How does one develop artificial stabilization for aircraft uncontrollable without it? In an answer about the F-117 Peter Kämpf writes

  F-117 (...) is unstable and uncontrollable without artificial stabilization.

I wonder how such an aircraft comes to existence, specifically how one develops the program without which the aircraft can't be flown.

How one establishes (presumably in a wind tunnel using a scaled model) that the plane will not be stable or controllable, but it can be helped by an artificial stabilization system well enough to be flyable? How does one develop the system itself? How it is determined that the stabilization system has became mature and reliable enough to permit test flights?

Sorry about the question being somewhat blurred, I realize I don't even know what exactly to ask about. Any suggestions or reasonable edits welcome (in worst case I can roll the edit back).

Most modern high agility ariplanes (such as fighter jets) are unstable in the sense that (among other things) their center of gravity (nearly) coinsides with their center of lift (i.e. they are wildly out of balance). The benefits include the ability of the airplane to react to control inputs nearly instantaneously. That the design will be unstable can be determined at a very early stage in the process (during the 'sketch' phase, really). Given a fast enough control system, nearly anything can be controlled (and modeled) so to answer your second question, to design a control system, all one needs to know is how complicated the system is (the order of the system) and how fast it should be. The first question can be answered based on very general principles (most aircraft can be modeled by a fourth order system). The second question is also easy to estimate (100 Hz is usually enough, don't ask me how I know this).

Of course, tricky regimes (supersonic, transonic, supercritical angle of attack, etc.) might require some time in the tunnel but these are to determine some parameters of the general model that has been decided on at the design stage.

That the system works is usually not in question before the plane is tested. Whether a human pilot will find it controllable is another matter. This is why the design of a modern high agility aircraft is done in parallel with a simulator design to see how the pilot would react. The simulator of course, is based on the same model that the actual design is (and is constantly modified as more experimental data becomes known).

Finally, even if the airplane is well designed, and everything works, the certification stage is where most accidents happen. For example, to test the single engine performance of a twin engine airplane the tests must be carried out at near sea level pressure, quite low to the ground. Not much time to react when the plane gets too slow.

A good book to read about these issues is Aircraft design: a conceptual approach by D. Raymer. You will quickly discover that one can practically calculate the look of the plane (controls location, high-wing, low-wing, where the engines go, etc.) based on its mission.


Is an ADS-B IN able to receive signal from ADS-B OUT of the same aircraft? Suppose I have an ADS-B IN on my aircraft and I am interested in receiving also data sent from ADS-B OUT of my own aircraft. Is it technically possible or are there any issues due to the relative position between transmitter and receiver or anything else?

On small aircraft many operators use portable ADS-B IN receivers which typically do not have the ability to sense suppression signals from their own transponder/UAT transmissions and so they will pick up their own signal.

Most installed systems have that suppression functionality so this won't happen.

The suppression is simply a wire between the two units that if the transmitter is transmitting, it goes high, which tells the receiver to ignore whatever it is receiving at the moment.

So if you want to "receive" yourself, simply snip the suppression wire if you have one...


Is "Tally-Ho" used in ATC radio communication, specifically in the UK? This question Are "Tally-ho" and "no joy" acceptable ATC terms for civil operations? concerns the use of the phrase "Tally Ho" to mean "target in sight" in civilian air-traffic-control communication.

It was largely answered (and somewhat asked) in relation to the US/FAA, where it is not acceptable but sometime heard. However "Tally Ho" is/was a British phrase, and my question asks the same specifically of British/NATS/CAA communication. Again, I'm interested as to whether it is actually heard, and whether or not it's generally considered acceptable.

I am aware that some efforts are made to keep English usage international, though most feeds/transcript I've heard from the US have an awful lot of regional idioms and phrasing in them, so I'm not sure the extent to which it is adhered to.

(For people outside the UK, "Tally Ho" is generally considered archaic and comically mannered in general life these days, even within the UK, which is why I'm particularly interested as to whether its usage survives here in aviation).

I have never heard "tally ho" used in civilian aviation and it not a recognised phrase so should not be used.  A civilian ATCO would not think positively about anyone using that phrase.

It used to be used in military comms in combat.  Its usage arose during WW1 when the Royal Flying Corps (and later the Royal Air Force) drew its crews mostly from the "officer classes" who, in general, were also fox hunters or supporters of the same.

"Tally ho" is the cry shouted out by a huntsman when the fox is spotted and the hunt is on.

I served in the RAF from 1976 to 1986. Then as a civilian in the military until 1993 and in that time never heard the phrase.  I suspect that it has died out since it is not standard phraseology and perpetuates stereotypes which have no place in modern, professional organisations.

A non-standard, but common phrase is a simple "visual", but the correct phrase in both civilian and military use is "traffic in sight".


What is the typical voltage and frequency of the electrical generator in an airliner? How many volts does the typical airliner CSD (Constant Speed Drive) produce?  I've read the CSD's output is typically 3-phase 115 VAC @ 400 Hz. Is that correct?

I've also read the typical airliner generator puts out "in-phase 28 VAC". Is that correct?

Does the CSD provide more voltage than the generator needs and the generator reduces the voltage via a transformer?

A constant speed drive (CSD) converts the rotation from the accessory gearbox to a predetermined constant rotation rate.  The rate of the accessory gearbox is directly related to the high pressure spool (N2) rate and changes with power adjustments.

The CSD will ensure its rotation rate is constant.  The Challenger 604 CSD rotates at 12,000 RPM and outputs 115 VAC @ 400 hertz.  Its max output is something like 30 kVA or kilovolts-amps.  Since we are talking AC power, it is not appropriate to call that watts.  The AC power is converted to 28 VDC by the way of transformers called transformer rectifier units (TRU).


Why do many light aircraft have a trim tab on one side of the elevator only? Most light GA aircraft, like this C172, have pitch trim tabs on one side of the elevator only. Why not on both? Doesn't the asymmetry add inefficiency in the form of a slight rolling moment?

A small propeller driven plane like that will have a bit of adverse yaw, rolling to the left, because of propeller effects (P-Factor, slipstream, torque, etc)

The trim tab's effect is minimal because it is very small and is close to the centerline of the plane.  Any drag it creates causes a slight roll and yaw to the right, specifically to counteract the propeller effects.

That keeps the overall effect almost un-detectable.


Is Rutan's method of foam core construction obsolete? Is Rutan's method of foam core construction obsolete, or is it used on other fibreglass designs as well?

I was a regular at Oshkosh during Rutan's heyday in the 70s, and had a chum that was building a Dragonfly, which used the same construction method.  Rutan's mold-less process is only suitable to one-off projects, like prototype parts, or amateur built airplanes.  It's a sub-optimal process from an engineering perspective because it is very time consuming to produce an ascetically decent component, and tends to be somewhat heavier than molded sandwich parts.
Rutan advertised extreme speed at creating a structure compared to metal or wood/fabric.  It was much faster to create a wing or fuselage... but then all the time saving was eaten up in finishing.  The method leaves a rough and somewhat uneven outer surface (due to the hot wire cutting process, where the wire bends under stress as it's cutting, producing a dished result when cutting a curved profile) that requires hours, and hours, and hours of filling and sanding (using looooong sanding blocks) with a lightweight filler made from epoxy and micro-spheres (hollow glass beads) to get anything close to the finish you get with a mold.
The hot wired foam cores, done by hand, always have surface continuity imperfections that need lots of filler, plus the outer surface of the epoxy tends to be rough.  You have to sand very carefully to avoid going down into the glass cloth.  The original layup method didn't use vacuum bagging and produced a heavy part with excess epoxy resin.  Later on builders started to vacuum bag the layups and use peel-ply fabric (polyester fabric similar to aircraft covering fabric, which epoxy won't stick to) with an absorbent material to soak up excess epoxy and provide a better outer surface (which ended up with about the smoothness of the peel-ply fabric, the outer surface being an impression of the peel-ply weave.
My chum sold the Dragonfly project before finishing it.  He just didn't have the time to do all the finishing work.
If you are going to build your own airplane from glass and foam, that method is pretty much the only option.  Beside Rutan's designs, the EZ's and Quickies, and the Viking Dragonfly, I can't think of any other designs off hand.  There was another scratch built homebuilt that used sheets of polyurethane foam bent to the skin curvature on a form, then layups done on the skin, that produced something closer to a modern fibreglass sandwich system without using molds, but I can't recall the name.
Here's is a complete guide to the Rutan moldless method.


N registration in Europe Do someone knows a way how to arrange an American(N) registration of aircraft located in Europe? I mean there should be an agency that you can entrust with carrying out an American registration of an aircraft that is in Europe. I am looking for this office.

The FAA handles all US aircraft registration:  FAA Aircraft Registration  Note that there are ownership requirements related to US citizenship etc.  See FAR 47.3


How is rotor pitch best managed in helicopter autorotation? In a recent stack exchange question writers were comparing autorotation to an aircraft engine out rotation prop when something rather startling occured to me:

In an engine out the airflow will continue to rotate the prop on the same direction as with power.  When helicopter loses power, WRT to rotor, it falls "backwards", creating force AGAINST the powered direction of rotation.

The thought is to go negative pitch and allow the rotor to build up angular momentum in the same direction as powered rotation, then switch to positive pitch to use that energy to flare and land.

Is this how it is done?

No, that is not how it is done: minimal collective blade pitch is not negative. The procedure is to lower collective immediately to the full down stop to minimise blade drag, upon which the helicopter will start to descend. The air upflow tilts the local lift vector forwards. Indeed, just like in a glider.

The bit that is not intuitively clear for fixed wingers is the reference frame of the airflow, which is the local blade velocity.

The inner bit of the rotor blade has a lower rotational speed than the tip. The up flow has a relatively larger influence on the lift vector tilt here, and  upflow through the rotor drives the inner part of the rotor blade forward, while the outer part still experiences decelerating torques. From Leishman:


When approaching stall, why does the center of pressure move back? In an answer to a previous question, Peter Kämpf stated:

  Before the lift coefficient of a stalling airplane peaks, the flow over part of the wing will start to separate. Ideally it will do so near the trailing edge of the wing root, and the separation will slowly progress forward and outward as angle of attack increases. This separation will shift the local center of pressure back, such that the aircraft will experience an increasing nose-down moment as it approaches stall.

