How to calculate estimated flight time? I am trying to calculate estimated flight time accurately. I calculate the distance with the great-circle distance method. I have some specifications of the aircraft such as max speed, cruise speed, stall speed, rate of climb, takeoff over 50ft, takeoff ground run, landing over 50ft, landing ground.

I tried the most straightforward approach(distance/speed), but the result was not nice. How should I calculate the estimated flight time?

I assume that you're a passenger travelling on a flight and you want to know how long the flight will last, from takeoff to landing.

First you would need to get the route you're going to fly. Sometimes the published route is available on flight-tracking sites, for example this one. After that, you can plot the route on an aviation chart to get the distance. If the route is not available, you will need to construct the likely route by guessing which airways the flight will take.

Next you need to figure out your plane's groundspeed. The groundspeed is not constant. When the plane is climbing, part of the engine thrust is directed upwards, so ground speed would be slower. Engine thrust also varies with altitude, so the ground speed will vary throughout the climb. You will need an aircraft performance table, which typically has data at 1000 feet increments. You need to repeat the calculation for each 1000 feet until the aircraft reaches cruise altitude.

We will assume the aircraft remains at a constant cruise altitude for now. You can easily work out the flight time by looking up the cruise groundspeed (not airspeed).

Now we need to figure out when the plane will begin its descend. From the route you obtained earlier, you'd have known the flight's cruise altitude. Flights typically descend 1000 feet per 3 nautical miles. Work your way backwards from the destination, you will have your Top of Descent point. Again, the ground speed will be slower because the velocity vector has a component in the vertical direction. The plane will descend at its cruise airspeed until 10,000 feet, where it will slow to 250 knots.

For a start, this very simplified version would give you a closer estimate to reality. I have left out:

Winds aloft. At high altitude, usually there is a significant wind blowing at one direction. Depending on whether it is head wind or tail wind, it will slow you down or speed you up. Different areas will, obviously, have different wind conditions. Different altitude in the same area will also have different wind conditions.
SIDs and STARs. These are the waypoints which connects flights coming from all directions to a common arrival point for final approach at the destination airport (or the opposite at the departure airport). These waypoints will increase the total length of your route.
Altitude changes. Throughout a flight, the cruise altitude may change. With a new altitude, the engine performance (hence cruise speed) will be different.

In fact, flight time estimation is so complicated that we use software to calculate them. No sane human-being will go through all the data to calculate flight time today.

Note that I did not take into account:

Taxi time: the time it takes to move the aircraft between the gate and the runway on the ground
Holding time: the time which flights are asked to fly in circles near the airport when the airspace is too busy

Your airline will have included these estimates in the quoted time on your flight ticket.


How do piston engines deal with sea spray (saltwater ingestion)? From my related question, I want to know about sea spray being ingested into a piston engine during flight. Will this immediately corrode the engine unless there are special materials?

I'm asking this for piston engines, not jets or turboprops. I'm especially interested in WW2-era planes.

Obviously this is primarily about naval aircraft built for saltwater environments, but I'd like to know the protection measures so I can see if a land-based aircraft can be easily modified, or if it's going to need an entirely new engine.

Note: At very high temperatures, corrosion is much faster. I don't know the math for corrosion, but my worry is that sea spray in the engine could cause some cracks somewhere after just a few flights, unless things are made from a much higher (more expensive) grade of stainless steel or aluminum alloy.

Seaplanes operate from coastal saltwater all the time, ingesting lots of seawater, and they don't do anything special to the engines to deal with seawater ingestion.  The effort is mostly on keeping salt rinsed from the airframe and engine externally and they normally get a fresh water rinse at the end of the flying day.  Some operators may spray nicely atomized fresh water into the engine intake while running it to rinse salt from intake passages and carburetors etc, when they are doing the external bits.

The other measure is keeping the protective coatings intact, including the paint on the engine.  You really don't want salt on an engine where paint is flaking off aluminum fins or areas of the head, because corrosion pitting results that leads to cylinder head cracks.

So the main protection measure is, keep the salt rinsed off as much as possible with fresh water, and maintain the protective coatings. Even with all that, coastal aircraft operating daily from seawater year-round often require airframe rebuilds to deal with corrosion maybe every 10 years or so.

In any case there aren't any physical modifications you can do to an engine.


What is the minimum set of registers to fill for ADS-B? I am researching about Automatic Dependent Surveillance-Broadcast mode of Mode S currently. On a third party document, I encountered a list of transponder registers to be filled; 05, 06, 07, 08, 09, 0A, 61, 62, 63, 64, 65 as the list of registers.The source is Grappel 2007- ATC 334.

05(Airborne position), 06(Surface Position), 08(Aircraft Id), 09(Airborne velocity) registers seem to be carrying most important data for ADS-B.

