Where is the spanwise flow? How does the span wise flow point the air towards the wingtip? If you search Google for span wise flow, the first result under the pictures tab is a image of a wing with the span wise flow at the trailing edge:

(Source)

If you look at some other photos you notice others pictures that show to span wise flow at the leading edge:

(Source)

Is the flow at the leading or trailing edge?

At first I though the flow must be at the trailing edge because this would cause the airflow to point more and more towards the tip of the wing. After a little thought I realized that the air(span wise flow) should be flowing along the leading edge because that is where the air splits up in the first place.

So basically I am asking if the span wise flow is flowing along the trailing or leading edge of the wing.

How would this air cause the airflow to gradually point towards the wingtip?

The two pictures in the question only show how to decompose the speed vector into the normal and parallel components. They are nowhere near "real" flow vectors.
First I recommend to read this answer so you get an idea how air is accelerated and decelerated when flowing around an airfoil. Note that the low pressure area over the wing will suck in more air. This answer might also be helpful because it looks closer at the boundary layer.
With a swept wing the same happens, but now the flow will not stay at one wing section but show an added sideways component. Again, air is sucked into the low pressure area, but because that area is a little to the side of the direction of motion of the wing, the flow lines will initially move towards the center of the wing on the upper side in case of sweepback.
But that cannot last: The pressure recovery over the rear part of the airfoil reduces this bending, so the flow lines become almost parallel to the direction of motion of the wing. The lower the local pressure, the more the flow lines bend inward. Higher than ambient pressure at the trailing edge or on the lower side of the wing will bend the flow lines outward accordingly.

Let the red color denote suction: It bends the flow lines (black) towards it, creating a spanwise flow component. The higher the suction (the redder the color), the bigger the bending. The cyan arrows show the flow lines inside the boundary layer. By losing some of the early acceleration to friction but being subject to the same deceleration in the region of the pressure rise, the boundary layer flow shows a stronger tendency towards the tip. To be precise: The acceleration and deceleration acts in a direction perpendicular to lines of the local relative chord. The sketch above exaggerates this bending for the sake of clarity.
Now we must look at the boundary layer to get the full picture: Here the flow speed is reduced by friction, so the result of that initial inbound acceleration is worn away gradually. The result is a twist of the local speed vectors over the thickness of the boundary layer such that the inbound component is gradually reduced the closer you get to the wing's skin. Now the deceleration in the area of pressure recovery will slow down the already decelerated air close to the wing, and again that slowing will happen in a direction perpendicular to the lines of equal chord. If this deceleration is sufficient, it will leave the boundary layer flow with an outward speed component. Such a deceleration brings the flow close to separation so it occurs only at higher angle of attack when the wing approaches stall. Note that only the boundary layer will exhibit a tipwise speed component.
Only when the flow separates, the boundary layer thickens past the separation line (blue line in the sketch above) and the spanwise flow will become significant. Separated flow is characterized by a local forward speed component, and now, past the separation line, you get a layer of air which is left with mostly a spanwise component; outboard in case of a sweptback wing and inboard in case of a forward swept wing.

EDIT
This paper presents a CFD study on the PRANDTL-D P-3C model aircraft using the OVERFLOW software. Figures 10 and 11 show the boundary layer flow on the lower and upper surface and illustrate nicely what happens. Note that this particular wing has a washout of -10° towards the tips which is essential to keep tip flow attached.

Lower surface streamlines. Only the 12° AoA case is shown because all others look quite the same.

Upper surface streamlines for angles of attack from 0° to 17.5°. The development of spanwise flow with the ever steeper pressure recovery at higher angles of attack should be evident. Note that flow direction past the separation line (the area with yellow-greenish tint) is from back to front.


Are there any airlines that fly a route that has an arrival time earlier than its departure time? Imagine the scenario where an airline flys a short 30 minute route which crosses a timezone going west. They could theoretically leave at 00:15 (local) and arrive at 23:45 local the previous day. I'm wondering if there are any airlines which fly such a route.

P.S. I'm specifically interested in how the airline allows booking such a flight and the UI that is presented, but am leaving this question more general for any flights that do this.

P.P.S. If someone could help with the tags for this question, it'd be much appreciated.

I found such a route for you. KATL (Atlanta) to KHSV (Huntsville, AL) is a 57 minute hop from the Eastern time zone to the Central time zone. It appears how you would expect:


Why does the takeoff N1 limit start to decrease below 30°C OAT? It's easy to understand why the takeoff N1 limit decreases with an increase in the OAT in the chart above; the engines get less dense air at higher temperatures, and less dense air means less engine power capability. But why does the takeoff N1 limit start to decrease with a decrease in the OAT below 30 degrees Celsius? For your information, the chart is an airport analysis chart for Boeing 737.

There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed).
Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the amount of fuel you can add to the system reduces as you get over-temperature, when the OAT decreases you could add more fuel, but that will increase spool speed which is also limited. From this figure you see the effects on SOT (turbine Stator Outlet Temperature, equivalent to Turbine Inlet Temperature, TIT):

But why does the takeoff N1 limit start to decrease with a decrease in the OAT below 30 degrees Celsius?

The limit on the left is the decrease in spool speed, note that the corrected spool speed is constant (N / √(T)) so for a lower temperature, the N also decreases.

It's easy to understand why the takeoff N1 limit decreases with an increase in the OAT in the chart above

Yes it is, this is limited by the turbine inlet temperature (or a maximum exhaust compressor temperature as used in the image above). The maximum turbine inlet temperature is fixed, the higher the inlet combustor temperature (exit high pressure compressor temperature), the less fuel can be added to heat to the air in the combustor, the less energy is available to expand in the turbines.
The constraining of the power is called flat rating.

This effect can be simulated with an engine model. If we would vary OAT for a certain power setting, e.g. constant (corrected) spool speed for a large bypass turbofan engine with imposed compressor exit pressure and turbine inlet temperature limit a model would look like:

After the variation of OAT you will yield the blue performance curves for a non-limited engine, and the black dashed curves for a limited engine:

Note that the compressor outlet pressure is the limiting factor in the graph on the left, but spool speed and compressor pressure are linked. The question is which of the 2 is reached first, this depends on the input and the design constraints.


What are the advantages and disadvantages of having a single castor wheel as compared to 2 wheels in an airship? I am working on a project to create a mid-size airship having a payload of 25kg. I am in a bit of problem regarding choosing the appropriate landing gear design for it.

Which design would be better, considering the tradeoff with weight, which is a very important parameter. Is using 2 wheels to stabilize my system further worth its weight penalty, or a single wheel would be better like Zeppelin?

I would appreciate any  help.

Two wheels side-by-side would only be needed if the craft is in danger of tipping over sideways. Airships should stay upright because the lifting gas will still provide most of the lift, and the wheel is only for carrying the remaining load. It can swivel for ground maneuvering when the airship needs to swing into the wind while mooring at the mast, but will neither absorb big landing loads nor carry the craft at high speed, so it can be kept small.

On the lower tailfin you should only provide a bumper for unintended ground contact; adding a second wheel would only make sense when the ship touches the ground with both wheels when level. This would be a bad design, however, because maneuvering will be much easier when the lower tail fin provides enough ground clearance to allow pitch angles of approx. 3° - 5°. Note that the lower fin on most Zeppelins was smaller than the upper fin; only the last, less slender ones could afford equally sized fins. In that case, however, you need sufficient propulsion power to keep the cruise speed well above the reversal speed.


XFY-1 Vertical take-off mechanics Hi I saw this video the other day of an XFY-1 doing a vertical take-off and landing and moving left to right while hovering.

See the video Here.

After watching this I had a few questions on how this is accomplished.

How does the plane go from vertical to horizontal and from horizontal to vertical flight? Is this done by moving the elevators to their respective extremes.
How can the plane move left to right forward to back while in vertical mode?
Is this done by some sort of helicopter rotor type of contruction in the props?

I am not sure if this is the right place to ask, neither am I an expert when it comes to planes or the engineering behind them (I just know the basics).

Would love to know how this works or resources that explain this, could not really find any myself :)

The plane takes off vertically, with the prop wash blowing over conventional control surfaces. To tilt forwards the pilot pushes the stick forward in the usual way to "lower" the elevators and tip the nose "down". As the plane tips progressively over, it progressively gains flying speed and lift transfers to the delta wings. At the same time, the elevators are brought back to the level for conventional flight. Landing is the same thing in reverse, with the elevators being raised.

Manoeuvring in vertical mode is again just the same, but only a small control movement is needed for a gentle tip in the right direction: elevators in one axis, rudders in the other. The elevators also act as ailerons, which allow the pilot to gently spin the plane round even when vertical.


Does extension of flaps 1, 2, 5 or 10 increase descent rates? Let's say you need greater descent rates than you can get with the full use of the speed brake in an airliner. Does it help to use the above flap positions in getting greater descent rates?

It depends what you intend to do with the speed. If all you are looking for is absolute descent rate:  a high-speed, spoilers out and high speed dive will get you the best rate.

What you are usually interested in a normal approach however is not descent rate (vertical speed) as much as descent angle. In this respect, all/any flaps will decrease the L/D ratio, which is going to increase you descent angle. Each stage of flaps will have increasingly higher descent angles, but mind you at a lower speed.


Can self-study and FSX substitute for the majority of PPL training? I am 16 and am very interested in becoming a private pilot, and fly flight simulators. I do not have enough money for the entire training. I can only afford 5 flying lessons.

Is it possible to learn majority of flight maneuvers using a flight simulator and just take a few lessons to get used to the feel of a real aircraft?

What are the list of topics that you need to learn when gaining your private pilot license?

Unfortunately, no.

If you learn and practice and master the flying skills in a flight simulator, you will become a certified pilot only for that simulator, not for real life.

You didn't mention the country, but you can look at this answer to find all the requirements to become a private pilot in USA.

