Is it common for panels on a plane's body to not be perfectly aligned? I was recently on a RyanAir flight. As far as I know, they use Boeing 737.

When boarding, I noticed a that one of the panels' edge, roughly where the head / chest part of the logo is, was sticking out maybe 4-5 mm compared to the edges of the neighboring panels.

This wasn't something new because the paint above was in perfect shape.

Is it common for panels on a plane's body to not be perfectly aligned like this? I never noticed this on any other plane I boarded.

The best illustration I could find is a bathroom tile picture:

The skin on the right of the door looked exactly this: one panel had one side not flush with the others and intact paint on top of the whole thing.

It looks like you were seeing simple skin lap joints.  Flush skin butt joints are generally used on high speed aircraft, but these joints are more complex to make (you have to use a joining splice strip under the butt joint).

A lot of the time the manufacturer determines that the local airflow isn't smooth right at the surface (there is a turbulent boundary layer that is maybe 1/8" deep) and a small step in the surface has a negligible effect on drag.  If this is the case, a simple lap joint, that doesn't require the joining splice strip underneath, is used.

There usually isn't a hard 90 degree step on lap joints because it's normal practice to put a fillet of sealant along the edge to make the higher skin taper toward the lower one and this mitigates any drag effects further.


If left stationary at a specific height, would altimeter change the displayed value over time? I have been reading a lot about how altimeters work, and from what I understood, most common altimeters are based of atmospheric pressure to calculate the altitude.

The question that I still have though is: If left stationary at a specific height, would altimeter change the displayed value over time? Something tell me there would be slight variation, because air pressure changes almost daily, or am I totally off with this one?

You are correct in both regards -- as Terry's comment points out, an undisturbed pressure altimeter's reading (imagine one sitting on a bench outside the local hangar) will change with the local barometric pressure.  TSO'ed "sensitive altimeters", i.e. ones with a hundreds pointer, not just a thousands, are required to have a knob and window, called a Kollsman window, for setting the local barometric pressure, or standard pressure (29.92) when above the transition altitude (18,000' in the US, check with the local aviation regulator for where you are in the world).  "Glass cockpit" systems have a knob/function for the same job -- whether it be a dedicated knob, or a mode on a multi-function knob.


Why does a force on a propeller act 90° "ahead" due to gyroscopic precession? This is a common explanation:

A tailwheel airplane is on the takeoff roll and the tail comes up, the
propeller disc rotates forward. That's akin to someone pushing on the
top of the disc. The force results 90 degrees ahead, or on the right
side, pushing the nose left.

I do not understand the term "90 degrees ahead".
In my mind, in this example, we are viewing the propeller spinning clockwise.  If a force is applied at the top (12 oclock), then the resulting gyroscopic force on the right (3 oclock) should be called 90 degrees after.  If the force is applied at 12 oclock, spines 90 degrees and then a resulting force at 3 oclock, why is that called before instead of after?

If I say "my brother was born ahead of me", you might reasonably assume that my brother is older than me.  That his birth took place at an earlier point in time.
You seem to be assuming that "ahead" refers to the position that any given blade of the propeller occupied at an earlier point of time.  I.e., a position displaced opposite to the direction that the propeller is rotating.
But if you are driving a car and your passenger says "the light's green, go ahead", would you take that a suggestion to go backwards?  Or to go forwards?
The meaning of "ahead" in the passage quoted in the original question is correct and reasonably unambiguous-- it means forwards along the direction that the propeller is rotating.
Note that the original passage did not contain the word "before".


How does Boeing park several airplanes on a single runway at Paine Field? On DigitalGlobe, I saw the following picture in an article, which talks about some of the best aerial shots of 2014.

Paine Field, Washington

Questions

1) Did the plane, highlighted with a yellow circle, need to reverse to line up so neatly even though it just turned (I assume). Is the turning radius really that small that it takes very little space for a big plane to change direction so quickly on ground?

2) Is there any special reason that all the planes are on one side of the runway and not centered on the markers? They all seem to be lined up towards the left side of the picture.

3) Isn't that too close when there is another plane parked so close in proximity? Provided there is ample space available.

4) Is it just that the scale of this picture, taken from a satellite, making them look closer than they really are?

5) What does that Double marking 7/91 Mean on the North East end of the small runway? usually that is a single number indicating the heading. That can't be 91, then what does that 91 mean?

6) Why are  there a bunch of planes that are about stacked on top of each other towards the bottom right of the photo, on the ramp to 29?

P.S: I know they are parked and are not taking positions to take-off.

It was tugged to that position via tug.
Angle of photo + possibly trying to offset weight, from centerline.
These planes are just being stored there. They'll be tugged to another location
before they even bother to start them up, and I believe most don't
have engines on at present.
They are quite close together.
(from @ratchetfreak's answer:) Turn the image around as if you are coming to land; it's actually 16L a perfectly normal runway number.
Its just a very compact way of parking aircraft in minimum space. Compare the space taken up by those 4 planes versus any 4 nose-to-tail on the runway.

These aircraft are mostly 787s that are the early teen aircraft that were rejected by the original airlines they were slated for, due to so many out of tolerance issues/rework required. They are just being stored on the runway until Boeing can get them into mod/rework status and get them to spec to be able to sell. Right now the mod line seems to be working on one of these aircraft, at a time, and each one taking many, many months to get fully reworked.


What's this red cabin light indicator inside a Boeing 737? What's this red light indicator inside the cabin, its flush with the sides of the plane. The exact location would be a couple of rows ahead of the overwing exit seats.

This picture was taken before take-off. Forgot to observe the light after take-off. This is not a flight-attendant call indicator.

I found this pertaining to the new interior design of some Boeing737(s), I can only guess the coloring is from one of the mood lighting (Blue or red) has a defect in the connections.https://www.airlines-inform.com/flight-reports/amarchukov-1754.html


How to calculate the velocity gradient at the wall normal to the wall (Boundary layers)? This problem is from "Fundamentals of aerodynamics" by John D. Anderson, Jr (Fifth edition, page 101):

Consider a flat plate at zero angle of attack in a hypersonic flow at Mach
10 at standard sea level conditions. At a point 0.5 m downstream from the
leading edge, the local shear stress at the wall is 282 N/m^2. The gas
temperature at the wall is equal to standard sea level temperature. At this
point, calculate the velocity gradient at the wall normal to the wall.

I already know: $\tau_w = \mu\left(\frac{dV}{dy}\right)_{y=0} = 282$
but how to get $\mu$ ??

Also I don’t know what this is for: “The gas temperature at the wall is equal to standard sea level temperature”

In page 72 you'll find the value of µ at standard sea-level temperature...


Were there any instances of aerial combat between helicopters? Were there any cases in history where two attack-helicopters like AH-64 and MI-24(for example) were involved in an air fight between each other?

Yes, there has:
In the Iran-Iraq War, Wikipedia:

The war also saw the only confirmed air-to-air helicopter battles in the history of warfare, with Iraqi Mi-25s flying against Iranian AH-1J SeaCobras on numerous occasions. The first instance of these helicopter "dogfights" occurred on the first day of the war (22 September 1980): two Iranian SeaCobras crept up on two Mi-25s and hit them with TOW wire-guided anti-tank missiles. One Mi-25 went down immediately, the other was badly damaged and crashed before reaching base. The Iranians won a similar air battle on 24 April 1981, destroying two Mi-25s without incurring losses themselves. According to unclassified documents, Iranian pilots achieved a 10-to-1 kill ratio over the Iraqi helicopter pilots during these engagements and even engaged Iraqi fixed-wing aircraft.

These would be pretty loosely classified as a "dogfight" since the Iranian AH-1J's "crept up" on the Mi-25's. It is doubtful that the Mi's did any kind of evasive maneuvering or that the AH's did any kind of aggressive pursuits, but it is air-to-air combat between helicopters.
The page from the Mi-24 page gives a little more detailed report:

This war also saw the only confirmed air-to-air helicopter battles in history with the Iraqi Mi-25s flying against Iranian AH-1J SeaCobras (supplied by the United States before the Iranian Revolution) on several separate occasions. In November 1980, not long after Iraq's initial invasion of Iran, two Iranian SeaCobras engaged two Mi-25s with TOW wire-guided antitank missiles. One Mi-25 went down immediately, the other was badly damaged and crashed before reaching base. The Iranians repeated this accomplishment on 24 April 1981, destroying two Mi-25s without incurring losses to themselves. One Mi-25 was also downed by an IRIAF F-14A.
The Iraqis hit back, claiming the destruction of a SeaCobra on 14 September 1983 (with YaKB machine gun), then three SeaCobras on 5 February 1984 and three more on 25 February 1984 (two with Falanga missiles, one with S-5 rockets). After a lull in helicopter losses, each side lost a gunship on 13 February 1986. Later, a Mi-25 claimed a SeaCobra shot down with YaKB gun on 16 February, and a SeaCobra claimed a Mi-25 shot down with rockets on 18 February. The last engagement between the two types was on 22 May 1986, when Mi-25s shot down a SeaCobra. The final claim tally was 10 SeaCobras and 6 Mi-25s destroyed. The relatively small numbers and the inevitable disputes over actual kill numbers makes it unclear if one gunship had a real technical superiority over the other. Iraqi Mi-25s also claimed 43 kills against other Iranian helicopters, such as Agusta-Bell UH-1 Hueys.

I don't believe there has been any recorded instance since the Iran-Iraq war, but during that conflict there were multiple instances of helicopter-vs-helicopter battles.


Why is powering on the APU so low on the A320 emergency checklist? In the movie Sully we have this section of dialogue

  He wasn't following proper procedure at all.
  And I know because I had the QRH in my hands.
  He switched on the APU immediately after
  engine blowback.
  According to the Airbus that's the 15th thing on the list to do. 15th!
  If he had followed the damn rules, we'd all be dead.

Now, another question discusses the importance of the APU to the crash, but my question is, why is it so far down the list? I suspect the above is dramatized somewhat, but it does seem odd that it's so low. What does Airbus consider more important in their handbook?

