What is the longevity and replacement schedule for turbine engine blades? I'm trying to understand how frequently hot section turbine engine blades are changed out. Is this a calculation based on flight hours, material, engine size or maybe something else?

Apologies if this is an obvious question, conducting some research for work and do not have any prior background in this area.

Thanks.

There are two primary modes of turbine blade failure, that can be predicted, and hence are used to determine the lives of turbine blades at which they need to be replaced.

One is called "creep". The Wikipedia article describes the basic mechanism. The life of the blade is purely a function of "time at temperature". If the blades experiences flight time above critical material temperature limits, it accumulates a portion of creep fatigue. The total (cumulative) amount of creek fatigue is tracked, and when the blade hits that limit, it is removed and replaced with new blades. The type of algorithm used to measure creep life is called Robinson's rule, and looks like this (where c1, c2 & c3 are coefficients related to the relative damage that occurs at the associated temperatures):

Total accumulated life = c1 x time above T1 deg + c2 x time above T2 deg + c3 x time above T3 deg.

Typically, each stage in the turbine will have its own life. (Each blade on the same stage is not individually monitored and lifed, but rather they are replaced as a set.)

Another failure mode is low cycle fatigue. This is caused by changes in centrifugal stress, caused by rpm changes from throttle cycles. When the pilot moves the throttle a significant amount, large changes in stress occur. These cause the blade material to behave like bending a wire in your fingers back and forth to make it break (you don't break it by just bending it once, or pulling hard on both ends). LCF lives are counted in a similar fashion to creep, but rather than counting time spent above a temperature limit, throttle cycles are counted. A throttle cycle might be defined as a change in rpm from one specific value to another.

If the engine is modern, the electronic control would count the creep and LCF lives consumed. If the engine is old, and only has a hydro mechanical control, the engine manufacturer will do an analysis on the type of flying the plane is expected to do, and determine the life limits in terms of flight hours. Periodically, they will try and confirm how the plane is actually being flown, and if the flight time before replacement needs to be shortened or can be lengthened. Generally, either the creep life or the LCF life is much shorter than the other, so only one failure mode is tracked for each engine stage.

This document gives a good description of turbine blade failure modes, and how they are monitored. I doubt you get information on actual lives, that is likely proprietary to the engine manufacturer. The only info you might find is probably for very old engines that they don't care about anymore. However, if you are lucky, you might finds some numbers burried in NTSB or other countries aircraft accident reports related to blade failure.

There are also periodic bore scope inspections, that may cause blades to be replaced for unscheduled (unpredictable) failure modes (FOD damage, coating loss, thermal distress) as also mentioned in the above document.


What powers the air-conditioning on commercial planes? How is the air conditioning system on airliners powered? I expect that the air conditioning is powered by the APU powers, but I wonder if there are other power sources used like the main engines?

What powers the airconditioning on most airliners is APU or main engine compressor bleed, which is being supplied at fairly high pressure, say, 80 psi, and temperature, around 3-400F, to the air conditioning packs.  You normally have a choice to select APU or main engine bleed.  You will obviously use APU bleed on the ground before engine start, but also may opt to take off with APU bleed running the air conditioning instead of the engines for slightly better takeoff performance.  On some airplanes, packs on APU bleed can be a requirement for departures in icing conditions because of the performance drain of bleed being used for anti-ice and air conditioning at the same time.
Large jet aircraft (and large turboprop) air conditioning doesn't use a gas vapour cycle like regular air conditioners (some smaller jets and turboprops, that can't produce the bleed flow, as well as piston a/c do however); they get the same result using just the high pressure, high temperature bleed air using a "bootstrap effect", where air is cooled off while still at high pressure, than letting it drop close to ambient pressure after a lot of the heat has been removed, resulting in air that can be very cold.
At the pack, part of the flow gets chilled and part gets bypassed while still hot.  Temperature regulation is by mixing the chilled flow and bypass flow. Electrical power is only required for the control system.
The air being chilled goes through an Air Cycle Machine. An ACM takes high pressure air, drops the temperature using heat exchanger pre-coolers (air-to-air radiators) while the pressure is still high, boosts the pressure and temperature again with a compressor, takes out some of the heat from that pressure boost with more heat exchangers, and you end up with air that is still at high pressure but the temperature is way less than the original bleed.  Then you suddenly let the pressure drop close to ambient by allowing it to expand into a much larger discharge duct.
Air that started off at 80 psi and 400F is now down to 150F while still at 80 psi.  When you let it drop to ambient, the temperature drop from the pressure drop will take the ACM discharge temperature down to just above freezing (theoretically, if you put all the bleed through an ACM, you can get the air down to deep sub-zero temps, but the control system will normally keep the discharge from going below freezing).
The compressor is powered by a turbine in the discharge flow where the expansion temperature drop happens. It's like a big turbocharger unit (the compressor driven by a turbine in the same flow is what constitutes the bootstrap effect).  The final temperature that goes to the cabin is a mix of chilled air and hot air that is bypassed from the ACM by the control system.
You may also be able run the system from a high pressure ground bleed air source (called a "Huffer Cart"), but airlines usually use Huffer Carts for engine starting and instead will run ground air conditioning units that connect directly into the aircraft low pressure ducting (when you see the yellow flex pipe hooked up to the fuselage at the gate, that's a ground low pressure air conditioning source).


Why are jet aircraft never designed with a slower cruise speed? All jet airliners have a cruise speed between Mach 0.82 and Mach 0.85.

At those speeds the aircraft are flying at their maximum subsonic speed. Any greater cruise speed would only be possible with major design changes.

My question is about fuel efficiency.

As drag increases with the square of speed, would a slower cruise speed lead to better fuel efficiency?

So why are jet aircraft never designed with a slower cruise speed?

If transonic phenomena didn't exist, would a faster speed be more fuel efficient?

Currently some airliners, after flying a 12-hour route, stay on ground for a long time before returning. Wouldn't a slow flight (if it is more fuel efficient) be better in this scenario? So not to waste time on ground?

On long-range flights, any fuel saving significantly increases the flight's profitability.

There is indeed a benefit in flying more slowly, but the aircraft needs to be designed that way to profit from slower speeds. You can see that wing sweep has been slightly reduced in the more modern airliners, but this is also due to better airfoils with higher Mach drag rise onset.

But some things will also cost more when you slow down: You have to pay the crew for more hours, and the aircraft cannot be utilized as often. To transport the same number of passengers requires more aircraft when they fly at a lower speed. The cost versus speed relationship is not linear, but it should have a minimum somewhere below today's operating points. Aircraft with a slower design cruise speed can be lighter and smaller for the same payload, but for now airlines don't have a choice of slow or fast jets; all manufacturers try to design their long-range airplanes for at least Mach 0.83.

This has a lot to do with marketing: The aircraft with the higher speed will show a shorter travel time for the same distance, so it will show up first in the booking systems of travel agents. Of course, now you will argue that most people book on the internet and will try to get the lowest cost, regardless of travel time. True. But this is not the sort of customer the airlines are after. Their profit comes from people in First and Business class, and these still book predominantly via travel agents. Therefore, Airbus and Boeing try to market their planes as the ones which come first on the Amadeus and Sabre screens.

If you would single-mindedly optimize a configuration for the best fuel efficiency, you would arrive at something close to the MIT D8 design or Boeing's SUGAR study. Note that both are designed to fly quite a bit slower than current airliners at Mach 0.72. These designs are for a world in which fuel costs 200 USD per barrel or more, while the current designs are based on the expected prices at the time of their design start.

When fuel prices were low, the optimum speed for the best transport performance was indeed above Mach 1. The Vickers VC-10 of 1964 sported a maximum cruise Mach number of 0.886 and still holds the record for the fastest commercial Atlantic crossing. When Concorde was developed, the general consensus was that future air travel would be supersonic. Only the jump in oil prices after the 1973 oil crisis put those plans to rest. Large business jets today can cruise at up to Mach 0.935 because their owners are less concerned with saving money by flying more slowly. Technically, there is still room for cruising above Mach 0.85; it's economics that keeps airliners back.

If airplane companies were free to pick the optimum Mach number, they would today choose a speed between Mach 0.78 and 0.82 with fuel prices around 50 USD per barrel. Note that this is the design Mach number range for regional jets which fly short hops in which the increase in trip time from the reduced speed is insignificant. Also, especially in times of high fuel prices airlines operate their gear at lower speeds in order to conserve fuel. But if you follow this link, you will learn that airplane companies don't have that freedom.

Regarding drag:

Drag does not go up with the square of speed. When plotted over speed, the drag of an airplane will first go down, reach a minimum and only then go up. Airliners fly close to this minimum, and they fly high to shift this minimum to the highest possible flight Mach number. They do this by flying in less dense air, which requires to fly at an altitude around 30.000 - 40.000 ft. An additional benefit to flying this high is the lower air temperature which makes the engines more efficient.