If separation starts at the back and progresses forward it seems to me that center of pressure ought to move forward. Why does it move back?

The center of pressure is generally not the best way to characterize the aerodynamic forces on an aircraft since it varies with AoA.  That is why the neutral point or aerodynamic center is used instead.

Having said that the center of pressure is the location of average pressure force acting on the wing or aircraft.  Mathematically it is found by taking the sum of integral of the magnitudes of pressure and shear stress acting on the surface and multiplying it by a distance (for an airfoil from the LE) on the upper and lower surfaces resulting in a moment at the LE, or reference point.  This is then divided by the sum of the integrals of the pressure and shear stresses acting on the upper and lower surfaces.  My explanation does not do it justice see http://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf

It can be somewhat complicated to calculate for an entire aircraft but in general for an airfoil it does move forward up until the point of stall or critical point after which it then moves aft.  See https://www.reddit.com/r/aviation/comments/2kibfo/why_does_the_centre_of_pressure_move_rearward/ for a good simple explanation of this.

The movement of center of pressure is highly dependent on the configuration of the aircraft (airfoils are much simpler) and unless you are certain of the specifics of the configuration generalizing the center of pressure movement is inaccurate...


Where to find approach plates for European aerodromes? Where do I find the approach plates for European aerodromes? Is there a website intended for pilots updated and certified from the authority?

Eurocontrol has the charts for all participating countries. You'll need to register for a free account.

https://eadbasic.ead-it.com/cms-eadbasic/opencms/en/login/ead-basic/

"The EAD Basic solution is specifically targeted at:

private pilots and general aviation;
members of the public searching for aeronautical information;
training schools;
everyone who has an interest in aviation and aeronautical information."


Do pilot uniforms always include a shirt with stripes? On airliners, pilot's uniform usually include a shirt with stripes on shoulder. I cannot find examples of pilot without this kind of uniform while in service (either in the airport before departure/after landing or on the flight deck). It seems that during long break they can wear T shirts but use their formal uniform when working.

I fail to see why they should wear such a uniform (wearing a shirt with stripes don't make you a better pilot).

For flight attendants, the uniform reflects the image the company wants to convey. Flight attendant uniforms are thus diverse and different for different airlines. But a pilot's uniform is more uniform from one airline to another.

Do airline uniforms for pilots, which do not use shirts with stripes on the shoulder, exist?

Uniforms are to make it easy to identify members of a group who have a separate function or authority from a larger more random group.  The stripes are to easy to identify the chain of command within this group at a glance.

Most of the time it's all just costume peagentry.

When the shmoozle hits the fan however, being able to identify who's in charge and next in charge and so on, without having to, you know, ask around, becomes a big deal.  That's why the bars.

You won't find a first or second tier carrier in a Western airline without a uniform shirt and rank bars.  I can't find any specific regulations that require that, but it's basically an old convention carried over from the marine world (it's why pilot uniform suits look like naval uniforms), where many of aviation's traditions originate.


Are helicopters reinforced asymmetrically? Helicopters are under a constant torque due to the action-and-reaction forces caused by the rotation of the rotor(s). In the tail rotor and tandem rotors configuration, the airframe is experiencing tension on one side and compression on the other. A weak airframe would be twisted by the strong torque produced by the rotors.

Are opposite sides of a helicopter airframe reinforced differently do due with the difference in stress?

As far as i know, the helicopter airframes are not reinforced asymmetrically due to this reason. Note that the helicopter has to yaw in both ways- so there is no point in reinforcing only one side.

The helicopter airframes are not weak- they are designed to withstand the full envelope of expected loads. Also, it is easier to design and manufacture a symmetric helicopter structure.

That said, I've personally seen at least one case where the torque has affected one side of the helicopter more than the other locally (especially in tail fin area), though the effects were not considered serious enough to warrant a design change.


What RA will be generated if intruder traffic is at our level in TCAS II? If the other traffic is at the same level, and neither climbing or descending. Will the RA say "CLIMB" or will it say "DESCEND"? How will it be decided and what are the possibilities?
Assume:

Mode S is not available
Only one aircraft has TCAS

Will it be random?

The TCAS algorithms are specified in RTCA/DO-185B (not freely available).
In case of an encounter at same altitude with a non TCAS equipped aircraft, assuming that neither climb nor descent are inhibited, the TCAS will produce a descent advisory (part 2, sec. 2.2.4.1.2.1.5 - sense selection).
This is a consequence of the somewhat subtle signs used in the specification (<= for descend, > for climb).

A non mode-S intruder will broadcast altitude with a 100ft resolution, and a TCAS equipped aircraft will normally use barometric altitude with 25 ft resolution, so the two aircraft do not need to be at exactly the same altitude - in the worst case, a difference of up to 62 ft will be interpreted as "same altitude".

There are also provisions for climb and/or descent inhibits. In particular descend RAs are inhibited below 1000ft radio altitude.
Climb RAs are inhibited when the aircraft performance are not compatible with a 1500 fpm climb (for example, one engine out). The exact mechanism through which this information is provided to the TCAS computer is aircraft dependent.
In your case of same-altitude encounter, if descend RAs are inhibited, a climb RA will be generated.

EDIT
As requested, more clarification.
A simplified description of RTCA-DO185B initial sense selection follows. This description assumes that the intruder is not TCAS equipped, and that the firmness (confidence in intruder altitude and position) is sufficiently high.
First, choose the direction (climb or descent) that will produce the highest separation at CPA (see below).
If equal, select descent (see 3.52 Noncrossing_Biased_Climb and 3.53 Noncrossing_Biased_Descent).
Denote this direction with D

If a RA in the D direction does not result in a crossing (see below), then issue an RA in the D direction.
Otherwise, if the opposite direction still provides at least ALIM separation, then issue an RA in the direction opposite to D.
Otherwise, issue an RA in the D direction.

Separation: The TCAS algorithm models a RA response as follows:

5 seconds of reaction time (pilot maintains current airspeed and rate-of-climb)
A segment with a constant acceleration of 8 ft/s^2 in the direction of the RA
A final segment at constant rate of climb (usually + or - 1500 fpm).

To select the sense, the TCAS algorithm considers:

Distance from intruder
Time derivative of distance from intruder (closure rate)
Time derivative of the closure rate
Ownship altitude and rate of climb
Intruder altitude and rate of climb

With these 5 parameters, the vertical separation at the CPA (Closest Point of Approach) can be computed.
In this scenario, the relative altitude is zero, and the rate of climb of both intruder and ownship are 0. Due to the symmetry of the model, the separation will be the same for both climb and descent.

Crossing
Crossing is defined as either:

A climb RA, with ownship at least 100 ft below the intruder
A descent RA, with ownship at least 100 ft above the intruder

In this scenario, ownship and intruder altitude are equal, so neither direction will result in a crossing RA.


Flare: low, slow, and uncoordinated? I'd like to rephrase the question:

Often I forward-slip on final right right rudder.  During this time, I use the ailerons to keep the aircraft headed down the center-line; the aircraft's bank varies slightly from left wing down, to right wing down.

At some point during this procedure, I have right rudder in and a slight right bank.  I believe this is a skid; Is this something I should worry about?  I would very much like to avoid skidding at all times, especially on final!

Previous question, for reference:

Assuming there is a crosswind and the pilot executes a crabbed approach, during the flare the pilot is going to have to use the rudders to avoid a side-load.   My understanding is that this will result in an uncoordinated flying condition and is prone to stall-spin.

However, this (the flare) does not seem to be much of an issue among pilots.  What am I missing?  How is this any different than a skid?

Crosswind landings are always uncoordinated. I'm not familiar with big jets, but I believe they simply kick the rudder to align the nose with the runway and let the momentum carry it in the right direction until it touches down, but I might be mistaken here.

In a smaller aircraft (like a piston single) you're taught to drop a wing into the wind and effectively land in a side slip.

This is all perfectly safe, the dangerous situation you're talking about is a skidding turn, where you apply too much bottom rudder, letting the 'inner' wing slow down to the point of stalling. This only applies to an actual turn however, if you're flying straight, coordinated or not, there's still more or less the same airflow over the wings. You could however experience other aerodynamic effects where fuselage and airfoils effectively perturb the air for other airfoils and might cause a wing drop or pitch change, but it depends on the properties of the aircraft.


Why don’t aircraft undergo contaminated-runway landing-distance and rejected-takeoff certification testing? All aircraft have to undergo testing to determine how much runway they need in order to safely land and come to a stop, or to safely reject a takeoff.

They do not, however, have to undergo this testing on anything other than a perfectly clean, dry runway.  Instead, to get landing/RTO distances for contaminated runways, they multiply the dry-runway distance by some arbitrarily-determined factor (precisely which factor depends on what the runway is contaminated with - water, slush, ice, wet snow, dry snow, sand, Teflon, Jet A-1, ping-pong balls, whatevs), and then everyone hopes that this actually gives them enough of a margin for landing on contaminated runways.

Why not simply run landing-distance and rejected-takeoff certification tests on contaminated runways, as well as on dry runways, and directly measure how far it takes to stop on a wet or icy or whatever runway?

Baseline performance data was developed some years ago by an FAA test program called the Joint Winter Runway Friction Program to develop mathematical models of the effects of the different types of surface contamination.  Manufacturers apply the models to develop their own aircraft's performance correction factors for various contaminated runway conditions.

Otherwise, as Ron comments, the manufacturers would have a massive test burden to develop customized data for a huge range of contaminants, and would have to go find the test conditions somewhere for each, adding eleventy jillion dollars to the typical development program.  The mathematical models, along with the safety factors built into the calculations, do the job just as well.

This Advisory Circular talks about it.


