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.

Air density or pressure altitude must be factored in when comparing engine thrust figures.  At cruise, FL400, air density will be about one fifth.  Max thrust (or max HP for piston engines) will also be one fifth, as will air resistance/parasite drag.  Air mass flow will be one fifth, so will fuel flow/burn.  Even lift will be one fifth for a given air speed (TAS).

I assume you meant TAS in your question, however, if we consider IAS/CAS, that is a different story.  At FL400, you would have numbers like Mach 0.82, 470kts TAS and 250kts IAS.  Airliners rarely do level flight except at cruise altitude, however it would be possible to maintain 250kts IAS/CAS with 20% of static thrust (give or take) at altitudes lower than cruise, such as in a hold.

Playing with these might be helpful:

IAS/TAS vs Altitude:  https://aerotoolbox.net/airspeed-conversions/

Engine power vs Altitude:  http://www.csgnetwork.com/relhumhpcalc.html


How do airlines handle communication/notification after an accident? I'm a frequent subscriber of airport messaging services where you get notified of check-in queue times, delays in takeoff, ETA + 10 minutes, landed timestamp, baggage claim time and these kinds of information, but not life-dependent messages.

However, if something bad happens I assume I won't get a "Flight X crashed and burned" message on my cell, but most likely a "Contact the airline at 555-1212 for more information" or something similar.

What's the standard operating procedure in these cases?

If an airline has an accident, the chances are you (as the next set of passengers to board that plane) will be the least of their concerns - if there's an incident then it tends to be a case of all hands to action stations to deal with the media, emergency services, families of any potential victims, provide assistance where possible etc. That said, they do have some procedures they attempt to follow. These vary a little from airline to airline.

Most will make an attempt to contact you via text/email if they have it or via the GDS to your travel agent, depending on how far away you are from your departure day/time. It also depends whether you gave contact information to the carrier or your travel agent, whether your travel agent passed it on or, indeed, whether you used a travel agent at all.

The next set of passengers rightly tends to be second priority after dealing with the incident, however. Often if you're on the next flight, therefore, the communication will be a little slow and the first you'll know about it will be when you arrive at the airport - also remember that there's a good chance you're already at the airport, or are on a connecting flight.

If you get to the airport, you'll likely be informed by the staff there who will attempt to get you onto another flight with the same company or another carrier.

As an addition from the comment below, if you're a family member etc who's signed up to flight alerts from the airline in order to hear about delays/diversions etc (some airlines provide this as a service to avoid inconvenience and avoid too much waiting traffic at the terminal), then you'll likely be one of the first priorities. The system is normally set up to send automated or manually-triggered pre-written messages, but in an emergency will usually be used to send a manual message to relevant family members etc.

Obviously this only applies if you've signed up to the service, and only usually applies to the airline's own service: if you've signed up with a third party flight tracking service, they likely have no more information than you do.

"Poorly" would be the direct answer to the title.

While each airline certainly has internal policies they are not utilized all that often (that's good) so when things do go boom everyone is out of practice. Having a well-rehearsed policy could be considered a liability issue - they expect things to crash often.


Should landing light be off during line up and wait position? What is the proper position for the landing lights when on the runway in a line up and wait position?

Counter-intuitively, the FAA answer (see chart on page 1-9 and text on page 1-10) is actually, they should be off during the line-up-and-wait phase.

When cleared to ...

  “Line up and wait”—when entering the departure
  runway without takeoff clearance, turn on all exterior
  lights (except landing lights) to make your aircraft
  more conspicuous.

Then when takeoff clearance is received, the expectation is that landing lights will be turned on.

The best rationale I've seen for this is so that Tower controllers (and other aircraft) have an immediate visual cue that an aircraft "on the numbers" understands that it has been cleared for takeoff, or not.

Another FAA document, SAFO 11004 (these are "best practice" recommendations, not requirements):

  Exterior Lighting:

  o Taxi with taxi light on when moving, off when stopped.

  o Turn on all exterior lights when crossing any runway.

  o If cleared to “Line Up and Wait”, turn on all exterior lights except landing lights.

  o When “Cleared for take-off”, turn on all exterior lights, including landing lights

  o If you see an aircraft in take-off position on a runway with landing lights ON, that aircraft has
  most likely received its take-off clearance and will be departing immediately.

Let's take this question from a position of logic- arguably you are in the most vulnerable position with traffic approaching from behind and your recommended to sit in the dark? That does not make a lot of sense to me. The landing light would not blind the inbound traffic, but rather cast a gentle light down the runway. I see two benefits from having the light on – The controller would advise the landing traffic that there is an aircraft on the active and waiting, why not make the visibility of said aircraft a little bit easier? Also as previous mentioned the taking off traffic that is one less thing to do. Do your lights camera action when taking the active runway including the landing light


Why do planes always land on rear wheels instead of the nose wheels? Various times, I have seen and felt myself that the plane always lands on the rear wheels, but not the front wheels. Why do the pilots land it on the rear wheels instead of the front? Does it have some advantage to it or is it a rule?

Is this required for all the planes (irrespective of size, type) to land only on the back wheels?

Aircraft land on the main wheels. For aircraft with nose wheel it is the back ones, but for aircraft with tail wheel (also called “tail-draggers”) it is the front ones. In either case the main wheels are very close to centre of gravity and carry most of the aircraft's weight, the nose or tail wheel only carries a small fraction of it.

The aircraft must land on wheels that are close to the centre of gravity (longitudinally). If they were not, the force on the wheels would create a moment that would violently pitch the aircraft. Actually, tail-draggers tend to bounce on landing a bit because there still is some moment left and in case of tail-draggers it pitches the aircraft up.

Early planes were almost all tail-draggers, because that layout is more robust, handles unpaved runways better and has less drag when the gear is fixed. It however provides poor directional control especially once the tail lifts off the ground during take-off roll. Later most designs started to use nose-wheel, because it provides better directional control, is directionally stable, makes the floor level when the aircraft is standing so loading is easier and visibility during taxi is much better and with invention of retractable landing gear the drag didn't matter any more.

While there is certainly a lot of truth in the above answers they all sort of treat the question in a fashion similar to: "Why does water freeze at the top and work down? Because otherwise fish would freeze." While there are a number of reasons why landing on the back wheels works better the single most important one is that when airplanes land their nose is higher than their tail so their back wheels are closer to the ground. The reasons for this is because as planes slow down their wings generate less lift. They have to do something to make their wings generate enough lift despite the airspeed reduction so as to keep flying else a sub-optimal condition involving bent metal and maybe fireballs will occur. They do this by pointing nose up which increases the wings' angle of attack and by changing the shape of the wing (flaps and slats) to make the wings generate greater lift at the expense of introducing more drag. By the time that they get close to the ground they have slowed as much as they dare which means that the speed alternatives are being used as much as possible. That means that the aircraft is leaning back as much as it safely can and flaps are fully extended. Right at the point that a safe landing is guaranteed then the greater danger to the aircraft's safety shifts from falling out of the sky to hitting the ground at too fast a speed so the pilot does things to dump speed. That means leaning the plane back just enough to increase air resistance but not enough to make it bob up in the air. A good pilot does this just a very few feet above the ground and adjusts things so that the plane levels out for a few brief seconds as the speed bleeds off. A not so good pilot briefly gains altitude and then the plane falls out of the air from 10 or 20 feet up and the passengers get to experience three or four very short flights before the plane finally settles down on the ground. All of this puts their back wheels closer to the ground which means that's where the rubber meets the road.


Is a propeller with two long blades more efficient than one with three shorter blades? Is a longer, two-bladed propeller more efficient than a three-bladed propeller with a smaller diameter?

As Troy E and Dave said, most of this question is already answered here.

In addition to this previous answer, just remember that a propeller is a technological solution to a simple question: how do we make the aircraft fly ? Therefore, there is no single solution, but multiple compromises that can be found depending on various parameters, mostly based on the type of operations the aircraft should take part in :

Speed
Altitude
Mass of the aircraft
FOD / injestion risks and tolerance
...

Since your question only states the efficiency of the propeller, you actually miss at least part of the real problem.
Let's say you manage to make a propeller with an efficiency of 1, but that it weighs 1 ton or is impossible to manufacture, you won't get far.

In that sense, only considering the efficiency to say what is the best solution is not good enough. The best solution is the best compromise between all of your requirements, that's why it is so difficult to find ;) !

I don't have a definitive source, but I read this recently:
"The rule of thumb is that you need 100 HP to justify each blade. Two blades are considered adequate through 200 HP. Three-bladed props seem to come into their own on efficiency at or above 300 HP. "


How do aircraft altimeters calculate altitudes accurately while airborne? While an aircraft is airborne, the local barometric pressure can change, causing an altimeter to no longer display the correct altitude. How are altimeters re-calibrated to display the correct altitude, or there are other methods to accomplish that?

Altimeters are calibrated to a standard atmosphere model (International Standard Atmosphere, ISA).

What the altimeter shows you is the vertical distance between the altitude equivalent to your current pressure (pressure altitude) and a reference "pressure altitude". The reference pressure altitude is set in pressure units (hPa or In. Hg) on the altimeter subscale. This is known as the "altimeter setting", set in the "altimeter setting window".

