How does one test a stall warning during pre-flight? As a pilot, if your preflight inspection includes testing the stall warning system, how is that done?

A recent question about ADs brought up AD 68-17-04, which requires pilots to test the stall warning system for proper function as part of the preflight inspection.

How would this test be done? How would other types of stall warning systems be tested?

Stall warning systems generally function on the principle of measuring pressure distribution or angle of attack. As each aircraft will have a different type of stall warning system, the necessary test will vary by aircraft as well. Here, I will answer based on aircraft that I have experience with, but these concepts should cover the majority of light aircraft.

Aural stall warning systems can generally be divided into two types, pneumatic and electric.

Pneumatic

Pneumatic stall warning systems are found in many light Cessna aircraft, including the 172 and 177. The following system description is from the Cessna 177 Maintenance Manual:

  The system is composed of an adjustable plate on the left wing leading edge, connected to a reed type horn by means of plastic tubing. The horn is actuated approximately 5 to 10 miles per hour above stalling speed as a negative air pressure area at wing leading edge causes a reverse flow of air through horn.

See this excerpt from Figure 15-6 in the C177 Maintenance Manual depicting the pneumatic stall warning system and it's location in the aircraft:

To test this type of system, air must be drawn out of the system through the leading edge scoop located in the left wing. The maintenance manual and AD 68-17-04 call for a test procedure as follows:

  Prior to flight, test the pneumatic stall warning system and determine if it is functioning properly, [...] by covering the opening in the left wing with a clean cloth, such as a handkerchief, and applying a slight suction by mouth to draw air through the horn. A properly functioning horn will provide a sound that is clearly audible.

The sound is something like that from a kazoo. In actual practice, speaking personally and from my observations, the "clean cloth" is typically dispensed with.

Alternately, a Stall Horn Tester is available for a small price if you would rather not have to put your mouth on a strange airplane.

Failure of these systems will typically be due to a problem either with the horn—such as a loose reed, or with the leading edge scoop—such as a cracked or misaligned scoop.

Electric

Electric systems operate on some of the same principles, but use a movable tab—instead of pneumatic suction—to sense the change pressure distribution. This type of system is found on aircraft such as the Cessna 182, Piper Cherokee, and Beechcraft King Air.

The system operation can be generalized as follows: pressure changes cause the tab to move up thereby sending an electrical signal which will sound a horn in the cockpit. In some cases the tab actuates a microswitch. In more complex systems the tab actuates a transducer which signals a stall warning computer.

The simpler of these types of systems can typically be tested with aircraft electrical power on by manually lifting the tab and listening for the warning horn. With the more complex King Air system, testing is accomplished with a switch in the cockpit which tests the circuitry and horn. In this system, signals from the stall warning transducer are blocked out by a weight on wheels switch when on the ground, with the effect that lifting the tab will not allow for system testing.

Flight Safety (FSI) offers the following system description in their King Air 200/B200 Pilot Training Manual:

  The stall warning system senses angle of attack
  through a lift transducer actuated by a
  vane mounted on the leading edge of the left
  wing (Figure 15-10).

  Angle of attack from the lift transducer and
  flap position signals are processed by the lift
  computer to sound the stall warning horn
  mounted on the copilot’s side of the cockpit.
  The horn sounds when the following conditions
  are present:

  Airspeed is 5 to 13 knots above stall,
  flaps are fully retracted.
  Airspeed is 5 to 12 knots above stall,
  flaps are in the APPROACH (40%)
  position.
  Airspeed is 8 to 14 knots above stall,
  flaps are fully extended.

  The system can be tested prior to flight by
  placing the STALL WARN TEST switch, located
  on the copilot’s left subpanel, in the
  TEST position. This simulates a stall condition
  and sounds the warning horn.

Figure 15-10 from the FSI King Air 200/B200 Pilot Training Manual

Other Systems

Other types of systems exist with which I have no actual experience, either as a pilot or mechanic. I offer the following information without much real knowledge of it, but hopefully with good sources. Others more knowledgeable than I might be able to verify it.

The Boeing 737 has a stick shaker type stall warning system to provide the required warning, rather than an aural warning system. The SmartCockpit.com B737 NG Warning Systems document gives the following system description:

  Stall Warning System

  The stall warning "stick shaker" consists of two eccentric weight motors, one on each control column. They are designed to alert the pilots before a stall develops. The warning is given by vibrating both control columns. The system is armed in flight at all times. The system is deactivated on the ground.

  Two independent, identical stall management yaw damper (SMYD) computers determine when stall warning is required based upon:

  • alpha vane angle of attack outputs

  • ADIRU outputs

  • anti-ice controls

  • wing configurations

  • air/ground sensing

  • thrust

  • FMC outputs

  • mach compensation

  The SMYD computers provide outputs for all stall warning to include stick shaker and signals to the pitch limit indicator and airspeed displays and the GPWS windshear detection and alert.

  Two test switches are installed in the aft overhead panel. Pushing either of these initiates a self-test of the respective stall warning channel. The No.1 activates the Captain stick shaker, and the No.2 activates the F/O stick shaker. Either stick shaker vibrates both columns through column interconnects.

The same document further defines the test procedure:

  STALL WARNING TEST Switches

  Push – on ground with AC power available: each test switch tests its respective stall management yaw damper (SMYD) computer. No.1 SMYD computer shakes Captain's control column, No.2 SMYD computer shakes First Officer's control column. Vibrations can be felt on both columns

  • inhibited while airborne


Why don't all cylinders have priming lines? Questions about the primer for carburetor engines.
Why does the primer only spray fuel to three (sometimes just one) of four cylinders. (Like Piper Cherokee or Seminole)
And is it true that priming engine during engine start (cranking engine) may cause engine fire?
That would be great if you could answer those questions! Thank you!

The airplane maker will not normally install any more primer lines than necessary. In warm environments it's not usually necessary to prime more than one cylinder, so you will see airplanes operated in the southern US with single primer lines more often than in the northern US or Canada.  My old '68 Cardinal only had a single primer port because it had been imported from the US that way (I was in Toronto at the time), and it was murder to get it started even in moderately cold weather without preheat. I had primers run to all 4 cylinders by my mechanic.
Also, you will often see one of the threaded holes at the intake port on a cylinder being used as a tap-off for a manifold pressure gauge, another reason for stopping at 3 lines.
When priming, the objective is to get a good coating of raw fuel in the intake pipes close to the port, that will take 5 or 10 seconds to fully evaporate after starting, to provide that temporary extra-rich mixture.  But not too much.
Over-priming can start a fire when so much gas is squirted in it starts to collect in the intake pipes and drain down into the airbox.  A backfire into the intake system during starting, because an intake valve doesn't quite seal, can ignite the fuel in the airbox, and potentially fuel that is dripping into the cowling.
That's why, when cranking and you hear a "fump!" sound under the cowling, keep cranking!  You need to try to draw any ignited raw fuel back into the intake.  Only stop cranking if you start to see smoke gushing out, and then it's time to leave and tell the owner he can have his plane back (or try to fight the fire if you want).
It's when you start to smell gasoline that you know you've over done it.  Best to stop and wait a while for all the gas to evaporate.
A worse habit is throttle pumping to spray fuel from the carb's accelerator pump into the intake.  You might do it when you start a warm engine; it's not necessary to prime with the primer and little shot of throttle while cranking is enough.
But it's critical to only pump the throttle during cranking. If you pump it while things are stopped, gasoline runs straight down into the airbox immediately.  Probably most cowl fires are from people pumping throttle several times, waiting a few seconds, then starting, then it's "Fump!".... "do you smell something?".


Are there any 100% renewable energy aircraft in development? There seem to be several options for aircraft R&D regarding cleaner fuels for the time when fossil fuels are no longer economically viable.

Some of these include:

Algae. A drop-in fuel. These fuels are being used and developed in co-operation with the USAF. They have had several successful test flights.
Batteries. Doesn't seem viable at this stage.
Nuclear reactor on board. Doesn't sound safe.
Hydrogen. This wiki page says they could be built by 2020. That's only a year away now...

I've found a few examples of test prototypes & flights using fuel mixtures that improve emissions and gasoline consumption, but are there any commercial aircraft in development, with an expected launch date that utilize 100% renewable energy?

Contrary to one clause of your question, there are a number of battery-electric light aircraft (or what the USA calls "light sport" class) in development, with prototypes flying.  This includes both fixed-wing conventional types with single or multiple electric powered propellers, and "drone" style craft using what amounts to thrust hover with electric powered propellers.  Flight time is limited, but better with the fixed wing types -- enough so that I'd call an electric motor glider a very feasible design.

Hydrogen has limited usefulness, for the same reasons it doesn't work well for cars: it's technologically difficult to store enough for reasonable range.  Either extreme pressure, extreme cold, or a chemical storage method that limits the rate at which you can draw fuel are reqired.

A submarine type nuclear reactor was operated aboard a modified B36 in the 1950s -- the project, intended to lead to a strategic bomber with a flight time of weeks or months, was dropped due to lack of need as well as cost and safety concerns (most of the safety concerns, such as "roll-up" explosions, were later resolved or found to be overblown).  This kind of power isn't "fully renewable" anyway -- fission fuel must be mined, it isn't grown.

Algae fuels, as drop-in replacements for kerosene, may have some usefulness, but there are storage issues (hard to stockpile something that can mold or rot) -- as you note, however, testing is in progress, and we'll know in a few years.


Since jet fuel is cheaper, why do pistons still use av-gas? Since jet fuel is cheaper, why do pistons still use av-gas? Why is 100LL used at all?

First off, there are some light aircraft powered by diesel like the Piper Archer DX and the Diamond DA42. It should be noted both come in a 100LL powered version as well. As it stands now the FAA is looking for a replacement for 100LL but has not yet decided/found one. There are also some airplanes out there that can run on mogas. But anyway, more to the point, you cant simply put diesel into a regular engine as the ignition system and compression requirements are different. Thus to convert a plane to diesel you need a different engine all together. With that in mind most certifying bodies dont just let you swap airplane engines to any engine you see fit. The engine would need to be certified for the air frame which is a costly proposition. Thus it is sensible and economical to keep the current engines running. There are some efforts like those made by piper and diamond to slowly make diesel aircraft available.

