somewhat expensive, upgrade from the SR-2X aircraft.  Just don't expect it to be a personal 747; it's not and you do get what you pay for.


What is the difference between an aircraft main battery and standby battery? I have just seen a diagram where it shows both a main battery and a standby battery however I am unsure what the differences are?

I thought the main battery was there as an emergency power source / APU starting etc.. so what is the standby battery there for? Or is it a secondary back up?

Depends on the aircraft.

In most piston driven aircraft, the main battery is there for starting. The main battery also sort of runs all electrical equipment, lights and avionics. The only thing it does not run is the ignition system of the engine. I say “sort of” because the alternator supplies the battery and aircraft with more than enough electricity to supply the aircraft’s needs and recharge the battery simultaneously and at a slightly higher voltage. If the alternator goes out, it should not interrupt the total supply of electricity to the aircraft systems.

Think of the battery as a bucket with a hole in it, placed under a faucet (the alternator/generator). As long as the faucet is open enough to supply more than the bucket needs to remain full, you are getting all of the faucets water. If you partially or fully close the faucet, you are only going to get the water that comes from the bucket. But, your water flow is never completely interrupted.

On the other hand, the standby battery is a separate, isolated supply of electricity. As long as the main battery/alternator combination is supplying electricity, the relay is self supporting to keep the circuit closed to supply electricity from the main battery to the aircraft’s Bus Bar (electrical connection to the aircraft’s systems). If the main battery/alternator combo is not supplying electricity because it is dead, the relay will open the circuit from the main battery. It will simultaneously close the circuit supplying electricity from the standby battery to the Bus Bar. This would happen if the alternator/generator was offline or inop long enough to drain the battery.

The relay is probably a spring and electromagnetic set up. The state of being ON makes the main battery active. The state of being OFF makes the standby battery active.

Think of it like the faucet and bucket in the previous example, if the bucket were attached to a spring loaded shut off valve for another faucet. That second faucet was being fed by a container of water. It also had a hose bypassing the bucket. Once the bucket became light enough, the second faucet would turn on to supply you with water. At least, temporarily in a no water emergency.


Can a 737 be on final at 160 knots, and also touchdown at 160 knots? Would the 737, going 160 knots, float a bit more before touching down?

You fly final approach at the "reference speed" (Vref) for the flap setting which varies with weight.  You will normally set up the reference speed for full flap on the speed bug on the speed tape on the primary flight displays, and on final while the autopilot flies the glide slope you will modulate thrust to hold the approach speed (or let the autothrottle do it if equipped).

For a given landing weight, the reference speed for full flap is worked out from the "speed book" or "speed cards" which the crew usually has handy on the center console.  The airplane on short final with landing flap will be going at Vref, which is usually well below 160kt, typically between 110 and 150 kt for different airliners at different weights.

As you cross the end of the runway and pass through 50 ft, you should be at Vref.  As you get close to the surface and start to flare to land, you reduce thrust to idle at some point (the technique varies with the airplane) and as you hold it off the speed will bleed off as you trade inertial energy for reduced sink rate in getting a gentle contact with the runway.  In most jets you want to be at idle before touchdown, with exceptions.

So in the interval from when you started to flare and reduce thrust, and the actual touchdown occurs, you are coasting along, settling gently as you pitch up to trade inertial energy for reduced sink, with the speed bleeding off as you do so. At touchdown you will be below Vref, perhaps 10-20 kt.  So if a 737s Vref for a given landing weight was some number like (wild guess) 135 kt, by the time it makes ground contact, depending on how much effort the pilot put into holding it off trying to get a gentle touchdown, it'll be doing perhaps 125-130 kt.

In any case, touchdown speed is always below the approach reference speed, unless the pilot just flies into the runway like a carrier landing, which will get lots of people excited.

160 kt on short final is pretty fast for most jets, but you could be going that fast if the flaps had failed while retracted or at an intermediate position and you had to use a higher approach speed (on the CRJ200, the reference speed with flaps failed fully retracted, which they do from time to time, is about 165 kt +/-).  Same thing though; during the landing itself, the touchdown speed should be less than the final approach speed due to the energy conversion process of the flare and touchdown.

If your reference speed was 140 kt and you crossed the threshold at 160, yes you would float quite a bit if you used normal landing technique. You would also get sent for remedial training.  If you tried to drive it on at that excessive speed, there is a good chance you would have nose wheel contact first (called a wheelbarrow), which would get people even more excited.


Should I do bachelor first or pilot training first? I am a student now going to grade 12 residing in Nepal. I am perplexed about should i do my training first  get a job and do bachelor alongside with my job or should I do bachelor first and then training and then get a job.

If you have ready access to university, do the degree first. You won't have time later, and if you get a pilot job in future and lose your medical for whatever reason, you will have an education to fall back on.  In any case, the aviation industry is going to take a number of years to recover from this whole "plague" fiasco, so you shouldn't be in any hurry to get a pilot license.  Thousands of pilots are on furlough and will likely continue to be for quite a while.
Long term, the future is bright, but get a degree first.


Why are there age limits to becoming a pilot in the armed forces? This may be more related to the armed forces as opposed to aviation but I have to ask:  Does anyone know why most armed forces put an age limit on being a pilot in the service?  Most set the cutoff at 26-30 years of age depending on the service in question.

One hypothesis I always had on this subject is the large amount of money that it costs to train a military pilot (2-3 million USD from basic flight training through advanced training and assignment to a fleet squadron) combined with a long service commitment (7-10 years once training is completed) where the individual must maintain good health and good eyesight to meet the medical standards required to fly and the mid 30s- mid 40s is when a lot of medical problems begin to crop up and can interfere with this.  As such the service limits the age so they enter this range at the tail end of their military flying careers.  But that's just a guess.

Another is that the competitive nature of military flight slots allow the services to be choose and only select young candidates to weed out the applicant pool.

In the civilian world there is an age limit of 65 for being a commercial airline pilot as the only real restriction to flying commercially but people frequently enter into that career in their 30s and 40s.  It does seem abnormal, if you meet the other medical requirements for the armed forces to limit a career as a pilot considering that other military occupations allow people to enter well into their late 30s or early 40s.  Does anyone have any information on this limit and the reasons why?

Largely for the same reason there is an upper bound on how old you can be to start training as an FAA Air Traffic Controller: Training people is expensive and time-consuming (flight training moreso than most military specialties), and we want to make sure we're going to get a certain number of years where the pilot is operating at peak performance out of that investment.
Combat flying - particularly in fighters - is a young man's game: It is physically demanding, and it's much easier to maintain the kind of physical conditioning required when you're younger.

Pilot contracts are also typically longer than other enlistment contracts (I believe the going term these days is 10 years of active service, after you "earn your wings"): A pilot who enters training at the maximum age (currently 28 for the US Air Force) will be 38-39 upon leaving the service, if they re-enlist for another 9 year contract they will be in their late 40s (when the natural process of aging will likely begin to limit the duties they can perform, particularly for more demanding combat roles).
Conversely if they entered at age 35 they would be 45-46 at the end of their first contract, and may already be limited by the natural effects of aging. They would be well into their 50s if they re-enlist, and while we've had active-duty pilots in their 50s they are by far the exception rather than the rule.


Why would a plane drop 14.000 ft mid flight? Just noticed American Airlines flight 221 dropped 14.000 ft mid flight today. Curious, what could have caused this?

Here are the flight details.

I looked up the flight on Flightradar24.com. The descent starts at 16:10 UTC and ends at 16:29 UTC, going from 38.000 feet to 24.000 feet. So, the aircraft descended 14.000 feet in 19 minutes.

14.000 divided by 19 makes roughly 750 feet per minute, which is a relatively normal descent rate for airliners.

The cause for this is probably an ATC instruction. The descent starts around 20 minutes ahead of the border of the U.S.A and ends directly in front of it (SSM VOR is the entry point to US Airspace).

Source

So, this is probably the cause for the descent: The pilots contacted the US controller around 20 minutes ahead of them entering US airspace. The controller then instructed the pilots to descent to 24.000 feet and reach this altitude at the SSM VOR. So, no uncontrolled drop there.


What is the difference between TAS and GS? I'm looking for an explanation of true airspeed and ground speed.

TAS = True Airspeed = speed that you get on radar gun as airplane flies by, when radar gun is held by someone in gondola of balloon in same airmass (wind motion) as airplane.

GS =Groundspeed = speed that you get on radar gun as airplane flies by, when radar gun is held by someone on ground.

