Why would maximum fly-time (endurance) not coincide with max L/D operating point? This question is motivated by another SE Aviation answer where @Peter Kampf writes the following two useful bits of information:

  Aircraft like to fly near their optimum L/D ratio, where drag reaches
  its minimum.

  Observation aircraft which want to optimize flying time will fly
  slower that what optimum L/D requires, especially if they use
  propellers.

Do aircraft use the majority of their fuel to overcome friction?

I'm trying to understand this more: Why would maximum fly-time not coincide with max L/D operating point?

Is this something to do with the jet engine fuel consumption characteristic? i.e. Does the minimum fuel-consumed-per-unit-time point not coincide with the optimum L/D point? But if your payload is fixed, so is your lift & hence your velocity choice that gives you that lift right?

i.e. For a given payload, lift is fixed. For a given lift, minimum drag leads to max fuel economy? And max economy means max dwell time? What gives.

For simplicity I assume we can ignore the climb / descent / acceleration etc. for now. And only consider the level flight segment at fixed speed.

Maximum dwell time or maximum endurance occurs when the power required is minimum. Hence, in this case, the maximum endurance speed is one where the power required is minimum, while in case of maximum range speed, the thrust required is minimum.

For maximum endurance, we must minimize the fuel consumed per unit time i.e. the fuel flow. For maximum range, we must minimize the fuel used per unit distance traveled.

In case of propeller aircraft, the fuel flow rate is proportional to the power produced. Hence, the maximum endurance occurs at a point where the power is minimum. For (turbo)jets, the minimum fuel flow occurs when the thrust is minimum. Hence the maximum endurance occurs when the L/D is maximum. For turbofans, it is somewhere in-between.

Consider a propeller aircraft in a steady, level flight. For determining the condition where the energy expenditure is minimum, we have,

$P = W (\frac{C_{D}}{C_{L}})V$

is minimum. For steady flight, we have,

$V = \sqrt{\frac{W}{\frac{1}{2} \rho S C_{L}}}$

This gives,

$P = \sqrt{\frac{W}{\frac{1}{2} \rho S}}(\frac{C_{D}}{C_{L}^{\frac{3}{2}}})$

Thus, for propeller aircraft, the minimum power and maximum endurance occurs when $\frac{C_{L}^{\frac{3}{2}}}{C_{D}}$, rather than $\frac{C_{L}}{C_{D}}$ is maximum. Due to this, the minimum power (maximum endurance) condition occurs at a speed which is 76% of the minimum drag (maximum range) condition.

Image from eaa1000.av.org

Also, see here and here

Thrust is a force that moves the aircraft. In steady, level flight, this is equal to the drag (if it is more/less, the aircraft will accelerate/decelerate). Power is the rate of doing work i.e. the energy consumed per unit time or rate of energy expenditure (by the a/c powerplant). This is why we are considering minimum power i.e. rate of energy expenditure for determining endurance.

Power is the product of force (thrust) and velocity. Think of it in this way- as speed increases, the drag decreases, reaches a minimum and then increases. However, as the power is product of drag (i.e. thrust) and velocity, it too follows the a similar path; however, the minimum is reached before the minimum drag. That speed gives the maximum endurance.

For jet engined aircraft, the speeds are different. In this case, the speed corresponding to minimum $\frac{C_{L}}{C_{D}}$ gives maximum endurance, while the speed corresponding to $\frac{C_{L}^{\frac{1}{2}}}{C_{D}}$ gives maximum range. Also, see here


Do helicopters need a class B entry clearance when air / hover taxiing? You're a helicopter pilot at a class B airport. You'd like to taxi to another parking spot. The ground controller could either give you a hover taxi clearance or an air taxi clearance, depending on the situation. Either way will get you off the ground and (technically) gets you to enter the class B airspace. Would you need to obtain a class B entry clearance prior to requesting your taxi clearance?

No. Hover taxiing with a helicopter follows the same regulations as taxiing with an aircraft (or helicopter) on wheels.


How can air launched UAV's be decelerated? I am working on disaster relief drones for a college project. I was thinking about having multiple set of drones launched from an aerial vehicle over the disaster prone area. That's when I came across a big challenge.

How would you effectively decelerate a UAV that is air launched from an aircraft which is flying at a much greater speed than the maximum allowable speed of the UAV?

The stall speed of the aircraft is much larger than the maximum speed of the UAV (assuming, it has to be a very low cost UAV and hence the idea of having a parachute for deceleration is not very effective).

I would be glad if anyone could suggest me simple and robust ways to do achieve this.

......the same way you decelerate any other kind of aircraft.  Once released from the Mothership at idle power, it slows down until reaching an appropriate cruise speed thence power is increased to maintain speed.

If the release speed of too high for the UAV, one might use some kind of a frangible capsule equipped with a drag chute or other high drag device of some sort to enclose the drone during launch, thence the capsule fractures once the speed is slow enough, releasing the drone to begin flight under its own power.


Why is the fuselage on an airliner circular-shaped? Consider the fuselage of an A300. I have noticed this not only with airliners, but also for private jets and turboprops: why is the fuselage circular-shaped? Why don't aircraft have square-shaped fuselage? Does it have to do with too much drag?

If you pressurize any hollow structure, it will try to assume a round shape. If you want to create a lightweight pressure vessel, again a sphere will be the most efficient result, because there the stresses in the skin will be equal at every point. Blow up a party balloon if you are in doubt.

A sphere is not the most efficient shape for aerodynamics, so fuselages are elongated spheres with a nice fairing at the end. If you make them cylindrical in the middle, you can build most bulkheads on the same jig and can interchange sections of the internal fairings. Plus, if you need a longer or shorter fuselage for the next version of the plane, you can easily add or remove sections - the parts will still fit after the modification.

Note that the fuselages of unpressurized aircraft don't follow this logic A Short Skyvan or a Dornier 228 have fuselages with a rectangular cross section, so large cargo can fit in.

Also, some fuselages are combinations of cylinders. The Boeing 377 Stratocruiser used the lower fuselage of the B-29 and had an upper, larger cylinder on top to give the passengers more room.

Boeing 377 cross section (picture source)

For the same reason, rocket stages are cylindrical, too. They also need to be aerodynamic, have to tolerate high internal pressure and need to be lightweight.


Why do planes not fly "backwards" (up from the planes POV) when in a vertical climb? As suggested by a user in this question, I'd like to ask this question:

One of the most basic aerobatic maneuvers is a vertical climb. This can be achieved by just pitching up to what appears to be 90°.

Assuming it is really a 90° pitch up maneuver, wouldn't the lift of the wing (though smaller due to less speed) and the missing gravity to counteract the lift make the plane go "up" (again, planes point of view!)?

Short answer: Yes. But only for a moment.

As you suggest, let's focus on the 90° case, because many things will become simpler. Intermediate climb angles can be covered by a linear superposition of horizontal and vertical flight.

Now we shoot up at 90° flight path angle (engine thrust permitting) and leave some angle of attack for the wing to create lift. Lift acts orthogonally to the local flow direction, so the lift will pull the aircraft sideways (up in the aircraft's frame of reference). Since there is no other force which will oppose this lift (drag, weight and thrust all act at 90° to the lift, parallel to the flight path), the aircraft will accelerate sideways (again, up in the aircraft's frame of reference). The angle of attack will immediately decrease and lift creation will stop — or the pilot forces the angle of attack to stay constant by pulling the elevator, and now the aircraft will further increase its flight path angle.

That is a complicated way of saying that the pilot forces the airplane into a loop. If he wants to maintain this 90° flight path angle, he better avoid creating any lift.


What safety rule replaced the now-rescinded two-person rule (after Germanwings 9525)? In the aftermath of Germanwings 9525 in 2015 many airlines instituted a two-person rule for the cockpit (aka "two-man rule") - with a flight-attendant replacing whichever pilot left the flight-deck.

In 2017 the industry rescinded that rule. Articles about the repeal of the rules don't mention what alternative processes are being used to prevent another occurrence of sole pilot murder-suicide - or a hypothetical where both pilots have suicidal intent - or when a single pilot is incapacitated while the other pilot is out of the cockpit for medical reasons or other reasons?

The problem being that while the reinforced cockpit door can be opened with an access-code, the occupant of the cockpit can countermand (indefinitely?) the access attempt. Has that policy been changed?

What safety-rule or practices are being instituted to protect against this - or is the pilot murder-suicide scenario being removed from the threat-model entirely? (so if I see one of the pilots banging on the cockpit door while in flight I should resign myself to my fate?)

The French accident investigation board, Bureau d'Enquêtes et d'Analyses pour la sécurité de l'aviation civile (BEA), investigated this incident. The report was completed in March 2016. Among the recommendations for EASA was better medical and psychological monitoring by the aero medical examiners. Explicitly not recommended was the already mentioned practice of the two person rule, which leads to the security door being open too many times.

An aero medical examiner recommended that the pilot should have been committed to a mental facility two weeks before the incident. This fact only surfaced during the investigation. So his mental state was detected but because of lax institutional rules, nothing happened. The report can be found at bea.aero.


Where can I find details on the control surfaces of the F-15C? Me and my team were working on a short animation and I need precise details on the following if you guys can point me to a website or answer them here it'll save hours off my search cause Google isn't helping much.

The different stages of the flap extension with their degree of rotation.
The maximum degree of the ailerons going both ways
The maximum degree of rudder both ways
The maximum the elevators can go both ways

And as a big bonus can you provide me with the landing gear specs as in how long it takes to extend, retract, open the great doors and so on? Or a video of an actual F-15C retracting and extending would be great. Not one of those mini-prototypes.

Aileron: +/- 20 degs

Horz. tail: 26 deg up, 15 deg down

Rudder: +/- 30 degs

See: https://www.nasa.gov/centers/dryden/pdf/87906main_H-1073.pdf


Gage or Gauge? Which is correct in the aviation industry? Gage or Gauge?

