How to refute this geometrical method to find MH370? A geometrical method to find MH370 has been published on the Net (see images below).

I am sure it is wrong (because if it were that easy, thousands of people would have had this idea back in 2014). However, it has attracted some attention.

My question is:
How can this method be proven wrong in a simple, straight-forward way ?

Here is the method:

source: This method was first presented here.

EDIT:
How is this question off-topic while How can a computer model yield two possible flight paths of MH370 in South Indian Ocean? is on-topic? Isn't this having it both ways?

The picture says that "If [the above facts are] true, the plane can only be at one of two green dot locations." However, it doesn't give any reasoning or evidence to support this claim. So, there's no reason to think that the claim is correct.

You asked if itʼs possible to prove that the method is wrong. Itʼs not possible to prove that the method is wrong, because there is no method here. The picture is merely a collection of facts and unsupported claims.

However, the picture does seem to rely on a particular premise. Specifically, the picture seems to be based on the premise that the departure airport, the Inmarsat satellite, and the crash site must form a right triangle. That premise is false; the locations of airplane crashes are actually not affected by the positions of overhead satellites (or vice versa).


What is the difference between the Maximum Glide and Minimum Sink ratios? In this comment a brief explanation is made of the differences between Max Glide Ratio and Min Sink Ratio:

  Sink rate is how much altitude you lose over time, for example, how many feet per second. So minimum sink is the lowest sink rate you can get. Glide ratio is how much distance you can travel per given loss of altitude, for example, how many feet per feet. So max glide ratio is the shallowest angle you can glide at.

However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at. If I can design wing A to sink at only 1 foot per minute, wouldn't that automatically get me a longer glide ratio from altitude X than wing B that sinks at 2 feet per minute?

Glide ratio is the ratio of the distance a glider can travel horizontally to the altitude lost in transit. For instance, if a glider can travel 40 miles horizontally while losing one mile of altitude, the glide ratio is 40:1 (typical for a medium-high performance glider). The best glide ratio or maximum glide ratio is simply the best ratio a glider can achieve. Best glide ratio is achieved at a specific airspeed, which varies depending on the glider type. One flies at best glide speed in order to maximize the distance covered. When gliders are flying in a straight line and want to go as far as possible, they fly at best glide speed (typically 55-65kts).

Minimum sink is the minimum vertical speed the glider is capable of flying at in still air (typically 100-200ft/min). Minimum sink is also achieved at a specific airspeed, depending on the glider type (typically 40-50kts). One flies at minimum sink speed to maximize altitude gain when flying in lift. If you're circling in a thermal that is going up at 500ft/min, and your minimum sink speed is 100ft/min, then the glider will go up at 400ft/min.

To review, the key differences are:

Glide ratio specifies how "flat" the glide angle is
Minimum sink specifies the minimum vertical speed the glider can fly at
One flies at best glide speed to maximize the distance covered vs altitude lost
One flies at minimum sink speed to maximize altitude gain in lift
Best glide ratio is achieved at a higher airspeed than minimum sink

Any aircraft can be a glider with its engines out, so this applies to all aircraft.

  However, I don't understand how the lowest sink rate you can get isn't the same as the shallowest angle you can glide at.

Remember that minimum sink comes at a lower airspeed than best glide. At minimum sink speed, the glide ratio is worse because even though the aircraft is sinking slower, it is also moving forward slower.


How does TACAN work? How does the Tactical Air Navigation (TACAN) system work? How is it different from the VHF Omnidirectional Range (VOR) system? Please explain in the simplest possible way.

How is TACAN different from the VHF Omnidirectional Range (VOR) system?

A very short question, but the answer calls for describing several techniques which are by themselves difficult to summarize without taking liberties with reality, so the post is rather long and should be read by sections of interest rather than totally at once. And for those not interested in the design techniques, fortunately there is a...
Short answer
Design principle:

A TACAN uses UHF to increase bearing precision. It consists on a single integrated system performing bearing and distance determination at once. In this system, the ground station is a transponder and the interrogator is on-board the aircraft (contrary to the SSR transponder). Frequency is like graduations on a scale, when frequency increases the graduations on the scale are more dense, and readings are more accurate.

A VOR works on VHF for bearing determination. No aircraft action is required to trigger the ground station signal which is permanent. To determine distance, another independent system, the DME, is used. The DME has been borrowed to military and is actually a TACAN without its bearing components (so it is a transponder interrogated by the aircraft).

When selecting a VOR frequency in a civilian aircraft, the avionics actually sets the VOR receiver on this frequency, and the standalone DME interrogator at some "paired" UHF frequency obtained from the ICAO standard pairing table (page 6). VOR and DME share nothing on board beyond the frequency pairing table.

Station antennas:

The original TACAN antenna is made of two small rotating drums with parasitic antenna elements (see details below). A TACAN can be installed on ships or mobile stations. The TACAN antenna is externally similar to a conventional VOR. A TACAN in Alaska during an exercise:

(Source: Wikipedia)

The cylinder contains the rotating antenna system. In more modern TACAN, the mechanical rotation has been replaced by electronically scanned arrays, reducing the size:

Transportable TACAN, source

Doppler VOR (DVOR) are more common than conventional VOR (CVOR), because they can be located on airfields (CVOR: See below for details). The DVOR antenna is a large circular array with a central reference antenna and a large counterpoise under the array.  VOR are sometimes colocated with a DME station, in that case the DME vertical antenna is above and coaxial to the VOR system.

Lambourne VOR/DME, DME antenna on top of the central VOR reference antenna. (source: Wikipedia)

Because the DME part is common to VOR/DME and TACAN, it is technically possible to associate a VOR to a TACAN to obtain a VORTAC station. Military use the TACAN, civilian use the VOR and the DME information of the TACAN:

A full TACAN in place of the previous DME antenna. Source

In addition VOR (CVOR/DVOR) uses Alford loop antennas which are horizontally polarized and radiate low on the horizon. They are sensitive to reflection on obstacles. An electrical counterpoise is required to hide the ground and raise the radiation angle. This artificial ground plane can be very large:

PFN Vortac (decommissioned), source

Signals:

A VOR transmits bearing information continuously.

A TACAN only sends reply pulse pairs when interrogated (see the explanation below). These pairs encode both the bearing and DME information.

A TACAN is usually more powerful than a VOR and has a larger range of use.

I'll focus on explaining bearing determination systems, and explain the DME as an integral component of the TACAN. In addition there are two types of VOR, conventional and Doppler, which work very differently even if they deliver compatible signals to the (unsuspecting) common receiver.

Bearing determination principle
The common principle of bearing determination is to send two signals from the ground station:

A reference signal telling any receiver around the current orientation of the active signal.

A variable signal allowing a particular receiver to determine when the active signal is "pointing" to the receiver (pointing is not the accurate word as the DVOR signals are omnidirectional, see more in this answer).

The receiver determines its relative bearing by comparing these two signals. Both signals are sine functions, the orientation value is represented by the current phase of this function. Both VOR and TACAN use this basic principle, albeit they realize it differently.
The signal phase has the major role in this story, so let's be sure we agree the meaning:

Any periodical (repetitive) signal can be seen as the result of a vector turning at some rate $\small \omega$. The sine function for angle $\small x$ is $\small y = sin(x)$. Applied to a sinusoidal wave of frequency $\small f$ and peak amplitude $\small A$, this becomes $\small y=A.sin(\omega t+\varphi)$ where $\small \omega = 2 \pi f$. The angle $\small \omega t+\varphi$, has been split between the phase $\small \omega t$ and the phase at origin $\small \varphi$. $\small \varphi$ is null if we start a cycle at time 0, this is usually the case. More simply, the quantity $\small \omega t$ represents how much the vector has turned at time $t$. It is an angle, reset after a full turn, therefore it indicates in the end which portion of a full cycle has already been spent (which phase we are in the cycle). Visually:

Phase angle of a sine wave
From that, it is clear that comparing phases of two signals with the same frequency (simple to do with electronics) is equivalent to comparing how much time one lags behind the other (time is actually difficult to measure).
VOR types
Conventional VOR (CVOR) and Doppler VOR (DVOR) stations are perceived identically by the receiver, though they transmit very different signals. The DVOR use tricks to mimic a CVOR and deceive the CVOR receiver. CVOR have nearly disappeared from sight because, due to their sensitivity to reflections, they cannot be located on airfields or close to roads. However en-route/high altitude CVOR can be found at isolated places, the reason is they are more compact and have a smaller cone of silence than DVOR and reflections can be minimized, e.g. when the VOR is located at the top of a hill.
Understanding the DVOR tricks without knowing how a CVOR works is difficult, and doesn't give clues on how the bearing is really determined. So I'm afraid we need to understand the CVOR before the DVOR.
Conventional VOR: CVOR
Early CVOR antenna used to be an array of four Alford loops at the corners of an imaginary square, known by their conventional names: NW, NE, SW and SE. NW+SE loops form the first pair, NE+SW loops form the second pair.

CVOR with four Alford loops
Alford loops are horizontally polarized and very sensitive to reflections on surrounding obstacles (multipath).
Recent generations of CVOR use a slotted antenna, which is a fixed cylinder with vertical slots (generally four slots):

CVOR with slotted antenna and the DME antenna on top. Source: AviaTecho.
A counterpoise is placed under the array to hide the VOR shelter and the ground and raise the radiation direction, it has the dual effect of minimizing undesirable reflection on the shelter and the ground and reducing the cone of silence above the VOR.
CVOR creates and uses reference and variable signals this way:

A low frequency generator creates three 30 Hz signals, identical except their phases. Two audio signals are derived from a reference signal: The sin signal has a phase at origin of -90° and the cos signal has a phase at origin of +90° (the point is sin and cos signals are in phase opposition).

The phase of the reference conceptually represents a direction and is often called the goniometer. As this signal frequency is 30 Hz, the imaginary direction it represents sweeps 360° 30 times per second (1,800 rpm, this is pure abstraction, there is no rotating parts in a CVOR).

A low frequency generator creates a 9960 Hz signal which is FM modulated by the reference. This signal is known as the reference subcarrier.