As an example: TAS of 200 knots and a headwind of 20 knots gives a GS of 200-20=180 knots.  That is: the plane travels at 180 knots over ground but the air is flowing past the plane at 200 knots.


Why do some aircraft have to have the landing gear pinned prior to towing? The Phenom 300 require all three gear to be pinned before towing. I assume there are other aircraft with the same restrictions.

The nose gear is understandable, due to the link being pinned out of the way to allow a larger range of movement of the nose gear.

But why the main gear? Does it have something to do with the trailing link style of the mains?

The most direct answer is that gear pins are installed to prevent accidental retracting during towing. I'm not typed on the Phenom, but the landing gear actuators are most likely hydraulic. Even if they are electric, the airplane is expected not to have any form of power established during towing, making accidental retraction more possible, even if the gear is held in the down position by locks, not its actuators.

The requirement to install the gear pins is not related to the trailing link design. No matter what the design was, that requirement would be there for each gear.

This requirement is ubiquitous on larger aircraft. During airline operations, I have had an aircraft towed over to me many times. I have never seen a plane in tow, at least at my company, that did not have the gear pins installed during towing.


If I fly a balanced turn long enough, will the AI show I'm level? Attitude Indicators are essentially gyroscopes. Mechanical gyroscopes have a "rigidity in space" property, which allows them to preserve their orientation when the support rotates. However, like spinning tops, gyroscopes will also slowly right themselves up due to gravity; otherwise the AI will show pitch down as the plane travels forward.

If I keep an airplane in a balanced turn long enough, is it possible that the gyroscope will right itself to the fake gravity, and show that I'm straight and level?

What about ring laser gyroscopes?

A gyroscopic Attitude Indicator will not build up long-term error simply because of its erection mechanism in an extended coordinated turn. However, all gyroscopes will accumulate error over time, which must be corrected in some fashion.

A gyroscopic AI will display a turning error of 3-5 degrees in a 180 degree turn. However, this precession is reversed in the second 180 degrees of a turn, so after 360 degrees, the precession is eliminated and the AI is correct again.

To explain why this is, first consider the design of the attitude indicator: the pendulous vanes exert an erecting force when the gyro is not vertical with respect to downward acceleration.

(FAA AC 65-15A)

This causes the gyro to erect itself over time, and would in fact do so incorrectly if the aircraft were kept on a constant heading (if, for example, flown uncoordinated on a constant heading.)

However, in a coordinated turn, the case of the AI (including the pendulous vanes) rotates around the gyro. For the first half of the turn, the gyro is erecting in a particular direction. However, as the pendulous vanes move more than 180 degrees around the gyro, they add error opposite the original direction - removing the original error.

Note that while this fixes turning error in the long term, other sources of error are still in place. Precession caused by the fact that the gyroscope gimbals are not perfectly friction-free, which will vary in each individual instrument, will still happen - and the erection system will be unable to correct for it, as it's too busy causing and removing its own error.

A ring-laser gyro would be subject to the same problems, because it must still have a correction process of some sort. All gyroscopes, even perfect ones, are subject to apparent precession:

  A freely spinning gyro tends to maintain its axis in a constant direction in space, a property known as rigidity in space or gyroscopic inertia. Thus, if the spin axis of a gyro were pointed toward a star, it would keep pointing at the star. Actually, the gyro does not move, but the earth moving
  beneath it gives it an apparent motion. This apparent motion is called apparent precession. The magnitude of apparent precession is dependent upon latitude. The horizontal component, drift, is equal to 15° per hour times the sine of the latitude, and the vertical component, topple, is equal to 15° per hour times the cosine of the latitude.

(FAA Flight Navigator Handbook, p. 3-7)

Any gyro, whether a ring-laser, MEMS, or gyroscopic, in a coordinated turn, will exhibit apparent precession and will have to be corrected over time if their only source of correction is acceleration.


How are type ratings created for new aircraft types? When a new aircraft is developed that requires a new kind of type rating to fly, considering that FAR 61.195(b)(2) and (e) require the instructor giving a type rating (or any sort of instruction) in an aircraft that requires a type rating to hold a type rating in that aircraft, how is the first type rating for that new aircraft type issued?  Is it implicitly acquired by the flight test pilots, who then can give type rating exams and checkrides to other instructor pilots?  Or is there some procedure tucked into a dark corner of FSIMS that is used when "we need a brand new type rating, now what?"

Seems you guessed right, actually.  FSIMS 5.9.3 does indicate that you can get a letter of authorization in lieu of a type rating:

  B.    Requirements for Aircraft Undergoing Certification. During the type certification process, it is sometimes necessary for industry pilots or FAA inspectors with airman certification responsibility to be qualified in a particular aircraft before a pilot certificate type rating designator is established for that aircraft. In these circumstances, and at the discretion of the manager of the aircraft evaluation group (AEG), an LOA (Figure 5-187) may be issued in lieu of a type rating. This LOA and Letter of Application (Figure 5-188) must be in the pilot’s possession when exercising the authorized privileges. The LOA can eventually be surrendered to a district office in exchange for a temporary airman certificate bearing the category, class, and type ratings.


Who manufactures the landing gear on the Airbus A350 series aircraft? Where are they made? I assume that they are made out of titanium. Right?

The landing gears are manufactured by Messier-Dowty, a Safran group company. According to their site,

  Safran Landing Systems is responsible for the design, development, qualification testing, manufacture and support of the main landing gear for the A350-900 aircraft.

Also,

  Safran Landing Systems also provides wheel and carbon brakes for the A350, as well as the landing gear extension-retraction system, monitoring system, steering system and braking system.

The landing gear design does use large amounts of titanium:

  The A350-900 main landing gear ... includes the increased use of advanced materials, in particular high strength titanium, corrosion resistant steels
  and HVOF coatings.

A350 landing gear materials, from A350 XWB family & technologies


Do airplanes need wings? Couldn't you just generate lift with a long body? Maybe a little broader than a normal plane.

As a design enhancement, we would need  a heavier bottom, so the plane doesn't flips to a side.

Do airplanes need wings?

Wait a minute! Airplanes have to have wings by FAA regulation, so you won't find any airplane without wings in the US.

FAA definition:

  Airplane. An engine-driven fixed-wing aircraft heavier than air, that
  is supported in flight by the dynamic reaction of the air against its
  wings.

My advice here: If you encounter an airplane without wings (an unapproved case) do only your duty, call FAA immediately at 866-TELL-FAA (866-835-5322):

(Source)

Aircraft on the other hand has benefited from FAA's leniency, and don't have to have wings:

  Aircraft. A device that is used or intended to be used for flight in
  the air.

As you can see, they are not required to have an engine, nor large rocks in the cargo hold to ensure they are still heavier than air. No, the only requirements for an aircraft are that:

At the beginning you intend to make the thing fly. It's perfectly legal to try to see if you could save wings from the bill, as long as you are honestly expecting it to fly.
You want it to fly in the air, not in the water, certainly not in the vacuum of the space. That's the part requiring all your attention during the design. The lift must be carefully monitored, you need to remain in this tiny layer called atmosphere, else you leave the aircraft category, and maybe the aviation field too.

This is a wingless aircraft:

Full scale Lippisch's Aerodyne, a VTOL aircraft built at Collins (Source)

This one is fully in the law, don't call FAA...

It's certainly a stupid answer, I did my best for, but it's correct :-)


How do I keep my home address private following new aircraft registration? I'm looking at filing out an 8050-1 for a new airplane in the US and the FAA is asking me for my name, and address. My concern is that once I register these bits of information, specifically my home address, become publicly available. So I'm wondering is there anyway of registering an aircraft tag in a way that keeps that information private? Are there any proxy registration services that exist for this purpose?

The 'usual' alternative is to have the aircraft owned by a legal entity like an LLC or trust of some kind (trusts are also used by foreigners who own N-reg aircraft). But that only adds a layer of obfuscation: if someone really wants to know who owns the aircraft then they can dig into the LLC or trust ownership details. That information is usually also public although it's at the state level not federal level.

The best thing here is to talk to an attorney in your state, preferably one with aviation experience. He can tell you what legal entities might fit your needs, as well as the pros, cons and costs of each one.


What is the advantage of the two-part rudder and how does it work? I saw that some aircraft's rudder is composed of an upper rudder and a lower rudder (B727, A380, ...) as illustrated:

Image Source

I wonder how does it work compare to a normal rudder and what are the pros and cons of the two-parts rudder over the one-part rudder?