In a slip or skid, how does one wing have a higher angle of attack and more lift? I've discussed this with my instructor numerous times, and I have the concept memorized, but from an aerodynamic point of view, I can't see how the angle of relative wind to chord line is different per wing (assuming a Cessna 172, if that makes much of a difference).

The wings are essentially a straight line over the top of the plane, and relative wind is a single vector - to me, it looks like they should have identical AoAs.

If you know of a graphic or video, I think it would help explain best.

You are mostly correct, but the fuselage distorts the airflow in sideslip such that the windward wing sees a higher angle of attack near the root, and the leeward wing a lower angle of attack. Please see the drawing below for an explanation.

The blue lines show the sideward component of air flow and the red arrows the resulting rolling moment.

This is why low wing airplanes need more dihedral, and high wing planes like your 172 almost none. Wing sweep adds more directional stability, and then you even find aircraft with pronounced anhedral, like the Harrier jump jet or the F-104 Starfighter. See this answer for more detail on this.


Why does JBU1825 from Ft. Lauderdale to Lima show a large zig-zag in the route? Why oh why is this non-stop flight flying such a zig-zag? (screenshot from FlightAware)

It isn't, as a screenshot taken later in the flight will show:

The dashed line is FlightAware's attempt to plot the planned route of flight. It's not always accurate, and FlightAware sometimes includes strange zig-zags. If you go to skyvector.com, choose "Flight Plan," and paste in the route shown by FlightAware (BEECH5 BAHMA URSUS UL780 TBG UL780 VAKUD UL780 TRU UV1 ATATU ATATU2), you'll get the more logical route that appears to match the actual flown route:


F/A-18E and F/A-18F Rhinos: What's the difference? The answer to this question seems intuitively obvious: One is a single seat strike fighter while the other has two crew stations.  But is that all?

When it was under development I judged that the E variant was an upgrade designed to replace legacy F/A-18s in Navy and Marine Strike Fighter squadrons (the Corps ended up never buying the jet and held out for a newer design) while the F variant was intended to be a multi-role replacement for both the duties held by the F-14 and the A-6 aircraft, while providing a optional arrangement with dual flight controls for training.

But from the NATOPS manual as well as other literature on the Super Hornet, it appears that the aircraft are one in the same, save only for the second crew station.  Does anybody know specifically why the USA purchased x number of E models to equip specific strike fighter squadrons (eg VFA-14) and y number of F models for others (eg VFA-2)?

The only major difference between the E and the F is the extra seat and its consequences (lower internal fuel capacity, lower landing weight etc).

  Fuel Capacity internal: (F-18E) 14,400
  lb (6,530 kg) (F-18F) 13,550 lb (6,145 kg)

http://www.aerospaceweb.org/aircraft/fighter/f18ef/
(data and own sources on the bottom of the page)

It also has a lower "Carrier Bringback Payload".

  Carrier Bringback Payload F/A-18E: 9,900 lb (4,491 kg) F/A-18F: 9,000
  lb (4,082 kg)

http://www.boeing.com/defense/fa-18-super-hornet/#/technical-specifications


Do you have to worry about airborne disease on an international air flight? Flu? Pneumonia? Anthrax? smallpox? Do you have to worry about them or there are designs in place to prevent them? Like some sort of sophisticated ventilation system?

Models to work with: Most common ones: Boeing 737, Airbus A320

I'm afraid you're probably not going to like the answer! And really, the model makes little or no difference.

The MITRE Corporation did some studies on this and a good summary can be found in the article
Detecting Aircraft Pathogens Before It's Too Late

The summary points are

  the researchers determined that most particles stayed suspended in the aisle, so when booking a trip, take a window seat

and

  Particles also did not travel far outside the contaminated row, and if they did, it was across the row. Previously, it was thought that contaminants would travel front to back, or back to front.

Most worryingly:

  The need for such sensors was evident in the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS), which originated in an Air China flight from Hong Kong to Beijing, spread through 18 countries, and resulted in 774 fatalities. Asian economies suffered $11 billion in damages

As for controlling pathogens on board an aircraft, the CDC provide guidance which involves the same procedures you'd use on the ground (Hand washing, PPE, handling of bodily fluids).

Although air is no doubt filtered, as far as I know there is nothing special about the filters used on Aircraft that would be unique to aviation, nor offer any additional defense against airborne pathogens.


Can passenger movement in an airliner make it stall? This question shows a couple of air hostesses dancing during mid-flight and the co-pilot came out of the cabin to record it.

This made me wonder:  Can flight crew or passengers moving around in flight cause the airplane to stall?

I used to teach sport parachuting, and we occasionally pranked the pilots by shifting the weight around.

A Cessna 206 set up for parachuting will have all but the pilot's seat removed, the jumpers sit on the floor. Everyone moves forward for takeoff and then sits back for the ride up. The instructor (me) sits on the floor where the co-pilot would normally be, facing aft, everyone else faces forward. I would wait until the plane was trimmed for climb (and well above 1000 feet) and then motion the passengers to come forward a bit (the pilot cannot see this happen). He adjusts trim as usual, thinking nothing of it. Passengers come forward some more. Trim adjusted again. Eventually we have 5 people wearing 20kg packs all jammed up into the first 2 "rows" and the trim almost fully nose up.

One signal, and 1/3 of the airplane's gross weight quickly moves almost 2 meters aft (in a 4 meter cabin), followed by a sharp pitch up in both the nose and the pilot's blood pressure. The stall alarm usually goes off too.

Mid-size tailgate aircraft used for jumping are regularly stalled by too many people on the tailgate for too long. The rule there is stay forward until it's your turn, then go aft and get out (if the buzzer goes, you get out NOW).

So it's possible to stall a small aircraft by moving the passengers around.

Scaling this up to airline sizes, we find that the passengers are a rather smaller percentage of the airplane's gross weight, and the presence of things like seats, bulkheads and galleys at the rear prevent 300 people from gathering rapidly at the back. The rather large hydraulically powered elevator also has substantial authority over the aircraft's attitude.

So no, a small group of passengers won't be stalling an airliner out of the sky no matter where they stand. Noticed by the pilots: possibly.


Do airliners have an "anti-pilot" warning system? In February 2014 a co-pilot hijacked Ethopian Airlines flight 702 and took it to Switzerland.

Now in March there is some speculation that Malaysia Airlines flight 370 may have been hijacked and destroyed by the pilots - maybe they took a nose dive into the Andaman Sea?

So my question is this: is there an automatic or say anti-pilot warning system on commercial airliners?

In other words, a system that is non-maskable (can't be disabled by the pilot) and which will automatically warn ATC about unexpected conditions (like a sudden decrease in altitude)?

The system you're asking about is called radar: ATC should monitor it to see what the plane is doing and ask what is going on when something unexpected happens.

It's hard to "automate" these kinds of alerts. For example, a sudden dive can also be cause by an uncontrolled control-surface hard-over (like the boeing 737 rudder problems that brought down 2 planes), or a loss of cabin pressurization (which requires a rapid descent to an altitude where your passengers can breathe).

The other option is having a second person in the cockpit who can wrest control away from the disobedient pilot (or let people in the cockpit to help).


Can I join the Navy after a corrective eye surgery and still be a Naval Aviator? I currently do not pass the requirements for uncorrected distant visual acuity according to the Navy physical standards document. However, if I were to receive eye surgery to correct my vision before joining and then apply to OCS/Flight School, would I be cleared as Class 1 aviation personnel and be able to fly as a Naval Aviator? Thank you!

(Here's some of the information from the linked pdf)

Class I: Naval Aviators and Student Naval Aviators (SNA). Designated Naval aviators are
subdivided into three Medical Service Groups based upon the physical requirements of their
specific flight duty assignment

Medical Service Group 1: Aviators qualified for unlimited or unrestricted flight duties

Medical Service Group 2: Aviators restricted from shipboard aircrew duties (include
V/STOL) except helicopter

Medical Service Group 3: Aviators restricted to operating aircraft equipped with dual
controls and accompanied on all flights by a pilot or copilot of Medical Service Group 1 or 2,
qualified in the model of aircraft operated. A waiver to medical Service Group 3 includes pilot-incommand
(PIC) authority unless PIC authority is specifically restricted.

Service Group 1, 20/100 or better each eye uncorrected, corrected to 20/20 or better
each eye.
Service Group 2, 20/200 or better each eye uncorrected, corrected to 20/20 or better
each eye.
Service Group 3, 20/400 or better each eye uncorrected, corrected to 20/20 or better
each eye.

The first time distant visual acuity of less than 20/20 is noted a manifest refraction (not
cycloplegic) shall be performed recording the correction required for the aviator to see 20/20 in
each eye (all letters correct on the 20/20 line).

I had this explained to me by an OSO a few years back. The answer is a qualified, yes, but the devil is in the details.

At the time, no procedure that involved cutting a corneal flap was approved. This means that LASIK and similar procedures were not approved for aviators. (This may change, but I think its unlikely). So that leaves PRK and similar procedures that only modify the the cornea's surface. This follows recommendations my medical professionals that recommend people that play contact sports to opt for PRK instead of LASIK.

However, here is the detail that was explained to me that I consider to be "the catch", you must already be in the Navy. In other words, you must have already made a commitment, then once inside, you can apply for the waiver and use a Navy approved opthamologist to perform the procedure. The problem with this is that if you are a person that is only interested in being a pilot, you must sign up for some other reasons (it might be aviation related, but not pilot) then once commissioned, then you can ask them to waive the rule and allow you to have vision correction surgery.

Also, at the time it was explained to me, you needed to wait for 1 year after the procedure before being considered for an aviator position. This is logical as they want to see if the procedure was successful and healed properly.

If you're already in the Navy this is great, but if you want to join the Navy for the sole purpose of being an aviator, I would look very closely at the details, and get it in writing before you commit.


How does a jet fighter with water-skis land safely? I just read about the Convair F2Y on the obvious encyclopedic site. It's a delta wing jet fighter from the 1950's, with hydroskis so it can take off and land on water.