Using a flight simulator, you can learn many very valuable skills which will be very useful in your actual flying (and many many pilots do a lot of practice in a simulator), but in no way it can completely replace actual flying. There are many things you can only feel in actual flying experience. You can make any mistake in a simulator and no harm is done, but not in actual flying.

Everything in flying is expensive. Even after you get your PPL, you still have to pay for flying afterwards. Since you are young enough not to have chosen a career path yet (probably), why don't you try to become an airline pilot?

For the answer to your second question, a few topics are:

the airplane and its physics
its engine
weather
effects of weather and altitude on pilots
navigation
planning
... and many other topics

There are tons of material available covering these topics. FAA has a wide array of reading material which you can download for free.


What is the relation between velocity and rate of pitch? I saw the answer to airspeed and rate of turn but this I think isn't what I want. I've seen it many times that aircraft will pitch up much slower when at higher speeds particularly fighter aircraft. What is the mathematical relation of these two characteristics if any?

I am trying to make a game about fighter aircraft and want to find a realistic solution to this problem.

A constant speed pull-up is basically a circular motion, albeit in vertical plane:

$$q=\frac{(n_z-1)g}{V}$$

where $q$ is pitch rate, $V$ is true airspeed assuming no wind, $n_z$ is the load factor (lift to weight) and $g$ is gravitational acceleration.

The amount of load factor is limited by the airplane and its flight condition. As you can see, the faster the speed, the smaller is the obtainable pitch rate for a constant load factor.

Note: a constant speed pull-up is a theoretical idea that only approximates a pull-up at the very beginning, before the airspeed begins to decay. But the conclusion is valid.


Was the Boeing 2707 design flawed? The mach 2.7 capable Boeing 2707 has an interesting design. I'm curious if there has been any studies done that concluded that this aircraft would simply not have worked, given the way it was initially designed, even if sufficient funding existed.

If so, what was the problem with the design? By problems, I don't mean it being financially feasible or sustainable, I mean technical limitations and restrictions imposed by physics. Could this sort of aircraft ever have achieved a sustained mach 2.7, and carry 277 passengers?

I'm personally inclined to believe it might have worked, considering other notable aircraft of the time, such as the military planes; the XB-70, the SR-71, or the civilian Concorde, and Tu-144. The civilian planes mentioned here are somewhat slower, but the military planes are somewhat faster, and puts the 2707 right in the middle.

However, it would be nice to see an actual evaluation of its design, if such a thing exists.

The three competing supersonic designs are discussed in this video – The Forgotten American Concordes. Two competing designs from the USA were proposed to the US government, which was going to fund 75% of the development cost: one from Lockheed, and the B2707 from Boeing which was the winning design.

The B2707 was meant to be bigger and faster than Concorde, and this was one of the three issues that eventually saw the project being cancelled:

Concorde flew at Mach 2, heating up the nose to 127 °C and the wing leading edges to 100 °C. The whole plane was considerably longer at cruise than it was on the runway due to heating up from the supersonic flow, but at this speed aluminium could still be used as the main construction material for most of aeroplane. The B2707 would be heating up much more, ruling out aluminium and having to use titanium, skyrocketing the construction price per aeroplane.
The oil crisis of the early 70s raised fuel prices a lot, with unfavourable consequences for the operating costs. It was thought that since the travel time is shorter there could be more daily flights, but:
protests about supersonic booms prohibited supersonic flights over most land, meaning that the only viable market was the US east coast to the European west coast.

  If so, what was the problem with the design? By problems, I don't mean it being financially feasible or sustainable, I mean technical limitations and restrictions imposed by physics. Could this sort of aircraft ever have achieved a sustained mach 2.7, and carry 277 passengers?

Image source

It might have, there were military aeroplanes flying at M 2.7 or faster at the time like the XB-70 Valkyrie built out of stainless steel, but the technology was still immature. Passenger transport implies high reliability and long testing times to verify if the safety requirements can be met, and this was a major issue to be crossed out at the time of cancellation.

So the question: "Could this sort of aircraft ever have achieved a sustained mach 2.7" at the required reliability rate was never tested.


In what conditions is the APU used in midair? In flight, engines provide all the needed energy (bleed air, hydraulic and electrical power). So the APU can be shut down shortly after engines are running. But I imagine that starting the APU in midair is a well known procedure because:

I understand that starting the APU while the engines are still running is common given the answers to this question (even if done when the aircraft has landed)
In the US Airways 1549, the APU was started in midair [1]

How is the APU used in midair? When is it started/shut down while the aircraft is not on the ground?

If needed, the answers can be restricted to airliners.

[1] NTSB report p.19

This answer applies to the AVRO RJ (BAe 146), because this is the aircraft I've flown. Other aircraft may or may not have similar features/issues/restrictions.
Reasons for APU use in mid-air

For takeoff, the APU bleed-air can be used to supply the cabin air pressure system to reduce load on engines. This leads to a significant reduction in runway length required. This is routinely used on short runways.
Similar reason for the landing, where, for the missed approach/go-around phase, a high climb gradient is required. For example in mountainous areas or combined with icing conditions, where use of engine bleed air is limited, the APU bleed air can feed the cabin pressurization.
During flight, when in icing conditions. Similar reason as above. The air-frame anti icing system uses that much bleed-air, there is not much left for cabin pressurization (or the other way round). If the APU bleed-air is used to pressurize the cabin, the (already low) climb gradient is improved.
During flight, in the event of a generator failure. The generator of the APU can replace one failed generator of an engine.
To save engine life. If APU bleed air (- to pressurize cabin) is used for normal takeoff, the peak engine temperatures will be lower during takeoff. This increases engine life as the peak engine temperature is the main factor of engine life. It's significantly cheaper to replace an APU than an engine.

In all of the above cases, there are restrictions on the operation of the APU. It may not light-up above 15'000 ft or operation above 25'000 ft is not allowed or not recommended. General operating rules for the APU (common sense when using turbines) recommend to use bleed air only after couple of minutes warm-up after startup. Likewise, on shutdown, it should be left running without load to cool down for a few minutes before switching off. The exact timings differ greatly between APU models.
Procedures

Before Takeoff, If the APU is not used during climb, the APU is normally shut down during taxi checks. If the pilots decide to use the APU during climb, it is started during normal engine start. In this case the APU bleed air is used to feed the cabin pressurization.
After Takeoff depending on the model of the APU (and following airline procedures), the APU usage is limited to a specified time or altitude. Before reaching this limit, the APU has to be shut down. (cabin air changeover to the engine bleed air, cool-off period of the APU, switching off)
During Flight In case of a technical problem or emergency, the Abnormal Checklist will suggest or dictate the use of the APU for the remainder of the flight. For example: A generator problem, the APU generator replace the failed generator. This may impose limits to flight altitude and duration.
Before Landing Usually after passing 10'000 feet, the APU is started and before the final checks, the cabin pressurization system is switched to the APU.


What is the significance of the exclamation mark (!) on ACARS page of a Boeing 737? For a while now I've noticed an exclamation mark on the ACARS inflight page of the B737NG at the beginning of a flight. It is located in the top left corner of the page and it just disappears after a while. I've been searching high and low what it means but can't find it.
Any ideas?

Okay guys, the verdict is in.

It is a symbol to indicate the crew alerting chimes are inhibited.
From OUT (off block) until OFF+15 and estimated ON-10 until IN (on block) the chimes to alert for new data and messages are suppressed to avoid nuisance warnings during critical flight phases.

Happy landings!


Do airport fire departments have to have airstairs? On the video of the JetBlue 292 landing (at 4:27), one of the first emergency vehicles to approach the Airbus is a fire department airstair. I've never seen a fire department with its own airstair. I found a standalone picture of LAX's Stair 80. I don't see a stair among ORD's fire apparatus but DFW and DEN have them. Why do some airports apparently have airstairs but others don't?

The only reference I found from the FAA is a 2012 article about New Large Aircraft (e.g. A380). Relevant passage:

  Several airports are now acquiring mobile air stairs and modifying
  them for ARFF use with hand lines, ventilation equipment and fire
  fighting tools. Some European airports have even acquired customized
  platform vehicles for rapid firefighter access into aircraft. Ideas
  being discussed are various air stair designs, scissor lift catering
  style trucks and modified ARFF vehicles with stairs or ladder that
  will accommodate all sizes of aircraft, including double deck designs.

I found this manufacturer page touting the importance of fire department airstairs. So obviously they exist. Here are my questions:

Does the FAA require fire department airstairs?
What are the primary uses? Getting passengers out or getting firefighters in?

Sorry unfamiliar with FAA regs, but since Canada's regs are often just rewrites of US or ICAO rules:

In Canada, I cannot find any direct reference to airstairs as required minimum firefighting equipment : Canadian Aviation Regulations (SOR/96-433) section 303.09
there is however a requirement for holding an airport certificate in Canada requiring a safety management system be in place, this involves doing a detailed risk assessment.

logically having the ability to deplane any aircraft expected to land might be seen as a necessary risk management solution to a number of potential possible issues. There are similar requirements for "air operator certificates", meaning some of the accountability may be place on the airline. Like so much of Canadian law, we will probably not get a definite answer until a large number of people die and the courts get involved.


How does a jet fighter landing flare compare to a civilian plane? How does a jet fighter flare compare to a civilian plane?

How low before flaring?
Do pilot keep track of pitch or AoA to know how much should be flared?
Is flare at last moment like in airliners, so after crossing runway threshold?
Will touchdown happen during the flare, or must the pilot bring the aircraft out of the flare to land more flat?

It's just 1 question. The bullet-points are just there to illustrate things in the answer I would like clarification of (which are all flare related).

It all depends on the aircraft for specific handling qualities, but for the most part they are flown just like any other large turbojet aircraft.

