Would the Wright Flyer have flown better with thicker airfoils? The Wright Flyer(s) flew with very little wing thickness,would they have flown higher, faster and longer if the wings had greater thickness?

Secondly, how do thickness and camber affect a  wing's performance?

There were several other limitations on the first Flyer besides the airfoil -- the engine weighed four times what the brothers had been promised it would (because the block was cast, rather than fabricated from sheet), and would have been underpowered by today's standards even if it had delivered to spec.  The whole machine came in just under 300 kg; a modern ultralight that size would weigh less than half as much and have nearly twice the power.

Given those limitations, the airfoil is a relatively minor consideration.  Yes, there are modern airfoils that would perform better in terms of coefficient and lift-to-drag -- but the thing that would have helped the most would have been saving 40 or so kg. on the engine weight.

It's worth noting that through the First World War, with aircraft flying at four or five times the speed of the first Flyer, at altitudes up to 4000+ m, inverted and other aerobatic maneuvers, what we'd call relative thin, cambered airfoils persisted.  Look at the wing section of a Sopwith Camel -- though slightly thicker and more deeply cambered than the original Flyer's airfoil, it was still thinner than anything you'll find today except on single-surface hang gliders and ultralights.

How thickness and camber affect flight characteristics is subject matter for a full length college level textbook, but short version: more camber tends to increase lift coefficient over some range of angle of attack, but increases drag in proportion; adding thickness increased coefficient over a broader AoA range, with less drag increase, but has less effect at low AoA than increasing camber.  There's a compromise, with both increased by similar amounts, that includes the well-known Clark Y, NACA 6409 and 6412, and many other common airfoils used in general aviation, especially when wood and fabric were the building materials.


How can I find the location and coverage of aviation VHF transmitters? In the USA, is there a way to find locations of VHF transmitters that are used for air traffic communication, and the frequencies they transmit/receive on? There are some places (Northern Bay Area, CA) where I fly where I have good signal/communication with a Terminal Radar facility, but have trouble contacting area approach in the same area - especially when low (<2000ft AGL). The area is hilly.

I'm interesting in seeing range and altitude support for a given frequency (in a given area).

This is a useful site for the Oakland (ZOA) Air Route Traffic Control Center (ARTCC), and the locations of these match up with the RCO locations on the sectional. It seems Mt Tamalpais RCAG is the one that applies to my area. The list seems a little out of date, however, as the location of "Priest - RCAG" doesn't have anything on the sectional, or in Google Maps.

Further, there is an FAA site depicting RCOs and frequencies for Alaska.


How are small airplanes steered on the ground if they don't have tillers? I have noticed that many small planes do not have tillers like in big aircraft.  So how does the pilot steer them?

Most small planes (there are exceptions!) have three wheels - two fixed wheels, one under each wing, and one under either the nose or the tail.

When the third wheel is under the nose ("tricycle" layout), that wheel is usually (but not always) steerable with the same pedals as the rudder, enabling accurate steering on the ground.

(Image Source: AvWeb - Author: Joseph Burnside)

The rudder itself will also add an element of steering moment depending on the airflow over the fuselage (taxiing speed is usually too low for this to have much effect, except that on single engine aircraft applying power will push air over the fuselage and tail, adding effectiveness to the rudder).

In addition, for sharp turning at low speeds, differential braking can be applied - press the brake pedal for the starboard main wheel only, and the plane will tend to pivot about that wheel - ie turn right.

When the third wheel is under the tail ("tail-dragger" layout), the tail wheel is not usually steerable (it usually pivots freely - but again there are exceptions!). In this case the other two techniques mentioned above - rudder and differential braking - are the only methods of steering on the ground. I've never flown a taildragger myself but I understand the taxiing technique takes some getting used to.

(Image Source: WikiMedia Commons - Author: RenioLinossi)


Why at some airports/airfields you need to fly a non-standard circuit/pattern? Why at some airports/airfields you need to fly a non-standard circuit/pattern, what are the specific reasons for this and what are the differences between them while flying?

The specific reasons will likely vary with each individual airport, but the most common reason I've seen is noise abatement.

If the north/south runway is on the west side of town, using right traffic when landing on 18 will keep the planes from flying over the houses.

From the pilot's point of view you fly the circuit making right turns instead of left turns.  It's easy to do and is covered in initial training.


In an emergency near a remote island is it better to land resulting in the airport closing or ditch in the water? Consider a landing at a remote island (like Easter Island or perhaps one with a calm lagoon, like YLHI) in which the aircraft is likely to crash and close the airport. (For example a gear-up landing at a single-runway airport.) Under these circumstances would it be better to ditch than to attempt the landing at the airport, thus keeping the airport open for emergency aircraft?

If the above question is unanswerable: Has there been advice issued from an aviation safety authority regarding such a scenario?

In general, it's a bad decision to ditch if there's an alternative, even a gear-up landing (Sully ditched in the Hudson because the alternatives were much worse: virtually certain total loss on board, plus significant ground casualties due to dense population).  The safety of the crew, passengers (if present), and people on the ground is the first concern of a captain or PIC.  Ditching seldom ends well, and often results in 100% fatalities, because even with a perfect ditch, rescuers often can't reach the aircraft quickly.

For a sufficiently remote location that incoming flights may not be able to return or divert (like the example, Easter Island), there may be an option to land alongside the runway (potentially a better choice for gear-up anyway, in a location without a large fire crew, since grass is less likely to start a fire than tarmac), both for safety and to keep the runway open.


Who invented the cabin chime? What aircraft was the first to use it? While looking for one to set as my ringtone, I became interested in the history of the cabin chime - the "bing-bong" noise heard on practically every commercial airliner today. I've found tons of downloads for the sound itself and a few posts by pilots and cabin crew on what different chimes or sequences of chimes mean, but nothing on its origin. Anyone?

I can't definitively identify the true origin of the chimes; but the earliest reference to them I can find is in:

ARINC Characteristic 715-3 Airborne Passenger Address Amplifier (PA AMP)
This standard describes the characteristics of a PA AMP designed for
installation in commercial aircraft. Its function is to amplify control voice,  music and internally generated chime audio signals to drive the aircraft cabin  loudspeaker system.

The original ARINC 715 was published in 1978.
The standard they defined is in section 3.4 of the document:

3.4 Chimes
The PA Amplifier equipment should include a chime generator capable of
providing two tones in the four remote controlled sequences that may
be deployed through either the auxiliary or main amplifier utilizing
aircraft strapping. Internal controls shall be provided to adjust the
individual tone levels for balancing the related outputs.
3.4.1 Tone Frequencies
The high tone should be 587 Hz (musical Note D) and the low tone
should be 494 Hz (Musical Note B) signals of a pleasant sounding
musical nature.
3.4.2 Tone Harmonics
The chime tone harmonics should be minimized with the second harmonic
level at least 30 dB below that of the fundamental and each succeeding
harmonic should be a further 3 dB below the preceding harmonic.
3.4.3 Tone Decay
The single high and low tone levels should decay to a level 30 ±6 dB
below the peak level within 900 milliseconds from the peak. The second
tone of the high/low combination should begin 900 ± 100 milliseconds
after the peak of the first tone.
3.4.4 Sequence Control
Five tone sequence control lines should be provided to actuate
individual or two tone sequences as follows:
a. Passenger Call (High Tone)
b. Attendant Call (High/Low Tone)
c. Attendant Emergency Call (High/Low Tone Repeated Three Times)
d. Seat Belt Sign (Low Tone)
e. Smoking Sign (Low Tone)


When does the airline check if an airplane can reach its destination? It is known that - for example - LaMia Airlines Flight 2933 needed to travel 1,605 nautical miles (2,972 km), which exceeds the range of the Avro RJ85 at 1,600 nautical miles (2,963 km).
At what point in flight planning does the airline calculate and determine if an aircraft has enough fuel/range for its intended flight?

The spec-sheet range is not a fixed value—

The range of the RJ85 (BAe 146-200) is 1,570 NM with standard fuel capacity.
Range values are only valid for certain (marketable) payload/fuel levels.
If you have less payload and/or more fuel, you increase the range. Example here.
If there is tailwind, the [air] distance to destination is reduced (apparent range is increased). That's why marketed range circles are not circles, as shown below. Example here.

  The wind may provide a head- or tailwind component, which in turn will increase or decrease the fuel consumption by increasing or decreasing the air distance to be flown.— Wikipedia

Corrected distance (after the average-wind is taken into consideration) is checked in the flight planning phase, and the plane is loaded accordingly (fuel and payload). If the aircraft can't reduce the payload to reach its destination, then a technical stop to refuel is planned.

Google Flights shows all the airlines stopping at least once on the way.

(Source) Range circles take into account the trade winds.


There is any technical reason for the new Airbus shades? There is any technical reason for the new Airbus shades? Or they exists just because it is sexy?

It's not just for the looks. The A350 is the first Airbus ever to feature curved cocpit windows to reduce drag, and according to Airbus, the black "mask" contributes to ease of maintenance and harmonization of the thermal condition of the windows:

  "The perfectly curved shape of the nose helps the air flow hug the surface, in the least turbulent manner, thereby reducing drag. The emblematic “Ray-Ban” like black windshield eases the window’s maintenance and contributes to harmonising the thermal condition of this temperature-sensitive window area. ..."

Virgin Atlantic blog - Why does the Airbus A350 wear a Zorro mask?


Why do airplanes bank sharply to the right after air-to-air refueling? Whenever I see videos on YouTube they bank really sharply to the right after disconnecting from the tanker. Why wouldn't they do a more shallow 30-degree bank?

See this video for reference:

Fighter jets are very maneuverable, so they may make anything from a shallow bank to a breakaway maneuver. The breakaway is a standard way for fighter jets to exit a formation. It provides a way to safely and quickly gain separation from the other aircraft.

In this case the bank is fairly shallow, but when the aircraft disconnects they are already in a shallow right turn, so the total bank angle is a bit larger.


What do these temperature data listed in the EASA Type Certificate Data Sheet mean? Could you tell me what is the Maximum Permissible Indicated Turbine Temperature (ITT) and the ambient limit temperature for sea level static thrust?