Image credit: FAA Pilots Handbook of Aeronautical Knowledge

Flying at lower altitudes (below "transition altitude"), the altimeter is set to airport/regional QNH, which is a pressure calculated so that if you were standing on the measurement point, the altimeter would indicate your exact elevation. At other altitudes, it would approximate altitude above mean sea level, although it would show incorrectly if temperature differed from ISA.

Above transition altitude, the altimeter is set to QNE, or standard pressure (1013.25 hPa, or 29.92 In. Hg). The altimeter will now only indicate correctly if both temperature and pressure are in accordance with ISA. You are now flying on "flight level" (where the FL is your QNE indication in hundreds of feet, ie 20,000 ft = FL200).

For the purpose of avoiding obstacles, both QNH and QNE have to be corrected using various methods, which are described in various manuals. (QNH for temperature, QNE for both temperature and pressure, both for other possible factors such as wind etc). For instance, this diagram shows three different aircraft, all flying the same pressure altitude, but with three different true altitudes:

Image credit: FAA Pilots Handbook of Aeronautical Knowledge

However, altitude is also used for the purpose of traffic separation, and for this purpose, neither QNH or QNE is corrected. This might sound counter-intuitive, but as long as your altimeter is showing the same error as any conflicting traffic, you can be safely separated, therefore: The altitude shown on the altimeter is not corrected at all (except by calibration for possible aerodynamic interference, such as compressibility, venturi-effects etc).

In technically sophisticated aircraft, barometric altitude input is used for vertical navigation. Since vertical navigation is concerned with the true altitude of the aircraft, the aircrafts Air Data Computer (ADC) will calculated a true altitude based on information available to it (either through sensors, or through pilot input). The true altitude is however, as stated, not shown on the pilots primary flight instruments.

I think that the question has been answered by a couple of great answers, so there's only one thing I can add.

In my opinion the main reason for the transition from QNH to QNE is so that above the transition altitude, the airplane, engine and systems always perform the same way.
For example, for every possible load-out, there's a certain optimal altitude for the plane to fly at. In truth, this altitude is really a pressure, that only converts into an altitude by assuming a standard pressure above a simulated reference plain.
In other words, it doesn't matter how hot or cool it is or where around the globe you are, if you convert your outside pressure into a reference altitude, you can always fall back to the same performance tables.


Can a pilot transition from a localizer approach to an ILS approach? I was recently looking at the approach to RWY 8L at Honolulu, Hawaii (PHNL).

Looking at the approach IAFs, I see the LOC RWY 8L approach has many different options, and has an intercept altitude at 2,300 ft. The ILS RWY 8L approach has three IAFs but none are south of the airport, and the glide slope intercept altitude is 2,700 ft.

In the scenario where a visual approach isn't appropriate and an ILS may be necessary, are there procedures/rules in place for a pilot to transition from the LOC approach to the ILS with glide slope once they are lined up? I know the LOC intercept is at a lower altitude, so can a pilot just intercept the glide slope from that altitude or are the two procedures incompatible and radar vectors are required to line the pilot up for the ILS?

I'm looking specifically at the JULLE5 STAR into the LOC 8L approach. JULLE5 ends at ALANA. The LOC has a IAF from ALANA, while the ILS doesn't (only BOOKE).

You can't legally switch from the LOC to ILS approach without ATC approval in this case, since they are different enough and are on different plates. However, if you're outside the FAF you can ask ATC to switch from LOC to ILS, and proceed with the ILS if approved. However, in lost comms, if you got initial approval for the LOC approach but no mentions of ILS, you can't switch to the ILS. Unless you exercise your emergency authority.

For the first part of your question you need to understand the parts of an ILS. ILS stands for Instrument Landing System.

The ILS system may be divided functionally into three parts:
   (a) Guidance information: localizer, glide slope;
   (b) Range information: marker beacon, DME; and
   (c) Visual information: approach lights, touchdown and centerline lights, runway lights

The localizer provides horizontal guidance. The glide slope provides vertical guidance, and the DME and marker beacons provide distance guidance. Notice the straight-in minimums on the ILS RWY 8L approach chart. They are 213-½ which means that an aircraft can fly down the glideslope until it is 200' above the ground. If they see the the approach light system (which in this case is a MALSR) they can proceed another 100'. If they don’t see the runway at 100' above the runway then they need to fly a missed approach.

There is actually a list of things they need to see in order to continue the approach—runway end identifier lights (REIL, the flashing strobes on either side of the threshold)
the runway (itself, the markings, or lights)
the threshold (itself, the markings, or lights)
visual glideslopes (VASI, PAPI, etc.)
the touchdown zone (itself, the markings, or lights)

The localizer approach uses the same horizontal guidance as the ILS but it does not provide vertical guidance. Notice that the straight-in minimums are 460'. That’s because the pilot is relying solely on the altimeter for vertical guidance and it is not as accurate as the glide slope.

Both approaches use the outer marker. The ILS uses it as a way to verify that the aircraft is at 1994' when it crosses the marker. The Localizer approach uses it as a step-down fix—the aircraft can begin its final descent to landing. In this approach the pilot must not descend below 2000' unless the items above are seen. At 0 DME they begin the missed approach.

Most aircraft that are equipped with IFR instruments will have a glideslope. However, it may be inoperative or the glideslope at the airport may be inoperative in which case they would fly the localizer approach.

I don’t know if they do it at this airport, but often airports with two parallel runways will use the localizer approach to one runway with a side-step to the other. If ceilings are high enough, they may do it here.


How does maximum speed vary with altitude? Question: How does a commercial jetliner's highest possible groundspeed vary with altitude (assuming wind=0) ?

My understanding is that drag decreases with altitude, therefore maxspeed increases with altitude.

This is also what Cpt. Simon Hardy, a B777 instructor, says:

  « Aircraft [referring to airliners] go faster at altitude than at low
  level. At sea level the aircraft [referring to a B777] can only do 330
  kn. And at altitude, 30.000 ft, it can do Mach 0.86. So it's a sort of
  sliding scale – as you descend, you get slower. » (source)

But today I read exactly the opposite on an aviation blog, which I found curious:

  « When a plane gets so high, you have to fly slower, since the speed of sound (and hence true airspeed for a given Mach number) decreases with altitude. » (paraphrasing, source)

  « Real basic point: You’re flying higher, you’re flying slower. You want to go fast […] you just fly lower. » (source)

... so which is it ?

The perfect answer for me would ...
• include a graph (Y: altitude, X: maxspeed)
• include citable references

EDIT (in response to the comments below):
I specified maxspeed as the highest possible groundspeed (assuming wind=0), although I'd rather prefer not to make that distinction, because from my experience this complicates the issue more than necessary: Of course the altitude will affect the different types of speed more or less, but isn't it nonetheless possible to make an approximate statement like "higher=faster, lower=slower" and explain why that is?

EDIT2
For a concrete example, let's assume:

aircraft = B777
wind = 0 km/h (for argument's sake)
weight = typical weight of a B777 ~45min into a 9 hour long haul flight
aircraft has already reached cruising altitude

If at that point the pilot wanted to "get away" as fast as possible (let's say from waypoint IGARI), what altitude should be chosen?

Is the quoted blog post correct in saying that at an usually high altitude, the plane would fly slower, therefore it would have to go lower to fly faster?

This question isn't clear because "speed" means different things to different people, and this is also engine specific.  Every aircraft will have a different speed vs altitude profile.  In general...

Mach will go up, as you increase in altitude.  This is because the lower density air lowers the speed of sound (in an absolute sense).

True airspeed will go up as you maintain a constant indicated airspeed.  Conversely, maintaining a constant true airspeed will increase your indicated airspeed as you descend.

Normally, a jetliner should not be seeing true air speed at sea level.  For jetliners, even if your engines are good enough to get you there, the aircraft has an indicated airspeed structural limit that keeps you from accelerating.  You can keep the same speed (at structural limit) through higher altitude, which increases your TAS.

There is a chart here which should help show the relationships between altitude and speed:

A simplified chart for determining Mach number and true airspeed from airspeed-indicator readings:

OK, your fastest get away in a B777 disregarding fuel consumption would be based on three things: 1. Never exceed Indicated Air Speed 2. Speed of sound at the Warmest altitude available.  3. Stall IAS.  Get away speed would be measured in True Air Speed.

If you were already at cruise altitude and PREPARED for this, the options would be as follows:

First rule out all (lower altitudes) where IAS Ne is LESS than your Mach limit.  Think wading through water is harder than wading through air.

Next rule out all (higher altitudes) where stall IAS is MORE than your Mach limit.

Now find the warmest part of your altitude "sandwich".
This is where Mach limit TAS is highest.  Then calculate IAS to get Mach limit TAS for that altitude AND temperature and make your escape!


How to calculate the amount of thrust of a autogyro during auto rotation? When a autogyro/gyrocopter is in a horizontal steady flight the rotor should be able to provide enough thrust using the auto rotation principle if the horizontal speed can be regulated by the vertical propeller.

I was wondering how the amount of thrust provided by the main rotor of a autogyro during auto rotation can be calculated. All the characteristics of the autogyro and his blades are known.

The horizontal speed of the gyrocopter can be regulated by additional engines and they do not need a specification.

