What happens to en route flights when a major earthquake etc. destroys facilities? Suppose you're flying a big plane in to an airport like Mexico City, already in final descent, when an earthquake strikes.  Large cracks appear in the paved infrastructure catching some vehicles by surprise.  The airport suspends operations.  Planes planning to take off are stuck on the tarmac for at least several hours.
Planes planning to come in from farther away are told not to, maybe from an external seismic monitoring service if communications are out.  Those already in the air but far enough away to be able to divert can land at other airports.

Yesterday, the main infrastructure was inspected and found to be OK, and the airport reopened several hours later.  But what would have happened if the earthquake had been strong enough to destroy the runway or other landing-critical infrastructure at this airport and at the nearest alternative, so that neither "rack & stack holding patterns" nor "just go to the alternate" (as in e.g. a terrorist takeover at one airport) are viable options?  Are there special rules/considerations in place to help protect safety in such a rare region-affecting event?

Continuing the distinguishing analogy from the terrorist takeover question, this situation is more like a widespread cyber-attack, solar flare, or electromagnetic pulse that knocks out ground facilities and prevents landing at multiple airports without destroying in-air flights in a way that e.g. a sudden volcanic eruption or nuclear explosion might. This question also focuses on emergencies that arise quickly and without the kind of notice that would permit a planned closure (as e.g. a hurricane would).

This is a valid and good question. The answer is all the different entities will work together to solve the issue.

It's called a system wide event, be it a natural disaster or otherwise.

  [It's an] event that affects a flight and a sufficiently wide area that all alternate routes and airfields briefed during pre-flight preparation have become unavailable. Ground facilities such as navigation beacons and air traffic services may also be affected.

A somewhat recent example is the closure of many airports in Japan after the 2011 Tōhoku earthquake and tsunami.

As you can imagine, this only affects a tiny portion of flights that are too near the center of the event. The ATC, flight-crew, and company (airline) personnel will all work together. Even a simple thing such as using the internet access aboard planes to gauge the situation on ground from the different news feeds can be used.

Best case scenario is there would be enough fuel to turn around and exit the radius/area of the event toward the most suitable location. There usually will be enough fuel, and using the performance manuals the flight crew can make their plane very efficient. Airlines balance fuel and time using what's called a Cost Index, in this case, they will push to save on fuel and forget time (flying slower but farther).

In conclusion, the affected are few, the events are far between, each flight will be very unique, and this is where human ingenuity, training, and decision making skills come in.

For the facilities part, there are a few posts on that here, for example see: Is there an ATC radio backup? Even if the radio is down, handheld radios can be used. Most of the aviation systems have many built-in redundancies.


Is there truly no way to get information about medical application status? I had an event in 2005 that has not happened in the 10 years since.  When I went for a 3rd class medical the AME I went to seemed very skittish and wanted to send my records to Oklahoma City for review.  They received them the first week in March.

As we near the two month mark I have called and asked for a status once each week and have been struck by how little information they provide.  My application, for example, has been in the "handed over to the Doctors" status for almost a month with no new notations on the file.  This means I can't tell if it is sitting in the queue waiting to be processed, or is actually being processed by a Doctor who just isn't putting any notations in.  If I'm in the queue I have no way of knowing whether there are two or two hundred apps in front of mine.  In short, calling and checking the status is a worthless exercise.

In every organization I've ever dealt with there was always some way of going up the chain and getting some kind of information.  I'm told by the call center reps that is not possible.  I wanted to verify that information with the community here, though... is this just the way things are?  Is there no way whatsoever to get some useful information?  If anyone has been down this road and has tips they would be greatly appreciated.

Dealing with the FAA medical is always a tiring process. Best places to consult would be services like AOPA medical assistance, or through Aviation Medical Advisory Service (AMAS).

There are other options probably, but I know AMAS is the go to consulting group for controllers and a lot of airline pilots. AMAS will often help guide you with knowing ahead of time what information you need and what forms to have filled out to try and make the process go quickly.


What are the differences between the different types of TCAS? In discussions about TCAS I've overheard mentioning different levels such as "TCAS 1" or "TCAS 3." What are the different types of TCAS and how are they different?

The Traffic Collision Avoidance System TCAS (also called ACAS - Automated Collision Avoidance System) is used in aircraft to alert the pilots about possible collision.

The basic system in all the levels are the same. As per the FAA what differs is the type of warning issued to the pilots. As per FAA:

  TCAS is a family of airborne devices that function independently of the ground-based air traffic control (ATC) system, and provide collision avoidance protection for a broad spectrum of aircraft types.  All TCAS
  systems provide some degree of collision threat alerting, and a traffic display.  TCAS I and II differ primarily by their alerting capability.

TCAS I issues 'traffic advisories' (TAs) to the pilot, alerting him to the impending collision.

TCAS II provides the pilots with resolution advisories(RAs) in addition to TAs. Basically the TCAS provides the pilot with the required escape manevores in th vertical direction.

TCAS III and IV were supposed to provide horizontal resolution RAs, but development was stopped due to limitations of the available TCAS antennae.

TCAS is expected to be replaced by automatic dependent surveillance – broadcast (ADS–B)


How is Spitfire landing gear powered? I have seen a WW2 movie where the pilot of the Supermarine Spitfire has difficulties to extend the landing gear as the engine is no longer running.

This would be reasonable if the gear normally needs hydraulic or electric power to be extended, otherwise probably just an attempt to add more tension to the plot. Would running out of fuel (no engine power) make the extension of the Supermarine Spitfire landing gear more difficult or different from usual?

Early models of Supermarine Spitfire (prior to the 175th production aircraft) had no hydraulic gear extension system, according to Wikipedia:

  At the same time the manual hand-pump for operating the undercarriage
  was replaced by a hydraulic system driven by a pump mounted in the
  engine bay

Source

The movie could show the normal extension of the undercarriage, using a lever, but a lost engine would not impact the operation.

After the 175th aircraft, the gear was hydraulics powered, so without engine running, the pressure could be insufficient. The design included an emergency extension using a $\small \textrm {CO}_2$ cylinder:

Source: 1940 Spitfire Manual

The system is visible on this image (black bottle with red handle "Emergency Only"):

Source

The hydraulic gear control box is at the center.


Why does the US Marine Corps fly the Harrier, a VTOL or operationally STOVL aircraft? The UK flies the Harrier because they have STOBAR carriers, but why does the United States Marine Corps fly the Harrier?

Because when selecting aircraft the requirements were for close air-support and to be able to operate from amphibious assault ships like the USS Tarawa and the USS Nassua.

These ships have very small decks that aren't geared towards launching larger aircraft like the F-18.


Is it possible to order or purchase a hardcopy of an NTSB report? I am an independent researcher working closely with several NTSB aviation accident reports and would prefer a printed hard-copy. I realize I can print out all of the relevant sections of each report but was curious as to whether hard-copies are published by NTSB?
*Edit: I am working on a museum display which would include the presentation of a hard-copy of a report. The specific packaging by NTSB would be valuable to me in this context.

I'm not sure that this is on topic, but the short answer seems to be that you need to contact the National Technical Information Service. NTSB reports used to include this text:

Accident reports, safety studies, special investigation reports,
statistical reviews, and safety recommendations may be downloaded from
the NTSB Web site, or printed copies may be purchased from:
National Technical Information Service
5285 Port Royal Road   Springfield, Virginia 22161
(703) 487-4630 (Subscription Orders)   (703) 487-4650 (Single-Copy
Orders)
(Subscription rates apply to certain recurring publications)

The current wording is:

Copies of NTSB publications may be downloaded at no cost from the
National Technical Information Service, at the National Technical
Reports Library search page, using product number PB2020-101001. For
additional assistance, contact—
National Technical Information Service
5301 Shawnee Rd.
Alexandria, VA 22312
(800) 553-6847 or (703) 605-6000

Alternatively, since NTSB reports are in the public domain (with some limited caveats) you could just get them printed at your local print store, presumably much more quickly and cheaply than by going to NTIS.


How can pilots identify, in flight, the cracked layer(inner/middle/outer) from an impaired windshield? Depending on the cracked layer, there would be some actions from the crew.
The inner layer seems to be simple to identify, but if the middle and/or outer layer is cracked, how will pilots identify the right crack's position?

It is possible to identify whether the crack affects the inner layer by sliding over it with nail or something sharp like pen. If the layer is cracked, the slight indenting in the surface can be felt.

As far as I can tell, there is no way to tell whether the outer or middle layer is affected with any useful degree of certainty. It is also not possible to ascertain how many layers are cracked. So usually the procedure is so that if the inner layer is not affected, you can continue and fix the issue at destination and if the inner layer is affected, you have to conservatively assume all layers may be affected and descend.


Does the sound barrier apply to a silent aircraft? For my summer duty, I was looking at the sound barrier effects with a glimmer of hope to understand them:

"A supersonic aircraft is one that can travel faster that the speed of sound. As the plane approaches the speed of sound, it catches up with the sound waves traveling in front of it and pushes them up against each other. The result is a barrier of squashed air in front of the aircraft." Source

Is it theoretically possible to design an aircraft totally silent or at least quiet?

In that case would the problems associated with transonic/supersonic speeds disappear, or at least are reduced?

  As the plane approaches the speed of sound, it catches up with the sound waves traveling in front of it and pushes them up against each
  other.

Or is there a silence barrier too?

The sound barrier has nothing to do with the noise the aircraft makes. It has to do with the fact that if it is going fast enough the air can't get out of the way. This creates a shock wave where the aircraft is pushing the air aside. A shock wave is where the speed of the air transitions from faster than sound to slower than sound and involves the release of considerable energy. You can control the shockwave to some extent by careful shaping of the surfaces, but a plane going fast enough can't be silent.