The Maximum Permissible Indicated Turbine Temperature (ITT) is the maximal operating temperature that you can accept without reducing power or shutting down the engine. This temperature is taken at the turbine entry and the limit is there to ensure good cycle endurance of the turbine blade. Higher temperatures and you will have increased fatigue degradation due to reduced mechanical properties. Now your question is why are there 3 different ones:
The maximum continuous is the maximal operating temperature of the engine for infinite life cycle endurance. At that temperature the engine turbine should not wear for the whole life of the engine. But in certain condition it might happen that the temperature exceed this value and in specific cases it was taken into account during the design.
The take-off temperature is what is going to dictate your power input, obviously you want 100%, but high AOA, slow relative speed means that your engine is not max performing and as you are applying full power, you might end up with a richer air-fuel ration than normal condition and see a temperature above your maximum continuous quite often. But the 5 first minutes of the flight at takeoff are critical so reducing power would be not good for safety, thus you allow for slightly higher temps, for a short amount of time.
And for the Maximum starting temp is the absolute limit you should never exceed. During startup the airflow inside the combustion chamber is not perfectly installed and it might happen that the combustion flame come closer to the turbine in addition to the richer air-fuel ratio than in normal condition, thus seeing increased temperature. But this should not extend for more than a few seconds or you'll have to shut down the engine.
Concerning the sea level static thrust this is the maximum output you can ask from the engine and only up to the max temperature indicated. Above that temperature you would see a decrease in performances to prevent exceeding ITT. This would then resume in reduced takeoff performances.


How does the use of Leading Edge Root Extension help in increasing stall angle? Leading Edge Root Extensions were first used in Northrop F-5A/B Freedom Fighter, which first flew in 1959. After that, many fighter air-crafts have used it, some of which are mentioned after the question. The reason behind using such extension is delay of stall at very high angles of attack. How does the use of Leading Edge Root Extension help in increasing stall angle?

Northrop F-5A/B Freedom Fighter

author unknown, public domain (source)

Sukhoi Su-27

photo by Dmitriy Pichugin, GFDL 1.2 (source)

McDonnell Douglas F/A-18 Hornet

photo by LCPL John McGarity, USMC, public domain (source)

The idea is to generate a vortex near the fuselage. In most cases we do not want any vortexes, as the unnecessary air movement always causes an increase in drag. In this case however, you need them.

As the air flow separates from this small triangle wing while at large angles of attack, it gains a rotary motion (if you look at the left wing standing in front of the aircraft it will rotate clock-wise). This vortex inducts further swirling movement across the wing.

Now, if you are familiar with fluid dynamics, you can imagine that this will decrease air pressure at the top of the wing (not much impact on the bottom side), which is what we want - higher lift is generated.

What is more important in this the case, is the boundary layer. If you provide it with energy, making it more turbulent, it will pay off with sticking to the wing surface even while flying at large angles of attack. Obviously the larger is the angle without stall, the higher lift can be generated. Therefore an airplane with wing extensions can perform sick maneuvers like cobra and other more useful in aerial combat. Of course it requires a huge amount of spare thrust, but you do not have to worry about that in military jets.


How many aluminum panels on the top of a 172M wing from 1972? I'm looking into getting a wing to do some experiments prior to modifying an actual aircraft. Thanks to awesome folks on this forum, I've been able to find the wing of a 172M from 1972 that I can pick up now.

However, I still need to know approximately how many panels are on the top of this specific wing. For example, anything like this 172S or this 172P would be perfect for me, while this PA28-140 configuration wouldn't work (too many panels). Do you know how I could find out about those panels for the 172M from 1972?

Thanks!

You way want to take a look at this video of a 172 wing rebuild. The wing is not lots of tiny panels but what you are seeing is the areas that the rivets are placed to hold the larger panel on.


What DAL are Humans? I am no expert at the Development Assurance Process, but hypothetically speaking what Design Assurance Level (DAL) would a human pilot receive if they were classified as one (i.e. DAL A, B, C, D, E). It seems absurd to try and answer this question since the plane can't even leave the gate today without a pilot or pilots, but I am hoping to learn more about the DAL process thru any answers received.

I'm oversimplifying a bit, but more or less the DAL level of a piece of software is decided by asking the question, "What is the worst that could happen if this software fails to operate as designed?"  Note that "failure" in this case could mean that it simply stops working, but it could also mean that it does the wrong thing, like pitch up when it should pitch down. So for example,

The software that runs the in flight entertainment system.  Worst thing that could happen has no safety effect whatsoever, therefore level E.
The software that runs certain cockpit indicators, like the engine vibration display or engine oil temperature display: Usually considered a "major" safety effect, but not more than that, so level C.
The software that controls the engine thrust: if that fails to set the thrust according to the pilot demand, that could result in the plane crashing, so "catastrophic effect", thus level A.

It may be a little subjective, but I'd say that if we consider the pilot to be "software", if the pilot fails, the worst thing that could happen would be "Catastrophic" condition, and thus I would label the pilot as level A software.

Edits: clarified that DO178 applies specifically to software


How (and why) does engine thrust change with airspeed? I'm interested in how the thrust of a turbofan engine is affected at higher airspeeds (TAS). I know (I believed) that engine thrust(at constant N1) was relatively constant like in the following graph (only slight deviations):

This graph is usually in the books/manuals describing engine performance with reference to speed.

Then I came across the data from CFM56-5C turbofan engine which states that max engine thrust at cruise is approximately 29,360 Newtons while it's max thrust when stationary is 140,000 N. That's almost 5 times more power on the ground than in cruise. Here is the link: How much air, by mass, enters an average CFM56 turbofan engine cruising per minute?

These are apparently contradictory statements or I am missing something. Which one is corrrect and why? Why is engine thrust being changed with speed? Also, on the graph above what are those two curves that when added form a net engine thrust?

After I did a few calculations using the thrust equation (F= mass flow * difference in exhaust and inlet velocities denoted as delta V -> we will disregard the fuel mass flow and assume exit pressure is equal to the free stream pressure thanks to a nozzle) and following data mentioned above in the link, I found out that delta V term in cruise and on takeoff is constant (at full power) and its value is 295 m/s, which states that exhaust velocity of the engine will always be 295 m/s faster from the inlet velocity(for a maximum power setting at any speed). I think that's logical because work done by the engine is used to increase kinetic energy (delta Ek) of the airflow which increases speed always by a constant amount at specific power/N1 setting (of course less power equals less delta V).

The first diagram you link to shows three lines but does not indicate what they represent. I guess the bold line is thrust over speed. Then this diagram is correct for a turbojet.

Thrust $T$ is the difference between the engine's exit impulse minus the entry impulse: $$T = (\dot{m}_{air} + \dot{m}_{fuel})\cdot v_{exit} - \dot{m}_{air}\cdot v_{entry}$$
The exit speed $v_{exit}$ of a turbojet engine is almost constant over flight speed (relative to the engine of course), so as the engine accelerates, a larger entry impulse must be subtracted from a nearly constant exit impulse. Thrust drops slightly over speed.

At higher Mach numbers, precompression from the ram effect at the intake raises the pressure level (and hence the mass flow $\dot{m}_{air}$) inside the engine, so it will develop more thrust than in static conditions. This effect causes the thrust line to bend upwards at higher speed, and since the precompression grows nonlinearly with speed, the initial drop in thrust is soon reversed. Of course, now the fuel mass flow $\dot{m}_{fuel}$ will grow in the same way, so the fuel efficiency (thrust per fuel used) will continue to drop as speed increases.

Only when flight speed approaches the exit speed of the jet will thrust go down again. The typical exit speed of a turbojet is easily supersonic, therefore this type of engine is well suited for supersonic flight.

  max engine thrust at cruise is approximately 29,360 Newtons while it's max thrust when stationary is 140,000 N

Here you have two effects combining to lower thrust. One is the reduction in the difference between entry and exit speed. This is more pronounced in a turbofan engine because the bypass flow will be accelerated much less than the core flow, and a higher flight speed will cause a proportionally bigger drop in thrust.

The second effect comes from the difference in air density between ground and cruise: Air density at a typical cruise altitude of 35,000 ft is only 0.38 kg/m³ or 31% of the air density at sea level. The original source for the cruise thrust number does not say for which altitude the figure is valid, but you can be sure that it is for about one third of ground density. Mass flow $\dot{m}_{air}$ is directly proportional with ambient density, and both effects combine. However, most sources give only a drop to a quarter of static thrust - the last table in the linked answer looks like someone mixed the values for the CFM56-5A and the CFM56-5C.


How thick is the skin of an aircraft like the Airbus A350 or Boeing B777? We can evaluate how thick walls are by looking at door or window frames, however that isn't representative of an aircraft fuselage, which appears to be very thin:

Airbus A350 XWB forward fuselage, source

How thick (thin!) is the fuselage?
Do some areas have a thicker skin for some reason?
What about glass, doors, wings, floors, and other elements than the walls?

(I'm talking about the aircraft skin itself, not stringers or other structural stiffeners.)

From whats written here

  f100 skin is 1.0 - 1.4mm

  b747 skin is 1.8 - 2.2mm

  a320 skin is about 1.1mm

According this article on the 757

  ...Although Boeing specified that the skin in that area of the
  fuselage must be 0.039in (0.99mm) thick...

Some interesting info here as well.


What are the factors considered while placing a propeller on a plane? I have read the post where it describes about the push and pull configurations of the engine, advantages/dis-advantages in them but not the purpose.

I have seen images of a figter where the propellers are placed both at the front as well as back, such as the Dornier Do 335.

(Image Source: WikiPedia - Author: Ad Meskens)

Later there are some downsized planes which are remotely controlled vehicles like the MQ-9 Reaper where the propellers are placed at the rear, Miniature UAV have the props in the front (sometimes even the direction of the propeller also vary)

Is it the size which makes the difference or the purpose?

Also what was the scenario due to which the designers had to go in for a varying propellers why not stick to the simple front facing propellers (which are placed either on the wings or in the front why to go for propellers with backward design/rear placed)?

The biggest factor is familiarity: What has worked before?