As to why 100LL is used, this stems mostly from history. The vast majority of the general aviation fleet both physically and simply in design, stems from the dawn of the jet age. When the lines for the 172 were first drawn there was plenty of 100LL to go around, it was for lack of a better term, the logical choice.


Is the landing gear controlled by the pilot or is it automatic? I have been watching a few aircraft landing videos in crosswind situations and I noticed that if the airliner missed the runway the landing gear would go back into the aircraft. So I'm wondering if this system is automatic after a takeoff or if it's controlled by the pilot manually?

The landing gear system is operated by the pilots. The extension and retraction, depend on the action from the pilot monitoring, on 2 pilots operation.
The pilot has a lever on their panel, on which they select the extension (down) and/or retraction (up).
Some aircraft has a system to avoid extending them on high speed, to avoid some damage, and on the ground, avoiding retracting, for obvious reasons.
There are lights, green and red, indicating their position.


Can I request VFR flight following in a practice area and without a flight plan? Can you request VFR flight following for your local non towered practice area?  Can you have flight following without filing a flight plan? Can you also receive FF when flying low (800 AGL) locally for separation from pipeline patrols and ag spray planes?

(I've assumed you're asking about the US because you used the term "flight following". If you ask about regulations or procedures, please always tell us which country it is and use a country-specific tag if appropriate.)

Can you request VFR flight following for your local non towered practice area?

Yes. I did a tower tour at Huntsville, AL (KHSV) once, and the controllers asked us to request it in the local practice areas. First and most importantly, it helps improve safety and traffic management. But as a secondary point, it counted as an "ATC contact" or whatever they called it, and that helps to justify the existence of the ATC services in that area. That's a good thing for both controllers and pilots.

In general, ATC strongly prefers to be in contact with aircraft in or around their airspace, and I've heard that in multiple presentations from them. If they're really too busy to provide the service then you won't get it, but it never hurts to ask.

Can you have flight following without filing a flight plan?

Yes. They're complementary but not the same thing and they're managed by different groups: Flight Service manages VFR flight plans and ATC manages flight following. In fact, ATC doesn't even have access to VFR flight plans.

Can you also receive FF when flying low (800 AGL) locally for separation from pipeline patrols and ag spray planes?

Maybe. ATC needs to see you on radar to provide flight following and if you're too low you may literally be below the radar.


Why do commercial flights avoid overflying Shandong Province? I noticed this a few months ago on a flight to Shanghai. Since then I've been looking on FlightAware occasionally, and every flight I've seen on a similar route has made a turn to the east just before reaching the coast of Shandong. My first thought was noise abatement, but that far out I believe they'd still be at cruise altitude. Any ideas?

This is a guess, but it may be to avoid the restricted area ZSR-522 that covers airspace just north and east of the Shandong Peninsula. It's hard to say exactly, but it looks like your flight path more or less skirted around it, and the IFR chart suggests that the flight might have followed A326 east and then south from the ANRAT waypoint.

The Chinese AIP says (or said; that link may be out of date) that ZSR-522 is only up to 18000ft so in theory it's still well below typical airliner cruising levels. I've read several times, though, that the Chinese military control their airspace very tightly and they typically don't allow direct flight paths so perhaps they simply insist that aircraft must follow the airways because of 'national security' or some similar reason. Example:

  And the military's control of nearly all the airspace between Chinese
  destinations means that flights within China, even by the favored
  national carriers, sometimes must fly indirect routes that are the
  equivalent of going all around the city on a ring road.


Can a civilian purchase a new (Not Retired) fighter jet? Let's take a jet like the F22 for example. Strip all the classified systems/weaponry out of it (civilian wouldn't need that), and just leave the basic aircraft itself.

Could a civilian technically purchase this aircraft for personal recreational use/travel? Or are these exclusively contracted to the military? How does that work legally/technically? I've always been curious.

There are multiple cases of wealthy civilians purchasing USED fighter jets, sure, but I'm talking about a wealthy civilian who can afford it ordering a shiny new (not retired, there is a difference) personalized, non-armed F22 for personal use. Is that even a legitimate premise?

Certain jets, yes.

But not the F22.

See current listings:

https://www.controller.com/listings/aircraft/for-sale/list/category/10072/turbine-military-aircraft


How much pressure does the compressor of an engine of B787 exert on air? I have been intrigued by this new airliner, especially about the engine. Can anyone tell me how much pressure is the compressor given by the Boeing 787 and how it can be calculated?

Wikipedia will tell you that two engines are offered for the 787, namely the General Electric GEnx and the Rolls-Royce Trent 1000. Their overall compression ratios are given as 52:1 for the Trent 1000 and 41:1 for the GEnx.

Note, however, that this just one point which will change over the whole operating envelope. In case of the Trent 1000, the point is given as "top of climb", whereas the number for the GEnx is given without context. At high speed, the intake will already raise the pressure at the compressor entry by approximately 20%.

If we now assume that ambient pressure at the (not specified) top-of-climb altitude is a quarter of sea level pressure and add the intake pressure increase, the total pressure of the air inside the engine at the compressor exit of the Trent 1000 is 1,580,000 Pascal or 15.6 times sea level pressure.

In the static case (at rest, on the ground), the engine needs to actively suck in air, so the pressure at the compressor entry is lower than ambient (approximately 88% of ambient pressure). If we now assume that the compression ratio is around 45, the absolute pressure at the compressor exit would be 4,012,500 Pascal or 40 times sea level pressure.


How do pilots avoid thunderstorms at night over the pacific en route to Australia where there is no radar? And what if a large front develops? While there is some limited radar on board for pilots, we hear that there is no radar over the ocean which is why flight locations are estimated. As the distance between the United States and Australia is lengthy, how do pilots avoid thunderstorms over the Pacific en route to Australia at night, and what did they do about them If a large front develops in their way?

You get by on your airborne weather radar which is good out to about 80 miles.  You may also get help from other pilots ahead of you on your route providing PIREPS to ATC on HF, or talking to you directly if you are close enough to use VHF between each other.  But as far as picking your way around cells, besides being able to see them if you are in the clear, your own weather radar is all there is.

The main limitation of airborne WR is shadowing due to signal attenuation in rain, where the really nasty stuff is hidden (looks black as if the sky was empty) behind less nasty stuff.  Even with the modern radar there is a bit of skill and experience required to interpret the returns and not get fooled by shadows.


Can I pass the EASA ATPL exams by studying FAA materials? If I have the theoretical knowledge required for an FAA ATP, could I use that knowledge to pass the EASA ATPL exams?

If you are asking about knowledge - you can use some points, but generally FAA ATP knowledge is insufficient. EASA has 14 subjects, and requires much stronger theoretical background. Just go through the EASA learning objectives and you will see it.

Moreover, there are some differences in Air Law (EASA exam pretends to be ICAO-based only, but in fact it asks about European regulations as well), IFR and VFR communications, and even units of measurement (for example, EASA has some questions where you pick altitudes in metres, not in feet).

And, finally, you cannot even sit the exams without taking a theoretical course in some EASA-approved flight school. That means that you usually need a course completion certificate before going to CAA. There is another way if you have 1500 hours under your belt, but that is a completely different story, and in that case sometimes you don't need to sit the exams at all.


What happens when VFR conditions change over a longer flight? If I am understanding correctly, you're not really supposed to use VFRs if the weather is not clear enough that you can ensure you're not going to run into things. Usually you do this by looking at the weather reports around your route and going "nice and sunny, VFR should be fine". Then picking an altitude that meets your other needs, but keeps you out the clouds (if there are clouds).

Then, because you are flying VFR, when you're outside of certain air spaces, you're "on your own". No ATC, or things like that. Maybe some flight following, where the ATC guys goes "Ummm, you see that big mountain right? you're not going to hit that are you?" But also no guarantee of that either (because you're supposed to be using your eye balls to see that you don't smack into the mountain).

But what happens when conditions change, either because the flight is taking longer than expected, or because they just do, and stuff happens? You seem to be in a rough spot. You should not continue your flight using VFR, but you can't land. So what do you do, specially if not rated for IFR flight?

Specifically, conditions are no longer acceptable for VFR flight, and you're not really near your destination, and the smaller airports around you are not currently staffed.

You wrote:

"you should not continue your flight using VFR, but you can't land"
"and the smaller airports around you are not currently staffed."

Why can't you land?   If you can't land, you should not be flying!
Why do you need an airport to be staffed to land there?
Pilots land at unstaffed airports all the time following CTAF (Common Traffic Advisory Frequency) procedures.

If weather conditions are deteriorating below VFR minimums, then landing at an airport is 100% an appropriate course of action.


How can I get back up into flying after not flying for over 20 years? I'm a former USAF pilot and former Bonanza owner. Haven't flown private in more than 20 years, but looking to get back into it. A friend who owns a 182 and will fly with me and I know some instructors. I don't even know where my old certificate and log book is, so this is really starting from scratch. What steps do you recommend?

I do not have a sanctioned answer, but I can give you my take on an overview of what I would plan for someone in your situation.

Your certificate duplicate can be ordered.

You will need a through review of airspace (things have changed), weather gathering (also changed), AIM stuff (changed runway markings, phraseology, etc.).  I would urge FAAST courses for various topics you might feel you can get brushed up in.

On the air side, if we were flying, we would do a "checkout" in the 182, covering the systems, cowl flap operation, targeted power settings, trim usage, etc.  A little airwork, steep turns, turning to headings, power on and off stall recoveries, emergency procedures and landings, takeoffs and go arounds.

Everyone is different, but the last guy, similar to you, flew C208 and stopped in 2003, and did not fly since then.  This summer he hit the books, watched a few videos, took a practice commercial written, and flew 3.4 hours with me.  I was real comfortable signing him off, and his wife, who was the concerned party, was real happy with the results.