Why doesn't the Convair 580 have check valves to prevent inadvertent fuel transfer? The Convair 580's fuel system is composed of two wing-mounted fuel tanks (one in each wing), each with its own shutoff valve (which can be closed to isolate its respective tank from the rest of the fuel system and prevent fuel from entering or leaving the tank) and boost pump (which forces the fuel, under pressure, to the respective engine(s)), plus a pair of crossfeed valves (one on each side of the aircraft) to allow an engine to be fed from its contralateral fuel tank if necessary.

Normally, the crossfeed valves are kept closed, and each engine is fed from its ipsilateral fuel tank.  However, if crossfeeding does become necessary (for instance, if one fuel tank drains due to a leak [in which case both engines must be fed from the non-leaking tank] or if one engine fails [in which case the remaining engine must be periodically switched between the left and right tanks to prevent the weight of the fuel in the tank ipsilateral to the failed engine from unbalancing the aircraft and causing a loss of control]), the crossfeed valves are opened, and the tank shutoff valve for the tank not being used must be shut off; otherwise, the boost pump for the tank being used can end up pumping fuel into the tank not being used rather than to the engines.1

Transferring fuel from one tank to the other is expressly prohibited for the Convair 580, as doing so could lead to, in increasing order of severity:

Fuel being lost through the overflow vents in the recipient tank (which is bad because it wastes fuel);
The aircraft's entire fuel supply being pumped into the tank not being used (which is bad because it can result in both engines flaming out despite the aircraft's fuel system containing plenty of fuel);
The recipient tank (and its associated wing) bursting (which is bad for reasons that should be immediately obvious).

However, there is no mechanism to prevent fuel from transferring between tanks if the pilot forgets to close the shutoff valve for the tank not being used, which leads to the Convair 580 being an aircraft where fuel transfer is, simultaneously,

Prohibited under any and all circumstances, as doing so could cause a fuel tank to overflow and/or burst; and
Quite easy to make happen, simply by opening the crossfeed valves and forgetting to close the shutoff valve for the tank not being used.

This seems almost trivially easy to solve, by placing one-way check valves in the lines leading from the fuel tanks so that fuel is physically prevented from transferring from one tank to the other, but, according to the NTSB (footnote 46, page 25/35):

  One-way check valves are not typically installed on airplanes, and the accident airplane did not have one installed. According to a Kelowna Flightcraft representative, one Convair 580 operator modified its fleet of about 30 Convair 580 airplanes in the 1960s under an engineering order by installing a one-way check valve, which prevented fuel from flowing back into the fuel tanks when the fuel tank shutoff valve was left open. However, the representative stated that this operator was no longer in business and that its airplanes had represented a small percentage of the entire Convair 580 fleet.

I can see no reason not to use one-way check valves in the Convair 580's fuel system, given the potentially-lethal consequences of transferring fuel between tanks and the frightening ease of inadvertently doing so.  So why doesn't it use them?

1: The boost pump for the tank not being used must be shut off to keep fuel from being pumped from that tank, rendering it incapable of preventing fuel from transferring into the tank not being used.2  Even if the pilot forgets to turn it off, fuel can still be forced through the pump in the reverse direction if it (the pump) happens to be less powerful than the boost pump for the tank being used.

2: And, if the pilot closed the shutoff valve without turning off the associated boost pump, the force of the pump pumping against the closed valve could potentially cause a fuel line to burst.

A check valve is another failure point in the fuel system (they can stick) so you have to do a risk analysis to decide if the benefit of adding them outweighs the risk of the added failure mode introduced, with sufficient weight to justify mandating them by AD.  With no AD, it's up to KFC to decide if it wants to do anything on its own.

Assuming this was a one-off event (the report only mentions one previous less serious  but similar event) that is more or less attributable to bad piloting, the balance of risk, when added to the burden of forcing an AD on operators, would have been determined to not fall within the threshold needed to mandate such a change (I've participated in these kinds of projects over the years).

If another crash or two happens later with more or less the same circumstances, it's more likely to trigger an AD.  But these airplanes have been flying a very long time like this.

Yes there is a tendency to think the need to do something is obvious, but if ADs were issued for every potential design deficiency to cover every eventuality, there would be eleventy jillion ADs out there.  That being said, there was probably an internal debate with discussions between Transport Canada and KFC, with pro and con factions arguing it out, and TC was probably convinced that mandating check valves would not provide sufficient risk mitigation to justify the burden of issuing an AD (The FAA could do something on their own, but usually they will work with TC on this and go along with TC's resolution).


Why are the AOA sensors placed outside the aircraft? Why should the AOA sensors be placed outside the aircraft? Is it not the angle between the chord line of a wing and a vector representing the relative motion between the aircraft and the surrounding air, in which case, is it not possible to measure the AOA with a simple instrument inside the cockpit, which eliminates malfunction due to exposure?

No, it is not possible to place the sensors inside the aircraft, because the angle-of attack sensors need exposure to the surrounding air, in order to measure, as you mention yourself, "relative motion between the aircraft and the surrounding air".


Exactly where can a stowaway hide in a wheel well? My question is exactly where a person could contort their body so as not to be mangled by the gear retraction system and very limited space. Some lived and died in the nose well and as I remember it, most aircraft at least provide half the nose wheel diameter (for several feet) to the nose strut (between strut and gear doors). The nose doors suddenly opening would explain why they fell to their death.

Presumably most people attempt to stow in the main gear and I just don't understand how?

WIKI lists approximately 150 known wheel stowaways. A cursory review shows about 20% percent lived through the ordeal. Most die of hypothermia, hypoxia, trauma from falling on the ground or in the air, but a few are crushed or mangled. Despite my knowledge as a pilot and A&P, I could not make an educated guess what a person would need to do as the gear retracted to keep from being crushed.

Medical issues are not part of this question and were covered by WIKI and reviewed in a  related question that asked "How can a stowaway hide in the undercarriage?" on SE. No precise aircraft volumetric space capability was given or the "aerobatics" (I could not resist) needed as the gear retracted.

I am particularly impressed that stowaways survived DC-3, DC-8, and most surprising is the B737; these are all aircraft I have worked on and can not imagine how it is done. No one listed attempted B757 but there is a somewhat spacious 12-18in in front of the retracted main gear for cables, fuel lines and valves that could possibly allow a human space.

For the 737, I think this rather sad photo provides one answer:

(Source)

  The body of a stowaway was found in a Kenya Airways flight from Kinshasa in the DRC on Sunday.
  The body was found after the plane landed at the JKIA, preliminary reports indicating the person froze to death.

And, as you can see in this photo, the ledge there (on the left forward side of the main wheel well) is indeed just wide enough for a slim person to contort themselves into:

(Source)

But, as you can see at the end of this video, there certainly isn't a lot of room for the stowaway's legs.


Are there requirements for ice protection for general-aviation aircraft? Does anyone know if any regulations exist making it a requirement for general aviation aircraft to have ice protection systems?

And any insights on why general aviation aircraft tend to use rubber boots and fluid based systems as opposed to electrothermal solutions?

Does anyone know if regulation exists making it a requirement for general aviation aircraft to have ice protection systems?

In specific answer to your question: Yes, there are regulations for general aviation aircraft to have ice protection systems if the aircraft is to be flown
into known icing conditions.

FAR Part 23.2540 (applies to general aviation type aircraft) does specify certification criteria if the manufacturer is requesting approval
for flight into known
icing conditions.
FAR Part 91, Subpart F- applying to Large and Turbine-powered mulitiengine airplanes and Fractional Ownership Program aircraft, does specifiy the
conditions, equipment, etc. required to fly into known icing conditions. (business jets/turbo props, etc)
General Aviation aircraft operating under certain regulations, FAR 135.227 for example, are subject to specific regulations pertaining to operations into known icing conditions .
FAR Part 91.9, applying to all Part 91 operations, while not uniquely applicable to operations in icing conditions,
does require compliance with all operating limitations published in a flight manual, placard,
POH, etc.  So, if the POH for the aircraft in question prohibits flight into known icing conditions (as many general aviation aircraft manuals/POHs
do) then it cannot be done. However, there are also many general aviation aircraft that are permitted to fly into known icing conditions in accordance
with stipulations published in the POH (or similar).

  And any insights on why general aviation aircraft tend to use rubber boots and fluid based systems as opposed to electrothermal solutions?

The only insight I can offer regarding boots/fluid systems versus heated solutions is the manufacturer's choice based on performance considerations,
functional utility, and interoperablity with associated systems.  For example, an engine driven air pump for deice boots creates a performance overhead
that may not be worth the expense, assuming it's an option, based on how the airplane will be used.  Electrically heated props (heated near the hub)
are another option for many general aviation airplanes that can be added assuming certification for flight into icing exists.  Bottom line is, I don't
think there is a "preference" for general aviation airplanes, I believe design considerations and after-market options are the predominate factors underlying the type of deicing/anti-ice equipment available in most general aviation airplanes.

Here is an excellent FAA Advisory Circular that pertains to your question:

AC 91-74B - Flight in Icing Conditions


Do you need a commercial rating for aerial application work? So to be qualified to do aerial application work do you have to have a private license with a commercial rating?