Other factors are:

Propeller size: Slow-turning propellers driven by powerful engines have a big diameter. This is desirable because it improves efficiency. However, now the propeller cannot be placed behind the wheels if the rotation angle should not be restricted.
Engine location: Driveshafts cost weight and run the risk of resonance problems, so the propeller should be directly in front or at the back of the engine. The engine location is dictated by the desired mass distribution, cooling, the pilot's field of vision, and space demands.
Interference: This includes all factors like prop wash and swirl. Putting the propeller ahead of control surfaces or flaps improves their effectiveness at low speed. Conversely, the increased flow speed increases friction drag on the surfaces wetted by the propeller's slipstream.
Stability: The propeller acts like an additional small wing and creates lift and side force when not exactly perpendicular to the inflowing air.
Engine-out performance: Multi-engined aircraft should still be flyable with one dead engine. This is an interaction between propeller location and control surfaces, and to minimize drag, those control surfaces should be as small as possible. Thus, the propeller should be close to the center of gravity.

The rear propeller is more efficient, because it helps to prevent separation on the surfaces ahead of it, and its slipstream will not increase the friction drag. However, integrating a propeller at the back of an aircraft is hard, so few designs (apart from those in this answer) made use of it. Notable examples are:

A range of British fighters during the early years of WW I, when only the Germans had mastered the technology of synchronized machine guns.
The Zeppelin airships. Their gondolas had rear-mounted propellers.
The Convair B-36. The pusher location in the wing was selected for better efficiency, but created cooling problems for the engines.
The Learfan, started during the first oil shock and designed for efficiency. Two PT-6 drove a single pusher propeller.
The Piaggio P-180 Avanti, which was designed a few years later and much for the same reason as the Learfan.

Also, Claude Dornier had used two engines in one engine pod, driving a propeller each at the front and at the back, as early as 1915, when he worked for Zeppelin on a giant metal flying boat. This became a common theme on many Dornier aircraft, especially the flying boats, and on the Zeppelin Staaken giant bombers. The last example of this range is the Dornier Seastar, designed in the late 1970s.


How to navigate at night after a complete navigation systems failure? How do pilots deal with incorrect navigation readings at night? How do they know where to fly?

At low altitude do they have to know the shapes of cities and villages?
At high altitude and speeds over an ocean, what is the procedure?

This is no different then flying during daylight. Your first choice is GPS ( for most people :)). Failing that you fall back on radio navigation ( VOR ). Failing that you fall back on your charts and NavLog. NavLog is essentially pilotage and you rely upon your airplane speed and direction and checking waypoints along the way. If you chose your waypoints carefully ( on the ground when creating your NavLog, I use airports so that I can land of them in case of an emergency) and you know how they look like from air ( I take satellite photos ), assuming that your waypoints are 10-15 minutes apart and you took care of wind speed and direction in your navlog, there is really a minimal chance of getting lost.

Ocean is a different matter. Maybe you will need to look at stars or something. In VFR we are told not to fly over the water, day or night.


What kind of microphones does aviation use? Do they use the dynamic ones or cheap ribbon ones? Also why are these microphones so small?

I guess they dont use condensers. (I am yet to see an 3.5mm jack (which is used both on the C172 and the B747) that can supply 48V phantom power to the one of those)

Microphones used in aviation are usually electret condenser microphones that do not require any phantom power (obviously there are different types as well).

While General Aviation Aircraft and many airliners use the dual PJ connectors (PJ-055 for headphones + PJ-068 for microphone), the Airbus and ATR aircraft (and probably some others) use XLR connectors. Most helicopters use the combined U-174 connector for both headphones and microphone.


How would you explain Bernoulli to a nine year old? This is a aviation principles question coming from my nine year old daughter that I need help with. I was explaining to her the principle of flight and the fact that you need airflow under the wing in order to create lift and that this is why an airplane in a steep climb will stall etc. So she said, why not have two large fans that blow air across the wings (we are not talking about the propellers here, but rather an external fan to create air-flow) so there is always un-interrupted airflow!!!! I am not sure exactly how to answer this. The best explanation I could think to give her was just as a sail boat cannot sail simply by placing a big fan on the boat.....but I don't think she was convinced.

Bernoulli's principle is probably not the best starting point to explain lift generation, especially if you do not first explain the origin of the Bernoulli effect! Naïve invocations of Bernoulli have led to a number of incorrect 'explanations' that are quite widely circulated, such as the idea that air flows faster over the upper surface of a wing because it is more curved that the lower, and thus creates a longer path. If this explanation were correct, paper airplanes and inverted flight would be impossible.

Bernoulli's principle is a consequence of Newton's laws of motion applied to incompressible fluid flow. To explain lift, you can start by invoking Newton directly, by observing that lift is the reaction to the wing's downwards acceleration of the airflow around it. Bernoulli enters the picture when you get to calculating the changes in pressure and velocity within that airflow.

From this perspective, we can see that while blowing air over a wing would create some lift, as the wing will deflect at least some of that airflow downwards, you would get more lift by pointing the fan directly downwards. This is, of course, what a helicopter does, and it takes a lot of power, so wing-blowing, being a less-effective way of generating lift, would require even more. If, instead, you use the fan as a propeller, you only have to produce enough thrust to counter the drag, which may be as little as 1/50 of the lift.


Distance to the runway/ touchdown point Trying to learn about aviation, and looking through all the displays in the cockpit, and also the Garmin G1000,  I am not sure where I see the distance to the runway touchdown point, or the start of the runway. Sure, I see the ILS glidescope indicator, telling me I am too high/low, but where is the runway?

Ergo, my question is this: If flying in heavy fog, dark night, let's say visibility down to 2 meters! how does the pilot know where to land? Some indication of the distance would be great.

Thanks a lot, much appreciated.

There is no distance to the touchdown point because there isn't a set touchdown point. there's no runway start distance because that's not what you are aiming for. The purpose of the ILS is to get you close enough to the runway to complete a landing visually. If the visibility is so bad you cannot see the runway even when you are right on top of it then you need an airplane with a completely automated landing system to legally shoot the approach.

What you do have a DIS (Distance) reading in nautical miles to the ILS at the top of the screen, which usually tells you all you need to know. This distance is what you use in the approach procedure which you get from the approach plate for the runway and category of approach you are doing. If you get to the missed approach point, which is a set distance from the ILS, without seeing the runway lights then you must go around.

I'm going to assume you are working with a simulator like x-plane or MFS, in which case you aren't bound by procedure, and you can give it a try without endangering anyone. The ILS in most airports is placed right next to the runway, about where you'd want to aim a touchdown, so in a white out condition you would follow it down as far as you can until you contact the ground. It's been successfully done in emergencies, there's a good deal of pucker factor involved.


Why are FDR's called "black boxes" when they are actually orange? Why do they call flight data recorders a "black box" if it's orange in color?

Just curious about this since I've seen several images of black boxes but they all come in orange colors, and not black.  I can understand orange being used to help brightly identify the box , but then why call it a "black" box?

The Wikipedia article on Flight recorder offers a possible explanation for the origin of the term:

  ... they were essentially photograph-based flight recorders since the record was made on a scrolling photographic film. The latent image was made by a thin ray of light deviated by a mirror tilted according to the magnitude of the data to record (altitude, speed, etc.). Since the inside of the recorder was pitch black, this may be the origin of the "black box" name, often used as a synonym for a flight recorder.

Interestingly, the Wikipedia article on Black box claims that the term entered the English language around 1945, which is a few years after the first flight data recorder was built.


How many crew does a P-8 operate with? According to Reuters, Iran says it refrained from shooting down U.S. plane with 35 on board. The article goes onto say the aircraft was a P-8 Poseidon with 35 on board:

  Iran refrained from shooting down a U.S. plane with 35 people on board
  that was accompanying the downed drone in the Gulf, a Revolutionary
  Guards commander said on Friday...

  Amirali Hajizadeh, head of the Revolutionary Guards aerospace
  division, was quoted as saying by Tasnim news agency: “With the U.S.
  drone in the region there was also an American P-8 plane with 35
  people on board. This plane also entered our airspace and we could
  have shot it down, but we did not.”

I thought the P-8 had a crew of about 12 or so. I thought two or three were flight crew, and the remainder were technicians working with the electronic warfare packages.

Is 35 an accurate number for the P-8?

This diagram clearly shows it would be almost impossible to fit nine crew plus an additional twenty-six people in the aircraft. There are no seats for the additional twenty-six. At most you can fit two additional people in the observer seats behind the power distribution packs. The additional seats in the middle are for crew only during takeoff and landing. The aft cabin only has one seat for the mission planning workstation, and that's one of the nine crew seats.


How do UAV designers improve visual quality without compromising computational efficiency in mini UAVs? I'm designing an Unmanned Aerial Vehicle(UAV) similar in size to the RQ 11B Raven.

This UAV can carry color and infrared cameras to capture outside scenes in real-time. The quality of imagery that these UAVs relay to the operators is directly related to how much of information they can extract from the frames being captured in real-time. So far the captured video looks jittery and not appealing due to vibrations of the platform. I am encountering two issues here

issue of oscillations ; and
low frequency vibrations

Can you guys please suggest how UAV designers have managed to address these 2 issues to improve visual quality without compromising computational efficiency?

First look at the frequencies involved. If your camera frame shakes so badly that the picture quality suffers visibly, I would suspect that your camera mount has an eigenfrequency close to whatever causes the vibrations (the engine, most likely). Maybe it is enough to shift the eigenfrequency of the camera mount away, either by changing the mass of the camera or by changing the stiffness of the mount.

It could also be that some structure other than the camera mount is in frequency neighbourhood with the engine and the camera is mounted near an antinode - maybe it is enough to shift the camera location a little.

Vibration problems are hard to diagnose from the little information you give, so all I can do is to guess the most likely cause of your troubles.


What is the benefit of a horn aileron? Like the one above. Is it just so that you can hinge closer to the leading edge of the aileron? Why did the designers of the ATR-72 elect to use this feature?