In auto-rotation, either flying level, or with a constant sink speed, the vertical component of the thrust provided by the rotor is exactly the same as the weight of the gyro...

In Wikipedia  https://en.wikipedia.org/wiki/Disk_loading#Momentum_theory  you can find the derivation of the following expression:

$T = 2v^2· A·  rho$

where T is the thrust of the rotor, v the sink speed in vertical auto-rotation, A the 'disk area' (that swept by the blades) and rho the air density.

The rotor of an Autogyro does NOT provide Thrust, it provides Drag. This is what differentiates Autogyros from Helicopters, and is why an Autogyro needs a separate engine-driven propeller. So the question needs clarifying as to what you actually want.


Why is the King Air avionics switching system wired normally closed? In the King Air 200 and B200 (perhaps other King Air models as well), the avionics electrical bus relays are normally closed. With the Avionics Master switch in the "OFF" position—which is electrically closed—when the Battery master switch is turned on, DC power is supplied through the closed Avionics Master switch to the avionics bus relay solenoids causing these relays to open and disconnect the avionics buses from DC electrical power. Subsequently selecting the Avionics Master switch to the "ON" position opens the switch which disconnects the DC power to the avionics bus relay solenoids, allowing the relays to default to the closed position thereby supplying DC power to the avionics buses. The checklist action item for the Avionics Master switch power failure is to pull the 5 amp Avionics Master circuit breaker, which electrically opens the avionics master switch circuit thereby restoring DC power to the avionics buses.

See the following circuit diagram:

(Source: own work)

This system works, and works well, by all appearances. However, the design does seem to be somewhat counter-intuitive.

My question is, what is the purpose of designing the avionics master switching system normally closed?

Despite being counter-intuitive, I can understand that the electrical loads required by the avionics demand the use of relays, and that the design allows for the relay solenoids to be powered for a minimal time. I think this is the most reasonable explanation for the design, but I am looking for documented confirmation. Is there some other design consideration that I have overlooked? Does anyone know of a documented reason for this design?

NOTE: While you are welcome to speculate with me on this issue in the comments or chat room, I am not looking speculative answers. I am looking for an answer coming from persons familiar with the King Air electrical system and preferably backed up with documentation, such as the maintenance manual, or other sources that I have overlooked.

Design elements like this generally have to do with failure modes and how you want the system to fail when (not if) it fails. In this case, if there was a momentary or worse, permanent, loss of DC power to the relay or it failed for another reason you would want them to stay closed (and continue in the "ON") position so you would not lose your avionics (provided the power supply was some how still operational to them). Unlike most other electric components (semiconductors) relays are actually electro-mechanical components and they do have moving parts with in them that can fail. In my experience (non avionics mostly) relays have a much lower cycle lifetime than their semiconductor friends.

If we dive even a bit deeper into them, a relay is effectively an electromagnet coil that actuates a small metal rod that generally moves a copper (or similar) strip to close a switch connect. Applying a steady state voltage to a coil to generate a magnetic field (and keep the relay in a desired position) also drives current through the coil. The coils, being very thin gauge heat, act like resistors and can generate a substantial amount of heat. Due to the very thin gauge of the wire this can lead to melting and shorting of the coil. The wire also tends to be resin coated which can heat and melt as well causing a short. In turn you are correct that they don't want to he help on for a long period of time.

This is the electrical equivalent of many mechanically similar ideas in aircraft design. For example some landing gear in planes is held up using positive pressure so should the pumps fail the landing gear may simply deploy or be able to be shaken down by releasing a lock, the idea being its better it can come down in the case of a system failure than be stuck up.

My guess:

From a functional and electrical point of view the task could be done by a single, 30A, double-pole, manual switch taking the place of the relay contacts in the circuit.  One chunky switch will probably be just as reliable as a dual-feed bus, 5A switch and two relays.

I reckon that Beechcraft might have chosen this configuration so they could (i) have a lighter, smaller 5A single-pole switch on the panel and (ii) avoid routing an additional four 30A wires to that panel.   This choice becomes even more sensible if a highly-reliable dual-feed bus was already available.

Disclosure: I am an electronics engineer but have no aircraft engineering experience.


Do I have to follow airways to meet the IFR cross country 250 NM flight requirement in 61.65? I understand that there is cross-country flight of "a distance of at least 250 nm along airways or ATC-directed routing" requirement for the IFR rating, but I'm not sure if understand what the second part of that sentence means.

Do I have to follow Victor airways? Will obtaining flight following from ATC be sufficient?

14 CFR 61.65(d)(2) says:

  (ii) Instrument flight training on cross country flight procedures,
  including one cross country flight in an airplane with an authorized
  instructor, that is performed under instrument flight rules, when a
  flight plan has been filed with an air traffic control facility, and
  that involves—

  (A) A flight of 250 nautical miles along airways or by directed
  routing from an air traffic control facility;

  (B) An instrument approach at each airport; and

  (C) Three different kinds of approaches with the use of navigation
  systems.

The wording is saying that you must follow airways or a route given by ATC. But since the flight must be under IFR, you'll get a route from ATC in your clearance anyway so I don't know why the FAA has the airways reference in there.

(As a complete guess it may be left over from a time when there was less radar coverage and the FAA wanted to make sure that instructors weren't taking any shortcuts.)

I don't know what flight following has to do with this because that's VFR, and the flight must be IFR.

No, you must file a flight plan, and fly the route you are cleared to, or as directed by ATC (such as for traffic clearance).


Are the guns on a fighter jet fixed or can they be aimed? Are the (built-in) guns on fighter jets fixed, or can they be elevated up and down or panned right and left for aiming?

In other words are fighter jet guns aimed by pointing the whole aircraft at the target or can they be aimed independently?

They are fixed, and many attempts were made over the years to have an aiming capability - and all failed. That should tell you something.

The earliest aereal combats used indeed handguns which were pointed at the adversary by the pilot. But it soon became obvious that only the rapid fire of a machine-gun will give the pilot a realistic chance of hitting a fast-moving target.

During the Italo-Turkish war of 1911 and 1912, the Italians used aircraft in war for the first time. Since the Ottoman Empire had no air force yet, the Turks engaged European aviators who flew their own aircraft, taking a handgun along to shoot at the Italians.

The same was tried in the opening months of WW I, but it soon became obvious that only a machine gun was effective. Before gun synchronisation was invented, pusher designs were fielded, some of which carried a dedicated gunner in a seat ahead of the cockpit.

Vickers F.B.5 "Gun Bus" (picture source)

Of course, the much smaller and nimbler Fokker single-seaters with gun synchronisation made such designs soon obsolete.

When Great Britain prepared for WW II, the firing power of bomber turrets impressed the Generals so much that they ordered several dedicated fighters with a gun turret mounted right aft of the cockpit. The resulting Bouton-Paul Defiant was again too slow and easily outmanoeuvred by the single-seaters, and the Blackburn Roc and the Hawker Hotspur were even worse. The Defiant's only successful role was as an early night fighter; when attacked by day it could only join others in a Lufbery circle to defend against being shot down itself.

Bouton-Paul Defiant F. Mk.I (picture source). Note the retractable belly-mounted antenna masts which gave the gunner a free field of fire in the upper hemisphere.

Another attempt was made by the MiG design bureau in the 1950s. In a dogfight the pursuer needs to get inside the circle of its opponent in order to score a hit with a fixed machine gun. The Russians thought: Why not pivot the gun into the circle, so the pursuer can fly the same circle as its opponent and still shoot at it? A MiG-17 was modified accordingly, but flight testing soon revealed that the recoil from the machine gun would push the nose of the fighter down and outside of the circle when the gun was not aligned with the longitudinal axis of the fighter, so this idea was dropped very soon.

Now I need to mention that rarely are the guns completely aligned with the longitudinal axis; instead, they point slightly up to compensate for the ballistic bullet drop at the expected distance of the target. Also, wing-mounted guns point slightly inward, again to ensure the bullet path is straight ahead when it catches up with the target, a procedure called gun harmonisation. This also means that interceptors will have their guns pointing up slightly more than those of air-supremacy fighters. One particular case involved the MiG-15: Being designed to fight large B-29 formations, the guns were badly aligned for fighting the F-86 Sabre during the Korea War. The MiG-15 armament consisted of two 23mm guns and one 37mm gun with very different ballistics. In order to ensure that the path of all bullets would meet at the right distance for fighting bombers, the 23mm guns pointed more up, and when used against the much smaller F-86, the Russian and Chinese pilots opened fire at a shorter distance so that the 23mm bullets would fly over the target while the 37mm bullets went below it. This was a major reason for the poor kill ratio of the MiG-15 vs. the F-86 despite the MiG's better performance.

Gun harmonisation on a jacked-up P-47 (picture source)

Modern high-off-boresight missiles with helmet-mounted cueing (or sight-controlled missile targeting) have turned that longtime dream of shooting at what the pilot is looking into reality, and the gun is carried largely as a low-priority weapon of last resort.

Pilots can aim by looking at their targets-even in High G situations.

  These days, fighter pilots' helmets are nearly as complex as their
  airplanes. And with good reason: If the helmets can't tell where the
  pilot is looking, many of the airplane's systems, including weapon
  targeting, are useless.