This is called Split Rudder and it provides redundancy. They run on different systems so if one fails, the other one can be used.

Here is a picture of a split rudder:

Split rudders also provide a finer high speed control, in that only the lower one moves at high speed, reducing the exposed surface area and therefore the control effect.

Very importantly, there is a structural benefit. When we use the rudder it imposes a twisting load on the vertical fin. By only using the lower rudder, when the aircraft is at high speed, we reduce the twisting moment and transfer that load to a bigger, stronger, part of the airframe. The principle is the same as why the outboard ailerons are disabled at high speed.

The patent information is here.


What are the radio characteristics of communications between tower and aircraft? I have a few questions on the types of antenna used for tower-aircraft comms.

What type of antennas are used today for tower-aircraft comms? Are they directional or omni-directional?

What's the range of data rate (in kbps) that these antennas can provide?

Also, is there any safety issues related to power transmission between tower and aircraft? What's the power limit for this transmission? I know that the tower controls the plane until the cruising altitude and after that it hands it over to enroute. Before enroute takes control, how does the tower ensure that the transmit tower is not too high while ensuring it can still reach the plane?

They are omni-directional:  Anyone, in any direction, can hear the broadcast.

They are analog (non-digital), AM signals, so there are no bits, and thinking in terms of kbps or data-rate doesn't make sense.

These have similar transmission power to many AM radio stations.  Do you have safety concerns about local AM radio stations that broadcast baseball games?

Finally, the tower does not "control" the airplane.  The pilot on-board is the one in control.  The tower may simply give traffic directions, such as what heading to take, or what altitude is appropriate.


Can noise from contra-rotating propellers be reduced by having a different number of blades on each propeller? Heads up: This came from Wikipedia's page on contra-rotating propellers and it explicitly says citation needed:

  However they can be very noisy ... It is also helpful if the two propellers have a different number of blades (e.g. four blades on the forward propeller and five on the aft).[citation needed]

Any truth to this? Can a variable number of blades per propeller really help reduce noise? If so, how?

(It doesn't explicitly say if it's piston prop or turbo prop. AFAIK, both can be used for contra-rotating props, and I have no idea which one would be more noisy, or if this supposed variable-blade idea will work on one but not the other.)

It does help a little, yes.

One source of noise is the fluctuation in dynamic pressure experienced by a blade of the rear propeller when it cuts through the wake of a blade of the forward propeller. Friction in the boundary layer on the first blade will reduce the dynamic pressure locally and create pressure fluctuations on the second blade, and noise.

By using a different number of blades, the pressure fluctuations will not happen all at the same time, but sequentially, which reduces the peak power of the noise. The effect of the second blade cutting through the wake of the first is still there, but is distributed over time.

Thanks to @MSalters for pointing out another effect. I copy straight from his comment:
The effect is not just distributed over time, but also shifted up in frequency. At 6000 RPM (100 Hz), with 4 blades per propeller they pass each other 8 times per revolution (800 Hz). If the second propeller has 5 blades, this moves up to 4000 Hz.

P-27 prop on the D-27 turboshaft of the An-70 transport, picture by Marina Lystseva. The forward propeller has 8 blades and the rear propeller has 6. For optimum noise reduction, the rear propeller should have just one blade less than the forward propeller. The leading edge sweep helps to stretch out the contact of a single blade with the wake over time, which again reduces peak noise.


Are there aircraft exemptions for CPDLC in the NAT area? Another question lists the aircraft that are exempt from the EASA CPDLC rule.

Is there a similar exemption for aircraft operating in the North Atlantic?

There will be no exemptions for the CPDLC rule in the North Atlantic Track area.

The Datalink Mandate will be implemented:

starting 5 February 2015: FL350 to FL390 (inclusive) on all tracks within
the NAT Organized Track System (OTS)
starting 7 December 2017: FL350 to FL390 (inclusive) throughout the ICAO
NAT region.
starting 30 January 2020: FL290 and above throughout the ICAO NAT region.

ATS Surveillance Airspace, airspace north of 80°N and the New York FIR are excluded from the mandate.
This means that aircraft equipped with a suitable transponder / ADS-B transmitter will be able to cross using the Canada - Greenland - Iceland ADS-B corridor without having to carry FANS 1/A equipment.


Does the Cessna 152 have a name? I'm wondering if the Cessna 152 has a name? There's the Cessna Skyhawk 172 and the Cessna Skylane 182.

Cessna started to apply marketing names to their line in the early 60s (along with goofy marketing gimmicks like the "Land-O-matic" name for the spring steel gear).  Up to then they were just known by model numbers.  The 150 was the "Commuter". For some reason the name wasn't applied to the 152 (which is a 150 with a Lycoming engine), which just has the model number.


Any unique characteristics to identify aircraft engine? I love to find out the engine manufacturer and make, used in all of the aircraft I fly.
Can anyone provide hints on how to identify an engine uniquely?

The CFM56 on a Boeing 737NG is easy. It has a flat bottom.
The GENx was at first easy to identify due to sawtooth pattern chevrons near the exhaust. But now the Trent 1000 too uses it.

Any easier ways to identify most popular engines?
Pratt and Whitney? Rolls Royce?

Some specific aircraft-engine combinations can be identified, but the engines themselves, no, because you don't see most of them—the nacelle is designed for each aircraft-engine combination. For example

  The CFM56 on a Boeing 737NG is easy. It has a flat bottom.

the CFM56 on a Boeing 737NG is easy mainly because it is the only engine type that aircraft uses. And the engines from the same series when used on A320-family don't have flat bottom!

For A320-family, the CFM56 engines use separate exhausts, with shorter nacelle and hot section jet extending behind it, while the IAE V2500 ones use mixed exhaust with long nacelle, so you can tell them apart on those aircraft. But on NEO both engine types used have separate streams, and the only visible difference is the slight one in shape of the blades.

It is actually quite common for aircraft to only use one engine type, and using more than two types is rare, so most of the time if you can tell the aircraft, including variant, you should already know.


What kind of certifications are required for a plane manufactured for personal use? If I make an airplane (lets say for research/recreational purposes), what kind of certifications would be required to fly it? Moreover, would I need special permissions to actually fly the airplane? What are laws regarding this to a situation specific to India?

As Andy pointed out, this question is very broad (even if we're only talking about India). Your starting point should be the Directorate General of Civil Aviation (DGCA) because they can tell you definitively what would be involved in getting a "homebuilt" aircraft approved to fly under Indian law.

Information about this is available on the DGCA website and is broadly similar to the FAA's "Experimental: Amateur-Built" certification requirements:

The aircraft needs to be constructed in accordance with the DGCA's acceptable methods, techniques, and practices. Basically this means you can't use chewing gum as a structural fastener: The aircraft needs to be safe to fly.
The aircraft will be inspected by a DGCA representative prior to being allowed to fly.

  Note that the DGCA inspection is mainly looking for "obviously unsafe" conditions - they don't do a teardown inspection so if you used chewing gum as a structural fastener and subsequently that area is covered up by a wing skin the inspector won't know, but you will when the wing falls off on your test flight.

You will need the DGCA to issue you a Certificate of Airworthiness before you can fly the aircraft around - that certificate is contingent on completing required test flights at the airport where the aircraft is based/built.
You will need an appropriate pilot's license (and medical certificate if applicable) in order to fly the aircraft (this is also required to conduct  the Certificate of Airworthiness test flights mentioned previously).


How are the hours that count toward fatigue limits calculated by the FAA? Which part of the flight actually counts toward the flight hours limited by the FAA? Is there any accounting for the time spent in flight preparation in so far as compensation or hour limits?

Flight prep and other non-flying duties are accounted for, but not under “flight time.” There are seperate regulations for time “on-duty” and time actually flying.

Flight Duty Period

The flight duty period begins when a flightcrew member is required to report for duty with the intention of conducting a flight and ends when the aircraft is parked after the last flight. It includes the period of time before a flight or between flights that a pilot is working without an intervening rest period. Flight duty includes deadhead transportation, training in an aircraft or flight simulator, and airport standby or reserve duty if these tasks occur before a flight or between flights without an intervening required rest period.

Flight Time

The regulations define flight time as: when the plane is moving under its own power before, during or after flight.

Rest period

Rest periods are “measured from the time the flightcrew member is released from duty.”