There are two coefficients which determine the forces needed to deflect a control surface (besides the physical parameters like area and dynamic pressure):

Change in control surface hinge moment coefficient $c_r$ with angle of attack $\alpha$ : $c_{r\alpha}=\frac{\delta c_r}{\delta\alpha}$
Change in control surface hinge moment coefficient $c_r$ with control surface deflection $\eta$ : $c_{r\eta}=\frac{\delta c_r}{\delta\eta}$

By shifting the hinge line back, an overhanging nose of the control surface is created which reduces hinge moments because the forces here will counteract the forces on the rear part of the control surface. This overhanging nose will reduce both coefficients equally.
In contrast to that, the horn, while also creating counteracting forces, will affect $c_{r\alpha}$ more than $c_{r\eta}$. This allows the airplane designer to tailor the ratio between both coefficients. Why this is important becomes clear once you look at the floating angle of a control surface:
$$\eta_f = -\frac{c_{r0}+c_{r\alpha}\cdot\alpha}{c_{r\eta}}$$
Normally, both coefficients are negative and the absolute value of $c_{r\eta}$ is a bit higher than that of $c_{r\alpha}$. And that is good: A positive $c_{r\eta}$ means that propelling hinge moments will increase with flap deflection, making the flap unstable and run into its stops immediately. But a positive $c_{r\alpha}$ just means that the control surface will deflect into the airstream when angle of attack changes, increasing stick-free stability over stick-fixed stability. So it is desirable to make $c_{r\alpha}$ very small while keeping $c_{r\eta}$ decidedly negative. This can be accomplished with a horn on the control surface.


Do pilots adjust the throttle setting during a Red Bull air race? During a race do the pilots adjust their throttle settings to control speed, or do they leave the throttle wide open the entire time?

The only reference I could find is here (watch the video too):

  Pilots keep throttle on full for racing.

It is a race. Only the fastest one wins.


What would happen if a Cessna flew into a power line? When I was going through my private pilot training, I asked my instructor what would happen if a Cessna flew into a power line. His response was that the power line would go through the single engine plane like a cheese slicer, no challenge.

How accurate is this statement? I - luckily - haven't come across any evidence to support one way or another.

There are a lot of variables so its hard to make a flat out call. Here is at least one example of it happening that was caught on film. In that instance it seems to have ripped through the cords (which also appears to have started a fire). However here is an example of a similar situation where the plane simply got tangled into the cables and hung there. This one from my home field (ill go do some more digging) but it looks like the cable ripped it apart.

I would say you have a few mitigating factors,

Is the engine running? the prop is sure to do some slicing on impact if your engine is running and your prop is turning.
The cable type. As Ron mentions in the comments the type of cable will have an effect on this. The higher voltage cables tend to be solid twisted cable securely affixed to the larger towers. While the stuff out side your house is of a much smaller gauge and possibly is a phone line or other coax line etc.
Speed and size. There are lots of aircrafts that are all capable of very different types of flights. A Mooney Mite on departure is going to impact a power line in a far different manner than a King Air on departure. Its simply a matter of inertia.
Insulation Joints. Power lines are generally connected to towers via some kind of insulated connector. Often times the connectors used can break away under enough force allowing the line to drop to the ground. This of course depends on the type of connection joint and direction of force applied.


On the A320 why do we follow this particular sequence of taking flaps and gear? First we take flaps 1 then 2 after that we lower the gear followed by flaps 3 and full. My thinking is that this will avoid any GPWS warning is that it or there is something else also to it.?

You would want to start with flaps/slats. The landing gear slows the aircraft down like an airbrake, and you'll need a higher lift coefficient to retain enough lift. Slats are a safety feature that allow for a higher angle of attack for the main wing, flaps provide a higher $C_L$ by themselves. So you start with a higher $C_L$ and the safety of the slats, then deploy the gear which slows you down, then delay more flap/slat at the lower velocity.

Deploying full flap/slat first and then deploying gear is also not a good idea: speed will be slow already at max flaps, gear deployment brings a significant further deceleration which may have an impact on safety, definitely on continuity throughout the landing process. You are in a queue with lots of others at a busy airport, and want to keep the array of craft flowing without sudden decelerations.


What are the pros and cons of a lift fan versus a helicopter? F-35 uses lift fan which can create 20,000 lbs (9 tonnes) of lift in a small compact platform. I can see three or four of such lift fans that could lift a heavier plane and is seems better than a helicopter because it can travel at higher speeds.

So what are the pros and cons of each design?
Which one is more efficient in lifting, lift fan or rotate wing?

To answer your second question first:

Generally, it is always better to accelerate a big amount of air a little than a small amount a lot. Therefore, the helicopter rotor is much more efficient when creating lift, similar to a propeller which is more efficient than a jet. In the same way, more wing span reduces induced drag since more air can be used for lift creation.

The advantages of a lift fan are mainly due to its compact size:

Low mass, low inertias
Compact size, therefore easy to tuck away for fast flight
Low torque for a given amount of lift

and the disavantages stem from the same source:

Low efficiency
High exit speed, so beware of the blast.
No autorotation possible when the engine fails.
Lift acts at the center of the fan, so you need additional jets for pitch and roll control. A helicopter can shift the center of lift away from the rotor axis (within limits) by using cyclic changes to the angle of attack of the single rotor blades.


How do elevons work to roll a flying wing? I have an RC Flying Wing that uses elevons for control.  To go up both elevons move upwards and the wing pitches up. Both go down and the wing pitches down. I’m OK with this.

However, if I want to roll right, looking from the rear of the wing, the left hand elevon goes down and the right hand one goes up.  I can see this when I fly it that it rolls right but don’t understand why?

If an upward pitch angle produces lift to go up, (like when the elevons act as elevators) then why doesn’t the right wing produce more lift and thus the wing roll left? Either a flap produces lift or doesn’t?

Good question! There's a bit of a misconception: when the elevon moves up, it actually decreases lift. It pushes air up which pushes the wing down.

This explains the roll behaviour, but how does decreasing the lift make the plane go up? The key here is that the lift is reduced only at the rear of the plane. In other words, the rear of the plane is pushed down, but only the rear. This makes the nose point up (just put the plane on the floor and push down on the tail, the nose will be up).

In this new orientation, the angle of attack (the angle of the wing relative to the airflow) is increased, and consequently the lift increases (the little bit of downforce at the rear of the wing doesn't really make a big difference). The plane will start to climb, which will reduce the angle of attack (the wing will be pointed the same way as the direction of travel), until the plane is in a steady climb.


When and why is the SELCAL (Selective Calling Radio System) check being performed? When and in what scenario is the SELCAL (Selective Calling Radio System)  check being performed by crew?

If you are unsure whether your SELCAL equipment is properly working, you can request a SELCAL check on a HF-enabled radio frequency. The ATC unit will send out your SELCAL code for testing purposes.

I'm not sure my selcal unit is working, is there anywhere I can test it?
Yes, several places.  If you are located in:

Europe or east of 30W on the North Atlantic, call Stockholm Radio
Eastern USA/Canada or west of 30W on the North Atlantic, call New York ARINC
Western USA/Canada or Pacific, call San Francisco ARINC
Asia - daylight hours, call Singapore or Manila Radio on 8942 KHz USB

[...] All selcal checks are free of charge - just call the station with the aircraft registration mark and when you receive a reply ask for a selcal check and give your selcal code.  The operator will send out your selcal tone and - assuming the selcal unit is working - you will hear a chime in the cockpit.  Acknowledge by replying 'selcal check okay' and you're done!   If your selcal check doesn't work the first time it is worthwhile asking for a 'double' which means you get 2 sets of tones sent out back-to-back and often cures any issues.
(Source: www.selcalweb.co.uk)


Why will the 777X be tested for crosswind handling with wingtips raised? Wikipedia says:

As existing regulations do not cover the folding wingtips, the FAA issued special conditions, including proving their load-carrying limits, demonstrating their handling qualities in a crosswind when raised

Why?
Especially when this question says:

The 777X family (777-8/-9/-10) features wingtips that fold up on the ground to let the aircraft fit in tight spaces. These have to be extended and locked in place before a revenue flight can take place,

So, what is the point of certifying the 777X for wingtip up flight? I haven't found a reason for this in Wikipedia either. Is it for ferry flights or something?

If you read the details in the Federal Register, you can see that this refers to crosswind handling on the ground, not in the air:

The folding wingtips and their operating mechanism must be designed
for 65 knot, horizontal, ground-gust conditions in any direction as
specified in § 25.415(a). Relevant design conditions must be defined
using combinations of steady wind and taxi speeds determined by
rational analysis utilizing airport wind data.

And:

The airplane must demonstrate acceptable handling qualities during
rollout in a crosswind environment, as wingtips transition from the
flight-deployed to folded position, as well as during the unlikely
event of asymmetric wingtip folding.


What are the components of a crew pairing? I need to do some modeling for an FRMS (Fatigue Risk Management System). By crew pairing I mean a sequence of flights that starts and ends at crew base. From my understanding, crew pairings are made of:

Duty periods, which include flight duty periods (where the crew actually flies).
Between duty periods, rest periods (overnight rest for example), or
You can also have deadheading before starting another duty period.
So basically when the crew finishes one duty period, he/she either has a rest period or some deadheading before starting another duty period.
Is there anything else that should be included in the pairing?