Some things to keep in mind about their specific handling:  Jet fighters with the thin, low aspect ratio wings are really power dependent and glide about as well as a brick.  Therefore they will be flown with power all the way to the runway until the roundout commences.

Like other large jets, fighters are flown at optimal angles of attack, the speed at which to achieve this is dependent upon the aircraft's landing weight.  This can vary between 110-160 or so knots depending on the aircraft in question.  The optimal angle of attack is available to the pilot in the form of an optical AoA indexer and either an AoA indicator or a graphic reference in the pilot's head up display (the 'E' Bracket).

Pilots will line up for final at Vref + 10 kts or so, align with the optical glideslope eg PAPI, etc and establish the optimal speed for the approach and power settings.  An ideal sight picture from the jet will be to line up on runway centerline, have the Flight Path Marker in the HUD aligned with the PAPI or numbers and the E-Bracket centered on the flight path marker.  Power corrections should be made to hold this sight picture as needed; in the case of a fighter, gently walking the throttles should be all that is required.  Roundout will be commenced considerably higher than one would in a light GA aircraft so that the jet arrives at the correct roundout altitude of 1-2 feet and correct attitude for the approach, once the roundout commences, power will be pulled to idle, allowing the jet to gently settle on the runway.  Flaring is not done quite like a commercial jet due to the difference in handling characteristics between the two due to the high wing loading, stores drag and a few other features found in fighters.

Most terrestrial based fighters are flared; naval fighters are often not and are flow to impact with the runway so the pilot can maintain proficiency in the techniques required to do arrested landings on aircraft carriers or log practice approaches with FCLP or Field Carrier Landing Practice.  Even terrestrial air forces who operate naval fighters in their air force - one I can think of right now is the Canadian Air Force - teach no flare landings to their pilots; not surprising as the CAF CF-18 syllabus was copied and adapted from the USMC F-18 training syllabus from MCAS Cherry Point.


What is this scoop found on the core of a PW1127G engine near the CC/turbine section? The view is back to front, the scoop is at 10 o'clock on engine 1. What would be it's function? I need supportive literature as well if possible.

That scoop allows air to go through the Engine Air/Oil Exchanger which cools the engine oil with fan bypass air.
Engine oil that is cooled by the air/oil heat exchanger decreases the quantity of heat that must be moved from the oil to the fuel in the fuel/oil heat exchanger.


Can a human get sucked into a jet engine? Can a human get sucked into the jet engine of a normal modern airliner? if so has it ever happened and what is the minimum or maximum weight that these engines can suck? As a safety measure, wouldn't light weight but high strength retracting safety covers that cover engines when the aircraft is parked protect both humans, equipment and engines? Of-course it would have to take into account the need to minimize weight. Also it could have the option to be controlled by the ground team supervisor remotely as an extra safety measure.

Yes. Human beings can get sucked into jet engines if they are close enough- this has happened multiple times in a number of aircraft ranging from A319 to A6E intruder.

However, it happens only in rare cases- usually in case of miscommunication or a mistake, when safety procedures are not followed. The following image shows the safety hazard area for aircraft engines.

A warning poster, available from Boeing, reminds ramp and maintenance workers
about the dangers of engine ingestion; image from Boeing Aero

In addition, the engines also have hazard-area warning decals on the nacelles to warn the ground personnel of the dangers.

Warning signs on engines; image from Boeing Aero.

As for the maximum weight that can be ingested, it will vary with the engines, though they have been known to ingest items from safety pylons to baggage containers, and of course people.


Do modern aircraft automatically manage the deceleration to Vref? Typically in most aircraft, the pilot manually reduces the speed gradually until reaching the landing zone/flare zone. Modern aircraft have FMC. Does the FMC automatically manage the speed gradually from a Vapp speed to Vref speed as calculated by an FMC in Autothrust mode?

This depends on the aircraft.

Airbus aircraft have a managed speed mode in the FCU (flight control unit), where the speed is completely controlled by the FMGS (flight management and guidance system). This includes slowing the aircraft down when the appropriate flaps are selected by the crew. No manual action is necessary to reduce airspeed for landing until you hear "RETARD, RETARD" during the flare and pull the thrust levers to idle. See here for some details.

Boeing aircraft can also manage speed automatically in VNAV mode, but not all of them slow to reference speed automatically, only to the flap maneuvering speeds when flaps are selected by the crew. When using other modes than VNAV for the approach (like APPR for an ILS approach), speed must always be set manually in the MCP (mode control panel). For an RNAV approach, this may be possible, see e.g. from the 737 NG FCTM (5.46 Approach and Missed Approach):

  When on final approach, VNAV may be used with speed intervention active (as
  installed) to reduce workload. Adding speed constraints to the final approach
  waypoints is normally not needed and causes extra workload without providing
  any safety benefit. This also reduces the ability to make last minute approach
  changes. However, if needed speed constraints may be changed if the default
  value is not suitable.


Are pilots legally required to verbally declare an emergency to invoke §91.3(b)'s deviation authorization? Related:Do you have to explain what your emergency is?

I am aware that pilots are required to declare an emergency and inform ATC of as much information as possible during IFR flight due to this rule:

  §91.183    IFR communications.

  Unless otherwise authorized by ATC, the
  pilot in command of each aircraft operated under IFR in controlled
  airspace must ensure that a continuous watch is maintained on the
  appropriate frequency and must report the following as soon as
  possible—

  (a) The time and altitude of passing each designated reporting point,
  or the reporting points specified by ATC, except that while the
  aircraft is under radar control, only the passing of those reporting
  points specifically requested by ATC need be reported;

  (b) Any unforecast weather conditions encountered; and

  (c) Any other information relating to the safety of flight.

Is there any rule that requires the declaration of a VFR emergency for it to be legally valid for the purpose of rule deviation?

My assumption is no because a 2-way radio is not even required in all locations.

The short answer is no, there is no specific requirement for VFR flights to state an emergency, however you may be expected to explain yourself and your actions once you reach the ground.

  My assumption is no because a 2-way radio is not even required in all
  locations.

To expand on this you don't need a radio on board for VFR flight (FAA FAR 91.205) so they would have a legally contradicting issue if they required you to announce when you don't have a radio. As mentioned by Ron in the question you linked the rule is "Aviate, Navigate, Communicate" If you are too busy with the first two that is far more important.

Im not sure where they are pulling the quotes but this NASA presentation on emergencies sums it up well.

  “…an intent of 91.3 is to ensure the PIC will handle the emergency in
  a manner necessary to save lives and not be worried about regulatory
  compliance.”

  “I’ve never seen a pilot violated for deviating from a
  regulation when that pilot has either declared an emergency OR has
  stipulated in ANY written response to the FAA that an emergency
  existed at the time of the deviation.”

This article has some interesting points from a lawyers perspective on the situation,

  Madsen defended a case for a pilot who deviated from an assigned
  altitude due to extreme turbulence while flying a Cessna 150. The
  pilot had dropped his handheld mic, so he couldn't initially
  communicate his situation. He stabilized the airplane and was about
  600 feet off his altitude when ATC called him. At that point he was
  able to find the mic and he told the controller what had happened.
  "It's unrealistic in some situations to communicate your emergency
  before you secure the aircraft," Madsen said. "You want to control the
  aircraft first." Madsen said the case was ultimately dismissed, but he
  felt it would have been much easier to defend had the pilot declared
  his emergency.

Here we can see that ultimately the pilot was in the right and while he should have declared, there was no legal grounds that required it.

On some level you are at the mercy of the FAA with this one and its clearly a case by case basis (as is everything in aviation) and the lawyer in the above article goes on to say

  Some FAA field offices take the wording very seriously. Such was the
  case with the Cessna 150 pilot who deviated from his altitude. "They
  questioned why he didn't state an emergency and call out mayday, and
  they took great issue with the fact that he didn't use the accepted
  terms," Madsen said.

Again the case was ultimately dismissed whether or not they were happy about that.


Do lights on aircraft flash in a specific pattern? On a clear night, everyone can spot planes quite easily: they have flashing lights. I have noticed that the pattern seems different for different planes, and tried to find a meaning.

What I saw so far is that they seem to have three lights that flash, two at the wings, one at the tail. Those at the wings flash two times (approximately per second, in quick succession), and that one at the tail flashes only once, along with the first flash of the wing.

Now that explains two patterns I seem to see from further away: Two flashes and one flash. It is probably that I am just not seeing the wings flashing for those where I only see one.

So far so good, but then again sometimes I see planes with more flashing, and those are faster. I had difficulties to count them, but it's at least three, maybe four quick flashes.

Do these different patterns have any meaning? I tried to match up the planes with the flight radar information, but since it is quite crowded around here, I wasn't really able to identify them. But whenever I saw that quick flashing ones, there was a military plane around, is that just coincidence?

Civilian aircraft have flashing lights based on manufacturer design preference.  As others have stated, aircraft like Boeing and Airbus have different patterns.

However, military aircraft can often select the pattern they want to use.  This helps military pilots identify key tactical aircraft during night operations, such as the tanker flying around the carrier.  It is definitely not coincidental that you noticed military aircraft with a unique pattern, although I'm truly shocked at your attention to detail.


Why does the thrust limit page display the thrust in percentage (« % ») and not in pound-force? When the pilot sets the takeoff engines thrust on the Flight Management thrust limit page that displays on the CDU (or MCDU) interface, the selection will finally show as a percentage.

The direct consequence of this is that, « at first sight », the pilot does not know the exact thrust that the engines will develop in pounds-force.

Considering that all operators do not necessarily choose the same rating for a similar combination of aircraft-engine, does it mean the same percentage of takeoff thrust might provide different thrusts according to the operator?

Does it mean the same percentage of takeoff thrust might provide different thrusts according to the operator?

Not just per operator, it is different per engine installation in one fleet. That's why calculations are done according to each airframe's AFM. And as the engine ages, its thrust decreases* (and fuel consumption increases), and this is reflected in the calculations. (The fuel flow (and drag) offsets are based on data from previous flights, and are presented to the pilots as corrections/biases to input.)