Two months later we did a instrument proficiency ride, and that was a little more difficult for him, but he completed the ride in two flights and less than 4 hours.

Good luck and enjoy. Have your instructor put a new high performance endorsement in your new logbook for the 182.


What is an optimum engine-out bank angle? I've heard three methods you should use in a multi-engine aircraft when engine-out, each recommending a different amount of bank:

Wings level, hold aileron to maintain roll and correct using rudder.
Ball centred, ailerons flush and correct using rudder.
5 degrees bank angle, hold aileron to maintain roll and correct using rudder.

Which one of these will result in the highest climb rate, and are there any advantages to other methods which don't give the best climb rate?

This will vary from plane to plane depending on design, and the best method to use for a particular airplane would be listed in the engine out procedures in the POH/AFM.

When the manufacturers are flight testing the airplanes they try various combinations and tell you the absolute best one so that you can get the best performance out of the airplane in a potentially dangerous situation.  Fly your particular airplane by the book to ensure that the airplane does what the POH says that it will do.

Examples:

The Seminole POH says:

  Trim the aircraft as required and maintain a 3° to 5° bank toward the
  operating engine. The balI wiII be 1/2 to 3/4 out for minimum drag.

The Diamond Twin Star POH says:

  Establish minimum / zero sideslip condition (approx. half ball towards good engine; 3° to 5° bank).

The Cessna 441 POH says:

  The one engine inoperative best rate-of-climb speed at various altitudes
  is shown in Section 5. Best single-engine climb is attained with the wings
  banked approximately 3° to 4° and with a 2/3 to 3/4 ball slip into the
  operative engine when the airplane is at low airspeed and heavy weight. As
  airspeed increases and/or airplane weight is significantly reduced, the 2/3
  to 3/4 ball slip becomes less important.

I'm seeing a trend here among the light twins.  :-)

On the other hand, in the jets that I have flown, we are taught to remain wings level and center the ball with the rudder:

Here is what the Lear 60 AFM says:

  Use rudder pedal force as necessary to maintain slip/skid indicator centered.

The Falcon 50 AFM doesn't mention it at all.

This isn't the case for all jets though.  The A320 Flight Crew Training Manual says:

  ENGINE FAILURE

  In flight, if an engine failure occurs, and no input is applied on the
  sidestick, lateral normal law controls the natural tendency of the
  aircraft to roll and yaw. If no input is applied on the sidestick, the
  aircraft will reach an approximate 5 ˚ constant bank angle, a constant
  sideslip, and a slowly-diverging heading rate. The lateral behavior of
  aircraft is safe.

  However, the PF is best suited to adapt the lateral trimming
  technique, when necessary. From a performance standpoint, the most
  effective flying technique, in the event of an engine failure at
  takeoff, is to fly a constant heading with roll surfaces retracted.
  This technique dictates the amount of rudder that is required, and the
  resulting residual sideslip.

  As a result, to indicate the amount of rudder that is required to
  correctly fly with an engine-out at takeoff, the measured sideslip
  index is shifted on the PFD by the computed, residual-sideslip value.
  This index appears in blue, instead of in yellow, and is referred to
  as the beta target. If the rudder pedal is pressed to center the beta
  target index, the PF will fly with the residual slip, as required by
  the engine-out condition. Therefore, the aircraft will fly at a
  constant heading with ailerons and spoilers close to neutral position.


How can I create a map of all paths of flights incoming to an airport in a given day? Is there any where I could find a map overlaid with the paths of all flights that came into a given airport on a given day?

I've found a lot of maps that show the current location of all flights, or the path of a single flight, but I haven't been able to find any that have the paths of many flights at once.

I don't really want to scan through every flight manually, but if I could see all of their paths I could pick out the ones that have interesting tangents for further study.

Background: I recently moved in to an apartment near an airport. I enjoy watching the flights come in, but one of my new roommates is less thrilled. He remarks that some of the flights seem to be pointed directly at our building until their final turn, so if the pilots were somehow incapacitated, we'd be in trouble. I think that's geometrically incorrect, but would like to prove it.

Well you didn't state which airport you are interested in but obviously you only want data for a single airport.

You might want to look into a noise abatement reporting system. For instance, London Heathrow maintains the great website Webtrak (https://webtrak52.bksv.com/lhr4) to communicate with the pubic about changing SIDs, STARs and noise levels. (They already have much of the TMA over the heavily populated area, and they want to add another runway)


Are there any speed and altitude limitations on the autopilot? This comes from a flight simulator experiment ages ago in the Boeing 737....

  I'm on the runway. I engage the heading switch, followed by the altitude and thrust. The aircraft lifts off, although hardly in a clean rotation.

Now I'm wondering: While it did work in the flight simulator, will the autopilot follow these commands in real life as well if I  repeated this in a business or commercial jet?

Is there anything stopping the autopilot from engaging a specified altitude or from following instructions which are utter nonsense?

It depends on the autopilot and you need to read the AFM to know what the limitations are.

All of the business jets that I have flown prohibit the use of the autopilot below a specific altitude (between 50 and 1,000 feet depending on the autopilot and the configuration of the airplane). Nothing prevents then from being used lower but it isn't allowed.  They will engage on the ground (in fact it is part of the required preflight tests) but have a specific prohibition against use for takeoff and landing.

Other airplanes and autopilots however are approved for autoland all of the way down to a full stop, and some for takeoff as well.

Speeds also depend on the autopilot.  Some automatically disconnect if the aircraft receives a stall warning or overspeed signal but others don't.


How is the pushback tug connected to the airliner? If I understand correctly, there are two way to connect a tug: with or without a bar; and the tug can push back or pull forward the airliner.

When using a bar, how is the tug connected to the airliner? Is there a standardized fixation? Is there any articulation on both sides of the bar?
When using a towbarless tug, the tug lift the nose wheel. How is it done? Is there limitation on nose-wheel dimensions?

You're correct that there are two ways to connect the tug with an aircraft.

In case a tow bar is used, it is fixed laterally at the nose landing gear, while allowing slight movement for height adjustment. Usually, the tow bar may pivot freely laterally and vertically at the end that attaches to the tug rather than the aircraft end, which is fixed. The following image shows the tow bar attachment point in Airbus A380.

Image from alamy.com.

The attachment points differ from aircraft to aircraft, so the tow bar basically operates as an adapter between the aircraft and the tug.

"Pins for pushback". Licensed under Public Domain via Commons.

When using a towbarless tug, the nose wheel is simply scooped off the ground using hydraulic (for large aircraft) or electric power. The tug has a movable arm, which is opened and the tug is moved and placed so that the nose wheel fits inside the fixed arms of the tug. The movable arm is then closed. The movable arm and the opposite side has sloping edges, over which the wheels slide and the nose is lifted off as the arm closes due to hydraulic power.

This video shows how its done in case of Airbus A380.

Video from Youtube: How to lift up the world's largest commercial aircraft the Airbus A380. by user Kasavubu

The nose wheel dimensions and aircraft weight do matter and tugs are certified for different ramp weights.


How does a DC-10 flight engineer manually raise the cabin altitude? Reading into Federal Express Flight 1406 and I noticed when the flight engineer said he was raising the cabin altitude “manually” I assume this has something to do with cutting off the oxygen, but how can it be done manually? This also makes me curious to know if there is another way of doing it, but would be disabled under certain emergency scenarios?

Tip: accident investigation reports cover the system controls related to an accident. The one in question is here (ntsb.gov), from which:

  According to the March 20, 1994, edition of FedEx’s DC-10 Flight Manual, page 8-2-2/8-2-3:

    Cabin pressure is controlled and maintained by metered release of conditioned air in either an AUTO, SEMI-AUTO, or MANUAL mode…. The cabin pressure controller maintains the desired cabin pressure in relation to the altitude set…. In the AUTO mode, cabin pressurization is automatically controlled during takeoff, climb, cruise, and descent conditions to the lowest cabin altitude compatible with aircraft and flight requirement.

    When MANUAL control is selected, the control wheel engages a locking mechanism to secure the outflow valve in its existing position. After selection, manual control is achieved by pressing the control wheel inward (to disengage the control wheel locking device) and rotating the wheel to position the outflow valve to attain the desired cabin altitude. When the control wheel is released, the outflow valve is again locked in its existing position. Selecting the valve at more than two-thirds open can cause a negative pressure and create a noise in the cabin. At touchdown, the Second Officer [flight engineer] must manually depressurize the aircraft. [emphasis added]

Regarding the outflow valves, see:

What is this device below the door of a 737 (and other aircraft)?
What are these "portholes & panels" on the fuselage of the Omega Tanker DC-10?

And the aforementioned wheel and readout:

Source: wikimedia.org


On a constant speed propeller, how is blade pitch related to airspeed and efficiency? I'm trying to wrap my head around what the PHAK (Pilot's Handbook of Aeronautical Knowledge from the FAA) states for constant speed propellers.

First, why is it that if airspeed increases, for a fixed RPM, your blade pitch will increase, and conversely decrease if airspeed drops? Is it because it wants to maintain that low AOA (2-4 degrees according to the PHAK) with the relative airflow? If its AOA were to increase, such as not changing blade angle as airspeed drops, would drag increase causing the RPM to fall? And if airspeed is too high and AOA is too low, there's not enough drag to slow down the blades?

On takeoff, you want max thrust/power. By setting the blades to low pitch, you get max RPM because the blades take less of a bite out of the air. This also keeps blade AOA low which in turn reduces drag and allows RPM to build? Also, is max thrust in this case the result of the blades generating a lot of lift because of their high velocities from the higher RPM (versus thrust from moving larger chunks of air)?

Why is low RPM preferred in climbs/cruise? If you increase the blade pitch in these flight regimes, you get a smaller AOA with respect to wind which from my understanding so far reduces drag? But then doesn't the blade taking larger bites of air require more power anyways to maintain that RPM? Is it more efficient to give power to blades to move larger loads of air than it is to run at lower blade angles and higher RPM?