Conventional VOR block diagram

A HF generator creates a carrier of frequency f (f being the VOR frequency), this carrier is split in three parts:

one part is AM modulated by the reference subcarrier.
another is AM modulated by sin.
the last part is AM modulated by cos.

The HF signal with the reference subcarrier is sent to all antennas. This way the reference can be received identically regardless of the position of the receiver around the CVOR.

The two other HF signals have first their carrier removed, so that only the sidebands subsist. This is to prevent carriers to interfere in space, interferences must occur only between sidebands.

Then one signal is sent to the NW+SE pair of antennas, the other signal is sent to the other pair (remember the two pairs are perpendicular).

The space modulation magic does the rest. The sin and cos sidebands are added as field vector values, sometimes the individual amplitudes are added, sometimes they are subtracted, in variable proportion. This results in an unbalanced cardioid pattern (more precisely a Limaçon de Pascal) which rotates around the VOR antennas at 1,800 rpm, the direction being linked to the phase of the reference (or sin or cos, as they are all linked by fixed values).

The signal resulting from space modulation appears like being a carrier AM modulated according to the direction of the virtual "rotating antenna". The AM modulation is also a 30 Hz signal, and is known as the variable signal.

The resulting signal also contains the unmodified (and constant amplitude) carrier with its reference subcarrier.

CVOR spectrum

To determine the bearing (the radial) of the receiver relative to the CVOR, one only needs to compare the phase of the variable signal with the phase of the reference signal. Both are contained in the resulting signal. The phase of the reference signal and the phase of the variable signal are equal when the reference "points" to north (per principle at this time both phases have a value of 135°, the sum of 45° and 90°, but the actual value has no influence, only the phase difference is meaningful):

VOR: Determination of the bearing by comparing phases

Now we know the principle of the CVOR, it more easy to understand the principle of the DVOR. The DVOR was created to make up for some weaknesses in the CVOR: The CVOR is not terribly accurate unless the installation site is very carefully selected (no obstacle). That means isolated points, not airfields. That's not the preferred option for maintenance and this often precludes having the CVOR aligned with the runway for a VOR approach.

From CVOR to Doppler VOR, ensuring retro-compatibility
The lack of precision of the VOR is rooted in two design choices:

The antennas are close to each other, any default in their placement has great consequences in the accuracy.

The variable signal is AM modulated, AM modulation is deadly subject to errors created by electromagnetic noise and multipath.

In the Doppler VOR (once again... there are two types of DVOR, the single sideband and the double sideband, I'll describe the DSB here):

The two active antennas are at a large distance from each other (diametrically opposed).

The variable signal is FM modulated.

In order to be compatible with the CVOR receiver, other changes had to be made:

As the receiver still compares two signals, one being AM, the other FM, the reference signal must be AM modulated.

Because the result of the phase comparison is now inverted (variable minus reference becomes reference minus variable), the direction of rotation of the pattern must also be inverted (anti-clockwise instead of clockwise).

Because the pair of antennas used for the variable signal creates deliberately a Doppler effect, the reference must be sent on a specific central antenna preserved from the Doppler effect.

Doppler VOR: DVOR
The principle of a Doppler VOR is to create the frequency modulation by Doppler effect rather than by electronic modulation. The Doppler effect occurs with a moving wave source: In spite the source has a constant frequency, when it comes closer to the receiver the apparent frequency is higher than the actual frequency. How much higher depends only on the closure rate.

Doppler effect on train noise: Sound is higher-pitched at the front than at the rear
In the DVOR, pairs of opposed antennas (still Alford loops) are constantly switched on/off, scanning the full array anti-clockwise, the full scan being done 30 times per second. Actually there are two groups of antennas rather than two antennas involved to allow for blending (smooth transition from one pair to the next), but let's simplify for a second. From a receiver standpoint, the signal seems to come from a moving source, and therefore a Doppler shift will occur in a proportion which depends on the apparent direction of the move.

DVOR Doppler effect
To allow for compatibility with the CVOR receiver, this shift must be at most 480 Hz, 480 Hz being the FM swing of the subcarrier in the CVOR. A simple calculation shows the array diameter must be about 14 m (46 feet).
To generate the FM modulated signal, the unmodulated 9960 Hz subcarrier is sent on the "rotating" pair of antennas. The Doppler shift is maximum when the receiver direction is tangential to the pair trajectory and minimal when the pair is perpendicular to the direction of the receiver. This shift is exactly representative of the aircraft bearing and is the variable signal modulation we need.
From a radio signal perspective, only the sideband frequencies are used to transmit the 9960 Hz subcarrier (VOR frequency f +/- 9960 Hz). The carrier is itself sent on the central antenna, AM modulated by the reference signal. This way the carrier is not subject to the Doppler shift.
Bottom line... Like in the CVOR, the receiver sees the composite signal: A carrier AM modulated with a 30 Hz (which is the reference instead of the variable signal), with a subcarrier FM "modulated" as the result of the Doppler effect, at 30 Hz (the frequency of the scan, it now represents the variable signal instead of the reference) and with a swing not far from the expected 480 Hz.
Blending: If a pair of antennas were used one at a time, the number of measurable bearings would be equal to the number of antennas in the array (about 50). To create a more continuous scan (and therefore a larger number of measurable bearings), the antennas which precede and follow the main antenna are also fed by the subcarrier signal, but with a smaller power. This "blends" the transition from a scan position to the next one.
See also What causes the phase to change in a VOR? for a better explanation of the DVOR.
TACAN bearing
A TACAN is based on a stationary antenna plus a rotating parasitic system. The base antenna is vertical and common to the distance and bearing measurement instruments.
Parasitic elements in the aerial field refer to passive antenna elements added to the actual active radiator. A reflector decreases the gain on its side, a director increases the gain on its side (more). The well-known Yagi directional antenna (here in an horizontal polarization) has the two types of parasitic elements:

(Source, modified)
These elements are used in the TACAN, but they are rotating around the active element:

(Source, modified)

The central element, which is the one also used for the DME portion, transmits a constant amplitude signal.

A rotating drum with a reflector electrically adjusts the radiation pattern, adding a signal dip (low gain) that rotates at 900 RPM, which is equivalent to a 15 Hz amplitude modulation. The radiation pattern in the horizontal plan takes the shape of a cardioid:

(Source: Advances in Electronics and Electron Physics, Volume 68, modified)

Another drum with a set of 9 directors, mechanically linked to the first one, creates a 135 Hz (9x15) additional amplitude ripple over the 15 Hz modulation:

(Source: Advances in Electronics and Electron Physics, Volume 68, modified)

Now we need to start again the reasoning taking into account that the TACAN signal isn't transmitted permanently, but only keyed (switched on/off) by bursts of information. Burst are of two kinds:

Reference bursts
DME responses.

Reference bursts are generated according to the orientation of the modulation pattern:

When the 15 Hz peak faces North a main reference burst is sent. The burst consists of 24 pulses

with an asymmetric duty cycle.

When any of the 135 Hz peaks faces East, an auxiliary reference burst is sent. The burst consists of 24 pulses with a symmetric duty cycle.

(Source: Advances in Electronics and Electron Physics, Volume 68. Modified)
The duration of these bursts is only a portion of the 15 Hz cycle, meaning that if there is few aircraft DME interrogations, most of the time the TACAN signal is not keyed, therefore not transmitted. This lack of transmission would create a difficulty for the aircraft receiver:

To adjust its receiver gain (AGC) to counter fading.
To identify the 15 Hz and 135 Hz modulations.

To maintain the capability of reception, the TACAN signal is instead keyed at a constant rate of 2,700 pairs of pulses per second, adding squitter pulses if necessary to fill the blanks. The more the DME interrogations are received by the TACAN, the more DME reply bursts are sent, the less squitter pulses are necessary (more in MIL-STD-291).

The 135 Hz signal has been removed for simplification (Source)
The 135 Hz modulation is used for the bearing determination. By comparing the time between an auxiliary burst and the subsequent reception of one of the 9 signal peaks, it is possible to determine the aircraft bearing relative to the ground station. The main burst (15 Hz) is used to disambiguate which of the 9 lobes was used, and therefore which of the 40° (360/9) sector is actually relevant for the bearing.
In theory the use of the top end of the UHF band and the 135 Hz ripple increase the bearing accuracy by one order of magnitude compared to the VOR. In practical this is less, but still better than the VOR.
The DME principle is to measure the time a radio signal takes for a roundtrip to the ground station. As radio waves travel at light speed, knowing the time is knowing the distance. "The aircraft interrogates the ground transponder with a series of pulse-pairs (interrogations) and, after a precise time delay (typically 50 microseconds), the ground station replies with an identical sequence of pulse-pairs." (Wikipedia).


Why are helicopters less efficient than planes? At higher speeds, advancing helicopter blades will operate at velocities approaching mach 1, reducing efficiency. But what about lower speeds?

A helicopter is basically a wingless plane with a giant propeller. Why is it more efficient to move around using propellers to provide forward trust while using separate fixed wings to provide lift?

The answer lies in the difference between energy and momentum. Both have to be maintained, but energy goes up as the square of velocity, while momentum goes up linearly. To impart a given momentum, you can either move a lot of air slowly, or a little air quickly -- but as you move the air more quickly, it requires disproportionately more energy.

In other words, the bigger the wing, the more efficient (from an energy/momentum perspective, anyway).

A rotor's size is limited, both because its fast rotation means it needs to be light (and thus can't be very reinforced, etc) and because the larger its radius, the faster the end spins. That means it has to touch less air, but move it more -- which requires more energy.

There's more detail at this question on Physics StackExchange (disclaimer: I wrote that question).


What is the "tennis ball between cylinders" trick used by flight instructors? From this answer, "A tennis ball stuffed in between two cylinders -
a common flight instructor trick to see if students are doing a thorough preflight".

A place between the two cylinders is not the expected place for the tennis ball to be, but how does in general this "trick" work? I tried to Google but only pull out the mentioned answer.

From aircraft flying handbook (page 2-9),

  The pilot should also check for loose or foreign objects inside the cowling, such as bird nests, shop rags, and/or tools.

This tennis ball is a tool for instructor to see if a pilot does a proper walkaround of the airplane. A pilot should be able to detect this strange object inside the cowling easily if he/she does the walkaround properly.