You can make very quiet sub-sonic aircraft, but they tend to go slowly so that the noise of the air flowing over the craft is minimized.


What is the difference between RTK GPS and GBAS? My understanding is that real-time kinematic (RTK) is also a differential GPS technology to enhance accuracy by using a ground-based station to provide the correction data to the onboard GPS receiver. Is it similar to GBAS? Then what is the difference between RTK and GBAS?

There are two primary differences between RTK GPS and GBAS, function and implementing technology.

Functionally, RTK is a differential GPS (DGPS) positioning system. It allows for very precise position measurements (~1 cm). Its primary application is for survey and related measurements.

GBAS also provides a DGPS function but it adds specialized support for aircraft approach guidance. To that end it provides via data broadcast to the using receiver reference path data for a number of runway approaches. It also provides signal integrity data (e.g. fault detection and alerts) to the user.

The difference in implementing technology is that the RTK reference station and using receiver determine position by carrier-phase tracking and the GBAS reference station and using receivers use code tracking.


What was the reason for replacing the non-smoking sign with a "no electronics" sign? When reading the following comment on question "Why do lavatories in modern planes still have ash trays?":

Andra: November 05 2014 at 16:00

  I have flown on an Embraer 195 where the no-smoking sign was replaced by a no-electronics sign

What is the reason for exchanging the non-smoking sign with a "Please Turn Off Electronic Devices" sign?
Of course Smoking is prohibited on all flights around the globe, so
the answer where the comment is, is untrue. But was this sign unnecessary
because everybody knows it's prohibited?

I did some searching on this and it's been discussed on some aviation forums (e.g. here). I couldn't find any 'official' sources from airlines or regulators but on the other hand there are a lot of comments and observations that seem to make sense.

A simple summary would be that it's a result of increased public awareness of airline smoking bans combined with a massive increase in the use of portable electronic devices (PEDs). That has made some airlines decide that it's more important to focus on passenger behaviour with PEDs rather than smoking. Wikipedia has an unsupported (citation needed) statement along those lines:

  Due to the ubiquitous prohibition of in-flight smoking and the
  increasingly widespread use of electronic devices, the illuminated
  no-smoking signs have sometimes been re-purposed to inform passengers
  to switch devices off for take-off and landing. Where this is the
  case, the no-smoking sign is permanently printed on the overhead
  panels.

Smoking on aircraft has been banned by most (all?) airlines and regulators for some time now (since 1988 in the US, for example) and the number of people travelling by air has increased significantly over the same period (e.g. UK foreign travel trend here). That means that more and more people know that smoking isn't allowed on aircraft and probably by now that fact has been 'absorbed' at a cultural level, at least in Western countries.

In the last 10+ years the numbers of PEDs in use has increased massively and they were banned too. I guess that at some point some airlines decided that the 'rising' issue of PEDs was more important than the 'declining' issue of smoking, and started adapting their signs, briefings etc. accordingly.

It's interesting that you saw the sign on an Embraer 195, which is used for regional not long-haul flights (as far as I know). Possibly the airline decided that on shorter flights there's less risk of passengers getting desperate for a smoke. But I couldn't find any information on which airlines and aircraft have these signs; that's up to the airline to decide when it orders the aircraft interior.

Of course now the US allows the use of PEDs in all flight phases and that makes it very likely that other regulators will follow. At some point the only sign needed may be the seatbelt one.


Why is the speed brake lever not a "spring" one that returns to the default position when released? I was watching this video in which are described various systems on B737 but at around 6 minutes, it talks about speed brakes and necessity for pilots to maintain their hand on the lever to avoid forget to release them.

It seems perfectly sensible, but I'm asking why this is not a simple spring lever... if is very important that pilot don't forget lever in extended position, and for this reason is necessary that he's maintaining his hand on the lever, why don't make a simple lever that returns to default position after release?

This would sound even more secure... and in both cases pilot hand would be "busy".

Indeed spring-loaded or not, the pilot will keep a hand on it. But in the latter case, they can still use that hand, and return to the lever, without abrupt changes to the plane's pitch/configuration.

An example busy scenario is a rapid decompression descent.

Source: YouTube; annotated

If you continue watching the video past step 4 (deploying the speed brakes), there are other things the captain attends to with his right hand: pointing something out, making a call to the cabin, fine tuning the mode control panel, putting on the oxygen mask, inputs to the CDU, etc.


What is the absolute ceiling of a 737-200? We're trying to find the official absolute ceiling for the 737-200 for a school project.  So far have only been able to maximum and service ceilings for this craft. Can anyone point us to the correct information for absolute ceiling?  Thanks!

Find the pertinent  performance charts for Vx (Max climb angle)and Vy (Max climb rate). You may have to extrapolate, but the altitude where Vx and Vy converge is the Absolute ceiling.


Do low-cost airlines make more profit than luxury airlines? I was watching this TV show and it said that currently low-cost airlines e.g. Ryanair/Easyjet make more cumulative profits than more expensive long haul airlines such as British Airways and Virgin airlines. This confused me because long-haul flights are more expensive and from looking at the type of aircraft British airways uses, those flights can carry more passengers than a 2 propeller Ryanair flight. So can someone please tell me how low-cost airlines are making more profit than long-haul airlines!!

It is hard to say for sure - profits will always be relative and in many cases not clear to anyone without advanced economics/accounting qualifications. However, there are a number of differences in terms of customer experience and service that can make a lot of difference to profit. Examples might be:

Crew size - it usually states that the crew on a no frills airline
are there for customer safety rather than service - on a legacy
carrier they are (technically) there for both so there are more of
them
What's included - rather than charging separately for things like
seat selection, baggage and meals, a 'frilly' (as opposed to 'no
frills) carrier will bundle them together, earning less than an
airline that charges them separately. Note - BA and a few other
airlines do charge separately for seat selection
Operating efficiency - a no frills aircraft typically spends less
time on the ground being turned around, so this expensive asset is
utilised with a higher percentage air time
Seat pitch - a smaller seat pitch means two or three additional rows
of seats in an aircraft - so greater profits per flight
Less straightforward pricing models - in many cases, a 'low cost'
airline charges more for a ticket than a standard airline, hiding
behind clever yielding of fare and additional costs added on later in
the booking process. In the travel industry I have seen time and
again people being fooled by the term 'low-cost airline'

I am sure there are many more. The operational efficiency of individual aircraft can be quickly worn away when a 'frilly' airline is required to compete with a 'no frills' on a similar route. Also, some 'flag carrying' airlines, which tend to make much greater use of joint ventures and codeshare agreements than no frills airlines, sometimes fly routes because it is a requirement of a larger deal - and if that is not very profitable (or even loss-making) it can drag down overall profit.


What does "Boeing" mean? We know about the existence of Boeing planes like the 737, 747, and other planes. But where did the name "Boeing" come from?

It's named for one of its founders William E. Boeing which is the American spelling of his father's German surname "Böing".

To answer the question directly: it does not mean anything in particular.


How much weight does a thrust reverser add? How much weight does a thrust reverser add to an engine?

Let's take the CFM Leap 1B as an example, which powers the new version of the Boeing 737. This engine can produce up to 130 kN of thrust for takeoff, and has a dry weight of 2.78 tons. I could not find how much weight is the thrust reverser system. Closest thing I found was this FlightGlobal article which at least talks about it, but doesn't give numbers.

One purpose of this question is to compare it to old-school drag chute system. I will open a separate question on that.

For the LEAP-1B example, the weight you found is a dry weight that does not include the reverse system per the type certificate. Such weights are very hard to come by, even from the respective system manufacturer. However, statistical data can help.

According to NASA, the reverse system of a GE90 class engine can weigh upwards of 30% of the nacelle weight.

  [Studies] conducted by General Electric Aircraft Engines
  indicate that the thrust reverser system accounts for more than 30 percent of
  the nacelle weight (not including engine) for an engine having a fan diameter in
  excess of 100 inches. This could be as much as 1500 lb [680 kg] for a GE 90 class
  engine.

As for the nacelle, an older NASA paper says the weight is in direct relation to its surface area constructed from simple cylinders.

  [For the nacelle, the surface] area is used to correlate weight, and, rather than introduce
  the complexity of the actual nonaxisymmetric shape of the nacelle, simple
  cylinders are assumed, using the fan diameter, the turbine diameter, and the
  lengths from the previous section.

  [And] a unit weight of 17.1 t o 19.6 kg/m2 appears reasonable.

From the same paper, here are some examples for the weight of the reverse systems of different engines:

Since the CF6 days manufacturers were able to reduce the weight of the nacelle and its components due to the "extensive use of composite materials."


Are ATC assigned altitudes temperature compensated? For the purposes of commercial air traffic operating in a terminal/approach space area that has temperature compensation applied to waypoints in the active flight plan, if ATC assigns an altitude, will the assigned altitude be a temperature compensated altitude?

If so, how does ATC know what factor to apply to the procedural altitude?

If not, then an ATC assigned altitude (non-compensated) could be interspersed with compensated altitudes and possibly cause non-monotonic descent or ascent.

Answering for the US. Controllers in the US do not assign altitudes differently based on temperature.
From the AIM:

5−1−17. Cold Temperature Operations Pilots should begin planning for
operating into airports with cold temperatures during the preflight
planning phase. Instrument approach charts will contain a snowflake
symbol and a temperature when cold temperature correction must be
applied. Pilots operating into airports requiring cold temperature
corrections should request the lowest forecast temperature at the
airport for departure and arrival times. If the temperature is
forecast to be at or below any published cold temperature restriction,
calculate an altitude correction for the appropriate segment(s) and/or
review procedures for operating automatic cold temperature
compensating systems, as applicable. The pilot is responsible to
calculate and apply the corrections to the affected segment(s) when
the actual reported temperature is at or below any published cold
temperature restriction, or pilots with automatic cold temperature
compensating systems must ensure the system is on and operating on
each designated segment. Advise ATC when intending to apply cold
temperature correction and of the amount of correction required on
initial contact (or as soon as possible) for the intermediate segment
and/or the published missed approach. This information is required for
ATC to provide aircraft appropriate vertical separation between known
traffic.