The assumption was that a multiple engine failure happens at altitude, for instance in cruise while flying through volcanic ashes. This happened to British Airways Flight 9 and to KLM flight 876, with all four engines switching off due to the ash cloud. Total engine failure at altitude buys time, and if time is available first priority is to attempt to re-light an engine. APU will probably be off during cruise, and given the choice to re-light an engine or to light the APU it is clearly the engine that is the most beneficial to start.

All-engine-failure in airliners is very very unlikely. It is a function of number of engines - in a four-engined aeroplane, the chance of all four engines failing is assumed to be super extremely incredibly unlikely. With a statistical failure rate of 10$^{-5}$ per flying hour per engine, a four-engined aeroplane would experience all engine failure due to statistical causes once in $\frac{10^{20}}{24}$ flying hours1: a billion times a billion times four. So very uncommon that the only practical consideration would be a common mode failure from external causes, such as the volcanic ash cloud.

For a twin-engine like the A320, chance of all engine failure due to random causes is 2 * 10$^{-10}$, still considered an Extremely Unlikely event. Procedures are written for the least unlikely events, and updated according to events that actually happened. Now that a dual engine failure during take-off has actually happened, updating the procedures will be the result of failure analyses: how likely would it be to happen again, or what other procedures can be put in place to avoid this situation.

 1: The chance that any of four engines fails is 4 * 10$^{-5}$ per flying hour. Then 3 * 10$^{-5}$ per flying hour for the remaining engines etc.


If we use upper surface blowing and externally blown flap on the same wing will lift correspondingly be very high? With two engines, one above and one below the wing, will the lift be the sum of two lift increments (produced by upper surface blowing and external blowing)? I am trying to make a jet suit and I am interested in incorporating this phenomenon in it.

When a jet of air blows over or under an airfoil with a deflected flap, it gives an immense amount of lift due to the change in direction of the jet via the Coanda effect. The engine can be placed either above and below and can achieve lift coefficients up to 7 (when any one of the methods - upper surface blowing or externally blown flap - is used)
What if we use two of them at the same time? To which value can the lift coefficient increase?

Two Engines Would Probably Be a Waste

First the basics about lift. Lift can be seen as occurring from one of two effects, a pressure difference across surfaces, or in reaction to the acceleration/deflection of the air (or whatever fluid you are moving through). Any case can be calculated from either method and you will come to the same result. You can't exploit a pressure differential without accelerating air, and you can't accelerate air without creating a pressure differential. Since force in these cases is governed by $F=\dot m\Delta v$ and required power is $P = \frac{1}{2} \dot m (v_{out}^2 - v_{in}^2)$. This means that to maximize efficiency you should try to accelerate the largest quantity of air by the smallest amount.

Now let's talk about flow control. Blown flow control systems increase lift in two ways, first is that they help keep the flow attached, and second, especially in propulsion driven systems like you are describing, is by deflecting part of the thrust downwards. Attaching the flow helps to allow greater angles of attack without stalling, once you have have generated this flow attachment, further actuation of flow control will not help generate any more lift by this method, though it may allow further angles of attack. If you are trying to improve low speed characteristics by allowing extreme angles of attack remember that the increase in lift will approximately follow a sin curve as it results from increasing the angle of deflection for flow across the wing, the first little increase in angle will give lots of lift with little drag, the last will give almost no lift for tons of drag if you want to take it as far as 90 degrees. Also lift is related to speed to a power of two, that means you need a lot more deflection as your speed drops.The second method that blown systems generate lift is by deflection, basically you gain some lift by sacrificing thrust. In the end this is no better than just pointing the engines slightly downwards.

If you want to perform flow control you also have to remember how much area on the wing your exhaust is going to effect. Your jet will likely be small in comparison to the total size of the wing.

At the end of the day, it is probably better to make a system that provides some flow control to the entire wing rather than a massive amount of flow control to a small portion of the wing so spread out those engines rather than trying to stack them; and remember, flow control will give no improvement over simple thrust vectoring at a standstill.


How much do the blades on the V22 or AW609 weigh? It seems that the blades on the V22 or AW609 would have to be heavier than conventional helicopter rotor blades since they have to be substantially stiffer when in fixed-wing mode. Normal helicopter blades would flap beyond acceptable levels when exposed to such high forward airspeed. It also appears that since tiltrotor disc loading is much higher, conventional thin helicopter blades would sag excessively.

So, how much do the blades actually weigh?

" However, two Osprey 38-foot rotors weigh 4,654 lbs (JANE's, 1998-9, p. 557.)" this quote is from http://www.freepatentsonline.com/8382030.html

That would put the mass of a single V22 rotor at 1050 kg. The quoted source dates from 1998, so some improvements may have been made, but it is unclear to which parts of the rotor this refers, so it's only a guesstimate anyways.


Should the helicopter be washed after any sea mission? I was seeing this image:

The caption of this image says (source):

Here a Belgian NH90 NFH returning after a flight over sea. The
helicopter is washed by the fire brigade to remove the salt water...

Should any helicopter be washed after any sea mission? Are those guys washing the engine? If so, should the engine of helicopters be washed after any sea mission?

The helicopter shown is an Anti-Submarine Warfare Helicopter. ASW helicopters generally have the ability to detect submarines either Passively or Actively. Passively, sonar-buoys are dropped into the sea and act as ‘microphones’. The returns from these can be triangulated to track submarines.
To achieve this, the helicopters altitude will generally be 1500ft amsl in this role. Unless the winds are very strong salt accretion is minimal. In the active role, the helicopter hovers above the sea and lowers an active sonorhead into the water. The height for this is normally between 40-80ft. The aircraft will stay in this position as long as it takes to establish a 360deg sweep, which is typically less than 5 minutes, or if in contact could remain in the hover for 30 minutes or longer. This process is repeated for the duration of the sortie, which may be up to 2-3 hours.
During this time, the helicopter is experiencing recirculating sea spray. Depending upon the wind / sea state, will determine how much salt accrets (windscreen wipers are required on occasion). The engines experience salt ingestion, which accrets on inlet guidevanes and compressors. As the sortie progresses it is possible to see a reduction in engine performance (the advice for this used to be to find a rain cloud to fly through).
On completion of a days flying over water the engines receive a compressor wash or after a single flight if particularly bad. The airframe will also be washed on a regular basis, perhaps every couple of days. Many bases operating ASW helicopters have dedicated ‘drive thru’ washes. The helicopter will ground taxi over an area with water jets set into the ground and water will spray the airframe and underside of the rotor blades to remove the salt, sometimes known as a ‘bird bath’.


Can air travel cause packaging air pads to burst or deflate? This question is a new cross-post from the physics stack exchange: https://physics.stackexchange.com/questions/265884/can-a-decompressed-airplane-cargo-hold-cause-packaging-air-pads-to-burst-defla

I received a parcel from Germany to Chile. The contents were severely bruised due to a lack of packaging material. On second inspection I found a bunch of deflated air pads.

It was a personal shipment, the air pads were used and potentially not in pristine condition, but there were several brands of them, a manufacturing defect can be ruled out. It seems most of them burst, while at least one still had some air remaining and just had a leak that required a bit of pressure to release more air. Surprisingly there was very little air remaining, maybe 20% of the maximum volume.

The contents were light, I doubt they could have damaged a pad at all, let alone destroy them all. Could this phenomena be caused by a pressure difference between the air inside the packs and the cargo plane's hold? It seems a bad explanation considering the popularity of the packaging material.

I did check for needles and dead chicken. ;-)

Simple answer: yes, air travel can deflate or pop air pillows.

Longer answer: Cargo holds are at the same pressure as the rest of the airplane, it's part of the pressure vessel. Generally airplanes are pressured to about 8000ft, so the air is a lot thinner and therefore there will be a higher pressure differential between the air in the pads and the air in the hold. Thin plastic is somewhat porous and will leak some air, the rate this happens will be faster when there's a higher pressure differential. This differential is known to pop these air pillows to the point many will not use them when shipping by air. Styrofoam, cellulose pops, or crumpled paper are the preferred method.

You can see these forces at work next time you fly yourself by taking an empty disposable plastic water bottle on board. Seal it with the cap at airport altitude before departure. After takeoff as the airplane climbs the plastic will bulge as the cabin pressure drops, and when you uncap it you will probably hear a hiss. Seal it back up and then during descent the bottle will crumple as the air pressure increases.


Why don't aircraft fly over Tibet? If you open www.flightradar24.com or their app, then this is what the scene commonly looks like:

As you can see, the South-Central China area is strangely empty and the aircraft look to take a circular route around it instead (like this Emirates A380 to Incheon):

So, my question is why don't aircraft fly over that area? Also, I occasionally see some China Air Force aircraft going to that area..

While Him's answer is correct, part of the reason that there aren't many airways over central China is just that there's rarely a reason to fly there. Where possible, aircraft prefer to take paths along great arcs, which are the shortest distance between two points on the surface of a sphere (such as Earth.)

In short, there simply aren't many common air routes between city pairs where the shortest path passes over central China. If you look at the first image posted in the question, you will see that the flights into China are primarily flying either to or from Europe or Eastern North America (the ones crossing over Mongolia,) Japan or Western North America (the ones crossing over the Sea of Japan and East China Sea,) India (the ones flying over South China,) the Middle East (the ones flying over North China,) or Australia/Philippines/Thailand/Indonesia/Oceania/etc. (the ones coming in from the South.)

For your great arc path to pass over Central China, you'd need to be going to or from central China itself (which isn't terribly populated,) Siberia (which is even less populated,) the Central Indian Ocean (which is open ocean with Antarctica on the other side,) or the mountainous region between China and India (which also doesn't have a lot of air traffic.) So, there's normally just not much reason to fly there and, thus, not much reason for China to have airways there.

For the specific example that was shown in the question of the Emirates flight, the arc on which its flight path is shown across Northern China is, in fact, very close to the great arc between Dubai and Incheon, which looks like this:

Great Circle Path from DXB-ICN Source: gcmap.com

This map should give you something of an idea for how unpopulated Western China is:

China, as seen on Google Maps Source: Google Maps


How do I choose the vertical tail's airfoil? I'm designing a fixed wing UAV and I'm having some doubts about how to determinate the vertical tail airfoil. It's important to say that we've already sized this surface. And till this moment we're using the software XFLR5 for the analysis.