Deadheading is considered duty, so it is part of the duty period, not part of the rest period.
Each duty period is composed of several parts:

a report time, defined in a specified timezone (perhaps GMT, perhaps local, perhaps "home" time of some sort)
time from the report until the first departure, which is typically standardized although it may be variable (45 minutes domestic, 60 minutes international, 75 minutes international overwater, for example)
a series of flight legs, each with its start & stop times, duration, start and end airports, and deadhead status
time between each leg
time from last arrival until release from duty, which accounts for the time on duty required to post-flight the aircraft, clear customs, etc. This is typically standardized in some fashion; xx minutes if clearing customs is not required, yy minutes if it is

For each duty period that does not end back at the base, there will be the intervening crew rest period, or "overnight", which consists of the start and end times of the rest period, its duration, and in practice a specified hotel where the crew will stay.
The pairing itself will typically be published with some summary values, such as total flight time during the pairing, total duty time in the pairing, etc.  If the crew contract specifies "overrides" for pay purposes, these will generally be published as well.  As an example, let's say that the contract specifies pay credit of 1.2 minutes paid for every minute of block time between the hours of 2300 thru 0500, and a particular flight operates from 2200 to 0100.  That is 3 hours of block time, but the last two hours pay at 1.2x, so the pay credit for that leg would be 3.4 hours, or 3:24 of pay even though it's only 3:00 of block. Other contractual rules may specify a minimum pay per duty period, such as 4:00 pay, even if less flying than that takes place. The pairing summary would generally include the total pay credit for the trip; each day's summary might include how that day's pay works out.
Part 117:
The rules of FAR Part 117 specify legal requirements for duty and rest periods; they include the definition of a "Flight Duty Period", which begins at the required report time, and ends at the block-in of the last operating leg. Thus, FDP ends even when a crew remains on duty.  If the last leg of the day is a deadhead, then the FDP might be six hours but the duty day might be longer -- say an hour's ground time, and hour's flight, and 30 minutes to clear customs & leave the airport. When the crew is off duty, then the rest period begins.
Depending on the operation and the contract, the pairing details may align closely with Part 117, or there may be enough additional considerations that what's published on the pairing may go way beyond the Part 117 definitions.  The FDP as defined in Part 117 (along with the other limitations in that regulation) are still tracked, although in some cases contractual constraints may be more limiting or require more bookkeeping (i.e. accounting for pay rules and contractually required rest periods).
FRMS:
When you're looking at things from the perspective of a Fatigue Risk Management System (FRMS), there are other factors to consider.  Not everything that is legal is necessarily safe, and just because some sequence of duty periods is legal doesn't mean it won't be really fatiguing.  For example, a sequence of 11-12 hour duty days flying in the same time zone may be fine, while the same sequence of 11-12 hour days crossing 5-6 time zones will shortly have the crew operating on the "back-side of the clock" in local time, which has its issues.  Worse, if each 11-hour day is followed by a 16 hour overnight, the crew could be waking up later & later (relative the their body-clock time), but operating earlier and earlier in the day (as measured by daylight/nighttime looking out the window) -- which is a serious setup for "not knowing what time it is".  An FRMS would probably need to consider how well or poorly a pairing or set of pairings works when considered against circadian rhythms and similar factors.  This would involve tracking data beyond what is published for the typical crew trip-sheet.
At a minimum, you might want to see flights and days and rest periods considered in both a constant (say, "home" time) reference and also a "local time" reference -- even if what is published to the crews is all stated in Zulu time.
Block Time:
The question was asked in comments, when does Block Time begin & end? When the schedule is being built, Block Time corresponds to the scheduled departure and arrival times of the flight. The book definition of block time is that it begins when the aircraft begins forward movement under its own power for the purpose of flight, and ends when it comes to rest with no further movement intended. In practice, block time is almost always recorded by ACARS, which uses "all doors closed + brakes released" -- i.e. the start of the pushback operation -- as the block-out time, and "entry door open" (i.e. pulled into the gate and door is opened to let people out) as the block-in time.
This practice slightly over-states the block time as compared to the book definition, since one can release the brakes, then begin the pushback (under tug power, not aircraft power, and not "forward movement"), start engines, and so forth, and several minutes may elapse before the aircraft actually moves forward.  And, if you arrive at the gate but the jetbridge isn't quickly pulled up to the aircraft, you may be stopped for some length of time before the ACARS records the "in" event.  These distinctions are evidently acceptable to the FAA, since this seems to be a fairly universal practice to use ACARS times.
Typically, the reporting from the aircraft back to Dispatch or headquarters will have "out" (i.e. out from the gate, block time began), "off" (i.e. off the ground, flight time began), "on" (i.e. on the ground, landed), and "in" times. The aircraft logs are mainly interested in flight time, from "off" to "on", while crew recordkeeping is pretty much exclusively interested in block time, from "out" to "in". When FAR 117 refers to flight times, it is actually referring to block time "out" to "in" and is unconcerned with actual takeoff & landing times. One explanation being, the crew is as much responsible for the operation of the aircraft while taxiing around JFK on the ground, and needs to be alert in order to avoid mistakes (entering a runway without clearance, for instance), as much then as when they're in the air. So all "operating" time counts the same, on the ground and in flight.


For a coded departure procedure (SID), Fly-At-Altitude or At-Waypoint instruction has priority in case of conflict? I am examining various RNAV departure procedures and especially those whose purpose is to overlay the existing conventional ones. I noticed that some of them begin with a "Flight at Altitude" instruction, immediately followed by either a DF or CF leg which end at a Waypoint very close to the associated airport. As a consequence, an aircraft whose initial climb gradient is small enough will reach the specific altitude well after it has passed the aforementioned waypoint and as a result, if the CF/DF leg must be followed, it must fly backwards in order to comply.

Take, for example, ANEKI9F overlay departure procedure of Frankfurt Airport. The respective conventional procedure is charted as follows:

Below I provide its equivalent description, as it is included in Eurocontrol's AIP library for German procedures:

  Eurocontrol Charts AD 2 EDDF 5-7-34.
  ANEKI NINE FOXTROT:

  On RWY track to 4.5 DME FFM/1.5 DME FRD or 800, whichever is later; LT (MAX IAS 210 KT until established on R356 RID), on R356 RID to RID (); on R183 RID to ANEKI ().

  GPS/FMS RNAV: [A800+] - DF134 (25C)[L] / DF135 (25L)[L] - DF141 (25C) / DF142 (25L) - DF143[L] - DF137[K 210-; R] - RID[R] - ANEKI.

The problem is that fixes DF134 and DF135 are near runway's end. As a result, if the aircraft climbs even with 500ft/NM, it will still fly over those fixes. Or in other words, the aircraft will reach those fixes well before it climbs at 800ft.

My question is, which of those two instructions has priority? If the aircraft reaches DF134/DF135, will the "Fly at Altitude 800ft" order be negated and the aircraft will fly at the next waypoint (DF142/DF143) or should it ignore the initial waypoints (DF134/DF135) and keep climbing until it reaches at 800ft altitude and then fly at the next waypoints (DF142/DF143)?

If someone could point me in the right direction I would highly appreciate it.

EDIT: As J. Houggard aptly stated, an assigned adverse minimum climb gradient should exist for preventing this conflict from taking place. However, no such climb gradient is assigned, according to the following table (source: Eurocontrol AIP Library).

Even if the table implies that the assigned climb gradient of CINDY1F procedure also applies to ANEKI9F (315ft/NM), it is still not enough.

The textual description says clearly:

  RWY track to 4.5 DME FFM/1.5 DME FRD or 800, whichever is later

The equivalent encoding is:

  [A800+] - DF134 (25C)[L] / DF135 (25L)[L]

The meaning of this is climb to an altitude of 800ft or above, then after passing DF134/DF135 turn left.

From this EUROCONTROL document (Guidance Material for the Design of Terminal Procedures for DME/DME and GNSS Area Navigation(PDF)):

  [Speed, track and altitude constraints are contained within square brackets.]

  If [A Set of Constraints] is not preceded by a waypoint name, the last calculated track must be flown until the constraint is reached

The turn will thus not be initiated before both the 800 ft altitude and the waypoint are reached.


How can I calculate the velocity of air drawn by a pusher configured propeller? I am attempting to construct a model plane using Custer channel wings. In order to calculate (or at least approximate) the lift generated I need to find the speed of the air through the channel due to the propellers. Is there an equation I can use to do this given pitch, diameter, rpm, etc?

Well, the textbooks tell you how to design a subsonic shroud for a turbofan and a propeller, but they usually don't tell you how to design half a shroud for a propeller. Besides, you need to pick a profile for the wing section in the channel also.

There are theoretical equations and heuristic ones for a propeller with not too many blades, but these are not nearly as useful as a run of CFD. The key part is to translate the number of blades and RPM, given pitch, chord, span, wash, and wingtip device into $\Delta p$ across the plane of the propeller. This is better done through simulation and experimenting than working with pen, paper and slide rule.

The roughest of the rough approximation is, of course, assuming $\alpha\approx0$ for each of the blade and $v_\mathrm{i}\approx\frac{1}{2}\cdot\omega\cdot\left(r_a\tan\theta_a+r_b\tan\theta_b\right)$ and assume $v_\mathrm{o}\approx v_\mathrm{i}$, where point $a$ is the point where the blade starts near the rotating axis, and $b$ the wingtip, $\omega$ the angluar velocity of the propeller, $r_{(\cdot)}$ the distance from the rotating axis and $\theta_{(\cdot)}$ the pitch angle.

The pusher configuration means the air hits the engine nacelle before it meets the propeller, and the propeller, therefore, works in disturbed air, if it's got a gas engine it will work also in the pulsing exhaust of the engine.


How are both squawk and altitude transmited when operating a Transponder in Mode C? I know that in Mode $3$ (which includes Mode A and Mode C) the aircraft transmits $12$ information pulses. That corresponds to $2^{12}=4096$ squawk combinations in Mode A which are sent to ATC to identify the aircraft. In Mode C, all $4096$ combinations are used to send the pressure altitude of an aircraft to the ATC.
Since there are $12$ usable information pulses, when operating in Mode C, there is no way to transmit the squawk code, which in my mind raises an obvious question: What if I suddenly squawk $7700$ due to an in-flight emergency? Or $7600$, radio failure? If I continue to operate in Mode C, it seems that there is no way that ATC would know that I am in any kind of trouble. How is this issue resolved and is my understanding correct?

Squawk codes (Mode A replies) are transmitted in response to Mode 3/A interrogations. Altitude codes (Mode C replies) are transmitted in response to Mode C interrogations.
Both Mode A and C replies use the same 12 pulse encoding, but the radar that asked the question knows how to interpret the answer (assuming there is no FRUIT or garbling, but that is another topic).
If the transponder is in altitude reporting mode (i.e. it replies to Mode C interrogations), it will also reply to Mode 3/A interrogations.
Thus, if you squawk 7700 and you transponder is switched on (either Mode A only or Mode A/C), then code 7700 will be the response to the next Mode A interrogation from a radar. If your transponder responds to Mode C interrogations as well (i.e. altitude reporting is on), the radar will receive altitude information in response to Mode C interrogations.