All pilots in the same general area of airspace are issued a common altimeter setting, which ensures everyone is on the same reference point of pressure as it relates to the weather, but not the temperature, specifically. ATC separates using altitude or lateral primarily and has a separation standard for this reason, so that if someone is off by a few hundred feet, it doesn't cause a collision due to being off as a result of atmospheric conditions. If nobody is correcting their altitude and basing it off ATC assignments and a common altimeter, there really shouldn't be much deviation.
Source: former ATC'er.


Is stall recovery training mandatory for Airbus pilots? The recent reports about the Air Asia Flight 8501 crash mention that the plane was stalled and that the pilots could not recover from the stall. Indonesia's National Transport Safety Committee apparently had this to say:

  Subsequent flight crew action resulted in inability to control the aircraft ... causing the aircraft to depart from the normal flight envelope and enter a prolonged stall condition that was beyond the capability of the flight crew to recover.

Online information that I could find suggests that Airbus pilots were not required to learn stall-recovery and other upsets recovery as part of training, due to the aircrafts "inability" to enter those conditions. This may have changed, at least for stall recovery, at some airlines, after the crash of Air France Flight 447.

Is stall recovery mandatory training for Airbus pilots? Was it ever not mandatory? How about for pilots of Boeing, Embraer, and other large passenger aircraft?

According to the official investigation report, both the flight crew on this flight had received stall recovery training, but not upset recovery training, on the A320. The Flight Crew Training Manual for the A320 (issued by Airbus) did not mandate upset recovery training. The operator's training manual (issued by Air Indonesia) did mandate upset recovery training, but it was not delivered by the airline for pilots of the A320 because of Airbus's statement that it was unnecessary. Only upset prevention training was given. Both the below are from the linked report:

  The  approved  Operation  Training Manual covers  the  upset  recovery
  training  in Chapter 8. The  module  consisted  of ground and simulator training. The ground training provides the flight crew with the background, definition, cause of  aircraft upset, aerodynamic and  aircraft systems in
  relation with aircraft upset. Recovery methods consider various aircraft attitude and speed including post upset conditions.

  The  upset recovery training  had  not  been  implemented on Airbus  A320
  training, since it  is  not  required  according  to  the  Flight  Crew  Training  Manual  and  has not been mandated by the DGCA.

and later

  The Upset Recovery training was included in the aircraft operators training manual. The aircraft operator advised the KNKT that the flight crew had not been trained for the upset recovery training on Airbus A320, and this referred to FCTM Operational Philosophy: “The  effectiveness of fly-by-wire  architecture,  and  the  existence  of control  laws,  eliminates  the  need  for  upset  recovery  maneuvers  to  be  trained  on
  protected Airbus”. There was no evidence of DGCA findings for this incompliance of training.

Upset aircraft is where the attitude or airspeed is greatly outside the normal flight envelope, such as might happen after a mid-air collision or loss of control, or (as in this case) prolonged wrong inputs when fly-by-wire is disengaged. A stall is not necessarily an upset aircraft condition.

The report also notes that the stall recovery training doesn't necessarily cover the developed stall, but rather recovering when the stall warning sounds before actually reaching the stall. The exercise is carried out from level flight by reducing airspeed and pitching up progressively to trigger the stall warning, and then pitching down to recover. (This is the same way stall recovery is taught to private pilots in smaller aircraft.)

Recovering from a developed stall where the aircraft's attitude is level but it is descending rapidly (stall at zero pitch) is not mandatory training (p119), and this is the situation the flight crew were unable to recover.

As there are too many crashes resulting from unrecovered stalls, the whole industry has worked together to produce better training programmes. One of the Safety Actions resulting from this crash is that Air Indonesia now gives upset recovery training to all its pilots, to comply with its own policy. In addition, the airliner manufacturers have produced a new training aid (applicable to most airplanes) for teaching upset recovery, and Airbus has already issued an Operations Training Transmission with instructions for carrying out suitable simulator training.


Why does the DC-10's horizontal stabilizer have such a pronounced dihedral? If you look at this picture, you'll see that the horizontal stabilizer has a very pronounced dihedral in comparison to the wings. Could somebody explain in a simple way why? I understand that a dihedral wing makes an aircraft more stable on the roll axis, but why on the horizontal stabilizer, which produces a downforce?

This question is not just about the DC-10, because there are also other planes that are not trijets and have this feature, I just used the DC-10 as an example

See also this schematic:

Image excerpted from original, by Julien.scavini, used under CC BY-SA 3.0

In a general sense, there are multiple reasons for the horizontal stabilizer to have dihedral/anhedral.

To clear the tail from the downwash/propwash. A good example is the Cessna 425, which was developed from the Cessna 421. The 421 had a straight horizontal stabilizer. However, with the addition of dihedral, the tail is kept out of the propwash, improving ride quality. From a report citing Cessna engineers:

  Cessna engineers say the dihedral improves Dutch roll qualities and moves a major portion of the tail out of the prop wash, thus providing a smoother ride.

The Dassault Falcon 50 had anhedral so as to clear the tail from the wing's downwash. Another good example of this is the F-4 Phantom. The prototypes had a straight tail. However, the anhedral tail was adopted so as to keep it clear of the disturbed air from the wing at high angles of attack while at the same time keeping it out of engine exhaust.

The anhedral in BAE Hawk is explained by the same reason. From The Hawk Story:

  This model gave the first indications of high speed problems due to interference between the high intake and the low wing, and the
  position of the tail.

  This resulted in the adoption of a low intake, ... .The rear fuselage was cambered downwards  to bring the tail down, and the tailplane was
  given pronounced anhedral for the same purpose.

Stability- The dihedral/anhedral in the stabilizers affects the aircraft's stability. In short, for a conventional tail, an anhedral improves the yaw stability of the aircraft while reducing the pitch stability (slightly), while a dihedral will reduce the tail's contribution to both pitch and yaw stability.

However, the use of dihedral in the horizontal tail influences static roll stability in the same way as that of dihedral in the wing. Dihedral affects the roll response in the same way as that of the wing- it causes the lift on the right and left semispans to respond differently to downwash.

A good example of this is the Vickers Viscount, which had a dihedral horizontal stabilizer, which improved its stability.

Due to this, it is better for the designer to add a dihedral to the horizontal tail rather than wing especially if problems are found in later stages of design as the impact will be lesser.

From Roskam's Aircraft Design Vol III, Chapter 5.1:

  The overall lateral stability is usually dominated by the inherent lateral stability designed into the wing. Wing sweep, dihedral angle and (wing) location on the fuselage dominate the magnitude and the sign of the stability derivative. Since the wing is designed by performance and operational considerations, the empennage is often used to fine tune the lateral stability of the aircraft. Examples include the F-4 and the AV-8B.

It should be noted that, as with any other design consideration, the dihedral of the horizontal stabilizer depends on the design choices made and perfomance characteristics required; as a result, the configuration required will change from aircraft to aircraft.

In case of DC-10, the main reason appears to be to keep the stabiliser out of the wing influence, especially when high flap settings are used. The more pronounced dihedral (compared to, say Boeing 737) appears to be used as the stabilizer is mounted lower due to the presence of engine in the tail.


If everybody jumps in an airliner, would the pilots notice? I often have this thought when stretching my legs on a long haul flight: jump up and down in the cabin and see whether the pilots complain. Alone, it is unlikely my weight would have any noticeable impact on the plane's attitude, and I'd likely annoying some flight attendant or passenger around.

But let's say I manage to get all passengers on the flight to cooperate. Everybody jumps up pretty much the same instance (within reasonable human reaction time). Will the pilots notice its effect?

30000 lbs of people leaving and landing on the floors at the same time?  They'd notice something.

Physically, the weight of the passengers on the floors would increase a bit as they pushed off and accelerated vertically.  Maybe 30000 lbs would increase to 35 or 40000 lbs for a split second, then once everybody's feet are clear, all the passengers are basically in a ballistic arc, so their weight is removed from the floors and the airplane is now 30000 lb lighter for a split second, then they would land and their vertical acceleration would apply maybe 40000 lbs to the floors for another split second, before returning to the steady state 30000 lbs.  So I would say it would feel to the flight crew like they hit a very small bump of turbulence, and there were be some kind of dull thud coming from behind from 300 feet landing at once.


What produces thrust along the line of flight in a glider? After reviewing discussions and vector diagrams of gliders in flight, the vertical lift component and the vertical drag components seem to produce a steady state, 0 acceleration balance with gravity. The glider is on a straight line flight towards the ground. Would the forward motion of the glider be accounted for by combining the horizontal lift vector and gravity? Would any other forces be involved?

V is the airspeed. U and w are the horizontal and vertical components of V. The rest of the vectors are forces; W = weight, K = aerodynamic force, L & D are the orthogonal components of K. Those are all the forces involved in a glide.


What is the effect of fuselage weight on a model glider? I've built a model glider and wanted to see the effect the weight of the fuselage had on its performance. It flew the further with more weight which seems counter intuitive. I can exert more force into the glider with more weight which results in greater airspeed, would this be the reason for the increased glide ratio? How does this compare to real life gliders? Thanks.

Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it.

The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio.

If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.


Are there examples (current or historical) of commercial aircraft seating arrangments beyond the standard today? Excluding various first-class seating arrangements, are there any examples in history or today, of an airline that has implemented a non-standard seating arrangement?

For instance, most military transport aircraft contain just rows with passengers' shoulders inline with the bow and aft. Are there any examples of where a commercial airline has tried this arrangement -or any other type?