What level of information does a PIC have about a missile crossing their aircraft route? Recently North Korea launched a missile out of its national airspace on a trajectory which crossed the route of an aircraft from Air France. The aircraft wasn't in danger at any time, the minimal distance between the missile and the aircraft was always higher than 100 km (7 minutes separation).

Source: CBS

ATC was not able to inform the aircraft, or any other aircraft in the vicinity. If the timing was such the missile could hit the aircraft it seems this would have made no difference.

From a pilot in command, responsible for on-board safety, stand point, what kind of information is available regarding such events? Is there some protection possible yet, or expected in the near future? If not, can (or must) a PIC asks for something to be put in place?

Pre-announced missile launches will often be published by responsible countries in NOTAMs (Notices to Airmen), which are notices of potential hazards to aircraft in flight or on the ground. These are generally things like closed runways, inoperative radio navigation aids, cranes on approach or departure paths and the like. They can also warn pilots about conflict zones, missile/rocket launches, and satellite re-entry paths.

NOTAMs are published publicly and are read by pilots as part of flight planning. I've heard of important ones being read off by ATC, but that's extremely rare. Commercial flights can also be sent warnings using the ACARS system by their corporate support groups.

The NOTAM system relies on organizations/governments posting these notices. North Korea does not announce missile launches before they happen, or even after they happen (unless they are successful or course), so nobody in civil aviation knew about it until it was all over.


Why do simultaneous-ops parallel runways need to be so far apart? In the answers to this question about Denver's widely-scattered pinwheel of runways and this question about Schiphol's runway out in the middle of nowhere, it's mentioned that parallel runways are required, by both European and U.S. regulations to be over a kilometre apart in order to be able to have simultaneous departures or simultaneous arrivals on both runways.  In contrast, if one runway is departing an aircraft while a parallel runway is landing an aircraft, the runways need only be a few hundred metres apart.

Why is there such a large runway-separation requirement for simultaneous arrivals or simultaneous departures from parallel runways?  It can't be for aircraft separation, since an aircraft is allowed to land on a runway quite close to a parallel runway that is simultaneously departing an aircraft, despite the possibility of this resulting in a head-on collision if one or both aircraft drift off track, so what, in fact, is the reason for this?

The simple answer seems to be for wake turbulence. This FAA document, ORDER JO 7110.308C, seems to say it is for wake turbulence reasons in para. 9:

  The geometry of the approach, as well as the lateral separation between the two approaches and prevailing local meteorological conditions, provide the wake turbulence avoidance necessary for reduced separation dependent approach operations.

Figure 3 shows the aircraft arriving in a staggered fashion, 1 nautical mile diagonal separation with runways 2500 ft apart (parallel runways separated by less than 2,500 feet, also referred to as, Closely Spaced Parallel Runways (CSPR)).  The text also describes how the lead aircraft follow a lower glide path than the following aircraft such that  the lead's wake turbulence isn't a factor for the following aircraft.


Can you safely operate a 777 with diesel? Is it possible to safely operate a Boeing 777 on diesel instead of Jet-A? Are there any regulations that prevent this? Would it cause any damage?

No. The fuel system would risk to clog up with gelled diesel fuel in the low temperatures at cruise altitude. If you had omitted "safely" in your question, the answer might be different.

Before I repeat what others have written before, I refer you to this excellent answer by @voretaq7. The essential part is:

  Why don't we burn highway diesel or heating kerosene in a 747? In a
  word, purity. The additional controls imposed on Jet-A fuel by the
  associated standard produce a product with well-known properties. When
  you fuel a plane with Jet-A you can be confident that fuel won't
  freeze in the tanks at altitude (at least not unless you cold-soak it
  below -40 degrees). You also know the fuel will be "clean" and won't
  gunk up fuel filters or leave deposits inside the combustion core of
  the engine that can cause problems later.


Does making a mistake mean you lose your cert? Disclaimer: I haven't been in, nor do I plan on having an accident. That said, this has always been a question of mine.

Let's say I'm flying a little Cessna or ultralight around Ohio. Maybe the wind is stronger than expected aloft and I run out of gas before making it back home. Clearly I need to put it down on a road or field if I can.

After this happens, does the FAA revoke your certificate?

Probably not.  Depending on how poor your planning was (or appeared to be) you may have nothing happen, have letter placed in your file (not a violation), receive a violation, have to make a 709 ride with a Fed, or have your cert suspended, or maybe even a combination of some of these.

If you were a real goober and just went flying without checking fuel, and then lied about checking fuel, and then get caught lying to federal investigators then, yeah, revocation is a realistic possibility.  They don't like liars.  They can deal with stupid mistakes, but they will not tolerate a liar.

//thanks for the edit but was written as a runon//


When outbound from an IAF on an approach to a procedure turn, may the pilot choose whether to use time or distance? I was trained that if a fix to be flown over and then outbound for an approach is off-airport, then fly out for 1 minute prior to making a procedure turn.  If it is over the airport, then fly outbound for 2 minutes instead of 1.

Would it also be proper to simply decide on a minimum distance to be flown such as 5NM outbound from the fix as long as it is within the published maneuvering area for the procedure such as 10NM from the IAF?

I know the times are only guidelines, but basically, what I am asking is would not using time at all and simply using DME as the guide be considered improper procedure?

Here is an exemplary approach to add something tangible to this question. It has an off-airport procedure turn with a requirement to remain within 10NM.

Using DME only is perfectly acceptable the FAA does not actually specify how exactly to execute the procedure turn but where and at what altitude. This is covered in the AIM:

  5-4-9 Procedure Turn and Hold−in−lieu of Procedure Turn. A procedure turn is the maneuver prescribed when it is necessary to
  reverse direction to establish the aircraft inbound on an intermediate
  or final approach course. The procedure turn or hold−in−lieu−of−PT is
  a required maneuver when it is depicted on the approach chart, unless
  cleared byATC for a straight−in approach. Additionally, the procedure
  turn or hold−in−lieu−of−PT is not permitted when the symbol “No PT” is
  depicted on the initial segment being used, when a RADAR VECTOR to the
  final approach course is provided, or when conducting a timed approach
  from a holding fix. The altitude prescribed for the procedure turn is a
  minimum altitude until the aircraft is established on the inbound
  course. The maneuver must be completed within the distance specified
  in the profile view. For a hold−in−lieu−of−PT, the holding pattern
  direction must be flown as depicted and the specified leg
  length/timing must not be exceeded.

The guidance is pretty straight forward that you need to be at the prescribed altitude and within the "remain within" distance but the type and execution of the maneuver is largely up to the PIC as they go on to elaborate.

  On U.S. Government charts, a barbed arrow indicates the maneuvering
  side of the outbound course on which the procedure turn is
  made.Headings are provided for course reversal using the 45 degree type
  procedure turn. However, the point at which the turn may be commenced
  and the type and rate of turn is left to the discretion of the pilot
  (limited by the charted remain within xx NM distance). Some of the
  options are the 45 degree procedure turn, the racetrack pattern, the
  teardrop procedure turn, or the 80 degree, 260 degree course reversal.
  Racetrack entries should be conducted on the maneuvering side where
  the majority of protected airspace resides. If an entry places the
  pilot on the non−maneuvering side of the PT, correction to intercept
  the outbound course ensures remaining within protected airspace. Some
  procedure turns are specified by procedural track.These turns must be
  flown exactly as depicted

The standard 1 minute timing is a convenient way to execute the maneuver for light aircraft since 1 minute at approach speed is usually ~1.5 miles (assuming a 90 Kt maneuver/approach speed) and will often keep you in the protected area.

Depending on how you read the regulation in reality a pilot may only use distance, although you may use time as an estimate and helper to execute the maneuver.


What are the lettered and numbered signs next to airport runways and taxiways? What do these numbers signify?

Can pilots see them while landing?

Do they matter while taking off?

Do Control Tower mention these while guiding planes on runway?

From left to right in your image

  16 CAT II / III

Refers to the runway number, and also the ILS Category that particular runway supports. However that is just secondary information. The primary purpose and position of this sign is actually to indicate the hold-short position for the numbered runway.

  E6 34/16

Refers to the taxiway designation (E6) and the runway(s) it leads to (34/16 being the same runway 34 one way and 16 the other)

  {Unreadable} and E6 L→ 10→

Both of these signs are the same, one is pretty much unreadable the other is again on taxiway E6 and points that Taxiway Lima (L) is to the right as is runway 10.

Pilots are unable to see these signs on landing, signs like this on taxiways are almost exclusively for ground operations - not just the aircraft but the variety of vehicles that operate on the apron of an airfield.

These signs are equally not useful at the point of takeoff, none of the information presented on them is relevant for that phase of operation.

Pilots will have a "map" of an airfield with them which will indicate the same information as presented on these signs. Ground controllers will typically mention which taxiways to use when guiding aircraft (and other vehicles) using the apron and taxiways. For example, at the airfield pictured the ground controllers may have instructed the pilot that they were cleared to "hold short of Runway 10 via Taxiway Echo 6" or may have told a landing aircraft that they should "clear the active runway at Taxiway Lima".

Here is a graphic that many websites use a variation of:

Source: americanflyers.net


What are the specific side step minimums for an approach? This question piques my curiosity a bit on this subject.  In regards to circling approaches the AIM 5-4-19 states that

a. ATC may authorize a standard instrument approach procedure which serves either one of parallel runways that are separated by 1,200 feet or less followed by a straight-in landing on the adjacent runway.
b. Aircraft that will execute a side-step maneuver will be cleared for a specified approach procedure and landing on the adjacent parallel runway. Example, “cleared ILS runway 7 left approach, side-step to runway 7 right.” Pilots are expected to commence the side-step maneuver as soon as possible after the runway or runway environment is in sight. Compliance with minimum altitudes associated with stepdown fixes is expected even after the side-step maneuver is initiated.
Note: Side-step minima are flown to a Minimum Descent Altitude (MDA) regardless of the approach authorized.
c. Landing minimums to the adjacent runway will be based on nonprecision criteria and therefore higher than the precision minimums to the primary runway, but will normally be lower than the published circling minimums.

However I’m not aware of any approach plates which list specific side step minimums on them for airports with parallel runways where sidestepping is commonly used.  Examples below are from Boeing Field in Seattle, WA, and John Wayne in Santa Ana, CA.