Are snatch pickups used today? John T. Downey, who spent 20 years imprisoned in China, was captured when his plane was shot down during an attempted "snatch pickup". The spy they were supposed to retrieve had been turned by the Chinese, and they were waiting for the plane with anti-aircraft weaponry. This 1951 attempt was the first time the snatch pickup was tried in a war, according to this article about John T. Downey:

  The snatch pickup was a bizarre maneuver, and an untested one. The
  process called for an aircraft flying at low altitude to hook a line
  elevated between two poles. Connected to the line would be a harness,
  into which an agent would be strapped. The contraption resembled a
  swingset, if that swingset was designed to be born aloft by a cargo
  plane. Two men in the back of the plane would operate a pulley,
  dropping the hook intended to catch the agent and then reeling in the
  line; both the operators and the pilots required extensive training.
  Describing the operation in his 1984 book Perilous Missions, a history
  of CIA covert operations in Asia, William M. Leary writes that the
  agent was forced to sit impassively in his harness, awaiting possible
  decapitation, among a litany of other potential injuries. The pickup
  of Downey’s courier would be the first time the CIA ever carried the
  plan into action. The Air Force deemed the operation too risky to try.

Whatever happened to this technique? Is it used today?

No, although they were used up into the 1990s. Wikipedia has an article on this - called the Fulton Recovery system. Your quoted paragraph is incorrect - U.S. Air Force Special Operations Command (AFSOC) did use the system, on modified MC-130s. Despite the danger, Wikipedia only mentions one death using the system, though given the clandestine nature of many of the missions it was used on, it would not be surprising if there were other as yet unrecorded instances of its use.

Modern special operations helicopters have a much greater range, and some, like the MH-53, MH-47, and MH-60, are actually air-refuelable, extending their range. Additionally, the CV-22 Osprey has a much greater range and speed than a traditional helicopter. These improvements in extraction vehicles made the 'skyhook' system obsolete.

Note that this article mentions the British used the system in 2001 to rescue an injured soldier behind enemy lines.


What is the typical weight distribution ratio between nose gear and main gear? On an airplane with tricycle gears, the main gear supports most of the weight and is therefore the strongest. The nose gear prevents the nose tipping down, since the center of gravity is forward of the main gear.

What is the typical ratio of weight distribution between the nose gear and all main gears?

For the purpose of the question, I'm mostly interested in airliners, although an answer comparing that with GA would be useful as well. I understand that the weight distribution varies with loading, so perhaps a range would do (e.g. on most airliners the max nose gear loading cannot exceed X:Y).

Look at the sketch of the recommended gear placement relative to the center of gravity (cg) from this answer:

We can calculate the load distribution for both extremes. For simplification we use the same angle of 60° for the nose wheel in both cases:

Main landing gear 6° aft of cg: $\frac{\tan 6°}{\tan 60°}$ = 6%
Main landing gear 20° aft of cg: $\frac{\tan 20°}{\tan 60°}$ = 21%

Add some dynamic loading from the application of wheel brakes. In this answer it is shown that the horizontal braking force is less than 30% of the maximum static weight of the aircraft, and applying this knowledge to the nose gear at 60° from the cg adds 0.3$\cdot\cos 60°$ = 15% of the maximum static load of a fully loaded aircraft to the nose gear load.

In extreme cases the nose gear would carry almost half of the aircraft mass, but then rotation would be impossible. In case of the Go-229 it was 45%.

Semi-finished Go-229 V3 (picture source)

On the other extreme you find nose gears with zero static load which were only intended for taking the braking forces.

Zeppelin Staaken R VI (picture source)


What are the FAA currency requirements for a foreign based certificate? I hold a FAA private certificate that is issued based on 61.75. I have single engine land and instrument ratings on the foreign license (EASA). My FAA certificate also has these ratings listed.

My question now is: What are the currency requirements to execute the privileges of the FAA certificate to fly in the US?

61.75 (e) states:

  (3) Is subject to the limitations and restrictions on the person's U.S. certificate and foreign pilot license when exercising the privileges of that U.S. pilot certificate in an aircraft of U.S. registry operating within or outside the United States; and

This can be interpreted two ways:

a) The currency of a rating on the foreign license is a limitation or restriction and therefore the pilot needs to be current for both the foreign license (e.g. EASA 2 year check for SE land rating, and EASA 1 year check for instrument rating) in addition to the FAA required BFR and IPC/IR currency based on approaches etc.

b) The limitations and restrictions mean explicitly mentioned limitations (e.g. EASA PPL does not include night flying, so the US cert would have "VFR day only" listed as a restriction). The EASA license itself does not expire on its own and is valid as long as the pilot holds a valid medical. Therefore only the BFR and FAA instrument currency requirements apply to fly legally in the US.

So which interpretation is correct? Is there some other regulation which makes it more clear?

Practically "to be safe" I could just fulfill the EASA requirements every year/two years, but that is quite an effort (this can only be done by an EASA ATO which there are few in the US). It also adds additional costs.

I could also take the test(s) to get a FAA private and IR independently and everything would be FAA based only. That's an option but also requires time and effort.

If you have that situation, you should ask FAA's Office of Chief Counsel. There is no univocal position about that.

The first position is that your EASA license should not be expired (which is not a case because EASA licenses do not have expiration date) or revoked, and should not list explicit limitations (according to Krauzs 2012 and Grossman letters). In that case your 61.75 license is good to go even with expired ratings in the EASA one. The only limitation is a current medical (because the license is invalid without it).

The second position is that all your EASA ratings should be current too, so they can be considered as limitations of your EASA license.

The second position is just safer. Nobody will have questions to you: neither FAA inspector, nor an insurance company.

In any case, it is required to be current by the FAA rules (BFR, medical, approaches/holdings etc.). For example, here (especially see Dave's comment) one had a problem with foreign-based license without current US medical.

In your case I would take FAA tests and obtain a standalone license, and forget about the problem (it's what I actually did recently). If it is not an option, either keep your EASA license current (just in case) or request an official answer in paper from the FAA.

Related topics on pprune: here and here.


How do SIGMETs arrive in a cockpit on a modern longhaul flight? What is todays medium of choice (Voice Radio, ACARS,CPDLC,…) to receive SIGMET messages on a commercial longhaul flight, for example from continental Europe to the US westcoast?

Are there any differences in the regions that are passed (European Landmass, Oceanic, North American Landmass)?

This varies between airlines. In my company (large European Carrier) SIGMETs are uplinked to us via ACARS and then automatically printed on the cockpit printer.

The differences in regions is the format of the SIGMET. In the USA the points of a polygon are mostly described as list of DIR/DIST of VORs (eg 15NM NE STL). Very hard to read if you are unfamiliar with the area. Rest of the world uses coordinates (N3905W09046). Way better to locate on a chart.

There are efforts to automatically download (via satellite) and parse SIGMETs and display them in our electronic charting application in the aircraft EFB. This works currently so-so as there are surprisingly often typos or similar mistakes in a SIGMET text which kills the parser.


Can helicopters perform the Split S maneuver? In the animation movie "Planes 2" we see the helicopter (Blade character) performing first a 180° roll (getting inverted chassis up and rotor down) and then the U-shaped half loop down into the normal orientation. From Wikipedia it looks like the Split-S.

Are helicopters actually capable of doing this? The concerns are

An inverted wing still may provide a lift upwards, but the inverted rotor will pull downward. Hence the inverted orientation seems risky.
Wings provide air support while making the sharp U-turn. Can the helicopter rotor do the same?

If a helicopter would manage this figure, would it trade altitude to speed as the fixed wing planes do?

If the model is important, the helicopter looks somewhat like Augusta Westland.

Yes, but you need a rigid head.

A rigid head does not have any hinges, instead, the blades flex up and down.

Wings don't know or care if they are upside down or right way up, indeed, most helicopter blades are symmetrical.  It only cares how much airflow there is and at what angle it hits it.

Here's an EC-120 performing a split S and a Lynx showing what's possible.

Perhaps the best around is Chuck Aaron, displaying for Red Bull.

Here, Dennis Kenyon shows about as good as it can get with a semi-rigid head.  He gets close to inverted but never actually flips over.


Why use different data bus for civil and military avionics? There are various type of data bus in aviation, the two most common in the '90 were ARINC 429 for civil aviation and Mil 1553 for military applications. They are not very different one for the other, the Mil is a little more complex.

My question is: why use two different data bus? The physical cable is the same (as far as I know, you can correct me if I am wrong) but the protocol is different. I think that what a data bus must do is the same in civil and military avionics, am I wrong?

There is actually quite a lot of difference between ARINC-429 and MIL-STD-1553.  They are different because they were developed by two different groups with different needs.

Both started in the 1970's. Before then there was almost no digital data of any kind on a an aircraft.  It was almost all analog or discrete. The biggest issue was noise and interference of the signals. The introduction of microprocessors in the early 70's opened up the option for reasonably priced digital bus technology.

The MIL-STD-1553 bus was a major component of the F-16. Like most military programs, this was a push for maximum performance. ARINC-429 came out of the Boeing 757/767 development effort. Civilian aircraft certification doesn't like a lot of risk and the simpler bus architecture made it easier to design, analyze, and certify.

Since then there have been a number of new buses introduced into aviation. Some have lasted longer than others. B777 introduced ARINC-629 and Fibrechannel. B787 uses ARINC 664 for the main buses with a large mix of ARINC-429 and ARINC-825 (CAN bus) as well.  There are a number of data concentrators/converters on the aircraft to get data where it needs to go.

The primary reason for moving away from ARINC-429 is that it is a single transmitter/multiple receiver architecture. This results in a large amount of physical wire. That adds weight and requires maintenance. A bus such as ARINC-664, which is based on Ethernet, can significantly reduce wire. The down side is that it costs a lot more and isn't very efficient when all you need is a small, low rate connection to an odd corner of the aircraft.  So you end up with a mix of buses.

And B787 opted for fiber optic cable for the ARINC-664 network to reduce interference from high-intensity radio frequency (HIRF) and lightning.