Registers from 61 to 65  are called event driven registers, carrying slowly changing data. SHALL these registers have to be filled with correct values to serve ADS-B IN capable systems? What happens if I don' t service these event-driven registers?

Is there a definitive list of registers to fill current settled ADS-B standards?

Note: I have access to many documents, so feel free to refer some well-known ADS-B documents.

ADS-B IN systems do require information from the 0x6X registers. These registers hold information like ADS-B protocol version, system design assurance level and position source quality indicators.

Without this information the ADS-B IN system can establish a track, but the quality will be flagged as too low to display.

For reference, get a copy of RTCA DO-260B or EUROCAE ED-102A, whichever is cheaper. They are the Minimum  Operational  Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance – Broadcast  (ADS-B).


Is true altitude vertical separation between airplanes dependent on temperature? Does the "true" altitude vertical separation between airplanes (using the same altimeter setting) vary as the temperature changes?

( I fully realize that the "true" altitude of both airplanes is lower when it's colder than ISA and higher when it's warmer than ISA)

Yes, the vertical geometric separation is temperature dependent.

If it cooler than ICAO Standard Atmosphere (ISA) conditions, the geometric vertical separation is lower than the pressure altitude separation. If the temperature is above ISA temperature then the geometric separation is higher.

The ICAO atmosphere assumes a sea level temperature of 15 degrees Celsius and a lapse rate of -6.5 degrees per 1000 geopotential meters altitude. At FL250, according to the model, the temperature is
-34.5° Celsius, at FL260 it would be -36.5°C.

Because the ISA is based on geopotential height, the geometric separation between FL250 and FL260 under standard temperature conditions would be slightly over 1000 feet. In fact it would be 1002.1 feet, because gravity is about 0.2% lower at that altitude.

If we would take the same mathematical model as in ISA, but would offset the temperature by -20° Celsius (-5°C temperature at sea level), the geometric difference between FL250 and FL260 would be only 932.7 feet.

If we offset the temperature by +20°C, the difference would be 1072.7 feet between FL250 and FL260.

This can be approximated by looking at the ratio of temperatures expressed in Kelvin. The average temperature between FL250 and FL260 is -35.5° C or 237.7 Kelvin. For 20 degrees below ISA temperature a rough estimate would yield:

$\frac{237.7 - 20}{237.7}×1000$ft = 915 feet.

For separation between aircraft and terrain, temperature effects are taken into account by adjusting the minimum safe altitude.

For vertical separation between aircraft no adjustments are made by ATC, but the effect is taken into account in safety cases.


How can I tell from the location of flow separation if an airfoil is stalled? I am in the process of completing a carbon airfoil for my track car (sedan) and I am looking to determine stall angle visually.  AKA, wool tufts and highway runs with my GoPro pointed upward from the trunklid.

Since I don't really know the angle of incidence at which the air coming off the rear window will hit the wing, visually looking for flow separation is really the only way for me to determine my max angle and work down from there based on the car's balance at the track.

Is there a paper (or common knowledge) regarding over what % of chord length flow separation must occur for the wing to be considered stalled?  Highly cambered airfoil, Selig S1223, 11.5" chord, 66" span.  I have searched for papers on the topic but cannot find anything.

Thanks!

The boundary layer separates usually at somewhere around 10-15% of chord near stall AOA (which is where you place vortex generators when using them to delay stall AOA).  Separation will start at the trailing edge and move forward.  If it was me I'd tuft the entire lower surface from just behind the LE, maybe 10% so you can really see everything that's going on.  Doesn't hurt to put too many on.


How is wing area defined? When the plain "wing area" is given in basic aircraft specifications, what area does it usually refer to? Is the same area used for calculating the wing loading?

For example I have a fuller specification in front of me which gives areas for:

Main plane with ailerons
Main plane with ailerons and body centre section
Aileron, total
Tail plane
Elevators, two

If this were being condensed to a single value for "wing area", which of these would be used? Obviously, for something with a very long root chord such as a delta-wing jet, the areas with and without centre section will be very different, also leading to very different calculated wing loadings.

Has standard practice changed over the years?

Yes, the plain wing area is also used for wing loading. How the value is calculaed depends on the method used. In all cases the center section and ailerons are included but the horizontal tail is excluded. Common is also the use of the projected area of the clean wing in the x-y-plane, because that is the area relevant for lift.

For airliners, the two most important methods are the Wimpress and the Airbus method (the Wimpress being used by Boeing). The difference is the triangles you get by extending the wing's leading and trailing edges forward into the fuselage for Wimpress versus connecting the points where the leading and trailing edges meet the fuselage on both sides by two straight lines for Airbus. This discussion on airliners.net lists a comparison of the wing areas you get with both methods for a range of models.