Short answer: you can do aerial application on your own crops with a private certificate, but if you're going to do it commercially then you need a commercial certificate. In both cases you also need an agricultural aircraft operator certificate.

An agricultural aircraft operator certificate is required for aerial application, per 14 CFR 137.11. The airman certification requirements are in 137.19:

  (b) Private operator—pilot. The applicant must hold a current U.S.
  private, commercial, or airline transport pilot certificate and be
  properly rated for the aircraft to be used.

  (c) Commercial operator—pilots. The applicant must have available the
  services of at least one person who holds a current U.S. commercial or
  airline transport pilot certificate and who is properly rated for the
  aircraft to be used.

The FAA's AC 137, Certification Process for Agricultural Aircraft Operators has all the details about how to get an operator's certificate and the airman requirements are in section 2-3. It says:

  Pilots designated to conduct operations for the commercial operator
  applicant must hold a current commercial or airline transport pilot
  certificate with the appropriate ratings

A private operator is limited to working his own crops:

  Private agricultural aircraft operators may not conduct operations for
  compensation or hire, or conduct operations over a congested area. The
  operator must provide proof of property ownership or other property
  interest in the crop located on that property where the operation will
  be conducted.

By the way, there's no such thing as a "commercial rating" in the US; private and commercial are both certificates.


What is the difference between leasing and chartering? As written above, what is the difference between leasing and chartering? Is it the same thing and if not what are the differences?

Leasing is just a long term rental of the chattel, the airplane itself, where the lessee entity does whatever it wants with the airplane during the period of lease. So you lease an airplane from a leasing company like GE Capital, and do whatever you want with it until the lease is up, then return it.
A charter is a more restrictive and objective-focused contract: it's an agreement to use aircraft for a specific purpose defined in the charter contract.
So if I want an airplane for a year to be at my beck and call to do whatever I wish, I lease it for a year.
If I want an airplane for a specific purpose, like transporting my football team around between cities X, Y  and Z for a month, with no other uses allowed unless specified in the contract, I charter it.
Usually, the charter contract is "turn key" and will include crew and support services, whereas with a lease, the user is only getting the airplane and has to provide all that themselves.
You can also have all-inclusive leases, called "wet leases" that include crews and maintenance, possibly even fuel, built into the lease rate, but you still have freedom to use the plane however you wish, as opposed to a charter where the use of the aircraft is specified in the charter.
An airline might wet lease an airplane when they need a short term replacements for grounded aircraft, especially if the type is different from their fleet.


When the brakes are checked, what do the firemen actually do? Watching this video (at 06:58 mark), this plane had just made a mayday landing, due to a bird-strike. Because of the right engine being shutdown, it was forced to land immediately, with a full load of fuel. The added weight of the fuel, combined with loss of the additional stopping power of reverse thrust, caused the pilots to bear down on the brakes harder and longer than would be normal in an end-of-flight landing.

The pilot requests to hold on a taxi-way after vacating the runway, during which time the firemen come up to the plane to "inspect the brakes."

What do the firemen actually do when inspecting? Are they looking for fire? Glowing brake pads? Do they use an infra-red heat gauge?

There are specific protocols that procedurally are required to be followed. Insofar as brake inspection is concerned, it would have been obvious from the trucks that there was no brake fire. It's also obvious that there's not a major amount of smoke coming from the brakes. As they approached the landing gear, they would have smelled serious brake heating. Glowing brakes would be seen then if not sooner. A close visual inspection for very light smoke and sensing the heat coming off the brakes would give them an idea of the rate of brake vaporization. It's a subjective judgement based on what they see and feel.

As I understand it, tires that dangerously overheat will explode before melting, and that's why thermal plugs are used which melt and deflate the tire to prevent their exploding.

At a landing in Nandi, Fiji (short runway) with what turned out to be a tailwind in a 747-200 at near-max landing weight, we had to use max braking, and I called for huffers to cool things down, and of course for an inspection. I exited the aircraft as quickly as possible to see how bad things were. I was amazed and alarmed to see thick dark grey smoke pouring from the body and wing landing gear wheels. I thought we were close to having a fire. However, the ground personnel assured me that there was no cause for alarm, that the smoke was just brake vaporization that would shortly stop as they blew air on them. It did, and after refueling we departed without incident.


What happens when an aircraft breaks the sound barrier? What happens when an aircraft breaks the sound barrier? Why can't it break the sound barrier near the ground?

The expression "sound barrier" was created maybe 70 years ago when approaching the speed of sound made aircraft react in unanticipated ways. Actually, there is no fixed barrier, and in reality the transition can be rather smooth, provided the aircraft and its pilot are prepared for it.
The speed of sound is the maximum speed with which small pressure changes will propagate through a medium, so at subsonic speed the air ahead of the aircraft can react to the approaching aircraft. Also, while local air density changes only little at subsonic speed, air density changes become dominant at supersonic speed. To make way for an approaching aircraft, subsonic air will speed up while supersonic air will slow down so that density increases to make way for the supersonic aircraft.
At subsonic speed, pressure and speed will change smoothly while air flows around the aircraft. As a consequence, the center of local pressure changes (its lift force) acts at around one quarter of chord, such that the aircraft is balanced when its center of gravity is at the same location.
At supersonic speeds, the air will be taken by surprise - at one moment all was calm and quiet, and suddenly the air molecules get kicked around by an unknown intruder. Pressure changes suddenly, through a shock, so instead of a smooth transition, at supersonic speed there are regions of similar pressure, separated by sudden drops or jumps. As a consequence, the center of pressure changes shifts backwards to 50% of chord. If the center of gravity remains at a quarter chord, the consequence is a strong pitch-down moment: The aircraft will nosedive.
To make matters worse, a control surface deflection, which could redistribute the lift between wing and tail surface, will not necessarily work in the same way as it would at subsonic speed: the aircraft might become uncontrollable. See this answer for a mode detailed explanation.

The cone you see in the right picture is a Mach cone, which would be caused by a supersonic aircraft. The picture was shamelessly copied from this blog.
The trick is now to give the air some advance warning where it counts, even when the aircraft travels at supersonic speed. This can be achieved with wing sweep, because if the sweep angle is larger than the cone angle in which pressure changes will propagate at supersonic flight speed, the air flowing over the wing will be forewarned, thus reacting similarly to subsonic flow. To correct for the inevitable shift in the center of pressure, the tail surfaces are bigger and full-flying in supersonic aircraft, so they work in trans- and supersonic flow. Also, by pumping fuel, the center of gravity can be shifted backwards, so less trim change is needed.
The sound barrier can be broken at any altitude, if the aircraft has a sufficiently powerful engine and is stiff enough. Normally, to save weight, the designers set a limit for maximum dynamic pressure (= air density times air speed squared, divided by two), so the structural deformation at this maximum dynamic pressure is small enough. Note that the deflection of its ailerons will deform the wing of the Eurofighter at maximum dynamic pressure to an extent that three quarters of aileron effectiveness is lost - the ailerons cause a twisting moment which warps the wing such that it works like the wing warping mechanism in the Wright Flyer, only in opposite direction to the aileron input.
Since density drops with increasing altitude, the same dynamic pressure is reached at higher speed, allowing aircraft to fly faster the higher they fly. The next limit is given by the local heat near the stagnation line. If air is decelerated, its temperature will increase with the square of the speed difference. The maximum continuous speed of the F-22 was reduced from Mach 1.8 to Mach 1.6 to avoid overheating the sensitive composite wing structure.


How does autobrake work? An autobrake is a type of automatic wheel-based hydraulic brake system for advanced airplanes. The autobrake is normally enabled during takeoff and landing procedures, when the aircraft's longitudinal deceleration system can be handled by the automated systems of the aircraft itself in order to keep the pilot free to perform other tasks - Wikipedia

How does the aircraft "know" when is time to activate the autobrake
systems on a rejected takeoff and landing?
Does it apply full brake to all the aircraft's wheels?
Is it really used by commercial jets?

Not all commercial jets have autobrakes, and the ones that do usually have a company policy dictating when it will be used.

The criteria for activating the brakes are different depending on the manufacturer and model of aircraft, but are typically some combination of:

System must be armed
A certain takeoff power setting must be set by the throttles
A certain airspeed must be obtained
Thrust levers must be pulled back to idle
Aircraft must still be on the ground

Usually for a rejected takeoff it does use max braking, however the autobrake usually has different settings for landing along the lines of "light/medium/max" braking.  Each setting will use a different amount of runway for landing, much like lightly pressing on the brake pedal in your car versus jamming it to the floor.


I would like to know where ideally it is to be sat on an Emirates A380 to get a nice picture of the wing and wingtip as well as the outer landscape? I would like to know where ideally it is to be sat on an Emirates A380 to get a nice picture of the wing and wingtip as well as the outer landscape?

LHR-DXB 7:50 P.M

The best seats would probably be in the back because you can see the wing and the ground without having to twist or having the wing in your view.