  The folks at San Jose, California, who have designed advanced U.S.
  military helmets for the F-15, F-16, and F/A-18, make their living at
  the nexus between man and machine. VSI's flagship program is the Joint
  Helmet Mounted Cueing System. It uses a magnetic field in the cockpit
  to sense the orientation of the helmet, then feeds information on the
  current line-of-sight to the aircraft's flight computer. VSI's helmet
  has an accuracy of about four milliradians, an angular measure
  commonly used in the world of shooting and targeting. One milliradian
  equates to one one-thousandth the distance to the target. So if the
  target is 1,000 feet away, you'd be accurate to within a foot.

  Determining where a pilot is looking by tracking eye movement is a
  much taller order. "You would not believe some of the human factor
  issues you have to overcome to have a successful eye tracker," says
  Louis Taddeo, VSI marketing director. "Although we have research
  projects into eye tracking, it is a very difficult task both from a
  technical [standpoint] and the physiology."

  So for now, VSI uses helmet position to achieve that four-milliradian
  accuracy. Pilots need only turn their heads to aim their weapons, even
  during high-G maneuvers, freeing their hands for other tasks.

Read more.


Are ducted fans more efficient? As far as static thrust, does a shrouded/ducted propeller outperform an un-shrouded propeller? It seems that the guide blades on the bottom of the duct would allow little more thrust to be extracted from the vortex the prop generates by redirecting the air flow. Also the duct seems like it could "focus" the propelled air (like a rocket thruster) resulting in better thrust.

(Source)

Of course I can see how an improperly implemented duct could really kill efficiency (creating more interference and turbulence), but what about a well designed one? And if they are better, why wouldn't we seem most propeller aircraft use them (what is the advantage about not having the duct)?

there is a lot going on here.  Short answer is that a ducted fan (what you have pictured) can produce a lot more thrust (experiments from one paper say twice as much) than an open rotor of the same size.  The big thing that these ducted fans have going for them is the fact that tip losses become negligible because there simply isn't room for the tip vortex to develop because there is a duct in the way!  Simple, easy solution that works well.

However, nothing comes for free.  One, you have to have the duct there, which is weight & money (you have to build the duct and the support structure, which adds some drag too).  Think of scale too--as propellers get larger, the duct has to grow with them.  Imagine the difficulties in putting a 20' duct on a B-36 propeller!

Second, the manufacturing tolerances on the duct are critical--the tighter it is, the more efficient you are (to a limit, naturally)...but the tighter it is, the more susceptible you are to having your blades strike the duct due to flexing of the duct or the blades.  And, speaking of flexing blades and the necessary support structure you'd need, this could make pitch control (on a propeller, variable pitch, and on a rotor, cyclic and collective) difficult to implement, depending on which of those schemes you're looking to do.  Then again, for a small implementation using RPM control...maybe you don't need those anyhow.

Similarly, while a crosswind on an open rotor leads to a sudden decrease in power, a crosswind on a ducted fan leads to a sudden pitching moment due to the lip of the duct and the asymmetric inflow to the fan (which, since you're now producing more thrust to begin with, asymmetric inflow effects become even more pronounced).  Now, if you stick this on the front of a typical Cessna 172, say, and throw a crosswind at it (which, if you think of takeoffs and landings, which since they must be in a certain direction, having a crosswind is actually quite likely) imagine the yawing moments that would be induced by the propeller (proportional to the static thrust) -- even before the aircraft got off the ground and before you have a lot of control authority to counteract it!  A similar scenario in the air could be as dramatic.  And, yeah, you could throw some guide vanes at it to help, but nothing comes for free.  Guide vanes, which is a workable solution, now mean that you need to have actuator(s) for those vanes, power & maybe hydraulics for the actuator(s), mounts for the actuators, hinges for the vanes, linkages from the actuator(s) to the vanes, redundancy for the actuation or some means of facilitating control in an emergency...it's a lot of work.  Or, by contrast, you could just bolt a fixed pitch prop on a shaft at the front of said Cessna 172 and call it a day.

In the long run, and this is some speculation, I think it just leads to a more complex system, particularly in full-size implementations, which is a maintenance, cost, and manufacturing headache that isn't worth having if you can get plenty of performance from a regular open propeller.  If you venture into the world of diffuser augmented wind turbines, there have been similar debates going on for decades...and a ducted wind turbine has yet to be successful on the large scale.  Similarly, while ducted fan aircraft have flown...

(Ryan XV-5A Vertifan, circa 1950.  The ducted fans are underneath the circular domes on the wings and another under the shuttered area on the nose for pitch control. Source: http://www.aero-web.org/specs/ryan/xv-5a.htm)

...there aren't any non-MAV examples around today that I know of.

Source:
https://theses.lib.vt.edu/theses/available/etd-05262005-170916/unrestricted/WGraf_Thesis_2005.pdf

It's nonsense that ducted fans are more efficient. It always gets bandied around the internet as if it were the truth, but it simply isn't so. Ducted fans look cool and they make props look and sound like jets, so people WANT it to be true, but it doesn't make it so. Ducted fans have about 50-60% less static thrust than the same size prop if designed badly. If designed absolutely perfectly, they might have 90% of the efficiency of a prop. But that's in a static setting. In the air, where you have drag created by the duct and duct inlet, that gets even worse.

We have to stop perpetuating this myth. It hurst innovation.


Are "Tally-ho" and "no joy" acceptable ATC terms for civil operations? I keep meaning to ask this question. I heard "Tally-ho" used for the first time by a pilot on Liveatc.net. I know these are pretty standard phrases in military aviation but I wondered if civil pilots use them. Are they acceptable under either FAA or ICAO standards? If not, what's the likelihood a controller will know what you mean?

As a private pilot, I have heard "Tally-ho" and other pseudo (British?) military phrases used when talking to ATC.  I understand "Tally-ho" to be equivalent to "target(s) in sight" or "inbound" or even in some cases "affirmative", and that's the problem.  It is important to be clear and precise when communicating on the radio, however, the folks working at ATC seem to understand these phrases.

FAA:

I've read the AIM - Aeronautical Information Manual and Pilot's handbook of aeronautical knowlage (Both available for free download from the FAA's website) and I don't remember seeing these phrases anywhere.

Other texts caution against using these colorful phrases instead of standard phraseology because problems stemming from varied interpretations.  It is important to be clear and precise when communicating on the radio.

I think the correct terminology is: "Traffic In Sight" and "Negative contact"

They aren't "quaint" terms, but are pulled from military terminology that IS approved for use in the air:

https://en.wikipedia.org/wiki/Multiservice_tactical_brevity_code

Example: No Joy, Bogey, Angels


Did Clement Ader make the first flight even before the Wright brothers? Clement Ader invented in the late 1890s, a plane that looked like a bat. It had a steam engine. Witnesses said it went airborne at least once and recorded a distance just over 50 meters. The Wright brothers made their first controlled flight in 1903.

Lots of people flew before the Wright Brothers.  Some of them even survived!
The Wright Brothers are credited as inventors of the airplane not because they flew, but because they were the first to take flight in a vehicle which:

Lifted itself using its own power (which gliders and kites did not)
Was aerodynamically controllable by the pilot (which Ader's aircraft was not)
Remained in sustained flight, rather than taking momentary flight before returning to Earth

Fact: Man kind only flew in 1906. That glider wasn't able to fly. Check this article

US patent was only granted the "invention" to the brothers many years later on a new "re-invented version" with the purpose of allowing airplane to be produced for military purposes.

Now check the video of first airplane really flying, greatest moment for human kind can't be stolen by fake papers:


Why do I get an overspeed warning at 250kts below 10,000ft in Microsoft Flight Simulator? I'm flying the Cessna CJ4. I get overspeed warnings at 250kts when below 10,000ft, and the speed indicator has red lines at 300kts. Are these warnings triggered because it'll damage the plane to go that fast, or because it's illegal to go that fast that low? What's the distinction between 250kts and 300kts?

Most likely Cessna introduced a speed switch that is controlled by an aneroid.  Anytime you are below 10,000 FT MSL, the aneroid activates that switch.  If you go above 250 knots, the overspeed warning will sound.
As far as why that is the case, in the United States you are limited to a speed of 250 KIAS below 10,000 FT.  I would imagine this is also the case for most other countries.  The FAA realizes that most small airplanes with a much slower airspeed travel only below 10,000 FT MSL so the FAA is providing a safety margin to prevent in-flight collisions.
The structural limit of the airplane would not be a factor at 250 KIAS hence the red line at 300 KIAS.  I would imagine 300 KIAS deals more with structural stability than anything else.
My question... can a slow-tation go that fast?  :-)

As V(MO) is approached in a jet aircraft, the overspeed warning is activated to protect the structural integrity of the aircraft and ensure it remains inside of the acceptable structural envelope. Pilots need to react without hesitation to an overspeed warning - usually by reducing the throttles to flight idle and therefore bringing the speed outside of the warning envelope. As Dean F mentioned in his comment, the FAA prohibits flight over 250kt below 10,000 ft MSL as a measure for traffic control. That can only be exceeded when the controlling ATC agency issues a speed clearance, often in the form of a "high speed climb out approved" clearance. In addition, manufacturer guidance needs to be followed when approaching the high end of airspeed in jet aircraft as to how to recover from an overspeed or approaching V(MO)/V(NE) airspeed.
See https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/17_afh_ch15.pdf page 15-9 for more information.