Can such a landing be made reliably safe? What is the angle of attack, landing speed, and other techniques of such a landing? I'm concerned because it's such a small craft, and if the skis catch a wave the wrong way, it cloud flip over and disintegrate. It seems to me that even a tiny amount of waves on the high seas would make this thing very dangerous to land.

It looks like the Convair was canceled for a different kind of accident in mid-air. I could not find how many landings it performed safely. For all I know it could have been only 1 or 2 landings before it was canceled.

In the specific case of the F2Y, this video report from the US Navy and General Dynamics shows how it was done.

In the video: It lands on water the same way as any other amphibious aircraft. The only difference is that the engine intakes must be protected from water because too much of it will cause the engine to flame out and shut down.


Must I log aircraft checkout flights as dual received? 14 CFR 61.51 says:

  (h) Logging training time.
  (1) A person may log training time when that person receives training from an authorized instructor in an aircraft, flight simulator, or flight training device.
  (2) The training time must be logged in a logbook and must:
    (i) Be endorsed in a legible manner by the authorized instructor; and
    (ii) Include a description of the training given, the length of the training lesson, and the authorized instructor's signature, certificate number, and certificate expiration date.

If I am rated in the aircraft and just getting a local FBOs checkout or familiarization flight with their instructor, must I (also) log it as dual received? Yes I see the word "may" in (h)(1)

Long story short, maybe. The FAA doesn't define "dual time" so it depends how you define it for your own purposes, but in practice most people use it to mean "training time". There's no definition of "dual" or "dual time" in 14 CFR part 1 or 61 and part 61 never mentions it, apart from a couple of references to "dual instruction". "Dual" is also not used in form 8710.

But "training received" is clearly defined, and 61.51 says you should log time in general if you need it to satisfy certification or currency requirements:

  (a) Training time and aeronautical experience. Each person must
  document and record the following time in a manner acceptable to the
  Administrator:

  (1) Training and aeronautical experience used to meet the requirements
  for a certificate, rating, or flight review of this part.

  (2) The aeronautical experience required for meeting the recent flight
  experience requirements of this part.

So in your example, do you need to log the time for a) certification/currency purposes, and/or b) as training received? And if you do log it, is it "dual"?

For certification/currency, it's usually best to log everything: even if you don't need to show the FAA something, your insurance company or FBO might ask you for details of your flying time.

As for training, in order for a flight to be "training received", the instructor has to endorse your logbook:

  (2) The training time must be logged in a logbook and must:   (i) Be
  endorsed in a legible manner by the authorized instructor; and   (ii)
  Include a description of the training given, the length of the
  training lesson, and the authorized instructor's signature,
  certificate number, and certificate expiration date.

That's a simpler case: if the instructor didn't sign your logbook then you didn't receive training.

But let's say that you decide to log the flight for one or both of the reasons above. Is it "dual"? Well, since there's no official definition of "dual" then it depends what you use it to mean.

Again, practically speaking, most people use "dual" to mean "training": my logging software uses it that way, and many paper logbooks seem to as well. So I only log dual if I have a corresponding endorsement from my instructor. (The software also has options like "instructor on board" or "introductory flight" that I can use for the scenarios you described.)

As a specific example, I did an FBO checkout ride in a new aircraft type a few months ago. I was already rated for the aircraft category/class and the instructor with me didn't endorse my logbook, he just updated the FBO's internal documentation to state that I'm now allowed to rent it. So I logged the time as PIC, but not dual.


How long to receive FAA wings lapel pin? Did anyone request FAA wings lapel pin? If yes, how much time do they take to ship and receive? Do you know if I requested all three phases at once will I get three or just one pin?

I requested all three FAAST WING lapel pins around, but not at, the same time. I think the first two pins were at the same time. It took over three months to receive the first one. When I enquired about the others, I was told that Avemco Insurance (the sponsor of the awards program) orders them in bulk. They ship out the awards only when they have them in stock. So, your experience my vary depending on Avemco’s ordering cycle.


Why hasn’t the Wankel rotary engine seen more use in aircraft? The Wankel rotary engine seems to have several advantages over traditional reciprocating cylinder-and-piston designs. In general, they have a much higher power-to-weight ratio, less vibration, and drastically reduced rate of catastrophic failure, among many other benefits. I can only find disadvantages related to emissions and fuel efficiency. So my question is, why hasn’t it seen more use in aviation even though it seems very well suited to this application?

They do not have drastically lower rates of failure than comparable piston engines. Their manifest unreliability in automotive applications is one reason they failed so badly in the marketplace. My college materials science TA owned one and had it rebuilt four times. His record for the shortest distance before failure on a zero-timed Mazda rotary engine was 150 miles.
The breakdown mode was failure of the apex seals at the tips of the rotor due to poor lubrication, leading to excessive wear and scoring and the complete destruction of the seals. According to my TA, Mazda went through more than six fundamental redesigns of the apex seal system but never could completely solve the basic problem.


Which "warnings" are issued when a military aircraft is going to enter foreign airspace? There are reports in the media saying Turkey shot down a Russian military aircraft after ignoring several warnings and after they entered Turkish airspace.

Which "warnings" are these? Are they standardized somehow?

This depends on the rules of engagement for a country and how the intercepting aircraft are instructed to act.

Radio Contact

The intercepting aircraft will try to establish radio contact on the distress frequencies available. 121.500 MHz or for NATO aircraft 243.000 MHz.

Visual Contact

Unless there is an imminent threat, the intercepting aircraft can position next to the intruding aircraft and establish visual contact with the pilots. There is also standardized procedures, such as the infamous rocking of wings.

(Image Source: www.cfinotebook.net)

Flares

Unless at high altitude, ground based units can shoot flares to attract attention and inform aircraft that they are entering prohibited or restricted airspace.


What options would Qantas Flights 7/8 have in case of emergency? Route For Qantas 7&8

As this route shows, once the flight exits SYD, then all it has nearby is just the great pacific ocean. In such a situation, if it hits some emergency, what all options would it have, if it wants to land, due to some emergency like a patient on board?

Although it appears that going from Australia to North America, there is nothing but the great Pacific Ocean underneath. However the truth is (as Federico mentioned) that there are many airports along the way which can allow an A380 to land there, and many more close enough that an A380 can divert to in case of an emergency.

During the early days of this flight (Qantas 8 to be exact), it had to stop at Noumea, New Caledonia for a fuel stop as the pilots thought that it would be safer to do that.

Wikipedia has a list of diversion airports in Pacific. If you turn on the options to view nearby airports on FlightAware and zoom in on the map, you can find many airports.

It should also be noted that not all emergencies require an immediate landing.


What is baro corrected altitude? I have data from an aircraft which I am trying to understand. One column contains the baro corrected altitude (in ft, e.g. 256) and another the pressure altitude(also in ft, e.g. -44). The two example data points given were measured at the same time.
I have issues finding a definition of the baro corrected altitude. How is it defined and how does it differ from e.g. pressure altitude or other types of altitudes?

Although, I can not find an FAA reference for baro corrected altitude, my understanding is that baro corrected altitude is the Indicated Altitude on a pressure sensitive altimeter when the Kollsman Window is set to the correct current barometric pressure. This may be different than the Indicated Altitude on a non-adjustable/settable altimeter. It may also be different than the Indicated Altitude on a pressure sensitive altimeter set for a non-current altimeter setting.
In contrast, Pressure Altitude is the height above the standard datum plain set at an altitude where the barometric pressure is 29.92 inches of Mercury. You would get Indicated Altitude to match Pressure Altitude by setting the altimeter Kollsman Window to 29.92 inches of Mercury. 5is may not represent the height above Mean Sea Level.
True Altitude is the height above the 19 year average Mean Sea Level. It is consistent regardless of barometric pressure, temperature, humidity, geographic location, etc.
Pressure Altitude is the True Altitude adjusted for non-standard barometric pressure. Indicated Altitude may differ from True Altitude based on what barometric pressure (if ant) is used to calibrate the altimeter. Having to use an altimeter setting from a weather observation (ATIS) not located at the field of use or one that has not yet updated to current.


Is it viable to put an engine at the rear of the plane like in this photo? I saw this image on a NASA page talking about hybrids and I was wondering if the back-engine placemente is actually viable and if it is why hasn't been done yet?

As you look closely to the picture, there are also engines located under the main wings. Thus this is a trijet, a concept that has been used in the past by for example the MD-11, Lockheed tristart and Boeing 727. The main reason for choosing a trijet configuration was the ETOPS requiring twin engine aircraft to always be within a certain range of an airport. This was in a time when engines where less reliable. With three engines the aircraft did not have to bother with ETOPS regulations allowing it to fly routes impossible to fly for twin engine aircraft. But as engines became more reliable and the ETOPS got lengthened the trijets became redundant.

A big downside of the trijet is the structural complexity of an engine in the back, the fuselage needs to be reinforced to withstand the extra forces. From a maintenance perspective it is difficult to access the middle engine because it is high of the ground requiring a stair for visual inspection. Then there is the economical aspect, if you can do it with less engines it will always be more fuel economical. The next iteration of 777 can carry as many passengers as the 747 with half the engines.


Why are AMRAAMs often mounted on the F-16's wingtip hardpoints? If an F-16 is armed with AMRAAM and Sidewinder missiles, with regards to inertia and roll-rate, it would seem to make sense to mount the heavier AMRAAM closer to the fuselage, and the Sidewinder's IR sensor would have a smaller blind spot (created by the aircraft's fuselage) if the weapon were mounted farther away from the fuselage.
Why are AMRAAMs often mounted on the wingtip hardpoints (e.g. as in this picture), instead of the lighter and line-of-sight dependent Sidewinders?