Some official aviation sources seem to use the spelling "gage" whether referencing cockpit instruments or aircraft maintenance tooling.

However, more broad English sources note that "gage" is either interchangeable with "guage," or flat out incorrect.

Is either spelling technically correct, or only one?

Do the different spellings hold different meanings or connotations in aviation?

Are there official examples that could make the argument "gage" could be correct in specific circumstances (patents, manuals, industry standards, trademarks)?

Is there a history in early aviation using "gage" that has been carried over unofficially to today (like how older aircraft operating today may have mph on their airspeed indicator)?

Gauge is correct. As in Oil temperature gauge, Fuel Pressure gauge, Fuel Level gauge, Cylinder Head Temperature gauge, Exhaust Gas Temperature gauge, etc.

Anyone use gage is misspelling the word.  Same with Hangar. (Not hanger)


Why does Vmca decrease with increasing altitude? A question from a mock MEP class rating test:

Many thanks for your help.

For airplanes with normally aspirated engines or turbocharged engine above their critical altitude, the engines produce less power as altitude increases as atmospheric air density decreases as altitude increases.  As power decreases, the operative engine requires less rudder authority to maintain directional stability due to the reduction in thrust, which allows the airplane to maintain directional authority at lower indicated airspeeds.  Therefore Vmca decreases as altitude increases.


Could a blimp maneuver using a sail? How do you build a sail for a blimp/hybrid airship and what would it look like? Similar to a sail boat how could a blimp maneuver without propellers? Can a blimp sail against the wind? Wind can travel in opposite directions over the same location depending on altitude.

Jet stream patterns.

https://earth.nullschool.net/#current/wind/isobaric/500hPa/orthographic=-41.18,34.22,610/loc=-58.714,40.895

On worldbuilding.SE: Could a city be built out of Balloons?

Sailing vessels work by exploiting the motion difference between the air and the water.  The sail holds the air and the keel holds the water.

Unless you have some severe wind shear (which comes with its own problems), or you have truly massive sails, there's no gradient for you to exploit because it's all air.


What is the purpose of D-Bag in skydiving parachute? Recently I had the opportunity to take a closer look at the skydiving parachute. And I could not explain to my self the reason to use D-Bag and not pack main canopy directly in backpack compartment.
I know that the reserve canopy is packed directly into the backpack. And the sequence for deploying reserve canopy is:

Sky diver pulls release cord, which pulls out the pin that holds the backpack flaps close
Spring loaded drogue parachute jumps out into the airstream
Drogue chute pulls out the main canopy which opens
Sky diver does not flatten him or herself on the ground.

More puzzling is the main canopy assembly. As far as I could understand the main canopy is packed into so called D-Bag. This D-Bag is then inserted into container in the main backpack. Why this intermediate step?
The deployment sequence of main canopy is described here: http://adventure.howstuffworks.com/skydiving4.htm

I wonder what happens to the D-Bag after deployment. Does it hang loose? Does it remain close to the canopy on the upper side?
As far as I understand the drogue chute remains attached on the back side of the canopy and acts as a sort of stabilizer. But what about D-Bag? Does it just flap around?
Why not pack main canopy in separate compartment of backpack directly?

Qualifier: I am a master parachute rigger.

The bag serves two functions - it allows the suspension lines to straighten out in a controlled manner, and it prevents the canopy from inflating before the lines are fully stretched out.

It is not always required - people who want a very fast opening can pack without a bag, I have done this many times myself. However, you do NOT want to do this after a drop of more than about 5 seconds. These jumpers open the main canopy immediately after exiting the aircraft.

On main canopies the bag and pilot chute are connected to a ring on the top of the canopy. The assembly just follows behind. Some canopies have a mechanism to collapse the pilot chute and/or retract it onto the top of the canopy.

The bag for the reserve canopy is not connected. This allows the reserve to still deploy if the pilot chute gets caught on something else, the bridle is extra long (longer than the suspension lines). It falls separately and usually lands within a kilometer of the airport, near the main canopy if that one was released when it malfunctioned. As they cost about $200 a set the jumpers will put considerable effort into finding it. We usually don't find it in high crops, and we usually don't try to find it at some places in Florida where there is an alligator ranch beside the airport.


Why not use flaps in take off from High Density Altitude airport? Why can we use flaps in short field take off but not in high density altitude airports?

Why can't they be used in high density altitude to shorten the take off distance as in short field take off?

Why short field take off with flaps doesn't apply to high density altitude airports?

With more flaps, you will lift off at lower speed, but will have worse climb performance due to the increased drag.

On short field you are limited by the field length, so you need to lift off early and therefore need to use more flaps.

But in the mountains you are limited by the climb performance. Therefore you need to use less flaps.

Of course, this is just general rule. You should look up performance for given density altitude and compare it with available runway length and required climb gradient.

Update: Extending flaps never improves rate of climb. Up to some points it does improve angle of climb though, because it reduces Vx and the engine can produce more thrust at lower speed. The setting for best angle of climb is higher than for normal take-off and is used on obstructed field.

However as excess power reduces (with density altitude), Vx increases towards Vy (at absolute ceiling, Vx = Vy and rate of climb is 0) and the flap setting for best angle of climb decreases. So even for best angle of climb you shouldn't use as much flaps at high density altitude as at low.

And most mountain airports are not obstructed. In most cases the valley ahead is large enough, but you want to get above the tricky winds fast and you want high rate of climb in case you encounter a downdraft. And that means no or little flaps.


What are these panels underneath the wing root of a A380? What are these open panels underneath the wing root of a A380 (right above the person standing under the aircraft)? Are they ram air exhausts of the air conditioning packs?

They are indeed the Ram Air Outlet Doors.

(source, showing Ram Air Inlet (RAI) and Ram Air Outlets (RAO))

From the A380 FCOM (21 - Air Conditioning):

  PACK DESCRIPTION
  The hot bleed air flows into each pack, via the two pack valves, then enters the heat exchanger. This
  heat exchanger precools the air using external air. This external air enters through a ram air inlet door,
  and is then discharged overboard through two ram air outlet doors.

The reason they are located in the wing root is that the entire packs are in the wings. From this article:

  The A380's wings are big enough that they can hold the plane's air conditioning packs. That, says Airbus, is an advantage over most other large planes because the packs don't have to be stored in the A380's belly fairing. That, in turn, means the plane's fuselage is more streamlined than it would otherwise be, reducing its drag.


What kind of Gyros are being used in smaller regional jets? As per my understanding, most of the Gyros in big jets are electric driven but I am not sure if the smaller planes also have the electric driven or vacuum/pressure driven Gyros?

Yes small planes have electric gyros as well. It can be as sophisticated as Garmin 1000 in Cessna 172 or Russian made electric artificial horizon gyro powered by accumulator in a glider Blanik L13. I heard battery didn't last very long though:) Also all turn rate indicators have gyros and the ones I saw were electric powered. This ensures that you can still survive if a vacuum pump fails. Turn indicators used in the eastern block made gliders are powered by simple 4.5V battery.


Are there types of instrument approaches besides precision and non-precision? In the 1990s there used to be two basic types of instrument approaches: precision and non-precision. How many different kinds are there now, and what are the differences.

All approaches can still be categorized as a precision or non-precision.  Some of the terminology has changed, radar approaches are becoming less common and and GPS approaches are becoming more common, but the fundamentals are the same.  Precision approaches still provide glideslope guidance, and non-precision approaches do not.

It may be important to note that many flight computers blur the lines some with GPS approaches.  There are approaches where the plate still calls the bottom altitude an 'MDA' (instead of 'DH' or 'DA') as you would in a normal non-precision approach, but the flight computer will generate a custom glide slope to that point.  Because this is generated artificially inside your aircraft and not schematically planned by a certified official, this still only qualifies as non-precision.

All in all, if you're trying to pick up instrument flying after a long break, you'll still be good to go with your basic understanding of precision versus non-precision approaches.


What aerodynamics forces are acting on a jet fighter during slow flight demonstration? I have long been contemplating the forces involved in certain maneuvers in an aeroplane and some of those maneuvers begin to break down my understanding of the forces that specifically oppose gravity. Typically this is described as lift and that is how it is explained in ground school, but they did not go over other forces that can be involved keeping a plane in the air (i.e. a non-horizontal component of thrust etc.). They probably didn't go over this as it doesn't apply (significantly) to the type of planes you're one would fly right after basic ground school.

One of those maneuvers I've thought about is a "slow flight" when performed at an air show by a jet where the jet appears to be going slower than their stall speed and at a very high angle of attack. It seems that a portion, if not all of the force keeping the airplane airborne is coming not from the wings in the form of lift but rather the thrust produced by the turbine. (Example:

            )

Is my understanding accurate?
Is there a good, well known reference to aerodynamics that does not gloss over the entire gamut of forces involved during a wide range of flight situations?

break down my understanding of the forces that specifically oppose gravity.

There are no separate forces that oppose specific other forces. There is simply a set of forces and they all add up and the sum divided by the inertial mass of the plane equals acceleration.

Gravity always points down, so to prevent downward acceleration, the sum of the other forces must point up. But whatever force points up can do that.

  Is my understanding accurate?

Mostly yes. Thrust always points forward along the engine axis, so if the engine axis points up, so does thrust.

  Is there a good, well known reference to aerodynamics that does not gloss over the entire gamut of forces involved during a wide range of flight situations?

There is really just a few forces, always:

The airfoils produce force that is approximately perpendicular to them. It grows with angle of attack (angle between the airfoil and the air flow) with one peak at the at the stall angle of attack (usually 10–15°) and another at 90°.

This force is normally described as two components, lift, which is perpendicular to the air flow, and drag, that is parallel to it. That means the second maximum of lift is at 45°, because at 90° the force is parallel to the flow and thus all drag.
Form (parasite) drag. This is parallel to the flow.
Thrust. This is always approximately forward along the engine axis.
Weight. This always points down. It is actually composed of gravity, which points towards the centre of Earth, and small contribution of centrifugal force that points away from the Earth axis of rotation, but that's just nitpicking.