Why must Vr be set so precisely? Why must VR (Rotation Speed) be set so precisely? I mean why can't they just say:

  Rotate the Aircraft at 120 to 150 knots

And then, the pilot thinks:

  OK, I have a full plane and half full tanks, I think I will rotate at around 135 knots

And then rotates at around that speed?

This answer is written for air transport category aircraft.
Introduction
During take-off there are three operationally significant speeds that ensure a safe take-off:

V1 - the take-off decision speed
VR - the rotation speed
V2 - the take-off safety speed

In addition there are three technically important speeds:

VMU the minimum unstick speed
VMCG the minimum controllable speed on the ground in case of an engine failure
VMCA the minimum controllable speed airborne in case of an engine failure

V1 is the take-off decision speed. Below V1, take-off can aborted safely without overrunning the end of the runway. Above V1 the aircraft is unlikely to be able to stop before the end of the runway so in case of an engine failure it is safer to get airborne. V1 needs to exceed VMCG to ensure the aircraft doesn't veer off the runway when accelerating to take off with a single engine.
VR is the rotation speed. At this speed the aircraft should be rotated and will get airborne soon after. The rotation speed ensures that, in the case of an engine failure, lift-off is possible and
V2 is reached at 35 feet at the latest. At the end of the rotation manoeuvre, the aircraft should be airborne. Otherwise the tail will hit the ground. Therefore, VR will need to exceed VMU.
V2 is the minimum speed that needs to be maintained up to acceleration altitude, in the event of an engine failure after V1. Flight at V2 ensures that the minimum required climb gradient is achieved, and that the aircraft is controllable. V2 speed is always greater than VMCA to ensure the aircraft is controllable in flight if an engine failure should happen.

Answer
What will happen if we just pick a VR that "feels good"?
If VRfeel is lower than VMU, we will end up with a tail strike. Even if the rotation would stop just before the tail strike, the increase in aerodynamic drag (lift is generated) will cause the take-off run to be longer than needed, risking not being able to climb over objects at the end of the runway. In addition, if the aircraft has an engine failure during rotation, it may not be able to reach V2 and could become uncontrollable and crash.
When VRfeel is too high, the aircraft risks exceeding the maximum safe tire speed. There is also a risk of not reaching VRfeel before the end of the runway.

References
Airbus operations briefing note on takeoff and departure operations; understanding takeoff speeds
Boeing article: exceeding tire speed
rating during takeoff

It is not as exactly as you may think. The optimal speed may is even higher/lower due to unknown factors (human load weight distribution, exact winds etc...).

But the speed you calculate is, roughly, the optimal speed. There is really no meaning in choosing another one.

Imagine you are pressurizing your cars tires. There is mostly a value written on it which is optimal. Then you go for that value, don't you? Why would you just give away free optimization?

In aviation, this is even more critical, as you never ever just make things unnecessarily worse due to safety reasons. Accidents don't happen due to one bad thing, but due to four, five not quite optimal things (a controller, who is a little tired, a radio which is slightly to noisy, wind which is slightly stronger then expected, slightly more resistance on the runway due to heat... that's how accidents happen)


What minimum radio communication equipment is required in a homebuilt aircraft in India? Let's say I am building a single seat aircraft for flying in one's own country(India). What should be minimum set (Not only from compliance prospective but to have core set of communication for security and safety reasons with ATC or other receivers ) of things I must have on my aircraft?

Background : We are planning to provide this set of Radio systems for Hobby Aircraft builders.

Update : As per the answer, It is cleat that one do not need to have radio system always in the flight.

According to DGCA Rule 63 you need a radio onboard to operate both VFR and IFR

  63. Aircraft for which radio apparatus is obligatory-

  (1). All aircraft registered in India and required to be operated in accordance with VFR/IFR condition shall be provided with communication equipment which
  is capable of conducting two-way communication at all times with those
  aeronautical stations and on those frequencies as prescribed by the
  appropriate authority. The communication equipment so provided shall
  be of a type approved by the airworthiness authority of the country of
  manufacture of aircraft and acceptable to the Director General.

  (2).   An aircraft shall be provided with navigation equipment which will enable it to proceed :

(a)   in accordance with the flight plan; and

(b)   in accordance with the requirement of air traffic services, except
      when, if not so precluded by the appropriate authority, navigation for
      flights under the visual flight rules is accomplished by visual
      reference to landmarks at least every 110 kms (60 Nautical Miles).

DGCA Rule 9 also adds some stipulations to the operation of the radio its self

  9. Radio-telegraph apparatus -

  (1) No person shall operate radio transmitting apparatus in any aircraft registered in India unless he
  holds a licence of the type required by the provisions of Part V in
  respect of aircraft required by these rules to carry radio-telegraph
  or radio-telephone apparatus, as the case may be, and issued in
  accordance with those provisions.

  (2) Radio-transmitting apparatus carried in aircraft shall be operated
  under the conditions defined by the International Tele-communication
  Convention (Madrid, 1932) and the general radio-communication
  regulations annexed thereto as far as these apply and shall be
  operated only during the hours in which such operation is permitted by
  the Central Government.

  (3) Aircraft registered in India and required by these rules to carry
  radio-telegraph apparatus shall be fitted with such apparatus in
  accordance with the provisions of Part VII.

  (4) Nothing in this rule shall exempt any person from those provisions
  of the Indian Telegraph Act, 1885, and the rules made thereunder which
  require that radio-telegraph apparatus shall be licensed.

You can find the rest of the rules here. I went through them briefly and cant find a whole lot on certifying home built aircraft. Or what rules they would actually fly under. There is a short list of things related to actually getting an airworthiness cert here but it does not touch on VFR/IFR classifications related to that.

As for the safety aspect of it, that depends on your countries aviation infrastructure and if carrying a radio benefits you. If no one is listening on the other end then its not going to add much but weight to the aircraft. From a regulatory standpoint you may need it to enter certain airspaces (you may also need a transponder for that as well) I cant find a whole lot on the DGCA website about airspace regulations so its hard to say exactly when you would need one. I can only compare it to places I have flown and here in the US while you don't legally need a radio in uncontrolled airspace its still a good idea to have one and to use it as you should (CTAF announcements, flight following if you like, 121.5 if you need it etc.)

The minimum? Nothing.

Requirements may be different in each country (but I'm fairly certain this is standard throughout the world) here in the U.K. there is no requirement to carry a radio in an aircraft - this of course limits which airports you can fly into and which airspace you can fly in.


Is it possible to land a commercial aircraft with an elevator trim stuck at a high degree? I know this is not practical (considering there is normally not much use to use up to 30 degrees of elevator trim), but if you were on a Boeing 777 or 737, or similar, and the elevator trim was set into said position, and stuck there, would it be possible to safely land said aircraft in that condition?

From the Boeing 737 emergency procedures:

After all attempts fail to free the stabilizer...

  Anticipate higher than normal elevator forces during approach and landing.
  The thrust reduction at flare will cause a nose down pitch.
  Note: Elevator control is sufficient to safely land the airplane regardless of stabilizer position.

Note on terminology: Most jet-liners (including the 777 and 737 as the question requests) use a trimmable horizontal stabilizer (THS), in which the pitch trim is achieved by positioning the whole rear wing up/down -- as opposed to using trim tabs.

(airteamimages.com) THS of an Embraer ERJ-190.

The answer is a resounding Yes! You can safely land an aircraft with Elevator Trim jammed either up or down.  Maintaining straight and level flight with Elevators alone can be difficult because the aircraft is sensitive to other conditions, airspeed, wind, etc These can be compensated for through the use of Trim. Trim is a much smaller version of an Elevator, often adjusted up or down by a small wheel between the front seats.

Use of trim can give you much more stability by making smaller adjustments so the Pilot doesn't need to fight the Elevators to maintain height.

During takeoff or landing the affects of a jammed trim can be compensated for by applying small amounts of opposite Elevator.


Is a propeller a wing? An aviation expert claimed to me that a propeller is a wing.

While I understand that propellers use similar principles to generate force, it muddied the definition of a rotary wing aircraft vs a fixed wing aircraft.  If the propeller of a fixed wing aircraft is considered a wing, what is the distinction between the two?  Is it how much surface area each type of wing works on?  Is it the direction of force?

I think most people would say the answer is no. Consider the dictionary.com definition as it relates to Aeronautics:

  9. Aeronautics .
  a. one of a pair of airfoils attached transversely to the fuselage of an aircraft and providing lift.
  b. both airfoils, taken collectively.

The wings are the big airfoils which are responsible for generating most of the vertical lift.

A colloquial test of this definition would be to grab a CFI and ask him/her to "point out the wings on this airplane to me." They won't point out the propeller. (Not a scientific experiment)

However, both wings and propellers are airfoils (and so are the other wing-like surfaces such as the vertical and horizontal stabilizers). You might get some disagreement about whether those other surfaces are wings, but I believe the answer is still no. There is nothing to be gained by calling them wings, and only leads to ambiguity. We already have a word which describes lifting surfaces in-general, and again, it's airfoil.