The AIM-120 weighs in at about 335lb (152kg), and the AIM-9 at about 190lb (85kg). While heavier, the Stores Management System (SMS) and the Fire Control Computer (FCC) know what is installed where, and compensate/adjust flight characteristics as needed.
You put the most capable missile, the AIM-120, out the outboard stations. You'd use that first. Only later might you want to use the shorter range AIM-9.
So, if you were to shoot the AIM-120 from Sta 2 (left inboard), you'd still have the AIM-9 out on the wingtip.
Do it the other way around.
Also, the AIM-9 since the AIM-9G can slave the missile seeker head to the aircraft radar. Target acquisition is not solely dependent on the IR in the missile and line of sight.

The AIM-9G was the first version to include SEAM (Sidewinder Expanded Acquisition Mode), basically an improved AIM-9D seeker head, but was quickly made obsolete by the AIM-9H. The latter was the first model to offer limited all-weather capability, as well as solid-state electronics and double-delta control surfaces resulting in a superior maneuverability compared to older models. SEAM slaves the seeker head of the missile to the radar when in "dogfight" mode, which enables the seeker head to be uncaged, slewed toward a specific target by the aircraft radar, and made to track that particular target before being launched.

(from F-16.net - http://www.f-16.net/f-16_armament_article1.html)


Can TODA ever be shorter than TORA, under any circumstance? I've always been told that TODA* = TORA† + Clearway.  I can't really imagine what a negative clearway would be, so that leads me to think that it is required that TODA >= TORA.  Interestingly, a colleague pointed out a certain airport (Invercargill, New Zealand - ICAO code NZNV) where the declared distances published by a certain source describe a TODA that is in fact less than the TORA.

My colleague and I are under the impression that some kind of typographical error has occurred when this source was publishing the data.  However, I can't help but think back to strikingly similar conversations we had after seeing a previously unheard of case where ASDA‡ < TORA.  As it turned out, the FAA had changed how Runway Safety Areas were considered, in such a way that it is totally possible for ASDA < TORA (and we've been seeing it more and more frequently ever since).

*TODA = Take-Off Distance Available†TORA = Take-Off Run Available‡ASDA = Accelerate-Stop Distance Available

TL;DR

I wanted to see if anyone has ever heard of any other case where TODA < TORA, under any regulatory agency.  What would the rationale be if that were the case?

No

Takeoff Distance Available (TODA) can never be shorter than Takeoff Run Available (TORA).

AIM 4-3-6

  Takeoff Distance Available (TODA) – The takeoff run available plus the length of any remaining runway or clearway beyond the far end of the takeoff run available.

I had previously downloaded the FAA runway database and determined there is no TODA less than TORA.  The mysterious airport may have incorrect data or the database is incorrect.

FAA Airport Information

The same database shows that approximately 14% of the runways have an ASDA shorter than TORA.  Multi-engine pilots need to be aware of how their aircrafts performance was created and how to apply the correct declared distances.


How did the Wright plane takeoff/land? I don't understand exactly how the Wright brothers handled takeoff with the first gliders and powered plane. I know it has something to do with an elevated rail track. Also, if the flying position was head first, body parallel to the ground, how did they land without injury?

Seperate question: for the initial glider flights, did they start at a high elevation and simply glide to the ground, or was there any method of increasing altitude during flight?

As you said, they used a monorail for launches. The aircraft was pulled forward by a pulley which was connected to a weight which hung from a derrick. First the weight had to be pulled up, the pulley was run to the tip of the rail and back to the Flyer, and once the engine was running and the pilot in place, a helper released the weight and the pulley pulled the aircraft forward.

The pictures here are taken from this site; I recommend it for further reading!

Below is a picture of the rail, note the uptick at the end which gave the airplane a gentle push upwards, similar to the ski jump technique of British aircraft carriers.

For landing the Flyer used two skids. Depending on the ground, this could mean a gentle slide to standstill. Rougher landings would mean bruises for the pilot, like the ones suffered by Orville on August 23, 1904. But in most cases of a rough landing, the elevator assembly absorbed most of the impact energy. Elevator repairs were quite frequent.

I know less about the glider launching technique. Most glider tests were done with an unmanned glider held by strings into the wind. Glider flights would be downhill; thermalling was discovered much later (in 1913). However, even Otto Lilienthal already mentioned the uplifting effect of gusts, but neither he nor the Wright Brothers could circle tightly enough to stay inside a thermal.


How are the navaids that are on the airport ground powered? How are navaids (such as the ILS localizer) that are on the airport ground powered? Are they connected to the airport's power? Do they have their own backup power?

They use the airport power. The airport and its important equipment have backup power and/or a dedicated power plant (which itself will have backup plans).

This is what the FAA says:

  The FAA recognizes the need to have a reliable power source to operate NAVAIDs, even during utility power outages. Order 6030.20 establishes Continuous Power Airports (CPAs) that provide continuous operations in the event of an area-wide utility failure. Backup power to designated runways at these airports must be able to supply power for at least 4 hours for runway lighting as well as navigation, landing and communication equipment. In addition, FAA policy also requires that power systems used for support of Category (CAT) II and CAT-III operations must be capable of transferring to an alternate source within one second.

  Source: AC 150/5300-13A

Basically it's to avoid having to shutdown a major airport. Select airports and runways have to be able to operate.

ICAO recommends the same thing and lists the criteria airports need to meet in Annex 14, section 8.1 Secondary power supply.

For example:

  8.1.5 Recommendation.— Requirements for a secondary power supply should be met by either of the following:

    — independent public power, which is a source of power supplying the aerodrome service from a substation other than the normal substation through a transmission line following a route different from the normal power supply route and such that the possibility of a simultaneous failure of the normal and independent public power supplies is extremely remote; or

    — standby power unit(s), which are engine generators, batteries, etc., from which electric power can be obtained.

For navaids in general that are not within an airport, then ICAO Annex 10, Volume I, Part I, Chapter 2 handles the recommendations.


What was the average duration of a Me-262 combat mission during WWII? Me 262 in 1945

Maximum speed: 900 km/h

Range: 1,050 km

Service ceiling: 11,450 m

Rate of climb: 20 m/s at max weight of 7,130 kg

Thrust/weight: 0.28

Could a Me-262 fight for (1050 km) / (900 km/h) = 70 minutes?

This depended strongly on the flight altitude. Staying at low level for the full mission meant that the Me-262 would run out of fuel within 40 to 50 minutes.

Climbing above 6 km (20,000 ft) would extend that time to 90 minutes. Time to climb to 9 km was 13.2 minutes, so that would leave a time for engaging the enemy of 60 minutes. Considering that typical armament were 24 unguided rockets and 4 cannons with 80 or 100 rounds means that the 262 would run out of ammo in much less time.

One pilot told me that a typical training mission would start in Munich, continue to a radio station near Berlin and back to Munich, all within 60 minutes.


When was Kai Tak permanently closed to cargo service? Kai Tak Airport in Hong Kong was closed to both passenger and cargo service early in the morning of 6 July 1998, with all flights moving to the brand-new airport at Chek Lap Kok.  However, shortly afterwards, Kai Tak was temporarily reactivated for cargo service (quoting Wikipedia; emphasis added):

  Government reports later revealed that Chek Lap Kok airport was not completely ready to be opened to the public despite trial runs held. Water supply and sewers were not installed completely. Telephones were installed, but the lines were not connected. The baggage system did not undergo extensive troubleshooting and passenger baggage as well as cargo, much of which was perishable, were lost. The government decided to temporarily reactivate Kai Tak's cargo terminal to minimise the damage caused by a software bug in the new airport's cargo handling system. During this period, the airport was given temporary ICAO code VHHX.

The article makes no mention of how long this arrangement lasted.

When did Kai Tak close as a cargo airport for the second and final time?

The cargo terminal was closed for good by 24th August, 1998.

First Anniversary of Hong Kong SAR: Infrastructure Development

  Air cargo handling was more seriously affected by problems with the automated handling system at the new 'Super Terminal 1' operated by Hong Kong Air Cargo Terminals Ltd (HACTL). This forced HACTL to re-open its air cargo facility at Kai Tak until the new terminal at Chek Lap Kok - one of the world's biggest - was running smoothly. By August 24, all incoming and outgoing air cargo was being handled at HACTL's new Super Terminal 1.


What is this square hole for? When waiting to depart Edinburgh today I noticed a square hole (looked to be something that can be opened and closed) on the rear of the right side of the aircraft.

Never seen it before and no idea what it's purpose is. Anyone know?

The opening is below and to the right of the rear door on the right hand side of the aircraft

(image source)

Cabin pressurization outflow valve.

Cabin pressure in an airliner is maintained by bleeding air from the engine compressors into the cabin, after conditioning (usually cooling and humidifying) it. Current designs are beginning to use separate air compressors for this.

In either case, the cabin pressure is regulated by controlling the difference between inflow, provided by the bleed or compressor, and outflow, which is a simple motor-controlled valve.

Generally the inflow is set to maximum and the outflow is varied, as this provides the maximum fresh air flow for the passengers.

See also:

How does the outflow valve work on a cabin pressurization system?

How does the cabin pressure controller work in a large commercial airliner?

How does the cabin pressure controller work in a large commercial airliner?

https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/

Can an aircraft re-pressurize at high altitude?


How to find the location of the Aerodynamic centre? Everywhere I look I'm only able to find information about airfoils.

How do I find out the location of the Aerodynamic centre of an arbitrary aircraft irrespective of its type or shape?

What you refer to as the aerodynamic centre is also called the neutral point, the point where pitch moments do not change at all angles of attack with attached airflow.

If you ask for an unswept body of high aspect ratio, the answer would be easy: At the quarter chord point in subsonic flow and at the half chord point in supersonic flow. Unfortunately, the neutral point shifts forward with decreasing aspect ratio until it sits right at the leading point (not edge) of a slender body, a body with infinitesimally small aspect ratio. It shifts slightly backwards with positive sweep, so you normally have to calculate correction factors which depend on

Sweep angle
Taper ratio
Ratio of fuselage width and span
Lengthwise position of the wing-fuselage intersection
High wing or low wing configuration (negligible influence)

If you don't have good wind tunnel data or a validated CFD model, you would use a collection of formulas and diagrams like DATCOM (see this link for a computerized version) to approximate a solution. Sorry, but I cannot give you a simple formula which would work out of the box.