    The selection will finally show as a percentage.

The final selection will be percent N1, EPR, or for the latest planes (A350 and 787 with R-R engines) as an actual available thrust percentage. So it depends on the plane/engine/technology.

As to why, because lbf or kN mean nothing at first glance.

The calculations are very complex to do by hand.
Density altitude would make a correct number feel incorrect. In other words, 95% N1 (for example) is a different thrust force at SL at 15°C from at 5,000' MSL at 35°C.

The number simply shows without guesswork or confusion** how much of the available thrust will be used. And it makes the AFM (or EFB) easier to present/use.

* As an example, a heavily used fan will not be in a good condition, so two similar fans (but different ages) running at the same rotation (N1 percentage) won't deliver the same force. (The same for the conditions of the rest of the stages and combustor.)

** Plus, the engine controllers have no way of measuring force, this is another reason why the selected level during flight prep must match the engine instrumentations, otherwise the crew can't verify the takeoff thrust that's been preset, has been achieved.


What is the difference between a crabbed and de-crab landing? Both landings are done when an airplane's nose is not in inline with the runway. How are these two different? when should one technique be preferred over other?

Consider where the center of gravity is in relation to the main wheels. If you are in a tail dragger, you definitely would prefer to de-crab (never heard that term before now) before touching down. If you touched down while crabbed, you'd have a moment created by the center of gravity around the main gear to ground loop the airplane.

In a tricycle gear airplane, that same moment would work to straighten the airplane since the center of gravity is forward of the main gear.

In a crosswind from the right, for example, when you apply left rudder to align the aircraft with the runway, the right wing will want to come up (you just gave it a little extra speed), so you will want to use right aileron to at least counter that as you certainly don't want the crosswind component to get 'under' the wing.

Both 747 operators I flew for recommended touching down in the crab. Personally I didn't like that and used a combination approach in a heavy crosswind, take out some of the crab but not all. Also, while they did caution against lowering the upwind wing, I always lowered the wing a little. I once figured out how much the wing would have to be lowered before ground contact, and it was quite substantial, more than I would ever think of using. Also, as I remember, It would not be the wingtip or an outboard engine pod that would make first contact, but rather the outboard corner of the outboard wing flap when set to flaps 30.

Finally, when your touch down speed is 140 knots, for example, you'll have to crab much less for a given crosswind component than if your touchdown speed is 70 knots. Cosine or tangent of the angle, I forget which.


When does the overspeed alarm goes off in a plane? I want to know when does the overspeed goes off in aircraft. How exactly is overspeed limit decided? Is it fixed for all the situations, or does it keeps changing? In a game that I play, (Infinite Flight) it goes off at 350 knots air speed for a Bombardier CRJ 200 or Cessna Citation X. But, it seems unrealistic as real planes often easily fly over 450 knots air speed.

The overspeed goes off when either the indicated airspeed exceeds that limit or the mach speed exceeds the mach limit. At least that's how it worked on 747-100/200 aircraft.

The 350 knot figure for the max operating airspeed sounds reasonable to me as long as you're low. From memory that was around the max operating INDICATED airspeed for 747-100/200 aircraft. Remember, though, that indicated airspeed decreases with air density. A 747 cruising above 30,000 feet at around 550 knots TRUE airspeed is going to show an indicated airspeed of less than 300 knots. However, that cruising true airspeed of 550 knots or so is going to be around mach 0.86. The max mach number for the 747 was 0.92.

If you were at, say, 15,000 feet and speeded up to 350 knots, the mach number would be much less than 0.86.

Take your sim up to 35,000 and see what you get.


Is it possible to use normal headphones for passengers in a light aircraft? I'm just finishing up my private license and am looking forward to taking some friends up in a Cessna 172.  I've invested in a nice aviation headset for myself, but, as none of my friends are pilots, they don't have headsets of their own.  Even the cheapest aviation headsets (at least that I've found) are around $100; friends going up for an occasional flight don't want to spend that kind of money, and I certainly don't want to shell out for several spare headsets.

My question is, are there adapters available so that normal headphones could be plugged into a standard 2 jack connector?  I have not been able to find any online.  While certainly not too effective at reducing noise, this would at least give my passengers some comfort and would allow me to communicate with them without shouting during these short and infrequent flights.  Worst case scenario, I could give them all earplugs, but it would definitely be helpful to have some kind of headset solution so that I'm not shouting through the sound of the prop and the earplugs and they're not shouting through my ANR.

There are some solutions to convert a normal set of headphones into an aircraft headset, but in total they are going to cost you more than a base level headset would.

One popular (available in TSO and non-TSO options) option is UFlyMike which is an adapter for Bose noise cancelling headphones that adds a microphone and connects to a standard 2 jack airplane audio panel.  But back to your issue at hand, this is not cheap.  The UFlyMike will set you back around \$200 and the Bose headphones are another \$300, making this adapted headset cost a total of \$500.  This is a bargain compared to an actual Bose aviation headset, which runs around \$1000, but is well above an entry level non-ANR headset.

Your best bet is to ebay some old clamp style headsets (e.g. DC) to get some cheap headsets and then keep around your current headset when you eventually upgrade (unless those trade-up deals from e.g. Lightspeed are too hard to resist) to build up a stock.  Alternatively, if you are renting, you may be able to rent a few headsets with the plane so everyone can be equipped for a trip.


Do jet aircraft have a better glide ratio than propeller aircraft in general? If so, why? Being fond of aviation, I play with software simulators a lot.
I have noticed that while jets seem to be able to glide for a long distance even at zero throttle, propeller aircraft need to be throttled right up to the runway.
Is this accurate in real planes? If so, why?

"Glide performance" is measured by "Lift-to-drag ratio":

In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the amount of lift generated by a wing or vehicle, divided by the aerodynamic drag it creates by moving through air

When you look for examples you'll see that for the most part larger airliners do indeed have a more favourable L/D ratio

Boieng 747 = 17:1
Airbus A380 = 20:1
Cessna 172 = 11:1

As to why, that is the way they are designed. A good glide ratio is a favourable feature of an aircraft as less drag will imply better efficiency, and airlines are more concerned with efficiency than, for example, a training aircraft which might be more concerned with stability.
More information can be found here: How far can airplanes glide?


Is there a formula to convert SFC to air-fuel equivalence ratio λ? Speaking about aviation internal combustion engines, is there a way to get the current value of air–fuel equivalence ratio (λ) given current specific fuel consumption (SFC) and current power?

Is the relationship between SFC and λ fixed or does it vary with engine displacement and/or power?

The data I currently have is:

The engine SFC at best power as watts/Kg*sec
The current engine power output in watts
The current engine fuel flow in Kg/sec
I know that λ is about 0.85 when SFC ratio is about 1 (best power)

No, there is no formula to calculate the air-fuel equivalence ratio from specific fuel consumption (SFC). To calculate the equivalence ratio, you need the air mass flow, and the fuel mass flow. From this data, you can calculate the actual AFR (air fuel ratio), and lookup the stoichiometric AFR for your given fuel.

$AFR = \frac{m_{air}}{m_{fuel}}$

The air-fuel equivalence ratio is then simply calculated from these two AFR values.

Air-fuel equivalnce ratio: $\lambda = \frac{AFR}{AFR_{stoich}}$

You don't have a value for the air mass flow. You only have fuel flow from the SFC and current power.

If you had the equivalence ratio and SFC at two different operating points, not just one, you could assume linear interpolation, or extrapolation, to the SFC in question.

With the equivalence ratio and SFC at just one point, you would have to find a characteristic curve for SFC vs equivalence ratio for a similar type of piston engine and the same fuel type, and assume the shape of this curve is applicable to your particular engine.


Do any flight simulators go completely inverted? From the pictures I have seen of airline simulators, it looks like the range of motion is limited to somewhat level flight. Are there any flight simulators, possibly military trainers, that can do a complete barrel roll?

Pulseworks makes some full motion sims that are capable of inverted flight and infinite barrel rolls.  These sims fall into the entertainment category more than the training category, but they are full motion, in cockpit sims.  I have also been in a similar type of sim at a Harris/Boeing sponsored party in Seattle made by a company whose name I cannot remember that did the same thing.  That one in fact was capable not only of rolls but also loops and could be pretty violent (I wanted to see if it would spin... it did).

Here's a video of one of the Pulseworks models.  They have some at the Udvar-Hazy center at IAD (free admission, and a shuttle from the terminal).  They cost a bit of money to get into.  They are also often setup in pairs so you can go into a dogfighting scenario with the box next to you.  Bring a friend.


When must full aircraft registration be used as radio callsign? What are the FCC or FAA regulations that require one to use their full registration as radio callsign, unless abridged by air traffic control, when using an aircraft radio for transmitting?  It seems it is becoming popular to use type of aircraft and color only for transmitting.

The AIM discusses communications with ATC in Chapter 4.

  4−2−3. Contact Procedures a. Initial Contact.
  1. The terms initial contact or initial callup means the first radio call you make to a given facility or the first call to a different
  controller or FSS specialist within a facility. Use the following
  format:

  (a) Name of the facility being called; (b) Your full aircraft
  identification as filed in the flight plan or as discussed in
  paragraph 4−2−4, Aircraft Call Signs; (c) When operating on an airport
  surface, state your position. (d) The type of message to follow or
  your request if it is short; and (e) The word “Over” if required.

  c. Subsequent Contacts and Responses to Callup from a Ground Facility.
  Use the same format as used for the initial contact except you should
  state your message or request with the callup in one transmission.

  4−2−4. Aircraft Call Signs
  2. ATC specialists may initiate abbreviated call signs of other aircraft by using the prefix and the last three digits/letters of the
  aircraft identification after communications are established.…The pilot may use the abbreviated call sign in subsequent contacts with the ATC specialist.