After doing a little more research, I have come to the conclusion that the answer is probably no. Since wings produce lift, I would argue that a propeller is not a wing since it does not produce lift, but thrust.

Since thrust is pretty much always perpendicular to lift, I can't see how the propeller would ever actually produce lift.


Why do airlines require passengers to return to their allocated seats for landing? Airlines which allocate seats often make announcements prior to landing to request passengers return to their allocated seats. What is the motivation for this requirement, especially since I have never seen the requirement rigorously enforced by the crew (by requesting sight of the boarding pass).

The requirement is not simply a polite way of requesting standing passengers return to their seats – for two reasons:

Crews are typically explicit in their language when giving directions, such as requiring standing passengers be seated. It is clear from the phraseology and emphasis given in the announcements that returning to the allocated seat is a critical component of the request.
On a recent long-haul flight, a couple seated in my row were offered to relocate to vacant seats in an exit row for the duration of the flight, provided they returned to the allocated seats for landing. (They were not, at the time, standing.) Furthermore, in the event, only one member of the couple returned; the other remained in the non-allocated seats (an example of non-enforcement).

Some possible motivating factors I have considered:

Weight distribution – ensuring the flight envelope of the aircraft is not exceeded during critical phases of flight. However, I have encountered this situation on wide-body jets on long-haul flights (e.g. A346, A388), where a handful of passengers, of nominal mass, being situated elsewhere in the plane does not seem cause for concern.
Collection of personal effects – if passengers relocate after takeoff, they may leave personal belongings at their original seat, unwittingly or not.   A requirement to return to the original seat may:

trigger passengers to locate & take with them items left/forgotten at their original seat, reducing the cost of the airline returning these to passengers later.
improve the efficiency of disembarking by avoiding passengers who need to move against the flow to collect belongings elsewhere in the aircraft.

However, I have encountered this after long-haul flights with tens of hours of turnaround time on the ground, so eliminating a few minutes from the disembarking time of some passengers does not seem a high priority (and, in any event, the effect of reducing the overall exit time would be in the noise and difficult to quantify).

Limitation of privilege (hence, revenue protection) – passengers in a higher cabin tier may benefit from certain privileges upon landing, such as the possibility to disembark the aircraft more quickly. However, crew are normally hot on ensuring passengers do not self-upgrade to a higher cabin, so this would not seem to apply.
Emergency preparedness – for example, ensuring there is a sufficient cabin crew to passenger ratio in all areas of the aircraft to meet emergency / evacuation requirements. Such ratios may be upset in the event passengers relocate far from their original seat. (If so, why is this not an issue when in the cruise?)
Passenger identification – air crash investigators often use seating manifests to identify remains of deceased passengers, so this requirement could be to ease any future investigation in the event of an incident on landing. (Similarly, what about the rare likelihood of an emergency which rapidly develops while in the cruise, without opportunity for passengers to relocate?)

None of these reasons seem to entirely justify the requirement for this routine request or its existence across all manner of aircraft types.

My questions

What benefit do airlines or the wider industry gain from this requirement?
Why is it not rigorously enforced by crews?
How do airlines which do not allocate seats to passengers satisfy the same requirement when there is no published seating manifest in advance of passengers entering the aircraft, or why are they exempt from such requirements?

Its a matter of legal requirement (under the FAA, and I presume the EASA has a similar reg). Strictly speaking its really more important that the passenger returns to a seat. Generally you are asked to return to your seat as it can be assured its empty and available for you. Under FAA regulations you are required to be wearing a seatbelt for landing and thus must be in a seat

  14 CFR 121.311 - Seats, safety belts, and shoulder harnesses.

  (b) Except as provided in this paragraph, each person on board an
  airplane operated under this part shall occupy an approved seat or
  berth with a separate safety belt properly secured about him or her
  during movement on the surface, takeoff, and landing. A safety belt
  provided for the occupant of a seat may not be used by more than one
  person who has reached his or her second birthday.

Weight and balance is important but not on a single passenger level i.e. on an airliner of decent size if a single passenger moved around it would be ok. If half the plane was empty and the passengers rearranged them selves there might be an issue.

  How do airlines which do not allocate seats to passengers satisfy the
  same requirement when there is no published seating manifest in
  advance of passengers entering the aircraft, or why are they exempt
  from such requirements?

They are not exempt from the requirements but there are enough seats for everyone on the plane and as noted above its really important that you are belted in so taking an available seat will do in this case. On these flights generally speaking, you take a seat prior to takeoff and that somewhat becomes your seat for the flight. In my experience there is little if any mid-flight seat swapping.

  Furthermore, in the event, only one member of the couple returned; the
  other remained in the non-allocated seats (an example of
  non-enforcement).

As noted above the policy is to be belted in, so the regulation was indeed enforced. The other member of the couple may have remained in the seat if they were the only person capable of preforming the exit row duties.

I always heard, informally within the airline industry, that it was in case the aircraft crashed, meaning it was important everyone was in their 'departure-control' allocated seat so they could be identified in the case of an accident. Weight and balance is less important at this phase of the flight (compared to takeoff), and the rule doesn't seem to be religiously enforced regardless.


What is the fuel/air ratio of the CFM56-5A1 engine (in the A320 model)? I could find the fan-bypass ratio, which was 6:1 (on some models 5:1) but I was unable to find the fuel to air ratio used in this engine. For reference, it is a high-bypass turbofan engine.

The ratio can be different for different phases of flight. It is easy to calculate for the Takeoff (TO) case because the required numbers are easily available:

TO Thrust: $ 25 \, 000 \, \text{lbs} $ [1]
TO TSFC (thrust specific fuel consumption): $ 0.3316 \, \text{lbs/h} $ per $ 1 \, \text{lbs} $ of thrust [2]
Airflow: $ 852 \, \text{lbs/h} $ [1]
bypass ratio: $ 6.0 : 1.0 $ [1]

The takeoff fuel flow $ \text{FF} $ is then given by

$$ \text{FF} = 25 \, 000 \times 0.3316 \, \text{lbs/h} \approx 8290 \, \text{lbs/h} \approx 2.30 \, \text{lbs/s} \approx 1.04 \, \text{kg/s} $$

and the airflow $ \text{AF} $ through the core is given by

$$ \text{AF} = \frac{1}{6 + 1} \times 852 \, \text{lbs/s} \approx 122 \, \text{lbs/s} \approx 55.2 \, \text{kg/s} $$

This results in a fuel/air ratio of

$$ \frac{\text{FF}}{\text{AF}} \approx \frac{1}{52.9} \approx 0.0189 $$

Sources:

[1]: www.cfm56.com
[2]: Wikipedia CFM56


Use of a METAR when $ is present I fly out of an uncontrolled airport that has an unmanned, automated weather reporting site with a precipitation sensor (AO2).  The METAR often has a \$ symbol at the end of the report, indicating that maintenance is needed on the equipment.  Can I use this METAR, or any METAR with a "\$" as a reliable indication of the current weather?

Do the indications match up and make sense compared with indications of a nearby airfield and other weather sources?

Legally the FARs require that pilots obtain "all available information" (FAR 91.103). $ could mean a number of things and since Metars are still available, you should be able to trust them, but I recommend to check more then one source regardless.


Why did some aircraft use hand-crank instead of gravity for alternate gear extension? I wondered whether the final scene in The Memphis Belle where they hurry to extend the gear by hand crank (because their electric system failed) is realistic, but according to this answer the B-17 indeed used a hand crank for manual gear extension.

But why did it (and other aircraft of the era that used similar system) need to manually crank the gear all the way down rather than just pull a latch out and let gravity do the rest, like modern systems generally work?

They had to manually crank down the gear because the electric system was damaged.

"As the B-17 limps closer to base, Dearborn orders the landing gear dropped. Only one wheel descends due to electrical failure caused by battle damage, but the crew are able to manually lower the malfunctioning wheel just before landing."

Free fall only works for hydraulic powered landing gear systems. The B-17 uses an electrically driven motor to raise and lower the gear.

Not all hydraulic landing gear can free fall. PBY-5A's have hydraulic gear and emergency gear extension used a backup hydraulic pump. Additionally you could manually break the up locks and then manually push the gear to the locked position.

I bet many other WWII era aircraft with hydraulic gear were similar. Free falling hydraulic gear probably came much later.

Because of wind pressure

Disengaging locks will result in landing gear to partially open but gravity is not strong enough to make the gears lock in their extended position. Wind blast or wind pressure will oppose the movement of landing gear and to actually make the gears extend fully when main system has failed, hand crank is the better option as opposed to relying on gravity.

Also modern systems do not solely rely on gravity!


Does the AP disengage if the plane is upset and how would the flight controls react if not being held? Does the AP (autopilot) essentially detect a rapid change in speed/altitude as a wild maneuver? If so, upon disengaging, if the pilots are not holding the control column in a neutral position, will it move in whatever direction the plane is going?

For example, a steep climb that wasn’t anticipated, and no one holding the Column, will it fall back towards the pilots, ultimately making the steep climb worse?