When departing IFR, when do I switch from Tower to Departure? When departing IFR from a Class D airport, must you wait until the Tower instructs you to change to Departure?

When operating within class D airspace (operating Air Traffic Control Tower) pilots are to maintain communication with the tower until instructed otherwise.

On an IFR flight departing from a tower controlled airport, the tower controller will instruct the pilot when to change frequency to contact the next controller (the IFR Departure Controller).  Remain on the current frequency until instructed to change.

Here is an except from the pertinent regulation (FAR Part 91):

  §91.129   Operations in Class D airspace.

  2) Departing flight. Each person—

  (i) From the primary airport or satellite airport with an operating control tower must establish and maintain two-way radio communications with the control tower, and thereafter as instructed by ATC while operating in the Class D airspace area;

Note: It should be noted however, that in the U.S., departing IFR military turboprop/turbojet aircraft (except transport and cargo types) will be instructed to "change to departure control frequency" before takeoff. [ref:FAA JO 7110.65, paragraph 3-9-3 a. 2.]

This requirement allows pilots of single-seat high-performance aircraft, as well as certain other military aircraft, the ability to avoid being distracted by changing to another (e.g., departure control) frequency immediately after takeoff.  Further, many military fighter type aircraft used to have (probably some still do) radios positioned in the cockpit in a location that requires looking down or moving the pilot's head/eyes in such a manner that might cause momentary disorientation, which is not a good idea right after takeoff and close to the ground.

U.S. military control towers and some (perhaps most) joint-use military/civilian control towers have an "override" frequency (same frequency as the departure controller uses), for use in case some emergency situation that occurs just before, during, or immediately after departure requires urgent communication with the pilot (e.g., engine fire on takeoff noted by the tower, or similar type situation).


Why does a turboprop lag? Compared to a [modern] turbofan/turbojet, why is the throttle response slow in a turboprop?

A constant speed propeller mounted on a piston engine responds much quicker.

Is the underlying reason mechanical (e.g. size of combustion chamber), or electronic (e.g. ITT protection)?

Turboprop engines typically have a core which includes a compressor and turbine that rotate as a unit (although an engine might have more than one such "spool"). The gas generated by the core drives a "power" turbine which, via a gearbox, drives the propeller, which by the way has a non-trivial rotating mass. The core and power sections rotate independently of each other. So, upon a commanded power increase, the core will first spin up, throwing more gas at the power turbine which will then speed up. The delay is therefore due to the rotating mass of the propeller and the fact that it rotates independently of the engine core.

For what it's worth: turboprop engines are just a specific application of a turboshaft engine - where the energy of the gas produced by the core is captured through a power turbine to drive a shaft, which could be used to power anything - propeller, helicopter rotor, electric generator, etc.

I should add that there are exceptions to the above, like the Garrett TPE331 which have only a single shaft.


How does the B737's 'throttle offline' setting work? I'm concerned about this article:

Throttle online means that the crew can override the autothrottle logic, while Throttle offline means that the logic cannot be overridden.

So for example - rejected take-off:
Table shows that at 60 knots 'throttle mode is offline' and we cannot override.
So at 60 knots we can't first move throttle to idle (because it's blocked?) before turning off A/T? Or we can same as at 80 knots?

That article is about somebody's simulation. In the real aircraft, the pilots can always override the A/T and can always turn it off entirely. The A/T will push the throttles toward the target position, but not so hard you can't move them easily yourself. (You may need to turn them off to get them to stop trying to push the power up, but for an action like pulling them to idle for an aborted takeoff before THR HOLD engages at 84 knots, the pilot can easily override them.)
Also, the terms "online" or "offline" aren't used with reference to the authrottles in any Boeing documentation that I've seen.  They may be off, or they can be on -- in which case the mode may be "armed" or any of various other modes. But not on/off line.
Mostly the info in that article is okay, but the table referenced seems to reflect some misunderstanding about what the pilots can/can't do with the thrust levers... at least, with regard to the actual aircraft. That info may be spot-on with regard to the simulation the author is writing about.


Do modern airplane controls provide force feedback of external forces on the control surfaces? As I was sitting inside the cockpit of an old AN-26 at the aviation museum, long grounded and no longer flightworthy, sudden gusts of wind outside would sometimes move the control surfaces and make the yokes move around all on their own, spooking visitors with creepy thuds and groans of the hull and acting all creepy on the dark windy day. It seems in the days of old, with mechanical controls, it wasn't just the pilot acting on the control surfaces through the rudder - when attacked by winds, they would feed the external forces back to the yoke through the same system - likely both providing the pilot with important knowledge, and causing trouble by wringing the yoke in his hands.

I wonder how that looks like nowadays in modern planes, where the control is electronic, hydraulic, and a feedback like this would need to be specifically, additionally implemented, as an extra complication of the system instead of being in its inherent nature. Well, is it? Do the yokes of, say, modern airliners push harder on the pilot's hands when entering a turbulence or such?

On some. The Fokker 100 is one, where the aeroforces on the elevator are directly fed back to the pilot. There is a hydraulic actuator helping him, but contrary to the usual solution it is a reversible system where the hydraulics merely amplifies the direct force that the pilot exerts.

The F100 has manual reversion for elevators and ailerons, and it can still be flown reasonable comfortably without hydraulics. The 737 also has manual reversion, but the flight controls are quite heavy without hydraulics - it is about the largest high-subsonic aircraft that can still be hand-flown. Larger planes just have triple redundant hydraulic controls, some with q-feel which artificially replicates the higher air stiffness felt at higher airspeeds. No romantic gusty creaking for those aircraft in a museum.


Do pilots use the pedals in flight on planes with a sidestick (Airbus)? Do the pilots flying sidestick aircraft with fly-by-wire control systems, such as the Airbus A320, use the rudder pedals while in the air? Or does sidestick input affect both the rudder and the ailerons?

The rudder pedals are not usually used in flight, even with the autopilot off.
The rudder on the Airbus A320 can be controlled via a mechanical connection to the rudder pedals in the flight deck, but also electrically via the fly-by-wire mechanism. The following graphic shows an overview of the rudder control:

(Airbus A320 FCOM - Flight Controls - Description)
As you can see the sidestick input goes to the ELAC (ELevator Aileron Computer) and from there a Yaw Order is sent to the FAC (Flight Augmentation Computer). This will then be passed on to the rudder via the Yaw Damper Servo Actuator.

Electrical Rudder Control
The yaw damping and turn coordination functions are automatic. The
ELACs compute yaw orders for coordinating turns and damping yaw
oscillations, and transmit them to the FACs.
Mechanical Rudder Control
The pilots can use conventional rudder pedals to control the rudder.

(Airbus A320 FCOM - Flight Controls - Description)
Since the yaw damping and turn coordination are fully automatic, there is usually no need for the pilot to use the rudder pedals in flight.
There are however some situations, where manual rudder input via the pedals is required:

Takeoff Roll: The rudder pedals move both the rudder and the nose wheel steering (NWS, see "NWS ORDER" in graphic above) on the ground to maintain directional control during takeoff and landing. After initiating the takeoff roll, the pilot flying (PF) will use the rudder pedals to control the aircraft:

The PF should use pedals to keep the aircraft straight. The nosewheel steering authority
decreases at a pre-determined rate as the groundspeed increases (no more efficiency at
130 kt) and the rudder becomes more effective.

(Airbus A320 FCTM - Normal Operations - Takeoff)

Landing: When landing the aircraft in a crosswind the direction in which the aircraft points is not aligned with the runway. To align the aircraft before touchdown (called de-crab), the PF needs to apply rudder input via the pedals during the flare:

During the flare, rudder should be applied as required to align the aircraft with the
runway heading. Any tendency to drift downwind should be counteracted by an
appropriate lateral (roll) input on the sidestick.

(Airbus A320 FCTM - Normal Operations - Landing)
This is the only time (for normal operations) where the rudder pedals are actually used in flight.

Engine Failure: After an engine failure the asymmetric thrust of the remaining engine needs to be overcome with the rudder. For an engine failure during the takeoff (but after V1), this needs to be done manually with the rudder pedals:

On the ground: Rudder is used conventionally to maintain the aircraft on the runway centreline. [...]
When safely airborne: [...] Since
the lateral normal law does not command the full needed rudder surface deflection,
the pilot will have to adjust conventionally the rudder pedals to center the beta
target. [...]

(Airbus A320 FCTM - Abnormal Operations - Operating Techniques)
Once the aircraft has accelerated, the rudder is trimmed and no pedal input is required any more.


Where can I find sample data required for Engine Condition Monitoring? I'm trying to find some sample data required for Engine Condition
Trend Monitoring to analyse and predict when an engine might fail.

I went through a lot of research papers and (kind of) understood the parameters needed for Engine Condition Trend Monitoring. From what I've understood, I need Exhaust Gas Temperature, Fuel Flow, Low Pressure Fan Speed, High Pressure Rotor Speed, Engine Pressure Ratio and EGT Margin data.

I'm doing this as a part of my college project and I need some sample data to start working.
Also, please correct me if I'm wrong because I'm very new to all this.

Your list of parameters is on the right track, and is the minimum set. If you can add a few more temperatures and pressures at other “stations” within the engine, the diagnostic accuracy improves.
No one on the internet will send you real data. OEMs and operators don’t give out data, because it contains proprietary information. That’s why 90% of the papers you have read probably use simulated data. Also, with simulated data, you actually know what the fault was. With a real engine, you don’t because when you pull an engine apart, there is no simple dimension you can measure to tell you what the change in efficiency of the fan actually is, for example. But, the trick with simulated data is to add a realistic amount of noise.
To generate simulated data you need a gas turbine performance program, like GasTurb or GSP. The latter of these is free for personal use. Maybe if you google “gas turbine performance simulation software” you will be lucky and find something free that does enough for your research.