Normally, you would pick a symmetric airfoil. The thickness should be between 9% and 15% to find a good compromise between structural efficiency and maximum side force. Choose a thicker airfoil at the root, a thinner one at the tip and interpolate linearly between them.
Of course, for supersonic designs a thinner airfoil should be selected.
In some special cases a cambered vertical helped to improve flight characteristics, but this concerned mainly very powerful, single-engine propeller airplanes. I would expect your UAV is outside of this category.
Depending on the Reynolds number range of this vertical, a member of the Wortmann FX71-L series might be a good choice.


What button is this pilot pressing with his thumb? In this video (marker 4:23), what button is the pilot thumbing on the yoke?

It's the elevator trim control to relieve control forces in pitch.  It does this by changing the neutral position of the elevators so that the pilot flying does not need to maintain a constant pressure on the yoke to maintain pitch.  On the 747 family, this is achieved by moving the stabiliser.

Although this image shows a home cockpit yoke, the representation of the controls is accurate:


Could an airliner be as efficient lower and slower? Could an airliner ever be as efficient at 15 or so meters using ground effect as it is at 50,000 km even if it has to go slower and be shaped differently?

It's important to note that an airliner cruising at 50000 ft (I assume that's what you meant, and that's actually business aircraft territory; airliners are generally below the low 40s).  Is going pretty slow from an indicated airspeed perspective.  At 50000 ft an airplane is going roughly twice its indicated airspeed (it's actual speed through 3D space is twice the dynamic air pressure acting on it - if you stick your hand out, you'll feel the force of a 200 kts acting on your hand, even though you're actually going 400 kt).  So, such an airplane is actually optimized to only fly at 240 kts anyway, if it's cruising at a true airspeed of 480 kts.

What it means is, however you want to optimize for sea level flight, the higher you go the faster you can go in reality for a given fuel burn, whatever the configuration, burning far less fuel per unit of distance in the process.


Is precipitation static a concern for small drones? I read a bit about Precipitation Static buildup on the outside of aircraft flying through dust, rain, or snow.  Can this also happen on a small drone and if so, could it interfere with controlling it?

The FAA has a question on their drone certification test that addresses this and I'm curious if a small/medium size drone would be affected by this.

Static electricity buildup could affect communications. Static wicks on trailing edges of flaps, ailerons, rudder, elevator dissipate the static electricity.  I would imagine drones could be mostly fiberglass construction, so some additional details need to go into construction to provide a path for the electricity to flow, vs an all metal craft where the outer skin can be that conductor.


Do the elevators generate vortices? I want to know whether the elevators generate vortices like the wings do? If yes, then where do the vortices form?

Yes, they do. The elevator vortices I have observed are pinned to the trailing edge corners of the elevator and are made visible when the aircraft is traveling through moist air that is very close to the dew point temperature, and at a significant angle of attack. There's a hotel at LAX where you can look out the windows and watch planes fly by on final, just before touchdown; on a foggy afternoon they are all trailing vortices behind them.


When and how can pilots waive wake turbulence holds in the US? According to FAA rules, controllers are required to hold aircraft 2 or 3 minutes for wake turbulence under certain conditions. When is that wake turbulence hold required, and when are pilots allowed to waive it? What is the correct way to request a waiver?

The AIM 7-3-9 lists the required separations. Behind a heavy aircraft, the departure hold is 3 minutes and it may not be waived. Behind a small aircraft, the hold may be waived on pilot request only:

  3. Additionally, appropriate time or distance intervals are provided to departing aircraft:

  (a) Two minutes or the appropriate 4 or 5 mile radar separation when
  takeoff behind a heavy/B757 jet will be:

  (1) From the same threshold.

  (2) On a crossing runway and projected flight paths will cross.

  (3) From the threshold of a parallel runway when staggered ahead of
  that of the adjacent runway by less than 500 feet and when the runways
  are separated by less than 2,500 feet.

  NOTE- Controllers may not reduce or waive these intervals.

  b. A 3-minute interval will be provided when a small aircraft will
  takeoff:

  1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft,

  2. In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach.

  NOTE- This 3-minute interval may be waived upon specific pilot
  request.

  c. A 3-minute interval will be provided for all aircraft taking off
  when the operations are as described in subparagraph b1 and 2 above,
  the preceding aircraft is a heavy/B757 jet, and the operations are on
  either the same runway or parallel runways separated by less than
  2,500 feet. Controllers may not reduce or waive this interval.

EDIT: The AIM 4-3-10(f) (intersection takeoffs) also addresses this question, including suggested pilot phraseology for waiving the hold (emphasis mine):

  Controllers are required to separate small aircraft (12,500 pounds or
  less, maximum certificated takeoff weight) departing (same or opposite
  direction) from an intersection behind a large nonheavy aircraft on
  the same runway, by ensuring that at least a 3-minute interval exists
  between the time the preceding large aircraft has taken off and the
  succeeding small aircraft begins takeoff roll. To inform the pilot of
  the required 3-minute hold, the controller will state, “Hold for wake
  turbulence.” If after considering wake turbulence hazards, the pilot
  feels that a lesser time interval is appropriate, the pilot may
  request a waiver to the 3-minute interval. To initiate such a request,
  simply say “Request waiver to 3-minute interval,” or a similar
  statement. Controllers may then issue a takeoff clearance if other
  traffic permits, since the pilot has accepted the responsibility for
  wake turbulence separation.

Wake turbulence in general is considered a key safety issue by the FAA and there is a large amount of information in the AIM, the ATC guide and elsewhere.


What aircraft has a single constant orange light? I live under a fly path in Oldham near Manchester and I see planes passing by every day/night as they approach the airport. However today around 21.20 I have seen something different too.

A plane just passed above our house to start its approach to Manchester then when it was further away roughly above Ashton-under-Lyne I noticed a contrast single orange light passing above the house.

It was somewhat higher than the passenger planes around this area and it's travelled with constant great speed. I followed with my eyes until it's disappeared on the horizon above the peak district (took about 1.5-2 minutes). It's travelled in a direction about SSE (roughly towards London). I couldn't find anything on the Internet suggesting anything flying with constant orange light. BTW it had no sound whatsoever and it wasn't showing up on flight radar either as I checked that straight away. I do not drink and do not smoke funny things. Any idea what this could be?

From your description it sounds more like the International Space Station than an aircraft. That looks like a small orange or yellow light because it's the reflection of the sunlight For that reason you're most likely to see it in the hour or two after sunset, and it's clearly visible with the naked eye in good visibility. It's much higher than aircraft and takes about 90 seconds to travel horizon to horizon. Obviously you won't hear it or see anything on aircraft tracking sites.

That said, ymb1 suggested looking at this ISS tracker site. Assuming your 2120 was in British Summer Time (UTC+1), the ISS would have been somewhere over the South Pacific Ocean at that time, which would rule it out.


What is the most severe engine failure for modern turbofan? Today I read again something about the Southwest Airlines Flight 1380 and the related engine failure resulting in one passenger being sucked out of the window. As I understood it was a fan blade out incident and normally the engine are certified for it.
Are the engines certified for all failure possible ?
If not what would be the most sever engine failure possible on modern engines ?

Here are my initial finding so far, and I will add information that I find along the route:
There seems to be two types of engine failures. Tested and untested ones. The ones that the engine are certified for through test are the ones that are likely to happen in the fleet life. And therefore, the engine manufacturer must show that those kinds of failure will not put the life of the passenger at risk. The other are unexpected incident that shouldn't have happened and are certified through numerical analysis.
The most extreme scenario in the tested failure category seems to be the fan blade out. The unbalance created by losing a fan blade creates huge vibrations and breaking torque when the fan rubs against the casing. In addition to that, the compressor is stalled so you have backward flow in the engine and huge ball of fire coming out. (see this video).

To obtain the engine certification, the manufacturer must demonstrate during a dedicated test that all broken blades remain in the engine and that no heavy parts will exit either in direction of the aircraft or fall to the ground.
Sadly, it doesn’t work as expected and recently two CFM56-7B have had FBO event that didn’t end as well as planned.
Southwest Airlines Flight 3472 and 1380.

Now the untested failure options. Two different examples came during my search, rotor disc rupture and engine loss. Both should never happen but cannot be mitigated if they happen and that's why they cannot be certified.
Let’s start with the American Airlines Flight 191, which engine number one was lost during takeoff. Whereas it is not the engine that failed but the fixations, the consequences where tragic.
Secondly the rotor disk rupture. This time it is not the blades (fan turbine or compressor) that fell off but the whole disk retaining them that fails. This seems mostly due to manufacturing defect with crack propagation not detected during maintenance. The kinetic energy of those disk is such that it would require a tank shield to prevent them from escaping from the engine which weight would be horrendous. Sadly, they are more example of those:
Delta Air Lines Flight 1288 (MD-88, the NTSB determined the most probable cause of the accident was a fracture in the left engine's front compressor fan hub)

National Airlines Flight 27 and United Airlines Flight 232 (DC-10 Tail engine fan disk rupture leading to flight control losses, loss of cabin pressure and passenger sucked out)

Qantas Flight 32 (A380, turbine disk disintegrated, causing extensive damage to the nacelle, wing, fuel system, landing gear, flight controls, engine controls, and a fire in a fuel tank that self-extinguished)

Air France 88 (A380, Fan disk failure. The examination determined that the hub failure was caused by a low cycle fatigue cracking process which originated in the part’s subsurface. 21 month later a 150kg piece of the engine was located 4 meters below the snow and was recovered in Greenland.)


What are the disadvantages of fitting underwing flaps to a Mirage III/5 fighter jets to lower landing speeds? The Mirage III / 5 series of fighter aircraft are versatile aircraft, but has a high approach speed of 180 knots and a high landing speed, requiring long runways.

Underwing flaps, like those of the Gloster Javelin, would surely work, and the fact that they are close to the CG would not cause an acceptable pitching moment. The tailless swept wing Messerschmitt P.1111 also had flaps on its trailing edge.