For non FVW airplanes:

Yes it will disengage if certain pitch and roll attitudes are exceeded.  When it disengages, the controls are "let go" if the A/P was applying an input at the time, and the control column will move to whatever its natural trimmed state is at that time if the elevator is manually operated with trim tabs.

If the airplane has irreversible hydraulic controls and an artificial feel system and a trimmable stabilizer, the column will just spring back to neutral and the airplane will seek its existing trimmed speed.  If the trim speed is the same as the current speed, pretty much nothing happens, if the trim speed is much different that trim speed, the nose will pitch up or down to try to accelerate or decelerate to the trim speed set by the current state of the pitch trim system.

It's as if your copilot was flying, minding his own business, and you shot him dead.  The column will just go to wherever it wants to go when no-one is holding it.  The airplane has natural static "stick-free" stability so it will seek its natural trimmed state and if the A/P input was different from that, it'll just start going where it trim state wants it.  As if you pulled on the column and then let go.

On an airplane with control cables, the autopilot is a little computer running a bidirectional electric winch (the A/P servo) back in the tail who's winch cables are tied into the elevator cable run.  It drives the cable circuit from back there instead of from the column.  The autopilot also usually works the pitch trim that same way a human pilot would, to relieve the need for the servo to hold control pressures beyond a small threshold same way a human would.

So if the airplane was more or less in trim when the A/P kicks off, not much happens.  If the airplane was in some crazy attitude because of some outside event, the A/P kicking off is like you were flying and got flipped over and you took your hands off the controls to cover your face in fright (that's what I would do lol). The controls will just return to where they are trimmed to go to in that state.  On most airliners with powered controls and a moveable stab, the column always springs back to the same neutral position regardless of trim state.

If the a/c is FBW, the A/P is integrated into the computers that actually run things and it gets a lot more complex, although the end results are not that much different.

It's hard to provide a specific answer because a lot of important context is missing from your question, but here goes.

In a conventional airplane, with any manner of control system, when the stick is released from a held position where there is deflection of the control surfaces, the stick should return to centre.

In salt-of-the-earth cable driven control systems, the act of pushing the stick in any direction causes control surfaces to move up into the airstream. The drag that this produces creates resistance, a weight that is felt in the stick. When you let go, the stick will spring back due to this resistance, and probably over centre into the opposite side and cause an oscillation, but that will dampen out and the stick will rest neutrally.

This self-aligning force can be felt when you're driving a car at relatively high speed. Imagine you're on the highway and to follow the road, you need to make a gentle

Even in systems where there are not direct connections between the stick and the surfaces, engineers add artificial means to produce this feedback. For example, in some fighter jets the stick has an extension under the floor which has a deadweight attached to it, which the pilot has to pull against to hold the airplane at a certain level of G-loading when turning.

This self-aligning force is very important to how the aircraft feels in the hand. It can also be felt when you're driving a car at relatively high speed. Imagine you're on the highway and to follow the road, you need to make a gentle curve. Then imagine you turn the wheel and it's as light as a feather.. it would be very difficult to find and hold the precise input required to produce the required turn.

In your scenario, when the airplane is climbing and the stick is released, it may fall backwards a little bit initially, but it will not accelerate like a dropped object straight to maximum deflection because the airplane itself is resisting the force gravity as it's applying itself to the stick. it will find a neutral point just aft of centre stick

None of this means to say that the airplane will climb or descend, or even maintain the same angle of bank. You haven't defined enough variables in this scenario (Airspeed, pitch angle, power setting, trim setting, engine type) to tell us what might happen next.

I will leave the autopilot part of the question to somebody who's even remotely qualified to talk about that.


How is the steepness of turn bank properly defined? In aviation we often refer to the bank of a turn as shallow, medium  or steep. I remember reading somewhere that the definition is not universal as to how many degrees it is, but it has to do with the airplane and whether the bank tends to roll back to level, remain where it is or continue to steepen. Does anyone know if this is a correct definition or have any references to share?

I don't know if this is only definition out there, but according to the FAA's Airplane Flying Handbook:

  Shallow turns are those in which the bank (less than approximately
  20°) is so shallow that the inherent lateral stability of the airplane
  is acting to level the wings unless some aileron is applied to
  maintain the bank.

  Medium turns are those resulting from a degree of bank (approximately
  20° to 45°) at which the air- plane remains at a constant bank.

  Steep turns are those resulting from a degree of bank (45° or more) at
  which the “overbanking tendency” of an airplane overcomes stability,
  and the bank increases unless aileron is applied to prevent it.

Also very closely related is the turning radius, which also depends on the speed. For example, a Rate 1 turn is defined as a turn that changes the aircraft heading 3 degrees per second, so it takes 2 minutes (120 seconds) to turn a full 360 degree circle.

If you are in an ultralight, doing 50 knots, you need approx. 50 / 10 = 5 + 7, or 12 degrees of bank to achieve a full circle in 2 minutes.

If you are in a 747 flying at 180 knots, you need 180 /10 = 18 + 7, or 25 degrees of bank to achieve a full circle in 2 minutes.

Both turns might be classed as "shallow" (although on that definition I would hate to do a steep turn in a 747).


In modern commercial aircraft, is it possible for pilot inputs to exceed the tolerable structural loads? It seems that modern fly by wire systems should / could limit pilot inputs to prevent structural damage.

For example, American Airlines 587:

  From the NTSB report of the accident:

  The National Transportation Safety Board determines that the probable
  cause of this accident was the in-flight separation of the vertical
  stabilizer as a result of the loads beyond ultimate design that were
  created by the first officer’s unnecessary and excessive rudder pedal
  inputs. Contributing to these rudder pedal inputs were characteristics
  of the Airbus A300-600 rudder system design and elements of the
  American Airlines Advanced Aircraft Maneuvering Program.

Airbus fly-by-wire aircraft have what is called envelope protection which is to prevent pilot inputs from exceeding structural limits as well as keep them from getting the aircraft into an upset flight regime (stall, overbank, etc.). The philosophy behind it is that, if the pilots need to perform an extreme maneuver they can be as aggressive with the controls as they want, confident that they won't break the plane.

Boeing generally disagrees with this approach saying that, in an emergency, the pilot needs to have all possible force at his disposal, even if it breaks the plane. See the question Federico linked to in comments for more on this.

Of course, the Airbus system can only provide these protections when systems are all functioning properly. If part of the system is not working the protections are removed. Examples: AF447, XL Airways 888T, Indonesia AirAsia 8501,

As far as AA587, the A-300 is not a fly-by-wire aircraft and doesn't have envelope protections. It's debatable, though, if envelope protection would have helped in that situation. None of the rudder inputs by itself was outside of the structural limitations of the aircraft so the computer would probably not have prevented them. The failure was due to the pilot making alternating inputs in quick succession which caused the sideslip angle to build up to twice the certified design strength of the vertical stabilizer. The A300 system was designed to limit the rudder travel available as speed increased, which is probably fairly similar to what the fly-by-wire computer is programed to do in other Airbus planes. But it would be difficult to determine what complex combinations of inputs might exceed the limits and prevent them.

A fly-by-wire system is going to be controlled by software. All software has bugs and conditions that it is not designed to handle. Thus, even in a fly by wire system it is possible for the control inputs to exceed structural loads.


Does the downwash created by induced drag increase or decrease lift? According to the theory I know, lift is produced by accelerating air downwards at the trailing edge (downwash). Hence, as downwash increases so does lift. Induced drag is due to vortices at the wing tips and they also create downwash. So according to the above theory induced drag should also increase lift.

But, the books say that induced drag increases downwash and reduces the effective AOA, which subsequently reduces the lift. Does the downwash from induced drag increase or decrease lift?

It is not "this causes that" - all is happening together. Let me explain:

For me to understand aerodynamics, it helped to disregard all that talk of vortices and induction, but focus on the pressure field around a wing. When the theory of flight was developed, electricity was new and exciting, and it just happened that electric induction could be transferred to lift. Now every author still copies the explanations from a century ago, but they are totally unintuitive.

Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects. Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise. Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they experience. Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly, the packet will change course and follow the wing's contour, but not without spreading out (= pressure loss). Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.

A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.

What is lift?

Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.

Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.

Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

Induced drag

The misconception about those "wingtip vortices" and induced drag is hard to eradicate. Most authors copy what has been written before without clearly understanding the issue. Therefore I repeat it here again: Induced drag is the backward-pointing part of the pressure force vector. The vortices are only a consequence of downwash, which in turn is a consequence of lift creation. At the same speed, more induced drag is indeed linked to more lift, but the causality is different: Lift and induced drag are both part of the pressures acting on the wing. If you add up all the pressure forces acting on a wing, their resulting vector will point slightly backwards. The streamwise component is drag, and the component orthogonal to the direction of movement is lift. This is just a defininion, made for simplicity.

There are actually two sources of induced drag (see wikipedia article.  One source is described well by Peter's post.

But the other source is vortex drag, from the wingtip vortices (turbulence), which depend on the aspect ratio of the wing as well as on wingtip treatments like winglets.   In the end the wingtip vortices lower the glide ratio (L/D) of the aircraft.  So if they produce a little bit of lift, it doesn't help because the drag has increased by a bigger factor, lowering the L/D.