Here is a picture I took from the back end of a Boeing 757-200. Is it the view that you want?


How does the throttle position affect the speed of airflow to the engine? It is said that the less we open the throttle the slower is the speed in Venturi tube, thus less reduction in pressure and less fuel is being sucked from float chamber, however, actually if we reduce the area of the tube by closing throttle ,according to Bernoulli theorem, the speed of air in the same tube should increase, shouldn't it?

That's because it's not the restriction created by the throttle plate itself, that you see as manifold pressure (or vacuum), that is creating the suction that pulls fuel from the carburetor float bowl.  It's the restriction of the venturi, where the main discharge nozzle is, which is upstream of the throttle plate.  Since it's in in the upstream flow, the suction created at the venturi, being a fixed cross section, is a function of air velocity entering the carburetor, which is a function of throttle opening and engine speed.

In other words, the fuel metering system at the venturi doesn't care about the pressure differential created by the throttle plate farther down, it only cares about the velocity of the total mass flow that engine is able to pull past the venturi upstream of the throttle.

Open the throttle, the velocity of the air going through the venturi goes up, the suction generated by the venturi increases, and more fuel is pulled into the discharge nozzle.  And vice versa.


How are runway incursions avoided? I'm wondering if there's ground radar on planes, or any fallback system other than relying on a controller, for avoiding runway incursions?

Have any studies or reports been made that show statistically how the majority of these incidents occur?

At least three of the "fallback systems" I know of are looking out the window, being familiar with the airport and its markings (as well as associated hotspots, as airport diagrams may indicate), and also paying attention to the radio, if you're equipped.  Of course, no one of these methods is foolproof, so it takes a combination of alertness and prudence to avoid incursions.  I was taught to look both ways before intersections, or to look both ways before entering a runway, just to make sure I didn't miss anything.  Also, using lighting on your aircraft appropriately helps make you more visible.

Other planes may not have a radio, so you may not hear a radio call of someone else in the pattern.  Also, at a complex intersection in a new airport, it may be confusing where you are, or what hold short lines you're about to cross -- so being aware of your surroundings is important here.

There are some technologies the FAA has been implementing to reduce the risk of runway incursions, such as Elevated Runway Guard Lights and stop bar lights, plus ground radar technologies (for ATC) to assist (see ASDE-X and AMASS).  Note that your A/FD entry may list, for example, that not all taxiways are visible from the tower -- which is a good reason to stay extra alert.

Other sources:

Runway Incursion Avoidance
Wikipedia

An additional FAA training video:

Airport Signs, Markings, and Procedures: Your Guide to Avoiding Runway Incursions (Because John and Martha King.... and Harrison Ford!)


Do presidential aircraft use additional call signs? From the answer to another question I learned that the call sign Air Force One (or Marine One, Navy One or Executive One) is transferred immediately to the aircraft currently carrying the president of the USA.

When president Obama visited the Netherlands (and probably on other occasions as well), I remember him arriving in one of two identical helicopters, to not give away his aircraft to would-be attackers. Using the call sign Marine One for either would defeat that purpose.

So are additional call signs used in such a situation?

When multiple U.S. military aircraft are operating in formation, they do so as a "flight". This flight has a callsign, and in the case of Presidential transport flights, this callsign is the Presidential one, such as Air Force One or Marine One.

For POTUS transport using a fixed-wing aircraft, the flight typically consists of a single aircraft, usually one of the VC-25s specifically built for the purpose. Its fighter escort, if any (typically there is one), is a different "flight" that operates in tandem with Air Force One as the escort "element" of the complete transportation "package". Therefore, for Air Force One and for Navy or Executive fixed wing equivalents, the aircraft is the flight and has the callsign.

When the "Marine One" helicopters must be used, however, a very sophisticated "shell game" is played, as you already know. Not only are multiple helicopters used (I've seen up to 6 in use at the same time), they are externally identical in appearance, and they change position in the flight formation constantly. The Secret Service has even gone so far as to use body doubles that would make positive ID of the helicopter the actual President has entered difficult, even if an enemy faction had spotters in place to track each individual helicopter's position in the flight the whole time it's in the air.

This entire flight of multiple helicopters is what carries the "Marine One" callsign. Individual aircraft within the flight, if typical military radio procedures are followed, would be identified by adding a craft number suffix: "Marine One-One", "Marine One-Two", etc. I don't know whether the aircraft change craft numbers during flight to match formation position, or if the pilots just keep track of which chopper's where as they juggle.

So, a potential assassin would either have to take a 50-50 shot (or worse) at one of the choppers in the flight, or he'd have to coordinate with multiple launchers to attack all the choppers simultaneously (and the larger the effort, the more communication is required to plan and execute it, which increases the group's exposure and the potential for detection). If there is any reliable intel that the President would be targeted with this level of sophistication during a visit, he usually doesn't go; he has a VP, a Secretary of State and a cadre of ambassadorial representatives whose job it is to personally represent the President anywhere in the world the President himself cannot be, for any reason.


How often do runways need to have typical maintenance? At a large airport (think Heathrow, JFK, O'Hare), I would imagine that the runways take a lot of abuse from all the huge planes landing on them.  So, a quick multipart question:

How do large passenger planes damage runways?  Does it differ depending on the surface?
How long can a runway sustain this damage before the runway must be repaired or resurfaced?

Keep in mind, I'm talking about normal wear and tear.  So, I'm not really wondering how long it takes to repair a runway after a 767 lands with it's gear up, for example.  Rather just wondering how the normal maintenance cycle works.

The pavement requires frequent renewal of the top surface. The frequency depends on the type of pavement and on the weather. Asphalt for instance suffers a lot of damage when it snows and becomes less viscous when it is very hot, causing heavy aircraft to sink.

The main issue with repairing a runway on a major airport like JFK or O'Hare is doing so with little to no impact on traffic. Resurfacing an entire runway can take a long time. Therefore, rather than closing the runway for days at a time, airports try to do as many short maintenance operations as possible at night.
Some operations cannot be done in a single night but can be divided into several perfectly timed tasks, that can be done between 11 PM and 5 AM. Some operations can not be divided into such small tasks and require that the airport closes for a day or more.

The French airport Orly (ORY) for instance has one slab of concrete replaced every night on the runway. This way, the strip can be renewed every other year on average, without disturbing traffic. (I saw a video on this subject once, but can't seem to find it again.)

The results of each runway inspection are compared with the results of the previous runway inspection. Maintenance programs are revised based on the rate of deterioration of the runway. The FAA has various resources to help airport operators plan these maintenance operations.

But damage done by huge planes is only one of the many things that needs to be taken care of on a regular basis. Moreover, aircraft manufacturers try to design landing gear that reduces the impact of the aircraft on the runway as much as possible (note that airports can charge higher landing fees for aircraft that do more damage to the runway, so airlines want airplanes that don't damage the runway).
The most common maintenance operations are (not in any order, and indicated frequencies are only a rule of thumb, as these will change from one airport to another):

Removing rubber. This is an important one. When taking off or landing, aircraft often skid for a brief moment and leave rubber on the runway. This greatly reduces the adherence of the runway, which has a negative impact on stopping distances. This is usually done with high pressure water.
Cleaning the approach light systems and other visual aids. These get dirty with rubber, tartar, dirt... This operation can be done for instance with a mix of water and sodium bicarbonate, or by using dry ice.
Checking the quality of the approach light systems and other visual aids. Often, a truck with optometric sensors drives down the runway and measures the luminosity. Sometimes the lights need to be replaced. Airports usually have spares available for every type of light that is used on the runway.
Paint and markings somtimes have to be redone.
Evaluating the pavement and surveying the structural condition of the runway. Trucks are equipped with all kinds of sensors and go up and down the runway. The measures are used to make a map of the condition of the runway. Sometimes, the runway is divided into sub-sections and a different section will be evaluated every month or every other month, so that each section is inspected twice a year.
Checking for foreign objects on the runway. On all the airports I visited, this is done by the airport firetruck several times a day. Every few months, they sweep the runway to prevent grass, dirt, pebbles,... from accumulating.
Repairing cracks and resealing joints on the runway. This is to prevent water infiltrations that have long-term catastrophic consequences. A visual inspection of the runway is performed daily to look for cracks.
Cutting the grass around the runway and on the airfield. The grass has to be kept 2 to 4 inches high around the runway. This reduces the amount of debris that can be blown onto the runway. This also reduces the number of birds and other animals living close to the runway (because nobody wants birds to be ingested by an engine, or a squirrel to be run over).
Keeping all water evacuation paths clear. No water can be left to build up on the runway, since this would increase stopping distance for aircraft, and may freeze.

There also are seasonal operations, such as de-icing the runway or removing snow.


What is the purpose of a Victor Airway? I see lots of great questions related to Victor Airways, but I haven't seen any which indicate why someone would want to fly one or what really makes them special or unique.

I get that they are radial of a VOR to another VOR and that certainly provides an indication in determining what that radial is if you wanted to fly from one VOR to another, but I do not understand the value of defining it as an airway beyond that.