So my question here is as the plates do not specifically publish a side step set of minimums, should the approaches be flown down to the localizer MDAs or use the Circling MDAs?  Is this up to a pilot’s preference or is it a matter of just how far away the landing runway is from the parallel approach runway?

KSFO has sidestep minima for ILS/LOC 28R to 28L but not from ILS/LOC 28L to 28R.  It also has a sidestep from ILS/LOC 19L to 19R but no ILS at all for 19R itself.  The latter is probably the more normal case for sidesteps: giving a second runway (probably one normally only used for departures) lower minima than circling without needing to install and maintain another entire ILS setup that would rarely be used.  Notice that the first two also have (much higher) circling minima.

KDFW has sidestep minima for ILS/LOC 35L to 35C, ILS/LOC 35C to 35L, ILS/LOC 36L to 36R, ILS/LOC 36R to 36L, and all their opposites.  Since all the sidestep targets have an ILS of their own, this seems to be intended only for when one of them is out of service, i.e. not for normal usage.


How do I know when to read a number as (for example) 241 or 24.1 off the navigation computer? How do I know in what way I should read a number off the naviagtion computer scale on the slide rule side?

Eg. 241 and 24.1

If you are talking about an E6B, you follow the same rules for your result as you followed for your inputs. If you used a factor of 10 for your inputs, you use a factor of 10 for your results. If all else fails, you use the common sense rule. For your particular calculations, does 241 make sense? Or, does 24.1 make sense?


How does a flight or squadron communicate when radio silence is in effect? By MSgt. David N. Craft [Public domain], via Wikimedia Commons

How does a flight or squadron communicate when radio silence is in effect?

If one pilot spots an unexpected target or danger, how is it communicated to their flight?

Rocking the wings? Kicking the rudder? A gun burst? What are the different non-verbal ways of communicating in such scenario?

USAF Instruction 11-205 Aircraft Cockpit and Formation Flying Flight Signals details the guidance and procedures for Air Force formation flight. It also indicates distress signals that have been standardized for use with North Atlantic Treaty Organization (NATO) forces.

To get attentions, usually the wings are rocked:

  Attention in the air Execute rapid shallow rocking of wings.

  Loosen Formation Fishtail the aircraft.

The following are some of the distress reasons and corresponding signals:

  Bailing Out or Ejection One or both clenched fists pulled downward
  across the face to simulate pulling an ejection face curtain.

  Intercepting Signals The  intercepting  aircraft  positions  itself  in
  front of and usually to the left of the intercepted aircraft and rocks its wings.  This is a signal that the interceptor wishes the other aircraft to follow it.  A responding irregular flashing of all available lights in this case indicates distress

In case of unexpected danger/target or where time is critical, the flight is allowed to break radio silence. The instructions states:

  These signals reduce radio communications and permit pilots to communicate with each other during periods of radio silence.  Use the radios if the situation permits, when time is critical, or the visual signals are creating confusion. (emphasis mine)


What are differences between vortex, swirl, eddies and wake? These terms seem to have the same meanings. Can anyone differentiate them?

Wake is the general term for air disturbed by a passing aircraft, as might be guessed from the word's nautical origins.
Vortex (in aviation, usually a wingtip vortex) is a part of the wake that originates at the tip of a surface that has a pressure differential.  It's the preferred term in scholarly writing.
Eddy by now seems to be restricted to the name of a general model for turbulence, https://en.wikipedia.org/wiki/Large_eddy_simulation.
Swirl has a technical meaning that I can't quite fathom, but it seems to apply more to combustion in turbines than to airflow outside the aircraft.


In which direction should a turn-back be made after an engine failure on a multi engined airplane after take off? In case one of the engines fails after taking off (let's suppose the left engine is the one that fails), which direction should we turn to go back to the airport? A left or a right traffic pattern?

That  depends on a lot of factors. Are you declaring an emergency? What altitude are you at what speed are you at when this happens etc?   What kind of aircraft are you flying? Does it have enough power to continue to climb on one engine? Some trainers do not.

For a typical airliner, loss of a single engine is not a big deal. The pilot will simply declare an emergency and get vectors from controllers as to where to go land again. Most airliners have plenty of power with one engine out and they don't have to worry about that. Smaller twin engined GA aircraft are a different story and may not be able to achieve a positive rate of climb on one engine, especially with the gear and flaps extended just after liftoff.  At low altitudes where this performance factor may be the case for the specific airplane you are flying, you may have to execute a forced landing in a clearing either directly ahead or off to your side, as a 180 deg turn to the airfield would be suicide.

As to maneuvering once an engine is lost, there is no requirement for this, save a pre-determined VFR traffic patterns for the departure airport.

If the engine out is an emergency and you cannot comply with either a controller's instructions or uncontrolled procedures, you are authorized as PIC to disregard any regulation as required to deal with the emergency.  This means any kind of maneuvering you feel will safely bring the aircraft back to the ground is acceptable.

I've read a lot of posts and comments amongst pilots about not turning into a dead engine on a twin; this is nonsense with one caveat - you must have sufficient airspeed in order to do this.  It depends on the aircraft in question but a good rule of thumb is that if you are at or above Vyse you can GENTLY turn into a dead engine.  The threat comes in attempting to turn into the dead engine at or near Vmc, minimum controllable airspeed on a single engine, where it may not be possible to arrest the turn due to thrust asymmetry.


What's the story behind U-2 flying from aircraft carriers? This video from the CIA YouTube channel shows U-2 aircraft launching via catapult and trapping with a tailhook:

The description only offers this:

  After stopping U-2 flights over USSR in May 1960, the IC looked for new ways to use this unique reconnaissance asset. One idea was to modify the U-2 to fly from US Navy aircraft carriers.

What motivated these experiments? Were they successful? Was the plane ever used operationally from a carrier? What would a mission objective be?

From Wikipedia (emphasize mine):

At one time, in an effort to extend the U-2's operating range and to eliminate the need for foreign government approval for U-2 operations from USAF bases in foreign countries, it was suggested that the U-2 be operated from aircraft carriers. Three aircraft were converted for carrier operations by the installation of arrester hooks, and carrier-qualified naval aviators were recruited to fly them.
It turned out to be possible to take off and land a U-2 from a carrier. Testing in 1964 with the USS Ranger and in 1969 with the USS America proved the concept. The only operational carrier use occurred in May 1964 when a U-2, operating from USS Ranger, was used to spy on a French atomic test in the Pacific.
In 1969, the larger U-2Rs were flown from the carrier America. The U-2 carrier program is believed to have been halted after 1969.

It also provides the following resources:
http://www.kristinhillartist.com/FishHawk_article.html
https://theaviationist.com/2015/06/28/u-2-carrier-operations-story/


Why would a VOR be decommissioned leaving the DME? As can be seen on this question and this question the OJC VOR has been decommissioned but the DME is still active. What would be the purpose in having DME without VOR?

Many older Air Transport Airplanes have no GPS. I'm a pilot since 1970. I operated A330 for 5 years until 2000 without GPS. The average IRS position (Inertial Reference Systems) in the FMS was updated by DME/DME. On older airplanes we selected the DME Stations manually preferably with a 90 degree cut-angle to obtain an accurate update. A single VOR/DME station could be used for update as well but it's a lot less accurate. Example, if you are 60 NM away from a VOR, a 1° error in azimuth would cause an error of about 1 NM. All Airbus aircraft have an approval for RNAV Approaches down to an accuracy of 0,3 NM with 2 DME stations available. This is why DME's are still useful, many ILS have a DME. Conventional VORs are terribly inaccurate, Doppler VORs are much better even down to a lower altitude, but requires more space.


How much does weight really affect the operation cost? Hi given an information from a reliable new source article (https://www.nytimes.com/2008/06/11/business/worldbusiness/11iht-air.1.13628276.html), it was mentioned that "Every 25 pounds we remove, we save $440,000 a year,". I wonder the validity of the statement because it goes against basic math.

Here is what I got,

\$440,000/yr/25 lbs = $17,600/yr/lb

Let's say a passenger weight at 200 lbs, this means by removing one passenger out of the plane, an airline would save   $\dfrac{\$3.5 million}{year \ person}$.

That means that one passenger should be paying more than that amount over the year, but if we think calculate that by using a ticket from L.A. to New York as an example, that's not the case (calculation shown below).

$ \dfrac{\$262}{person}$ $\times$  $\times$ $ \dfrac{4 flights}{day}$ $\times$ $ \dfrac{365 days}{year}$ =
$\dfrac{\$380k}{year \ person}$

I know there must be something wrong in this simplistic calculation, but I couldn't figure out. I am hoping someone here could point that out (with a reference would be awesome).

Thanks in advance.

A couple of old references behind paywalls estimate commercial aircraft weight reduction is worth $300-800/lb over the life of the aircraft, and it is heavily fuel price dependent. This was back when fuel was half the cost it is today, so double it.

Your reference quotes a Northwest Airlines manager who may have been describing savings across their whole fleet of 320 planes. This drops the annual savings to 50/lb/aircraft, or $1500/lb across the 30yr life of an aircraft, roughly equivalent to my two sources.


Which Inlet Barrier Filter (IBF) is used in Airbus H145? Inlet barrier filter (IBF) is an optional equipment in H145 / MBB-BK 117 / EC145 helicopters, which company Inlet Barrier Filters are used on them Donaldson, Aerometals etc..

The H145 Helionix / BK117 D2 Differences Training Course (airbus.com/content/dam/channel-specific/website-/us/training/…) mentions the Inlet Barrier Filter as an optional equipment, but The Airbus Options Catalogue for 2019 doesn't list an Inlet Barrier Filter for that aircraft.

The only IBF listed is for the AS350/H125 family


How much does it cost if someone falls ill before take-off? A month or so ago, I flew from LHR to Australia, via Doha, on a Qatar Airways A380. Just as we were about to enter the runway at LHR, the captain announced that someone had been taken ill on the plane, and we'd be going back to the gate. That delayed take-off by (I think) about an hour, and required topping up the fuel tanks.

How much would this have cost the airline? Are there any circumstances where they would pass the cost onto the passenger who fell ill?