How are GA aircraft ferried from factories? It's relatively easy to transport jet aircraft like Airbus and Boeing: most of them have a long range, and a few hops at most would get them to the owner.

But how would a Cessna manufactured in U.S. be delivered to Dubai? It would not have the range to cross the Atlantic. Taking a brand new single-engine aircraft across a large ocean also sounds very risky (to the occupants). Same story applies to Hawaii.

Getting there may also require many hops (e.g. Texas -> California -> Canada -> Alaska -> Japan -> Taiwan -> Singapore), making the transportation long and costly.

Are they put on trailers and shipped like cargo? If yes, how is the aircraft taken apart? (I'd imagine even the wing span of a 172 would be too wide for roads in many countries)

EDIT: the linked question addresses airliners. I am specifically interested in GA airplanes.

As you've guessed, there are two options: a ferry flight, or shipping the aircraft in a container.

Ferry flights are quite common; the recent Cirrus ditching in the Pacific was a ferry flight to Hawaii. As well as planning a suitable route and dealing with permits and paperwork, refueling etc., the ferry company may install extra fuel tanks to give more range, and provide the pilot with specialized survival and communications gear. And an airline ticket home, of course :-) This might all seem expensive, but considering that a new top-of-the-range Cirrus SR22 has a list price of almost USD 800,000 you can imagine that ferry costs aren't necessarily a big deal in comparison.

The second option is to put the aircraft in a container and ship it, and there are some companies who specialize in this (the page has some small photos of aircraft in containers). The major concern in this case is removing the wings: gliders and many experimental (e.g. home-built) aircraft are designed to have their wings removed for storage and transportation (towing behind your car) but most typical Cessnas, Beechcraft etc. aren't. You need to have the aircraft professionally and carefully prepared and then re-assembled and checked after delivery.

Which option is best depends on the costs of each option and the value of the aircraft. There's also the consideration that if you buy a 60-year old aircraft from the other side of the world, ferry companies may decline to fly it to you because of the risks of subjecting it to the stresses and weather of a very long series of flights. Or their/your insurance company may refuse cover for a ferry flight for the same reason. Or perhaps the aircraft is certified for VFR only, which would make a long ferry journey much more difficult to plan and execute. In those cases, container shipping may be the only available option.


Why does the P-3 Orion use an engine driven compressor instead of bleed air for pressurization? In another question, an answer mentions that the P-3 Orion uses a gearbox driven compressor for cabin pressurization, even though the aircraft uses T-56 turbine engines that supply bleed air in other designs.
(How are turboprops and other propeller planes pressurized?)

Brief research suggests that the "EDC" exists, but I can not find any rationale for it.
(http://blueaero.com/wp-content/uploads/2016/11/Heico_P3_EDC_Repair.pdf)

Does using gearbox driven compressors help in some way with the mission, possibly in dealing with the environment? Is it an anachronism/carryover from the L-188 Elektra? Is it not actually gearbox driven, but somehow an exchanger for bleed air?

The T-56 engine, like all turboprops, is much more efficient at converting fuel into rotational energy than compressing air.  It takes approximately 50-100 shaft horsepower to run an EDC, depending on altitude and how cold of a temperature you select. Bleeding a small amount of air off the engine to run engine anti-ice costs you a 9% loss of SHP, which in flight, would be anywhere from 150-250 SHP.  I don't know the exact volume of bleed air you would need to run a pressurization system, but it is definitely more than the engine-anti ice system.  I'm making that assumption base on the size of the ducting.  The anti-ice tubing is much smaller than the EDC tubing, or the main bleed air manifold for that matter.
All in all, I think it's due to efficiency.  The EDC is more efficient for that motor, but is more complicated and introduces more possible malfunctions.  A P-3 requires a LOT of cold air to keep the computers from overheating, more than a C-130 (same motor) for sure.  I'm not sure if the C-130's back in the day also had EDC's, but they use bleed air nowadays.  So the designers went with whatever can move the most air.  I don't have my Orion Service Digests handy, but I bet there's a more thorough answer in there.  I'll check whenever I go back to work.


What happened to this airplane at Lombok Airport? Last month I was in Indonesia and I had to take a flight from Lombok International Airport (IATA: LOP). While I was waiting for boarding I noticed an airplane (wreck) next to the runway that was somewhat burned down at the front (fire?).

The abandoned plane at Lombok International Airport. (personal photo)
I was intrigued by the way that it was just placed there, so close to the runway. I thought about an accident where this aircraft was involved in, could've have just happened a couple of days or weeks before. But I couldn't find any source in collaboration with this particular airport. I maybe thought that the airplane was used to train fire fighters, as I see that more often on other airports in the world. I tried to find more info about it on the web, but no luck.

The airplane on the map
I also headed out to Google Maps, Bing Maps and Apple Maps to see if the plane was there.
Google Maps has the airplane on its map (see image below, top middle and slightly to the left), the data is from 2016, so says the footer.
Google Maps location of airplane on Lombok International Airport, data consulted on 2 Dec 2016.

Bing Maps isn't having the airplane on its map, but maybe that's because the map is outdated (?).
Bing Maps location of Lombok International Airport, data consulted on 2 Dec 2016.

Apple Maps shows an airplane in almost the same spot (it's just on the concrete I think), but is it the airplane?
Apple Maps location of 'airplane' on Lombok International Airport, data consulted on 2 Dec 2016.

Zoomed pictures of the plane
The above picture is zoomed in on in the following pictures. Unfortunately, the aircraft registration is not visible.

And now?
I'm eager to know what happened to this plane. What type is it? Who is the owner? Can anyone tell me? Thanks in advance.

(Source)

The aircraft's registration is PK-YVL. It's a Boeing 737-300 that was operated by Batavia Air before the airline ceased operations.

This plane is/was leased from Pacific AirFinance. It has been withdrawn from use and stored at LOP since 31 Jan 2013. After ceasing operation, some of the other 737-300's the airline had were delivered to Peruvian Airlines.

Due to its proximity to parked aircraft, it's not likely the plane has been converted to train firefighters. The darkened area could just be storm blown mud.

(Source) Same plane, previous operator.


Why is this type of rotor hub so rare...? I wonder why is this type of helicopter rotor hub so rare... It is a very simple construction, having no flap or lead-lag hinges. A universal, constant-speed joint leaves the rotor free to find his position in space, and the only complication is the swashplate, essential in order to control cyclic and collective pitch. This type of hingeless rotor hub needs little maintenance and is relatively easy to build with standard materials... But it's nevertheless nowhere to be seen...

It's basically the teetering Bell/Robinson 2 blade rotor concept applied to a 4 blade rotor.  Like a teetering system, the gimballed rotor head accounts for most of the lead/lag/flapping, with whatever isn't accounted for absorbed by flexing of the long skinny root shafts.

With a 2 blade teetering system you have the weight of the machine hanging from a big cross bolt - pretty simple.  This system appears to be a bit complicated in the manner that the load of the machine is suspended from the rotor head, and that may have worked against it when other designers considered it.

The designer may have seen it as the advantages of a teetering system and an articulating multi blade system combined, whereas other designers saw mostly the disadvantages of the two.  An example: a teetering rotor, unlike an articulated one, can't tolerate hitting the teetering limits in flight because it puts huge bending loads on the mast (mast bumping) and makes the mast come apart.  This is why you never let a 2 blade machine go zero G.  I would expect this system could have a similar limitation, which a conventional articulated hub doesn't have.


How common is engine failure? What are the failure rates for piston, turbine, and jet engines? (If more subdivisions of "engine" are necessary to make the data useful, please point them out).

Engine failure is not that common. For a gas turbine engine, a press release by GE gives some data as a reference point. In 1995 they quoted various engines as having a dispatch rate of between 99.99% and 99.89%. That is only 0.01 to 0.11 % of flights could not depart due to an engine issue. However, that doesn't identify in flight failures, best captured by the inflight shut down (IFSD) rate. Data on this is difficult to get for specific engines, as the OEMs consider it competitive information, so the best guide is probably as mentioned the ETOPS rating. The ETOPs fleet is reported as achieving a shut down rate of less than 0.01 per 1000 engine hours. Another way of defining reliability is the (repair) shop visit rate (SVR). Figures given for this (by GE in the same link as above) were between 0.085 and 0.231 visits per 1000 flight hours. However, this covers both scheduled and unscheduled maintenance (i.e. failures), so perhaps doesn't identify what you want.

For piston engines, the answer to this  question probably has some useful data. In addition, Continental claimed in 2014 that they had an IFSD rate of 0.0061 per 1000 flight hours, which they claim is the most reliable piston engine in general aviation. This compares with "less than" 0.01 for gas turbines, so, it seems very similar.

It maybe possible to find a better value for the IFSD rate for gas turbines. Given the rate is so low, it really needs to be quoted to more than 2 decimal places, or given per 100,000 flight hours. Ultimately, I would expect the IFSD rate for gas turbines to be lower than for piston engines, due to the huge economic incentive gas turbine manufacturers and the commercial airline industry has increasing reliability for turbofan engines, that does not exist to the same extent, for general aviation piston engines.


Are low wings less safe for aerobatics training? I fly a low wing (CZAW SportCruiser) and am looking for aerobatics training schools, however I can only find schools with high wings available for training. My plane has "no aerobatics" placards inside the cockpit, so apparently I can't use my own plane. Are low-wing aircraft less safe for aerobatics training?

No.

It has nothing to do with the configuration, but the structure, the engine and the fuel system of your aircraft. The SportCruiser is certified in the Light Sport Aircraft (LSA) category and is meant for flying right-side-up. For aerobatics, you need

a fuel system than can still feed the engine when the load factor is less than or equal to zero,
an engine which can still lubricate all moving parts when it is experiencing a negative load factor, and
a structure which can tolerate higher load factors than what you can expect from cruising along at 115 kts max in fair weather.

The wing location has no impact on aerobatic fitness. You might even argue that the unrestricted view up in a low wing aircraft helps to see better what is ahead in steep turns.