And then you just need to do vector addition.

So when the aircraft flies with high angle of attack, the thrust points diagonally up, cancelling some of the weight, while the force on the wing is tilted aft, cancelling the other part of weight, but also causing huge drag, so the thrust needs to be correspondingly large. That's why only fighters and aerobatic aircraft with their large thrust/weight ratios can do this kind of stunt.

For more details I would suggest See How It Flies by John S. Denker. It covers basic aerodynamics and explains reasons behind common piloting techniques. Chapter 4 discusses the forces.


Is the Runway Visual Range (RVR) communicated to the pilots? During CAT IIIA/IIIB approach where the decision height is 50 ft/0 ft, if the Runway Visual Range requirement for the approach is not met, what is a pilot to do?

I mean, 50 ft (or even 100 ft?) seem so low enough  that a go around can not be initiated. Unless the pilot knows the RVR beforehand how can he be sure to perform CAT III approach?

Pilots get the RVR values either from weather reports or directly from ATC. RVR also shows up on ATIS reports when visibility drops to less than 1 mile. ATC will have a realtime readout of RVR and can communicate these values to pilots at any time by radio.

ATC RVR display

Source: http://mtechsystems.com

The CAT III approach should only be commenced if the ceiling and RVR values meet the minimum for the approach in use and for the current aircraft equipment status. Some airlines will have their own minimums which can differ from the published CAT III minimums.

Pilots will often ask for the latest ceiling and RVR values while doing the approach. ATC will also announce any changes or reduction in ceiling or RVR values while aircraft are on approach. If the new values are below limits, the crew should commence a go-around.

A go-around can be done from any altitude and the wheels can touch the runway during the go-around. Low atlitude, "Low Energy" go-arounds are practiced during re-current Simulator training.


How can I calculate the Obstacle Free Zone of an aerodrome? How can I calculate the Obstacle Free Zone of an aerodrome?

AS per ICAO Annex 14, Obstacle free zone (OFZ). The airspace above the inner approach surface, inner transitional surfaces, and balked landing surface and that portion of the strip bounded by these surfaces, which is not penetrated by any fixed obstacle other than a low-mass and frangibly mounted one required for air navigation purposes.

For easiness it is a rectangular area around the runway which extends 60M for Code 4E runway and 77.5M for Code 4F runway from runway centre line sideways and 60M from the runway thresholds.


PBY-5A exhaust modification? What is the function of these “tubes” that cover the outboard exhaust stacks on some PBY-5A and PBY-6A aircraft?

Thanks for the suggestions in the comments.

Using the term “heat exchanger” I was able to find an Aircraft Modelling forum that gave the answer I was looking for.

Interpreting wartime photography: Catalina in Ceylon

It is indeed a heat exchanger system for leading edge anti-icing. The tail appears to have a separate anti-ice system that burns fuel for heat. This thermal anti-icing was introduced on late model PBY-5A. It probably was an optional system.


How can pilots minimize parasite drag? What can pilots do to minimize parasite drag?

As far as I know the effect of parasite drag really depends on the manufacturer of the airplane, but can pilots do anything about it?

Parasite drag is for the most part out of a pilot's hands, as it's a combination of:

Form drag: drag from an airplane's general shape
Interference drag: drag from airflow mixing around parts of the aircraft structure
Skin friction drag

You can reduce skin friction drag by keeping the airplane clean, dirt and smashed bugs cause friction, removing them reduces friction. You can also make sure all the little doors are flush. Other than that you can't do much without making changes to the airplane itself, for instance:

Flush fuel caps
Wheel spats: these aren't decorative, they reduce the wheels' drag by providing an aerodynamic cover
Seal kits: these reduce interference drag by smoothing airflow
Cowling replacements
Fairing replacement

Whether these are worth the cost depends on the airplane, Cessna 152/172s are very draggy to begin with because of their rivets and wing struts, you could dump thousands into additions for very little benefit. Other airplanes get measurable improvements in speed from a few seals here and there.


Why are Flight Directors turned on so early? I have noticed that the requirement to turn on the Flight Directors in a commercial aircraft is in one of the very first checklists. Why does this have to be the case when the Flight Directors are really just an indication of where the aircraft needs to fly to?

Why, for instance, is it not in the "Before Take-off" checklist?

For a jetliner, the 'why early' is to test the auto flight system. Turning on the F/D also turns on the Flight Mode Annunciator (FMA) and displays the armed modes.

So it's an early check that all systems are go.

From a 777 manual, any fault will be indicated on the FMA as it reads above.

Here are the flow checks pertaining to the F/D and FMA during the cockpit setup:


How do changes in flight affect L/D max and the speed at which I must travel to maintain it? I have an plane flying at L/D max and I want to maintain L/D max.

Firstly, if this plane burns through a portion of fuel and becomes lighter, how would I have to change the speed to maintain it?
My thoughts are that because the plane is ligher, less lift is required to keep it level and therefore Cl decreases, decreasing L/D and therefore I'd have to increase speed?

Also, how would an altitude drop affect L/D max?
I assume because of the higher density of air, you'd get more drag and therefore, lower L/D as such, would need to speed up to maintain?

Also, extending speed breaks during landing - again i assume because this increases drag you would need to speed up to get back to max ratio.

I'm quite confused with this concept and what parameters affect what.

The best way to fly a constant L/D max is to have an angle of attack indicator.  Otherwise you would be constantly re-calculating the best speed as you burned fuel and changed weight.  Because most most GA aircraft don't have AOA, and are not too dramatically affected by changes in weight, max range and max endurance are generally expressed as an airspeed.

For a constant L/D max AOA, airspeed would actually decrease as the plane got lighter.  Think about the fundamental forces acting on the aircraft - lift is produced by a combination of airspeed and AOA.  If AOA remains constant, less airspeed will be required to produce the lift needed to counter the weight as the airplane gets lighter.

The reverse is true of course, if you are flying a constant airspeed you would need to gradually trim nose down to a lesser AOA as the aircraft gets lighter to prevent it from climbing.

Regarding speedbrakes, they add drag so if you want to speed up you would not extend the speed brakes.  Speedbrakes are used to slow down, or to maintain a given airspeed at a higher power setting to improve throttle response and go around capability (upon retraction) during approaches.


How does a (non-pilot) arrange a tour of an ATC facility? Although the question has been asked for pilots, how does a civilian get access to a ATC facility? I regularly listen to Live ATC, and I'm incredibly curious as to what the inner-workings of a tower look like.

Even at that, is there a live video feed of ATCs?

For tours, call the facility and say you're interested in aviation, and would like a tour, same as pilots. Depending on the facility, business hour tours are preferred, though others will let you come any time they have available staffing to escort you around.

As far as I know, there are no video feed of any ATC facilities.


In a 777, what would happen if the auto-pilot and the altitude hold failed at the same time? I know nothing about a Boeing 777 flight controls.

In general,
I know that the speed of an aircraft is controlled by the throttle on the center console. I have never heard of an automatic speed control. I believe the pilot manually adjusts the speed.
I have heard of altitude hold control. You set it and the system keeps the aircraft at the same altitude. Is this independent of the auto-pilot?
And of course there's the auto-pilot. It makes the aircraft go in a direction you want it to go.

If the pilot does not intervene, what would happen if the auto-pilot and the altitude hold failed at the same time?

Does the aircraft climb or dive?  Dose it oscillate up and down?
Does the aircraft roll over?
Does the aircraft turn?
What about speed?
If it continues in some fashion, what is the effect of the fuel consumption in the different tanks? Is it automatically controlled? What happens if that control fails?

Regardless of whether it's a 777 or any other airplane where the altitude is being held by the autopilot and the altitude hold function stops, whether because of general autopilot failure, just altitude hold failure, or the altitude hold function is inadvertently or purposely switched off; the airplane will seek whatever pitch attitude it is trimmed for.

For example, using the 747-100 and -200 autopilot setup, let's say you are in cruise at 35,000 ft, the autopilot is navigating and altitude hold is on. Further, let's say we're beyond 1990, and many operators of those aircraft had disabled autothrottles because they were a maintenance hog. Thus the airplane is maintaining whatever speed has resulted from where you have positioned the power levers. The altitude hold function is maintaining altitude by using the elevator. You can look down at an indicator on the center console between the pilots to see the degree of up or down elevator being used.

As the flight progresses and fuel is burned, the c.g. moves forward and the altitude hold function adds more up elevator to maintain altitude. When a certain amount of up elevator amount is reached, the system automatically retrims so that no up elevator force is required.  You hear and see it happen because the large trim wheel on the right side of the center console rotates.

If you're a good captain or a good flight engineer, you can monitor the amount of up elevator force being held and trim it out manually before it reaches enough to trigger the automatic retrim, and save the company a little fuel money (or if you're JFK to Tel Aviv in a -100, possibly avoid a fuel stop in Athens).

Now let's say that you reach up and disengage the altitude hold sometime between retrimmings. The airplane will nose down, and actually quite abruptly if it's been awhile since it last retrimmed. So abruptly that normal practice was to always have the yoke in hand before disengaging the autopilot.

Let's say that you that you let the condition persist. The nose is down, so the airplane would descend, of course. The speed would oscillate during the descent. If there was a severe nose down trim ... well, you wouldn't want to go there.

For a humorous account (at least meant to be humorous) of the altitude hold function being inadvertently disengaged, go to http://terryliittschwager.com/hajj4contest.php.

Now for the case of total autopilot failure, I defer to the other answer already here when I posted this. Suffice it to say that the pilots would have to take control in short order to avoid an extremely serious condition, very possibly the loss of the aircraft. You can go to http://community.seattletimes.nwsource.com/archive/?date=19911219&slug=1323954 for information on an incident that involves such. The interesting thing is this airplane had started to do this a number of times (including once to me coming out of Hong Kong), but in previous instances the pilots quickly recognized that the autopilot had failed and had taken control before things got serious.