MIL-STD-1553 is a powerful bus, but it's not as good a fit as many of the newer buses available in the commercial market. The military keeps using it because they have a huge installed base of equipment, it's flexible enough to support their newer functional needs, and commonality reduces their logistics costs.

ARINC-429 sticks around because it's cheap and it has its own large installed base as well.


Is there a way for an ultralight or powered glider to smoothly enter a jet stream? Once inside the jet stream altitude 7-15 km is it all that turbulent or is there a way to enter it smoothly for an ultralight plane or a powered glider? More info

Height is not the barrier. People have flown off Mount Everest - see http://abcnews.go.com/Travel/paragliding-everests-peak-kayaking-indian-ocean/story?id=21097292 as one example - though not when the jet stream is blowing over/around the mountain.

The problem is at the boundary layer between jet stream and none jet stream air when the aircraft only have air speeds of 20-50mph. The mixing of air, and the sudden hit of jet stream air speed will be well in excess of the gliders max air speed.

It has however been done in a hot air balloon. http://www.businessinsider.com/richard-branson-on-hot-air-ballooning-2014-10?IR=T


What is this airplane that sits in front of Barringer High School in Newark, NJ? This airplane sits in front of Barringer High School in Newark,Nj. No one there can identify it. Anyone have any idea of the make and model? Thanks

It's a Fourney Aircoupe  that's seen better days. Much better days...


How does the F-22 Raptor radar reflector work? The F-22 Raptor (and other stealth aircraft) can be made more visible to radar by using radar reflectors, or in other words, radar cross section (RCS) enhancers. In the comments section of this answer to a question posted on Aviation Stack Exchange it was speculated, that the RCS enhancer used in F-22s is empty, sparking further wondering whether this was in fact possible.
So the question here stands: how can such a small device improve the visibility of a stealth aircraft sufficiently to make it a clear radar target?
The RCS enhancer on F-22 seems to be less than half a gallon (4 litres) in volume.

The F-22 RCS enhancers size and shape (to some extent), as well as this source for example, suggest it is a Luneburg lens. The Luneburg lens is a device that reflects electromagnetic waves very efficiently back to the source, making it possible for a small device to appear as a big target on a receiver placed at same position as the transmitter (for example a radar).
According to  Wikipedia:

Luneburg lenses are normally layered structures of discrete concentric shells, each of a different refractive index. These shells form a stepped refractive index profile that differs slightly from Luneburg's solution. This kind of lens is usually employed for microwave frequencies, especially to construct efficient microwave antennas and radar calibration standards.

This site further elaborates on the Luneburg lens:

The Luneberg lens is a passive radar augmentation device used to increase the radar reflectivity of a target without the use of additional energy The lens reflector is a sphere in shape, usually composed of concentric dielectric shells. By the proper selection of dielectric constants for each shell, radar energy incident on one of the faces of the lens is focused at a point on the rear surface of the lens. The rear conductive surface reflects radar energy back to the source.
Principal of Operation:
(1) Basically the Luneberg lens is made of concentric shell of polystyrene with varying reflective indices (dielectric constants) from 1 to 2, the highest refractive index being the innermost shell.
(2) A ray passing through the various concentric shell will bend towards normal and then focus at a point. If a metal reflector is placed around the focal point, the ray gets reflected and retraces the same path. This is the principal of Luneberg Lenses
(3) In order to have smooth bending of the ray theoretically, many concentric shells are required. In practice, a few concentric shell are used.
(4)The radar cross section augmentation of the lens is given by the formula( 4∏3R4/4λ )
The RCS is inversely proportional to the squire of the wave length and directly proportional to the 4th power of the radius."

[sic]  Note: the correct spelling is Luneburg, after Rudolph Lüneburg
To act as a Luneburg lens, the device cannot be empty. To properly "bend" the incoming and outgoing waves, the sphere must hold some kind of inner refractive structure to make it function properly as a radar reflector over meaningful distances. A single concave spherical layer of reflective material will not achieve this.
It should be noted that the RCS enhancer on F-22 is not spherical in form, but a cylinder with rounded ends. The principle of Luneburg lens could still be applied to the device. And as always, having the inter of net as source, it is totally possible the aforementioned device operates on a different principle. Operating principles of many of the systems on F-22 are not exactly public for obvious reasons... Please feel free to overthrow this answer if better knowledge is available.


What is the division of tasks among the control towers at Chicago O'Hare airport? I was at KORD this week for the first time in decades. I spotted 4 control towers (two near the terminals, and one at each end—and I am not sure if I got them all). I was not counting the short ones, obviously for gate control.

What is the division of labor?

Is it anything like KDFW?

I was not counting

As mins pointed out, the four towers you are asking about:

Old tower, no direct responsibility
Center tower
South tower
North tower

The airport directory (accessible from AirNav) breaks out the different tower and ground frequencies.

TWR N - 9L/27R
TWR C - All others
TWR S - 10R/28L

This makes sense, as the north and south towers shown on the AD are really only close to those applicable runways. The center tower is located where it can handle the rest. The old tower is not labeled.

The ground control is also split between the towers. I couldn't find a good reference for the division of responsibility there, but I would imagine that it would be similar to that of the towers, only controlling the taxiways associated with those runways, which are hard to see from the center tower.


Do glider pilots require normal headsets? When flying light aircraft, we use headsets which include ear protection from the noise of the fan. Being that unpowered gliders are presumably much quieter, are the same headsets used?

My assumption is that gliders are radio equipped, and pilots are still talking to ATC or their airfield traffic information. If this is not the case, are headsets still used for intra-cockpit communications or are they really quiet enough to talk normally?

No, but a hat to protect the head from the sun. You may call this the glider pilot's headset ;-)

Glider pilot with hat (picture source)

The flight noise is just a faint hissing sound, and the most noise is either produced by the electrical variometer or the radio. On a busy day and with everyone on the same frequency, I often switched the radio silent when high up. When you receive transmissions from tens of kilometers away, you often receive parallel transmissions, which result in a very loud and annoying crackling in the radio.


What is the official term of a plane flying under another plane to avoid radar? I was watching the news and, just now, learned that the Russians used a "trick" by flying a fighter plane under a cargo plane to Syria and successfully avoided radar detection for the fighter plane.

I wanted to learn more about this method (a Wiki article, for instance) but failed to find anything. So do the experts here know what this term is called, if there even is one?

My copy of "EW 101: A First Course in Electronic Warfare" calls it "formation jamming".

Mind you, an operator of a radar with non-cooperative threat recognition capability will at least suspect something is wrong (although not from all aspects and ranges).


What, effectively, does a navaid being unmonitored mean? I saw a NOTAM about a VOR being unmonitored. I think this means that the voice capability of the VOR frequency is not in operation/nobody is listening. Is that right? If so, it just seems like something nice to know but doesn't affect the navigational capability of the VOR, right? If a VOR is unmonitored can I still use it for navigation, such as for a VOR/DME approach?

Actually it doesn't have anything to do with voice capabilities:

5-3-5. UNMONITORED NAVAIDs
a. All VOR, VORTAC, and ILS equipment in the NAS have automatic monitoring and shutdown features in the event of malfunction. Unmonitored, as used in this order, means that the personnel responsible for monitoring the facility have lost aural and visual monitoring capabilities and cannot observe the status of the facility. It does not refer to the automatic monitoring feature.
b. When a navigational aid's operational status cannot be monitored at the controlling or monitoring facility, but all indications or reports are the facility is operating normally, issue a NOTAM placing the aid in an unmonitored status.
c. When issuing a NOTAM describing a facility as unmonitored, do not use the category of monitor, only the contraction UNMON.
EXAMPLE-
!DCA LDN NAV VOR UNMON

(Source: JO 7930.2M)
So basically it says that this VOR may malfunction without notifying a responsible party, or that a responsible party is no longer monitoring the VOR. A lot of times this is done with VOR's that are scheduled to be removed from service, they may be allowed to continue to operate but are no longer maintained.
You can still use the VOR for navigational purposes. The VOR is supposed to shut down in the event that it becomes unusable, so you can be (relatively) assured that if the VOR is operating, it is reliable and usable.


What are the benefits of using thick trailing edge flaps? I had been reading in numerous articles (see this answer and this one) that there are advantages of using thick trailing edge flaps. In these articles it kept on coming up that Airbus uses thick trailing edge flaps also for certain reasons but I couldn't understand them. Could someone please explain?

My main question is about the aerodynamic aspect: Why do the flaps have to be made thicker?

The best flaps (aerodynamically, as you ask) will be complicated and elaborate. But complicated flaps add weight and maintenance cost. Airbus wanted to have a variable camber wing for the A340 (Flight), but they didn't pursue it to the end, due to the complexity.

As for the aerodynamic benefits of simple flaps, there aren't any, as the other post shows: slower MMO and smaller MTOW.

Slower MMO is because of the smaller wing sweep because of the simple flaps because of needing a good [slow] approach speed. MTOW is because simple flaps don't carry as much off the ground.

The simple flaps are thicker because:

The flaps are shorter chord-wise: the aerodynamic load (lift and drag) acts on a smaller surface area, compared to the complicated multi-slotted flaps.
Thin flaps would produce even less lift (thanks to @PeterKämpf).
The lack of a thrust gate: they need to be thicker to withstand the jet blast.

Also see comment by Peter Kämpf.

Comparing the A380 and 747-400 flaps. Note the thrust gate and the triple-slotted flaps of the 747.


What to be considered in designing airplane tail (vertical and horizontal stabilizer, elevators and rudder)? I would like to make a small amateur triplane with it maximum weight approximately 400 kgs with maximum speed 100 knots (180 kph). I have read many discussions about how to calculate the wing requirement in relation to the weight, the speed, the wing shape, the airfoil shape, and so on. Unfortunately not so many discussion I found here about tail part.

What is the aspect to be considered in order to have the right design? Tail parts I meant here are: vertical and horizontal stabilizers, elevators and rudder, including the airfoil shape and the area. This question is about low speed airplane only, not sonic and moreover the supersonic one.