Gloster Javelin in landing configuration

Tested on a small free flight glider, I found that the flaps on a delta resulted in a noticeable increase in lift and decrease in turning radius, the elevons were not overwhelmed by the downward moment of the flaps. Using trailing edge flaps on a the same delta wing model resulted in an unacceptable downward pitch that could not be counteracted by the remaining set of elevons.

I would think the system would work, however my question is what sort of performance effects: increase in weight, for example, and possibly drag, would result.

Later models of Mirage incorporated canards and other devices to reduce landing speed did Dassault Aviation overlooked this option or did they evaluate it and decide that it would not be worth it?

I would have incorporated these in any delta wing fighter I designed, at least to offer it as an option to customers like the tailor made canards were to be offered.

Every airplane is a compromise of conflicting demands. While I am not intimately familiar with the Mirage family, I can easily see why flaps were never included.
In the early Fifties when work on deltas started at Dassault, the most important design parameter was top speed. High landing speeds were accepted, and given the design alternatives to a delta (think F-104), the landing speed could be considered low and another plus on the side of deltas.
When the first Mirage III were ordered, they had to serve in four different missions: As an advanced trainer (Mirage III B), an interceptor (Mirage III C), attack aircraft (Mirage III D and E) and for reconnaissance (Mirage III R). In order to allow for the widest variation in external loads, the lower wing should be free of flaps or other equipment which would restrict the attachment of tanks, armament or reconnaissance pods. Mounting any flaps at the straight trailing edge would require an opposite deflection of the elevons, negating any lift gain.
Flaps on naturally stable tailless aircraft are only helping if they are attached at a point ahead of the pitch trim flaps. This is possible with swept wings (see the Horten IX glider with its small flaps below) or with split flaps at mid-chord, like on the Gloster Javelin. With a straight trailing edge, no lift increase with a naturally stable configuration will be possible since the additional pitch moment of those flaps must be compensated at the same lengthwise location by the pitch control surfaces. What is gained with the flaps must be destroyed for pitch trim.

Horten IX V1 in tow, flaps lowered (picture source)
Since the Mirage line was never considered for use with aircraft carriers, its landing speed was accepted as part of the overall package. The Gloster Javelin was designed as a pure interceptor which put fewer demands on its lower wing. Also, the Javelin's tail helps to trim the aircraft in the landing configuration.
From the US translation of "Aerodynamique de la Nouvelle Generation d'
Avions de Combat a Aile Delta", published in the AGARD Conference Proceedings No. 241:

Its [the Mirage III] aerodynamic formula, a delta wing swept back 60° at the leading edge, allowed Mach-2 flight with a single engine of 6 tonnes thrust, while its competitors were equipped with a 7-tonne J79.
The trade-off to this supersonic performance was a large increase
in the approach speed, 180 knots instead of 140 kt for the Mystere family. This constraint was accepted for the Mirage III, but later program specifications from the General Staff of the French Air Force required an approach speed below 150 kt.

So the Mirage III users had second thoughts, after all. The result of this lower approach speed requirement became the Mirage F-1.
Note that artificial stability allows to lower trailing edge flaps on a delta wing; the Mirage 2000 could be flown down to 100 kts airspeed.


How (often) are jet engines cleaned? In the on-board magazine of my today's flight, there was a report on how fuel efficient the airline flies. They accomplish this by new scitimar winglets,  daily optimized route planning, and regular cleaning of the jet engines.

I never heard about this cleaning. How and how often is it done?

It used to be done launching crushed walnut shells into the running engine and the blades are polished by the process.  Known as Coke Blasting.  The process is uncommon these days.

Similarly they use a walnut shelll coated emery cloth to polish crankshafts after machining

http://www.dennisdawson.com/jetblast.htm

It is still commonly done on industrial turbines apparently


why do all planes hold position when Air Force 1 is taxiing? Why does everyone hold in position when Air Force One is just taxiing? This can cause a delay that could cascade to other airports.

No doubt security has a lot to do with it. With only one vehicle supposed to move on the airport premises, everything else that moves can be easily spotted and classified as a potential threat, and resources dispatched to handle it.

If there are dozens of other aircraft and hundreds of cars and trucks moving around, this gets a lot harder.


What is the optimal way to climb fastest between two points on a Cessna 172N? Cessna 172N’s POH has Vx = 59 Knots.

Let’s assume you need to climb from 1000 msl on traffic pattern just after you finished turning cross-wind and are clocking 85 Knots, to 2000 as fast as possible, say to clear the noise restriction zone.

Is it better to let the plane get to some 100 Knots then pull back and climb or just start the climb right away?

As an example practicing full speed stall maneuver I have noticed I climb at a rate of 1500 feet/min while on Vy my rate is never more than 650 feet/min.

Ron nailed it. (See comment on OP.)
It depends on what you’re trying to achieve.
Climbing “fast” in terms of maximum vertical speed = Vy (best rate)
If the noise sensitive zone is immediately surrounding the airport, you want to use Vy to put altitude under you as quickly as possible.
Climbing “fast” in terms of maximum feet per nautical mile = Vx (best angle)
If the noise sensitive area is a few miles away, you may want to use Vx to get to altitude before overflying the zone.
I gather from the question that the noise sensitive area is right next to the airport.  If that’s the case then Vy is likely your best option.
While accelerating to 100 and then pulling up feels satisfying, just pitching for and holding Vy (or Vx) is more effective.  Peter’s answer explains why this is.
In any case, watch your oil temps (or cylinder head temperatures if you have a CHT gauge) and prioritize keeping the engine cool over noise abatement.  While being a good neighbor is important, taking care of the engine and flying safely is even more important.


Why do aircraft (namely military) need/use formation lights? Why are formation lights used,and why are they largely/always used only on military aircraft and not civilian?

Formation lights are used so that aircraft can fly close formation at night.  They are typically luminescent strips positioned so that an aircraft can be in the desired position (i.e. fingertip formation) and have references fore-aft, up-down, and near-far similar to references as used for daytime formation flying.  Something along the lines of, "keep the position light just below the formation light (strip) under the cockpit and the aft fuselage light (strip) just touching the wing hardpoint, and you're in the right position.  If the position light goes farther below the strip, you're getting high; if the hardpoint is sliding back into the aft light you're getting close, etc."

Obviously these references are very specific to each individual aircraft.

You can see the formation lights in this picture of an F-14; the dots are actual position lights, while the strips are the formation lights.  Source

As to why only military aircraft have them... how often have you seen or heard of civil aircraft flying formation at night?  Yeah, me neither.  There are very, very few non-military reasons for formation flight, and essentially none that require it to be done at night.


What UK aircraft looks like an Islander but with twin fins? Driving on the M40 recently just south of Banbury, England, I saw a distinctive smallish plane I'm trying to identify — probably military from the dull grey/green color.

It looked a lot like the Islander, but:

It had a tailplane and twin fins
The tail boom was narrower in proportion to the body, and more distinct from the body than the Islander's is (I think I'm misremembering or mis-saw that part — I was driving on the motorway, after all)
The body may well have been a bit bulkier (or that may have been an illusion caused by the narrowness of the tail boom)
The wings had a normal-looking span for the size of the aircraft but were shorter front-to-back than seemed normal (to my very-much-not-expert eye)

Like the Islander it was a twin prop with a fairly pointy nose.

Does anyone have any idea what it was?

It sounds an awful lot like a C-145, or maybe the 'civilian' version (it's primarily used by militaries), the  PZL Skytruck.

Source

It's used by the U.S. Air Force, so seeing it in Britain would not be surprising - all 3 of the U.S. Air Force's RAF joint use bases are within 100 air miles.

It's twin-engine, with a tailplane and twin fins
Its tail boom does narrow somewhat and distinguish itself, as seen here
It does have somewhat of a chunky body
The wings are fairly thin (front-to-back)


What is a red airway on the VFR sectional? The VFR sectionals show a number of airways/routes marked in red. Here are several near KMTH, you can see B-646 extending east of the NDB:

There are others in Alaska that are lettered A, R or G, e.g. A-7 runs between the CMQ and MNL NDBs.

What are these airways or routes and how are they different from Victor airways?

These are Red Federal Airways, and can be found in Order 7400.9

Red airways are one of a 1950s-ish system of airways: Red, Blue, Green, Amber, which are designated by an NDB (rather than a VOR or VORTAC for a V or J route).

Green and Red airways go east/west, and Blue and Amber routes go north/south.

These routes were prioritized: GARB, with Green routes having the highest priority.


Are experimental aircraft exempted from the 2 year transponder and avionics checks? Experimental aircraft and LAS. The 2 year verification test for avionics , transponder and altitude encoder.  Are experimental aircraft exempted from this test or they must comply ?

When you operate in airspace that is under FAA regulations, FAR 91 applies (unless you are flying military)

FAR 91.413 a) states:

  (a) No persons may use an ATC transponder that is specified in
  91.215(a), 121.345(c), or §135.143(c) of this chapter unless, within the preceding 24 calendar months, the ATC transponder has been tested
  and inspected and found to comply with appendix F of part 43 of this
  chapter;

So when you haven't performed the two year transponder check, you are not allowed to use the transponder. The way to fly then is to fly VFR in airspace where no transponder is required AND not to use the transponder at all.

This is not specific to experimental aircraft, there is no exemption.

There is good reason not to have an exemption for experimental aircraft; a malfunctioning transponder can cause many problems to the ATC radar systems.


Why does the Harrier jet have four landing gears? I was looking at pictures of Harrier jets and I noticed something.

They have four landing gears. That seems pretty unusual for a fighter jet - most have only 3 in a triangle formation.

What is the reason for this 4 point configuration? It seems to just add complexity and cost without really increasing stability.

The main load bearing landing gear is in a bicycle setup, with 2 outriggers for stability.

This allows the gear to be lighter, stay close to the center and out of the way for the jet blast.

The Lockheed U-2 also has its gear in the bicycle configuration and takes off and taxies with detachable "pogo" outriggers for stability.


What API can I use to get FAA NOTAMS or will this one I found work? I am not sure if this is the correct place to ask this because it has to do with APIs with XML/JSON/SOAP, but it also has to do with aviation.