An extreme example would be an airline like a subway car (everyone standing up, just hanging on to something).

Some of the old flying boats had a seemingly haphazard arrangement

Source

Source

Source

But maybe all flying boat passengers were effectively first class?


Which is the technically correct term: Nose Gear or Nose Landing Gear? Talking about landing gear system, came across my mind the correct and technical name. Is it NOSE GEAR or NOSE LANDING GEAR?

Based on the ATA iSpec 2200, ATA Standard Numbering System, which provides a standard for structuring maintenance data by the manufacturers. The structure helps the airlines by providing a common structure regardless of manufacturer.  While voluntary, most aircraft manufacturers use it.

The spec is organized into chapters by aircraft system. ATA Chapter 32 is LANDING GEAR. Chapter subsections cover subsystems.  Subsection -10 is Main Gear and Doors and subsection -20 is Nose Gear and Doors.  So that's the 'common' terminology in the aircraft manufacturing and maintenance world.


What is the weight of the bulletproof cockpit doors of a Boeing 747? What is the weight of the bulletproof cockpit doors of a Boeing 747 built prior to 9/11? Do the doors weigh the same for later models, like the Boeing 747-8 specifically? How many fewer passengers do they have to carry in light of the added weight?

From federalregister.gov:

  The FAA also assumed that the average safety door system adds 100 pounds to a large airplane. This additional weight would have minimal impacts on weight and distance limitations.

For a large airplane, like say the 747, it seems the weight addition is around 100 lb (including the other safety features such as the surveillance). As for the exact weight before/after, this information is not readily available as the doors are not sold to the public. But feel free to get in touch with Boeing if it's for research/educational purposes and such.

If the information provided by the seller on this eBay item is correct, then the weight of a 747-400 door is 59 lb. And also from eBay, is a bullet proof 737 door shown below.


Can you identify the plane with number NC 14959 photographed in Boston in 1937? I have a photo of a plane with the number NC 14959 taken in Boston on December 31, 1937. The hanger behind it says "Occidental Airlines".

I am trying to find out what kind of plane this is?

I am sure of four things:

the date,
the NC registration number,
that it was taken at Boston, MA
the photographer's name is Carleton Parker, a prominent Boston area train photographer who photographed extensively from the late 1920's to the early 1950's, primarily trains but an occassional aircraft.

It is a Lockheed Electra 10A, and, by good fortune, quite a famous one now admired by hundreds of people every day. Details on the Electra Survivors Project. It was delivered to Eastern Air Lines on 24 September 1935.

It was later re-registered NC5171N and can be seen today in the Science Museum in London, displayed in the 'Making the Modern World' exhibition (as opposed to the aviation section). Its prominence there derives in part as being the same type as Amelia Earhart's aircraft on her final expedition, as well as the same type that British Prime Minister Neville Chamberlain used on his return to England after the ill-fated Munich Conference. (Note, this was not the actual aircraft in either of those events - but it is the one in your photo.)

 source


What does an Air Battle Manager do? I heard someone talking about a position in the U.S. Air Force called an Air Battle Manager or ABM. What does a person in this position do?

Air battle manager has the following duties:

Plan and execute theater air operations
Analyze national defense strategy
Develop and conduct flying and simulation exercises
Conduct, manage, evaluate and improve unit training and readiness
Create operational policies

Further information can be found at AirForce's website.

In USAirForce, there are seven categories of aeronautical ratings. They are:

Pilot
Navigator
Combat Systems Officer (CSO)
Air Battle Manager (ABM)
RPA Pilot
Observer
Flight Surgeon

For further reading about these ratings, read here.


How much time do pilots have to execute a TCAS Resolution Advisory? The Wikipedia entry for TCAS (Traffic collision avoidance system) states for resolution advisories (RA):

"When an RA [Resolution Advisory] is issued, pilots are expected to respond immediately to the RA unless doing so would jeopardize the safe operation of the flight" (Source).

Is there a general rule of thumb as to what "immediately" translates? How many seconds (or distance?) remain from time of resolution advisory to closest point of approach? Although RAs are usually accompanied with traffic advisories, I assume there is still some response time already calculated in...

It depends a bit on speed but your linked article has the answer

Traffic advisories are provided 35-48 seconds out and a Resolution will be provided 20-30 seconds out. Since the resolution is coordinated you only need enough time to clear the other aircraft however the greater separation the safer it will be so its advised to respond immediately upon a resolution being provided. Immediately means as fast as possible.

The FAA's TCAS II outline document states that modeling was performed assuming the pilots responded within 2.5~5 seconds

  In modeling aircraft response to RAs, the expectation is the pilot
  will begin the initial 0.25 g acceleration maneuver within five
  seconds to an achieved rate of 1500 fpm. Pilot response with 0.35 g
  acceleration to an achieved rate of 2500 fpm is expected within 2.5
  seconds for subsequent RAs.


Is the pilot required to keep visual lookout during IFR operations in VMC? During VFR operations, the pilot must always self-assure separation from other aircraft, and thus must maintain a visual lookout all the time.

For IFR operations but in VMC, are there any rules as to pilot visual lookout? Is the pilot still required to maintain visual lookout (might be difficult in clouds, though...). Even if ATC assures separation, some objects such as drones may not be known to ATC, so ATC cannot provide separation...

You didn't mention which country or jurisdiction you're asking about, but in the US 14 CFR 91.113(b) says:

  When weather conditions permit, regardless of whether an operation is
  conducted under instrument flight rules or visual flight rules,
  vigilance shall be maintained by each person operating an aircraft so
  as to see and avoid other aircraft

I assume other countries have similar regulations but even if they don't, it's still common sense and good piloting to keep a visual lookout whenever possible.


Could a Dyson fan displace more air without increasing the fan size? So the Dyson Fan is cool. But it has a thrust to weight ration of 0.6. Not very efficient for flight. Its great at sucking large amounts of air but not at a very high velocity. Thrust is small amounts of air at high speed correct me if I'm wrong. Is there any way to get the Dyson Fan to blow air at a faster speed without increasing the size of the fan.

Thrust is small amounts of air at high speed correct me if I'm wrong.

No, more air at lower speed is more efficient. Thrust is derivative of momentum added to the air, while power is derivative of kinetic energy added to it. Since momentum grows with velocity, but energy with square of velocity, accelerating more air by less creates the same thrust with less power.

Dyson fan works by blowing a thin stream of high velocity air along the ring, which then pulls in more air to create a very smooth air flow through the ring. To create this high velocity flow, you have to provide a lot of energy, which will then dissipate as heat when the streams mix, but you have no way to recover any of it as it happens already behind the fan. So it is very inefficient and while adding power will make it generate more thrust, it will only further reduce the efficiency as the flow speed increases.


Why do some fighters have two engines and other have only one but still give the same performance? The F-16, F-35, and Gripen have single engines.

On the other hand, the Ching-kuo (Taiwan) and F/A-18 have double engines, but the combined thrust of two engines is almost equal to the single engine thrust.

Why do some aircraft use double engines?

Some countries don't get the most powerful engines and have to use what is available in the market. To avoid upsetting China too much, the US would not sell the F404 or the F100 to Taiwan, and a derivative of a civilian engine had to be developed.

A second reason is redundancy: When the US Navy had to decide between the F-16 and what was to become the F-18, they preferred the design with two engines, because losing your only engine over water is a much more life-threatening experience than losing it over land. Similarly, the Eurofighter was designed more for peace than for war - having two engines will reduce training losses.

Generally, a single, bigger engine will be more powerful and more efficient per unit of mass, because manufacturing tolerances will be relatively smaller in the bigger engine, if the same technology is used for both. Single engines need to have more system redundancy than one of a pair of engines, so some of that advantage cannot be transferred into the finished design. Nevertheless, from a performance standpoint the single engine fighter will look better.


What is this abandoned aircraft in Thailand? A friend found an old photo of himself and an abandoned aircraft in Thailand. Can you identify the aircraft?

Curtiss C-46 Commando
First flight was 26 March 1940. 3,181 were built. The one you have was stored at Don Muang, and ended up in Chonburi, Thailand.
Identifying features are the nose light, the now faded blue/gray cheatline that turns upward behind the cockpit windows, the black antenna under the cockpit, and the white paint under the nose that is gone from the belly:
(airliners.net)
Matching engine:
(wikimedia.org)
Matching front view:
(wikimedia.org)


Can astronavigation be used during flight? In an aircraft, can you look at the sky to get your bearing and position like sailors do?

You sure can! It's not really done all too much any more in the days of GPS but it was done quite a bit in the early days of aviation. Historically, this was done by the "flight navigator" a position that no longer really exists. Some aircraft even had mounted sextants.

(source)

Here is a video of it being done in practice (fanta optional)

This was even automated on the SR-71's Inertial Astro-Navigation system which used the stars to align itself.


B767 antenna identification Can anyone identify which comms/nav system the circled antenna belongs to?

Thanks.

That is one of two heated drain masts for the lavatory sinks.

How does the drainage of the gray water work?


In an emergency, how much runway does an A320 need to stop safely? From time to time I land at Wellington Airport in an A-320.

The runway has sea at either end and is 2,081m (6,827ft) long.

There's never been a problem but I find landing there a bit nerve wracking (the frequent high winds in the area means just getting the plane over the fence is sometimes a little more exciting that most would prefer).

I imagine that getting the wheels onto the runway as soon as possible is a priority so you can start braking but I wonder in a flat out emergency how much runway does an A-320 need (full of passengers/cargo but not much fuel) to stop if nothing else matters but stopping ?

EDIT: I meant to say I do appreciate that wet/dry runway and head wind strength would influence this quite strongly but I'm just looking a ballpark figure.