It's hard to get an exact figure, there are fuel prices, the time to taxi, gate charges, medical charges for the removal of the person on the plane, staff costs, and more. I can't guess at most, however from a fuel cost perspective I know for the 747-400 at LHR many airlines use a figure of 1000kg in planning for APU and taxi to takeoff. An A380 would be more, I'd ball park a figure of 1380kg for the airbus based on the MTOW difference (a rough approximation). 1380kg is about 1750 liters, at about .50 GBP per liter that's about 875GBP, or about 1150USD at today's conversions. So the fuel alone would cost somewhere around a grand, I'd imagine you could comfortably double that by the time all is said and done.

As for passing the cost on to the customer I've never heard of that being done for a genuine illness, or even a suspected illness. I don't know if they could if they wanted to, however I cannot imagine them trying it, the reputational fallout would be enormous.


Do flight recorders need more efficient antennas? At a frequency of 37.5 kHz, a quarter wavelength is about 2 km (1.24 miles) long. How does the black box efficiently radiate anything if it is not attached to 2 km of antenna?

The short version of the answer is 37.5KHz is not "radio frequency", it's "audio frequency", so an antenna is not necessary - just a (very loud) speaker, which is basically what the pinger is.

Specifically, the 37.5KHz "pinger" tones used in the Underwater Locator Beacons that are bolted onto Cockpit Voice Recorder / Flight Data Recorder "black boxes" are in the low end of what we call ultrasonic frequencies. Water is generally a good conductor of sound (with at least one notable exception), so it is usually possible for sonar detection equipment to "hear" the pinger tones at reasonable distances (with the interpretation of "reasonable" varying based on the depth of the area being searched - much like in air the sound intensity falls off as distance from the source increases, so if we're listening at the surface of the ocean a pinger in 500 feet of water will have a larger coverage footprint than the same pinger in 5000 feet of water).


Where can I find standard aviation abbreviations for the "unit of measure"? I have been in the aviation industry specialising in Logistics all my life.  There used to be a standard, the ATA 200, for how to abbreviate the unit of measures, such a EA for each, KG for kilograms, HU for hundred, and so on.  I understand that the ATA 200 is now dead and has been replaced by ATA Spec 2000 but I can not find these abbreviations in the Spec 2000.

I suppose that these abbreviations are still valid but where can I find them?

This is defined in ICAO Annex 5 - Units of Measurement to be Used in Air and Ground Operations


Could the GPS be used to aid the autopilot with speed? In the AF447 incident, the autopilot system was offline due to the aircraft's pitot tubes being obstructed by ice particles.

Since the physical condition of the plane in air is cold and risky, the same physical obstruction could happen again and again.

Though the pilot is trained to fly the plane in bad weather, I'm wondering why not just use the GPS to aid the plane's computer? The GPS system could be a secondary source to providing the computer with information for flying, like speed and altitude.

If the GPS could've aided AF447 in preventing the autopilot system from going offline, would the pitot's issue not to initialize the system problem then crash a plane?

The main issue is that we can only get groundspeed from GPS data. Winds aloft aren't known exactly at a particular place, so they could be rather inaccurate -- certainly not enough to leave to the hands of the autopilot. The decision to use GPS data to verify airspeed would lie in the hands of the crew. Anyway, they were too preoccupied to go and check that their ground speed was sort of accurate.

It's not uncommon for winds aloft to reach speeds between 180 and 200 knots. While previous weather reports can be useful, winds can change quite frequently -- the chances of the wind changing is much more than the chances of all the sources of airspeed becoming unreliable.

GPS data is also not 100% accurate, so keep that in mind, especially if only some of the sattelites can be reached.

What the crash came down to was mainly a loss of situational awareness, combined with their lack of experience with actual stall recovery, was the main contributing factor to the accident.


Is there a 180HP conversion available for a 2011 160HP Cessna 172R with a Garmin G1000? I've recently purchased a 2011 172R (160HP) with a G1000 on the sellers promise that I can apply a propeller switch and modified air intake STC to get the derated engine back to 180HP. However, the holder of said STC, AirPlains, shows the kit is only applicable to 1996-2004 172Rs without a G1000. Are there any other manufacturers that can apply this conversion on a later-model 172R, or am I stuck with my 160HP?

Not a solution, but I looked at this a couple years ago.  I have a 2005 172R G1000 in a flight school and students tend to go to the 180hp planes, so I looked into the upgrade.  I did find that Cessna did a STC for a limited, sequential group of Serial Number 172R G1000's.  I believe they might have been done for Embry Riddle, but not positive.  I actually found this out from FAA when I was calling them about a STC or Field upgrade.  Got to the same point as your e-mail string.
Bottom line - it has been done.  It can be done.
When I called Cessna about the details they did tell me the block of SN's it applied to, but after that they would not share anything.
After looking into this again today, talking to Air Planes (they are very nice, and they would love to be able to sell a kit for this!) and remembering my conversations - I think I'll do some digging again.  I'll post if I find anything
Mark - 191CA


What are advantages and disadvantages of flaperons? What are some advantages and disadvantages of flaperons? Do they cause some issues or are they top-notch reliable?

The main downside of flaperons is you can't lower them as much as a regular flap because it has to still do its aileron function when "flaps down".  This means the downgoing flaperon has to be limited to its "lift generation" range when the flaperon mixer is moving it to full down flap plus full down aileron, and can't exploit the "drag generation" range of a normal flap because the resulting adverse yaw is extreme and the roll rate drops.

At the same time, the inboard end of the flaperon contributes marginally to roll control, the roll arm being short.  And you are also limited to the use of differential aileron gearing as an adverse yaw reducer due to the need to keep the flap function reasonably symmetrical.

You end up with a flap with limited extension and therefore limited utility as a flap, and an aileron who's inboard end doesn't do much for roll control.  These issues limit their potential effectiveness to being marginally better than a well sized regular flap and aileron.

The principal advantage is you can get a lower overall stall speed because the early high lift range of flap travel is available across the entire span, and you have a much simplified control system since you are combining aileron and flap on one linkage, with some form of mechanical mixing device in the fuselage.

So, like so many things in aviation, they may seem like an ideal solution and have their place, but the limitations and disadvantages are significant enough that a majority of designers prefer to go with the separate flap/aileron.


Does flight training have any impact on my H1B visa? I'm on an H1B visa, working in the IT industry and I need to know if I take flight training after getting an AFSP clearance from the TSA, does that affect my current visa or IT job?

Having flight training has no effect at all on your visa status. It's actually the other way around: your visa status can determine whether or not you can have flight training. But you can definitely get flight training with an H1B.

And flying has no special impact on your job: how you choose to spend your free time is none of your employer's business. There could be some very specific scenarios where flight training and work intersect, e.g. if your company is in the aviation sector you might be able to get them to pay for training. Or if you're a very senior executive at a very large company then the Board of Directors might have concerns about you spending your time on 'risky' activities. But otherwise, flying has nothing at all to do with your work.


"Report a 3 mile right base" - where is it measured from? At which position should you report to ATC?

It's neither of the above, it's a base leg which will converge onto the final approach leg 3 miles out from the runway threshold. In other words when you turn from base onto final you'll be on a 3 mile final.


Is TBO an airworthiness item or just a recommendation? In browsing Trade-a-Plane for my dream (light GA) ride, I often note the TBO (Time Before Overhaul) for an aircraft in relation to its asking price.  Obviously, the more time left before an overhaul is due, the greater the value of the airplane compared to one that is near or beyond its overhaul due date (hours).

I've always assumed this was an airworthiness item, meaning the aircraft was illegal to fly if it was overdue for a major overhaul.  But now I'm not so sure.  I've seen aircraft advertised as having a relatively recent top overhaul while being overdue for a major overhaul and I've seen a couple advertised that were overdue with no recent work done. Is a GA aircraft legal to fly if it's overdue for maintenance, and what are the exact rules for this?

Unless there is a TBO Airworthyness Directive for that particular engine/airframe/serial number, it is not illegal for GA operations to fly beyond TBO. I can't find an example of a TBO AD but I do remember seeing some.

TBO is a manufacturer recommendation for that particular engine, and does not have any legal enforcement with the FAA. An engine that is well cared-for and has good compressions and clear oil screens (along with other positive inspections) is perfectly fine to fly beyond TBO. There are even stories of people flying 1000 hours or more beyond TBO.

Here are some good reads from around the web:

The Savvy Aviator - Debunking TBO
AOPA Engine Overhauls
Aviation Law - Flying Past TBO, Negligence or Smart Economics

In any case, flying beyond TBO is controversial. Some say that an engine near or over TBO is less likely to fail than one that has been recently taken apart, or at least less likely to fail suddenly.

The key aspect though when buying used aircraft is you don't know how the engine was treated in its run up to TBO. Was it shock cooled? Used for training? Operated out of a grass strip? Poor maintenance? Flown rarely? There are way too many conditions to consider when buying an aircraft near TBO to make the judgement of going beyond it safely or not. The only person who can answer that question is an A&P. That is why TBO is tied so closely to the value of an aircraft. Engine overhauls are expensive, running $25k or more.


Why does pitching moment coefficient decrease with an increase in the angle of attack? Slope of Cm vs alpha graph is negative, why it is so?
The attached graph is from https://en.m.wikipedia.org/wiki/Pitching_moment , this graph is for cambered airfoil, as pitching moment is zero for symmetrical airfoil, as center of pressure remains constant for symmetrical airfoil.

My question is different from suggested question Should the pitching moment be up or down? as my question is talking about pitching moment coffecient which depend on pitching moment, dynamic pressure, and other parameter.

Cm= M/qSc

Where M is pitching moment, q is dynamic pressure, S is the wing area, c is the length of chord of airfoil. But I am not able to get it correctly how Cm decreases with increases in angel of coffecient. If we see pitching moment value then I think it should increases with increase in angle of attack as centre of pressure shifts away from center of gravity with increase in angle of attack.
 (source: wikipedia)

There are several, equivalent explanations:

The airplane has positive static longitudinal stability.
As the angle of attack increases, wing lift grows more slowly relative to tail lift because the coefficient of lift on the wing is larger than that on the tail.
The airplane exhibits a growing nose-down pitching moment as the angle of attack increases in order to return to its trimmed angle of attack of -3°.