Following the incident described, the airplane was repaired and reentered service, and we called her Christine.


How do pilots identify the taxi path to the runway? When the aircraft is readying to taxi, the ATC asks them to proceed to some runway number, say runway 09. In big airports, how do the pilots identify the correct path to their runway?

Does the ATC guide them? Or do they get a map of an airport chart?

Source: wikimedia.org

Guiding is called progressive taxi and can be requested. A follow-me car can also be used to guide the plane. In some airports the follow-me car is mandatory, along with its fees.
In most cases the pilots use charts. Either paper or electronic. Some electronic charts offer a moving map feature, whereby the crew can see their position on the map.
Lastly, there's the new follow-the-greens (video link), in which the ATC programs the taxi route and the crew are then guided by smart green lights. Follow-the-greens cockpit video / ATC audio in Singapore Changi can be seen here.
In all cases, except for follow-the-greens, the route is given. It's worth mentioning that the taxiways are marked by signs as well.
Coming soon by 2020 (2021 update: it's now ready for deployment; full benefits by 2023) is the taxi route display for pilots, in which the cockpit displays draw the taxi route via ATC datalink. See possible interface here (Boeing). Airbus interface below:

Source: SESAR JU D-Taxi provides datalink exchange solutions for flight crew and controller during taxi, YouTube


What is the difference between "closed traffic" vs "stay in the pattern" request? My instructor made "request right closed traffic..." call before practicing a series of landings. Just wondering, what is the difference between this request and a "stay in the pattern..." request? Which one should I use when?

Actually, "stay in the pattern" is non-standard phraseology and shouldn't be used, even though it means the same thing as "closed traffic".

Specifying the direction isn't required but it is a good idea to ensure that everybody is thinking the same thing.


Why don't modern jet engines use forced exhaust mixing? Jet engines are noisy[citation needed] - even a modern high-bypass turbofan is still deafeningly loud compared to pretty much anything except the even-louder low-bypass turbofans or a turbojet (much louder still than even a low-bypass turbofan).  Unsurprisingly, making jets quieter has been a major concern for the entire jet age; as the majority of a jet engine's noise comes from turbulence at the boundary between the hot core exhaust, on the one hand, and the ambient air (turbojets) / bypass airflow (turbofans), on the other (along with, for turbofans, a small contribution from turbulence at the boundary between the bypass airflow and the ambient air), the primary means of doing this without needing a turbine-incineratingly-high bypass ratio1 is by making it so that the two or three exhaust streams mix smoothly, and preferably do so before going out the tailpipe (so that any noise that is generated during the mixing process can be trapped within the nacelle).

To this end, early jetliners used intricate forced-mixer tailpipes, which were extremely effective at mixing the exhaust and the ambient air, and, thereby, reducing noise; they could either be built into the engine at manufacture time, or retrofitted later on as a "hushkit".  These could be seen both on turbojets...2

...and on turbofans.3

With the switch to high-bypass turbofans in the 1970s and 1980s, the dramatically-increased bypass ratio of the new engines was enough all by itself to produce a large drop in noisiness, such that even a plain, unmixed tailpipe was acceptable, and the forced-mixer tailpipes of the earlier engines slowly disappeared from view as the numbers of aircraft using the older engines dwindled.

However, since then, the NIMBYs have gotten more exacting and noise standards have continued to grow stricter and stricter, forcing jets to again use exhaust mixing to make them quieter.  Two methods for doing this are in common use:

Chevrons

These sawtoothed tailpipe rims enhance the mixing of the core, bypass, and ambient air, reducing noise:4

Shared tailpipes

These engines simply send both the core and bypass airflow through a single, long tailpipe, where shearing forces at the boundary between the two airstreams create turbulence that causes them to mix to some extent before exiting the engine:5

Both of these methods, however, are fairly inefficient at mixing the different airstreams; a scaled-up forced mixer would be far more mixy (and, therefore, more noise-reducingy).  So why do modern high-bypass turbofans still use inefficient methods of exhaust mixing, instead of the more effective forced mixing?

1: A higher bypass ratio requires (for a given thrust rating) more power per unit turbine, which requires the turbine to run hotter.

2: Exhaust mixer of a General Electric CJ805-3 turbojet from a Convair 880 (image by Thomas R. Machnitzki, via DoxTxob at Wikimedia Commons).

3: Exhaust mixer of a Rolls-Royce Conway RCo.12 low-bypass turbofan from a Boeing 707-420 (image by Alf van Beem at Wikimedia Commons).

4: Exhaust mixer of a General Electric GEnx-2B high-bypass turbofan from a Boeing 747-8I (image by Olivier Cleynen at Wikimedia Commons); note the chevrons on both the fan cowling (here opened to appease the peanut gallery), for mixing the bypass airflow with the ambient air, and on the core tailpipe, for mixing the core exhaust with the bypass airflow.

5: Exhaust mixers of two General Electric/SNECMA CFM56-5C high-bypass turbofans from an Airbus A340-300 (image by Hansueli Krapf at Wikimedia Commons); two more A340s, along with their engines and said engines' exhaust mixers, are also visible in the background (one each at upper left and upper far right; please ignore the 747 at upper mid right).

Source: wikimedia.org

They're back. Above is the General Electric Passport. Entry into service was 2018.

  Its core cowling, exhaust cone and mixer are made in ox-ox composites, with inorganic high-temperature-tolerant resins and oxide ceramics CMCs to withstand 1,000°C without deformation, saving weight and allowing complex molding.

The above hints at the hotter exhausts of the newer (since the mixers disappeared) engines, and that CMCs were the solution.

For the turbine inlet temperature, below is the trend:

Source: researchgate.net

From the respective flight manuals, an A380's max continuous EGT (Exhaust Gas Temperature) is 970°C, while it's 580°C for the older MD-80 (both don't have mixers, but it shows the trend). While you can bleed-cool turbine blades, there must have been no gain in cooling a mixer with bleed or they would have done it (bleed takes away from a combustor's efficiency, and at some point overall gains turn into losses).

Related: Why did Boeing remove the engine chevrons on the 777-X?


Database for NACA airfoil polars Is there anywhere where I can find a database of experimental or CFD-obtained NACA series four or series 6 airfoil data?
I'd like to have access to a relatively large set of high-fidelity drag polars/$C_L \times \alpha$ curves in order to train a neural network for a personal project, and I don't have the time to digitalize the plots in NACA's original contractor reports.
Any help is appreciated. =)

The technical report NACA-TR-824 has been digitized.  The digitized version includes Cl and Cd for 118 airfoils at 3 Reynolds numbers.  To get it, create an account on x-plane.org.  (It's been online sporadically elsewhere over the decades, e.g. here.  People associated with the digitizing include Gregory Peter, Gregory Siemens, and James Sonnenmeier.)

Convert plots to tables of numbers with Data Thief or WebPlotDigitizer.

At lower Reynolds numbers, http://airfoiltools.com/polar/index lists XFOIL-calculated polars for many of those airfoils, as text as well as plots, e.g. http://airfoiltools.com/polar/details?polar=xf-n0009sm-il-500000:

alpha    CL        CD       CDp       CM     Top_Xtr  Bot_Xtr

-13.500  -0.9606   0.06449   0.06155  -0.0264   1.0000   0.0073
-13.250  -0.9745   0.05737   0.05426  -0.0319   1.0000   0.0074
-13.000  -1.0011   0.04929   0.04597  -0.0377   1.0000   0.0072
-12.750  -1.0177   0.04406   0.04052  -0.0404   1.0000   0.0073
...

You may need some clever webscraping script to collect them all in a NN-friendly format.  Or you could run XFOIL yourself:  they describe exactly how they generated these tables.


Why does the F-15 have a swept-back trailing edge at its wing tips? The F-15's wing has an curious feature: a swept-back trailing edge near the wingtips, along with an unswept inboard portion of the wing.

(Modified from https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle)

This feature is not present on comparable teen-series fighters (e.g. the F-16):

(https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon)

Lots of Soviet-made aircraft (e.g. MiG-25, -29, and Su-27 families) appear to use a swept trailing edge. However, these fighters all have a constant sweep across the wing--not two sections of different sweep like the F-15.

(https://en.wikipedia.org/wiki/Sukhoi_Su-27)

Why then does the F-15 have such a seemingly unique feature (i.e. a trailing edge with two sections of differing sweep)? Clearly its designers must have thought it served some purpose--I presume they would not have added it for mere decoration.

EDIT: to clarify, my question seeks the physical reason(s) for why the trailing edge of the wing tip has a rearward sweep--not why the wings are cropped at an oblique angle, or a citation of historical precedent.

EDIT 2: I'd also like a well-sourced answer, if possible.

The F-15 wingtip is a "raked" tip configuration, which increases lift and decreases drag.1
[]2

This is the design decided by the USAF after discovery of several design deficiencies during initial flight test which included aeroelastic deficiencies (wing bending) and aerodynamic effects (buffeting). "In early June 1974, the Air Force flew the “raked” tip configuration and were completely satisfied
with the configuration’s buffet characteristics."  The Saga of the F-15 Wingtip


What is the relation between pressure and airflow speed above an airfoil? Does decreased pressure on the top surface of an aerofoil cause high velocity airflow or does the high speed airflow result in decreased pressure?

Above the wing, decreased pressure causes the air to accelerate. But it is the plane's velocity that causes the decrease in pressure above the wing.

Much like pressure gradients are the cause of wind.

Or when a fast truck on a highway punches a hole in the air, the air behind the truck rushes in to the lower pressure area.

So when a fast truck/airfoil causes low pressure, the air rushes in / speeds up.