I am in dire need to find an API associated with the FAA to get NOTAMs for areas and airports. NOTAMs are Notices to Airmen, and it will be a useful feature in a service I am creating.

I have done extensive searching for a NOTAMs API, but I cannot find anything except this:
https://notams.aim.faa.gov/notamWFS/

However, this website requires registration which is just a broken link. Any suggestions on how to use this?

Do you know of any other NOTAM APIs available?
Also, if possible, do you know of a weather API which has the same information included in an ATIS/AWOS reporting station?

I was able to find working APIs for US regulations and general weather (not specific enough) and pilot information, but I cannot find any for specific weather or NOTAMs.

EDIT: A friend of mine was able to click the registration button, and it didn't give him an error. He sent me the link and I am now trying to register on the site I found.

As @GdD mentioned in the comments, there is an API for NOTAMs here: https://www.icao.int/safety/iStars/Pages/notams.aspx

You also asked for metars, which you can get from here:
http://tgftp.nws.noaa.gov/data/observations/metar/cycles/${TIME}Z.TXT where ${TIME} is the hour of the Zulu time you are interest in, wrapped between 0 and 23 (eg. 16)


Throughout aviation history, what airships were metal clad? Throughout aviation history, what airships were built with metal skin instead of fabric or other material?

At least two all-metal airships are known to have existed, the first one invented by David Schwarz, and test-flown in Tempelhof, in 1897. It flew, but crashed when landing.

Another one, a really successful and better known aircraft, was an American airship, the ZMC-2, that first flew in 1929.

https://en.wikipedia.org/wiki/David_Schwarz_(aviation_inventor)

https://en.wikipedia.org/wiki/ZMC-2


What would be the ground roll and total distance to clear a 50ft obstacle given these conditions? I'm a student pilot, the question I am trying to answer is

  What would be the ground roll and total distance to clear a 50ft obstacle given an elevation of 5,000ft, altimeter 29.52, and a temperature of 25° Celsius, winds calm.

Disclaimer: I'm going to double check with my CFI, obviously, you should too if you are unsure.

So I understand how the chart works, I'm just unclear about the "Elevation 5,000ft. Altimeter at 29.52".

My inclination is to assume that the meaning is sea level pressure is 29.52, and therefore we should adjust our input for "PRESS ALT FT" to be the altitude that reflects that average pressure, which would be 500 feet.

We round up for safety, so assume PRESS ALT=1000' and TEMP=30° Celsius, we would have a ground roll of 890' and a takeoff distance of 1645', right?

Another question: Where do I find the official documentation for converting barometric pressure into the altitude that has that average pressure?

We round up for safety, so assume PRESS ALT=1000' and TEMP=30°
Celsius, we would have a ground roll of 890' and a takeoff distance of
1645', right?

Good thinking, but no.
Refer to the Pilot's Handbook of Aeronautical Knowledge, Chapter 10.  You want page 10-3 specifically.
When the altimeter setting is 29.92, the pressure altitude is the same as the field elevation.  When the altimeter setting changes, you must apply a conversion factor to the field elevation to get pressure altitude.
In this case, the altimeter setting is 29.52, which means that your conversion factor will be about 380 feet:

Therefore, the actual pressure altitude will be 5,380 feet.
If you round up for safety, the takeoff distances will be 1455 and 2855.

We can get a little more precise than that, though.
To calculate the ground roll at 25 celsius, we can interpolate between 20 and 30 degrees:
5,000 feet
$\left[ \left(1315 - 1215\right) \over 2 \right] + 1215 = 1265$ ground roll at 5000
$\left[ \left(2525 - 2320\right) \over 2 \right] + 2320 = 2422$ 50 ft obstacle at 5000
To correct for pressure altitude, we do the same thing at 6000 feet...
6,000 feet
$\left[ \left(1455 - 1345\right) \over 2 \right] + 1345 = 1400$ ground roll
$\left[ \left(2525 - 2320\right) \over 2 \right] + 2320 = 2732$ 50 ft obstacle
...and then interpolate again:
5,380 feet
$\left[ \left(1400 - 1265\right) * 0.380 \right] + 1265 = 1316$ ground roll
$\left[ \left(2732 - 2422\right) * 0.380 \right] + 2422 = 2540$ 50 ft obstacle


How does CDI scaling transition from terminal to approach sensitivity? I read in the AIM that WAAS vector modes will provide a linear course width of +/-1NM prior to the FAF and then provide a fixed width of +/-0.3NM to the point where an ILS splay would be 0.3NM and then tapers down to 700 ft total width at the runway threshold.

How smooth is the transition from the 1NM segment to the +/-0.3NM segment? Is it time-based, distance-based, or something else? Is it smooth at all, or is it actually an abrupt change?

Are all manufacturers following a common standard on how these scale transitions work?

Beginning 2NM prior to reaching the final approach fix of an activated approach, the GPS will smoothly taper down the CDI scale from 1-NM full-scale deflection (from the center) to 0.3-NM at full-deflection over that 2-NM flight distance and reach a scale of 0.3-NM as the final approach fix is reached.
AIM Page 1-1-25

"When within 2 NM of the Final Approach Waypoint (FAWP) with the
approach mode armed, the approach mode will switch to active, which
results in RAIM and CDI changing to approach sensitivity. Beginning 2
NM prior to the FAWP, the full scale CDI sensitivity will smoothly
change from ±1 NM to ±0.3 NM at the FAWP. As sensitivity changes from
±1 NM to ±0.3 NM approaching the FAWP, with the CDI not centered, the
corresponding increase in CDI displacement may give the impression
that the aircraft is moving further away from the intended course even
though it is on an acceptable intercept heading. Referencing the
digital track displacement information (cross track error), if it is
available in the approach mode, may help the pilot remain position
oriented in this situation. Being established on the final approach
course prior to the beginning of the sensitivity change at 2 NM will
help prevent problems in interpreting the CDI display during ramp
down. Therefore, requesting or accepting vectors which will cause the
aircraft to intercept the final approach course within 2 NM of the
FAWP is not recommended."


Detecting faults in CVR mic inputs Years ago my father worked for Eastern Airlines out of Miami (that should give you a timeframe) and he was an electrical engineer.  I recall once he told me that the pilots (one or more) used to intentionally damage the CVR microphone in the cockpit by twisting it until the contacts broke.  Their reasons are not relevant for this question.

My question is, knowing what model planes they flew back then, (727, Tristars, A300s, 757, etc) how did the mechanics know that the mic was not working?  Would the system show it as a fault or did the CVR tape review show that there was no data from that mic?  The former I'd think would allow them to track down the pilot(s) that were responsible for this act.

Also, on planes with more modern electronics, would this fault be detectable immediately or at least before the next flight?

Interesting question. On the Boeing 727 there is a panel named "Cockpit Voice Recorder Microphone Monitor". A headphone can be plugged into the labelled jack to test if the cockpit voice recorder microphone is working properly. I have not checked the other model aircraft you mentioned but I assume they have the same test capability.

These microphones are usually passive devices. I doubt there is a warning system even on modern airliners to tell you the CVR mike is not working. But I would also be willing to bet good money they are no longer so easy to disable.


Why do pilots refer to the Flight Service Stations as "Radio" in the the United States? When using an aircraft radio to call a Flight Service Station ("FSS"), why do pilots refer to them as "radio?" I understand calling control towers "tower", ground control "ground", Air Route Traffic Control Centers ("ARTCC") "Center", and so on, but why "radio" for a Flight Service Station? Why isn't it "Station?"

"Radio" is just a shortened version of "Aeradio Station" which was the term used before "Flight Service Station".


Axis of rotation of tail surfaces In cases where complete tail surface rotates (be it horizontal, vertical or at angle) what should be preferable axis of rotation of surface.

A "preferable" solution depends on many factors, both in aerodynamics and in design/construction. There is no single best solution for all situations.

On slow(er) subsonic aircraft with reversible control, the main concern is to provide convenient force on the yoke/stick. For symmetric airfoils, which are common for tails, there is a point at 0.25 chord where the hinge moment will be zero for all the operational range of deflections. To provide some positive force, you'll want to put the axis slightly ahead of this point.

However, there is another problem: stability with relaxed controls. If you move the axis too much forward, making the surface 'weathervane' with the stream, you reduce the control-free stability. This is an important consideration because when you are flying 'hands off' on a well-trimmed airplane, this is exactly the situation you are in. With classic hinged elevators, this effect is already quite noticeable, but with all-moving tails, it can be extreme, up to the total loss of stability. On the other hand, obviously, if you put the axis aft of this 25% chord, you'll have an outright unstable configuration.

So the ideal situation for such airplanes is to have the axis at 25% chord, but to create positive control force with springs or with an anti-servo tab, like on PA-28:

$\hskip 6cm$
Source

For supersonic aircraft, the situation is somewhat different.

First, their control is typically irreversible: the actuators take all the load. This means that the control-free stability problem doesn't exist; and the actuators can potentially control high (or even negative) hinge moments.

Still, you don't want to oversize the actuators unnecessarily. In addition, many (older) supersonic aircraft allowed for fully manual or assisted control in case of hydraulics failure, so the aircraft still had to be flyable by hand.

This again puts the best hinge location near the 25% chord point.

However, the second difference is the fact that at supersonic speeds, this magic 25% chord point (called aerodynamic centre) moves to 50%. On top of that, the relative efficiency of control (per degree of deflection) reduces, and this is actually one of the main reasons of using all-moving surfaces on supersonic aircraft. This, obviously, creates enormous hinge moments.

To reduce them, we could move the axis a bit back. This may create negative moment at subsonic speeds, but if we have a full-authority hydraulic control, this may be the optimal solution. Some 4th-generation fighters seem to follow it.

The third difference is that supersonic tails are often highly swept back. How do you align the axis then?

$\hskip 1cm$

Again, there is no best solution for all cases. Aerodynamically and kinematically, the straight axis (on the left) is a bit better: it creates direct linear control. Structurally, it depends on how the stabiliser and the tail section are designed.