The increasing slope of the moment curve at 12° angle of attack indicates stall onset on the inner wing. Here, the flow starts to separate on the rear upper wing root which reduces local circulation and, consequently, the downwash angle at the tail so the lift increase on the tail becomes disproportionally large. This is typical for a straight wing with a moderately high aspect ratio.

The two lines show the behavior at two different Reynolds numbers which seem to belong to a model airplane or a low-speed tunnel measurement. They also show the behavior of a full airplane, not an isolated airfoil.

  If we see pitching moment value then I think it should increases with increase in angle of attack as centre of pressure shifts away from center of gravity with increase in angle of attack.

It does grow, but in the negative direction. This is because a nose-up pitching moment is defined as positive and what we see here is an increasing nose-down pitching moment. The center of pressure of the wing-tail combination is shifting backwards with increasing angle of attack and the pilot would need to trim a negative elevator deflection to bring it back to the center of gravity.

  My question is different from suggested question Should the pitching moment be up or down? as my question is talking about pitching moment coffecient which depend on pitching moment, dynamic pressure, and other parameter.

No, it's not. As the constant Reynolds number indicates, this has been measured at constant speed and it does not matter if it is a coefficient or the "real" moment because both only differ by a constant factor.


What is the purpose of having different ILS categories? What is the difference between CAT I, II and III ILS approaches? I understand that the different categories are used in different weather, but what are the differences, exactly? Do flight crews have to act differently when flying a CAT III approach as opposed to, say, a CAT I approach?

Instrument landing system - ILS - is a very common precision approach system used in airports around the world. An ILS consists of two ground antennas and an airborne received in the aircraft. One of the ground antennas, known as a localiser, transmits a narrow beam along the runway, giving lateral guidance to aircraft approaching the runway. The other antenna, the glide slope, transmits a vertical beam at a specified angle, giving vertical guidance for aircraft approaching. Together, the localiser and glide slope provides aircraft with an exact path to follow toward the runway.

A localiser antenna

The main advantage of ILS is that is allows approaches and landings in poor weather conditions. Pilots do not have to visually see the runway until moments before touchdown, because the ILS can guide the plane down very precisely. However, there are different standards of ILS. These are named CAT I, CAT II and CAT III (CAT III has three additional substandards: CAT IIIa, CAT IIIb and CAT IIIc).

In order to fly a CAT I approach, the cloud based (ceiling) must be no lower than 200 feet and the visibility must be no lower than 550 metres. This is because the pilots must be able to visually identify the runway no later than 200 feet above the ground to be able to land.

For a CAT II approach, the ceiling must not be lower than 100 ft and the visibility not less than 350 metres.

CAT IIIa has a minimum ceiling of 50 ft and a minimum visibility of 200 metres.

CAT IIIb has no minimum ceiling, but there must be at least 50 metres of visibility.

For a CAT IIIc, which has not actually been implemented anywhere, there are no minimum limits for ceiling and visibility.

As you can see, CAT II or III approaches allow pilots to land is extremely bad weather conditions.

So if CAT III is so great, why don't we use it all the time?

In order to use CAT II/III approaches, a number of conditions have to be met:

The ground based ILS equipment must live up to very strict standards, and be very carefully calibrated. As a result, installing and maintaining CAT II/III ILS equipment at an airport is very expensive, so not all airports have it.
In addition to the ILS antennas, there are also strict requirements for other runway equipment such as lighting.
The aircraft must have special equipment that is certified to perform CAT II/III approaches. Again, cost is a significant factor. Equipping aircraft with such fine tuned equipment is very expensive, and if you mostly fly to areas with good weather, it is probably not worth it.
The pilots must be qualified. A normal pilot license is not enough to fly a CAT II/III approach, you need additional training and qualifications to be allowed to perform such approaches.
Air traffic control low visibility procedures result in reduced airport capacity. Because ILS signals are transmitted from antennas on the ground, there are risks of interfering with the ILS signal, resulting in imprecise guidance. The main issue is when an aircraft moves in front of the antennas, partly blocking the signal for other aircraft on approach. Because of this, airports have strict rules for how close aircraft may get to ILS antennas when CAT II/III approaches are in use. Aircraft on the ground must hold further away from the runway than they normally would, to stay clear of so called ILS sensitive areas.

An example of ILS sensitive/critical areas at an airport

Separate runway holding points used when CAT II/III approaches are carried out

We also need additional space between two aircraft approaching the same runway. This is because the first aircraft, once landed and on the runway, can block parts of the localiser signal for the following aircraft. Because of this, the first aircraft must be clear of the runway by the time the second aircraft joins short final. This means we need extra space between aircraft, resulting in a reduced runway capacity.

As you can see, there are a number of factors limiting the use of CAT II/III approaches, which is why they are only used when the weather is so bad that CAT I approaches cannot be carried out.


Why is the 737 so low? Why does the Boeing 737 sit so low on its wheels compared to the Airbus A320?

The 737 was originally designed to be a smaller aircraft serving more regional routes. The original JT8D low-bypass turbofans easily fit under the wings, and allowing the plane to sit lower to the ground made it easier for the plane to operate at smaller airports without support equipment. Passengers could use the built-in air stairs and the cargo bay was low enough that ground workers could climb in to load and unload baggage.

The low height also allowed shorter and lighter landing gear. The 727 had tail-mounted engines which allowed it to sit lower to the ground, so keeping the 737 low helped to keep some commonality there as well. Over time the airplane has grown longer, been fitted with larger diameter engines, and become capable of much longer flights.

The A320 was designed from the start for the larger family of CFM56 high-bypass engines. The main competition was the 737 Classic version, which was powered by similar engines and had grown substantially from the original 737 versions. Both planes ended up in a market where the low height of the 737 didn't have as many advantages.


What is relation between Reynolds number-turbulent flow-flow separation? can some explain conection betwen re number -turbulent flow -and flow separation ,in example at wing in the air and water?
1)does high numbers= turbulent flow mean higher resistance to flow separation and low numbers=laminar flow less resitance to flow separation?
(becuase we use vortex generator to make trubulent flow which delay flow separation on upper wing surface...)
2)why L distance of chord has something with which type of flow will be over wing?
so wide chord will have more turbulent flow but it will delay flow separation?
3)in formula i see density and velocity in numerator,so high density and speed will delay flow separatin compare to lower density and lower speed?

In most commercial aircraft applications the Reynolds number per foot of chord length lies between 1.5 and 2 million per foot. Major operations of takeoff and cruise for turboprop and turbofan airliners fall in this range. Consider the following mean aerodynamic chords:

Turboprop Aircraft = 6.9 ft < cMAC < 10.6 ft
= Reynolds number of 10-20 million

Turbofan Airliners = 13.3 ft < cMAC < 26.9 ft
= Reynolds number of 20-54 million

B747/A380 size Aircraft = 30.6 ft < cMAC < 40.33 ft = Reynolds number of 45-80 million

Note that for highly tapered wings the outboard chords may be considerably smaller than the mean aerodynamic chord and will experience lower Reynolds numbers. During low-speed operations like landing and takeoff, high lift devices such as flaps and slats must be used, because the characteristic lengths of these elements are considerably smaller than the local wing chord they will be operating at lower Reynolds number and therefore more susceptible to flow separation and stalling. The general trend is for cl,max to increase slowly, if at all, with increases in Reynolds number and for that increase to be more substantial for thicker sections. This is understandable because as the Reynolds number increases the boundary layer effects become relatively weaker allowing the flow to remain attached to the airfoil for longer distances along the airfoil surface. The lift of an airfoil depends primarily on keeping the flow attached to the airfoil while friction drag itself weakly influences the lift of an airfoil.
A more detailed explanation is available here


Is it legal for a balloon to land at a US international airport? Assuming the winds directed a hot air balloon near a US international airport, and the pilot has no other choice than landing, say near the runways/taxiways, would they face legal trouble/fines?

Assuming the winds directed a hot air balloon near a US international
  airport

By this I assume you mean a major airport in class B or C (possibly even D) airspace. An "international" airport is simply one that is an airport of entry and maintains customs officials and processing ability and may or may not be a "large/major" airport.

  would they face legal trouble/fines

This depends on how the balloon is equipped. As mentioned in the comments you need a transponder and a 2 way radio to enter certain controlled airspaces (usually those surrounding big airports) and if the balloon is properly equipped it would be allowed to enter and utilize public resources such as airports and landing areas. This advisory circular on ballooning would indicate it is legal to enter all airspaces in a balloon.

There are of course some practical problems here such as the fact that as a balloon you can't really divert around an airspace if you are not granted clearance. If there is no safe landing space and the wind is blowing such that your only option is to enter the class B you may be stuck declaring an emergency, or at least getting back on the radio and explain whats going on. Some FAA officials may consider this poor/dangerous flight planning (since you should have looked at the wind prior to departure) and you may end up in some trouble for that.

From the Balloon Hand Book

  As a general rule, balloons do not freely operate within Class B
  airspace. Equipment requirements are the same as for Class C airspace;
  however, due to air traffic congestion, the balloon pilot requesting
  entry to Class B airspace will likely be denied entry, as ballooning
  operations inside the Class B airspace constitute a potential traffic
  conflict....

But legally you can and the FAA prefers you coordinate before hand,

  Should it become necessary for operational reasons to fly through
  Class B airspace, that flight should be coordinated at least one hour
  prior, as provided for by 14 CFR section
  91.215. It is permissible, and perfectly legal, to operate a balloon under the lateral limits of the Class B airspace.

Same applies for class C,

  Balloon operations in Class C airspace, while technically feasible,
  are usually not advisable


Why do we need to starve the engine before turning it off? This question is for Piper Tomahawk, but I guess applies to any small aircraft.

Why do we need to use the fuel mixture to starve the engine before turning it off with the key? Why can't we just turn it off with the key like in a car?

The reason is the large spinning thing on the front. Residual fuel in the engine has been known to auto-ignite (i.e. combust without a spark), causing the prop to spin, causing serious injuries and deaths. A lean cutoff reduces the risk that someone handling the prop will get maimed or killed.