  Bernoulli's principle states that within a steady airflow of constant energy, when the air flows through a region of lower pressure it speeds up and vice versa.

Source: Wikipedia

More information here (NASA).

What happens in a stall, is the drag from the airflow separation outweighs the lift. But the pressure is still lower above the wing.

Leading-edge root extension and delta wings can make better use of high angles of attack.

The NASA link above explains the true relation between airflow speed and pressure, but that's not the only cause of lift, is what I mean.


What is meant by the term "Code on request"? I recently flew from a small airport under a Bravo shelf into the nearby Bravo airspace above. When I advised the ground controller that I was doing as such, he cleared me to taxi, then added the frequencies for the Bravo approach control and said "code on request". A moment later he came back with a discrete squawk code. I'm assuming "code on request" meant he had requested a squawk code from approach control and was waiting for the reply, am I correct? Is any response required by me? I got my PPL at a smaller airport and had never heard that before.

Yes, I think you're correct although the phrasing is unusual. The usual phrase is "clearance on request", which is used for IFR departures as explained nicely in this AOPA article:

  Experienced instrument pilots know that when they call to request
  their clearance, the controller may not yet have received it.
  "Clearance on request" is ATC's way of politely saying "I don't have
  it - I've requested it - you'll be the first to know when I get it!"

The only FAA reference for the phrase that I could find is buried in the glossary of the Pilot's Handbook of Aeronautical Knowledge:

  Clearance on request.  An IFR clearance not yet received  after filing a flight plan.

But as RalphJ commented, if this is a VFR scenario and you needed a class B clearance and transponder code then I assume that the controller said "code on request" to let you know that he was waiting for a transponder code for you, as you assumed.

It wasn't an instruction as such so you could just acknowledge it with "Roger" or whatever. See this question for more information on that.


How do you calculate the maximum distance to communicate with tower control? I'm stuck on the following riddle found on the geocaching web site:

A helicopter pilot is flying at an altitude of 1981 meters. He is heading inbound an airfield 335 meters high. There are no particular obstacles (such as mountains, ...) between the helicopter and the airfield. What maximal distance (in nautical miles) can the pilot expect in order to be able to communicate with the airfield control tower?

Unfortunately I don't know anything about air control rules and multiple Google searches came to no avail.

Please could you give me a clue (not the answer as it would spoil the fun) how to answer this riddle?

Edit

For those who are interested here is the link to the geo-cache

The formula for VHF range taught in flight schools1 is

$$ d = 1.25 * \sqrt{h} $$

where d is the range in nautical miles and h is the height in feet of the receiver above the transmitter.

So in your case it would be (spoiler: mouse-over to see hidden text):

   $$ 1.25 * \sqrt{(1981 - 335) * 3} \approx 87.84  $$

However, keep in mind that in some countries the range of Tower VHF radio tranmitters is limited so they only achieve a certain range. Uncontrolled airfields in Germany usually have a range of 30nm, which is achieved by limiting their power.

1: "Motorflug kompakt" - Winfried Kassera / 2012 - p. 296


What are the reasons for autopilot restrictions on instrument approaches? On some instrument approaches, the autopilot-coupled approach is not authorized (or not authorized below a certain minimum).
For example,

Autopilot-coupled NA (MRY: ILS OR LOC RWY 10R)

Autopilot-coupled NA below a minimum (LVK: ILS RWY 25R)

Moreover, on some approaches, the DA is lower if we have a AP/FD/HUD.

I guess the lower DA is because we can give more attention outside the window when there is AP/FD/HUD's assistance. I didn't come up with any idea or context why autopilot-coupling won't work.

If it is turbulent and the autopilot doesn't respond quickly enough to keep the needle centre, we may not use autopilot at all. If the air is calm, the autopilot may well outperform hand-flying. However, it depends on the quality of the autopilot.

It's the fact the autopilot works more accurately than a pilot which is actually the cause of the restriction.

The decision to restrict the use of the autopilot usually comes from the certification agency after the ILS inspection flight detected glideslope erratic variations or reversals. The false signals are likely due to interference from the environment or the nearby activities with the runway equipment.

Flight inspection volume

The ILS volume surveyed by certification agencies using ICAO recommendations is located below a 5.25° angle. It means G/S indications are checked and expected to be correct only below this angle.

Source

This is why G/S are usually intercepted from below unless specific procedures are in place and extra care is given to validate G/S indications.

Anomalies in flight inspection volume

However there can be false and reversed guidance areas within this volume. They are discovered when the ILS is commissioned and flight tested by the certification agency. Example of glideslope reversal during an inspection on the nominal slope:

(Source, page 15-97)

In these areas, the ILS signal may vary quickly and if the autopilot were active it could react to the variations, while a pilot will likely not notice the interference, or will disregard it unless it follows a trend for a few seconds. A pilot with a Vertical Situation Display can confirm the ILS error visually.

A reversal, which is a guidance given in the wrong direction, can lead to a serious incident or an accident, due to autopilot quick reaction time. When such reversals are discovered during the inspection flight, autopilot approaches are not authorized, or forbidden during the final segment where the consequences can be the worst.

Consequences of a reversal during autopilot approach

Using the erroneous signal from the ILS may divert the aircraft from the approach path. Being below the glideslope close to the threshold is indeed not desirable, especially for a jet aircraft for which the thrust increase required to recover the lost altitude cannot be obtained instantaneously.

Suddenly jumping higher is also dangerous. It leads to a loss of airspeed, and possibly to an unrecoverable stall. Below is an example of autopilot reaction to a reversal. While it didn't happen in the certified volume of the ILS, the principle is exactly the one found in any reversal.

Some explanation about the context it happened

It was believed for a long time ILS radio signals for the glideslope contain false guidance at angles multiple of the nominal approach slope (e.g. 6° and 9° for a 3° G/S), but it was also taken for certain these false glideslopes were not reversals and would anyway guide the aircraft to the threshold, at a different descent rate (which would be enough to warn the crew about them following a wrong glide angle).

However G/S reversals, especially after 9° has been observed later (2011-2013), for some types of ILS antennas, without a full understanding of the reasons. Among these cases (source ICAO):

Schiphol Airport, The Netherlands, 2011, KLM, Embraer E190
Murcia Airport, Spain, 2011, Ryanair, Boeing 737-800
Charles de Gaulle Airport, France, 2012, Air France, A340 (BEA report)
Treviso Airport, Italy, 2013, Ryanair, Boeing 737-800
Eindhoven Airport, Netherlands, 2013, Ryanair, Boeing 737-800.

Eindhoven serious incident, May 2013

A B738 on autopilot was flying a non stabilized approach where the G/S had been captured from above. When reaching the 9° glideslope area, at 0.85 NM from the threshold, the aircraft suddenly pitched up to 24° and climbed from 400 to 1,000 ft. Airspeed plummeted to 100 kt, the stick shaker activated:

Source. Original source: Dutch Safety Board report

From the investigation done by the Dutch agency, it appears the actual guidance provided around 6° and 9° is completely reversed:

Source

As a partial mitigation measure, Boeing changed the Flight Control Computer software to limit the pitch up rate in G/S mode. The BEA also recommended manufacturers would prevent the capture of the G/S by an aircraft not on the published glidepath.


What are the lines of triangles on the DCA RWY 19 River Visual approach chart? A recent update of a certain flight simulator made me revisit this chart and I had not noticed the lines of triangles (at the Chain Bridge / Roosevelt Bridge etc, but not at the Key Bridge) in a past version, years ago.

What are they?  I couldn't find the information in the FAA Chart User's Guide.

So, applying some local knowledge, the Key Bridge is a high arch bridge in that it is a good distance above the water level, and the other ones listed, Rochambeau, Memorial , Roosevelt, and Chain Bridge are all "low" bridges, more like causeways.
I tried looking for a chart legend that said this, but I wasn't able to find one.


Is C.G. of wing itself in line with hang point (hang glider)? Where is c.g. of hang glider wing,at hang point ,in front or behind hang point?
Is control bar "neutral" when glider is at rest and there is no wind or flip backward or forward?
At 0:45 control bar  lay at seat,slightly "nose heavy," so  this  wing c.g. is somewhere in front hang point(but to me seems very close to hang point,bar is almost neutral)

here is wing c.g.  excatly at hang point

The centre of gravity must imperatively be located between the nose and the quarter of the mean aerodynamic chord. The quarter of the mean aerodynamic chord is the point where applies the resultant of the forces and moments aerodynamic. One finds of it the definition theoretical low.
If the centre of gravity is placed behind the quarter of the average aerodynamic cord, the wing is unstable and completely impilotable... If the centre of gravity is too ahead, it is hardly less serious, the wing " marsouine " by carrying out oscillations around the axis of the pitching, oscillations which can, at worst if the control surfaces are not sufisament dimensioned to be able to increase the moment to pull up, not diminishing and leading to the crash..
Check out http://aeropic.free.fr/pages/modelisme/theorie/theorieenglish.htm#:~:text=The%20centre%20of%20gravity%20must,of%20the%20mean%20aerodynamic%20chord.


Due to the corrective calculations being ground-based, does the reliability of WAAS degrade with increases in altitude? Since the WAAS system utilizes stations at known, fixed locations on the ground to make determinations for positional correction, is there a loss in the precision or validity of that correction with an increase in altitude?

If there is, how does it scale?  Is the loss in reliability of the correction linear, exponential, is it simply a negligible degradation, etc?

Yes, but it is a fairly small error.
Some of the major sources of GPS error are able to be "corrected" by WAAS.  The WAAS stations are used to build a model to make these corrections.  These corrections are most accurate near each station, and less so the further away from each station.
A couple of the largest errors are due to Satellite clock error and ionosphere delay.  Satellite clock error is "correctable" by WAAS and will only depend on the specific satellite so would not change with receiver location.  (ie when using Satellite number 1, add 1 microsecond...).  On the other hand the ionospheric delay is somewhat location specific (when receiving satellite number 1, while standing at position X, the atmosphere is delaying the signal by Y microseconds). For locations that are not at one of the WAAS ground stations an approximation for the the ionosphere delay must be used.
However, being at altitude is probably not going to change the error by much.  I don't have data on how it scales.