Wing area methods in comparison. The shaded are is covered by the fuselage and determined differently; creating different areas for the same wing.

Note that the specifics of the method used is irrelevant for the purpose: This wing area serves as the reference area of most aerodynamic coefficients and either method is good enough for the purpose. Consistency is more important than precision here.

Both methods have been around for a while, so there was no change recently. But since both are used in parallel, there is no common standard.


Must quadcopters have the same propeller sizes? I was flying quads today and it just suddenly popped up into my mind — do the propellers all have to be the same size in quadcopters? Mine are, but What if two propellers were larger and the other two smaller? The drone would still be symmetrical, but would it have any effect on the performance of the drone? Could you explain the physics behind your answer?

The main point of a quadcopter is that it can be controlled by engine power using much simpler fixed-pitch rotors¹. Assuming this construction, you need to be able to separately adjust power of

forward and rear set of propellers for pitch control,
left and right set of propellers for roll control and
clockwise and counterclockwise-rotating set of propeller for yaw control.

If you had two bigger and two smaller rotors, then one of the combinations would have the two bigger ones in one of the sets and the two smaller ones in the other. That would make it really, really hard to balance all the parameters to maintain appropriate controlability.

I think it might not be completely impossible, but it certainly makes no sense.

¹ Of course once you add cyclic and collective control, you can fly with one rotor only (well, two, or one with anti-torque rotor or jet) as most full-scale helicopters do, but then you've lost the simplicity of fixed propellers.


Do crews look for "Dr." on a passenger manifest to identify medical doctors? With many airlines, when a passenger buys a ticket online, there is a space for them to select a "title", such as Ms., Mr., Dr., Rev., etc.    In most social circumstances, the title "Dr." is applicable to anyone with a doctoral degree (M.D., Ph.D., D.D., etc).

I have heard a rumor that in the event of a medical emergency on the plane, the cabin crew will check the passenger manifest for people who have given the title Dr., in hopes of finding a medical doctor who can assist.

As a consequence, goes the rumor, a passenger who has a doctoral degree, but is not a medical doctor (say, a PhD in mathematics), should not choose the title "Dr." when buying a ticket, since if there is a medical emergency, the cabin crew might waste time asking this mathematician for assistance.

I have also heard it said that this is nonsense, that crews would not bother looking at the manifest, and would simply make a PA announcement asking for anyone with medical training.  (This has the advantage that it would also identify medical professionals such as nurses, paramedics, etc., who would not use the title "Dr." anyway.)

Which of these reflects actual crew training or standard procedures?

What is the best practice for a passenger who holds a non-medical doctorate?  I've never seen any official guidance from an airline on when to use the title "Dr" on a ticket.

(I wasn't sure if this question would be best here, or on Travel.SE, or maybe even Skeptics.SE, but I thought this site would be most likely to have people with first-hand knowledge from the crew side.  I'm open to suggestions for migration.)

You are correct that it is done by PA rather than by reference to the manifest, at least at my carrier in my type of aircraft. As you surmise, the "Dr" notation has too many opportunities for both false positive AND false negative results to be where you want to start when time is of the essence.

Besides, if you need medical help at 40,000', you're probably much better off in the hands of a good E.R. nurse or paramedic than with a radiologist who hasn't done hands-on emergency care since his residency.

Most airlines use a service that's reachd by a phone patch to an E.R. doctor who specializes in evaluating these sorts of situations (and recommending, divert now or continue on), so any medical professional on the aircraft is working under his direction. He is also familiar with the contents of a medical kit carried on the aircraft just for these situations, so he can say "have the (nurse/paramedic/doctor/etc) administer ___ out of the medical kit" already knowing what is & isn't available.

Interestingly, a veterinarian can also administer the same medications the other medical professionals can in this sort of situation!


How do investigators always know the logged flight time of the pilots? Almost all incident investigation reports include the flight experience of the both pilots with hour accuracy. For instance, for MH370 just a few small fragments of the plane have ever been found, yet the investigators know the captain had 18,365 hours of flying experience. How do they know?

From that I have seen on a web, a pilot logbook looks like an unprotected paper document, often unlikely to survive. Do pilots actually keep the book at home or in the office, or maybe someone else apart the pilots logs the flight hours?

The methods that Dan referred to in his answer are valid, and might be used by investigators—the NTSB for example.

However, generally speaking the NTSB will not spend a great deal of effort researching pilot time. Instead they will resort to one of two methods of determining pilot experience.

Firstly, if a logbook is readily available—either found at the scene or made available through the investigation—it will be used to determine pilot experience.

Secondly, if a logbook is not readily available, the NTSB can determine pilot experience based on the pilot's reported flight time on his or her last application for a medical certificate. Since an application for a current FAA third class medical can be several years old, private pilot experience determined by this means may not accurately reflect an accident pilot's actual level of experience. However, a professional pilot's application for an FAA first or second class medical will only be six to twelve months old, and will much more accurately reflect that pilot's actual level of experience.