And lastly, with regards to blade AOA and relative wind, would a blade generate more lift if it had a higher AOA, but at the expense of additional drag on the blade which in turn requires more engine power? Is this drag induced drag because it's an airfoil?

A propeller has an optimum AoA, where the thrust/drag ratio is highest. A variable prop can maintain this optimum AoA at a wide range of speeds, thereby reducing fuel consumption. That's all it does.

Fixed wing propellers have a dilemma that at take-off they require maximum thrust, and in high-speed flight the engine will need to be throttled back to prevent overspeeding. Maximum engine power cannot be utilised at every airspeed.

  why is it that if airspeed increases, for a fixed rpm, your blade pitch will increase?

To maintain optimum blade AoA. Increased airspeed causes reduction of blade AoA at constant pitch.

  On takeoff, you want max thrust/power.

You want maximum thrust. This is generated by making the propeller spin as fast as possible (not exceeding tip Mach), with a maximum blade pitch where the blade does not stall. Reducing blade pitch to increase RPM results in no increase in thrust: Power = Thrust * RPM.

  If you increase the blade pitch in these flight regimes, you get a smaller AOA with respect to wind

No a larger AoA. More thrust and more torque on the propeller axis.

  Is it more efficient to give power to blades to move larger loads of air

The engine runs most economically at the RPW where maximum torque is generated, the constant speed propeller speed control wants to maintain that RPM.

  would a blade generate more lift if it had a higher AOA, but at the expense of additional drag on the blade which in turn requires more engine power? Is this drag induced drag because it's an airfoil?

Yes and yes :)


Is it possible for aircraft to transfer from low to high airways? Is it possible for aircraft to transfer from low to high airways?

In the US, you must be flying below 18000 feet to be able to use a Victor airway, while you must be at FL180 or above to use a Jet airway.

If it is possible to do so on one flight plan, at which point would you switch over, and what altitude would you need to be at?

I've looked at various low and high altitude enroute charts, as well as FAA guides on using these, but they don't seem to really point this out.

The use of airways is, as you already specified, based on altitude/flight level. To be able to transition from higher airways to lower airways and vice versa, they need to intersect at NAVAIDs, such as fixes, VORs or NDBs. Where no restrictions are in place for the use of an airway, the flightplan can just contain the fix where the transition from lower to upper airway will take place. Let's take a look at this flightplan route from EGLL - London Heathrow to EDDG - Münster-Osnabrück:

DET UL6 DVR UL9 KONAN UL607 SPI UY862 PODEN Y862 KENUM UZ907 ABAMI Z907 HMM

Lower and upper airways in Europe are distinguished by prefixing the airway designator with a U for Upper. At the end of our flight to Münster, we intend to switch from UZ907 to Z907, because UZ907 does not connect KENUM to HMM. The flightplan routing validator at the CFMU understands that HMM is a STAR/transition entry point to Münster and therefore considers the airways altitude restrictions to be met during descent.

If this was not the case and we would like to use airways restricted to certain altitudes/flightlevels, we would need to indicate this in the flightplan. Example:

Airways Z1 can only be used between FL195 and 245, airways UZ2 can only be used above FL245. We want to file our flightplan with a step climb and a final cruise level of FL290:

With RFL290 entered into the validator, the flightplan would not be accepted, as Z1 is only available below FL245. The portion of the flightplan to be accepted would need to be:

[...] ABCDE/F230N0260 Z1 FGHIJ/F290N0270 UZ2 KLMNO [...]

In summary, you can switch between upper and lower airways, if you can meet the altitude/flightlevel restrictions and any other restriction bound to the airway.

  ICAO flavored anwer added on request by question op, even though original question is tagged with FAA.


What are the relative internal noise levels in various aircraft? What are the relative noise levels in various aviation scenarios for observers seated inside an aircraft? I'm sure the specifics depend on model but I'm curious if any general trends can be drawn that are model agnostic.

e.g. Let's consider the following cases for a start:

Jet aircraft e.g. 747
Jet aircraft gliding with all engines out
Helicopter
Glider
Jet aircraft with noise insulation removed
Piston driven prop. aircraft

We could rate on an arbitrary 1 to 100 (noisiest) scale. Even better, in case data is available, I'd love to see the dB values. (@vortaq7's comment below)

I suppose speed will matter so we can probably use their typical cruise conditions for each case?

One motivation is to tease apart contributions from engine vs aerodynamic noise. Another is to quantify the effectiveness of sound insulation.

I suppose most of us never get a chance to really hear an all engines out jet nor to fly in a jet with no sound proofing.

PS. This question was motivated by some comments here:

What are these green "bags" on the SSJ100?

Let's start with the items for which hard data is easily available:

This FAA chart gives relatively average noise levels for passenger jets, GA turbine aircraft, and helicopters. In addition, this chart should start to give an idea of how much noise is generated aerodynamically in aircraft:

While this is older ground data and not cabin data, it still serves to show that the contribution of aerodynamic noise to total nose levels is not insignificant.

Based on this, I would say that a likely ranking of these environments, from loudest to quietest, is:

1/2: Helicopters and uninsulated jets. In both cases, the close proximity to large, powerful engines leads to extremely high noise levels.

3: General Aviation piston-engined planes. Again, these generally have little insulation,but smaller engines generating somewhat less sound.

4: Passenger jet cabin. Protective insulation reduces sound, but actual noise levels vary based on seat position.

5: Passenger jet cabin, gliding. As seen in the second chart, aerodynamic noise still contributes to a relatively loud environment.

6: Gliders. Slower speeds and cleaner aerodynamic profiles reduce aerodynamic noise, leading to a much quieter ride.


How does the FADEC prevent compressor stall during rapidly increasing thrust demand? My understanding is that if the demanded thrust is increased rapidly on a jet engine (for example going from nearly idle to TOGA during a go-around), the engine will likely experience a compressor stall: the combustion chamber is demanding a high quantity of air, but the front of the engine is unable to spin up and cope with the demand in such a short period of time.

This is the reason why engines spools up slowly: the FADEC is carefully controlling the amount of fuel increase to prevent a flameout; pilots should anticipate this delayed response compared to a piston engine.

My question is how exactly does the FADEC do this? Which parameters (air temperature / pressure) are being monitored, and what are the signs which the FADEC can detect such that it knows it has reached the limit of fuel increase?

For example is it something like monitor air pressure at X while increasing throttle, if X exceeds a certain value, stop increasing throttle until X drops?

Of course it depends on the engine, and the exact how is proprietary information. That being said, nowadays jet engines are designed to allow faster acceleration than before. So it's not just the fuel rate the FADEC controls.

COMPRESSOR AIRFLOW CONTROL
To limit compressor surge and to provide good acceleration, the engine
is equipped with a Variable Bleed Valve (VBV) system and a Variable
Stator Vane (VSV) system.
Both systems are fuel operated by the HydroMechanical Unit (HMU)
and controlled by the Electronic Control Unit (ECU).
— CFM56-5B training manual

As for the parameters being monitored, first let's look at the common stations:

(Source)

With the basic station numbers now known, let's look at what the ECU (part of the FADEC) monitors and controls:

During testing, data for the optimal accelerations for all conditions (forward speed, air density, engine angle of attack, etc.) is collected. From there the FADEC is programmed to stay within those limits.
If a pilot demands full thrust from idle on ground on a hot day, the FADEC will have an idea of the initial fuel flow and rate to supply, and how to control the variable stators and bleed. From there it will monitor the many parameters, and adjust accordingly.
The same can be achieved on older engines using purely hydro-mechanical systems, but of course not as efficiently. There are of course different ways to override the FADEC and demand more power than the FADEC was designed to allow.
For the CFM above, the fuel flow commands depend on:

engine oil temperature,
Fuel Level Sensing Control Units (FLSCUs) shut off signal,
A/C on ground and low fuel flow return level,
A/C in flight and low or high return fuel flow level,
N2 speed,
engine fuel flow demand.

This flightglobal.com article about the GE flying testbed discusses the benefits of fine tuning the FADEC from real data including extreme angles of attack.

We think of the FADEC monitoring system as being like a screwdriver. We're using it to adjust and fine-tune the turbine clearances in real time by using the data to modify the control software and make it more efficient.
(...)
[Testing at] angles of attack of between 30° and 32°. The high-inlet-angle test is required for fan-stress and inlet-stability evaluation.

Trivia: In the making of the 777, the management wanted to skip the in-flight testing of the P&W engine (the first 777 engine). As those tests cost millions. They eventually played it safe and tested it on a 747 testbed, guess what happened:

The engine surged on its third takeoff, despite the numerous static tests they made.


What is autofeather for? What is autofeather and when is it used?

Autofeather is an automated feathering feature, common in a lot of turboprops, which automatically feathers the propeller on an engine if it loses power without having to either pull the power or propeller lever to feather the prop.

It reduces crew workload in the event of an engine failure and, as a feathered propeller greatly reduces parasite drag on the side of the dead engine, reduces the asymmetrical thrust yaw moment about the vertical axis of the airplane.

It is particularly useful during takeoff and landing operations where airspeed may be very close to minimum controllable airspeed on one engine.


What is the role of the flight dispatcher? How do the dispatcher and the flight crew interact? The flight dispatcher is known as the ground pilot.

Air Canada flight dispatcher (source: On the job: Flight Dispatch)

It's not clear to me what is their role related to a given flight, or where are they working from, how the interactions occur between the flight dispatcher and the flight crew, or the maintenance team.

Have you ever been waiting for your flight at an airport and noticed that big roll of paper being printed up at the counter? That is called the "Flight Release". That roll of paper has all the information that the pilots need to fly their flight. It has their fuel, their route, the weather information, probable payload, and everything else they need. A Flight Dispatcher put that together for them.

The pilot looks it over and calls the Dispatcher if there are any questions. The aircraft cannot push back until both the Captain and the Dispatcher agree that it is safe to do so.