What are the circled features on the upper surface of the A380 wing? On the A380 wing are at least 3 red circle each side. There seems to be a black part inside. Is this a dip stick, or something else?

They are likely attach points for safety harness equipment. Workers working on top of the wing attach gear that functions as anchors for lines that run the length of the wing, then attach their safety harness to the line.


Is it normal to instruct an aircraft to squawk VFR if it goes around on an IFR visual approach? A pilot is flying IFR on a visual approach to a towered airport.  He elects to go around due to gusting winds and notifies the tower. The tower then asks his intentions. He replies that he would still like to land. The tower instructs: "Squawk VFR and enter a left downwind for runway 5."

Is the instruction to squawk VFR proper?

No, that scenario does not sound normal to me. For one thing, a visual approach is an IFR procedure. Instructing the crew to change to a VFR squawk makes it sound like the controller is suddenly treating the flight as a VFR flight, which makes no sense. A controller is never allowed to just cancel IFR; going from IFR to VFR must always be explicitly requested by the crew ("cancelling my IFR flight"). We are not even allowed to suggest a flight to change to VFR, the idea has to come from the pilot.

Besides, it makes no sense that the controller would change the squawk code of a (presumably) already identified flight to a non-discreet code (1200/7000) since radar identification would then be lost and it would be impossibly to continue to provide radar service.

If the example you provide is a real one, there is obviously a reason for the controller doing what they did, however, based on the limited information you give here, it is hard to see what the reason would be. Maybe if we knew the whole picture, it would make more sense.


What causes noise when a plane is on the ground? What's the loud continuous noise the plane is making when it's on the ground and the (jet) engines are visibly not running (spinning, at least) ?

I was boarding and I thought it was perhaps a compressor for the cabin air but when I entered the cabin it wasn't cool inside.

That would be an Auxiliary Power Unit, a small jet engine that is used to provide power on the ground when the engines are off and when there is an emergency in the air.


Is the stall speed different at -1G? This question makes me wonder what happens at -1G.
When the load factor of an aircraft increases above 1, the stall speed increases. As the load factor comes closer to 0, stall speed decreases and becomes "infinitely" low (basically 0 kts) as an aircraft can not stall at 0G. But what happens at a negative load factor?
For example, during inverted flight the airfoil is still producing lift when the angle of attack is negative (enough).  Thus the stall speed should change and as the efficiency of a inverted wing is (almost definitely) not as high, I am assuming that the stall speed decreases.
But is a negative G maneuver necessarily to be done in inverted flight? No!
As a glider pilot,  I can refer to negative Gs during winch launches, or rather when they fail. You often practice cable fails, where your instructor disconnects you from the winch. To maintain speed and prevent a stall you push the nose down, ideally flying a small Zero-G parabola (as this will prevent a stall). But what if you push even more? Where is the stall speed going to be at?

Only in aerobatic aircraft is the inverted stall speed identical to the upright stall speed in level flight. Airplanes registered in the Normal and Utility classes have cambered airfoils which have a higher positive maximum lift coefficient. In inverted flight, their stall speed is higher than their upright stall speed. The same goes for gliders and airliners.

When I had my last aborted winch launch in a ASK-13, I involuntarily checked how well the cockpit had been cleaned last time. I had to briefly close my eyes to avoid getting sand and dust into them. I can confirm that it is fully possible to fly a negative maneuver in upright position, albeit only briefly. However, the negative acceleration had as much to do with the rate of rotation and my position ahead of the cg as with negative lift.

If I had pushed a lot more, I could had stalled the wing by undershooting the negative lift minimum. But this would had just limited the downward acceleration, and the negative lift would not had been sufficient for inverted flight, because the inverted stall speed in level inverted flight was most likely higher than the speed at the time of the tow line rupture.

The negative stall speed is just your flight speed, and the possible negative gs grow with the square of speed. Stall is a matter of angle of attack, not of speed. A negative stall in upright attitude is rather uneventful - the aircraft will be at a high rate of rotation and will just hit the limit of downward acceleration.


Are aircraft separately certified for CAT III ops for +2500 feet elevation? Is it mandatory have separate certification for aircraft operating CAT III Operations on airports having elevation more than 2500 feet? If so do that aircraft are to carry any additional equipment?

One of our airline operator has submitted a list of their aircraft which are certified for CAT III operations at airports above 2500 feet elevation. We are yet to be equipped with CAT III, hence we would like to know if we have to have any additional ground systems to enable all aircraft to operate CAT III.

Yes. In certain jurisdictions.

Such as Europe, if it has not been demonstrated during flight testing.

This is covered in CS-AWOS / Subpart 1 (which covers Automatic Landing Systems).

  In accordance with CS–AWO 132, the effects of aerodrome conditions (e.g. elevation, ambient temperature, runway slope and ground profile under the approach path) are to be investigated and, if necessary, appropriate limitations derived for inclusion in the aeroplane Flight Manual. Guidance is given in paragraph 5.

Paragraph 5:

  Elevation and Temperature. The effects of aerodrome elevation and ambient temperature should be examined where operation is envisaged at aerodromes about 750 m (2 500 ft) or in temperatures greater than ISA + 15°C.

A performance demonstration would then be required (but not additional equipment).


What are L/D ratio of famous WWII fighters? I'm looking for L/D ratio of old warbirds. I'm particularly interested by the P-51D, the Supermarine Spitfire Mk.VII and the Mitsubishi A6M3 Zero, but I'm also quite curious about other warbirds that flew during this period.

What are L/D ratio of those warbird? The answer may include L/D ratio of other WWII fighters.

Please find the answers below from "World war II fighter aerodynamics" by David Lednicer.

And L/D, of course, depends on the CL you are flying at but you can easily calculate it from the unambiguous drag area given

Edit: Calculated L/D vs speed curves below for 4000m altitude flight. A fixed span efficiency was taken to simplify the calculation. Please consider this as an approximation only


Which compressed air and fluid are generally used by oleos? Which compressed air and fluid are generally being used by oleo-struts?

I'm sure it varies by aircraft type, but for the C172S nose gear oleo (for example) it's air and hydraulic fluid. The operating handbook says:

  Keep strut filled with MIL-H-5606 hydraulic fluid per filling
  instructions placard, and with no load on the strut, inflate with air
  to 45.0 PSI


What is the explanation of the apparent staining inside this airliner window [photo]? On a recent flight, I noticed a pattern of staining or dust particles inside the window, on the plastic reveal within the cavity.

It looked as though a long-term and consistent pressure differential between the cabin and the cavity had drawn a stream of air into the cavity, resulting in what looked like a sooty stain.

And even more oddly, as you can see in the picture, right opposite the vent, the mark is very concentrated, as though formed by a high-pressure stream - which I presume isn't actually the case.

I believe the stain is caused by air going through the hole relatively fast due to compression of the cabin. Assuming the window is 40x40 cm large, and there is 10 cm between the panels, the volume of air in it is 16.000.000 mm3. About 30% of this air comes out due to the lower pressure at altitude, that is 4.800.000 mm3. Assuming it takes 600 seconds (10 min) to pressurize the space again, the flow will be 800 mm3/s. Assuming the hole has a diameter of 2 mm, it has an area of 3.14 mm2. The average flow velocity is then 254 mm/s. It looks like the spot is only 20 mm away from the hole, so it would take a dirt particle less than 0,1 second to get to the spot from the hole. I think that is not much time to deflect, hence the concentration in the middle.


Do Gulfstream G650 rivets "smoke"? Any G650 operators seeing "smoking" (i.e. they leave a dark trail on the fuselage) rivets on the fuselage at various locations?

All rivets that are properly set will eventually "smoke" given a structure subjected to vibration that has been under-engineered, meaning not enough rivets per inch. The other cause would be improperly installed rivets. What looks like smoke is aluminum oxide produced by the movement of the surface, and the rivet rubbing a bit of it and sloughing it out and back along the airstream direction.


What is the meaning of "Steer 1-2-0" in WWII texts? I am reading some texts about WWII and English operators tell the pilots to "Steer 1-2-0". Can anyone help with the meaning of 'steer' here? Is it a synonym of 'direction', 'heading' or 'vector'?

To "steer" means to change the aircraft's track (the direction of movement of the aircraft) to a certain degree in relation to the lines of meridian (north–south lines). The units are degrees from north in a clockwise direction. North is 0°, east is 90°, south is 180°, and west is 270°. Note that, due to wind forces, track is not the same as the heading (where the nose is pointing).


Can hours at an FAA flightschool be credited to an EASA PPL? Does anyone know if it's possible to put FAA flight school hours towards an EASA (UK/CAA) PPL?

I've heard answers from "NO" to "Of course" and everything in between, but am struggling to find any documentation that gives a definitive answer one way or another.

From what I can see FAA hours can't be used for credit towards specific training but they can be credited towards the 45 hours total time needed for the license. If anyone has links to appropriate documentation I'd really appreciate it.

Assuming that "flight school hours" means flight time then CAP 804 suggests yes:

  3.6 Logging of Flight Hours gained in the USA   Some flight hour recording practices allowable in the USA do not comply with European
  and UK requirements. In particular:

  2 pilots flying together in a single pilot aircraft both claiming P1    hours;
  One pilot accompanying another on newsgathering or traffic    control flights and claiming P1/PIC when they have not acted as Captain or signed for the aircraft.

  Flying hours of this nature cannot be credited towards the
  requirements for Part-FCL or UK Licences.

The last sentence implies that if hours logged in the US do comply with "European and UK requirements" then they can be credited towards licenses. If you need something more detailed and explicit then it may be easiest to ask the CAA directly.