In a car when you turn the engine off usually it is in park or neutral, so if the engine turns a few revs the car goes nowhere, even if it is in gear the car may lurch but that's generally it. Modern car engines have electronic fuel injection and electric fuel pumps, some modern push-button gasoline engines are stopped by performing a lean cut-off, turning off the injectors before the plugs, most work by just stopping ignition. Diesels don't have spark plugs, they rely on compression alone, so cutting off fuel is the only way to stop them.


Do IFR / VFR weather minimums include ceilings? I'm not really sure how to ask my question, but there are classifications for weather conditions for MVFR, IFR, and LIFR. The FAA also has weather minimums for flying VFR in the different airspaces here.

What is the difference between when we would use one over the other? If I am departing from an airport in class bravo airspace, would I need three statute miles of visibility and remain clear of clouds (second link), or would I also need to adhere to the minimum VFR ceiling of 1000' (first link)?

The FAA does not define "LIFR" or "MVFR"
These are not terms that the FAA makes reference to.  Go look in the FAR/AIM, section 1.2.  You'll find VFR defined, and IFR defined, but not the other two.  Why is that?
Because those are terms used by the National Weather Service, not the FAA
That means that if you want to operate an aircraft, you obey the FAA weather minumums that are provided in your second link.
The information in the first link is about reading weather reports.  As far as the FAA is concerned:

VFR is VFR
MVFR is also VFR
IFR is IFR
LIFR is also IFR

So answer my question
Ok.  If you're departing from an airport, and you will be in class B airspace from the moment your wheels leave the ground, then as long as you have 3 miles visibility and you don't touch a cloud you can fly.
However, it's not likely that you'll be in class B airspace from the moment you leave the ground unless you're departing from the main airport (LAX, ORD, and so on) and if that's the case odds are you're on an instrument flight plan anyway.


How are the frequency pairings for localizer and glideslope decided? How are the frequency pairings for localizer and glideslope decided?

As always the country is responsible for the final decision, so FAA/FCC will decide in the US (but see @Gerry 's comment from an insider below), and I believe so does the MIC in Japan. However this local decision happens after an agreement already found in international bodies. For aviation, the international body is ICAO and ICAO Chicago Convention is the main framework used to ensure international aviation operations are possible, safe and fair:

The localizer / glide path pairing table is part of Annex 10 of the Convention, Volume I, paragraph 3.1.6.

  3.1.6.1 The pairing of the runway localizer and glide path transmitter
  frequencies of an instrument landing system shall be taken from the
  following list in accordance with the provisions of Volume V, Chapter
  4, 4.2.

ICAO decisions in radiocommunication matters are subject to the frequency allotment and protection decisions taken under the auspices of the ITU during the World Radiocommunication Conference (WRC), held every four years. It's ITU which is responsible for the global (civil) use of the frequency spectrum.

The pairing table was built back in 1956! (Fifth Session of the COM
Division, as visible on p. XI)

Pairing was carefully designed so that ILS could work without interferences (harmonic and heterodyne) in certain conditions without being below the radio horizon of each other.

A preferred pairing table is provided for such use:

  3.1.6.1.1 In those regions where the requirements for runway
  localizer and glide path transmitter frequencies of an instrument
  landing system do not justify more than 20 pairs, they shall be
  selected sequentially, as required, from the following list:

Attachment C of Annex 10, (2.6 Deployment of ILS frequencies) provides some details about this selection:

  2.6.5 In general, when international use of ILS systems is confined to the pairings listed in Chapter 3, 3.1.6.1.1, the criteria are such
  that, provided they are met for the localizer element, the glide path
  element is automatically covered. At certain congested locations,
  where it is necessary to make assignments in both the first ten and
  the second ten sequence pairings, it may be necessary to select
  certain pairings out of sequence in order to meet the minimum
  geographical separation in 2.6.6.

  Example: Referring to Chapter 3, 3.1.6.1.1, it will be noted that ILS
  Sequence Number 2 pairs the localizer frequency of 109.9 MHz with
  glide path frequency 333.8 MHz. Sequence Numbers 12 and 19, however,
  although providing wide frequency separation from Sequence Number 2 in
  the case of the localizers, assign frequencies of 334.1 MHz and 333.5
  MHz, respectively, for the glide paths, both being first adjacent
  channels (300 kHz spacing) to the Sequence Number 2 glide path
  channel. If selection of ILS channels is confined to either the first
  ten or the second ten pairings, then the minimum glide path frequency
  separation will be 600 kHz.


Why do flight control cables not slacken during the cold temperatures at cruise? Image source

Flight controls in airliners have long cable lengths in between the cockpit controls and the control surfaces. For instance in the picture above: the pilot operates the yoke, which deflects a steel cable running through the front fuselage and the wings, which then opens a servo valve for the hydraulic actuator moving the aileron.

The cables are made of steel, the aircraft from aluminium.
Thermal expansion of aluminium is twice that of steel.
The aircraft can be standing in the sunshine at 40°C, and half an hour later be at cruise altitude at -50°C.
The difference in thermal contraction would cause a couple of centimetres slack in the cables at cruise, with the pilot then experiencing a control deadband: turn the yoke some degrees and nothing happens. Or the cables would be appropriately tightened for the cruise situation, causing tight cables and high control friction at take-off: try to turn the yoke by pushing hard.

But the forces and (lack of) deadband are identical at take-off and cruise. How is this possible?

Edit

Yes there is a Cable Slack Takeup drawn in the picture, but the working of it is a little bit more complicated than it seems, I've added an answer below.

If you look closely at your drawing you will see the cables are not really simple pull cables but really act like steel belts. That is, there is a pulley at each end and when you actuate the control the cable is rotated around the system in a loop.

The end pulleys and or idler pulleys are spring loaded to maintain a predefined tension on the cables and should take up any slack in the system caused by operation, temperature, or aging of the cable.

If the top left pulley above is the yoke in the cockpit, and you turn it, the top cable rotates around the loop, like a belt, and turns the actuator bottom right. Note that also means when you turn the yoke, it does not pull significantly on the springs, the cable just rotates.

Interestingly, the Concorde had to handle both contraction and expansion. When cruising at supersonic speeds the aircraft stretched up to about ten inches.

ADDITION:

For non looped cables, and occasionally for looped cables where the cable needs to traverse between objects that move relative to one another, cable housings are also used.

You are probably familiar with these on bicycles. The trick to them is the ends of the housing are attached, or anchored, to the frame somewhere. The housing is flexible so can be arranged in a loop or s-shape that can be extended or compressed as the fixed end points move relative to one another.

The control cable on the other hand always has a fixed length within the housing despite the change in the end positions.

The issue with these though is friction is high compared to pulleys, the inside of the housing can become contaminated with dirt and debris, and it is impossible to inspect the cable within the housing.

As with the looped system, a spring is still required, usually at the business end, to pull back on the cable. In the single cable case, actuating the cable from the control end also involves pulling on the spring.


What is wind "CALM"? What is the exact criteria for wind speed to be considered "CALM".
Different countries might have different values and i do not mind getting answers from those. But an answer from an ICAO annex would be prefered.

Wind "CALM" means a wind speed less than 0.5m/s (1 kt) according to ICAO Annex 3, Appendix 3:

4.1.5.2    In local routine and special reports and in METAR and SPECI:
...
d)  when a wind speed of less than 0.5m/s (1 kt) is reported, it shall be indicated as calm;

Before Amendment 75 of ICAO Annex 3 this read:

d)  when a wind speed of less than 2 km/h (1 kt) is reported, it shall be indicated as calm;

See also Annex 3:

Appendix 5, recommendation 1.2.1
Attachment C

The FAA, in their Order JO 7110.65X (Air Traffic Control), defines a limit of 3 knots.

2−6−3.  REPORTING WEATHER CONDITIONS
....
b. Describe  the  wind  as  calm  when  the  wind velocity is less than three knots.


Does the profit from selling duty-free items on board justify flying the carts? Some airlines have duty-free carts (2) with items for sale on board. It seems that only a few items get sold each flight. Does the profit from selling a few items justify the cost of having this extra weight/volume on board?

Okay so there are a few points to this

Lots of different things are sold - perfumes etc, gifts, "in flight" convenience items such as headphones, plus food/drink. That means lots of sales opportunities.
"Luxury" items are high margin anyway: perfume is a big profit item even with duty. By knocking the duty off and making it "feel" cheap: they can potentially sell it for more than the RRP (minus duty), and thus make more profit than a regular shop, despite selling it for less (gross) to the passenger
You only need to carry one or two bottles of each perfume, as you won't sell a ton of them. They don't take up much space: perfumes etc are only sold to one or two people per flight, they're not the reason the cart is there... they're just an extra. Similarly headphones: you won't sell many, but you'll definitely sell some when somebody realises they left theirs at home: I bought a set of cheapo £2 headphones for £8 at a train station a few days ago because wasting £6 on a set of crappy headphones was better than the idea of a 5 hour train without any music. People will pay for convenience, and carrying one or two of these items doesn't take up much space at all.
Food/drink are the reason the cart is there. When on a flight for 1-4 hours, some people are going to be hungry/thirsty... and you have anywhere from 100 to 850 passengers. The markup on these items can be huge - £6 for a small can of beer, for example, which would cost <£0.30 from a wholesaler. You don't need to sell a huge number of pies/burgers/cans of beer/£3 cans of coke to make a decent profit. With the air-con on a plane, I find I get very thirsty and will go through a 500ml drink roughly once an hour. Similarly on a 4+ hour flight I'll tend to want a snack or meal at some stage. I'm certainly not alone in this.
Seats on these planes aren't always that expensive. On my last flight it was less than £40 for my one-way ticket on a 2 hour flight: if the trolley takes up roughly one seat's worth of space, they only need £40 in profit (or perhaps 20 cans of coke in 2 hours, at £2.50 per can) to be worth carrying a trolley. Similarly the longer the flight, the more likely people are going to be thirsty. With 190 people on board, even if only one in 10 buys a drink, that trolley has made more profit than my seat. Even considering an 8 hour transatlantic flight with 400 people on board a 767, that still only needs half the flight to buy one drink (which they probably will in 8 hours...) to make up for my £500 seat
It's expected, and is a service to some extent... people would start getting quite annoyed if they found they couldn't get a drink on the flight, even if said drink is expensive. People tend to expect to be able to get a drink, if nothing else, but usually food too. Notice that most non-local (2hr+) trains (and many local ones!) tend to have a trolley service or, more and more often, a shop on board? Even if it didn't make a profit at all, it may be worthwhile for customer happiness. This is a bit of a moot point, though, as trolley services are definitely profitable (trust me, if it wasn't, Ryanair wouldn't provide one)
Your cabin crew are there anyway for safety reasons, why not put them to use as sales people? Otherwise they're just sitting relatively idle for much of the flight. Not that I wish to discredit the work and important role that cabin crew play: they're far more important than most of us realise... but the fact remains that for much of the middle of the flight, on a routine flight and unless something goes wrong, they don't actually necessarily have that much to do considering the number carried.
The cart may be there anyway! On longer flights with meal services, the airline will already have the cart available for set times. Why not put a few bottles of perfume in the bottom for an extra profit?
Capacity limits. Aircraft aren't always limited purely by space or weight: they're also only certified to carry a set number of passengers due to things like the number of emergency exits. It may be that you can't actually carry more passengers even if you want to
The trolley can fit into space a person wouldn't. A trolley isn't person shaped, and can be stored somewhere near the back of the plane (eg where the fuselage tapers) that you couldn't fit another seat even if you wanted to. Why not use the space for something useful?