Cumulative hours

Cumulative hours are limited for both flight duty period and flight time. There are 7 day and 28 day limits on FDP; there are 28 day and 365 day limits on block time. The 7-day and 28-day limits are expressed as hours, so a rolling 168 hour limit (7 days) and rolling 672 hour limit (28 days). [thanks Ralph J]

This info was taken from the FAA Fact Sheet – Pilot Fatigue Rule Comparison


How to locate the owner of a 5' styrofoam wing found in a field? I found a 4-5' styrofoam wing in eastern Montana in a field. It has what looks like a registration number on it. How can I trace that number to try to return it?

If it has a US registration number on it (N followed by 1-5 numbers or letters) you can search the FAA registration database to find out what aircraft has that particular registration number, and who owns it.
In the case of a 4-5 foot styrofoam wing this would likely only be useful if the aircraft in question was a registered UAV.

Some common sense needs to prevail here: Model aircraft often have an N number painted on them for decorative reasons, so if you look up N12345 and it says "That belongs to a Piper Cub" you can safely assume this wing does not belong to that particular real-world aircraft (as the Cub's wings are not made from styrofoam, and are more than 5 feet long).


What special tyres (tires) are needed for high altitude takeoff and landing? I just watched this video of AA922 taking off from El Alto (LPB) to Santa Cruz.  The video's author mentions that the aircraft needs "... special tyres" due to the high altitude.

While I do understand the physics and aeronautics associated with high altitude takeoffs (extra ground speed; longer runway needed, etc.), I'm curious:

What specific modifications to the tires (special or synthetic rubber, extra tread, stiffer sidewalls, etc.) are needed for high-altitude ground operations?
Should I assume these tires are the exception, not the norm, because of economics / higher prices / more maintenance?

The higher takeoff speeds might exceed the speed rating of the usual tires. As a tire is rolling it deforms in two directions.

As each section comes in contact with the runway it is pressed inward by the weight of the plane. After it leaves the runway surface the deformation is in the opposite direction from centrifugal force. This constant flexing is what causes the tires to fatigue and possibly fail. The flexing also causes heat to build up in the rubber which causes the rubber to lose its flexibility.

The faster the tire spins the more centrifugal flexing occurs and the shorter the period between force reversals. This shortens the time the tire has to dissipate heat.

The two ways to increase speed rating of a tire is to reduce the flexing and to reduce heat retention. The way manufacturers reduce flexing is to add cap plies. These are usually of nylon or other synthetic material placed on top of the body plies on axial ply tires and on top of the steel belts on radial tires. These help keep the body plies from deforming outward from centrifugal force. The faster the tire must spin the more cap plies are used.

To reduce heat retention the rubber used between plies and at the shoulders must be made of compounds that have lower heat retention.

These things combined, more or stronger cap plies and better rubber compounds in key places, increase the cost of higher speed rated tires.

Image source: The Role Of Cap Plies In Steel Belted Radial Tires [pdf], David Osborne


How does auto landing work? Some aircraft are equiped with auto landing to assist on zero visibility and ceiling land. Actually, how does this system work?

Autoland is an autopilot feature which allows the autopilot to control the aircraft all the way to touch-down and even a bit further.
Basic idea is that the autopilot is accurate enough to keep the aircraft on the correct approach path. This is achieved by (on top of really good software engineering) with at least 2 autopilot/flight director calculators. One of the calculators is the one in control, anfd the other one is monitoring the first one. In case of disagreement between the systems, a fault is announced and the approach must be aborted (fail-passive system).
There are also fail-active systems that require 3 autopilot channels. These are capable of continuing the approach even if one system gets faulted.
Some systems have a roll-out guidance to keep the aircraft aligned on the runway centerline after touch-down during braking.
Functionality is the same as with autopilot. The calculator gives commands to the control surfaces and thus steers the aircraft. In addition a radio altimeter gives accurate altitude information to enble flare and thrust reduction.


What is the meaning of Minimum Flight Weight? In the Aircraft Flight Manual of the Bombardier CRJ700, the Minimum Flight Weight is listed as 42,000lbs. What is the meaning of Minimum Flight Weight?

This is the weight in an operational condition(with engine oil, hydraulic fluid, unusable fuel in the tanks[maybe also with a specific usable amount like 30 minutes], and some other misc items), zero cargo, with minimum crew.

The minimum weight at which the airplane can fly. You may also have maximum zero fuel weight, maximum landing weight, maximum takeoff weight, and maximum taxi weight.


Can a single engine on an A320 provide wing anti ice? Can a single engine on an A320 provide wing anti ice for both wings in the air ?

It would probably be a single point of failure if one engine wouldn't be enough to deice both wings.

For this reason there is a cross bleed selector so that the remaining engine can feed the wing on the opposing side.

Here is a link to a computer based training video of the A320 explaining that a single engine bleed is enough for both wings but not for both air conditioning packs and wing anti ice at the same time. So to use have wing anti ice with a single engine you need to turn off one pack and enable cross bleed.


Why is pitch measured with respect to the horizon and not the ground? Why is the pitch measured with respect to the horizon and not the ground? How different are the angles between the ground and the horizon? Would it be possible to fly with an attitude indicator that indicated pitch with respect to ground rather than horizon? What does it mean when you have a 0 degree pitch with respect to the horizon? What, exactly, are you flying towards in that case?

I've always had a hard time understanding this. Thanks for your time.

Pitch is measured with respect to a plane orthogonal to the local weight¹ force, which is properly called the horizontal.

That is used because:

It is equipotential plane of the gravitational field. Moving around it won't change your potential energy.
Is always well defined.
Is smooth and locally flat (it is still a spherical surface globally, of course).
Can be measured by averaging inertial forces on board without external reference.

It is called simply horizon, presumably because looking at the horizon is the easiest way to find that plane by human senses.

Using ground, which is not flat, as reference wouldn't work. Aircraft need to fly mostly along the equipotential plane, because climbing to a higher one requires adding more energy by the engines and descending below means the aircraft accelerates and needs to dissipate (= waste) the energy. So following terrain would require quite a bit more energy than flying horizontal. Plus it would be very complicated.

¹ By weight I mean the sum of gravity and centrifugal force in the reference frame rotating with Earth.


Why are turbofan inlets leaned/angled forward? (Image Source: Wikipedia)

Seeing that everything is radially symmetric (apart from the nacelle mount, naturally), I wonder why the inlet (the front part of the nacelle) is usually skewed?

The inlet angle is a compromise between cruise, when the aircraft has a low angle of attack, and the take-off and climb phase, when the engine runs at maximum thrust, and the angle of attack is several degrees higher than during cruise. Especially right after rotation, when the aircraft is heavy and slow, the angle of attack might reach into the lower two-digit territory, and flow disturbances must be avoided especially in this phase.

Note that the fuselage should be horizontal during cruise to avoid making the work of the cabin personnel harder than it already is. A few degrees of intake misalignment can be tolerated, and the misalignment is usually split between cruise and post-rotation climb.

With supersonic intakes, things are different: They are much more sensitive to misalignments, and the intakes of planes like the SR-71 have been carefully optimized for their cruise condition.


Was this Gulfstream way below the glideslope? I saw this YouTube video about a Gulfstream jet that came into landing on Princess Juliana International airport and the landing does not seem to be by the book at all. It seems like the landing gear is only a few centimeters from hitting the fence and a meter from hitting the people on the beach. It looks like to me that he is way below the glideslope, but since I'm not that familiar with Princess Juliana I really can't confirm anything.

Is there any more information available about this incident? Was it actually a normal landing, or was it caused by wind shear or pilot error? How often does this occur?

UPDATE:

I found this comment in the comment section from someone who claim to be a long time pilot:

  Juliana is Class D so 91.129 and 703 applies:

  FAR 91.129 (E) "An airplane approaching to land on a runway served by
  a visual approach slope indicator shall maintain an altitude at
  or above the glide slope until a lower altitude is necessary for a
  safe landing."
  See also FAR 91.703 (a)(1) "Each person operating a civil aircraft of U.S. registry outside of the United States shall: when over the
  high seas, comply with annex 2 (Rules of the Air) to the Convention
  on International Civil Aviation and with 91.117(c), 91.127,
  91.129, and 91.131;

  They were way below the visual glide slope indicators i.e PAPIs (for Juliana), and (from flying this approach myself) it's not necessary for a safe landing. Besides ...it's just not very good practice.