What is the purpose of indicating things such as width of the Victor Airway, or altitude or any other attribute? Is the concept that it would be frequently travelled and thus these attributes make it a little more controlled and thus safe?

In short: Other than providing an easy way to determine a radial from one VOR to another, what is the purpose of a Victor Airway?

An airway (Victor or otherwise) is just a standard route for aircraft to fly on in the national airspace system - for all practical purposes they are literally highways in the sky (with the lanes being stacked vertically rather than horizontally).

This is part of the paradoxical logic of how ATC provides separation services for IFR aircraft: "You have to put them together to keep them apart." -- In an environment with no radar, or in crowded airspace, having everyone following a standard routing ensures that you know what each plane is going to do. If you have 10 aircraft all flying along an airway and each separated by altitude or time you know they won't hit each other

We define the attributes (width, minimum altitude, etc.) of a victor airway mainly for purposes of obstacle clearance:

A VOR has a specific degree of precision in its measurement, and if we assume the maximum distance from the VOR (the widest degree of arc) and the maximum allowable "slop" in the VOR receiver/indicator in the aircraft that means an aircraft that thinks it's "on the airway" will be somewhere within a given width around the defining radial: For Victor airways that width is 8 nautical miles - 4NM on either side of the centerline radial - or 4.5 degrees if the VORs are more than 102 nautical miles apart.
Those numbers may sound  like they're arbitrary, but they aren't: If the VORs are exactly 102 nautical miles apart then the width of the airway at the 51 nautical mile midpoint between the two VORs is - you guessed it - 4 nautical miles. The further apart the VORs are the wider the center of the corridor becomes, accounting for the fact that a degree of arc gets wider as you get further from the VOR.

Airways are an IFR construct so if we continue on the assumption that the plane is in the clouds and the pilot can't see anything then within the airway corridor the aircraft needs to be clear of any obstacles (towers, trees, etc.), and of course it needs to be able to receive the VORs that define the airway. The combination of those factors and a few others will determine the minimum altitude at which a given airway can be used.

Note that much like ground highways airways are not always the most direct or convenient way to fly from one location to another. In particular the Victor airways are defined by VOR radials (or the intersections of such radials), but they're not always a straight line between two VORs.
As an example, take a look at this utterly contrived flight plan for the purposes of illustration: The airway I picked (V451) will take me from Groton VOR (GON) to La Guardia VOR (LGA), but it bends in three places on this route: CREAM, KEYED, and NESSI (not labeled on the sectional, but its the bend southeast of Bridgeport). A straight line between the two VORs would be the 265 radial from Groton direct to La Guardia: A much shorter distance, but not a "standard" airway routing.


What are the takeoff and landing distances of motor gliders? I'm doing some estimations of weight and thrust loading for a college project, and in the formula below the landing distance is not a variable, but should be estimated using similar aircraft and usual runways. (There is also a formula for landing with the same problem)

The similar aircrafts are 18-21 meter self-launching motor gliders like DG-505MB, DG-1001M or ASG 32 Mi (but with absolutely no data I have now, information about any other gliders would be appreciated as well).
The only thing I've found was that European Regulations CS-22 allow motor gliders a maximum takeoff distance of 500 metres.
Source; T.C.Cork, Design of Aircraft

You may wish to obtain POH data from the 18-21 meter motor gliders you have listed and compare it to your calculated results. Also look into information such as wing loading, aspect ratio, takeoff speed (or stall speed), and airfoil type in addition to power source to include in your report.

Motor gliders, true to their name, should lift off well before 500 meters.  Landing distance would be similar and also available in POH.


During VTOL landings, do Harrier pilots cut power before touching the ground? When vertically landing, do Harrier pilots cut power before they touch the ground?

The Harrier's NATOPS flight manual (A1-AV8BB-NFM-000) is available online.

Both the text and the diagram in Part III Normal Procedures confirm the opposite. Thrust is idled when "positively down". And not before. Which agrees with the dozens of soft landing videos on YouTube (base and shipboard alike). And as with shipborne helicopters, in rough seas the pilots wait for the pitching deck to catch up to them. Anything else would be an unlucky situation the pilot has found themselves in, but is not standard procedure.

  7.6.4 Vertical Landing

  (...)

  When positively down:

  Throttle — IDLE.
  Brakes — APPLY.
  Nozzles — AFT.
  Trim — 4° ND.
  Water — OFF (if selected).

Moreover, before step 5 there is this note:

  If strakes or gun pods are not installed some suck-down effect is present.
  A power increase may be required near touchdown to prevent excessive sink rate.

The only mention of idling (i.e., cutting/chopping) is step 5 and the diagram.


How and why do wake vortices come together to form two tip vortices? Wake vortices are created all along the wingspan but, the vortices come together to form two tip vortices. How exacly does this happen and why does it? I know that wake vortices push air to the side. Does this sideways moving air somehow pull the vortices to the tip?

Here is a picture of what I am talking about:

This picture shows that there is downwash and vortices along the whole wing span but, for some reason there are only two vortices at the tips.

One theory I have is that the up wash from the vortices follows the downwash from the rest of the wing and they spin around in a circle but, I am not completely sure

The picture in your question is an oversimplification. Maybe this photo of an MD-11 on a moist day will help to illustrate the point:

I also think you should not call the wake vortices "tip vortices". The tip vortices are an insignificant part of the whole wake system and can be seen in the picture above on both the winglets and the horizontal tail as thin lines of condensation. The wake vortices are the result of the vorticity shed over the wingspan rolling up. With flaps set for landing, the MD-11 has a big change in circulation at the flap tips, so here the shedding is intensified and shows as a thick line of condensation. There is more going on left and right of that thick line, but it is not visible because the gradient of circulation over span is too shallow to cause condensation in the wake (it does over the wing itself, due to the low pressure on the top side).

Maybe it will be best for understanding wake rollup if you watch it happen in a video. It should also help to confirm your theory :-). A good one is at 3:20 and again at 5:40. Also, watch how the wake changes when ground effect kicks in at around 4:00.


Is there a specific purpose for the silver "swoosh" on Air Force One? Air Force One, just ahead of where the wing joins, has a silver "swoosh" on the bottom of the fuselage:

Air Force Two, or other congressional aircraft painted with the blue/white livery that I was able to find do not have that same feature:

What is the purpose of this "Silver Swoosh"? I was bale to identify the following two paths of reasoning, which raise additional questions:

Option 1: Part of the Paint Job

If the silver is just part of the paint job, why don't other congressional aircraft have that same feature?
If it is part of the paint job, what does it signify or what drives its inclusion?

Option 2: It fulfills a defensive function

What defensive purpose does it fulfill?
Since it seems to be highly reflective, does it potentially serve as a mirror for missiles with certain seeker heads?

It's just for looks; an area of unpainted aluminum, polished.
In this close up you can make out the rivet lines and skin laps of the bare aluminum surface.  With a little work it's easy to get a finish like chrome plate, especially on a queen of the sky like that.

Back during American's natural finish paint scheme days, 80% of the fuselage was unpainted and polished.  AF1 just does it on a little patch so it's like a chrome accent.
There was significant weight saving in AA's case, but they finally gave in and painted their entire airplanes because the polishing has to be done regularly and is a pain, and over time the polishing, on an airplane that spends a lot of time outside and constantly re-oxidizes, slowly removes the pure aluminum cladding of the metal that is its corrosion protection.
Vinyl wrap is now starting to become common on airliners, and in theory you could probably do that sort of accent today with "chrome look" vinyl wrap, but AF1 has had that paint job for decades.


Why do airports remove/realign runways? When looking at Google Earth, I often find myself looking for airports. I often notice that no matter what size airport, over time many of them have abandoned old runways and built new runways in different directions.

It doesn't matter between rural and urban, so residential doesn't seem to be the cause. Is it that the wind direction changed from decades ago? Did they decide that sunlight matters more or less at some point in time? Anything to do with Earth's magnetic field?

In some cases the prevailing winds change and certain runways fall out of use. Since airports tend to be land limited they may wish to use the land for hangars or other facilities so the runways are eventually decommissioned.
In other cases it could a noise abatement issue, many airports predate the property development that often surrounds them. It may be in the best interest of the local neighborhood to change the runway orientation to one that creates less noise and pollution over homes or businesses.
In some cases runways may be moved to comply with changing regulations as was the case at Aniak Airport, where an existing runway no longer complied with obstacle clearance regulations.
In some cases it also becomes expensive to maintain a runway that may not see much use and it will be shut down.

Magnetic shift generally wont cause a runway move but it may cause a runway re-number since the runways are numbered as per their magnetic heading.


Is it possible for a plane to be over fueled (and over-weight) to take off? I've just reached my destination after 33 hours going from one European hub to another. A journey which usually takes a max of 6 from leaving my house!

On the final leg of the journey, after boarding, the pilot asked for volunteers to leave the plane as they were too heavy to take off.