Once the flight is airborne the Flight dispatcher keeps an eye on that flight and makes any recommendations to the Captain regarding any change in route, altitude, speed, destination or anything else relating to flight safety and the efficient operation of the flight. This is called "flight following". If ATC gives the Captain a route change, the Dispatcher will calculate the new fuel burn. If the flight has to hold, the dispatcher will calculate how long the flight can hold before it has to land somewhere.

The Captain and Dispatcher communicate with each other while the aircraft is in flight primarily by ACARS. Think of it as an instant messaging system between the Flight Deck and the Dispatcher's computer. They type messages back and forth to each other and each sees the message on a screen.

The Flight Dispatcher is also there to help if there is a problem with the flight, i.e., a sick passenger, a mechanical issue, a change in the weather, or any other problem that the Captain may be facing.

The Captain and Dispatcher have joint authority to initiate, delay, continue, deviate, or cancel a flight. Nothing can happen until they both agree on a plan of action. Because of this the FAA considers the dispatcher to be a "crew member" and the Dispatcher's base of knowledge is very close to that of the pilots. Because of this, Dispatchers are allowed to ride up in the Flight Deck if they want to and are required to do it at least five hours a year.

No, the Dispather is not an Air Traffic Controller. However, the Dispatcher's plan for the route is given to ATC. ATC then takes all the flight plans filed by all the Flight Dispatchers who work for the airlines and put them all together for the big picture and then make it happen.

In short, the Flight Dispatcher works for an airline to plan flights and give that information to both the flight crew and air traffic control, monitor the flights while they are airborne, and work with the Captain to solve any problems that may arise. Their main concerns are, in order, flight safety and flight efficiency.


What is black routing and red routing? In a meeting with aviation experts, the terms black routing and red routing were discussed. What are they? From the discussions I was able to put together that either the TMU (Traffic Management Unit) or the controller can see two routes for a flight, a cleared route (black route?) and an agreed route (red route?).

I don't know what display they were referring to and if it was the controller that was looking at the display or if it was some traffic management display.

Here's the history, at least for the US.
When I hired on, way back in 1985, the FAA had printers based on IBM Selectric mechanisms. They had a two-color ribbon, red and black, and a typeball with capital letters in two sizes, plus a few special symbols.
When a strip was printed, color could be selected by the computer for certain items. Callsigns on eastbound aircraft, for instance, were printed in red.
A red route appeared when an aircraft filed a non-preferred route to or from certain airports.
In the US, red is used for a PLANNED route or restriction - something the controller SHOULD do, but hasn't accomplished yet. As an example, if you filed KBFF..KDEN in your C650, the strip would reflect your filed route, but the computer would include a route over LANDR, like this - "KBFF.LANDR1.KDEN" (routing would be over AALLE, today)
That route would be printed in red, to alert the controller, and was the forwarded route. In other words, all subsequent positions would show that route in black, with no reference to the original filed route.
The controller issuing the clearance is expected to use the red coordinated route. If they choose not to (such as for a MEDEVAC), they must update the flight plan to force processing of the original route.
Today, there are no "red routes" per se, if you observe a preplanned route in EDST, it's highlighted in blue. Our strips no longer have two-color capability, so "red" text is now printed in black with a grayscale highlight.
The jargon, however, lives on!
To further address your example, if both a red route and black route exist in the system for a particular aircraft, that means the preferred route has NOT been issued, and some action must be taken. The controller with responsibility for meeting the restriction for that route will issue it, or force the computer to override the red route. Either way, it ensures that subsequent sectors get the right information.


Why does the 737 auto-land trim up at around 400 feet RA? From the 737NG FCOM:

400 Feet Radio Altitude
The stabilizer is automatically trimmed an additional amount nose up. If the A/Ps subsequently disengage, forward control column force may be required to hold the desired pitch attitude.
If FLARE is not armed by approximately 350 feet RA, both A/Ps automatically disengage.

For the media component, you can listen to the trim wheels in this auto-land video (YouTube).
While I understand the design reason may not be public, educated guesses wrt the aerodynamics involved, and/or answers from experienced 737 pilots, are welcome.
When the plane trims up while in auto pilot, the auto pilot would push the control column forward. So what I'm thinking of, is during a go-around, somehow it's better to relieve this control column pressure for a nose-up, rather than pull back?
I checked the A320 since it is of the same category, and as far as I've looked, it looks like the A320 doesn't use that system, which tells me it may not be a certification requirement (not 100% sure).

My educated guess is that they had to find a way to give the autopilot more pitch authority for Autoland.  It's possible that the pitch feel system force the AP servo has to work against to flare the airplane, which would be more than its normal in-flight range and would also need to include some margin to apply more than the normal flaring range if necessary, is more than the servo can handle for one reason or another.  Slip clutch limits, torque load limits, maybe the operating range of the servo capstan, something like that.  Possibly it was fine for 95% of conditions but there were extreme cases uncovered in testing that had to be catered to.

So the system sets stab trim to a slower speed, which requires the AP servo to maintain a nose down load to maintain Vref.  The servo now has increased effective pitch up range because the first part of the flare action is just to stop pushing, and from there is has the normal nose-up-from-neutral range available as additional authority.

Rather than go to all the trouble to certify a new, more powerful servo, they worked out this little cheat to get the extra pitch range they needed.

Just a theory, which assumes this feature is only active during Autoland.  An alternative theory (say if the same feature works in normal non-Autoland approaches) would be they wanted a positive pitch up tendency built in if the GA button is pushed due to the airplane's behaviour somehow.


What was the lowest ground speed landing? Planes fly if their speed relative to the air is high enough to produce lift. If there is no wind, the plane's air speed will be the same as its ground speed. If flying against wind, its ground speed will decrease.

That said, what was the lowest ground speed during landing ever recorded?

I think there should be some rules against landing with the wind from behind, as it would make the plane go too fast, but what about opposite? Has any plane landed with zero, or close to zero ground speed? How about 'negative' speed, pushing the plane backwards relative to the ground?

I found some information about the highest ground speed while in flight, but I'm interested in the exact opposite.

Highest points for answers with passenger airplanes (737 etc), helicopters are considered cheating.

Considering groundspeed as a vector, 0.

You can land vertically with strong headwind: Could a plane land vertically in a strong headwind?

As the answers to the question, previously linked in Flexo's comment -  Is it possible to fly backward if you have really strong headwind? - detail, the answer is "yes". It's even possible to have one's groundspeed vector directed tailwise.

The Antonov An-2, can be flown at as little as 25 knots of airspeed, and safely landed below that speed. When turned into decent headwind, it can be kept in hover relative to the ground. I haven't found a video of a hover landing, but it's been reported to be done. With a 12-passenger capacity, it's no 737, but it can count as a passenger airplane, and it's designed to do this.

Vertical landings of smaller planes have been filmed:

             (thanks bukwyrm).

P.S. Vector representation is most appropriate for groundspeed, as it's not normally directed along the aircraft's longitudinal axis due to side winds. So landing "backwards" can be considered positive groundspeed once again.


What are the "beta lines" of a gas turbine engine component? For a compressor in particular, what do the beta lines represent? I know for a compressor map you have the lines of constant spool speed, and the contours of isentropic efficiency. I also understand the purpose of the surge line and how it acts as a limit for stable compressor operation. I'm not entirely sure what beta lines do though, are they just fractions of the pressure to mass flow ratio that results in surging? Or are they something else?

Beta lines are either linear or parabolic shaped lines that are conveniently placed and spaced between the surge line and the choke line of the compressor (or fan) characteristic (also known as compressor/fan map). Such lines are also found in the turbine characteristics maps.
Basically, the Beta lines create a new axis system that prevent a solver from finding multiple (or the incorrect) operating point(s). The lines itself do not represent anything, they are simply added for convenience and calculation stability.
Compressor maps describe the relation between various parameters which are based on experiments or CFD calculations; corrected mass flow, pressure ratio, spool speed and efficiency. In gas turbine performance simulation, a new operating point needs to be found for a change in operating condition. Knowing that speed lines can contain a local maximum pressure, you cannot uniquely describe an operating point with a pressure and a speed, there can be 2 solutions for corrected mass flow. Hence, gas turbine performance engineers have created the concept of an alternative coordinate system that prevents that multiple solutions can be found; this is very useful when programmatically finding/iterating the new operating point.
Compressor maps typically are 2-in, 3-out parameter maps, to find the 3-out parameters (pressure ratio, efficiency and corrected mass flow) you require a unique pair of Beta and spool speed values.

Compressor map (Beta lines are hidden):

Compressor map (linear Beta lines are shown):

Example of parabolic Beta lines:


Why are propeller driven aircraft commonly built with a higher wing aspect ratio and a straighter wing angle? Why do propeller driven aircraft (especially smaller ones like fighters) tend to have a narrower wing with a higher aspect ratio and a more perpendicular wing angle in relation to the body, whereas jet powered aircraft have a broader wing with a lower aspect ratio (a more "delta" shape) and also more angled? Does it have something to do with the operating speed and aerodynamics?

Speed is the reigning factor here, and aerodynamics as well of course.

Speed.
Propeller driven aeroplanes operate at lower speeds than jet engines - propellers are more efficient until compressibility effects start to appear, and it is these same compressibility effects that influence the wing shape as well. Compressibility as in: give air enough time to move out of the way and follow the wing contour. Usually, propeller aeroplanes operate in this aerodynamically docile speed regime.
Aerodynamics.
A wing creates lift by deflecting air downwards, and is most efficient if there is a large amount of air deflected a bit: the least induced drag is created. So the wingspan is the most important parameter here. Lower speed aeroplanes will stick their wings straight out, as much as possible without creating large structural issues due to wing root bending loads becoming unmanageable.
Aerodynamics at higher speed.
Once shock waves start to appear, drag rises rapidly. But only the air flowing perpendicular to span concerns us for the lift creation, and therefore we can present the wing at an angle and use the lower perpendicular speed component for lift creation. But swept wings create many more structural problems than straight wings, and also since we're going faster already the argument of more air at less deflection speed is of less relevance. We have many more problems to tackle here than only minimise induced drag.