Airbus's charts for the A320 suggest that it requires approximately 4,500ft for a sea-level landing at typical landing weight. I've extracted this from this Airbus document (the A320 landing chart is on page 49), which is admittedly more of a promotional brochure than an operating manual.

Wellington's runway is 5,955 ft between thresholds, and 6,827ft overall, so there is some room for error. However, depending on the nature of the 'flat-out emergency' a pilot may approach faster implying a longer landing roll, or choose a firm landing and significant abuse of the brakes to shorten it.

Christchurch and Auckland airports both offer much longer runways at 10,787ft and 11,925ft respectively, so I'd speculate that a pilot faced with a known problem with brakes or thrust reversers might elect to divert there if range permitted.


How can an all flying rudder be almost 100% more efficient than a vertical stabilator and rudder? How can an all flying rudder be almost 100% more efficient than a vertical tail and rudder?

According to the Zenith school of aircraft design, a rule of thumb for the size of an all flying rudder is 7% of wing area. For a classic vertical tail and rudder, the rule is 12%.  That's an increase in area of almost 100%!!

A classic rudder foil, something like a NACA0012, has a max cL of roughly 1.5.

A vertical stabilator and rudder is like a wing and an unslotted flap, which adds cL of about 0.7. That's a total cL of about 2.2, which works out to an increase in efficiency of 50%.

So... why is an all flying rudder 100% more efficient, if a vertical stab and rudder produce 50% more lift?

Because on the Zenith the massively slab sided fuselage is the vertical stabilizer, and the rudder is still the rudder.  The surface area of the all flying rudder is about the same as the rudder on a normal plane, and it depends on the fuselage for weathervaning tendency.   There's no free lunch.  If you build a Zenith with a skinny boom tail and the all flying rudder, it will be quite marginally stable in yaw, like a Fokker DR-1.


Is an RT license required for EASA PPL holders? Is an RT (radiotelephony) license required in the EU for an EASA PPL holder acting as PIC in an EU-based aircraft?

EASA itself does not require a RT license (RT is usually legislated on national level), however, it gives the state issuing your license the possibility to include RT privileges as part of your license (item XIII in the license).

This is also the most common case, ie, your PPL will most likely include RT privileges, so you will not need an RT license in addition to this.

What EASA state are you planning to have your PPL issued in?


How does a jet's throttle actually work? How does a jet's throttle actually work? What does it do to the pressure chamber and the fire output controller?

The throttle (or thrust lever) in a gas turbine engine adjusts the thrust produced by controlling the fuel flow to the combustion chamber.

Most of the modern engines are actually controlled by the Full Authority Digital Engine (or electronics) Control, FADEC (or Engine Control unit)- so the signals from the throttle are sent to the computer, which regulates the fuel flow based on various parameters (including engine safety) thereby adjusting thrust. The computer handles the additional components of the engines like the thrust reversers and afterburners.

The image below shows the throttle controlling the fuel system through the engine control in the Pratt & Whitney F100 engine that powers F-15 and F-16.

Image from kaskus.co.id


Why is dual crossfeeding prohibited on DA42? Diamond 42 AFM says:

  The fuel crossfeed function can be tested simultaneously with both
  engines. Proper function can be tested by running the engines for
  approx. 30 seconds with CROSSFEED selected. The operation of both
  engines with both FUEL SELECTORS in CROSSFEED position, other than for
  this test, is prohibited.

I looked at the schematics and both fuels crossfeed pipes are completely independent and separate, including the fuel return lines and heat exchangers. Fuel flow looks almost identical (other than longer pipes) when both selectors are in CROSSFEED. So why is the manual prohibiting dual CROSSFEED ?

Maybe it's not clear from the resolution, but in the original you can see on the left image, that the cross fuel flow passes through all the filters, while the cross return line passes through the heat exchanger (the blue box on the lower right side)

As I also could find no satisfactory explanation within the diagrams, AFM, POH or Maintenance Manual, I sent an email to the technicians at Diamond Aircraft.

This was their response, so far:

  Hi Chris, I can only assume that if this procedure became too routine, eventually a pilot would select the second electric boost pump on with both systems in cross feed, in flight.  The high pressure created would damage fuel lines and possibly damage the HPPs for both engines.  That would not be a good day.  You can see the step prior to going to cross feed, Fuel Pumps check Off.  I have asked Austria for an explanation but I think I am close to the mark.

  Customer Support,
  Technical Services Specialist

Please note, that the customer service technician actually did include his contact information, but that I have removed it for privacy.


How can a hang glider take off from flat ground? There are a number of different ways of taking off with a powerless hang glider, the most commonly used being either running down a hill or jumping off a cliff/platform. This is how I learned to hang glide and is the standard way of getting airborne for most hang gliders.

However, I recently moved to Houston, Texas which is extremely flat. As far as I can tell, there isn't a single hill tall enough to take off from within a 100 mile radius of where I live.

How can I safely get airborne when I am on flat ground?

If you were an avid hang glider pilot at the time you posted your question, you no doubt have long since connected into the local hang gliding community and found the answers to your questions, and have spent many joyful hours soaring under the beautiful cumulus clouds over the Texas flatlands.

Aerotow (towing up behind a slow-flying ultralight or lightsport aircraft) is one way to launch hang gliders from flat ground.  A typical tow will be to around 2500' or 3000' up.  You'll start with the glider on a three-wheeled dolly so your own legs don't have to be involved.  If you've never done it before you'll get training in a "tandem" hang glider, flying with an instructor.  There is an active hang gliding business very near Houston that offers aerotow launches, and I hope you have made contact with them since you posted your question.  (Google "Cowboy Up Hang Gliding".)

Another method is called "platform launch".  The glider is placed on a platform on the bed of a pick-up truck.  The truck has a reel of line that pays out under constant pressure, regulated by a disk brake.  The truck starts driving down a long little-used road or on a dry lakebed or an airport runway.  When sufficient airspeed is reached to ensure a strong positive launch with a strong vertical climb rate, the  pilot pulls a trigger to release the glider from the truck and the line starts spooling out.  I've gained as much as 4000' of altitude during this type of launch, using a reel of lightweight spectra line, and a 3-mile-long road.

This type of launch is a very technical procedure and unless you are a willing to take high risks as a test pilot, you must use the exact equipment that other experienced flyers have carefully developed over the years.  You must get training in the techniques, use a  detailed checklist, etc.  I'm not sure if this is currently being done anywhere near you.  Aerotow would be the easier route in most cases.

Yet another method uses a stationary winch that reels the line in.  Often such a winch is made from a motor scooter, mounted on a trailer, with the back wheel replaced by a drum (reel).  This method can be good for beginner training via ground-skimming flights, but also can be used to launch to several thousand feet of altitude.

It is also possible to tow a hang glider from a car or truck using a fixed length of rope.  This is not as simple as it sounds-- for example a line tension gauge is essential.  Read on for more safety tips.

To safely tow a hang glider, you must avail yourself of community knowledge and training.  With many methods, it is vital to have some way to actively control the towline tension, and with all methods, a weak link of appropriate strength is essential.  Reliable release mechanisms that route the towline's pull to the appropriate points on the pilot's harness (and sometimes to a point on the glider as well) are also essential.  Simply tying one end of a strong rope to the glider and the other end to a fixed point on the ground and trying to launch yourself like a kite on a windy day would be extremely dangerous.  Several fatalities occurred this way back in the early days of hang gliding, before the community gained expertise in safer methods, and also developed a strong culture of safety-consciousness.  A phenomenon called a "lockout" is waiting to bite you if things go wrong.  If you've ever seen a tethered kite dive into the ground at high speed with the string under high tension, you'll have some idea of what this might be.

Generally speaking, hang gliders have less control authority than sailplanes, especially in the roll axis, so towing hang gliders is more complicated than towing sailplanes, and more care is needed to maximize the odds of a safe outcome.  However, if due caution is taken, towing can be a very effective way to get a hang glider up into the sky.

There are also motor units available for hang gliders.  The modern method is to incorporate the motor into the pilot's harness, not to attach it to the glider itself.  Using your own feet as landing gear while launching with a motor can be somewhat challenging and also dilutes the joy of pure soaring flight; you'll probably have a more enjoyable experience if you go the aerotow route.  But google "mosquito harness" or "hang glider powered harness" for more on the motor option.

A related concept is the "light trike", where there are wheels and a seat as well as a motor, all connected to a hang glider wing, but everything is kept light enough in relation to the wing size to allow a low sink rate in unpowered flight.  The motor is used to get some altitude, and then you shut it off and have a nice soaring flight.  The control "feel" may be a little awkward compared to a pure hang glider flown from a prone position-- most hang glider pilots would prefer to let the motor fly away with the towplane at the end of the tow, as long as the soaring conditions are good and a towplane is available.

I can't answer a question about hang gliding in Texas without referencing the famous flights originating from Zapata TX on July 8 2012-- one pilot flew 764 km (475 miles), and a second pilot flew just a few miles short of that distance.  Read more about it -- and see video of their aerotow launches-- here: https://xcmag.com/news/dustin-martin-and-jonny-durand-break-hang-gliding-world-record-in-zapata-4/

And yes of course it is also possible to run into the wind as fast as you can and then increase the angle of attack of the wing to do a little hop.  In terms of peak altitude gain, this is particularly effective on a beach, where the wind gradient tends to be especially pronounced-- the wind gradient will help you out as you start to climb-- but then will work against you as you come back down.  You'll never get more than a few seconds of flight time this way, but it can be a fun thing to  do a few times after you touch down on the beach after a nice long ridge-soaring or dune-soaring flight on a windy day.


Is a pilot's instinctive reaction to a stall counter-intutive? Is the correct stall recovery action somehow counter-intuitive? I've read about crashes like Colgan Air 3407 where pilots experiencing a stall have done the exact wrong thing and pitching the nose up. It makes me wonder if pitching down does not feel right to a person.