  Sincerely, Long time pilot. (:

I have to say that by my logic thinking: I agree with this guy. I am not a pilot myself, but here is why I agree with him:

Low approach method:
If the plane hits the fence: It could cause the gear to break of (And maybe hit someone) and when the plane touches down on the runway with broken landing gear at flying speed: It could cause a rapture in the fuel tanks, igniting it. And you are essentially a tumbling fireball down the runway.

High approach method:
If you fly higher to avoid the fence: The chances of hitting it and cause a tumbling fireball are greatly lowered. However: You may overrun the runway. But this would be preferred (In my book atleast). Since the plane has touched down on the runway and slowed down: You decrease the chances of fuel ignition when you overrun. And if the fuel ignites at the end: You will most likely end up with the fish. And the fire may not be put out completely, but greatly suppressed. And since the speed also is lowered: The chances of survival is greatly improved.

This is just my personal look on this. And some may or may not agree with me. But since the YouTube comment is there: I though there might be something to this.

There's no incident here. St Martin has a very short runway which spans a small stretch of land between 2 beaches, it's an airport where jets need to be on the numbers. It's famous among plane watchers and there are hundreds of videos of aircraft coming in low enough it seems as if you could reach up and touch the landing gear.

So if there was a glideslope this aircraft was certainly below it, however it's not an instrument landing so glideslope had nothing to do with it.


Can Hellfire missiles be used effectively as air-to-air weapons? Consider the following hypothetical scenario:

An MQ-1 Predator pilot sees an Su-27 closing in from straight ahead for a kill. The pilot prepares to write up his 10th loss of the week when he has an idea: He targets the Su-27 with the laser designator and fires two AGM-114 Hellfire missiles.

Would this work? Can Hellfire missiles be used effectively as air-to-air weapons?

While I can't say that I know the too much about Hellfires, the aiming capability appears extremely limited for any air-to-air combat.

The range is bad: ~8km (compared to ~35km for a 'proper' Sidewinder). Covering this does not take many seconds for jets travelling in opposite directions. I'd imagine you might be able to get a helicopter if you're lucky.
I'm incredibly doubtful that a sensible lock on any aircraft could be achieved from a Predator. Bear in mind that you only have a camera on the bottom, so you've got huge blind spots.
You got a fair bit of satellite/communication lag, in the range of 0.5 seconds either way to get any response. More likely that your opponent has already got you by the time you understand what's going on.
The majority of the Hellfire missiles appear to require a constant lock to seek its target; some can continue on autopilot with limitations if it were to lose it. Again, sustaining a lock with that setup does not have a good outlook.
Remember that the Su-27 has countermeasures (such as flares) to protect itself.


if your Seneca weight is 4000 pounds , and you have 800m visibility, would you take off? An examiner asked me if your Seneca weight is 4000 pounds , and you have 800m visibility, would you take off ?
I said by law the visibility is ok but still I would not because if we get an engine failure, We can't maintain VMC or altitude  , but he wants me to explain more and better !??

What he wants to know most likely is do you have accelerate-stop and accelerate-go distances computed for that airport and ambient atmospheric conditions at that takeoff weight. Do you also have adequate runway available for that as well as a climb rate adequate for obstacle clearance in the event of a critical engine failure after Vr?  And if that’s the case, do you have a plan in place for that emergency situation?  Multi engine flying is a lot about anticipation for these kinds of contingencies.  Having a plan for takeoff and departure and anticipating engine failure on takeoff is all part of safely flying light and medium twins.  If you can’t do that, you’re in dangerous waters and the examiner wants you to think about that.

I don’t have a Seneca AFM available to me, but you should probably go back and review your takeoff and climb performance data for OEI as well as information related to your airport and departure procedures.  This should give you a much better answer to this question.

From a legal perspective, it also all depends upon under what flight regulations the flight is being conducted under.  With approx 1/2 sm visibility and flying a two engined aircraft, the departure would be illegal when conducted under Part 121, 125, 129, and 135 commercial operations, but is legal under a Part 91 flight operations.  Make sure that is included in your decision as well.

Finally, just because you CAN do something does not necessarily mean you SHOULD do something.  These conditions are pretty tight and you are painting yourself into a really uncomfortable corner with little margin for error should anything go wrong.  Are you really that competent to operate a twin, including handling an engine failure, with such low weather minimums or in complete zero-zero conditions?  Are you willing to bet your life on that knowing that once you’re airborne in that environment and on course, there’s no turning back?  Can you maintain the correct airspeed, altitude, heading, and climb rate which the AFM lists all while juggling an engine failure or some other emergency while in IMC?  Are you sure these conditions don’t exceed your personal minimums given your experience with multi-engine flying and, in particular, operation of a Piper Seneca?  If the answer is no, then you should not fly under these conditions.


What is a jet engine's fuel flow at constant IAS? Here is my assumption:

In straight and level flight of a jet turbine engine:
One IAS = One drag = One thrust = One fuel flow

Therefore, the fuel flow will be approximately the same for a given IAS, regardless of altitude, temperature, or RPM necessary to maintain this IAS.
Is that correct? If not, where is the fallacy?
EDIT : Here is a screenshot from ATPL course material which supports this idea and led me to ask the question as it seems doubtful to me. Propulsive efficiency vs RPM at high altitude at the end is not clear as well.

The most obvious difference is due to the temperature of the air.

Both turbine and piston engines are heat engines. They work by converting thermal power into mechanical power. The theoretical absolute maximum efficiency you can achieve is called the Carnot efficiency, $$\eta = 1-\dfrac{T_C}{T_H}$$

This is the efficiency of an ideal engine using the Carnot cycle, that works by transferring heat from a hot reservoir with temperature $T_H$ to a cold reservoir with temperature $T_C$. A typical jet engine is approximated by the Brayton cycle, and a piston engine the Otto or Diesel cycle, but neither can ever by more efficient than the efficiency noted above. The efficiency of a Brayton cycle is $$\eta = 1-\dfrac{T_C}{T_E}$$ with $T_E$ the EGT.

When flying higher, the temperature of the cold reservoir (the atmosphere) drops lower. You can see that the maximum efficiency of the engine will also increase (even if $T_H$ or $T_E$ respectively drop simultaneously with $T_C$). This means that, even if the required power would stay constant for constant thrust, the fuel flow changes, because a single unit of energy from a drop of fuel can be converted into more mechanical power.


What would happen if blades are not pitched correctly on a propeller blades? What would happen on a 6 blade propeller if each blade opposite another was pitched in completely the opposite direction.

For a start, whoever built the propeller would get investigated for possible drug/alcohol impairment on the job.

In terms of trying to run the propeller, it would generate a lot of noise (from turbulence of the blades running in "reverse pitch", possibly stalled relative to the airflow induced by the previous blade) and very little or no thrust.  If it's on a piston engine (or electric motor, assuming a model aircraft or drone case), the engine/motor is likely to overheat after a short time due to lack of cooling airflow.

Finally, it's possible vibration induced by the noise-causing turbulence could lead to a structural failure of either the propeller itself, the engine/motor, or the engine/motor mounts.


Which instruments are assigned to Pilot Flying and Pilot Not Flying in the various flight stages? Is there a document which highlights the responsibilities of the Pilot Flying and the Pilot Not Flying? Specifically, I want to find out what instruments are assigned to whom in all the stages of flight.

I haven't had any luck in finding any documents which highlight these points.
So far I have read through the FAA handbooks but to no avail.

For example:

During takeoff, PF focuses on X and Y but PM monitors X, Y, A and Z
Then during descent, PF focuses on C and F but PM monitors C, F and A

Are there any resources which I have failed to look at? Does any document explain this?

The general duties of Pilot Flying (PF) and Pilot Monitoring (PM), also called Pilot Not Flying (PNF), are e.g. outlined in the following section of the Boeing 737 NG FCOMv1 (NP.11.2 Normal Procedures - Introduction, emphasis mine):

  Crew Duties

  Preflight and postflight crew duties are divided between the captain and first
  officer. Phase of flight duties are divided between the Pilot Flying (PF) and the
  Pilot Monitoring (PM.)

  Each crewmember is responsible for moving the controls and switches in their
  area of responsibility:

  the phase of flight areas of responsibility for both normal and non-normal
  procedures are shown in the Area of Responsibility illustrations in this
  section. Typical panel locations are shown
  the preflight and postflight areas of responsibility are defined by the
  "Preflight Procedure - Captain" and "Preflight Procedure - First Officer."

  The captain may direct actions outside of the crewmember’s area of responsibility.