They had flown a 50 min flight from my final destination to where I was and, without fuelling, were turning around to go back. They were adamant that the plane was too heavy and (with a print out from one of the flight computers in hand) were asking for 27 people to leave the flight.

Is this possible? Because the MTOW of the aircraft is greater than the max landing weight, how could they have landed under max landing weight but now be too heavy to take off?

Maybe they flew with no passengers, but exceeded MTOW after adding more passengers and luggage?

Any suggestions for question re-phrasing or additional clarity are greatly appreciated.

The aircraft you flew on was D-AIUX, a Lufthansa Airbus A320-214. On 10 February it was scheduled to fly from Munich to London, but the flight was cancelled due to the storm:

(flightradar24.com)

If the aircraft was fueled for this flight the day before, it would need:

3.13 kg/km x 944 km = 2.95 t of fuel for the trip
about 1.5 t of reserve fuel

After actually flying the 356 km to Vienna, it would have burned about 1.11 t of fuel, resulting in a total weight of:

42.6 t OEW (Operating Empty Weight)
3.34 t of fuel remaining
+any payload

The MTOW (Maximum TakeOff Weight) is 78 t, which would therefore still leave 32.06 t for payload, but the maximum payload is 19.9 t for the A320. So this cannot be the limiting factor.

It was noted that the runway could limit the takeoff. Your flight departed from runway 29, which is 3500 m long:

(flightradar24.com)

The A320 only needs 2100 m at MTOW for takeoff, so this was also not the limiting factor.

You said "they filled the plane in Munich the day before to give the plane more balast to cope with the storm winds", which is not a procedure I am familiar with. Increasing the aircraft weight will increase its inertia and therefore make it less susceptible to wind gusts. Let us assume they completely filled all fuel tanks in Munich.

The A320 has a fuel capacity of up to 27,200 l. Multiplying that with the density of Jet A-1 (0.804 kg/l at 15°C), gives about 21.87 t. This would make the total weight before payload at Vienna 63.36 t, therefore leaving 14.64 t for payload. Now this is lower than the maximum of 19.9 t. Using standard passenger weights in Winter of 195 lbs = 88.5 kg, this would allow 165 passengers. The A320 exit limit is 195 passengers. It is therefore possible that some passengers had to leave the plane to get below MTOW.


What did a maintenance person put into the engines after landing? I was recently flying on a A320-251N and directly after landing & coming to a halt at the gate, a maintenance person approached the engines carrying a box of something that looked like paint cans, opened some lid on the side and apparently put some of the content of the cans into the engine or so. What was it probably? (It was on an cold & icy airport, if that plays a role)

It was probably Engine Oil.

On modern jet engines, oil is a consumable in the sense that it gets burned up and needs topping up. A new engine will require less, a high time engine more. The JT-9s on older 747 were notorious oil-guzzlers. CF-6s were a bit better but they still needed topping up after an 8 hour flight.

Jet engines never get an oil change. The regular topping-up of burnt oil keeps the oil fresh and regular filter changes ensures material from engine wear is not circulated. Oil samples are taken at set intervals and sent for spectrographic analysis. Too much of a particular element will focus attention on a particular component. There are also 'Magnetic Chip Detectors' which trap ferrous metals and this also helps to pinpoint wear in the engine.


Would it be frowned upon that with a uncommanded nose up, the pilots just do a full loop? So what I’m asking is that if a uncommanded nose pitch up happened and was so bad that it reached a pitch angle of 50 degrees, the pilots were unable to stop the pitch up by applying the control column forward, would it be justified if the pilots just applied more back pressure, ultimately doing a full loop?

No airliners aren't stressed for loop maneuvers.  The proper action for that kind of extreme pitch attitude is to roll 90 degrees and let the nose fall, then roll level as the horizon is passing.  Of course if you had a nose up stab trim runaway that left you stuck with that pitch tendency (effectively a much lower trim speed), you're back to square one.

You have to slow down to as close as you can get to the trim speed that the nose-up-stuck stab wants to fly at.  You might find that you have enough elevator authority to maintain a level pitch attitude if you get the speed down to a minimum flaps up speed, say 180kt.

Then you can do other things to create a nose down pitching moment, like dropping the gear, and on most airplanes, extending flaps.  That should be enough to remove most or even all of the elevator input required depending on where the stab was when you killed its runaway.  In an extreme case where you've run out of options, you might even try getting passengers to move forward.

In recurrent sim training, a nose up stab runaway was always a more pleasant exercise than a nose down one.  Mostly because slowing down, and gear and flaps helped, and to the extent you had to hold continuous nose down elevator, one of you could brace your knee against the column to help with most of the push force and it was easier to hold it for extended periods.  A nose down stab runway was a much more difficult situation.


What is this jet aircraft appearing in the 2019 TV Series "Black Summer"? This looks like an F-22 Raptor, but I can hardly see it:

If this is in fact an actual plane (or a cgi representation of one, it is most likely a F-22 Raptor.
The picture is very blurry and somewhat distorted (probably due to panning), but features fitting to F-22 are:

Trapezoid twin rudders (F-35 has parallelogram shaped rudders)
Rudders fairly far from each other (twin engine)
Rudders canted outwards (rules out F-15)
Relatively high canopy (i.e. F-35 has the canopy positioned much lower in respect to aft fuselage
Sleek fuselage profile (i.e. F-35 has rather chubby fuselage proportions)
Air intake rather forward (rules out F-18)

Not being familiar with the series this is from, I'm willing to bet this is not an actual plane, but cgi. The 3D -model might be totally fictious, only resembling F-22 in the manner described above.


How much does it cost to add a glider rating to a PPL? In Europe, how much does it cost to learn to fly gliders if you're already a private pilot? I am one, so I already have the medical and theory etc. but what cost can I expect for a GPL on top of a PPL?

If you're in Romania, are a Romanian citizen and are under 23 you can do it for free in the Romanian Airclub.

Otherwise it's 15 eur/winch launch and  25 eur/tow at around 300m.
I'm not sure if there's any deduction from the glider training if you have ppl but I know that if you have the glider licence you get a shorter training plan for motorized aircraft.


How to maintain your pitch when looking for other traffic in the pattern? I am a student pilot and I fly in a desert region where turbulence often changes the pitch and airspeed of my aircraft.  I am having an issue maintaining pitch while scanning for other aircraft in the traffic pattern (I lose sight of the horizon while scanning).  I might scan for 2-3 seconds and by the time I bring my eyes back to the horizon my pitch has changed 10 degrees and I have lost 5 knots of airspeed.  My flight instructor then scolds me for not holding my airspeed.  Is there a way to scan for traffic while still maintaining reference to the horizon?

When starting out as a pilot you are normally overwhelmed by all the tasks you need to do. Most of these tasks become second nature with time and experience. Here are some tips you could try.

Make sure the aircraft is trimmed correctly for the manoeuvre so that you do no need any force on the controls to keep the aircraft straight and level. Sometimes you can create your own turbulence because you did not trim correctly but then fight the controls.

Listen to the air noise. You could easily pick up from the change of air noise if the aircraft is loosing or gaining airspeed. A ten degrees pitch change will cause an airspeed change you will pick up by ear. Remember you are flying VFR so looking outside at the horizon to judge your pitch is very important. Flying the correct power setting and maintaining the correct pitch will help you a lot in flying because the aircraft will settle at a certain speed all by itself.

Maybe schedule your flying lessons when the air is more stable and turbulence is minimum. As you gain experience you will be able to handle the turbulence better.

Also try another instructor. Maybe another instructor might be able to understand your weakness and strengths better and handle the situation differently.

Ask many opinions from other pilots in the area. You will find that there are more than one technique sometimes. Just find the one that works for you.


What is the difference between Angle of Incidence (AoI) and Angle of Attack (AoA)? Following discussion here, I am still confused about the difference between AoI and AoA. As I read in many references and watched in many YouTube videos especially this one:
1). AoI is tje angle between the longitudinal axis and the chordline as per design/manufacture. It says normally AoI designed/manufactured is six degrees. AoI remains constant as it is fixed relative to the fuselage.
2). AoA is the angle between the chordline and the RELATIVE AIRFLOW. AoA changes in flight.

My confusion is with the meaning of the RELATIVE AIRFLOW. This phrase confuses me. In my opinion, airflow produced by the propeller or the turbines flows parallel to the longitudinal axis, or at least it has fixed angle, also as per designed too.

To avoid a repetitive question, I want to give this situation.
1). During take off of an airplane, from the airplane start moving until it left the ground, what is the RELATIVE AIRFLOW?
2). After the airplane takes off and during climb, what is the RELATIVE AIRFLOW?
3). During level flight (cruise with constant altitude), what is the RELATIVE AIRFLOW?
4). During approach to land until touching ground, what is the RELATIVE AIRFLOW?

What is actually that AIRFLOW?

I apologize if there are too many questions. It is actually just one. Just to make sure that you will not confuse to read and I will not get confused to read your explanation as product of unclear question.

Before I post this question, I have search if someone have posted previously. But I didn't find the same question.

Angle of Incidence is fixed by the designer of the aircraft. It does not change in flight.