When operating at a non-towered field, many pilots have begun to identify their aircraft by type and color. I’m not sure when this started, but if you listen to popular YouTube flyers like steveo1kinevo, you will notice that when they are flying into non-towered fields, they just give the aircraft type, e.g. Caravan or TBM. From a practical standpoint, it makes a lot of sense. You can’t see the numbers but you can see the type and color. And unless there are lots of white Cessnas in the pattern, it is easy to keep track of who is where. What you want to know in the pattern is where are the other planes and how fast are they going.

Type and color gives you the necessary information. For me at least, it is easier to keep track of type and color than it is to keep track of N numbers.


Has any passenger airliner ever offered forward-facing windows? The empty, dolphin-like, forehead of the A380 seems like it would be an amazing place for the first and/or business class bar, especially if it had forward-facing windows.

I realize in the A380's case there are crew rest and storage compartments in that space, but wondered if there have ever been any passenger planes outfitted with forward-facing windows to enjoy the view?

Image source: lufthansa.com

We could start with the Tupolev ANT-20 with its rather daring forward-facing passenger gallery. [edit - the prime seat right at the front in the gallery was for the navigator and his equipment.]

source

Like the Tupolev, most examples are inter-war. Here is the moderately successful Latécoère 521 being assembled showing its gallery beneath the cockpit [edit - seems this was only for the navigator, not for passengers] :

source

... and the more esoteric Caproni Ca.60 which would have made for an interesting journey had it entered service -

source

-- EDIT - one special mention for the Junkers G.38 which also had a sizable glazed area in the wings. Unfortunately as far as I can tell (and despite what Wikipedia says) neither the nose nor wings were accessible by the passengers, but rather used by the navigators and engineers, which seems like a missed opportunity. [ed. the wing area was indeed intended for passenger, see comments]

source


What is the purpose of long endurance test flights? Well known is the last long endurance test flight by a Boeing B787-8, which drew its shape in the air for 18 hours while on a long endurance test flight: https://www.wired.com/story/boeing-787-8-drawing-test-flight/.

What exactly is the purpose of these long endurance test flights? What is tested, what shall be proven? - surely not that the aircraft can actually stay in the air that long, because that test flight happened long after the introduction of this type into the market...

On a side not, does every commercial airline type have to undergo such an endurance test flight?

This particular test flight tested the engines, not the aircraft:

  At 15:38 local time N7874, the fourth 787 built, departed Boeing Field
  in Seattle for an 18-hour, 22 state test flight. The crew spent the
  overnight hours above the United States performing ETOPS testing on
  the new Rolls Royce Trent 1000 TEN engine, which will power the
  787-10.

https://www.flightradar24.com/blog/boeing-uses-a-787-to-draw-a-787-dreamliner-during-etops-test/


Regular procedure followed by the pilots before takeoff? What are the list of procedures (checks) followed by A320 and A380? Are these procedures decided by the airlines and (or) the manufacturer, or by the Civil Aviation Regulatory Authority of the respective country?

Try to search "Before Takeoff Checklist". Manufacturer will provide standard checklist but airlines can modify it.


What are the differences between a helicopter engine (turboshaft) and an aircraft engine (turboprop)? Given the fact that most helicopters nowadays use turbo shaft engines and many aeroplanes still use turboprop engines (which are essentially turbo shaft engines with a propeller), how do you differentiate between a helicopter engine and an aircraft engine? What are the differences between the two?

p.s. I am not looking for differences in turboprop and turboshaft, but differences in helicopter and plane engines

There are no major differences between a turboprop used on an airplane and a turboshaft used on an helicopter. All power is collected by the turbine shaft which rotates the blades through a gearbox.

An example of a gas turbine used in helicopter, airplane, train, and even snowplow is the well-known Pratt & Whitney PT6. It has different versions. The two versions shown below are the PT6A for a propeller (top), and the PT6B for an helicopter (bottom, with a large gearbox). Each sub-family has variant for different powers.

(source)

(source)

On the two images, from right to left: The accessory box powered by the compressor section, the air intake recognizable by its filter, the compressor (not visible), the combustion chamber (yellow color on the bottom image) with its circle of igniters, the turbine (not visible), the exhaust (the big hole, best visible on the bottom image), the power gearbox with different output shafts.

There are different types for aero-engines, including turbojet, turbofan, turboprop and turboshaft (I exclude the ramjet which is not used in civilian aviation).

All types are based on a gas turbine, but the way the turbine participates to the motion is different.

(source)

While airplanes use all types, helicopters cannot use the jet action/reaction principle (tip jet excluded for simplification).

The gas turbine can be the same for turboshaft and turboprop engines, but will be fitted with appropriate elements:

The rotational speed of the shaft will be adjusted to accommodate the rotor or the propeller. This is done using a gear reduction system. An helicopter rotor turns slower than an airplane propeller.
Air intake and exhaust are adapted. It rather common to have the air intake on the opposite side of the power output shaft. For a pulling propeller, this means the intake will be aft, and air must be collected forward and ducted to the engine intake.
The power output shaft is usually used to bear the propeller tension caused by the lift, while on an helicopter, the rotor tension is borne by the frame.
The accessory box is usually powered by the compressor side of the engine.
A turboshaft don't provide the bleed air extraction, there is no cabin pressurization required.

(source)

Gas turbines used in both cases are frequently free turbines, where the compressor section and the turbine (power) section rotate independently of each other. This allows to produce electricity or hydraulic power on the ground without turning the blades.


Do parking brakes lock the wheels and airframe together? (YouTube) Wildcat undercarriage.

For planes with non-truck undercarriages, can they be rotated around the lateral axis of the main landing gear with the brakes applied without causing translation?

My answer is no, they must be locked and there must be translation (image below).

I had the following comment on a recent answer, and frankly I'm not quite sure, hence the question.

  [They] are wheels. The whole structure is free to rotate around this 'locked' axle...

I don't agree, imagine chocks are placed forward of the main landing gear:

A) Frame is locked, blue man can't lift as he can't cause translation.
B) Frame is not locked, plane rotates around wheels (from the wheels' perspective, they're free moving).

(Original) Above shows what I mean by translation, the plane moves forward (blue distance) with the wheel—plane frame (red lines) being locked. (Brakes are applied.)

Note: This question has nothing to do with deceleration or a running engine.

Yes, the parking brakes lock the wheel and the airframe together. If the brakes are not applied, the wheel can rotate round the axle.

If the wheel's position is fixed to the ground (e.g. by adding chocks, but the brakes are not applied, then the aircraft will rotate round the wheel axle when the tail is lifted by the blue man.

If the brakes are applied, but the wheel's position is not fixed to the ground, the aircraft will rotate round the wheel's point of contact with the ground. Since the contact point moves as the wheel rolls, this results in translation in addition to the rotation.

If both the brakes are applied and the wheels are on chocks, the wheels either have to roll over the chocks or something (the wheel, the chock or the brake) will have to slip.


Can the CG move out of bounds as fuel is used? I've never bothered to check the weight and balance when the tanks are empty. I always assume that if the Center of Gravity is within range when the tanks are full, then it will remain within range as the flight progresses.

Am I correct? Should I compute the W&B on both ends?

I know some airliners have complex fuel systems which require fuel to be moved around during flight (e.g. B747-100, Concorde). I am interesting in small aircraft in general, i.e. General Aviation and perhaps personal and up to regional jets.

With a straight wing aircraft with all the fuel in the wings, the fuel is close enough to the C of G across the span of the tanks that the C of G shift as fuel is consumed is insignificant.  When this is the case, the weight and balance chart won't have any special requirements to show C of G shift, relative to the fore/aft limits, from fuel burn.

If there is an aux tank that is forward or aft, the effects of its fuel level on C of G will be included in the airplane's weight and balance, and there may be limits on loading of cabin or cargo incorporated into the weight and balance data, and special instructions for C of G calculations when it's being used along with curve added to the wight and balance graph.

On swept wing airplanes, sweep and dihedral in wing tanks causes C of G to move forward as fuel is burned (level drops in the tips first, which are aft).  On swept wing airplanes with a center tank, the center tank is almost always burned first and it causes an aft shift of C of G as its fuel is burned, followed by a forward shift as the wings are consumed.

The C of G charts of swept wing airplanes will include a fuel burn curve to show the C of G location variation with fuel burn. If no center tank, the curve will slant or curve to represent the forward shift, and if there is a center tank it will have a kind of sideways V shape as the C of G moves aft and then forward as the pounds of fuel load declines.

Bottom line is, do whatever the weight and balance chart for the airplane says to do.  If it's something like a 172, there isn't going to be any major shift. If it's something like a Taylorcraft with a nose tank, C of G will shift aft as fuel is burned.  To keep things simple, there may be a baggage restriction that is intended to prevent exceeding the aft limit when the tank is empty at the end of a trip.


In an ATIS broadcast, is the visibility in statute or nautical miles? I think I remember seeing somewhere that since it's an observation at the surface, statute miles are used for the visibility. Would be nice if someone had a reference.

The FAA has this handy chart to decode METARs (which are the printed version of the ATIS) and TAFs.

  Prevailing visibility; in U.S., Statute Miles & fractions; above 6
  miles in TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as
  required, lowest value with direction)


What could cause the APU on a Boeing 777 to fail and also trip the GPU? I am a cargo aircraft loader at Heathrow and while we were offloading cargo, pallets and baggage containers from a Pakistan Airlines B777, the aircraft's Auxiliary Power Unit (APU) failed. It made matters worse when we attached the Ground Power Unit (GPU) ground cables to the aircraft nose because the GPU went into trip mode. This resulted in a complete loss of power and we were unable to open up the front and rear cargo doors. The loading systems' Power Drive Unit (PDU) also failed.