  The general PF phase of flight responsibilities are:

  taxiing
  flight path and airspeed control
  airplane configuration
  navigation.

  The general PM phase of flight responsibilities are:

  checklist reading
  communications
  tasks asked for by the PF
  monitoring taxiing, flight path, airspeed, airplane configuration and
  navigation.

  PF and PM duties may change during a flight. For example, the captain could be
  the PF during taxi but be the PM during takeoff through landing.

So monitoring instruments is primarily the responsibility of the Pilot Monitoring (hence the name). But of course the pilot flying will also monitor instruments like e.g. airspeed indicator when hand flying.

The exact duties for each phase of flight are also outlined in the FCOM, but they are of course aircraft specific. The following is part of the 737 takeoff procedure:

If you are interested in more details, I suggest you try to find an FCOM (Flight Crew Operating Manual) for the aircraft you are interested in online. It should have detailed lists of responisibilities for each phase of flight.


How much time is a commercial aircraft (e.g. A320 / B737NG families) flown manually? FMS/FMGS computers can fly many modern aircraft between waypoints set by the crew. They find their way using aids like IRS, GPS, DME, and ILS and are able to manage route, altitude and thrust.

Mode control panel (MCP) on a B737 NG (probable source: Alessio Meneghetti on Flickr)
Most of a commercial flight can be handled by computers when everything goes as planned, air regulations don't seem to forbid such automation.
On the other hand the crew ensures flight safety, programs, engages and monitors computers, handles changes, hitches and abnormal situations, takes decisions and re-programs computers with new instructions. Some configuration changes (e.g. landing gears) are also beyond computer reach.
I'm curious to know which actions are manual and how much time is spent for them from entering the takeoff runway to leaving the landing runway, e.g. for a 2,500 NM commercial flight? What is the current trend for airlines policies regarding the use of these automation capabilities?

Today's aircraft automation is just not set up for unattended operation.
There are too many individual tasks, that pilots must complete, to list here. The lowest workload for pilots is during cruise but even then ATC can instruct you to change speed, routing, or altitudes. Maybe one day this could be controlled from the ground but today only the pilots can make those changes.
I think you want to know how many minutes of an average flight are the pilots actually manipulating the controls either manually or through the A/P.

The A/P comes on somewhere between 30 seconds and 7 mins after takeoff and it stays on continually for the whole flight until maybe 2 mins before touchdown.
Every flight is different and sometimes there is lots of vectoring by ATC on departure/arrival and lots of step climb/descents, which requires the crew to "fly" the A/P. Sometimes the arrival/departure are linked up directly with the en route portion and you get a direct climb/descent.
So as a rough guess on an average flight the pilots might manually fly the aircraft for about 3–10 minutes and "fly" the A/P for about 5–10 mins. The rest of the time is spent talking to ATC, doing checklists, checking weather, fuel checks, and trying to keep yourself busy any other way you can think of. :)
I personally fly mostly ultra long haul flights of 13–15 hours as part of a 4-man crew so I also spend about 6–7 hours sleeping in the bunk.

In every phase of flight the crew is needed to do much more than just monitor the automation. The automation can not be used unless the pilots first enter all the required parameters, and then activate the automation at the right time, and in the correct order.
There is no aircraft that can take off on autopilot. The autopilot can only be engaged after takeoff. Once airborne at cruising altitude the aircraft could follow a programmed flight routing but any change in heading, routing or altitude must be manually entered by the pilot.
At the destination the approach must be manually programmed for the approach and runway in use. Once the aircraft in on the ILS approach an aircraft with autoland capability can land itself but it still needs the pilots to clear the runway.
Autopilot/autothrottle policies can vary with each airline. Typically airline pilots hand fly from takeoff to at least 200' but many hand fly to 10,000' or more for practice. When landing most pilots hand fly from about 1,000'.
Depending on the specific aircraft, autothrottle is usually engaged during the the takeoff roll and stays on until the flare just as it lands. There is no automation for landing gear and flap operation. It is 100% manual.


Can a company without an AOC hire out pilots to another company that does have one? Is it legal for a company that does not hold an Air Operator Certificate (AOC) (company A) to hire out a pilot a second company that does have an AOC (company B)? Company B would be the one actually carrying out the aerial operations.
Would this be considered a franchise arrangement?
Edit: Company B knows about a pilot who is a sole director/owner of company A (no other employees). Company B came to company A to have the pilot fly the plane for company B's operations/contract with a third party (their pilot was unavailable).

The simple answer to your question is "NO".
There are 4 different kinds of part 135 operating certificate, many of which have a limited number of pilot positions. These pilots must be listed on the paperwork for the certificate and must meet all training, experience and currency requirements as specified by the certificate.
On a Single Pilot Operator certificate, the holder is limited to using only one pilot for all part 135 operations. That specific pilot is listed by name and certificate number on the FAA issued Operations Specification (OpSpec) A040. The use of any pilot(s) other than the single pilot listed on OpSpec A040 is not authorized.
On a Single Pilot in Command certificate, the operator is limited to using only one PIC and up to a maximum of 3 Second In Command (SIC) pilots for all part 135 operations. The PIC and the SIC(s) are listed by name and certificate number on the FAA issued OpSpec A039. The certificate holder is only authorized to use those pilots in the specific duty positions listed in OpSpec A039. The certificate holder is not authorized to use any other pilots, nor are any pilots allowed to serve in a duty position (PIC or SIC), unless they are listed in that duty position in OpSpec A039.
On a 135 Basic, the operator is limited to a maximum of 5 pilots which includes SICs, but all 5 pilots may serve as PIC or SIC as required as long as they are all so qualified. However, they must still be listed ok the certificate as above and since this type of certificate requires the establishment of approved manuals and training programs, the pilots must also have completed the necessary training and comply with the standards set forth in all operation manuals.
On a 135 Standard, there is no pre-set limit to the size of the operation, but all pilots must be listed on the certificate as above and comply with all required training and manuals as above.
Basically, an operator can't just bring in a pilot to fill-in for a single mission. If their own pilots or aircraft are not available for the job, then their company is not available for the job. They would need to sub-out or recommend the client to another certificated operator whose plane and pilot(s) are available.


What are the duties of an ATC supervisor? We know that each air traffic control facility has a "supervisor", a person who is "in charge of the controllers", so to speak.

What exactly are the duties of the supervisor? Here are some of the things I've guessed:

Resolve conflicts between a pilot and a controller (by assigning a different controller to the position? Taking over the radio directly?)
Assist in emergency situations (coordinate with nearby airports / approach sectors?)
Deciding when to merge / split sectors, when to bypass noise reduction procedures due to weather etc.

Am I correct?

In the FAA, there are two types of "supervisors" who might be on duty at any given time: a genuine managerial supervisor, who is part of the managerial pay scale and is not part of the bargaining unit (union); or a "controller-in-charge," a "line" controller (a bargaining unit employee) who has been certified to perform most but not all supervisory duties if a supervisor is on break, not scheduled that day, etc. Sometimes the supervisor or CIC will be working a control position, or they might be working at a desk in the back of the operation not talking to any aircraft directly.
In either case, the general duties of the supervisory position are:

Running the shift (determining the break schedule, approving sick leave, etc)
Assigning controllers to combine or decombine control positions as traffic and complexity warrant
Assigning on-the-job training to ensure trainee controllers get sufficient training time
Monitoring the operation and, if they feel it is necessary, instructing a controller to take a specific action to ensure safety (this happens very rarely!)
Coordinating with surrounding facilities—not about individual aircraft (the controller on position does that) but about traffic management initiatives, contingency situations (e.g. telling people "we lost commercial power so we're on generators, be prepared in case we lose those too"), etc
Answering the commercial phone if people call (survey pilots informing where they will be operating, military ops calling to see if a plane can come do practice approaches, someone claiming they saw a UFO, noise complaints, etc, etc)
Keeping the daily log up to date (logging equipment outages, runway closures, etc)
Coordinating with other facilities or airport operations regarding emergency aircraft or crashes

The CIC authorized to carry "on the spot" corrections—telling the controller on position they need to do something else for safety reasons—but is not authorized to perform personnel actions like officially counseling a controller about a loss of separation. That's reserved for the actual supervisors.
Of note, if you hear a controller give an instruction and then another controller comes on the frequency and gives a different instruction, that isn't a supervisor. That's the instructor plugged in alongside their trainee controller. During training the certified trainer controller is responsible for the position, so they will give their trainee latitude to make their own mistakes but will step in to prevent an actual safety event.