Would a wireless fly by wire system be practical? Given that most modern aircraft are controlled using the fly by wire system, what is the likelihood or the drawbacks of transmitting those fly by wire instructions from the cockpit and or cockpit computer wirelessly? Wires add weight to the aircraft and hence increase the net fuel burn. I'm assuming a totally wireless system is not already in use today.

The fly-by-wire is absolutely vital for control of the aircraft, and the three dominating factors here are safety, safety and safety. Weight is not one of them. The fly-by-wire system is triple or quad redundant: instead of removing a set of cables, the manufacturers are installing 3 more cable looms, just to make sure that the system always works.

Wires are safer than wireless!. There are active transmitters and receivers in wireless systems, which can fail. Signal reception depends on the quality of the space between transmitter & receiver: can the signal always penetrate the aluminium bulkhead? What happens to the wireless signal when flying past airport radar or in the weather radar field of another aircraft?

Shielded wires are passive and relatively immune to electro-magnetic radiation, and are the means of choice for such a safety critical system.

Update

The OP was on fly-by-wire. In a sense, the flight control system is the most safety critical system on board and must be immediately available at all times. The presentation linked to by @mins in a comment reports on the progress of on-board wireless communication, for systems with a less critical safety aspect:

  Examples of Potential WAIC Applications

  Low Data Rate, Interior Applications (LI):

    Sensors: Cabin Pressure - Smoke Detection - Fuel Tank/Line – Proximity Temperature - EMI Incident Detection - Structural Health Monitoring - Humidity/Corrosion Detection
    Controls: Emergency Lighting - Cabin Functions

  Low Data Rate, Outside Applications (LO):

    Sensors: Ice Detection - Landing Gear Position Feedback - Brake Temperature - Tire Pressure - Wheel Speed - Steering Feedback - Flight Controls Position Feedback - Door Sensors Engine Sensors - Structural Sensors

  High Data Rate, Interior Applications (HI):

    Sensors: Air Data - Engine Prognostic - Flight Deck/Cabin Crew
    Images/Video
    Comm.: Avionics Communications Bus - FADEC Aircraft Interface - Flight Deck/Cabin Crew Audio / Video (safety-related)

  High Data Rate, Outside Applications (HO):

    Sensors: Structural Health Monitoring
    Controls: Active Vibration Control

From the same presentation: An A380 has 5,700 kg of wires on board, and 30% of them are potential candidates for a wireless substitute. So wireless communication on board of aircraft does make sense, starting with the non-safety critical systems first.


What are the advantages and disadvantages of elevons? I'm in as student, I wanted to know the advantages and disadvantages of elevons?

Image source

The advantage is: if there is limited real estate available for flight control surfaces, you don't have to divide it up into elevators and ailerons and only use half the available surface for pitch or roll.

The disadvantage is: you cannot go past the surface stop, and there will be situations where a combined pitch and roll cannot be commanded fully by the elevons because one of the surfaces will run into its stop. Resulting in smaller roll & pitch responses than commanded, in only certain situations.


How can this kind of confusion about taxi instructions occur? A few days ago, I (a student pilot on a solo) had landed on runway 9 for a full stop taxi back. The taxi instructions were "alpha to 27 remain on this frequency". I missed some words in between these instructions, but after responding "alpha to 27" there were no corrections, so I continued my taxi to 27. At this point I should have asked for clarification. A jet was landing and I was blocking the exit as I had taxied and was holding short of 27. ATC responded saying I was at the approach end of 27 and instructed me to do a 180 and taxi to runway 9 (they were very nice about it though).

I've been racking my brain trying to think what those missing words were. I can't imagine they'd have said departure end of 27 instead of just 9. This may be one of those moments I'll never figure out, but would anyone more experienced have any idea what might have happened?

Unfortunately the airport is not on LiveATC.net and my GoPro died from low battery so this is all from memory.

I continue my series of "not actually an answer but I got to say this":

The most valuable lesson you can learn from this is not figuring out afterwards what was said, but getting into a habit of always asking "Say again..." when you miss even a slightest part of a transmission. Ron stated it in the comments: no shame in that, not even a bit.

Whenever you allow yourself to guess the missing parts, you put yourself, and probably others in danger. Requesting repetition, every now and then you'll catch the grumpy controller that'll bitch at you for not paying attention, but never mind them. You don't want extra holes in your Swiss cheese.


To what extent can I deviate from the endorsement guidance given in AC 61-65? Specific case:  I have a student who is ready for his ride, but he has not yet scheduled it, and because I'm moving, I will be permanently unavailable to endorse it in a few days.  I want to add a stipulation to his endorsement that says the endorsement is valid only until a certain date, and it's only valid if he has had some number of hours of flight training within the preceding $x$ days.

Neither of these items are mentioned in AC 61-65.

General case:  How much can I legally deviate from the guidance for checkride endorsements given in AC 61-65?

Advisory Circulars by definition, are advisory and not regulatory in nature (Chapter 1, paragraph 3 of FAA Order 1320.46C). Therefore, AC 61-65 provides suggested language that will ensure compliance with the FARs.  You are free to word the endorsements differently, but if the examiner takes issue with your endorsement, the student may not have any recourse but to have the entries redone (by you, or another CFI).

Since you say your student is ready (I am assuming for their Private Pilot check ride), why not go ahead and endorse them right now? FAR 61.109(a)(4) requires the student to receive 3 hours of flight training within the two months prior to the test, this would cause your endorsement to automatically expire if they don't test (or receive additional training) within that window.

Alternatively, I would suggest contacting the local Flight Standards District Office and get guidance from them directly.


Why does part 135 require a maintenance log on board the aircraft, but part 91 does not? During a dispute within friends, it came to my attention that in 135.65 the maintenance log copies must be present in the aircraft, and that 91.407(b) and 91.417(c) have similar rules, but, meanwhile, as indicated in 91.203 the "maintenance logs" are not necessarily maintained and be present in the aircraft. Why is this?
Why didn't a school provide the maintenance logs in the aircraft, but only the airworthiness certificate?
I don't understand the reason behind this; if it must be present in the aircraft for 135 operations (air taxi) why it is not present in 91 operations? Safety wise speaking, I think any pilot who commands a plane, abused by many different people, would want to know what was in the maintenance logs?
I tried to understand the reasoning: maybe the regulators think that it will create another responsibility for PIC? Or maybe this added responsibility will diminish the part 91 owner/operator responsibility? If anybody can explain the underlying rationale for the legislation ıt will be great.

For the same reason truckers are required to keep logs in their trucks noting rest times, inspections, etc. but private vehicle owners do not. Part 135 is for commercial operations, part 91 is not.


What constitutes a wind gust in a METAR report? In a METAR report gusts of wind are shown with a G after the wind speed.  When will gusts be reported?  I have seen reports that show:

12017G20
12010G20
12003G20

The definition of "Wind Gust" for METAR reports can be found in the Federal Meteorological Handbook No. 1, chapter 5:

  5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the
  occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots
  or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.


How effective was WWII bombers self defense anti-air armament? Bigger WWII bombers like e.g. the B-17 Flying Fortress were equipped with lots of machine guns for self-defense against single-seat fighter aircraft.

Wikipedia says:

  Defensive armament increased from four 0.50 in (12.7 mm) machine guns
  and one 0.30 in (7.62 mm) nose machine gun in the B-17C, to thirteen
  0.50 in (12.7 mm) machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and needed to be flown
  straight and level during their final bomb run, individual aircraft
  struggled to fend off a direct attack.

The last sentence and the fact that fighter escort was developed and also personal impression from live footage etc. indicates that the self-defense armament wasn't very effective.
Is this true?

I can understand that bombers cannot do evasive maneuvers, but so does the attacking fighter plane when it is homing in on the bomber and aiming, isnt it?
In that moment the bomber actually should have the advantage of being able to point the (several) mobile guns, while the fighter must use the entire airframe to aim its guns and thus has a very predictable flight path.

And in case it really wasn't effective, why did they carry it then? Must have added a lot of additional weight.

Are there any figures to illustrate the effectiveness of bomber self defense?

The only figures I could find were the 60 bombers out of 300 lost by the USAAF in the Schweinfurt raid, most of those losses from German fighters. That convinced the generals that the onboard gunners weren't all that effective, and bombers couldn't defend themselves from a determined fighter attack. Major air attacks on Germany were postponed until the long range P51 could be deployed to protect the bombers better.

Gunnery from a bomber is far more difficult than from a fighter, that fires straight ahead. In a bomber, the gunner might be firing to the side, which means they have to factor in the bomber's speed that will propel the bullets sideways, and actually aim slightly behind the approaching fighter. The closer to the nose or tail, the less one aimed behind... bomber gunnery is very complex. This wartime training film illustrates the problems of firing on fighters approaching from the side. Try to calculate those ever changing angles when you're being shot at...

The same difficulties were experienced by ships being attacked by aircraft... very difficult to shoot down a small, fast moving aircraft approaching a ship. The Bismarck couldn't stop slow moving Swordfish biplanes from knocking it's rudders out. In the last major naval action of the war, eight Japanese warships (including the Yamato) were attacked by around 400 aircraft from US carriers. Seven of the eight ships were sunk, while only 14 aircraft were lost in the action, despite those ships having had quite a few extra AA guns added.

So, why arm the bombers? That was a question asked by Curtis LeMay in 1945. He devised a plan to take most of the guns out of B29's and operate them at night, because the Japanese hadn't demonstrated much in the way of night interception. He reduced the attack altitude to 12k feet from 30k feet, because that put the planes above small arms, but below the effective range of large anti-aircraft cannon. And, he used the weight saved from armament removal to carry extra incendiary bombs, to ignite the largely wood buildings of major Japanese cities.

The result was the fire raids, the most destructive bombing campaign of the war, even more destructive than the atomic bombs.

Guns on bombers have been dwindling since then, with the B52 having only one tail gun, while the B1 and B2 have no guns. Still, during Linebacker II, two MIG21's were shot down by B52 tail gunners, so the guns weren't completely useless.