If there is a possibility to use a common shaft for both halves of the horizontal tail (which is typically true for T- and cruciform tails), this is a enormous advantage and it naturally leads to the straight axis. On the other hand, it is usually easier to integrate the shaft (or hinge points) along the spar of the stabiliser, which favours the swept axis for most swept tails.


Thinking of changing careers! A little background: I am a senior software engineer with over 14 years of experience. I will turn 30 soon. I live in the UK but I'm neither a British citizen nor from EU member country. BSc in software engineering (abroad) and MSc in Advanced Computer Science (UK).

If I decide to pursue a career as a pilot, would I be eligible?

I'm not just asking this randomly, I genuinely want to know and seriously thinking of it. Aviation's been my dream for so long now I can't even remember the beginning of it. Not studying to become a pilot has been my biggest regret so far, and feels like this is one of those last moments I can do something about it. Not counting the satisfaction I can't get from my industry anymore - been doing this for some time now!

I hope this type of question is not against the rules of this community, but I don't know where else I can this. Also, if you were in my position, would you risk everything and do it?

Your previous education does not matter that much, it's way more about your suitability for the extremely demanding job of professional pilot.

You can enter a range of differing training paths that lead, should you successfully complete the hard training, to an atp licence.

The ones that offer some kind of promise of getting hired, have rigorous selection processes.

Any training towards professional pilot license is ridiculously expensive, you'll have to consider that. It'll be years before it pays back financially.

Only you can be the judge of whether or not to pursue this, but a word of advice, if I may: it's a job, just like any else. It's really about how well you fit the job and vice versa. If you must become a pilot, you will.


What is the ground idle fuel flow of an A320 (CFM or IAE or neo)? Standing on ground, what is the idle fuel flow of an A320? ISA conditions, both engines running?

I don’t normally post answers that I cannot personally verify, but I have a Facebook friend who is FO in an A320. I sent him a message and asked your question. I don’t have any official documentation to reference, so take this as anecdotal...
An A320 with CFM engines burns about 600lb/hr per engine at idle in ISA conditions.
An A320 with V2500 engines burns about 730lb/hr per engine at idle in ISA conditions.
If the APU is running, it will burn another 300lb/hr.


Can a 737 yaw while parked due to wind? How much wind force is required to push/rotate a 737 sideways while parked?

From own experience some years ago an empty B737-300 with ~2500kg fuel in the tanks can be moved by crosswinds as low as 55kt. Surface was a regular dry concrete apron with no slope. That's why my companys GOM (ground operation manual) required ballasting the aircraft with max fuel in high winds and chocks on the main wheels both sides.


What is the force to change the pitch of a variable pitch propeller? At the moment I am struggling with the problem that I do have an idea how to calculate this force by using the blade element theory and going over a whole propeller blade in order to calculate the torque inside the blade. I need this torque in order to calculate the force necessary for changing my pitch inflight. Is the blade element theory the correct way to calculate this?

Source: Nelson, Wilbur C.: Airplane Propeller Principles (John Wiley & Sons, 1944).

Also I do lack information regarding the airfoil data. Is there a good estimate on how high the coefficient of torque will be? Is there a good airfoil database for propellers online?

The assumption that cm=-1/4*ca does not work, as profiles on propellers are not even plates.

In a well-designed propeller the hinge axis is very close to the line connecting the centers of pressure, so the aerodynamic pitch moment is rather small. The main force is friction; note that the centrifugal pull of the blades at the hub will put a considerable load on the blade bearings, so friction goes up with rotation speed. Regular propellers have plain bearings; if you know the mass of the blade, the rotation speed and the coefficient of friction, you can calculate the breakaway torque of one blade.

Since propeller blades have some (but not much) camber, the center of pressure moves forward with increasing angle of attack. This will drive the blade away from a medium angle of attack; in other words, the aerodynamic moment is destabilizing the propeller pitch.


What is the legal altitude to fly through Yosemite Valley (US)? I have a question regarding flying in the Yosemite Valley national park, which I flew over last weekend in an open-cockpit gyroplane.

The sectional states you must be a minimum of 2,000 ft about the highest point within 2,000 feet of you. The famous valley itself, containing El Capitan and Half Dome, has a highest point of 8,800ft or so, so when I flew it, I was at well over 10,000 ft.

However, it occurred to me afterwards, that if the valley is actually more than 4,000 ft wide (which it is), then all I needed to do was to be sure I was more than 2,000 ft above the valley floor, which is at 4,000ft.

So instead of flying over it at 10,000, I could do it at 6,000ft if I'm in the middle of the valley.

This would put me way below the peaks of El Cap and Half Dome, but provided I'm more than 2,000ft away from them, I'd be legal.

Is that correct?

Short answer: you can fly in Yosemite Valley at 500ft AGL (most likely); there's no special regulation applicable there; the wording on the sectional is a non-regulatory 'encouragement' only.

By regulation, the minimum altitudes under VFR are in 14 CFR 19.119:

  (a) Anywhere. An altitude allowing, if a power unit fails, an
  emergency landing without undue hazard to persons or property on the
  surface.

  (b) Over congested areas. Over any congested area of a city, town,
  or settlement, or over any open air assembly of persons, an altitude
  of 1,000 feet above the highest obstacle within a horizontal radius of
  2,000 feet of the aircraft.

  (c) Over other than congested areas. An altitude of 500 feet above
  the surface, except over open water or sparsely populated areas. In
  those cases, the aircraft may not be operated closer than 500 feet to
  any person, vessel, vehicle, or structure.

I don't know what Yosemite Valley looks like and whether it's "congested" or not (see this question), but it's very likely that you could operate legally at 500ft AGL there, if not lower.

What the sectional says comes from AC 91-36, Visual Flight Rules (VFR) Flight Near Noise-Sensitive Areas. But as the AC itself says, it only encourages pilots to fly above 2000ft AGL (emphasis mine):

  This Advisory Circular (AC) encourages pilots making VFR flights near
  noise sensitive areas to fly at altitudes higher than the minimum
  permitted by regulation

If you read the whole thing, you can see the not-so-hidden message:

  The Federal Aviation Administration (FAA) receives complaints
  concerning low flying aircraft over noise sensitive areas such as
  National Parks, National Wildlife Refuges, Waterfowl Production Areas
  and Wilderness Areas. Congress addressed aircraft flights over Grand
  Canyon National Park in
  Public Law 100-91 and commercial air tour operations over other units
  of the National Park System (and tribal lands within or abutting such
  units) in the National Parks Air Tour Management Act of 2000.

  [...]

  Adherence to these practices is a practical indication of pilot
  concern for the environment, which will build support for aviation and
  alleviate the need for any additional statutory or regulatory actions.

In other words, 'please be responsible and considerate because if you aren't, Congress might force us to introduce more regulations like we had to do with the Grand Canyon'. Practically speaking, I guess that if you fly down Yosemite Valley at 500ft AGL every day, even if it's completely legal you can still expect a call from the FAA asking why you're doing it and 'encouraging' you ("cough 91.13 cough") to fly higher for 'safety' reasons.


Do jet engine exhausts usually glow orange? In these two lovely images:
Source: Wikimedia Commons
Source: wvi.com
The exhaust end of the engines are glowing bright orange from being so hot. Do they get that hot during normal flight operations, or are they hot because the engines are statically mounted for testing and have (relatively) minimal air flow to cool it?
If it's normally this hot, what is the exhaust made of, and how is it insulated to keep it from melting the nacelle around it?

Both images you posted are of Pratt&Whitney J58 afterburning turbojet that was used on the Lockheed A-12 and Lockheed SR-71 aircraft. That engine ran particularly hot.

The engine was designed for operation at Mach 3.2. The efficiency of turbojet increases with speed, but only to a certain point and somewhere above Mach 2 it starts to decline. However efficiency of afterburner continues to increase, so this engine was designed to operate continuously with afterburner and to generate higher fraction of power in the afterburner than other engines.

The afterburner combustor liner was able to withstand temperature up to 1,760°C (3,800°F). The core of the flame was much hotter still, but a colder air was fed along the walls, like in all turbine engines, as explained here. The engine was also cooled by incoming fuel similarly to how rocket engines are.

Additionally, at the top speed, the compression heating as the air was slowed to subsonic speed in the inlet heated it to over 400°C (800°F) at the compressor face already. Since in the static test the air was not that hot, the engine might have actually ran a bit colder in the test than in normal operation.

This was however specific to this one engine. The compression heating is negligible at subsonic speeds and grows faster at higher Mach numbers. Most supersonic aircraft fly around Mach 2, where the effect is already noticeable, but not nearly as pronounced. For most engines, they run at roughly the same temperature in test as they do in normal operation. It just isn't as high as in this special case.


What are the scale effects of GA wing tip airfoils? [ update: I am adopting an improved description provided by Peter Kämpf: "...only looking at the airfoils in isolation..." ]

The goal of this question is to determine if wing tip airfoil scale plays a significant part in stall characteristics for aileron control.

Lets assume we have a small Piper or Cessna aircraft with only a change in size from root to a tapered wing tip. There is no washin or washout at the tip. Lets further assume the wing tip has 50% chord to the root (if some other value is useful, feel free to use what is appropriate). If Reynolds number has any appreciable bearing on the result, lets use approx 50kt/mph as a ruff stall speed.

Q1) Will the wing tip stall at the same speed as the root (root vs tip), or is there an appreciable difference do to size & scale?

Q2) What would the stall characteristics be (root vs tip) if we scaled the tip chord 50% but did not change the thickness?

Illustration for reference, values are not exact.

If you only look at the airfoils in isolation, the shorter tip airfoil will stall at a slightly lower angle of attack and lift coefficient (I use angle of attack instead of speed, as both should be equivalent). It has only half the Reynolds number of the root airfoil and, therefore, viscosity forces are twice as big relative to inertial forces. This is made worse by the ailerons on the outer wing: A downward deflection will strain the boundary layer more, moving stall to even lower angles of attack.

The exact difference depends on the airfoil. Below I pasted Figure 18 from chapter 4 of Sighard Hoerner's book Fluid Dynamic Drag. Your wing has a chord of 6 ft at the root and 3 ft at the tip, corresponding to a Reynolds number of 3.25 Million at the root and half as much at the tip at 50 kts speed. Note the logarithmic scale of the X axis below: The 2 million point is halfway between the 1 million and the 10 million point.