What is the official term for a gas / petrol station for planes? I'm talking specifically about small airports where they don't have the fancy equipment and they actually have to go to a specific place to refuel for example cars go to gas station / petrol station so:

Is there an official term for a fuel station specific to planes?

At airport with a standalone self-serve facility, it's a Self Serve Fuel(ing) Station.  If it's a business selling fuel in addition to other services, that is a Fixed Base Operator, or FBO.


Is it possible for a modified hang glider to land safely coming from orbit? Can a glider be dropped from geosynchronous or other orbits and safely land?

That's pretty much how space shuttles and other orbiting craft work.

They are dropped from orbit, they do calculations to enter the atmosphere at a velocity and attitude that doesn't burn up the aircraft, then they glide to their landing airport.

The entire journey is controlled, but unpowered flight. The flight surfaces are there to allow the pilots the ability to reduce the velocity and attitude of descent in order to get controlled descent into the atmosphere.

So a glider can also land; assuming of course it had some way of slowing down and is sufficiently hardened and shielded against the atmospheric effects. I am not too familiar with gliders but from what I know they don't have any spoilers or other such controls.


What kind of jet plane is this? The cockpit can hold one person, is smaller than a compact car and as narrow as a motorcycle. What kind of plane is this and how hard would it be to fly?

Thats a Bede BD-5 a microjet famously flown by James Bond. Based on the tail that may be the propeller variant.

(source)


How does altitude affect sound? So I know that altitude affects the speed of sound because of temperature differentials in the atmosphere, but I am curious what else affects how we hear planes overhead?

When a plane flies overhead, you dont hear a very constant filter at all frequency bands. It seems that lower frequencies are more consistent, where high frequencies seems to fade in and out, sounding much like a low pass filter.

Is this the product of the temperature differentials between you and the plane changing as the plane moves, or are there other factors? Does moving air affect the sound in any way?

The answer to your question has a lot to do with how waves generated by a moving object propagate. You can see a full explanation here but the short answer is the Doppler Effect which causes pitch to change as the object moves buy you.

Here is a nice explanation

Edit:

My knowledge is a bit rusty but basically you are talking about a mechanical wave propagating through a fluid (the atmosphere) which will absorb some of the energy (eventually all of it). I would also thing that as the speed of sound changes the frequency of the wave would change much like the changes light has when it crosses different mediums.


How to determine individual wing loading of a triplane for stress analysis? How to determine individual wing loading of a triplane for stress analysis?

I understand that a monoplane must be able to withstand the weight of a plane x a load factor, say n=4, for a standard flight envelope, and also asymetrical loading.

What about triplanes?

Should each wing carry 1/3 the load?

Assuming 1 deg decalage each for the 2nd and 3rd top wings, they will stall before the bottom wing.

But.... you could have a situation where the top wing is stalled and the bottom 2 wings see the full load of the plane

And... you could have a situation where the top 2 wings are stalled, and the bottom wing is subjected to the entire weight of the plane.

So.... how strong do you have to make each wing of a triplane?
Of course, if the plane has struts, all 3 wings would be part of the equation.

My gut feeling is that the top wing should be 33% loading, middle wing 50% loading, and bottom wing 100% loading, and then model the wings as 3 joined cantilevered beams.

Is this correct?

A stalled wing will still produce considerable lift, so it is not on - off. That said, the best you can do is to model the lift distribution at least in a vortex lattice code which correctly computes the mutual interference. This will give you very good results for attached flow and unstalled wings.

But triplanes have successfully been designed before the availability of computers, so for a first approximation distribute the load according to the wing incidence. Lift will be proportional to the angle of attack of each wing, calculated from the airfoil's zero-lift angle. Stagger will increase the local angle of attack of the forward wing and vice versa. How much depends on the ratio between stagger and vertical wing distance. The first solid theory of lift distribution was developed by Ludwig Prandtl and his pupil Max Munk, whose tabulations can be found in NACA Report 458 which is for biplanes but can be extended for triplanes.

If you like more recent literature: Even a publication from 2009 refers to Munk's tables, so they should still be relevant today. However, it finds a lift difference between positive and negative stagger that – according to Munk – should not exist.

Note that with struts and inter-wing bracing the whole triplane is your bending beam and will collectively carry the lift loads. Only with cantilevered wings will you need to worry about the root bending moment on individual wings. Bending between struts needs still to be carried by the single wing but will be small compared to the root bending of a cantilevered wing.


What is the biggest airplane that does not need a paved runway? Since we don't need a control tower for instrument landings I was wondering what else that is thought to be needed could be skipped. I know some small general aviation is done on grass runways and I was wondering what is the biggest airplane that does not need a paved runway? Examples of manufacturer-approved usage or regular (e.g. daily or weekly) usage are preferred.

Military transports are designed to be operated from unpaved runways- by extension, any of these converted to civilian use can operate from unpaved runways.

For example, the Antonov Airlines operates a number of Antonov An-124 'Ruslan', which can and do operate from unpaved runways regularly. The Volga-Dnepr group specifically states:

  Multi-leg landing gears equipped with 24 wheels allow to operate the aircraft on unpaved runways ...


How do contrails compare between piston-powered aircraft and jet aircraft? Based off this question, I got interested on how contrails compare between jet engines and piston powered engines.

Seeing images like these from WWII, I am wondering what the significant differences are in the contrails made by the two different kinds of engines.

Image Source

As far as the contrails are considered, there is little difference between the ones formed by different aircraft. Contrails form when the ambient air (which is usually cooler) mixes with the hot, humid exhaust. This usually happens at high altitudes.

One difference may be that the different particulate matter (eg. soot) emitted by both the engines may be different. These particulates act as condensation nuclei for the water vapor at high altitudes. From FAA's Aviation Emissions, Impacts & Mitigation A Primer:

  The soot particles at cruise altitudes interact with other chemicals such as sulfuric acid and nitric acid to form small particles that act as nucleating sites for condensation of water vapor present in the upper atmosphere under certain conditions to form larger particles to form condensation trails or contrails, for short.

While I'd venture that the (older) piston engines emitted more of these particulates, the only study i could find online indicates that the particulate emission is similar in case of gas turbine engines and aviation piston engines using unleaded petrol (while significantly higher in case of older, leaded gasoline, as was used during WWII).

Both images from Microphysical and Chemical Properties of Nanoparticles emitted by Flight Engines by Claus Wahl, Theo Rindlisbacher and Lars Hjelmberg

This would mean that the effect of particulate emissions on contrail formation would've been the same. However, there is another factor- altitude. In general, the (modern) jet engines fly much higher than their piston engines counterparts- the contrails you see in WWII bombers are formed by supercharged engines flying at high altitude to escape from AA defenses. Also, as @ymb already noted, the size/shape and number of exhausts also vary between the piston and gas turbine engines.

Though the mechanism of contrail formation is the same in the piston and gas turbine engines, the difference between them lies in their size/shape per engine and their operating envelope.


Is there a way I can find out online what private jets are parked tonight at a nearby international airport? There is an international airport about 10 miles from my home.
Is there any way I can find out online what private jets are parked there tonight? I am not familiar with any of the lingo used here :) so if you could respond in layman's terms I'd greatly appreciate it.

There is not simple list available online, apart from calling the airport and asking them directly. Aircraft are expensive assets so when people own them they tend to keep them under the radar and out of sight and out of the elements by parking them inside a hangar. As an outsider you can only see the outside parked aircraft from the airport fencing.

Determining which aircraft are parked for a short term on an airport is relatively simple to find with the use of an aircraft tracking site such as FlightAware and FR24. On these sites you can enter an airport and see all the official flights in and out of that airport. Flights that have landed but not yet departed will be parked somewhere on the airport.
For example, Lubbock Executive Airpark (Lubbock, Texas, USA)
https://flightaware.com/live/airport/F82/arrivals

Starting from January 2020, the FAA dictates that all airborne flights in the USA should be equipped with an ADS-B transponder, when inside ADS-B airspace, used by the sites above and air traffic control to track flights. So from that day the tracking of smaller aircraft will be greatly improved.


What kind of cameras are onboard high altitude UAVs that aid in its control? I ask this question in the context of people flying their drones much higher than the usual hobby planes. Satellite based GPS navigated Autopilots using photogrammetry and/ or military drones.

What kind of cameras do first-person-view (FPV) pilots use and mount onboard to control the plane, and not for any sensors to collect data/ perform other tasks?