When is 'heavy' used in callsign? I have heared at ATC communication that some aircraft uses a heavy word in the call sign.

What does heavy means?

When it is used?

Is it used when aircraft is heavier than 200-300 tons?

The rules are slightly different in different countries, but common for all is that the "heavy" designation is used for aircraft in the heavy wake turbulence category, that is, aircraft with a maximum takeoff mass of 136 tonnes or more (see Is there a definition of light/medium/heavy aircraft?).

Some countries require aircraft to use the heavy suffix to their callsign in all radio transmissions. Others only require it on initial contact with a new ATS sector, whereas others still will additionally require it to be used at all times in the aerodrome environment, but only on initial call in the en-route environment. The local rules will be listed in the AIP for the country in question.

Also note that in some places, certain aircraft types are considered to be in the heavy category, even though their MTOM is below 136 tonnes. The typical example of this is the Boeing 757, which is put in the heavy category by some countries because it creates significantly more wake turbulence that other aircraft with similar MTOM.


Do surfaces ahead of propellers decrease thrust? I have had this question for ages, nearly impossible to find anything on the web. My experiments confused me even further! Its concerning the blockage effects of surfaces ahead of a propeller. For example lets  consider  the usual dual vectoring propellers on the sides of airshipcars like in the skyship 600 for instance.The gap between propeller disc and underside of blimp is 1diameter.

In vertical flight the propeller's  intake are faced by the underside of the blimp body itself. How are the propellers still able to generate thrust when the propellers' intakes are  "somewhat blocked" due the very close surface of the blimp?

As An experiment I  attached a foam sheet about  1.5 diameters above a big 28 lbs 8-rotored octo- multirotor in hopes to see that the multirotor would still lift.  At full throttle the multirotor  didnt even move or lift.  Obviously it lost almost all of its thrust due to the the sheet blocking the airflow above.. even though the foam sheet was high above the props,  about 1.5 diameters.

Can someone help me  on what could be going on here? does having a surface above  a propeller decrease its thrust? Why did my experiment showed different results than the blimp. Are there other factors involved? if so.. what is the relation between distance, disc loading, surface shape, other factors etc  and thrust decrease?
Thanks!
Best Regards
/Eric

does having a surface above a propeller decrease its thrust?

Anything that can prevent or reduce the acceleration of air will reduce the effective thrust of a propeller.  That might include obstructions in front of or behind the propeller.

  Why did my experiment showed different results than the blimp.

First of all, I don't see any information about the blimp.  Perhaps it too is greatly affected.  I don't know how much thrust is required in this configuration.  It may be tiny compared to the thrust available when pointing fore-aft.  If true, both propellers have reduced efficiency.

Second, I would think there's a big difference between a flat obstruction and a smoothly curving one.  Your experiment would be closer to the blimp situation if you put a large blimp shape over the propellers rather than a flat sheet.


Could a rear center of gravity position cause an airliner to lose control when a pitch-up is commanded? Modern aircraft cruise with the center of gravity close to the most rearward position to minimise trim drag, usually achieved with fuel in the tail surfaces. This is transferred forward before or during descent for landing.

Is it feasible that unexpected nose-up inputs could lead to partial stall with the aerodynamic center moving rearward, aggravating the nose high attitude to cause mushing? This is my suggestion that might explain the loss of AF447.

Without ASI (pitot icing) a pilot without experience of this might not recognise the problem. Further, in that attitude elevators could be ineffective and fuel transfer too slow for recovery that way.

This was a serious problem with T-tails and caused accidents to DC9, Trident, BAC1-11 and Tomahawk aircraft; maybe others.

The only solutions were stall warning and stick pusher systems, apart from streaming the braking 'chute by an RAF Victor crew.

On virtually any swept-wing design, "wingtip stalls" are a danger. Most light aircraft are designed with a straight wing and a leading-edge contour such that the inside of the wing stalls first. This has two advantages for light craft; first, aileron control is available as long as possible into stall entry, allowing roll control and thus the ability to stay coordinated. Second, the departure of airflow over the top inside of the wing will reduce or eliminate the downwash of air over the horizontal stabilizer, which normally corrects for a slight nose-heavy CG. With the balancing force gone, the normal response of the aircraft will be to nose down, restoring a lower AOA and building forward airspeed to let the pilot recover quickly.

With a swept wing as is seen on modern jet airliners, the wingtips tend to stall first instead, because the airflow over the swept wing lowers the stagnation point and thus the effective angle of attack of rearward sections of the wing, so the wingtips at the rear will exceed the critical AOA first.

When the wingtip stalls first on a swept wing, two things happen. First, aileron control is severely affected, making it harder to stay wings-level and avoid entering a slip or even a spin. Second, the center of lift from the unstalled portion of the wing moves forward, placing the CG further behind it. An aft CG works against pilot efforts to nose the aircraft back down to recover, and in an uncoordinated stall the aft CG will keep the tail of the airplane "outwards" from the center of the spin, hindering or preventing efforts to cancel out the rotation with the rudder as the vertical stabilizer will be stalled due to the extreme "sideslip".

This is a potential problem in any airliner, where the weight is distributed along the majority of the fuselage. Any significant deviation from mean values for per-passenger or per-bag weights can place the CG too far aft. Tail trim tanks can further exacerbate this.

However, Air France 447's crash, in my amateur analysis, was not caused by trim characteristics. It was primarily caused by the pitot tube icing. This had several detrimental effects on the flight crew's situational awareness:

The disagreement of the plane's two pitot-static systems cause the flight director to unexpectedly disengage the autopilot, setting the flight crew on their heels.
This same disagreement in the pitot-static data also caused the flight director to switch from "normal law" to "alternate law 2" which among other things disabled roll and pitch/AOA limiters, as the data used to calculate those limits is no longer reliable.
Lastly, the lack of reliable pitot-static data was reflected in cockpit instrument displays. We don't know exactly what they saw because information on the status of the right side pitot system is unavailable (it was not recorded by the black boxes). However, we do know the disagreement of data caused the red-flagging of practically all information reliant on such data, including the airspeed and altimeter tapes, the flight director, velocity vector, and all secondary indicators including AOA readouts, the stall horn, etc. The flight crew would have been totally reliant on external visible references to determine the aircraft was stalled.

A secondary instrumental cause was the stall indicator system itself. Counter-intuitively, the stall warning was issued when the aircraft was nosed down, and stopped when the aircraft nosed up. This was due to the flight computer treating the extreme AOA calculations (in excess of 40 degrees) as unreliable. Nosing down brought the AOA back into the "reliable" band, but the resulting alarms were counter-intuitive to the pilots; applying a control input and hearing an audible warning is usually a signal to back off that control input.

Finally, the flight crew themselves demonstrated a lack of familiarity with the behavior of the aircraft in exceptional circumstances; the loss of pitch/roll limiters and the counter-intuitive stall horn behavior caused the flight crew, in the absence of the captain, to make a series of critical and ultimately fatal overcorrections and inactions.


How do the fighter jets in air shows color their smoke trails? At some large military parades or airshows, there will be tiers of fighters arranged neatly. In order to increase the visual effect, white or colored smoke will be sprayed from the tail of the fighters. How is color added to the smoke?

For example for the Patrouille de France under the Alpha Jet there is a tank called the smoke pod. Inside, two tanks contain oil with a colorant if you want color, or without colorant if you want white. When the pilot opens one of the valves, the oil flows through a small pipe and exits only at the left reactor. With the heat, around 700°C, it immediately turns into gas and gives this thick smoke. See this (in French):

Source: pilote-virtuel.com


What is the official unit of airplane/aviation speed? Some units I read/found are commonly used in aviation: mach, knot (nautical mile per hour), mile per hour (mph), kilometer per hour (km/h or kph).

"Official" meaning like the units used by the flight data recorders, if a pilot makes a report of their speed, and so on.

Knot. Pilots think and communicate with ground personnel in knots. HUD tells you knots. Textbook tells you to increase speed to 500 knots after takeoff, and land at 150 knots.


What geographic reference system do airport towers use? What geographic reference system (WGS84, EGM96, EGM2008 etc.) do airport towers use when measuring things like aiport elevation and current QNH for airport? Is there even a commonly accepted one worldwide? I couldn't find an explanation on this subject online.

I've only done passing research on the matter before, so if anyone knows more please chime in.  As far as I know the FAA uses NSRS (National Space Reference System), and the NSRS currently uses NOAA GEOID models, although this is expected to be changed in 2022.  I'm not sure which regulation covers airport surveys, but you can reference AC 150/5300-18 (pdf). ICAO Annex 14 doesn't seem to mention a specific vertical datum for MSL, but specifies that, independent of the method, surveyed altitude for precision approach runways has to be accurate to "one-quarter metre or foot".
Interestingly, some airports and navaids aren't even using the WGS-84 lat/long coordinates, much less a recent international geoid model for MSL.  This is increasingly rare, but you can check a list of waypoints excluded by Garmin for non-WGS-84 compliance here and see a list of overall compliance by country here.
While it's always nice to have higher resolution MSL altitude models, we have to keep in mind their limited usefulness. Since airport's charted elevation is the reference point for barometric altitude in aviation, the only issues with an inaccurate airfield elevation are seen when comparing to elevation measured elsewhere. As a result airplanes a few miles from the airport may indicate less accurate elevation relative to the airport and local sea level (but would indicate accurately relative to other altimeters at the same location). An inaccurate survey would also affect GPS's geometric or "GLS" altitude, but that's basically only used as a cross-check.
I'd guess that in practice temperature and pressure variations over an area are greater than the variations in geoid elevation error over the area, and regulations already account for those inaccuracies, for example by requiring higher MDA when using an altimeter setting from a different airport. Additionally, some of the tools for measuring altitude on an aircraft have much lower accuracy than the <1 m accuracy of these geoid models and surveys.  Even an ideal radio altimeter can only measure 1.5 ft accuracy and then only when close to the ground (according to Arinc 707-7), and un-augmented GPS and barometric altitudes are frequently off by several dozen feet. Aviation is designed with imprecise altitude measurements so sub-foot precision isn't necessary or practical away from an airfield.