But there is more: With a 50% tapered and untwisted wing the highest loading is at maybe 60% of span - that is where the full wing will stall first. Next, you have to assume a fuselage at the root. Depending on the local pressure distribution, the root might even begin to stall earlier. You sure know comparisons like the one below for the lift distribution and stall behavior of different planforms (source). Yours is case C (moderate taper):

My recommendation is clearly against the proposed planform. Combined with aileron loads, it will be very prone to tip stalls. Given the "right" airfoil, it will show positive roll damping, that is, it will roll all by itself once the stall starts on one side. There are several steps to make the wing more benign:

Use a thinner airfoil (10%-12%) at the tip than at the root (here 15%-17% are a good choice). Why that helps is shown in Figure 23 below, again taken from chapter 4 of Sighard Hoerner's book Fluid Dynamic Drag.
Use less taper. 70% tip chord relative to root chord gives the best value for induced drag and leaves more margin against tip stalls.
Use an airfoil with benign stall characteristics. Definitely avoid 5-digit NACA airfoils.
Use at least 2° of washout. That sounds little, but has a profound impact on stall behavior.

  Q2) What would the stall characteristics be (root vs tip) if we scaled the tip chord 50% but did not change the thickness?

You mean the same absolute thickness, so the tip airfoil has twice the relative thickness than the root airfoil? This would guarantee very poor stall behavior and is clearly a poor layout. While the root will benefit from a thicker airfoil (lighter spar, more internal space for landing gear and fuel), the tip enjoys no such benefit. Fuel near the tips is more often a liability (roll inertia, tank internal pressure in spins). Manufacturing a spar with a single taper is only marginally more complicated than a spar of constant thickness. The flanges need to be tapered in both cases - this creates the most work.

In order to have the highest margin for aileron deflection, the recommended tip thickness is 10% - 12%, not more. Many designs even used a 9% tip airfoil successfully. Also, the use of differential aileron deflection is highly recommended. Let the trailing-edge-down movement of the aileron be only half as much than the trailing-edge-up movement. This will improve stall resistance and help to reduce aileron forces, especially at high angle of attack.

My favorite example for a poor wing layout is the Dornier Seastar. It was designed more than 40 years ago by a bunch of old Dornier engineers who basically repeated what they had done on the Dornier 17 another 40 years earlier. While the first prototype inherited a rectangular wing from the Do-28, the "new" wing design used a NACA 23015, no washout and had a taper ratio of 0.7. During an early test flight, the airplane flipped on its back during low speed trials. Fortunately, this happened with enough distance to the ground so this upset could quickly be corrected, but it necessitated the addition of a drooped nose on the outer wing.

Now look at the Beech Bonanza: Same planform and very similar root airfoil (NACA23016.5), but a NACA 23012 at the tip and 3° of washout. And much better low speed characteristics. Usage of a NACA 6-digit airfoil would improve those further, see the Piper Cherokee for example.


Can anybody confirm which aircraft is in this picture? Image Source

Does anyone know what airplane is shown in this picture? It looks like the Republic F-84, but I'm not sure because the front landing gear is under the nose on the F-84 and this one is further back.

Based on what I find from the marking on the plane of "8-MV", it appears to be a Dassault Mystère IV A. The image on the non-Wikipedia page looks identical to the one in your image (I can't put that image here because it is copy-protected).

Image Source


Can the load factor during glider winch launch be measured with the G-meter? I am looking at the flight envelope of a glider, the velocity - load factor diagram. I read that generally the air-frame, the G-meter and the pilot 'experience' the load factor as a G force (an acceleration).
My question is whether that is always correct. I think of a winch launch and believe that in this phase, because of the cable, the the G-meter and the pilot do not 'experience' the load that the air-frame has to deal with. Or in other words, there is load on the air-frame which is not due to acceleration. But I do not clearly understand the physics of the launch phase.
The question is important to me because if my believe is correct then during launch the pilot cannot judge the stress on the air-frame by the G-force he/she feels.

You are absolutely correct.
Since a string can only transmit tension forces (and no shear or compression force), the direction of the tow cable near the glider directly indicates the direction of its force. In the late phase of a winch launch the tow cable is pulling mostly downwards and has only a small forward component left. As long as the ratio between forward and downward component is still larger than the gliders lift-to-drag ratio, staying in the launch still adds energy to the glider, so a glider launch ends only when the cable shows an almost vertical direction.
All tow cables have a weak link at their glider end which limits the loads than can be transmitted. The picture below (source) shows a collection of color coded weak links with different limit loads (note the different center hole sizes), the strongest one being 10,000 N, mostly used for two-seaters.

The picture below (source) shows how the weak link is integrated: It sits in a U-shaped sleeve and is connected to the rest of the cable by shackles. On the right you see the rings which engage with the hook on the glider. In this particular case two weak links are used, and one has oblong holes at both ends so it can act as a fallback if the first snaps from some sudden jerk. Most tow cables have several of those weak links at their end and the appropriate one is selected depending on the glider type.

As you say, the load on the tow cable must be added to the weight of the glider to determine the lift force the wing has to produce in the late phase of a winch launch, which can easily be twice the weight of the glider for a total of 3 g. All while a g meter in the cockpit will read only less than 1 g (less because the glider is still climbing, with a nose-up pitch attitude). The pilot can only guess the load on the airframe, judging from indicators such as the wind noise, the stick force or the airspeed instrument. Another indication would be the sudden jump the glider performs when the cable is disconnected when still at high load. That happening, however, depends on the style of the winch operator.
When sitting in the winch on the ground, this load can be felt: During the launch the whole winch (normally a truck with a second engine and spools mounted at its back) will be lifted up a bit and drops back into its suspension when the tension on the cable is released. Still, this load is much less than the safe load the glider has been certified for which is 5.3 g for most modern gliders (see CS-22.337).
While the cable load is not dangerous, a more critical moment can occur during lift-off:
At the initial phase of a winch launch the force on the cable, acting in horizontal direction and at the bottom of the fuselage, creates a mostly horizontal acceleration and, in addition, a pitch-up moment which can stall the glider if not counteracted by elevator lift. Especially gliders with a fully-flying horizontal tail suffered crashes when their tail was still stalled during lift-off due to an excessive angle of attack on the tail. Therefore, it is important to keep the stick neutral and only push it forward when the glider raises its nose too quickly. The worst is to pull too early: A low-level stall will follow with the risk of an unsymmetrical wing stall, followed by a half roll and violent ground contact soon after.
If you are interested in the physics of winch launching: In 2008 a thesis has simulated the launch of an ASK-21 and describes what happens in detail.


Why is Landing Altitude displayed in amber on B777? When I set the landing altitude manually, an amber LDG ALT and Landing Altitude EICAS message is displayed. There are no failures nor has the landing altitude been set incorrectly.

Why do I still get the EICAS message ALT LDG in amber?

The Boeing 777 is a very automated aircraft.  The takeoff altitude, landing altitude, and cabin pressurization altitude are all automatically set when you enter the departure airport and destination airport in the Flight Management Computer.

The EICAS message ALT LDG appears when the FMC is not controlling the landing altitude. This can be caused by one of three different conditions:

1) There is no destination airport entered in the FMC

2) The FMC landing altitude function has failed

3) The LDG ALT knob has been used to set the landing altitude manually

It is also normal to see ALT LDG after landing when the route has been dumped.


Is it ever permissible to takeoff on an aircraft heavier than its MTOW? I'm thinking in terms of the Concorde accident in 2000 having just read part of the report which states that they were in effect several tonnes heavier than they should of been factoring in the tailwind and tyre speed restriction imposed for that reason. The crew took 3 tonnes of taxi fuel of which they only burned 800kg and that put them at least a tonne above MTOW not taking the tailwind into consideration which the report states would have been negligible. So I was wondering, is there any margin for takeoffs with weights marginally higher than MTOW? (obviously in this case they were ridiculously heavy amongst a whole host of other poor decisions taken by the crew)

The crew took 3 tonnes of taxi fuel of which they only burned 800kg and that put them at least a tonne above MTOW not taking the tailwind into consideration which the report states would have been negligible.

I can't speak to operations in today's environment, but at the two 747 carriers I worked for in the late 1980s and the 1990s, it was quite common to load more taxi fuel that you knew you would actually burn prior to takeoff. This was especially true for the 747-100, which had only a four bay center tank as opposed to the -200 which had a five bay center tank. Justification was easy. For example, if you were taxiing out at JFK to a trip to Tel Aviv, you well might burn 4000 lbs if you got caught in a line up. However, with any luck you'd get to the runway without delay, and you'd only burn 2000 (maybe even a little less), and you'd have the extra fuel for later. You wouldn't have the whole 2000 extra, because it would take about 20% of that to carry it.

Why was the extra fuel desirable? Well, in general as they say, the only time you have too much fuel is when you're on fire. Seriously, though, an extra 1500 to 2000 pounds could make the different between having to divert to Athens for fuel or going on to Tel Aviv.

  So I was wondering, is there any margin for takeoffs with weights marginally higher than MTOW?

There is a practical margin but it's a judgement call as to how much it really is. If on the 747 all four engines stay running, the airplane will march right out, albeit with reduced performance, even if seriously overweight, but you've compromised the airplane's ability to stay in the air if an engine fails, especially if the failure is catastrophic.

Sometimes you're overloaded but don't know it. Either accidentally or on purpose the cargo weight you're carrying is misreported. The worst case that I experienced was taking a full cruise line crew and equipment from Miami to Trieste, where they would board a brand new cruise liner and put it in service.

We were at max gross and had enough fuel for that weight to reach Trieste. Once airborne at at cruise, we realized we were burning more fuel that we should. Using our mach number, altitude, and fuel flow we worked backward to find our weight and discovered we were somewhere between 30 and 35 thousand lbs heavier than we should have been. Our dispatch confirmed our calculations when we contacted them, and we made a fuel stop in Paris.