The Bottom Line Up Front

Infrared and visual video cameras aid in navigation of larger class UAVs and are helpful for safety but do not satisfy the legal requirements for unmanned aircraft to operate in US/international airspace. Instead, the use of onboard navigation systems like GPS, combined with air traffic control services and common flight safety rules allow safe navigation of high and medium altitude UAVs.

What UAVs Operate at High Altitude?

Northrop Grumman's "RQ-4" variants (e.g. USAF RQ-4B Global Hawk, US Navy MQ-4C Triton, NATO RQ-4D Phoenix) are the only widely recognized UAVs flown at high altitudes (FL500-600). The RQ-4 is also the largest UAV with a wingspan greater than a Boeing 757.  All other UAVs, including the MQ-9 Reaper, operate at medium or low altitudes.

How Does the RQ-4 Navigate?

The RQ-4 has sophisticated, GPS & gyro-enabled navigation computers that ensure it always knows where it is and where it is going. To avoid airborne hazards like other aircraft or bad weather, it follows the same instrument flight 'rules of the road' that manned aircraft rely on when flying at night or in the clouds. Specifically, air traffic controllers use radar to track aircraft from the ground and alert the pilots if they see an unsafe situation on their radar screens. These instances are rare, however, because everyone follows their flight plan and obeys air traffic controllers' instructions in order to remain safely separated in the air--even remotely piloted aircraft.

Does the RQ-4 Have Any Cameras Used to Aid in its Control?

Yes. As a backup to air traffic control clearances to taxi, takeoff, and land, the RQ-4 has a forward-facing infrared video camera on the nose of the airplane.  Although it is not legally required to have this camera, it is used only as a backup for the pilot to ensure the runway is clear of obstacles during taxi and prior to landing on the runway. Radio transmission of the video camera signal is via line-of-sight antennas only (not over satellites), so therefore it is limited only to the airfield/runway environment where the ground control station is located and is not used while away from the airfield during climb, cruise, or descent.

Note there are two small windows in the pressurized camera compartment: One upper window for the video camera lens and the lower window used to display the pressure dial for the camera compartment.

What About Medium/Low Altitude UAVs?

Medium altitude UAVs like the MQ-9 predator carry a variety of infrared (day/night) and electro-optical (day only) full motion video cameras used primarily for their mission rather than for navigation. Unlike the high-altitude RQ-4, they operate at the same altitude levels as commercial air traffic.  They can follow the same instrument flight 'rules of the road' that manned aircraft do when flying at night or in the clouds. Often, their mission requires them to fly in areas where following these rules aren't required by all aircraft. In this type of airspace, pilots of smaller aircraft like gliders and crop-dusters don't have the radios to communicate to air traffic control and count on other aircraft to 'see-and-avoid' them instead. The MQ-9 does not have a pilot onboard to look out the window and avoid these 'visual flight only' aircraft, so it needs other solutions to ensure safe navigation.

The MQ-9 does have a powerful ball-mounted, full-motion video camera onboard. However, the USA's Federal Aviation Administration and the worldwide International Civil Aviation Organization has not deemed the use of this camera as a sufficient substitute for the see-and-avoid requirement. Therefore, despite the MQ-9's robust visual/infrared camera system, it must rely on other methods of deconfliction like temporary/military restricted airspace, special clearances, and even chase airplanes, in order to conduct its training in the US and its missions overseas.  The same challenges exist with smaller, low-altitude military unmanned aerial systems and commercially operated drones.

Are Cameras Any Good For Long Range Navigation?

That depends on what you consider long range. Navigation using cameras, especially to avoid other aircraft, requires low-latency. It does little good to see a rogue aircraft filling your windscreen if, by the time you see it on the screen, the plane has already crashed into your UAV. This usually isn't a problem for FPV hobbyists because they have a line of sight video signal, directly from the RC aircraft, that feels instantaneous to the pilot wearing FPV goggles. On the other hand, truly long ranges require operation beyond line of sight, relying on satellites to pass information to/from the aircraft. This extra architecture causes video latency, sometimes up to multiple seconds, making it nearly impossible to visually react to a time-critical safety event.

A Better Alternative to Cameras

In addition to the video latency problem, video cameras on medium/high altitude UAVs are specialized to focus on small targets at long range--not scanning the skies for unidentified aircraft. Imagine trying to spot a fly buzzing around you while looking through a toilet paper tube. An additional, wide-angle navigation camera/lens could be carried on the aircraft but it would not be worth the additional cost, weight, space, and power consumption. Instead, long-range aircraft like the RQ-4 and MQ-9 rely on a framework of aircraft regulations, ground-based air traffic control, and segregated airspace to safely navigate.

Note: This response was drafted from scratch, solely to answer Rajath's question about cameras on high altitude UAVs. Thanks in advance for your feedback.


Is engine sync to avoid beat frequencies also a concern for jet engines? Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?

Context:

This question on SE-Aviation discusses prop sync for multi engine prop aircraft  to eliminate beat frequencies that can be annoying. But it says nothing about any multi-engined jet aircraft.

On the other hand, this other question on SE-Aviation refers to why it is preferable to run all engines at the same RPM (and hence differential thrust is NOT used for directional control) and I presume the answer applies to both jet and prop aircraft.

Which begs the question above:

Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?

If it is, how is this done in typical modern jet aircraft? Is there an automatic system in the FADEC etc?

Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines.

MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged:

  Engine Synchronizer System

  The engine synchronizer system automatically matches the N1 or N2 speed or
  EPR of left engine to match right engine. The system includes a three
  position (N1, OFF, N2) ENG SYNC selector, ENG SYNC ON annunciation, and control
  system with actuator to trim left engine fuel control. The pilot can override
  the system at any time with throttles.

  When the ENG SYNC selector is placed in N1 or N2, signals from the tachometer
  generators causes engine synchronizer system to trim left engine fuel control
  until left engine RPM (N1 or N2) matches right engine.

(MD-80 FCOM Sec. 18 Power Plant - General)
CRJ: The Bombardier CRJ-700/-900 can also synchronize by either N1 or N2 via the FADEC:

  N1 and N2 Synchronization

  In the cruise range, thrust is set to a specific value by the FADEC based on throttle
  position. Synchronization allows the FADEC to match the fan (N1) or the core (N2)
  speed of the two engines for noise reduction. Synchronization is selected by using the
  SYNCH switch on the engine control panel . The left engine is designated as the
  master engine and the right engine will be slaved to it.

  For N1 synchronization to be enabled, the right engine N1 must be within 1.5% of the
  left engine N1.

  For N2 synchronization to be enabled, the right engine N2 must be within 7.5% of the
  left engine N1.

(CRJ FCOMv1 chapter 20 Power Plant - Thrust Control)

As far as I know, no Boeing or Airbus aircraft has an engine synchronization system. Presumably, the beat is less of a problem for wing mounted engines since the passenger cabin is shielded from direct sound by the wings. For tail mounted engines, engine noise is more of a problem in the cabin.


Why is Denver International Airport runway 16R/34L rarely used? From watching planes land at DIA on Flightradar24, none seem to ever land on 16R (longest runway in the US at a length of 16,000 ft), but instead land on a runway 4,000 ft shorter such as runway 16L (12,000ft). So why do even large planes like the 747 not land on the longer runway to have an easier landing?

KDEN has a lot of runways, so they have the option of not always using some of them. As FreeMan pointed out, 12,000 feet is plenty for just about any aircraft flying in or out, even at 5,300 feet elevation. Being such a long runway, it may also be good to leave it open so that no aircraft need to be moved in case an aircraft declaring emergency would need the extra length. Runway use will depend on the wind and traffic, so depending on when you are observing, you may not often see conditions where it is used.

North flow: arrivals on 34L could interfere with runway 7/25, and for departures it is further from the terminal than 34R. TomMcW also points out potential noise abatement concerns.

South flow: departures on 16R could interfere with runway 7/25, and for arrivals it is further from the terminal than 16L.

Even with 17L/35R currently closed it is not always necessary to use 16R/34L. 8/26 and 7/25 can be used for east/west departures and arrivals, and 17R/35L can be used as well.

Runway 16R/34L can still be used as a secondary runway though. It can certainly be used as 16R for parallel approaches with 16L, or departures as 34L with approaches on 34R.

To put things in perspective, Denver has more runways than both KLAX and KATL, which see more aircraft movements.

Also, while far from authoritative, the virtual Denver ARTCC lists 16R/34L as a secondary runway in all situations.