What is the maximum wind speed a Boeing 737-800 can deal with during takeoff and landing? I'd like to know how safe the Boeing 737-800 is during takeoff and landing when it's windy. What are the maximum wind speeds for cases of tail, head and cross winds?

EDIT

I've found this resource http://www.b737.org.uk/limitations.htm#Wind_Limits, but I'm quite unable to understand the data:

According to the table (which only shows x-wind data), the max wind speed depends on the so called "braking action", but I don't know what exactly is nor what it depends on.

EDIT 2

The same resource points out the max tailwind speed, as well as other info:

As your source shows, the maximum tail wind is fixed and fairly simple.

Getting slightly more complex, there is technically no maximum headwind I'm aware of, although most airports will close when the wind gets much above 50 knots, and even with the wind coming almost straight down the runway, you still get some crosswind component, which factors in. Expect to see many pilots divert in winds much above 40 knots even if the airport is open, especially if gusting to above 50.

Crosswinds are a little different as they depend on the braking action - basically how grippy the runway is. Simplified, you can think of it as a scale from dry weather, through slightly and very wet, then slush and ice. The grippier the runway, the more the tyres will grip and the higher crosswind the aircraft can deal with. The higher numbers in the chart you show are for a dry runway (good on your chart) the lower numbers for wet or icy (poor on your chart) runways.

Although it's written in terms of 'braking action', it's really a measure of grippiness, and it's important both when braking, but also for the ability of the tyres to prevent the aircraft sliding laterally off the runway. This job doesn't solely come down to the tyres, but they're part of it. This is also a large part of the reason why landings and takeoff have different maximum crosswinds on some aircraft.

If we are talking about 737 then there is maximum headwind. Landing speed for 737 is Vref + 1/2 headwind and cannot exceed Vref + 20. That means that maximum headwind component can not be more than 40knot


Does the A-10 Thunderbolt fly without the GAU-8? Has the A-10 Thunderbolt ever flown without the GAU-8? Can it be done with a ballast in place of the gun? I'm not referring only to the combat role of the A-10, a plane used for training or a ferry flight would also qualify.

Near as I know, the answer to the first question is no. The aircraft requires that a tail stand be fitted to it when the gun, magazine and electromechanical drive mechanism is removed for maintenance.  Operation of the A-10 without the gun would exceed the weight and balance envelope for the aircraft, making flight, as well as ground operations hazardous.

The A-10 has been proposed for use as a converted water bomber to fight forest fires but this balance problem without a counterweight installed in this region would make this very difficult.

http://fireaviation.com/2014/09/25/a-10-warthog-proposed-again-as-an-air-tanker/

If one was going to remove the gun for a ferry flight, it would have to be fitted with some kind of counterweight in its place, which would defeat the original purpose of removing the gun.

The A-10 has already been used as a fire fighter.   I don't know what mods were made but I know it didn't have a tank killing cannon in the nose.


Does ATC use mode S addresses to identify planes? By design, Mode S surveillance radar equipment uses global 24-bit airframe addresses to keep track of the aircraft it is tracking.

Are these addresses also used by higher levels of the ATC system, such as to fill in call signs, flight plans and so forth automatically when the plane first appears in a controller's domain? Or does that still depend on ATC assigning 12-bit squawk codes that pilots set manually?

Does the Mode S address appear in a flight plan when filed?

The Mode S address is not used for identification of aircraft in the higher levels of the ATC system such as the Flight Data Processing System.

Mostly the assigned 12 bit code (also known as Mode A code or squawk code) is used to correlate a radar track to a flight plan. The major exception is that in Europe, when the Mode A code is set to 1000 (octal notation), the Mode-S downlinked Aircraft Identity (ACID) is used for flight plan correlation. This is to free up Mode A codes. Usually planned flights are assigned discrete Mode A codes but using ACID / Flight ID correlation, multiple aircraft on a flight plan in the same area can use Mode A code 1000.

The downlinked ACID must be equal to the Flight ID used to file the flight plan.

The Mode S 24 bit address is only used in low level systems; in the Mode S radar, in multilateration systems, in ADS-B receivers and to some extend in tracking systems.

This depends on the specific Air Traffic Navigation Service (ATNS) provider providing services and the equipment the controllers are equipped with. US-based FAA controllers actually are mostly unaware of which types of surveillance data are currently contributing to the position the system is plotting aircraft on their displays or scopes.

While they have access to all elements of ADS-B data associated with an aircraft which has been received, the need to query any beyond what is included with a legacy flight strip would be the exception, not the rule. The FAA maintains the world's largest and most complex integrated air traffic surveillance system which offers multiple tiers of aircraft tracking capabilities. This network of air traffic surveillance data sources offer coverage within the vast majority of the US National Airspace System (NAS) designated as Controlled Airspace (Class E, D, C, B, & A).

The FAA does maintain a database of all US registered Mode-S 24-bit aircraft address codes, colloquially referred to as "Aircraft Hex Codes". As a function of the many Bilateral and Multilateral Aviation Safety Agreements entered into with a number of foreign nation states, the FAA also populates this database with the Mode-S Hex Code provided by these foreign Civil Aviation Authorities (CAAs) with regularly received updates.

Nearly all FAA ATC surveillance functions associated with an aircraft's Hex-Code during normal operations are automated and seamless to the controller's display or scope. These functions include the auto population of registration number, aircraft type, and last reported equipment codes, etc into the track data blocks of nonparticipating aircraft, among others.

Your question specifically asks about tracking aircraft movements using these Mode-S Hex Codes. As far as the US FAA's policy, the answer is an unquestioning YES! All air traffic track data, to include all elements of ADS-B data are recorded and retained for a range of defined duration as dictated by specific facility and role. If there is a request or need to retain any set or portion of FAA AT data beyond those disposal time-frames, this can be done indefinitely.

Every nation state CAA and ATNS provider would need to be queried individually to determine the extent of their use of collected Mode-S Hex Code data. The only specific detail I'm aware of is that the ATNS providers located in the UK (NATS) and some parts of the EU (EuroControl) generally seem to be more aware of the Hex-Codes associated with the aircraft under their control. In fact, they are known to recognize and take notice when an aircraft Hex-Code mismatch is operating within their specific sectors. An aircraft Hex-Code mismatch occurs when the aircraft's reported, filed, or communicated registration number can't be reconciled with the Mode-S Hex-Code being used and transmitted and/or the aircraft records database they maintain.

Probably more than you wanted but I'm not one for brevity!


What is the best design to stack wings of a triplane? I would like to design an ultralight airplane, which probably the total mass will be around 400kg. The speed should not exceed 200km/h. Forget about what the engine I will use. I intend it can land and take off in a small road and/or in a common road, which the road line not more than 2.5m each direction. If I have to occupy both direction, mean the road width is just around 5m, plus small space beside the road in both side. Beside the road, normally many electrical poles. So, not many space available.

To tackle the situation, I need to change wings design without reduce the wing area (A in m2). Was discussed earlier here some of the triplanes that were used during the WWII. If they were used during the war, that mean that such design was safe enough. But, I have a bit confusion regarding how to place the wings' stack? Here are the three options design.

The wings stack are lean to the tail.
The wings are stacked exactly over another.
The wings are lean to the nose.

Another thing. We know that the closer wing to the ground will produce what is called earth deflection (CMIIW about the terminology) during take off. That will make the plane requires longer time to take off. In my opinion, the same effect will occur if we stack the wings too closed one to another, especially if the wings are stacked lean to the nose, the deflection will hit the lower trailing edge if there is no enough space. So what is the best design in this case? And what is the best space?

Wing stagger was mostly chosen to improve pilot vision: In single-seaters, a positive stagger would allow to place the upper wing ahead of the pilot, improving the field of view.

Large bi- and triplanes had no stagger. Here the pilot sits ahead of the wing and large aircraft fly mostly straight, reducing the need for the pilot to observe the airspace around him.

However, if you look for a proven design of a very compact triplane, I would like to suggest the Italian Pensuti triplane. It had a wingspan of only 4 meters and a total weight of just 230 kg. Emilio Pensuti, a test pilot with Caproni during WW I, designed it as an easy to fly aircraft for the common man. Therefore, it is sometimes also called the Caproni-Pensuti triplane. The first version was powered by a 35 HP Anzani engine while a later development, the Breda-Pensuti 2, used a 80 HP engine.

Of course, its wings had no stagger.

Drawing of the Pensuti triplane (picture source). Yes, it did fly!

Pensuti triplane in flight (picture source)

If you build one, I would strongly suggest to change the wing airfoil to a thicker one with the same amount of camber.

A good starting point would be the Curtiss JN-4 "Jenny" biplane.  Plenty of wing here, no need for triplane.  You want long wings for easy slow handling.  Warplanes designs generally are not as stable and do not make good recreational aircraft unless you are a very good pilot.

Stacking wings leaning towards nose reduces interference and maintains airflow over lower wing at higher Angle of Attack, making for a gentler stall.  This design is the fore runner of slats, which are also very effective.  You also want wings spaced as far apart as you can for better lift (get rid of the third wing).

It is surprising that there are not many recreational biplanes around these days, as they offer simple, rugged, reliably safe construction and lower flying speeds.

I would recommend continuing to research this, particularly to get a good engine/aircraft match.  Good luck!