If you're flying freight, and pallets of uncovered dry goods are left out in the rain, they weight a lot more than when they were dry. That happened at Madras, India (now called Chennai) and that put us well over gross.

In a perfect world, neither experience should have happened. Welcome to the real world, at least as it existed in the 1990s.,


How reliable are stick shakers? I've read many accident reports, and seen reconstructions, where instrument failures, disagreeing sensor sources and auto pilot faults have led pilots into believing that they are over speeding when they are not (think AF447), are descending when they are actually climbing etc.

It seems that in all cases where this has led to a stall, the stick shaker has activated but the PF does not take the correct immediate action for stalls, instead continuing to believe the erroneous information they are being fed.

I understand the psychology of why this might be but is does strike me that many lives would have been saved if power up nose down had been applied.  I've not found a case yet where the shaker was wrong. This leads me to wonder...

How reliable are stick shakers?  Should they always be trusted?

Some older types even had an automatic stick pusher but this does not seem to be present in modern designs.  Is this a consequence of shakers not being infallible?

The Australian Transport Safety Bureau recently published a comprehensive research paper on reported stall warnings in high capacity aircraft - http://www.atsb.gov.au/publications/2012/ar-2012-172.aspx

The report says that 30% of all stall warnings were false. Note that stall warnings aren't limited to the stick shaker.

The report says that of all reported stick shaker events, 16% were false warnings. Have a read of it, you'll find lots of interesting bits of information.


What are the Takeoff Minimums for aircraft operating under Part 91? What are the IFR Takeoff Minimums for aircraft operating under Part 91?

According to the Instrument Procedures Handbook (FAA-H-8083-16A) page 1-8:

  "Aircraft operating under 14 CFR Part 91 are not required to comply
  with established takeoff minimums. Legally, a zero/zero departure may
  be made, but it is never advisable."


What is the purpose of the extra wheel on the XB-70 main gear? This photo of the XB-70 Valkyrie's main gear shows a small wheel between the two main wheels:

Courtsey of AirPigz.com

This smaller wheel doesn't touch the ground, though I can see where it might as the side walls of the main tires compress under landing weight.

The AirPigz site promised an explanation, but none has been forthcoming (in the 4+ years since that was posted), and I couldn't find any information on it on Wikipedia, either. Does anyone have any information (real or speculative) on what it's for?

It is a sensing wheel that is part of the antiskid braking system. It contains an electronic sensor to measure the true groundspeed of the airplane. See NASA Technical Note D-4836, Landing Loads and Accelerations of the XB-70-1 Airplane (1968), bottom of page 3 of the document (page 5 on the PDF):


List of aircraft which are not certified to be flown by one pilot Is there a list somewhere, where it says which aircraft are not single pilot certified?

My question is motivated by this question:

Fine for operating not single-pilot certified jet with one pilot

I dont know of any list off hand but the FAA publishes all their type certificates here which do contain the relevant information needed to compile such a list. As mentioned in the comments most small GA planes are single pilot certified so if we start with that assumption you will find a lot of lists that cover the topic of "the largest single pilot certified aircraft" like this one. These lists provide you with the ones that are on the fence size/speed wise.


How often do people fly SVFR? I was going over Special Visual Flight Rules (SVFR) with my instructor recently and he told me that he has only requested it a few times, and that it is mainly used if you are flying from deteriorating weather into improving weather. In other words, you are sure that where you are going is clear. I got thinking about it and felt that if there were ever an SVFR situation, I would most likely ground the flight (until I get my instrument rating that is).

That being said, how often do pilots request SVFR?

It seems like it would cause people to end up in situations that they can't handle i.e. weather deteriorates more instead of getting better, does this happen often?

It has been mentioned that SVFR rules vary from country to country, I am asking about peoples experiences in the USA but am happy to hear of other similar situations around the world as well.

From my experience in the US at the facilities I've worked, and the discussions I've had with other controllers, it depends heavily on the area's weather patterns. The facilities I've worked, it's been usually a tool, when people are trying to get in right before the weather hits, or they're seeing good improvement in neighboring airports and at the main one, and know that it'll be good enough to make it between the two areas.

Other places, like coastal or areas where fog banks are common, SVFR can be useful, esp if only half the field is covered in clouds, and you need to make it in or can easily get out.

Personally, I'd probably just file IFR and cancel when I got to a good spot VFR, if I just wanted to go out and enjoy, versus just flying to a destination.

Also, note, SVFR in terms of separation is only one SVFR aircraft(and really aircraft period) in the airspace at a time. So, you'll be delayed to come in if there's IFR, or other SVFR aircraft ahead of you.

Helicopters generally do request it more than fixed wing aircraft in my experience. Also, realize a pilot has to explicitly request SVFR, a controller can't prompt or hint at the correct terminology to use to request like saying "Do you have a SPECIAL request"


What does it mean for a wing's lift distribution to be elliptical when the planform is not? How would you go about finding an equation for the lift coefficient of this wing where the lift distribution is known to be elliptical but the planform is not?

The (local) lift depends on many factors - not only the chord but also the profile, angle of incidence - and even up to the actual local flow. Without knowing the details of the actual wing, the distribution of lift can be hard to calculate.


Is there a limit to how many hours a person can fly per month? I'm trying to figure out whether there's a maximum per person.  Of course factoring in eight hours of sleep and say maintenance time there is definitely a limit, but I don't see where it is.

With respect to air carrier operations in the US, 14 CFR 121.481 requires:

  (e) No pilot may fly as a member of a crew more than 100 hours during any one calendar month.

  (f) No pilot may fly as a member of a crew more than 1,000 hours during any 12-calendar-month period.

14 CFR 135.265 commuter:

  (1) 1,200 hours in any calendar year.

  (2) 120 hours in any calendar month.

Under Part 91, fractional ops, 14 CFR 91.1059 states:

  (a) No program manager may assign any flight crewmember, and no flight crewmember may accept an assignment, for flight time as a member of a one- or two-pilot crew if that crewmember's total flight time in all commercial flying will exceed -

  (1) 500 hours in any calendar quarter;

  (2) 800 hours in any two consecutive calendar quarters;

  (3) 1,400 hours in any calendar year.

Otherwise, there is no direct limit under Part 91. However, if you are fatigued enough to cause a problem, FAA can still charge you under general "unsafe operation" rules.


How can I get the ADS-B GNSS Antenna Offset and Position Offset? I'm reading the FAA's Advisory Circular AC No:20-165 appendix 1 point 28, which talks about GNSS Antenna Offset and Position Offset.

How would I receive this information from an aircraft? Is it transmitted?

The fields are transmitted when the aircraft has ADS-B protocol version 2 (assuming you are looking at 1090MHz) and is on the ground.

They are specifically added to the ADS-B MOPS (minimum operational performance standards) to allow for accurately depicting aircraft on the airport surface.

I don't know by heart which message type it is in, which bits are used and what the encoding is.

Best is to obtain a copy of the MOPS; RTCA DO260B or EUROCAE ED102A (they are essentially the same, get the cheapest).


Why is airspeed different in climbs and descents although the AoA doesn't change? Here are the background facts:
This excercise has the objective to familiarise the student with the slow flight characteristics.
The steps are:
1)Establish straight and level, 90% RPM, 15 units AoA, hands out trimmed. (No specific airspeed, it just happens to be appr. 110 KIAS)

Add 5% RPM (so i set it at 95%) and establish a climb with 15 units AoA for 500 ft climb. No bank, whatsoever. (Again no specific airspeed, it just happens to be appr. 100-105 KIAS)

Reduce 5% RPM so you fly straight and level at the new altitude (Same parameters as step 1)

Reduce 5 % RPM (so i set it at 85%) and establish a descent with 15 units AoA for return to the initial altitude. No bank whatsoever. (No specific aispeed during the descent, it just happens to be appr. 115-120 KIAS).

My question is simple. Why is the airspeed different? The answer might be simple. I simply can't figure it out.
Hope i clarified it as much as possible.

You should see the same airspeed for the same AOA and non-accelerated flight with a mild climb or descent.

Except with a Buckeye?

The Buckeye has its thrustline low enough so that at different power settings you have different pitch trim settings. Add power to climb and the thrust pitches the nose up. You now have to trim nose down. Nose down trim means less down force on the tail, which means less lift required by the wing, which means a slower speed for the same AOA.

Conversely, pull power off and the nose wants to drop, leading to nose up trim, more down force on the tail, more lift required by the wing, higher IAS for the same AOA.

I frankly never noticed this effect, and never taught it. Wish I could go back and check it out!


Does this type of stabilizer exist? If they do then what aircraft are they installed in? This stabilizer is not a T-tail shaped stabilizer.

As you can see there are 2 v-shaped vertical stabilizers with a horizontal stabilizer hanging on the each of the vertical stabilizers. So different from a V tailed. And what advantages would this design have over a t-tail for example?

I don't know if such a structure actually exists, but I find it difficult to see why anybody would build one.

Firstly, placing the horizontal stabilisers at the top of two separate tail fins will require that each fin is strengthened to support the extra weight and the aerodynamic loads,

Secondly, there will probably be any number of aerodynamic issues around the joints between the components, each of which will be doubled.

Thirdly, in a real aeroplane you will have introduced substantial complexity into the control systems that have to manage all of this.

There may be other issues, but I see very little benefit aerodynamically or structurally to such a layout.

A conventional V-tail involves some control complexity, but does away with the weight of the vertical stabiliser. A T-tail retains the vertical stabiliser, but avoids control complexity, aerodynamic issues that may occur close to the engines, and can use the extra length to improve the leverage available to the horizontal stabiliser.

All that said, aircraft design is about compromise and utility. There may exist some powerful reason for adopting a sub-optimal layout to fulfil some specific purpose. You can see such an example in the design of the AN-225 Mriya which was originally built to transport the Soviet Buran spacecraft. The vertical stabilisers were moved to the ends of the horizontal stabilisers to keep the clear of the wake from the spacecraft riding piggy back. The 747 used to transport the US Space Shuttle had a similar modification, but retained the original vertical stabiliser.

Soviet Buran spacecraft during transport:

US Space Shuttle Atlantis during transport (image source: Wikipedia/NASA):