My team leader played with the GPU leads and set them at a certain angle where the trip mode stopped and we offloaded the airplane. What could cause this APU failure, and why would the GPU trip? Is this a safety issue if airlines operate a non-functional APU on their aircraft?

Dispatching an aircraft with an inoperative APU, regardless of whether it's a passenger aircraft or a freighter, is a normal procedure. While an APU may be started inflight preparatory to it's use after landing, it is not needed inflight, so it's not a safety issue. In my experience (retired in 1999), dispatch of freighters with inoperative APUs is more common than dispatching passenger aircraft that way due to the passenger comfort aspect. Keeping passengers cool while unloading/loading is a priority. Keeping a freighter's crew comfortable is not.

An APU failing while in use is not unusual. GPU failure is also not unusual. If the APU tripped (as opposed to just quit) and the GPU tripped as well, a natural suspect is an electrical problem in the airplane. What is needed, of course, is a competent mechanic to troubleshoot.

APUs can fail for a number of reasons. They're just small turbine engines. In fact, for 747-100 and -200 aircraft, the APUs were often the same as the engines used for SA-226 and -227 Metroliners (with the addition of propellers of course) that I flew prior to flying 747s. Common causes of APU failure include overheating, loss of oil pressure, excessive load, and just plain mechanical breakdown.

The decision as to whether to dispatch a large airplane without an APU would be an economic one primarily. If you're sending the airplane to an airport that is known to have GPU power, it wouldn't make financial sense to delay the flight because of an inoperative APU. If, however, you're dispatching to an airport that doesn't have such, that would be unwise unless such an airport does have a huffer and you want to risk a battery start (never a good idea in my opinion).


What records should a CFI keep to prove they took due care in the quality of their instruction? I know there has been some cases where a flight instructor is sued after a current or former student does something that gets themselves (the student) killed or causes damage.

I know that there have been cases where a flight instructor has been acquitted by proving that they took due care in their flight instruction, and the accident was not a result of poor instruction.

What specifically would a prudent flight instructor do to ensure they are keeping detailed enough records to prove that they are providing adequate instruction and that a resulting accident would not be due to poor instruction?

What are the records a CFI is legally required to keep, and what are the records they should keep in addition to that?

This question is in context of CFIs in the United States.

Have a well defined syllabus, and then a daily activity sheet where you write down the lesson you did, the time it took, and how the student performed. If anything peculiar happens, make a note.

Have a well-written lesson binder too. Even if you don't need to use the binder to teach a lesson (like steep turns, that's not really much of a ground lesson), have it in there anyway to show that you know what you're talking about. If you have a page that only says "Teach steep turns" and a lawyer gets a hold of it, there's nothing to prove that you actually know how to teach steep turns.

AS for legally required records:

You must sign the logbook of anyone you have given flight or ground training.
You must maintain a record of the following:

The date and name of each person whose logbook/student pilot certificate you endorse for solo flight privileges (I keep a record of every endorsement).
The date and name of each person you endorse for a knowledge/practical test, and the kind, date, and result of the test.

You must keep the records for at least 3 years.


Is fuel in trim tank usable? Some aircraft (A380, Concorde,...) are equipped with a trim tank in the tail. Transferring fuel from or to this tank help controlling the CG. This feature can only be achieved if there is fuel to transfer. If tanks are nearly empty, and CG must be kept aft, this fuel could not be usable, except if there is another mechanism to compensate for the CG.

Is the fuel from trim tank always usable? Does it count when calculating minimum fuel requirement for a travel?

Here's a good pilot write up of the A380's fuel systems:

https://captaindave.aero/2017/09/07/the-a380-fuel-system/

Of particular note is this paragraph:

  Towards the end of the flight there are two additional fuel transfers. Any fuel remaining in the trim tank when the time remaining to destination drops below 80 minutes is pumped forward. Similarly, when the time remaining to destination drops below 30 minutes any fuel left in the outer tanks is moved.

So, clearly, on the A380 not only is the fuel usable but it's also a standard operational procedure to utilise it.

I would expect that any fuel trimming is either

Contingent on having more fuel outside of the trim trank. As that fuel depletes, the trim will change, necessitating (or at least allowing) the movement of fuel back away from the trim tank

or

The change in trim is about optimisation, rather than keeping the aircraft within safety tolerances. On a long trip in a large airliner, small changes in trim could have real impacts on efficiency and fuel usage without necessarily impacting control and safety.


Will the G1000 cut off transmission over 35 seconds? I'm new on G1000 system, it says;

  If the push-to-talk (PTT) Key becomes stuck, the COM transmitter stops
  transmitting after 35 seconds of continuous operation.

I'm wondering if we keep speaking on COM over 35 sec, will it stop transmitting? Or, if I keep PTT open without speaking, will it stop transmitting?

I haven't tried it for real with PTT, but from what I know about G1000, it has generic protection against any stuck keys (including those on remote keypads/panels). When a key is detected to be pressed for longer than about 30-35 seconds, it is declared stuck and the respective CAS message appears on the PFD. It is then temporarily disabled; however, when another press is detected, it will work again. You can try it yourself with any of the bezel keys.

In this vein, it doesn't matter if you speak or not. If you keep PTT pressed for longer than 35 s, it will cut the transmission, but if you release and press it again, it will start transmitting again.

(Note that most normal keys in G1000 are activated on release (this is very common in software). But if a key is stuck or you hold it for long, its function will activate after about 0.6-0.8 s anyway. Some keys have 'press and hold' operation (e.g. holding frequency transfer for 2 s tunes in the emergency frequency 121.5), and it will also execute after the elapsed hold time. And only much later the key will be declared stuck.)


What is the difference between frequency wild AC system and other AC systems? What is the difference between a standard AC system and an "AC wild" system? How does the latter work?

Frequency wild Unregulated AC is generated using alternators.
They were more common before constant speed drives were added to aircraft beginning in the 50's.

The alternator runs at various RPM (which varies the frequency) since it is driven by the engine.
— Wikipedia

The difference is that regulated AC is of fixed frequency. Better for motors, pumps, etc.
Unregulated AC was used for things like windshield wipers and windshield heating. DC inverters were used when regulated AC was needed.

Edit: On a plane like the ATR 72, it is indeed referred to as AC Wild.
It is used for heating the windshield, propellers, and probes. Among other systems that don't require regulated AC.

(Source)

ATR notes that discuss AC Wild can be found here. (Courtesy of FreeMan.)


What is the relationship between mountain breeze and air pressure? I am studying for my PPL and am having some problems understanding the mountain breeze phenomenon, especially when it comes to my understanding of pressure.

Basically, I've read that during the day mountainsides get warmer, resulting in the air flowing upwards. While at night this mechanism reverses : mountainsides get colder, and the air therefore flows downwards.

I could be ok with this, if I hadn't been told earlier on by my physics teacher, that in general high pressures tend to flow towards lower pressures. I.e warm temperatures flow in the direction of cold temperatures. This would imply that the breeze has to travel the other way around.

I can't really wrap my head around this. How can I reconcile both concepts ?

There is nothing to reconcile: the higher pressure is where the cool air is. The weather report shows pressure fronts, and the low pressure ones are the ones heated up by the action of the sun.

If the mountainside warms up, the air in contact with it warms up. Warmer air has a lower density and it wants to float upwards, like a hot air balloon on a cold day. Meanwhile, surrounding air not heated up by the mountainside flows into where the heated air was, going from cool to hot. The reverse happens when the mountainside cools down: air gets denser and it wants to sink - when it hits the mountainside it just follows that down.

In a vessel with a constant volume, the higher pressure is at the hotter gas. In the unlimited volume of the atmosphere, the gas can expand as it pleases.


Where to get information on FAA AIXM 5.0 versions of Special Activity Airpace (SAA) files? Does anybody know where to find documentation or someone to ask questions about the SAA database provided by the National Flight Data Center? I have some questions about the ambiguity of some of the data.

Or better yet, is there a better source for the boundaries and meta data for special-use airspace?

One possibility is to contact the FAA National Flight Data Center (NFDC). They provide information as part of the US Aeronautical Information Services (AIS).

NFDC Support Desk email is available on this login page, below the login data boxes.

See also this question: Where to find a waypoints/fixed points, navaids, airports database for other aeronautical data sets in case you need them too.


Can the FAA stop the municipality owned airport from closing down? I live in Wheeling IL. Our town owns the municipal airport there. Most of the residences here want to close down the airport. Can the FAA stop us from closing the airport?

Frequently, municipal airports are created and/or operated with funds granted by the Federal Government that contain contractual obligations that the airport remain operating until a particular date or certain conditions.

I don't know about that particular airport.

Even if a municipality owns the airport free-and-clear without contractual obligations to operate it, the FAA does have a lengthy process for shutting down an airport.

Since you're in Wheeling, you may be familiar with Meigs Field, which was abruptly closed against FAA rules, leaving airplanes stranded, charts incorrect, and airplanes attempting to land at an airport that didn't even have a functional runway anymore!


What are regulations for pyrotechnics on planes? Being in love with both fireworks and flying this is a question that I wondered about for quite some time.
Especially when watching videos like this (which is from 2009 showing a stunt plane flying stunts during night with sparklers on the wings and some single shots) or this (a glider with essentially the same effects like the stunt plane from 2015) I wonder what safety regulations apply.

Most certainly I am not allowed to duct tape some rockets to my wing and light them off remotely mid flight but I haven't heard about a firework rating or something similar either.

What requirements do I have to accomplish to be allowed to fly with pyrotechnics (mainly fireworks) on my plane? This would include anything additional directly in my PPL, permission by local authorities, special safety measures on the plane, etc.
This question is mainly regarding the european room (namely Germany) but I am also looking forward towards answers about regulations from other regions.

I'll answer for the FAA since thats the jurisdiction I know. Like most things some may consider this careless or reckless operation as per 91.13 which is the FAA's catch-all for odd occurrences that are not worth explicitly regulating.