Angle of Attack is the angle between the chordline of the wing and relative airflow during flight.

That is definitely something that changes during operation of the aircraft in flight. Consider slow flight, for example. You reduce power and raise the nose to reduce airspeed. The direction of the airflow remains the same, but the angle between the airflow and the wing chordline, the Angle of Attack, increases. Raise the nose too much (and hence increase the AoA), and the airflow will separate from the top of the wing and you will stall.

On aircraft like Cessna trainers, the stall warning is a very basic indicator of Angle of Attack, to warn you when you are about to stall.

More advanced Angle of Attack indicator systems use a vane to measure the angle of the relative airflow, and give you more information about AoA.


Can one ARTCC take over for another ARTCC in an emergency? Earlier today the ZNY ARTCC was shut down due to COVID-19. This resulted in a temporary ground stop for incoming flights while a contingency plan was being activated. When certain sectors within an ARTCC are shut down, as happened yesterday at ZID aircraft are routed aground the sectors, but it would be very difficult to route around an entire ARTCC facility’s airspace. In fact, as ZNY was shut down, aside from the temporary ground stop, flights over the ZNY airspace appeared to continue as normal.

I make the assumption that the centers are connected and that when one center has to shut down entirely for some reason that the other ARTCC’s are capable of taking over for them and providing the same radar service. How true is this assumption?

Note: There are conflicting reports at this time as to whether the entire ARTCC had to shut down or just some sectors. The question still stands as to what would happen if an entire center had to close.

Generally speaking, no.

As for the specific situation, New York Oceanic has been closed since midnight UTC:

  DUE TO AN OPERATIONAL EMERGENCY NEW YORK AIR ROUTE TRAFFIC CONTROL CENTER IS DISCONTINUING AIR TRAFFIC SERVICES IN THE NEW YORK OCEANIC FIR UNTIL FURTHER NOTICE.FLIGHTS CHOOSING TO ENTER NEW YORK CENTER AIRSPACE SHOULD EXPECT ONLY ADVISORY SERVICE.
  NOTAM A0249/20 refers.

Most ATS units have contingency procedures in place to dictate what happens if the unit needs to close on short notice, for example during an evacuation. It is quite common to have agreements with adjacent units to help clear the airspace ("clear-the-sky" systems). This is possible if adjacent units have RADAR and radio coverage that overlaps with that of the unit being evacuated. While not able to provide regular service, adjacent units can monitor traffic and make sure everyone gets out of the evacuated airspace safely, after which that piece of airspace is closed.

In order for an adjacent ATS unit to actually take over and provide normal service, several things would be required:

Additional staffing. It might not be possible to just call more people to work, as that would mess up the entire roster in days and weeks to come, seriously affecting the non-evacuated unit.
Additional working positions for the extra staff to actually sit and work, while still maintaining a number of backup positions in case of equipment failure.
Staff must have knowledge of local rules and procedures of the airspace they are taking over, and have valid and current ratings to be allowed to control it. You can't keep your rating current if you don't use it regularily. Just like you can't just take an Airbus-pilot and ask them to fly a Boeing-aircraft because the Boeing crew called in sick.
RADAR coverage must be sufficient. This could be achieved by sharing RADAR-data between facilities, but if one ATS unit needs to evacute, their technicians have probably evacuated too - which means the data cannot be used since there is no one to monitor the status of technical equipment.
Radio coverage must be sufficient (see above)
All telephone lines must be re-routed (which is doable, if the system is set up for it)

What is more likely to happen is that the unit evacuating will move into a contingency facility from where they are able to provide anything from a very limited to full service. Some airports have old control towers that are still being maintained as contingency positions, and area centres might have dedicated backup facilities, or possibly training facilities (simulator positions) that can be switched over to a live environment. Of course, if all the staff that happened to be on shift at the time of evacuation needs to be quarantined due to a contagious disease, you will have a pretty big staffing issue.


How come some aircraft have really cool registration numbers? I've seen registration numbers like

D-ICEY - A Cessna Citation owned by a German company that produces frozen foods
G-FILM - An Alouette II helicopter used in some movies and TV series for the flying sequences

and Aviation Humor, for example, lists many other examples like TF-MOM, TF-DAD, TF-SON, TF-KID (all of former WOW Air).

I imagined it might work similar to how license plates for cars are issued in Germany (I know it's different in other countries): Here the first one to three letters are determined by the town/area you live in (e.g. F for Frankfurt). The remaining part is more or less random but you can pick from a list of these random numbers (for a fee of around 20 €).

Does it work the same with aircraft registration numbers and those people simply were lucky because e.g. D-ICEY was still available?

Aircraft registration numbers are assigned by the country where the aircraft is registered. Each country has a prefix (see the full list on Wikipedia), like e.g. D- for Germany. The prefix is usually separated from the rest by a dash, but some countries deviate from this, most notably the United States, where registration numbers begin with N.

Whether or not custom registration numbers are available depends on the country. In Germany, the authority in charge of assigning registrations is the Luftfahrtbundesamt. It is possible to ask for a custom registration number via the form Antrag auf Vormerkung eines Kennzeichens (§ 19 Abs. 2 LuftVZO). The fee is currently 30 Euros. In order to get the custom registration, it must be available of course, but also follow these rules:

  D-AAAA to D-AZZZ for aircraft with more than 20 t MTOW
  D-AUAA to D-AZZZ (test registrations) for aircraft manufactured by Airbus at Finkenwerder
  D-BAAA to D-BZZZ for aircraft with 14–20 t MTOW
  D-CAAA to D-CZZZ for aircraft with 5.7–14 t MTOW
  D-EAAA to D-EZZZ for single-engine aircraft up to 2 t MTOW
  D-FAAA to D-FZZZ for single-engine aircraft from to 2–5.7 t MTOW
  D-GAAA to D-GZZZ for multi-engine aircraft up to 2 t MTOW
  D-HAAA to D-HZZZ for rotorcraft
  D-IAAA to D-IZZZ for multi-engine aircraft from 2–5.7 t MTOW
  D-KAAA to D-KZZZ for powered gliders
  D-LAAA to D-LZZZ for airships
  D-MAAA to D-MZZZ for powered ultralight aircraft
  D-NAAA to D-NZZZ for non-powered ultralight aircraft
  D-OAAA to D-OZZZ for manned free balloons
  D-0001 to D-9999 for gliders.

So it was at least in part luck that D-ICEY was available and fit the rules for a Cessna Citation.

In the UK, the authority in charge is the Civil Aviation Authority (CAA), which also allows custom registrations (called out of sequence here). The fee is currently £158.

This answer explains the procedure for the US and Canada.


How does thrust reversal work on a jet engine, and when should it be used? A two part question this time, though they are closely related enough that I figured I'd knock them out all at once.  I figure the first question probably informs the second question anyway.

Firstly, how does a jet engine flip over to reverse thrust?  I'm assuming the turbine doesn't actually switch directions.  Is there a set of blades towards the rear of the engine that divert the flow forward again?  How's that work?

Probably more topically:  When is the right time to apply reverse thrust?  I'm assuming you ought to be on the ground (that seems like a given), but are there checks that a pilot has to do before engaging the reverse thrust?  Are there times when it's not used, or when it possibly just isn't necessary?

The engine does not move, the air flow is only redirected. The method of redirecting the flow varies with the size, configuration, and manufacturer of the engine. The flow does not need to be directed completely forward; the flow is generally mostly outwards and partially forward. This is still enough to create significant drag and slow the aircraft down.

In smaller and older engines, the entire flow is redirected. This corresponds to the first two configurations below. Buckets or clamshell doors close over the jet flow to redirect the air.

In larger engines, particularly high-bypass turbofans, only part of the air is redirected. This corresponds to the last configuration below. The core air from the engine still exits normally, but the bypass air from the fan is redirected. Since the bypass air on these engines is a much greater flow than the engine core air, this results in a net reverse thrust. There are multiple mechanisms, but the general idea is to deploy doors that block the bypass air and send it through the sides of the engine cowl.

Turboprop aircraft just change the pitch of the propeller blades such that the propeller pushes air forward instead of backward. The pitch in which the blades provide reverse thrust is called the "beta range."

In civil aircraft, reverse thrust is only used when on the ground. There are typically interlocks in the system that prevent the thrust reversers from deploying if the aircraft does not sense that it is on the ground. Once the aircraft touches down, the pilot will deploy the reverse thrust. Some military aircraft like the C-17 can use reverse thrust in the air. This enables them to perform very steep descents.

Reverse thrust is more of an "optional" method of braking (see this related question), only to provide extra stopping power when needed. This particularly helps when braking force is less effective, such as in rain or snow. Before landing, pilots will take into account the wind, aircraft weight, runway length, and any contamination of the runway (rain or snow). Based on this they will know what kind of braking force is needed and whether they should use thrust reversers.

The thrust reversers don't always have to be operational for an aircraft to fly. Some operators choose to disable them, which decreases maintenance costs. The thrust reversers can also break, in which case they will be mechanically locked to prevent them from deploying until they can be fixed. Pilots will consider this when they are computing their landing distance.