How can I scale down a propeller for wind tunnel testing? I want to scale down my propeller for a wind tunnel testing. But I am unable to find any literature on it.  Please give suggestions.

The airfoil of the propeller should not change with size if you are scaling it up and down. As long as the airfoil is the same and the proportions are correct relative to both propellers (length, chord, pitch, etc.) that should be all you need to pay attention to.


Does a helicopter's boundary layer have different properties that are caused by the rotation? How does this affect stability of the helicopter? I recently got introduced to the boundary layer theory of an airfoil, and I couldn't stop thinking of how it would be affected by rotation as well as the varying speeds of rotation.

You are right, the rotation does affect the boundary layer.

Normally, as a wing approaches its stall angle of attack, the boundary layer becomes thicker and the flow starts to separate near the trailing edge. On a rotating rotor or propeller blade, the slowed-down boundary layer will experience a centrifugal acceleration, so it does not come to a standstill eventually (as it does in case of separation), but merely starts to flow tipwards. Since the speed of the rotor blade increases as the flow moves towards the tip, the boundary layer experiences an additional Coriolis acceleration. Therefore, flow separation is delayed on rotors and propellers compared to the two-dimensional case (for example, when the rotor airfoil is tested in a windtunnel).

From heli-air.net:

  The helicopter rotor experiments of Dwyer & McCroskey (1971) also
  suggest favorable effects on the spanwise development of the boundary
  layer, which tend to delay the onset of flow separation to a higher
  blade section AoA and thus serve to increase the maximum thrust of the
  rotor system.

The stability is affected by other effects; here the peculiarities of a boundary layer on a rotating wing make little difference beyond the higher stall angle of attack.


Are there schools that would hire a non-CFI ground instructor? Do flight schools have a strong preference for the CFIs giving BOTH ground and flight instructions? Would there be any interest on the part of some schools to have a pilot, but not a CFI do the ground teaching, while their CIFs would be entirely dedicated to the flight instruction?

I am a newly minted PPL and I would love to associate myself with some flight school and teach the ground as a Certified Ground Instructor. First, it would be really fun to teach this stuff, but also I hope that I could get some financial break on getting my instrument rating (i.e. the highest level of certification I’m interested in). I’d be also interested in staying with a school longer on a more permanent basis (unlike airline-bound CFIs)

Our local flight school had a dedicated AGI/IGI for a while. Lots of CFIs don”t like doing ground instruction and, if they are building hours, every hour on the ground is an hour they are not building time. I like teaching and so I just took and passed all three tests and need to fly over to a FSDO to get my certificate.

For the Fundamentals of Instruction Knowledge Test, study the Gleim book. If you have time, and if you want to be a CFI, at some point you will need to read FAA-H-8083-9A Aviation Instructor's Handbook but you don’t need to read it to pass the exam. A lot of the information in the book is wrong, so I had a hard time memorizing the wrong answer to questions. You probably won’t have to worry about that.

For the IGI and AGI it probably helps to read FAA books or whatever you prefer for getting your aviation knowledge. And you definitely need to study FAR 61 and 91 and know the VFR and IFR charts. I used the Gleim books Flight/Ground Instructor and Instrument Pilot and there were only a few questions that were completely new to me.

I’d say that about half of the questions are really stupid. e.g.

The force acting opposite of lift is:
A. Apples
B. Gravity
C. Oranges

Another quarter were interpolation/math questions that you will never ever need while flying. The study guides do a good job preparing you for them. The trick is to read the question really carefully. e.g. Are they asking you to look on the chart for landing distance or do you need to do another calculation to get ground roll.

There are a bunch of E6B questions that you just need to be really careful in your arithmetic and reading the question.

There are a few that I really had no idea what the question was, let alone what the right answer was. I got about half of those right. There were also a couple on each test where none of the answers was the correct answer.

I took the Commercial and CFI since they are good for two years and I won’t have to study them again if I decide to get a CFI.

My results: FOI 84%, AGI 88%, IGI 90%, CFI 87%, COM 92%.

I studied for a month, which was a bit of overkill.

Sporty’s and King Schools let you take practice tests for free. I put a lot of my notes up on my website so you can save time when studying by printing and memorizing them.


What makes the perfect landing in a tail-dragger? What is the 'perfect landing' in a tail-dragger? Do you try to float down to a main-gear landing at minimal descent speed, or do you attempt to float to the point where your tail will either start to come down, or does come down? In other words, is a two-point touchdown or a three-point touchdown more 'perfect.'?

They both have their uses.

A wheel landing (landing on the mains, keeping the tail in the air) gives you better rudder authority on the ground, which is useful in a crosswind or during touch-and-gos. However, there is a point where as your airspeed slows, your rudder authority falls off quickly. This can be dangerous in a crosswind for obvious reasons. Most pilots avoid this condition by keeping the tail up for as long as they can, then quickly planting it on the ground when it begins to fall. The rudder is kept up in the airstream for as long as possible, and then you quickly transfer your turning control from the rudder to the tailwheel.

A three-point landing is useful for getting on the ground and stopping quickly, because the additional wheel provides more friction. It also avoids the situation in the previous paragraph, because you land with the tailwheel down and have full turning authority the entire time.

So in general, there is no preferred landing style in a tailwheel airplane. This may not be true for specific models of aircraft, however.


What's an "altitude engine"? FAR 91.205(b)(8) says that you need a manifold pressure gauge for each "altitude engine", whatever that means.

What's an "altitude" engine?

An altitude engine is a type of engine that uses a turbocharger (and less often, a supercharger) to generate sea-level pressure in its intake manifold, up to its critical altitude, at which point power will start to fall off.

Non-altitude engines will lose power as they climb because the ambient air pressure drops off at a rate of about 1" Hg per 1,000 ft of altitude (up to 10,000ft).

The altitude engine allows aircraft to fly at higher altitudes at full power, and increases the aircraft's speed and efficiency, because true airspeed increases with altitude. It also allows an aircraft under IFR to fly at higher MEAs or under VFR over higher terrain, and allows the pilot to choose a favorable altitude for weather.

The first kind of altitude engine is called a turbocharged engine, which can generate more intake manifold pressure than at sea level. For example, a Mooney Bravo M20M can generate 36" of manifold pressure, and can generate at least 30" of manifold pressure up to its critical altitude of 22,000ft.

The other kind of altitude engine is called a turbo-normalizer, which develops a maximum of sea level pressure: 30”Hg or 1 Bar. These engines will perform as well at altitude (up to their critical altitude) as they do at sea level.

A turbo-normalized engine is considered less taxing on the engine than a turbocharged engine because the former doesn't develop above sea-level pressure or high turbine-inlet temperatures. The turbo-normalized engine is also easier to fly than a turbocharged engine, since it never develops more power than would be developed at sea level.

All altitude engines also typically have a controllable-pitch prop, and develop more than 200 horsepower, so they would require both a complex aircraft endorsement and a high-performance endorsement. See 14 CFR Part 61.31.


Do older aircraft require increased general maintenance? Does the age of the aircraft increase the aircraft's maintenance expenses, fuel consumption, and electrical shorts?
If so, by how much?
And is this a reason why planes get scrapped?

The very simple answer to your first question is just yes.

As something (or someone) age, more maintenance is required. It varies from case to case and you cannot have one size fits all rule or guideline.

FAA has published a thorough guide for maintaining older airplanes (PDF). For the relation of age on maintenance costs, this article (PDF) discusses it in detail.

Getting to your last question, it also depends on an individual case. Please also note that when an airplane is grounded, it is not required to be scrapped in all cases. Sometimes museums are interested in it, or it can be installed as a roadside monument or placed in an aircraft boneyard.

Regarding when an aircraft needs to be retired or scraped, these two articles are a good read:

When to replace an aircraft
How old is too old?


What does "radar contact" mean in ATC calls? When ATC mention 'radar contact', what does that mean?

In what conditions ATC might contact you and mention 'radar contact' in the call? and what would be the appropriate responses?

The FAA's Pilot/Controller Glossary gives both US and ICAO definitions:

  RADAR CONTACT- a. Used by ATC to inform an aircraft that it is identified on the radar display and radar flight following will be
  provided until radar identification is terminated. Radar service may
  also be provided within the limits of necessity and capability. When a
  pilot is informed of “radar contact,” he/she automatically
  discontinues reporting over compulsory reporting points. [...]

  b. The term used to inform the controller that the aircraft is
  identified and approval is granted for the aircraft to enter the
  receiving controllers airspace. (See ICAO term RADAR CONTACT.)

  RADAR CONTACT [ICAO]- The situation which exists when the radar blip
  or radar position symbol of a particular aircraft is seen and
  identified on a radar display.

  RADAR CONTACT LOST- Used by ATC to inform a pilot that radar data used to determine the aircraft’s position is no longer being
  received, or is no longer reliable and radar service is no longer
  being provided. The loss may be attributed to several factors
  including the aircraft merging with weather or ground clutter, the
  aircraft operating below radar line of sight coverage, the aircraft
  entering an area of poor radar return, failure of the aircraft
  transponder, or failure of the ground radar equipment.

Note that this is not a universal term, Canada uses "radar identified" instead. Their equivalent of the P/CG is the Glossary for Pilots and Air Traffic Services Personnel:

  “Radar identified”

  An expression used by ATC to inform the pilot of an aircraft when radar identification is established.

  Fr: « Identifié radar »

And:

  “Radar contact”

  U.S.: Expression for: “Radar identified”


How are temperature differences handled in a jet engine? Given the graph in this answer, the temperature are quite high inside a jet-engine, even greater that many metal's melting point. I understand high core temperature are efficient.  A jet engine is designed to work for several hours at those high temperature.

Moreover, starting the engine (i.e. going from ambient temperature everywhere inside the engine to more than 1500°C in the combustion chamber while not significantly changing the temperature at the first low pressure compression stage) may be quite stressful for material.