How much thrust is produced on an A320 at takeoff and during cruise? Where can you get the drag polar of the A320 at cruise and TakeOff?

A320 family's engine of choice is the CFM56-5B. Now there are multiple variants of this engine. But on average the takeoff thrust for the models -5B1/B2 and -5B3 are between 30,000 to 33,000 lbf.
Other variants have a lower thrust rating between 21,000 - 24,000 lbf and 27,000 lbf.
The cruise thrust is lower of course since the aircraft has already taken off and now in upper atmosphere, with low drag and running at its best efficiency. Cruise thrust is about 4,700 to 5,800 lbf, although these are the maximum rated cruise thrust, so the engine thrust is always lower than that.
Say if the engine is rated for 5,800 lbs the actual thrust would be roughly about 5,300 - 5,500 lbs, maybe even lower.
Feel free to correct me if I'm wrong!
Reference
Cfm technical data


Retreating Blade Stall, P-Factor, and Gyroscopic Precession Most airplane pilots are aware of P-Factor in propeller driven aircraft and that it tends create a torque in phase with the lift imbalance over the blade disc, resulting in a yaw moment about the airplane's CG.  Curiously I'm surprised that it does not act to cause a pitch up moment as one would assume that to be the result of gyroscopic precession acting 90° out of phase with the application of the reaction forces.  This occurs that way with the retreating blade lift imbalance in helicopters leading up to a retreating blade stall if the airspeed is high enough through the rotor disc.  Retreating blade lift imbalance is a result of lower relative wind speeds passing over the retreating blade than the advancing blade.  But instead of resulting in a rolling moment towards the retreating side of the rotor disc, it results in a nose up pitching moment due to gyroscopic precession.  Which raises the question:  Why is gyroscopic precession present in a helicopter rotor due to lift asymmetry over the disc but absent in a propeller?  Gyroscopic precession causes phase shifts in resultant forces on propellers when the propeller disc is pitched or yawed in aerobatics or tailwheel aircraft operations, but P-Factor does not cause this as well?

Related, especially footnote 2: What is the influence of yaw on P-factor?
The answer to your specific question is that when you are talking about the effect of P-factor on a prop, you aren't talking about actually moving the prop disk in pitch or yaw.  You are applying a rudder force as needed to prevent that movement, so there's no gyroscopic effect to consider.
Whenever the prop disk is actually pitching or yawing-- regardless of whether the cause of the movement is P-factor or something else-- then you have to consider the gyroscopic effect, which makes the force act as if it is applied to the blade disk at a point 90 degrees further along in the direction of rotation.
In helicopters, the individual blades can be moving in a way that creates the same gyroscopic effect as pitching or rolling the entire blade disk, even when the blade disk as a whole is not pitching or rolling.  This is due to the way the blade is hinged.  This wouldn't happen with a rigid rotor disk.  So that's why-- at least with a hinged blade system-- the P-factor effect acts 90 degrees further along in the direction of rotation from the point where it is actually applied, even when the body of the helicopter isn't pitching or rolling, and even when the circular path traced by any given rotor blade tip is remaining constant over time.


Could placing a fan jet further in front and closer to the wing create undesired pitch motion from its exhaust? So far in discussion of the Max 8 people have mentioned CFM LEAP cowling size and 737 tail design as possible factors in the need to develop a MCAS system to control tendency to pitch up at high AOA.

Since the tail has much more leverage than the wing (much longer lever arm), is it possible that the high bypass exhaust blast is affecting the tail at high AOA by pulling more downwash onto it by Coanda effect? At first I though the may be affecting the back of the wing, but then realized the 737 Max 8 is very close coupled (tail close to wing) and may be susceptible to stronger downwash effects.

I wanted to be sure before suggesting a larger horizontal stabilizer or considering lengthening the fuselage.  Is it possible?  Has it ever been seen before?

I seriously doubt the engine exhaust hits the horizontal stabilizer except possibly when fully stalled. Otherwise, the wing downwash follows approximately the extended chordline and takes the engine with it safely below the horizontal stabilizer, which is significantly higher than the wing.

The one thing I've seen mentioned was change in the centre of gravity generally decreasing the longitudinal stability—which is most pronounced near stall, because the centre of pressure moves slightly forward with increasing angle of attack (until stall, where it moves abruptly aft all the way to midchord).

The other possible effect is that the cowling generates vortices—possibly with help of vortex generators needed to avoid stagnation in the gap between the wing and the engine—that go over the wing at high angle of attack and increase the suction peak near the leading edge and making the forward shift of centre of pressure more pronounced.

The only point regarding tail I've read is that it is not large enough to allow moving the allowable range of centre of gravity forward to improve the stability ‘naturally’.


Is the highly optimized high subsonic cruise airfoil less safe for higher AOA flight? An examination of the 737 airliner wing profile and that of a blue whale leads to the question of whether the high Reynolds number subsonic cruising airfoil and the low speed, high AOA airfoil are compatible.

Supercritical airfoils are designed for efficient cruising flight, relying on bottom lift and minimizing acceleration of air over the top to avoid creating drag producing transsonic shock waves.

The blue whale is designed to cruise efficiently over thousands of miles, through a viscous, non-compressible fluid (water). Their shape includes a rounded, sloping lower fore and a flatter top, very similar to a supercritical airfoil!  Applying thrust to this shape would provide "lift" simply by directing water "down" (or pushing the whale "up").

Reynolds Number = Velocity×Chord/Kinematic Viscosity

Reynolds Number 20th century Type VII U Boat (submarine):  speed: 4 meters/second
                  chord length:  70 meters. Kinematic viscosity (sea water): 1.04 × 10e-6

      4 × 70/1.04 × 10e-6 = 269 million

Reynolds number Blue Whale:  speed: 5 meters/second, chord length: 30 meters
                       Kinematic viscosity (sea water):  1.04 × 10e-6

      5 × 30/1.04 × 10e-6  = 144 million

Reynolds number Supercritical Airfoil (approximate):  speed 300 meters/second,
Chord length:   4 meters    Kinematic Viscosity (air):    1.46 × 10e-5

    300 × 4/1.46 × 10e-5 =     82 million

Reynolds Number Cessna 172 Airfoil :  speed 30 meters/second
Chord length:  2 meters        Kinematic Viscosity (air): 1.46 × 10e-5

   30 × 2/1.46 × 10e-5 =         4 million

Reynolds Number Albatross (bird) airfoil: speed 10 meters/second
Chord length: 0.2 meters       Kinematic Viscosity (air): 1.46 × 10e-5

  10 x 0.2/1.46 x 10e-5 =        140,000

We can see the supercritical wing is more in the range of the blue whale!

When pitched up to a higher AOA, this shape would be a very poor wing in the classic Bernoulli sense, as the significantly contributing top lift would be lost at a lower AOA than a top rounded airfoil, behaving much more like a flat plate (see polars).

There are accounts of airline pilots extending their slats at higher speeds for better performance (and getting into trouble for doing so).

So, for modern designers, would it not be important to consider a more robust low speed system that could be left on (in varying degrees) at higher speeds (V maximum flaps and slats extended airspeed) to help avoid stalling?

This could create a wider safety margin for the climb-out and landing phases of flight. Hard earned efficiency gains in cruising flight (from the supercritical wing) would be kept, when they are fully retracted, but only at safe speed and AOA conditions (for example, above 10,000 feet or at cruising flight level).

The short answer, whales aside, is yes.  Such wings, called "supercritical"  because the critical Mach# is elevated, tend to suffer from leading edge stall that makes the aircraft's natural stall behaviour dangerous.  These airplanes typically require stick pushers (as @Peter Kampf mentions in this answer, the 737, even the later ones that got redesigned wings, doesn't really have a "pure" supercritical wing and I believe that's why they don't require stick pushers).

The Challenger business jets and the CRJ line of Regionals all have supercritical wing  profiles.  The profile promotes a pre-stall flow separation and immediate reattachment at the LE, called a Laminar Bubble, which would form at some AOA prior to stall.

This had an effect similar to having a stall strip or ice shape, with the bubble forming a trip wire so to speak.  It causes the wing to stall at the leading edge instead of advancing from the trailing edge, which meant the entire wing just let go all at once with no buffet or other warning, with a tendency to progress right into a deep stall.  The Challengers and RJs have stick pushers because of this; they must never be allowed to stall naturally, because it is probably going to be unrecoverable.  Bombardier did a lot of testing with various aerodynamic band-aids like vortex generators and vortilons and such, and they didn't help.

The CRJs larger than the 200 have LE slats and are just about unstallable slats out, but they also have the same LE stall slats retracted and still require stick pushers.  Flying with slats extended at high speed is out of the question. The drag they produce is massive, as you discover when you first land one after flying a hard leading edge CRJ200. Incorporating a fixed slot in the leading edge could be done but they are pretty draggy and would negate the benefit you're trying for in the first place, and I'm sure there are other mach related issues with a fixed slot.