Do tower-controlled helipads have a standard naming convention? Just wondering if anyone knows, or if it just depends on the airport.

Edit: This question is different than How does the FAA determine which format of location identifier to assign to an airport?.

My question is how the tower names helipads under its control at an airport, not how the FAA assigns an identifier to an airport as a whole. Runways use the runway heading. How are helipads named?

Normally helipads are allocated names depending on what taxiways they are adjacent to. For example, Helipad Alpha would be off or near taxiway Alpha. Helipad Charlie would be off of Taxiway Charlie. Results may differ depending on the airport/heliport layout, but this is how the normal naming system works.


Airliner headlights flashing alternately I live in Portland,Oregon.  I watched a large jetliner coming into the airspace heading to the airport.  The red,green and tail lights were normal. But the headlights were blinking alternately.  I have never seen that in all my years watching the sky. Is there a reason for the alternate blinking?Emergency onboard?

Some modern landing lights have a pulsating or alternating mode. It's just for being recognized more easily, has nothing to do with an emergency.

A constant white light heading right for you over a city full of lights doesn't get as much attention as a pulsating light which gets bigger each time as you get closer :)

Here is one example: https://www.preciseflight.com/pulselite-pulsing-aircraft-lights/

and their video:


In case of failure in the Electronic Flight Instrument System, how would they be transferred between each other? In EFIS, in case of failure in the PFD, where would be PFD shown? In case of failure in the ND, where it would be shown? In case the PFD & ND both fail, where would they be shown?

On the Embraer 170/190-series, the normal display layout is as follows:

There is an automatic display reversion logic when one or more panels fail. The following image shows this logic:

There is also a manual reversion switch located near each pilot's outboard knee, which controls what is displayed on the panel:

All images sourced from an Embraer Airplane Operations Manual.


Does the FAA prescribe radar cross-section requirements for civilian aircraft? Are there any FAA regulations stipulating the minimum permissible radar cross section for civilian aircraft (i.e., making sure that civilian aircraft are at least such-and-such cross-section visible to primary radar)?

No. Primary radar isn't a universal tool to begin with. Secondary radar and thus transponders are more commonly used. So there are regulations as to where and which aircraft need to install transponders. Ultralights and some GA can get away without.

Related: Is it legal to make a non-military stealth plane?

No RCS requirement, but you'll need a transponder regardless.


How to design the blade twist angle in propeller design? I am designing a scale propeller for small UAVs and I have computed blade twist distribution for the blade airfoil sections. While I was modelling the propeller I have set the angles to twist about the trailing edge of each section so that this point is collinear. However, the final result looks a bit weird.
I would like to ask whether I have to consider any other point of the airfoil to set for the twist angle (Maybe leading edge or C.G). Below is a screenshot for the final model.

You should be twisting the profile chord, like in the picture. Why is your tip so small, that is where most thrust is created.

 Source


Where can I find a list of internal airline designations for various aircraft/cabin configurations? Is there any way to tell what the internal designations of an airline mean? Looking at Delta's seat maps for the 757, Delta has a couple of kinds, but what's the difference?

Boeing 75A
Boeing 75M
Boeing 75S
Boeing 75G

I know they're slightly different configurations for different routes, over water etc, but can what type is used on what route/configuration or what the designations mean be found somewhere?

Is there a place where these are defined for airlines of the world?

Internal designations are usually used to differentiate various seating configurations or differences internally for an airline. From your example, all those codes are various codes for the 757-200 (at Delta, they code the 757-300 as a 75Y). You can often look up various seat configurations at Seatguru.com, a site that generally tracks seating information on airlines. Additionally, all would be sent to ATC using the ICAO type B752.

The reason for these variations can be broken down to a couple areas of differences. One is exit configuration; in the same type (757-200), there can be 2 or more different configurations of emergency exits (combinations of doors and windows, and various types). If you have a fleet that mixes these, you'll often have to have different seating configurations for them. Another differentiation could be whether there is a subfleet equipped and used for ETOPS flights. There can be other reasons an airline could want a subfleet and a different cabin. Also, if you're in the process of upgrading a fleet, you might want to create codes to differentiate them so you know which is going where and be able to plan your revenue models around that.


Does filing a flight plan with Garmin Pilot activate ACAS and SE-SAR for my inReach? I have my aircraft set up with a Garmin inReach for ACAS and SE-SAR on 1800wxbrief.com. If I file using the Garmin Pilot app, will the inReach ACAS and SE-SAR work or only if I file on the 1800wxbrief.com site?

SE-SAR and ACAS are only available for flight plans that are either filed directly through 1800wxbrief.com or through an app that's linked to your Leidos account (1800wxbrief.com > Account > Service Provider Authorization).
Garmin Pilot isn't on the list of known service providers, so FSS won't be able to match your flight plan with your online profile... meaning these services won't be activated.
Some apps (like Foreflight) have their own built-in ACAS feed, separate from 1800wxbreif.com's SMS-based interface. If your app has that, it would receive ACAS messages directly via push message. Unfortunately, Garmin Pilot currently doesn't have this feature.
See also this related question:
Will Lockheed Martin Flight Services send me ACAS, EasyActivate™, and other text messages if I file my flight plan through ForeFlight Mobile?


How do pilots manage to remember ATC instructions? Before takeoff, during flight, before landing and for taxiing, pilots are given several instructions by ATC.

Do they mentally remember all instructions? Or do they note them down somewhere such as in a notebook so that they do not forget?

It depends.  Some instructions are really simple and with a little practice, are very easy to remember.

Runway 27 right, line-up and wait
Turn left heading 250, descend flight level 120.

Other ones will be so common, they are just remembered:

Taxi to Gate 1 via Alpha, Alpha 1, Lima 3, cross runway 24, hold short Delta.

But this is where the danger creeps in.  It is possible, and has happened, that the crew think they heard something but actually, the instruction was different today.

Taxi to Gate 1 via Alpha, Alpha 3, cross runway 24, hold short Delta.

The prudent crew will write it down.  Large aircraft have tables, or boards clipped to the control column on which to write taxi clearances, complex airways clearances and so on.

In some large aircraft, many light aircraft and in helicopters, pilots might wear a kneeboard to write instructions on.  It is a minor advantage to me that I am left handed and as a helicopter pilot, this means that my kneeboard is on my left leg where I can easily write without taking my hand off the cyclic and having it right next to the collective.

TSGT LANCE CHEUNG, USAF. - This Image was released by the United States Air Force with the ID DF-SD-01-07429

Most instructions come in the same order, every flight.  For example, here is a simplified list.  This happens every time so the crew know what's coming next, what it will likely contain, and how to respond:

Clearance for the flight plan.
Permission to start and push back.
Taxi instructions.
Departure instructions.
Take off clearance.
Hand-off from departure controller to area controller.
etc etc.

Note that any instruction which will cause the aircraft to change it's configuration or location must be repeated.  The controller then also gets to check they've got it right.  This includes things like speed, direction or altitude changes, taxi and departure instructions, frequency and squawk changes etc.

There are also some words which are only ever used at certain times, e.g. "takeoff" is only ever said when an aircraft is cleared to take off.  For example, "ready for departure" tells the controller that the aircraft is prepared to take off immediately.  The crew should not say "ready for takeoff".

You can sometimes hear a pilot receiving a long instruction, start to read it back, pause, then ask for a repeat.  Very frustrating when you are sitting there waiting to get your request in.  You just know they thought they could remember it, but didn't.  Write it down if it's anything except a simple instruction or clearance.

Here's an example of a complex one.

  SuperAir 123, cleared to Barcelona via vector departure, fly runway track, climb altitude 5000ft, after passing 2000ft turn right direct Hamm VOR, route as filed, squawk 1234, information Alpha current, startup approved

The crew might write it down like this.  Each pilot develops their own preferred shorthand:

  VD RWY TRK 5000 / 2000+ RT HMM / SQ1234 ALPHA / SU

Very brief since they know what each part is and the order they come in.

Since it contains instructions, the crew will then read it back.

  Cleared Barcelona via vector departure, runway track 5000ft, after passing 2000ft right direct Hamm, squawk 1234, Alpha on-board, startup approved. SuperAir 123.

Finally, a good reason to write stuff down is because sometimes (and we've all been there), you think you remember, then turn round to your co-pilot 2 minutes later and ask "what was that altitude?".

Here's a great example of someone who didn't have a pen and an approach to writing down clearances.


Do pilots have their own in-flight entertainment? Besides flying the plane and talking to the person in the other seat, what other forms of entertainment are (also legally) available to pilots while on the job?

I'm thinking of 'activities' like reading, listening to music, watching a movie, or... playing a flight simulator game on a laptop.

I am not aware of any legal requirements, but rules may differ according to the country of jurisdiction.

Generally it is up to the company that a pilot works for to establish their own rules to cover "in-flight entertainment" in the flight deck. Most companies do not allow it. It has been my experience that the rules are generally ignored and common sense rules are used instead.

During non-critical phases of flight, pilots will often read newspapers, books, iPads etc. Many will do crossword puzzles or Sudoku. A few will even listen to music or watch a video, but it is pretty rare.

In 40 years of flying, I can't recall ever seeing a pilot play a video game or flight simulator.


What is the reason for setting flaps 1 on the ground at high temperatures? Yesterday I've seen an A320 standing at the gate with flaps 1. While this question answers, why the flaps should be up, is there any reason to retract the flaps to 1 instead of up?

In addition it seems to apply only in high temperatures (30°C, and rising yesterday). As well in this pilot eyes video the FO confirms the temperature (31°C) before setting flaps to 1 as they taxi to the gate.

I've seen this only on the A320 (yesterday) and on the A330 (in the video). Does this apply only on Airbus aircraft or as well on others?