Not being a pilot I can only relate it to driving a car. You're taught that to recover from a skid you should steer into it. An inexperienced driver will be in a turn, feel a skid and turn the wheel further into the turn resulting in loss of control. You have to teach yourself to overcome that instinctive reaction and remember what you were trained to do.

Is the reaction to a stall similar? When you feel the a/c lose lift and start dropping is it an instinctive reaction to try to climb? You're going down and you want to go up so you pull back. Do you have to teach yourself to overcome a reflex to use proper recovery technique?

If so do airline pilots practice stall recovery in a simulator? I know it took me a bit of practice in an icy parking lot to make skid recovery come naturally.

The correct way to recover from a stall is counter intuitive, not because pilots are trying to climb, but because the nose of the airplane drops due to the loss of lift and aircraft design.  99.999% of the time (when the aircraft is not in a stall), if the nose drops, you simply pull back in order to raise it and this muscle memory is built over time.

Unfortunately, if the aircraft has stalled, pulling back will only worsen the situation and you need to push forward in order to gain sufficient airspeed for a recovery.

Yes, stall recovery is taught in both primary training, and during simulator training for pilots, although not typically to a full stall in the simulator.  Instead, the aircraft is slowed to the first indication of a stall which may be a stall warning system instead.  At this point they are expected to apply the correct stall recovery techniques recommended by the manufacturer / their SOP's.

There is a fairly large difference between recovering from a stall in a training environment because you know that it is coming and already know what you are going to do when it happens.  A large part of stall training is for stall recognition so that hopefully if it were to happen outside the training environment, the pilot will recognize the warning signs before it happens and can apply the proper stall recover technique before the aircraft enters a full stall.

Unfortunately, out "in the real world" most unplanned stalls that occur happen because the pilot is distracted by something else and they aren't expecting it. That lack of anticipation sometimes causes them to fall back on muscle memory and pull up just because the nose fell without realizing what happened.  This can be more likely when the stall doesn't happen quite like it does in training.  In some situations there may be fewer warning signs, or a component of the stall warning system isn't working as expected and can delay the realization that they are in a stall.

For more information on what a stall is and why pilots practice them, see another question that was asked before here: What happens when an airplane stalls and why do pilots practice it?


Below what altitudes do shock waves form? For a shock wave to form, the air must have a certain density. Surely in Thermosphere or even upper parts of Mesospheres the air is very thin. I was wondering if there is a formula related to the Mach Number and the probability of shock wave.

Shock waves form any time the speed of the airflow over or around a vehicle exceeds the speed of sound.
The local speed of sound varies with ambient temperature in a well-defined way; in general it goes down slightly as you climb higher.


How does a pilot select the correct ILS when the airport has parallel runways? How do you not mess up ILS on parallel runways? There are many airports with ILS and parallel runways, how do you select which ILS for the runway you want?

For example there are RWY 22L AND 22R and both of them have ILS enabled. How does the pilot know on which ILS he is flying?

The two ILS approaches on parallel runways will have a different ILS Localizer frequency. The pilots will select the correct approach from the charts and then either manually tune in the correct frequency or (for more modern aircraft) select the approach in the flight management computer, which will then automatically tune the correct frequencies. Before actually flying the approach, the pilots will cross-check the morse code identifier either by listening to it or (for more modern aircraft) by reading it off the displays.

Below you can see the ILS charts for the Runway 27L and 27R approaches at London Heathrow (source). I marked the frequencies and the identifiers (as they will be displayed on the screen, the morse code is depicted below).


To determine wing loading, do you need to subtract the weight of the wing from the gross weight? Is it true that I must subtract the weight of my ultralight wing from the gross weight of my airplane inorder to determine the wing loading of my ultralight airplane? If yes,why is this so?
I read a pdf sometime ago stating that the weight of the wing is negligible when determining the wing loading.

No, you don't subtract any weight from gross when determining wing loading.

In airliners wings can be 40% of empty weight which is often about half of gross, so it would make a 20% difference in the loading calculation.


Would a vertical stabilizer that extends both above and below the centerline be benefit or hindrance? Ignoring for a moment potential schematic difficulties in creating an aircraft that can land and take-off without threatening a downward vertical stabilizer, would a vertical stabilizer that extends both above and below the aircraft's centerline improve or hinder in-flight stability?

As long as the vertical tail surface sits above the centerline (more precisely the longitudinal axis of inertia - for now we can assume both fall together), any side force will also create a rolling moment.  This is undesirable because now a yaw command will not only create the intended yawing moment, but also a rolling moment.

If you want to yaw in order to turn, the vertical tail above the centerline will even roll the aircraft in the wrong direction, so you need more coordinated aileron command than with a symmetrical vertical tail.

So the short answer to your question is: Terry is right: Extending the vertical tail below is clearly a benefit.

This can be seen by the widespread use of ventral fins in combat aircraft. Adding area below the centerline helps to reduce the roll contribution of the vertical tail in a sideslip, which improves handling characteristics (more directional stability AND lower yaw-induced roll). See the picture below for an example where the effects of additional vertical tail area are compared to the effects of a ventral fin of the same area.

At supersonic speeds, the low aspect ratio ventral fin has a nearly constant contribution to directional stability, whereas the wing-like vertical tail loses effectivity in proportion to the Prandtl-Glauert factor $\frac{1}{\sqrt{Ma^2-1}}$. The small ventral fin of the F-104 raised directional stability by 30% at Mach 2.

At high angles of attack, the vertical tail is in the wake of the fuselage which reduces local dynamic pressure and, consequently, effectivity. The ventral fin then is in ideal flow conditions, so it can help to stabilize the aircraft at high angle of attack, just when the fuselage contribution to instability is largest.


When did the military/Air Force start measuring fuel in pounds instead of gallons? Watching a training film for the B-47 from 1951, they mention (around 12:30) that the fuel is now measured in pounds instead of gallons on the instruments.   Was that a new concept with jet aircraft?  Did previous propeller aircraft instruments always measure gallons instead?

Good question! It makes sense for the conversion to have been done in 1951, as the film you saw states, because that is the year when JP-4 fuel was introduced. JP-4 has a lower freezing point (-60C) than earlier JP-x fuels, which were all derivatives of JP-1, first introduced in 1944. Lower freezing point means the aircraft can fly higher, and the changes in volume would have led to the change in measurement basis.

The reason is that the weight of fuel doesn't change with temperature or pressure, but the volume does. Rule of thumb is about one percent change in volume for every ten degrees centigrade up or down.

A little experiment to prove this would be to weigh a glass of water, cool it in the freezer for a few hours, and weigh it again.

If fuel quantity indicators on jet aircraft were based on volume they would indicate different levels depending on the aircraft's altitude and outside ambient temperature. That would not be very useful.

It does also make weight and balance calculations easier, otherwise the loadmaster would have to compensate for temperature when calculating center-of-gravity.


Do US state governors have jurisdiction over their airspace? Do local governments have any control over their local airspace in the US.  Could a governor singlehandedly close or restrict the airspace above his state?  I know the FAA controls national airspace but could a local government enact local measures?  And to what extent could they?

State governors do not have any control over their airspace but may have control over landing sites (read airports). From the FAA's brief on drone regulations:

  Congress has provided the FAA with exclusive authority to regulate
  aviation safety, the efficiency of the navigable airspace, and air
  traffic control, among other things. State and local governments are
  not permitted to regulate any type of aircraft operations, such as
  flight paths or altitudes, or the navigable airspace.

  However, these powers are not the same as regulation of aircraft
  landing sites, which involves local control of land and zoning. Laws
  traditionally related to state and local police power – including land
  use, zoning, privacy, and law enforcement operations – generally are
  not subject to federal regulation.


What is the Eights on Pylons maneuver intended to demonstrate about a pilot's skillset? This maneuver has the common requirements such as clearing the area, and it has the unique requirement among maneuvers to determine the pivotal altitude and select appropriately spaced pylons as references for the maneuver.  However, unlike others, this maneuver has no altitude, heading, bank (other than not to exceed 40 degrees), or airspeed tolerances to determine satisfactory performance standards.

Without these tolerances, what is actually intended to be proven of the pilot's ability upon successful performance of this maneuver?

This is the ACS page for commercial airplane eights on pylons:

The maneuver tests the ability of the pilot to hold precise altitude and airspeed. You need to make constant small corrections when flying into and away from the wind. In my opinion, it is the most difficult of the Commercial maneuvers.
Garry Wing has a good explanation of the technique.
As Garry Wing explained in the video. If you don’t hold precise altitude and groundspeed the point moves. So as the wind increases upwind it lowers your groundspeed requiring a lower pivotal altitude. Downwind you will increase your groundspeed requiring a higher pivotal altitude. So you need to remain coordinated, constantly vary the altitude, scan for traffic, and maintain situational awareness of your location around the pylons.
Your airspeed will vary a bit as you climb and descend and head into and away from the wind, but if you can’t maintain a consistent groundspeed your pivotal altitude will change and that will throw off the whole maneuver. As you can see in the chart below, if your groundspeed varies by 4 kts from 87 to 92 kts the pivotal altitude changes by 65'. This is the part of the maneuver that requires precise control of airspeed and altitude with coordinated use of the rudder and ailerons.

The FAA explains it well in Airplane Flying Handbook:

Eights-on-Pylons
The eights-on-pylons is the most advanced and
difficult of the ground reference maneuvers. Because of the techniques
involved, the eights-on-pylons are unmatched for developing intuitive
control of the airplane. Similar to eights around pylons except
altitude is varied to maintain a specific visual reference to the
pivot points.
Chapter Summary
At the completion of ground reference maneuvers, the
pilot should not only be able to command the airplane to specific
pitch, roll, and yaw attitudes but, while correcting for the effects
of wind drift, also control the airplane’s orientation in relation to
ground-based references. It should be reinforced that safety is
paramount in all aspects of flying. Ground reference maneuvers require
planning and high levels of vigilance to ensure that the practice and
performance of these maneuvers are executed where the safety to groups
of people, livestock, communities, and the pilot is not compromised.
To master ground reference maneuvers, a pilot must develop
coordination, timing, and division of attention to accurately maneuver
the airplane in reference to flight attitudes and specific ground
references. With these enhanced skills, the pilot significantly
strengthens their competency in everyday flight maneuvers, such as
straight-and-level, turns, climbs, and descents.