My question is in two parts:

How is the high temperature handled, especially in the combustion chamber and the high pressure turbine? I read somewhere that the turbine can be cooled by air flowing inside the blades but I cannot find the reference. I also wonder where this cooling air comes from.
How is the stress on material due to temperature changes (between full thrust and shut down, and maybe when changing thrust in mid-flight) handled?

1. The combustion chamber

Temperature here is usually controlled via design of the chamber shape, in particular of the location where combustion occurs vs the airflow.

Image source

In the image above you can see that the fuel line (2) terminates in a nozzle. The nozzle will turn the fuel into tiny droplets to increase the surface area and hasten the combustion process.

Indicated with (6) are instead the main air holes: the cool air coming from the compressor (on the left side) enters the chamber with high pressure from these holes. Note that the holes are distributed in an annular pattern: high pressure air will be entering from all sides of the chamber.

The result is that the flame will not directly touch the walls of the
chamber, we then got rid of conductive heat transmission.
Convective is a non-issue: cool air keeps coming and hot air goes
into the turbine.
Radiative heat transmission might be an issue, but the new air
incoming will cool the walls via conduction.

The engine can thus run without the combustion chamber melting. Note that the use of specific materials does greatly improve the conditions, as higher temperatures can be tolerated, and thus better efficiencies achieved.

2. The turbine cooling air

The air is usually bled from a stage of the compressor, usually a high pressure stage, in a similar manner as done for the air used for the passenger compartment.

3. Thermal stresses

a. Short term

Over the duration of a flight the blades will warm up when the engine is turned on and cool down at the end of the flight. Intermediate load variation do not have a significant impact.

Even the initial warming and the final cooling do not pose a significant problem. The only effect to be taken into account is that the rotating hot blades will become slightly longer: a special material is placed on the opposing stationary side so that the gap is minimal during operation and the losses due to the air going around the blades (instead of between them) are reduced.

b. Prolonged

The most problematic aspect here is the so called "creep": the blades, after prolonged use, will tend to change shape, because they tend to be more malleable at high temperatures, exactly when they are under high tensile load. This becomes a problem over the lifetime of an engine, not during a single flight.

To mitigate this problem the blades are nowadays manufactured as single crystals: each blade is a single solid block of metal and not made up af many tiny crystals as the metal tools we use everyday. This is because under tensile load each of the crystals can slide over its neighbors (and high temperatures facilitate this) changing the shape of the blade. To visualize it, think of a rubber band that does not go back to its original form.

Having a single crystal prevents this sliding, giving more strength to the blade.


How do the vortex created by wing tips affects the lift? I’ve been reading about how improve the aircraft performance and I found that for low aspect ratios, the vortex created by the wing tips decrease the lift generated by the wings. Why is this happening?

The lift over a wing comes from an imbalance of pressure above and below the wing. The higher the pressure, the more force is imparted to the wing. Thus, by generating a higher pressure on the bottom of the wing then on the top, a net upward force is created which offsets the pull of gravity on the airplane. The greater the pressure differential, the more lift is created.

At the end of the wing, there's nothing to prevent the higher pressure underneath from causing air to "spill over" the end of the wing into the lower pressure area. Thus, the pressure at the end of the wing tends to equalize itself, with a corresponding decrease in lift.


Can a Boeing 747 or A380 climb with only one engine? Assume that a Boeing 747 or Airbus A380 takes off at max takeoff weight. Minutes after takeoff, 3 engines fail, leaving only 1 engine operating.
Can the aircraft still achieve a positive rate of climb?

I doubt that either aircraft can climb on a single engine (though momentum and airspeed could help, as @RonBeyer already commented). The closest an aircraft came to this condition was the British Airways Flight 9 when it ingested volcanic Ash.

During this incident, the aircraft flew on one engine for around 90 seconds. In this case, a report from the flying magazine says:

  On one engine they could maintain height at 13,000 ft; over the next five minutes, the next three engines relit, and they begun a climb.

This seems to indicate that the aircraft was not able to climb with one engine. However, during this incident, the aircraft was not climbing, but was descending at 1800m per minute.

Note: There seems to be some confusion about the altitude. The captain's website gives the altitude as 12,000 ft.


Why does "V1 increase with decreasing density"? The statement made in the question is mentioned in one of the renowned publications.
My contradictions which I have already ruled out as a potential solution:

As Density decreases Vmcg decreases thus V1 is not limited by Vmcg.

As Density decreases Vmbe decreases as there's insufficient cooling
of the brakes, thus V1 should ideally be limited by Vmbe.

At lower density the aircraft needs to be at a higher groundspeed to
achieve the same CAS, but the ASDA would be limiting factor as it
would require a greater stopping distance, this would again limit V1.

I guess the statement is about V1 from the balanced field length, which is where the accelerate-go distance is equal to the accelerate-stop distance:

(image source: boldmethod > Engine Failure On Takeoff: Do You Stop Or Go?)
How do these curves change when the density decreases? Lower density means less thrust, therefore accelerating will be slower and uses more distance. This affects both curves equally for the acceleration to V1, but afterwards only the accelerate-go distance:

Let us call the force accelerating the aircraft initially $ F = 2 \times T $ (thrust from two engines).
In the accelerate-go case, you would only accelerate with a force of $ F = T $ after the engine failure.
In the accelerate-stop case, you brake with a certain force $ F = F_\text{brake} $.
Now, at lower density, the thrust $ T $ will decrease, but the brake force $ F_\text{brake} $ will be unaffected.
The accelerate-go case is therefore affected during the entire time by the lower value of $ T $.
The accelerate-stop case is only affected during the accelerate part, but afterwards the brake force is still the same.

Therefore, both curves will move up in the diagram, but the accelerate-go distance will move up more. This implies that their intersection (the balanced field length) will occur further to the right, resulting in a higher value for V1.


Are all engines fan blades made out of titanium? I was wondering if all engine fan blades are really made out of titanium.

Many, but not all. Rolls-Royce has been working on carbon fibre fan blades for a long time, using the material as far back as 1968 on the RR. Conway. They planned to use carbon fibre (Hyfil) for the RB.211 too, but had to switch to titanium when they couldn't get the Hyfil blades to work reliably during bird ingestion tests.

The GE90 uses carbon fibre fan blades, with titanium leading edges.


What does "alpha floor" mean? I was talking to a pilot, and he said that the alpha floor had been activated, and I did not have time to ask him to explain this. So, can you clarify my doubt, please?

Alpha floor is a thrust function which provides TOGA thrust regardless of thrust lever position in Airbus. From Airbus Technical Digest FAST 20:

  Alpha Floor is a low speed protection (in normal law) which is purely an
  autothrust mode. When activated, it provides TOGA thrust. As the aircraft
  decelerates into the alpha protection range, the Alpha Floor is activated,
  even if the autothrust is disengaged. Activation is roughly proportional to
  the rate of deceleration.

  ... at low speeds, if a rapid avoidance manoeuvre is required to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up
  to max Alpha, engage TOGA thrust and climb away.

Note that this protection is only available under normal law and is lost under alternate law.


Which clearances are issued by ATC without the pilot requesting them? Some clearances are only provided after specifically requesting them, such as the departure, engine-start, taxi or oceanic crossing clearance. Line-up and Take-off clearances are issued once the pilot relays to be ready for it (this could be viewed as "actively requesting" the clearances.

As I have learned after this question, sometimes descent clearances are issued by ATC without the pilot asking for it. Which other clearances are issued "automatically"? Specifically, I'm asking about the approach clearance and the landing clearance.

No clearance is issued without the pilot requesting it. But a request is not just something you do on the radio. The interesting questions here is: how do pilots request something?

The most common way is through a flight plan. By filing a flight plan, the pilot details the planned flight, from departure to destination, via a predefined route, with a requested speed and level. ATC will issue a flightplan clearance prior to departure or prior to entering controlled airspace. This covers the general flight, and basically lets the flight take place.

During the flight, specific details need to be requested by the pilot - such as taxi - but only if the controller is not already aware that the flight is ready for whatever it is they are requesting. For example, during radar vectors for approach, ATC already knows that the flight is requesting approach at their destination (since the destination is noted in the flightplan), so ATC will clear the flight for approach without needing the pilot to explicitally request it. Same goes for landing clearance. The pilot does not have to explicitally say "request to land", because this is already a part of the flight plan. ATC will automatically issue a landing clearance when appropriate.

ATC can issue instructions or clearances to flights that deviate from their flightplan if needed for safety or efficiency reasons. For example, ATC may instruct a flight to change level due to traffic ahead on the same level. But the flight plan is always the basis for all clearances.


What determines CFD accuracy for aircraft design? Do CFD results depend on the knowledge of the person who is working with software or do the final results just depend on computer power and CFD software?
Does one need to know aerodynamics to do serious CFD work or is everything in the CFD software?

CFD is a prime example for "garbage in – garbage out".
There is a large range of parameters that needs to be set correctly, from fluid properties to surface roughness. If the persons feeding the software do not understand what they are doing, it is very hard for them to judge the result. Lots can go wrong and will produce colorful but wrong results.
CFD is best suited when you know what to expect and can check the correctness with real-world data. Then it is fair to assume that the results over the whole flowfield are realistic and looking into corners that cannot be observed in an experiment makes CFD supremely useful.
Why else are wind tunnels still solidly booked and running? Today, it is mostly for validating CFD results and making sure the complex software produced realistic results.
Now I have answered your question body. Since you asked something rather different in the title: That answer follows here:
Accuracy depends on:

The code: Potential code cannot model separation, Euler codes cannot model viscosity. Navier-Stokes codes can, but still have trouble with turbulent, separated flows. For more detail, make sure to read the answer of @XRF.
The mesh geometry: It is hard to get the mesh right so solutions iterate well and results are robust.
Mesh resolution: A coarse grid (2D) or mesh (3D) will create poorer results than a fine mesh. Of course, computational load goes up with mesh size.
The domain being investigated: Subsonic, fully attached and steady state flow is easier to get right than separated flow or the exact shock position in transsonic, instationary flow.