The measurement of overall tail authority for stability and control (a function of tail surface area and its leverage or arm) is called Tail Volume and the link goes to a handy calculator page for working it out.


Why is the F-35B's elevator not in neutral position when in vertical flight mode? I was watching videos of F-35B taking off and landing vertically on ships when I realized that when transitioning from horizontal to vertical flight mode (I don't know the exact vocabulary to describe that), the horizontal elevator moves to "stick pushed full forward" position. The elevator is also put in that unusual position when transitioning from taxiing to a vertical takeoff. I know that the elevator position is not significant in vertical flight as attitude is controlled via non-aerodynamics means. Thus I wonder why this control surface is then put in that position for that flight phase instead of staying in its previous position or in a more neutral position.
I'm even more confused as this behaviors doesn't seems to be reproduced on other VTOL fighters such as the AV-8B or the Yak-141.

In this video you can see it better I think:

I drew on a screenshot of the video from the side where you can see the air moving under the aircraft.
Compare this to the Fig. 1 in the paper linked by "Anonymous Person" here in the comment section of your question: http://ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/finalworks2016/papers/03.2_3_366paper.pdf
As you can see angling the tailerons down significantly widens the gap between the leading edge of the tailerons and the trailing edge of the wings which allows air to be pulled along with the exhaust gases (drawn in blue). The paper calls this "entrainment".

Again I drew on another screenshot of the video and illustrated possible air currents that can freely move around the airframe: aft of the exhaust duct and to the left and right of the vertical stabilizers. As you can see the flaps are also extended which helps guide the air around the wings. There probably is a similar airflow around the front section of the fuselage where the wings are not blocking the flow and the fan is pushing the air down.
In a vertical descent or climb the tailerons also create less aerodynamic drag in this configuration than in a leveled attitude so the pitch inducing effects due to the large surfaces at the rear are reduced.
It could also be beneficial to angle the tailerons trailing edge down for a vertical landing or a short takeoff to increase the lift at the rear end by blocking the escape path for air trapped under the rear end of the aircraft. Essentially preventing the cushion of air to escape at least when in ground effect.


What is the cost per hour of a Gulfstream V? Could someone inform me about the exact operational costs of a flight hour for this specific type of aircraft?

The Gulfstream G550 is a variant of the Gulfstream V.

From aopa.org (November 16, 2015) for the G550, it costs $4,731.05 per hour, give or take. (Assuming the plane is paid for already.)

Adjusted for inflation that's $4,825.11.

Note that the GV is older than the G550, and would generally cost more to operate.

  This figure includes fuel, maintenance (parts and labor), engine reserves, auxiliary power units if applicable, and miscellaneous expenses to include crew travel, catering and cabin supplies, landing and parking fees—all boiled down to a single number.


Is it possible to determine if winds were high without groundspeed, wind speed, AoA or AoS? I have a data set that includes TAS, IAS, GPS Lat/Lon, Inertial Lat/Lon, ENU speed,  inertial speeds, attitude, and accelerations. I don't need to know the exact wind speed, and I don't think that's even possible to calculate at this point, but I would like to determine whether or not wind speeds were significant at a certain point in the flight. Is this possible?

In the inertial (Earth) frame, denoted by subscript $_E$, inertial velocity ($\bf{V^i}$) is related to air velocity ($\bf{V}$) and wind vector ($\bf{W}$) as follows:

$$\bf{W_E}=\bf{V}^i_E-\bf{V_E}$$

Air velocity is more commonly expressed in the body frame (subscript $_B$), and is related to the inertial frame via the rotation matrix, $\bf{L}_{BE}$, as a function of the Euler attitudes ($\phi,\theta,\psi$):

$$\bf{V_B}=\begin{bmatrix}u \\ v \\ w\end{bmatrix}=L_{BE} (\phi,\theta,\psi) \bf{V_E}$$

$\bf{V_E^i}$ can be transformed from the ENU velocity to NED, and you mentioned that attitudes are measured, so that leaves the air velocity components.

If you have AOA ($\alpha$), AOS ($\beta$) and TAS ($V_T$) measurements, they are related to the components as:

$$\alpha = \tan^{-1}\frac{w}{u}$$
$$\beta = \sin^{-1}\frac{v}{V_T}$$
$$V_T=\sqrt{u^2+v^2+w^2}$$

That's three equations with three unknowns. At this point, you have all the information necessary to solve for the wind vector.

Without AOA and AOS measurements, your only hope is to assume some reasonable values of AOA and AOS. For AOS, you can assume 0 if there is no lateral/directional input. For AOA, you may be able to use some offline data (e.g. wind tunnel, CFD) if your weight estimate and aerodynamic data are trustworthy.

An additional note, per your previous question on finding AOA and AOS without sensors, you have a real dilemma here: you cannot obtain both winds and flow angularities without sensors!


What is the unit for ground speed reported by ADS-B? Strangely enough, I'm having trouble finding the unit reported by ADS-B on the Internet. Many sites simply refer to it as "ground speed". Is it measured in KPH?

ADS-B* encodes the ground speed in NM/s (nautical miles per second) in $2^{15}$ increments of ≅ 0.22 knots each such that:

  0 ≤ Ground Speed < 2 NM/s

As of June 2015*, it is optional to report the ground speed (users can use two position reports to get the speed). Decoding to Knots (NM/h) is more common though for aviation for most of the world.**

* Eurocontrol Standard Document For Surveillance Data Exchange
** ICAO (Annex 5) recommends km/h but permits the 'temporary use' (as they put it) of non-SI alternative units for vertical/horizontal distances and speed.


Does the yoke move when trimming a THS like it does when using elevator trim? I would like a clarification from the operational point of view:
Both the THS and elevator trim tab is used to remove pilot pressure on the yoke / stick.
On the elevator trim similar to the C-172 the yoke deflection is a function of the speed: so in trimmed condition the yoke deflection depends on the IAS we what to fly.
What happens on the THS trimming? Does the yoke deflection always stays in a "center" position after trimming or the behavior is similar to the C-172 ?

When I was type rated in the CRJ200 with hydraulic controls and a THS, directly coming from light aircraft, the differences in technique are significant and it was definitely something new to learn.
With a THS, column neutral is always the same spot.  This means trimming to a speed is not a case of moving the control to location X and trim until it stays at location X. You have to apply a control input to achieve a speed/attitude, and trim until you can relax the control input and let it go back to its "permanent" neutral position while the airplane maintains the target trim speed.
So what you do is, say, you want to slow down from 200 kts to 180 kts, so you pull and hold to slow down. While you're holding the input, you start to blip the trim switch with your thumb as, at the same time you let the column move a bit back toward neutral.  The objective is to make the trim input as you relax the control input so that the stab incidence change is replacing the elevator deflection and the airplane holds a constant attitude and speed.
You keep doing this; blip-release-a-bit, blip-release-a-bit, blip-release-a-bit, until the column is back at neutral and the plane is holding your target speed with no input.  With practice you start to pull and blip the trim right away and it all just flows automatically.
It's murder on the trim screw jack drive systems, because they get many many start/stop cycles a flight from pilots going blip-blip-blip on the switch whenever they are hand flying.  The autopilot also does the same thing, sending numerous trim pulses to trim out speed changes to offload the AP drive servo.
Light planes with trimmable stabs, like the Cessna 180-185 family, are very similar, because the elevator just wants to "trail" when hands free, and the neutral position doesn't change the way it does with a tab, although it varies a little compared to a jet with hydraulic controls.


What would be the potential thrust of an "electric jet" like Lilium's? Lilium claims to fly with "electric jets". Throwing out the debate over whether a non-combustion compression of a stream of air is a jet or only a ducted fan, what is the potential thrust each of these could be getting?

Edit: since it's not compressing, what potential lift could the Lilium prototype engines provide, and are there examples of electric engines today that provide compression in a significant way?

Ignoring the stability issue of Lilium as well. Just curious about the ducted fan design they have. Also ignore the problems with it being electric and battery powered please :)

They look like 15 cm fans, thrust of which is competently ROM-ed in this answer.: about 40 to 60 Newton.

Note that the $C_T$ in the equation depends on fan solidity, the higher the number of blades and  the wider they are, the more thrust they produce, but also the higher power required. For a given fan power (and battery life) it makes more sense to increase fan diameter and decrease fan solidity.


What is the use of this sharp edge found on the leading edge of a DA 40NG wing? It is found on both wings and there is also another long one near the wing root too.

I'm leaving the aerodynamics to someone who paid more attention in classes but, as I remember, that leading edge is supposed to induce flow separation at lower angles of attack (for that portion of the wing), inducing a small buffet into the ailerons and a buffet feed-back to the pilot through the control stick.

Without it, the wing would begin separation (and stalling) at the root, and by the time you get the buffet in the stick the whole wing is basically already stalled.


What licenses / permits are required to perform slingload operations in a helicopter? Regulation-wise, what is needed to conduct slingload operations in helicopters in the USA?

I would like to know regulations about both the aircraft and the pilot.

This one I found in the FAA Helicopter Instructor's Handbook

In the United States, FAR Part 133 covers this.  For the Flight Crew, you are required to obtain either a commercial or airline transport pilot certificate - rotorcraft helicopter with additional ratings for external load carriage.  This rating can be obtained only at the behest of the chief pilot licensed for external loads and under the employ of an operator holding an external loads certificate and requires both a knowledge test and a practical test as prescribed in Part 133.

The helicopter in question must hold a valid restricted category airworthiness certificate and must have been designated for the carriage of external loads on its type certificate or by STC per Parts  21, 27 and 29.

The operator must hold a current Rotorcraft Helicopter External Load operator's certificate which must be renewed every 24 calendar months.


What's this tiny airplane? I found this very small airplane in the desert on Google Maps. It appears to have about
a 10 foot wingspan, and is about the size of an SUV (compare to cars nearby).

Location in Google Maps: Google Maps

I would make this a comment since I can't be certain, but I wanted to put some photos in. I concur with Ron that it's a target drone. But what target drone?

The one next to it appears to be a Ryan BQM-34 Firebee.

The most likely possibility is the Beachcraft MQM-107 Streaker.

Here is a chart of US target drones.

source