From the A320 Flight Crew Operating Manual (FCOM):

And the image to text:

  On ground, hot weather conditions may cause overheating to be detected around the bleed ducts in the wings, resulting in “AIR L (R) WING LEAK” warnings. Such warnings may be avoided during transit by keeping the slats in Configuration 1 when the OAT is above 30 °C.

(Emphasis mine)

Basically it lets the leading edge anti-ice [hot air] leak sensors breathe, and not display a false positive.

Note that moving the lever to 1 from 0 or 2 at slow speeds (on ground) is called config 1+F, which deploys the flaps as well as the slats (see below).


Details about video equipment used to help assemble X-59? On 2020 Dec 14 NASA reported
on Lockheed Martin's construction of the X-59.
The report's photos include an unexplained apparatus, comprising what looks like
two Panasonic video projectors and two video cameras mounted on a stand.

Green markings
on a wing being assembled, and on one of the worker's arms, may be coming from these projectors.
The stand might be mobile, because it's missing in this overview.
Any authoritative details are welcome, such as:
Does this apparatus have a name?
If it guides workers, how do they use it?
What are its capabilities and limitations?
What are the cameras for?
Why does it need more than one projector and camera?

The system may have many names depending on detailed configuration, but it is  basically a mixed reality assisted manufacturing system.
One example is described in this pdf document.
The system consists of at least one projector that projects a predesigned guidline on a workpiece, to assist human workforce in their tasks. Additional hardware may include

camera(s) to document work and/or follow the progress and enable switching to new guidline projections as work proceeds; recognize different workpieces
sensors to verify correct orientation of workpiece
manipulators to reposition the workpiece as the work progresses
text or audio output further guiding workers as the work progresses (possibly the aforementioned camera verifies each step).

Depending on resolution of the projectors, the accuracy of the system might reach accuracy of millimeter scale, for example for the purpose of drilling holes into a workpiece, but it is best suited for work such as in the picture:

highlighting necessary tasks
providing guiding for correct order of tasks
highlighting "no go" areas or such on a workpiece
etc...

The bigger the workpiece, and/or the more complex the shape of it is, the less accurate the guidline picture projection will be. This can be overcome to some extent by using multiple projectors, but the system quickly becomes very complicated especially with workpiece with complicated contours.
This could be called a poor man's AR goggle, with the benefit of being rather low tech, but the downside of the workers limiting the visibility of the guidline pic as they move around the workpiece.
There most probably are two projectors on this system simply to cover a lager workpiece. Another solution might be to verify correct 3d orientation/placement of the workpiece by overlapping two guidline images, but the orientation of the projectors in this picture are such, that the images cast by the projectors do not seem to fully ovelap.
I was unable to quickly locate it, but there is a fine video of this type of system being used on assebly line of the F-35 fighter. I will keep looking...


When was the Speed Trim System implemented on the Boeing 737? The Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged.

Was the Speed Trim System implemented on the original Boeing 737 or just the Boeing 737 NG series?

Boeing's patent (US4676460A) for the STS was filed on 1984-11-28, the same day of the 737 Classic (3/4/500 series) entering service.

So since the addition of the bigger engines on the Classic, it's been there.

Below is from a 737 flight manual of a company that had both the Classic and NG at one time, which highlights the differences between the two series.

The 3 (for -300) in a circle is non-EFIS, in case you're wondering how the two 3's differ.


What are "approved weather reporting services" and do they apply to part 91? When I look up Torrance Zamperini (KTOA) alternate minimums, it states, "NA when control tower closed, except for operators with approved weather reporting services."

My main questions are:

Can Part 91 pilots file this airport as an alternate when the control tower is closed?
Do the "operators" restrictions apply to Part 91 pilots?
What is an "approved weather reporting service"?

While we are on the topic, what exactly does the word "appropriate" refer to in the text below? FAR 91.169(c) seems to indicate that almost any source of weather information is acceptable (emphasis mine):

  IFR alternate airport weather minima. Unless otherwise authorized by the Administrator, no person may include an alternate airport in an
  IFR flight plan unless appropriate weather reports or weather
  forecasts, or a combination of them, indicated that, at the estimated
  time of arrival at the alternate airport, the ceiling and visibility
  at the airport will be at or above the following weather minima:

First, within the context of the question, the term "operator" in the Alternate Mins section refers to a person or company that has an operating certificate.  Not just Joe the Pilot who is "operating" an aircraft. So, a "Part 91 pilot" (taken from the context to mean: "pilot flying for pleasure or personal business") cannot file this airport as an alternate when the tower is closed.

Second, the term "approved weather source":  In the FARs, various Parts apply to various operators (121, 125, 135). They each have similar verbiage about approved weather reporting services.  For example, Part 121.101 (for domestic and flag operations) says the following (paraphrased):

  No certificate holder may use any weather report to control flight
  unless it was prepared by the U.S. National Weather Service or a
  source approved by the U.S. National Weather Service. Each certificate
  holder shall use an approved system for obtaining forecasts and
  reports on each route to be flown and at each airport to be used.

You can see this related question: What sources of weather are approved by the U.S. National Weather Service? for more information.

BUT... the important point from that FAR Part 121 quote is the second sentence. ALL operators must have Operational Specification (OpSpecs) approved by the FAA before they can operate. One of the items within the OpSpecs will specifically address how and from where these weather reports and forecasts will be obtained.

So, this is the bottom line answer to the question: only "operators" can use this airport as a filed alternate when the tower is closed, since that operator will have their own "weather source" that is approved by the FAA and listed in that operator's OpSpecs.


What sort of sensors are used for sensing the positions of the control surfaces for the ECAM F/CTL page on Airbus aircraft? During the pre-flight flight-controls-check how does the ECAM F/CTL page report on the actual positions of the control surfaces?

What sort of sensors are these? Is it a proximity sensor?

This is motivated by this related question about pre-flight flight-controls-check:
Testing control surfaces pre flight; what feedback does pilot recieve?

No, they are not proximity switches, which are suitable olny to detect when a moving part has reached one of few predetermined positions (as in a landing gear, in which only two predetermined positions – up and down – are of interest; any other intermediate position is transient and undesirable).

For those moving parts that are required to be positioned anywhere within a given range, such as the control surfaces of an aircraft, you need a sensor (aka transducer) that can accurately measure the position of the part at any point within that range, not just at the ends. As already mentioned, the device of choice is the linear variable differential transformer (LVDT), or its rotary variant RVDT.

For example, this is the schematic of a yaw damper servo actuator of an Airbus A320:

Element No. 4 is an LVDT that measures the displacement of the actuating rod attached to the rudder. Similar devices are used for ailerons and elevators. The elevators also have an additional RVDT fitted directly onto the elevator's torque tube, in addition to the LVDT part of the servo control.


What VFR cruising altitude should be flown to the west below 4500 feet? This is similar question to this question, but different scenario.

Let me describe the situation:

I am at KDYL in class G airspace. Accroding to ATIS the clouds are at 4700 scattered. I am going to fly west. I would like to fly below clouds. I will need to have 500 feet clearance below clouds in class E, so it is going to be 4200.

Do I have to comply 91.159? What altitude should I fly? Can I fly 3500 west in my scenario or I have to descend below 3000 and fly any comfortable and safe altitude?

Yes, you would have to comply with §§91.159.  If you’re flying VFR westbound (180°-359° magnetic) at >3000 ft AGL,  you will need to fly at even thousands + 500 ft, except while in a holding pattern or turning.  For a forecast of scattered clouds, you will simply deviate from your course for cloud avoidance, then return to the route.


Does "pendulum effect" apply to hang gliders or any aircraft? In discussion of aircraft roll stability in FAA paragraph on dihedral effect and in keel effect and in Why are high wing aircraft more stable the question of pendulum effect arises.

This is usually described as center of gravity displaced laterally from below center of lift during a roll in a high wing airplane causing a rolling moment which restores wings to a level orientation.

source

In any explanation you give, please consider the case of a hang glider which depends on weight shifting to control roll and pitch. (Don't need to discuss pitch)

source

For a proper discussion, we should first define what a pendulum is. Only then can be established if such an effect can exist in airplanes.

Let's base the definition on Wikipedia. It says that

  A pendulum is a weight suspended from a pivot so that it can swing freely.

Maybe it is also worth to look closer what a pivot is: A thing on which something turns.

So the pendulum is fixed to a fulcrum which keeps it suspended and allows it to swing freely. The ideal pendulum has all its mass in its massive bob and, therefore, pivot and center of gravity are not in the same place. If the center of gravity and the pivot would fall together, a pendulum could only rotate but not swing. And that swinging motion is what the pendulum is all about.

Now for airplanes: Here we have no pivot. All rotation can only happen around the center of gravity. This is equivalent to the pendulum with no length which is no pendulum any more. But what about hang gliders? Using Pilothead's sketch let's consider how a hangglider starts a rolling motion and compare that to how a glider does it. The top two sketches show each in steady straight flight while the bottom two sketches show both initiating a roll to the right:

In both cases, a lateral imbalance is needed to create a rolling moment (red round arrow). While the pilot of the hang glider shifts the whole wing sideways, the glider pilot commands a lift difference between both wings using ailerons. Note the shift of the hangglider to the right and with it the center of lift in the lower left sketch: The center of gravity stays where it is due to the conservation of momentum while the pilot will shift slightly to the left.  In case of the glider, no such lateral shift of the craft occurs; instead the distribution of lift is shifted in order to create the imbalance. The effect is the same: A lateral displacement between weight and lift which causes a rolling moment. Rolling happens around the center of gravity (because of conservation of momentum again) and in all cases there is no displacement between the pivot point and the working point of the weight, because both are the same: The center of gravity.

A hangglider is only superficially different from an airplane because the center of lift is actively shifted by displacing the wing laterally instead of a change in lift distribution (neglecting the influence of the pilot mass on the rigging of each wing), but in all cases the vehicle will rotate around its center of gravity. Weight has no lever arm to that center of gravity, so there can be no pendulum effect. Or keel effect, fo that matter.