  Sec. 91.13

  Careless or reckless operation.

  (a) Aircraft operations for the purpose of air navigation. No person
  may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another.

  (b) Aircraft operations
  other than for the purpose of air navigation. No person may operate an
  aircraft, other than for the purpose of air navigation, on any part of
  the surface of an airport used by aircraft for air commerce (including
  areas used by those aircraft for receiving or discharging persons or
  cargo), in a careless or reckless manner so as to endanger the life or
  property of another.

I cant find anything that says "you cant tie fireworks to your plane and set them off" but that is an unlikely thing to bother regulating.

The TSA does not allow fireworks to be carried on aircraft and this AOPA article would make it seem as though they are not allowed to be transported by air at all. I would think this includes mounting them to the aircraft so you may be out as a result of this. Your local FSDO may be able to provide some more info on this and what you may need to do to make this ok.

The other thing to consider is that the FAA may consider this "dropping" so you would need to make sure you conform to 91.15.

  Sec. 91.15

  Dropping objects.

  No pilot in command of a civil aircraft may allow any object to be
  dropped from that aircraft in flight that creates a hazard to persons
  or property. However, this section does not prohibit the dropping of
  any object if reasonable precautions are taken to avoid injury or
  damage to persons or property.


What is the proper procedure to change altitude when changing heading? I'm told that, depending on the heading of your aircraft, you have to fly at particular altitudes.  Which, of course, makes sense because having planes flying in opposite directions at the same altitude is just asking for a mid-air collision.

My question is this though:  What about when you are changing your heading?  Pretend, for example, that for some reason you need to go the exact opposite direction (so, we were going due west, but now we are going due east, for example.)  I know you have to gain or lose 1000ft.  But do you change altitude after the heading change, before, or during?

So: What's the proper procedure for changing altitude when you change headings?

Bonus points:

Is this made more complicated if you are flying Visual Flight Rules instead of Instrument Flight Rules?
Are all these rules thrown out the window in Class B and C controlled airspace?

TL;DR:

  Do you change altitude after the heading change, before, or during?

When flying under Air Traffic Control, you are expected to execute instructions after you receive them, following the motto Aviate, Navigate, Communicate. When receiving a turn and climb instruction at the same time, you are expected to start both, e.g.

 DELME, turn left heading 220, descend FL80

  What's the proper procedure for changing altitude when you change headings?

You do not change altitude or flight level on your own when flying IFR. When flying VFR, you should adhere to the semi-circular rules when changing your direction and end up in one of the semi-circular areas. On turn techniques that allow direction change and altitude change at the same time, see Tyler's answer.

  Is this made more complicated if you are flying Visual Flight Rules instead of Instrument Flight Rules?

Yes and No, because remember: You do not change altitude or flight level without being cleared or instructed to do so when flying IFR. Whether having to choose the correct altitude or flight level when flying VFR is more difficult than having ATC select one for you, is debatable.

Additional Information:

The rule you are talking about is the semi-circular flight level rules, which depending on the country, can either be applied West to East or North to South. Let's assume the example you have already used and differentiate between westbound and eastbound flights.

(Image Source: TU Wien)

The above picture shows that aircraft on a eastbound general track between 000° and 179° are bound to odd flight levels, such as FL210, FL230, FL250, etc.

Opposite traffic on a general track between 180° and 359° is bound to even flightlevels, such as FL220, FL240, FL260, etc.

This flight level you are bound to use is also the flight level you request in your flightplan, called the Requested Flight Level or RFL.

Imagine you have a flight going westbound and you have requested FL280 as your cruise level. You would maintain this flight level until you are cleared to descend by ATC. Even if a portion of the descent or arrival route leads in the opposite direction of your orignal track, it will be ATC who is responsible to assign the correct desend flight level, but these do not have to be necessarily following the semi-ciruclar flight level rules.

There is one scenarion where a pilot may want to initiate a flight level change due to changing jurisdiction and use of the semi-circular flight level rules, which is when crossing borders between countries that use west-east and north-south rules. If are on a westbound flight (approx track: 200° that requires even flightlevels, such as FL280 and cross the FIR border into a country with north-south rules that require an odd flight level, you would have a step climb or descent in your flightplan to accomodate this, e.g.:

[...] FIXXA UZ123 CROSS/N0265F290 UZ123 FIXXB [...]

It would still be ATC who clears you to climb to FL290 at that fix, but this way your flightplan is in accordance to local semi-circular flight level rules. Remember that when flying IFR, you never change the altitude or flightlevel unless you have been instructed or cleared to do so.

The same semi-circular flight level rules apply to VFR, just with the added 500ft to stay clear of the IFR flight levels. In most cases however, VFR does not entirely follow these rules, as the are VFR and their separation is based on visual identification and the see-and-avoid principle.

I am not familiar with airspace class B procedures, but in class C airspace, you require permissions to enter the airspace and will thus be assigned restrictions, which can be either a single altitude or flight level, an upper or lower restriction or a level or altitude band. If you are assigned either of the two latter options, you could apply the semi-circular flight level rules.

See below on an overview how other countries handle these semi-circular rules.

(Image Source: Delta Virtual Airlines - Not for real navigational use)

See also these related questions:
What is the difference between “flight level” and “altitude”?
What differences are there in Flight Level rules, internationally?


How should night time be determined and logged in a fast westbound plane? You take off from KFLL (Ft Lauderdale, FL) an hour before sunset in a B737. You're heading to KSEA (Seattle, WA).

4:35PM EST plus 6 hours 34 minutes is 11:09PM EST, or 8:09PM PST.  Seattle sunset was 4:22PM PST.

Since the flight went west quickly it delayed sunset. Since it went north in the winter it hastened sunset.

So how does the flight crew split their day and night flying portions?

Record UTC when the PIC declares it to be night, split accordingly
A computer just tells them the answer
The flight crew already have ATP ratings, they don't care for detailed logging anymore
Something else

All of my flying has been roughly north/south at C172 speeds, so it hasn't really mattered more than the rounding error of recording in 0.1hr increments.

I don't think many airline crew actually bother to log the day/night breakdown.

At my airline we have different pay rates for day and night flying, so we do care, but the company "Electronic Logbook" tracks the day/night breakdown for us. The system is not 100% accurate and we sometimes get paid more or less day or night then we actually fly in, but it tends to average out over the year.

In the summer, polar routes can be as much as 15 hour flights of daylight, while the same route in the winter can be 15 hours of night flying.

On some flights we will see the sun set, and then rise, and set again, as we chase the sun and our route changes from northwest bound to southwest bound.

Here are some samples of summer/winter, day/night Toronto to Hong Kong flying in my eLog:


Do jetliners carry radar that can detect other aircraft? If not, why not? For example, if a civilian jetliner is flying at night over the Ocean, or a remote area with no airports/ground radar stations, and another airplane is also heading towards it on a collision course, shouldn't the pilot be able to spot the oncoming airplane on its airborne radar to perform evasive maneuvers? My understanding is that military aircraft have such airborne radar, which I believe is called ACAS, so they need not depend on ATC/ground stations for collision avoidance.

Radar that can detect, resolve and track small targets as opposed to large areas of water (raindrops) has to be a lot more powerful and sophisticated, especially in ye olden tymes of the 50s and 60s when weather radar was becoming a thing and the search for a practical collision avoidance system was underway.
When the effort that eventually became TCAS got started in the late 50s following several notable midairs, on-board target detection was one of the early proposals but was rejected by the industry as too expensive and time consuming to develop (where to put the more powerful system with its larger antenna for starters).
Once TCAS was perfected, made possible by the development of Mode C altitude reporting by transponders, you got 95% of the benefit (because only non-Mode C-transponder light planes were still a risk - a very minor factor overall once Mode C was pushed out to GA aircraft) without a crazy expensive addition of on-board target detection and tracking radar PLUS a weather radar.


Could a jet engine be made from cast aluminium? What materials are used to make the high temperature-high pressure bearing parts in a jet engine? I'm asking about the combustion chamber, combustors, nozzle and diffuzers (as in ramjets) rather than turbomachinery. Could cast aluminium or cast steel do the job for a low thrust small model engine?

Short answer: No.  Unfortunately the yield strength of typical aluminum alloys drops off after about 300° F and long term exposure to high temperatures dramatically affects their creep and fatigue lives.  Typical aviation gas turbine hot sections run between 3000°-3800° F and rotate at angular speeds as high as 40,000 RPM, making Aluminum totally unsuitable for this role.  Most hot section components eg cumbustors, stator buckets, rotor buckets, etc.  are manufactured from exotic nickel-cobalt superalloys and employ novel single crystal casting techniques as well as passages to bleed compressor air as a boundary layer over them to protect them from that kind of intense heat.


How do you cope with an asymmetric flap extension? How do pilots cope with an asymmetric flap extension? What are the procedures for this type of occurrence which I suppose are similar on all aircraft?

I'd guess the procedures probably vary considerably by class and type. If the flaps on one side broke or got stuck on the light GA plane I mostly fly, the procedure would be to just set the side you can control to the same as the side you can't control and land with that amount of flaps. You can land it with no flaps; you'll just have to land a little faster than normal and with sub-optimal visibility of the runway (but still plenty to get the plane onto it safely.)

At least in the planes I've flown, it would be nearly impossible for the flaps on both sides to get stuck in different positions (because both sides are mechanically connected to the flap lever,) but even that could be dealt with by using the ailerons to counter the asymmetry of lift.

The other procedure involved is to do a proper pre-flight and make sure your flaps aren't broken, loose, or damaged before you leave the ground. :)


Is there an explanation for the measurement system used in aviation? Why is speed measured in knots (as opposed to kmp/h or mp/h), and distances in nautical miles? Is there a reason for following such a system?