How do solo fighter pilots sleep? Tonight I watched Nova's "The Impossible Flight", a documentary film about the around the world Solar Impulse project where a solo pilot set the world's record for a 5 day non-stop flight from Japan to Hawaii with a ~35mph TAS! The documentary showed the pilot practicing 20min naps and said "this is a technique military fighter pilots are taught...". (It is supposed to keep them from going into a deep sleep)

Many commercial sleep questions have been answered on stack such as;

Is a pilot allowed to sleep during a flight?

How are emergencies handled during In-Seat Rest?

So this question pertains to military pilots.

Is this practice common among most nations?
How is a fighter pilot woke up?
Is there a alarm in the radio system?
Does an air or ground controller wake them?
Or perhaps the documentary was misleading and only sleep is allowed with two or more crews?

Generally, the only time fighter pilots are asked to remain airborne long enough where this is a factor are when ferrying aircraft across the ocean, (or for long distances over territory where there are no intermediate refueling bases.)  Generally this is always done in multi-aircraft flights or formations or two or more aircraft, so even in a single seat fighter, you are not completely alone.
Obviously, the average fighter cannot carry fuel sufficient to last that long, so (again, generally), when this happens they accompany a refueling aircraft  (or have pre-planned rendezvous with refueling aircraft every few hours). So there is no need to "wake up" - they remain awake the entire flight. They are flying manually in formation off the wing of the refueling aircraft, or one of the other aircraft in the flight. If the flight is long enough where alertness is an issue, or if the flight occurred during periods where the aircrew would normally have been asleep, we were sometimes issued Dexedrine tablets to take to help remain awake and alert.

In peacetime, we would never deliberately plan a flight where from start of pre--flight planning, until engine shutdown would exceed 12 hours. This rule of course could be violated in the case of wartime or national emergency, etc. etc.


Why were turbojets the first kind of turbo-engines, instead of turboprops? During WW2, the first jet engines appeared en mass in combat aircraft. Britain and Germany made the first ones.

But why were turbojets first? Why not turboprops, which are easier since the turbo-machinery is smaller for a given thrust. Turboprops have propellers so it would've been a much easier evolution too. If I'm not mistaken, turboprops can fly higher and faster much more easily than piston-props, so it would've conferred an advantage.

So why were turbojets first? Did the concept of a propeller attached to a turboshaft just not cross the minds of engineers during the war? Was it tunnel vision onto the much more powerful turbojets?

Propeller planes already existed, and turboprops were an answer to a question that was not asked in times of war. The main driving factor behind development of the turbojet was to provide great climbing power and high airspeed for intercepting enemy bombers, which the turboprop would not have been able to do. Fuel economy and engine life were of secondary importance, the Junker-Jumo 109 of the Me-262 lasted only 10 flight hours!

The piston propeller engines were very sophisticated towards the end of WWII, the super-charged Merlin piston engine powering the Spitfire developed over 2000 hp.

But the Me-262 and the He-178 were way faster than the Mustangs & Spitfires that escorted the bombers, and could only be shot down when lined up for landing. From the wiki:

  Too fast to catch for the escorting Allied fighters, the Me 262s were almost impossible to head off. [Note 7] As a result, Me 262 pilots were relatively safe from the Allied fighters, as long as they did not allow themselves to get drawn into low-speed turning contests and saved their maneuvering for higher speeds.

Turboprops would not have resulted in such an advantage, since the propeller is the limiting factor for airspeed. The lighter weight of a turbine would have been beneficial: the Merlin weighed 600 kg, and for instance the PW123 turboprop delivers 2,380 hp and weighs 450 kg - but only beneficial in the lower speed turning performance which the jet fighters learned to avoid.


What emergency situation can pilots encounter in gliders/non-powered aircraft? Pilots have to be able to handle a wide range of emergencies in the air. For powered aircraft, the obvious example to me is recovering from an engine failure, but obviously this can't happen on an unpowered glider. Many gliders have no electrical systems, so an electrical failure can't happen. A stall, I believe, can, and I assume the recovery would be similar to in a powered aircraft. What other emergencies should a glider pilot be able to handle? I'm especially interested in any emergencies that powered aircraft don't have to deal with.

The number one emergency in a glider is a tow rope break at low altitude, less than 200 feet above the ground with no appreciable headwind. Below 200 feet, the proper technique is to land straight ahead, which usually means landing off airport.

Between 200ft and 500ft, it is possible to make a U-turn back to the runway, and land in the opposite direction in which you took off.  The main hazard with this kind of landing is landing with a tailwind, and once the wheel touches the ground, the flight controls operate in the reverse direction.

Above 500 feet, it is possible to fly an abbreviated pattern and land on the runway you took off from.

Add 50 feet of altitude for each 10 knots of headwind.

Some other tow emergencies are inability to release to tow rope, in which case the tow pilot needs to release the rope on his end; engine failure of the tow-plane; and a double tow rope release failure, which means the glider needs physically break the tow rope (it has a pre-determined breaking strength of 80-200% of the weight of the glider) or to land behind the tow-plane.

You can also run out of altitude and have to land in a field, which is called an "off airport landing." Someone has to come and get you with a trailer, and you disassemble the glider and drive it away. One hazard is the off airport landing in a field, and the farmer comes out with his shotgun.

Other hazards are flying into the clouds or into severe turbulence, which can happen when the glider pilot is flying "wave," a kind of lift generated by strong winds in proximity to
 mountains.

When thermaling, you are flying very tight steep turns, often with other sailplanes who are also trying to gain altitude. Everyone is supposed to fly the thermal in the same direction, and there is the hazard of colliding with other gliders.

Reference: FAA Glider Flying Handbook - Abnormal and Emergency Procedures


How are businesses able to use experimental aircraft? I saw businesses like this using military jets to sell rides to civilians. Since warbirds are considered experimental aircraft, does the FAA issue special certs to fly them commercially?

According to 14 CFR § 91.319,

  (a) No person may operate an aircraft that has an experimental certificate

    (1) For other than the purpose for which the certificate was issued; or

    (2) Carrying persons or property for compensation or hire.

Waivers available for the purpose of instruction. EAA can help with this.
You will not be able to instruct anyone who does not already have at least private pilot license.
You can of course use the airplane as a vehicle personal transport. If you a high value individual traveling to a client and get paid for travel time, paying for the use of your vehicle could actually cost the customer less.


Can hypersonic aircraft be agile without the G forces harming the pilot? I read that high G forces associated with hypersonic flight could kill the pilot.

Is there any way of making this survivable yet keeping agility at hyper sonic speeds (irrespective of the plane's capabilities) and how do they work.

Well, 2 G's is 2 G's, regardless of what speed you're flying.  A level (coordinated) turn with 60 degrees of bank is 2 G's, regardless of your airspeed.  What changes is your turn rate and turn radius; at 20 knots, a model plane at 60 degrees bank can probably complete a 360 degree turn in a few seconds and a radius of maybe 10 yards.  At 300 knots and 60 degrees bank, a conventional jet will need a minute or two and a turn radius of probably a couple of miles.  At 2000 knots and 60 degrees bank, an SR-71 might overfly a few states making that same 360!  But it's all 2 G's for each of those aircraft.

Now if you want the SR-71 or some similar highly supersonic aircraft to have the turn rate & turn radius of your 300-knot Lear jet, then you'll get a LOT more G's -- which can induce various other issues with your supersonic beast.  And if you want either of those to keep their speed while turning in the radius of your 20-knot drone... well, let's just say that you're talking about G forces beyond the realm of all human experience to make that happen!

There are all kinds of discussions that can be had about how hypersonic aircraft can be made more maneuverable without doing things that risk losing control, flaming out the engines, creating unacceptable amounts of drag, and so on, but all of those discussions take place in the context that high speed + high turn rate = more G's, and given any 2 of those factors that you choose (i.e. how fast you want to go & how much G-loading you're willing to sustain), the third is determined.

The max G-loading for a human is pretty well about 9 G's, so pick whatever combination of the other two factors you want.  Or go unmanned -- which with today's technology is probably cheaper.


Why do slats deploy first? I know on the 737, the leading edge slats deploy at the first flap setting, and the trailing edge flaps deploy after that at higher flaps settings. Why do the slats deploy before the trailing edge flaps?

LE Flap and slat schedule as it relates to TE Flap position for the 737-300:

The slats help to protect the outer wing from stalling when the flaps deploy. The outer wing carries the ailerons, so there are no flaps, but the flaps inboard will induce more lift on the whole wing, raising the effective angle of attack even in the aileron section. Slats push the stall angle of attack up. Without the protection provided by the slats, the outer wing could stall, and if this happens on one wing first (very likely, there are many reasons for flow asymmetry), the aircraft will roll violently. To do so during an approach is not good at all.

Look at aircraft with powerful flaps: They all have slats at least on the outboard wing to avoid outer wing stall. One goes with the other.