Are most/all flights affected by a noticeable amount of turbulence? How common is turbulence that is severe enough to feel uncomfortable? I don't mean a little shaking here and there, but a drop that lasts a second or two.

Last 2 flights I was on (Paris to Singapore, and Singapore to Sydney) had more than one big drop of turbulence, to the point where passengers screamed.

Is this common? I know it is relatively safe, but feel uneasy about flying again as it was uncomfortable and scary!

It's very difficult to answer this question as it depends upon so many different things:
 - Location
 - Time of day
 - Season
 - Flight level
 - Terrain below
 - Current weather (of course)
 - Type of aircraft
 - The crew and their knowledge / weather radar use.

What also makes it hard is that what 'feels uncomfortable' for one person might not for another.

In the tropics it's quite common for flights to be affected by turbulence. Most will see at least some light chop. In my experience of a few hundred long-haul flights through the tropics I've only had a couple with no moderate turbulence (what you might call uncomfortable). In a similar number of short-haul flights in Europe I'd say that around 50% had no significant turbulence at all and only around 5% had moderate turbulence.

You're more likely to experience turbulence in the tropics as there's a lot more convective activity there (air rising and falling). Away from the tropics the USA tends to generate turbulence whilst Europe does not (relatively speaking). SE Asia is particularly well known for turbulence encounters.


Why don't we have allocated cabin baggage space just like seats? There is always a hassle of finding overhead cabin baggage space. Why don't airlines, divide the space and allocate it for each seat, just like they have allocated space for cabin crew and demo stuff.

This way, each seat has their allocated cabin baggage space, so a passenger can board in peace knowing their cabin baggage space is reserved.

Passengers do usually have some allocated space - under the seat in front of them. No-one else is going to claim that...

For the overhead lockers, a quick calculation (as @mins suggests) - seat pitch averages about 30", so there is going to be approximately a 30" slot above each row for "its" locker. On a 3-4-3 widebody, this would be ten seats sharing 4x30" of locker space, equivalent to a 12" slot each. On a 3-3 narrowbody, the six seats would share 2x30" of locker space, or 10" each. Not all seats have an accessible locker above them - eg emergency equipment storage - and there is some space taken up by the opening/closing mechanisms, so in practice it would be an inch or two less than this overall.

Meanwhile, almost all airlines give a maximum smallest dimension of their cabin bags - ie narrowest width they can fit into - of 20-25cm, or 8-10". So one standard-sized bag per person approximately fills the space given, assuming they can all stack neatly on their sides.

So, in theory, yes, you could try allocating space, one ~8-10" bag slot per person, and the overall space is available. But this would almost certainly mean putting in dividers to avoid disputes (you can imagine the disputes, right? "That man's bag is taking up some of my space!").

The dividers would mean that you couldn't put anything weirdly shaped in there - say, a walking stick - and would mean there would be problems with bags that don't quite fit the limits. At the moment, they can bulge a bit and be squashed in with everything else, so the passenger in seat 11A with a slightly outsize or awkwardly shaped bag can benefit from the fact that 12C's is quite slim, and everyone is happy, but once you have rigid dividers that becomes impossible. 12C's would rattle around, while 11A's doesn't go in, and overall the space is used less efficiently.


Is there any way to estimate the weight difference between a trimmable horizontal stabilizer and a fixed tailplane? I am trying to estimate the benefits of using a trimmable horizontal stabilizer (THS) instead of a fixed tailplane. In my model I use a T-tail (Sh~30 m), and I can trim the aicraft in every condition so the main reason to go for a THS would be aerodynamic efficiency.

I've estimated the drag reduction but I'm having huge problems estimating the weight increment of the system.

Please, could someone shed some light on this? I've checked a lot of references but still couldn't find anything clear.

Hard to find a general reference for this one. You're adding the weight of the hinge, the actuator, and the supporting structure now connecting only to the hinge axis.

Torenbeek section 8.4.1 gives a weight estimation for preliminary design purposes for the tail group of high subsonic aircraft: it mentions a weight increase of 10 % over the weight of a fixed horizontal tailplane.


How do head- and tailwinds affect airspeed? I'm sure this was asked somewhere, I just can't find it. So if it's a dupe, let me know.

Anyway, I'm one of those guys that actually looks at the flight information when I'm flying. I fly transatlantic 3-4 times a year and they show it on the personal screens.

They display the groundspeed, headwind/tailwind, temperature etc. I'm wondering how to calculate the actual speed based on that. Is it the groundspeed plus/minus the headwind/tailwind? Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph? Meaning it will take us 10 hours to travel 3000 miles? Or are there more factors involved and more math calculations?

I'm wondering how to calculate the actual speed based on that.

Which actual speed? All of the speeds you mentioned are actual speeds. None of them is in any way artificial, virtual, or otherwise invented.

  Is it the groundspeed plus/minus the headwind/tailwind?

That would be airspeed, i.e. the speed at which the plane moves relative to the air around it.

  Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph?

No. If groundspeed is 500mph into a 200mph headwind, this means that the plane has to fight against the headwind, i.e. its airspeed is 700mph.

Have you ever gone swimming where there is current? Can you imagine riding your bike on a treadmill?

  Meaning it will take us 10 hours to travel 3000 miles?

No. If the groundspeed is 500mph, it will take you 6 hours to travel 3000 miles over ground.


What should you do when the pattern is crowded? If you're approaching a non-towered field and you find that the pattern is too crowded to enter what do you do?

Provided you have adequate fuel to wait for an opening is there a standard place and altitude to hold?

What if you don't really have enough fuel to hold? Do you call on ctaf and say that you need to enter the pattern right away?

As PIC if you don't feel comfortable entering congested airspace at a beehive airport, dont enter it.  It's that simple.  Divert to another airfield and attempt to return to the original airfield when the congestion abates.

If you anticipate high density traffic at an untowered airport, you should let other pilots know your intentions on the CTAF well in advance, generally at least 10NM from the field.  Closely monitor other CTAF pilot reports and start building a mental picture of traffic reporting their positions in the pattern, entering the pattern, or leaving it and begin to adjust your entry speed and position for appropriate separation and traffic flow.  Most other pilots will work with you on this and make changes to their traffic pattern to accommodate you as well.

If you have a fuel emergency or any other emergency for that matter and no other alternative is available, declare an emergency on the CTAF, indicate your position, altitude and intentions.  All other traffic in the pattern, by federal law, MUST give way for you


Aircraft data bus and computer system What is a line replaceable unit (LRU) in an aircraft data bus system?

What kind of components are defined as LRU in a data bus system?

The components needed for a databus depend on the specific bus standards chosen. The most common of the recent air transport (B787, B777X, A350XWB) aircraft use the ARINC 664 network standard.

These aircraft also use ARINC 653 IMA computing resources hosted in a cabinet. Within the cabinet will be ARINC 664 network switch modules. These connect the IMA modules via a backplane bus and also connect it to external systems via fiber optic or quadrax network cabling.

ARINC 664 is a switched Ethernet type of bus with each A664 end system connected directly to a network switch, so there is no contention for media access at the end system. All network access control is controlled within the switches.

Beyond the IMA switches, the network typically has 4 or 6 remote switches spread along the aircraft. Aircraft systems with A664 end systems can be connected directly to these switches, but many systems use legacy interfaces such ARINC 429, ARINC 825 (CAN), and even analog audio or differential voltage signals.

These other networks can be connected via generic programmable network bridge devices called Remote Data Interface Units (RDIU) or something similar. Specialized interfaces are typically identified as gateways (Audio Gateway unit or A825 Gateway). These have an A664 connection as well as a number of the legacy bus connections. These have internal software that translates the data between the bus types. The key benefit of this architecture is a significant reduction in network cabling.


When should I request clearance to return to altitude, after leaving cruise altitude? I can not understand the term”cruise” perfectly.
I know I should request ATC clearance to return to cruise altitude, if I leave cruise altitude to out side of the block.

Ex) Cleared cruise altitude 6000, MEA is 4000. The block is between 4000 to 6000. So I can descent or climb within the block without clearance. if I leave to out of block, i need ATC clearance for return to the block altitude

There are two which I am confused.
First, Can I descent/climb several times without clearance within the block or just once is possible?

Second, When I leave assigned within the block, still should I request an

The “cruise” terminology you are questioning is really not used anymore. It still appears in manuals, but it was very confusing and fallen out of use. The intention of a “cruise” clearance was to allow the pilot to climb or descend as required with the specific cruise altitude window. In your question that was 4000’-6000’. However, today people the same request would be made as “request an altitude block 4000-6000”. ATC would respond “(callsign) is cleared the block 4000 to 6000.” No further calls are required from you while maneuvering in that altitude block.
Sometimes a “cruise climb” clearance will be issued to heavy aircraft to allow them to climb slowly and level off for a while when they are too heavy to climb all the way to their final altitude. In that case they can climb as they are able but are not allowed to descend out of an altitude they have already climbed to.


What is the backup for a glass cockpit, if a plane loses power to the displays/controls? I'm assuming there is still power to the rest of the plane - maybe it's an unrealistic scenario.

I'll answer this question as "If the glass cockpit instrument fail, what are the backup? " This answer will also be General Aviation oriented (as I'm a PPL student)

The short answer is redundancy.