To quote from the Wikipedia article on the subject:

  Although the unit knot does not fit within the SI system, its
  retention for nautical and aviation use is important because standard
  nautical charts are on the Mercator projection and the scale varies
  with latitude. On a chart of the North Atlantic, the scale varies by a
  factor of two from Florida to Greenland. A single graphic scale, of
  the sort on many maps, would therefore be useless on such a chart.
  Since the length of a nautical mile, for practical purposes, is
  equivalent to about a minute of latitude, a distance in nautical miles
  on a chart can easily be measured by using dividers and the latitude
  scales on the sides of the chart. Recent British Admiralty charts have
  a latitude scale down the middle to make this even easier.[8]

  e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002


Can planes benefit from Mitsubishi's Air Lubrication System? Mitsubishi's Air Lubrication System blows a fine sheet of bubbles below the hull of a ship. The bubbles form a "carpet" on the hull, reducing friction. See Gizmag (yes, sorry) for some more information.

Let's assume that such a similar mechanism - something like the surface of an air-hockey table perhaps - would be feasible to implement on an aircraft.

What aerodynamic effect would it have? Could it, assuming it were feasible, provide any benefit to an aeroplane/helicopter/dirigible?

NASA tested a similar concept. They call it Active Flow Control, or AFC.

A full-scale 757 tail, equipped with active flow control, has demonstrated increased rudder effectiveness in wind-tunnel tests by Boeing and NASA that could lead to smaller, lower-drag vertical tails.
"Sweeping jet" AFC actuators were mounted on one side of the fixed stabilizer, just upstream of the rudder hinge line to blow on to the leading edge of the deflected surface. The 37 actuators were supplied with variable mass-flow pressurized air from an external source and were individually addressable so that different spacings and zones could be tested.

Tail fins are huge to help compensate for engine failures. But with such system blowing pressurized air out of holes upstream from the rudder, manufacturers can design smaller tails to save on drag and weight.

Another system that works by suction (not blowing) at the leading edge is on the 787-9/10. It is used for both the horizontal and vertical stabilizers on the -9, but Boeing dropped the horizontal stabilizer system on the -10. There were plans to use it on the upcoming 777X, but Boeing dropped it. That system is called hybrid laminar-flow control (HLFC).


Are cabin dividers used to "hide" the flex of the airplane? Recently onboard an aircraft, a fellow passenger told me that the cabin dividers (walls) inside an aircraft are there to limit your line of sight such that you will not notice the flex of the airplane.

I know that some walls are there because of the presence of a lavatory or are actually used to create a barrier between Economy class and Business class. However as can be seen in the photo below, the circled wall is doing none of the above.

Was my fellow passenger right?

Was my fellow passenger right?

No.

That photo is taken in a B777-300 (2 aisles, 4 seats in the centre and three at the sides, lavatories only behind the central rows, rows on the left side of the aircraft are one more than the central ones at that location):

As you can see the wall is just were an emergency exit is, and if you will look on the other side of the wall you will find a "jump seat", i.e. a foldable seat used by the crew, and the wall is there for it.

Also, walls help in dividing the aircraft in different classes.


Why are in-flight entertainment systems not very evolved? Why don't in-flight entertainment systems have the capability to connect to our own devices and play movies? Or why don't they have arena gaming?
Are there any regulations that prevent engineers from designing innovative in-flight entertainment systems?

I mean, what factors come into play - regulations, cost, weight, complexity?

It's expensive. If you wanted to build something like an XBox or Playstation into each seat,  you're talking about an absolute minimum of \$300 per seat (\$60,000 for a small-ish aircraft like a 737, \$160,000 for a typical A380, and probably much more). While that doesn't sound like a lot on the price of an airliner, it certainly adds up over a fleet of hundreds of aircraft... all of which means you have to add to the ticket price and become less competitive. And that's just talking about gaming.
Safety/security. Allowing the passengers to connect to the system, no matter how well designed, is a potential security hole. This can be avoided to some extent if you completely separate the systems, but there's always potential for someone to find a loophole you didn't think of (overloading the in-flight entertainment and messing up the electrical power, for example). Dealing with this would add significantly to the cost above.
There isn't much need to. Most people who fly are either happy with the in-flight movie choices, or bring their own iPad/Kindle etc.
While using Airplay to send movies from your iPad to your TV works pretty well for one or two devices at home... it quickly falls apart when you're trying to make a wireless network capable of handling 200-800 streams at once!
General safety concerns: despite the fact that we're now fairly confident that having electrical devices on with their WiFi/Bluetooth/4G off is safe, to send the movies they'd need to turn WiFi or similar on, which is a potential risk

In short it's a lot of risk and cost, for not much reward. If you have an iPad with you, why do you need to send the movie to the TV screen in front of you? You can just watch it on the iPad.

Note also that some airlines do have some basic multiplayer gaming built into the IFE: although it's more "Connect 4" than "Battlefield 4"


What are the FAA regulations on lighting for ultralight vehicles? I'm in the process of scoping out a project involving an ultralight vehicle and some Neo pixel LEDs.

In the US, there are many regulations for lights on cars. Things like the location on the vehicle, the direction, color and blink rate are all regulated.

What lighting laws apply to ultralight vehicles, as far as location, direction, color and blink rate?

Ultralights are regulated under 14 CFR 103 and there are very few regulations of any kind that apply to them.

The only specific lighting requirement is in 103.11(b):

  §103.11   Daylight operations.

  (a) No person may operate an ultralight vehicle except between the
  hours of sunrise and sunset.

  (b) Notwithstanding paragraph (a) of this section, ultralight vehicles
  may be operated during the twilight periods 30 minutes before official
  sunrise and 30 minutes after official sunset or, in Alaska, during the
  period of civil twilight as defined in the Air Almanac, if:

  (1) The vehicle is equipped with an operating anticollision light
  visible for at least 3 statute miles; and

  (2) All operations are conducted in uncontrolled airspace.

This question and this one have some information about what an anticollision light actually is (a beacon and/or strobe), but since ultralights are so lightly regulated I'd be surprised if anyone cares much about the exact form of lighting. However, you could check with your local EAA chapter or ultralight club to see what people typically use.


Engine options for the 777-300ER: GE90-110B1 or -115B? The Wikipedia page on the Boeing 777 says that the 777-300ER can have either the GE90-110B1 or the GE90-115B, however it was my understanding that the 777-300ER does not have engine options and is only available with the GE90-115B.

Furthermore, this unofficial production list confirms that there are no 777-300ER with the -110B1.

With that said, here's my question: has the 777-300ER ever been offered with the -110B1, whether as a factory option or as a retrofit/modification?

According to the EASA Type Certificate Data Sheet (TCDS) of the Boeing 777, only the GE90-115B engines are approved for the 777-300ER:

5. Engines:
Two (2) General Electrical GE90 Turbofan Engines
Models installed: GE90-115B,
EASA Type-Certificate No.: EASA.IM.E.002
Limitations: See Engine Data Sheet No. EASA.IM.E.002

The GE90-110B1 model is listed for the Boeing 777-200LR and 777-200F models. In the Wikipedia table, the engine entry is for all of these models together.


What does two 'high-low' chimes in the cabin of a 737-800 mean? Recently on a trip to Orlando on Sun Country, I came across a cabin chime that I'd not heard before. It sounded like two 'high-low chimes' in a row, so basically equivalent to someone ringing a doorbell twice.

I know that one 'high-low' means that the FAs are speaking to each other through the phone in the aircraft, but I've never heard it twice in rapid succession.

Are all 737-800s equipped with these chimes? What do they mean?

As far as I know, the chimes work the same on all 737 NG (and Classic) versions, although as was mentioned in comments, how they are employed will vary from airline to airline.

The two tone "high-low" chime described in the OP is the "crew call" system. The pilots have a button, the forward F/A station has one, and the aft F/A station has one.  Regardless of which button is pushed, the "hi-low" chime is the same.  In many, though not all, cases, this chime is a signal to the F/A's to answer the interphone.  If the pilots press their button, they're asking to speak to the F/A's; if one F/A station presses the button, they're asking to speak to the other F/A station.  (They can also press a different button that calls the cockpit, but passengers won't hear that chime. In the cockpit, we hear that "call" as the two-tone chime; we don't hear the chimes in the cabin unless we're listening to PA audio.)

At one carrier, a single high-low chime may mean "answer the interphone," while at another carrier, one chime might be used to mean "secure the cabin" while two chimes is "answer the interphone," three chimes is "answer the interphone - urgent," and so on.

You'll also hear chimes when the F/A Call Button above a passenger row is pressed; this is a distinct chime (a single, relatively high note).  There is also a chime when the Fasten Seatbelt sign is turned on or off, this is again distinctive, a single lower note.

Note that you won't necessarily hear the chimes when the interphone is in use; it's simply a "Hey!" call to the Flight Attendants in the cabin.  While it's often followed by a conversation on the interphone, they two are separate systems and one can be used without the other.


How can I calculate the cabin pressure following a decompression? What are the calculations to determine the level of cabin pressure at a given altitude once the cabin is punctured?

I was unable to determine the window size and the flow rate of the compression system on the recent Southwest accident aircraft, for example.

You should read through this paper it outlines a lot of great info on calculating valve size for cabin outflow valves in pressurized aircraft it also addresses your second point (to an extent) on the flow rate of compression systems. The issue you will encounter is that the calculations will only hold for a controlled situation i.e. a situation where you have precise control over the valve (hole) size and can account for small leaks.

In the most generic terms you can find the calculation for flow rate through a hole in a pressure vessel here.

Your specific example is going to be a bit more complex since after a cabin pressure failure the plane will begin a descent rapidly to 10,000ft. so your exterior pressure will be rising as your interior pressure drops (you will need to factor that in to make the above solution a complete result). You also need to consider the airflow over the airframe its self since you are talking about a situation where the aircraft is in flight you have some fluid mechanics to play in as well.

Its called explosive decompression for a reason, its going to happen very very fast.