At first, let's look at a G1000 (standard GA glass cockpit) architecture :

The most important components are :

One (or multiple for redundancy) AHRS : gives information about Attitude and Heading of the aircraft, thanks to electronic gyrometres / accelerometres / magnetometres.
One (or multiple) Air Data Computer: gives information regarding airspeed / vertical speed / altitude, thanks to static / dynamic pressure informations.
And two Integrated Avionics Unit, that collect data from AHRS / ADC, and generate the content displayed on screen.

Now, Although the two screens usually don't display the same informations (PFD and MFD), These two systems are standalone : in case of a screen / Avionic unit failure, the second can take other.

Here the goal is to remove all single points of failure.

  But what if the whole EFIS fails ?

It is required by law to have backup of essential instruments. The definition of "essential" depends of the type of flight (day VFR, night VFR or IFR?). For some aircraft, there backups are traditional (steam gauge) instruments / others have independent electronic systems, like the new Cirrus SR22 G6 :

all electric !

  And in the case of a complete electrical failure?

Depends of the aircraft. In my case, I fly the Robin DR401 (certified for night VFR):

The EFIS is a Garmin G500 (smaller version of G1000) along with a GTN650 GPS.

With such a cockpit, even if I have a complete loss of electrical power, backup instuments would still work :

Altitude, Airspeed and Vertical Speed indicator don't require any power to operate ;
So does the slip/skid indicator (a simple ball)
And the Attitude indicator gyro is powered by a vacuum pump, linked to the engine ; it would thus still operate.

In that case I'd be more concerned about the loss of radio / transponder / flaps control / trim than the loss of instruments!

To summarise, Glass Cockpits are designed to avoid the creation of single point of failure, to reduce the risk of complete loss of instruments, thanks to redundant system / backup instruments / displays... And for all-electric planes, I guess backup power supplies!


Why are airliners not inherently "speed stable"? In a light aircraft, once the pilot has trimmed for a speed, the power controls the pitch (climb/descent rate).

I don't know the technical name for this stability, so I chose "speed stable" for now.

The fly-by-wire (FBW) of the 787 (for example) mimics this behavior:

  In flight, the pitch trim switches do not position the stabilizer directly, but make inputs to the [Primary Flight Computers] to change the trim reference speed. The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes.—787 FCOM

I.e., once a speed is trimmed, the top sentence becomes true like in light planes.

So the related questions are:

What's the name of this stability?
Is it limited only to light aircraft?
Is it related to having a trim tab instead of a stabilizer trim? (Just a speculation.)
How can an airliner without FBW be designed to be "speed stable"?

An answer doesn't need to address the exact questions in order, I'm just communicating what I'd like to understand.

2019: The impact of the third point still confuses me:

Once the speed is trimmed (and left alone), and power is increased (for example), wouldn't having a trim tab (like on a typical light plane) have more impact on pitch response (speed stability) compared to the now-fixed stabilizer (jet-liner)? In other words, a light plane would pitch up quicker (elevator responding to the increase in airspeed), while a non-FBW jet-liner would pitch up much slower (stabilizer unaffected by airspeed) and gain more speed compared to the earlier trimmed state?

Light aircraft and transport aircraft behave exactly the same regarding trimming for speed. Trim is for angle of attack, but power affects that trim and phugoid oscillation occurs in speed and pitch, because the feedback is second order.

The Boeing FBW has no trouble mimicking anything. It simply translates the control forces and position exactly as a hydro-mechanical link would, just in electric signals. The aircraft is still stable.

Airbus aircraft are also all stable, but their FBW relaxes the stability by auto-trimming for current speed. The result is that side-stick controls flight path angle and power controls speed.

The only exception to the stability are fighter jets (starting with F-16). Those are aerodynamically unstable to get faster control response and their FBW makes them neutral (that is, the yoke/stick again controls flight path angle).


Why are some LNAV/VNAV mins lower than the LPV mins for the same approach? Why are some LNAV/VNAV minimums lower than the LPV minimums for the same approach? See the RNAV (GPS) Rwy 5 approach at Scott Municipal (KSCX).

From what I could find out, it looks like they're unusual cases produced by the obstacle clearance calculations. There's a lengthy thread on PoA about it and the main points I saw are:

LNAV/VNAV and LPV approaches are designed differently (the specifications are in FAA Order 8260.58)
The Obstacle Clearance Surface (OCS; an imaginary zone that must be free of obstacles) is shaped differently for each approach type
The OCS isn't at a fixed angle: it depends on the designer's choice of glideslope angle (e.g. for terrain clearance) and - in the case of LNAV/VNAV - on the prevailing temperatures at the airport
The two approach types have decision points at different distances from the runway threshold, which also affects the glideslope requirements
The resulting OCS may be at a higher angle for LNAV/VNAV than for LPV

The FAA Order has a lot of formulae to work it all out, but apparently the result is that if the calculations come out in a certain way and there's an obstacle close to the runway, it can penetrate the LPV OCS but not the LNAV/VNAV OCS. That means the LPV DA then has to be made higher.

In the PoA thread, every example of an approach with LPV minimums above LNAV/VNAV has an obstacle close to the runway. But the consensus was that the LPV minimums being higher than LNAV/VNAV is still very unusual overall.

Finally, although this isn't exactly the same scenario, an old FAA article on GPS approaches mentioned that the minimums aren't always what you would expect:

  While [varying the obstacle clearance for an LNAV/VNAV approach] occasionally results in minima higher than the LNAV
  minima, the added safety benefit of a stabilized descent outweighs the
  difference in minimums

(That all refers to altitude, of course. The other part of the minimums is the visibility, but that should increase anyway as the DA and/or category increases. A high, fast aircraft needs to be able to see the runway from further away in order to descend safely.)


How is a wing joined to the fuselage? I've seen this video from Airbus:

This is a capture from that video:

What is the process for attaching the wings to the fuselage?

The main wing comes in two pieces, left and right wings, and they are bolted to the lower center fuselage section, which also houses the center fuel tank.

Fuselage/wing fillets (fairings) are added after.

The frame you captured shows the rear wing—the horizontal stabilizer. It is slid in and then bolted, more precisely attached to the mechanism that moves it.

(Same video)


Is it possible to stall a plane so badly that the nose refuses to go down due to lack of airspeed? So basically, a stall from a high AoA to the point that the entire plane just falls with the rear end pointing straight down?

Because if wind is no longer passing over the wings, then this includes the elevator, and the elevator can’t deflect the wind with no forward speed?

Yes it's called a Deep Stall, and is mostly a problem with T Tail aircraft, especially jets with Supercritical Airfoil wings (like the CRJ Regional Jet line).

Such wings stall from the leading edge and the stall's flow separation spreads rapidly and completely across the entire wing all at once, so there is very little residual nose down pitching moment.  Plus the T tail ends up in a spot where it's in the wake of the flow from the wing, and gets blanked out, and loses its ability to pitch the nose over with positive lift since it's sitting there in the turbulent wake of the wing.

So the plane just mushes downward in an unrecoverable, stabilized, well, mush, and will pancake into the ground like that.  A CRJ200 test aircraft was lost in development testing when it got into a deep stall and IIRC, the stall/spin recovery parachute in the tail failed to deploy (or it didn't have one at the time; I forget which).

Such airplanes require, in addition to the usual stick shaker, a stick pusher to force the nose over, by shoving the control column for you if you do nothing while the shaker is going off, before the natural stall can occur, since the natural stall can be unrecoverable.  Generally if an airplane has a stick pusher system it means it has a deep stall mode.


Is there a way to calculate the perfect angle of incidence of a horizontal stab for a given airspeed? I have a Grumman AA1. It’s been modified several times through stc’s to increase power and speed.  The aircraft requires full nose down trim to fly level in cruise. Is there a way to calculate the perfect angle of incidence of a horizontal stab for a given airspeed? It’s set at -3deg now, I wonder what moving it to 0deg would do? I have all kinds of load data for the horizontal stab.

One clue may be increase in power and speed.  It is not unusual to trim down as plane accelerates from climb to cruise.  Since you are faster, the plane is generating more lift at a given AOA.  Greater speed will require more downtrim.  Your engine may be too powerful for your current airframe.

If the plane is safely flyable, you may try to isolate the problem by gliding power off to see if Hstab, elevator, and trim are working properly.  CG of course must be within limits.

If everything is ok, you may need a larger trim tab rather than resetting the Hstab angle.
Remember, the Hstab incidence compared to the wing AOA is critical.  I would not mess
with it.

If the problem isolates to the new engine, you may also check the thrust line.  A few degrees can make a big difference.  Your more powerful engine may need to be mounted with more downthrust.  That, and a larger trim tab, may do it.

I would definitely seek out people who have done similar modifications for expert advice.
Please, be safe.


What exactly happens to kinetic energy in the turbulent layer on an aerofoil? Sources I've read contains contradicting information.
As far as I know, when the laminar airflow breaks up, it becomes a thicker and draggy layer, causing a layer of what essentially is slowed air; this would suggest a reduction in kinetic energy, however some books have stated that it is actually an increase.....?
This makes what seems to be a simple concept to be hard to grasp, personally I see it as a reduction in KE, but the books say otherwise. So if anyone knows what happens to KE in a turbulent layer, that'd be greatly appreciated.

Energy is transferred from the moving airplane's skin into the air in the turbulent boundary layer. Because the layer is turbulent, all the momentum vectors for every little parcel of air entrained in the layer are pointing in random directions with some uniform component (in the direction of the plane's travel) superimposed.
Thus the air in that layer gets vigorously stirred up, and since air has viscosity, the stirring action gets fairly quickly transformed into frictional heat. So after the plane has departed out of the picture, the turbulence and net motion in its wake get dissipated and the air is left a little warmer than it was before the plane flew through it.