Similarly, FAA certificated pilots report their flight time on some IACRA applications for pilot certificates. This reported time will have been independently verified as accurate (unlike times reported on medical certificates). However, in most cases, a pilot will have applied for a medical more recently than for a certificate.

For actual examples of these two scenarios, see NTSB report CEN16FA333, in which both are detailed.

For the first pilot involved:

  The [first] pilot's flight history was reconstructed using logbook documentation. His most recent pilot logbook entry was dated July 31, 2016, at which time he had accumulated 135.5 hours total flight time, of which 48.6 hours were listed as pilot-in-command. All of his flight time had been completed in a Cessna model 172N single-engine airplane. He had accumulated 5.0 hours in actual instrument meteorological conditions, 12.9 hours in simulated instrument meteorological conditions, and 3.4 hours at night. He had flown 24.7 hours during the prior 12 months, 4.4 hours in the previous 6 months, 2.4 hours during prior 90 days, and 1 hour in the 30 day period before the accident flight. The pilot's logbook did not contain any recorded flight time for the 24 hour period before the accident flight.

For the second pilot involved:

  According to FAA records, the [second pilot], age 60, held a private pilot certificate with a single engine land airplane rating. His last aviation medical examination was completed on November 6, 2014, when he was issued a third-class medical certificate with a limitation for corrective lenses. A search of FAA records showed no previous accidents, incidents, or enforcement proceedings. A pilot logbook was not recovered during the on-scene investigation; however, on the application for his current medical certificate, he reported having accumulated 120 hours of flight experience.


Why can some runways only be used from one end? Is there any reason for why some runways can only be landed and taken off from one end and not the other, even though there aren't any obstructions (mountains, etc.) and apparently can be used from both ends?

Even if there are no physical obstacles visible - the airspace structure can be somewhat more difficult and one runway direction might run into other reserved airspace. Or just go over noise-sensitive neighborhoods...


Why is one exhaust directed upwards? This is the first Cessna Skycourier during certification. Why are the two exhausts oriented upwards?

The rotation direction of the propellers produces a rotating slipstream.  This slipstream pushes the exaust from the (engine's) right side downward, but the exhaust on the other side would be pushed up, producing potential heat problems where it impinges on the wing (as well as depositing lots of soot).  By bending the exhaust pipe upward as you see, the exhaust on that side passes over the wing, in an airstream that will both protect the wing surface from the exhaust heat and reduce soot deposition.


What regulatory bodies are there? Since planes traverse the Earth, and a country may deem another country's airline unsafe, yet that unsafe airline is deemed safe in its country, what is the hierarchy of the aviation regulatory bodies from the highest global level down to a state or even a town.

One example of how it moves top-down would be great.

Global

Starting from the top, we find the International Civil Aviation Organisation (ICAO), which is a United Nations agency. ICAO was formed in 1944 when the Chicago Convention was signed, and initially had 52 member states. Today, 191 states worldwide are members of ICAO. The headquarter is in Montreal, Canada, and there are 7 regional offices.
ICAO standards and other provisions are developed in the following forms:

Standards and Recommended Practises (SARPs):

Technical specifications adopted by the Council of ICAO in order to achieve the highest practicable degree of uniformity in regulations, standards, procedures and organization within air navigation. Member states will conform to standards and will endeavour to conform to recommended practises. States are invited to inform ICAO of non-compliance.

Procedures for Air Navigation Services (PANS)

PANS comprise operating practises and material too detailed for SARPs. They are suitable for application on a worldwide basis. Member states are invited to publish differences between national procedures when considered important in the interest of safety.

Regional Supplementary Procedures (SUPPs)

SUPPs are supplementary procedures which are only valid within one of the ICAO regional office regions. For example, there are supplementary procedures published only valid within the European/North Atlantic region.

Also worth mentioning are the International Telecommunication Union (ITU) and the World Meteorological Organization (WMO), both of which function on an (almost) global level.

Regional

Below ICAO we have regional organisations. As an example, in Europe, the European Union (EU) provides legislations for its member states. Non-regulatory organisations, such as the European Aviation Safety Agency (EASA), assist the EU in developing regulations (but the EU, not EASA, is the regulatory authority).

Some areas do not have regional regulators like the EU. In such areas, national regulations are handled directly by national regulators. Within the EU, countries also have national regulators – but they are bound to base their regulations on EU regulations.

National

Below the regional level we have national regulators, commonly referred to as civil aviation administrations (CAA). Each country will have their own CAA. One that you will often hear about is the Federal Aviation Administration (FAA), which is the national regulator in the USA.
Generally speaking, a national regulator will be following framework regulations published by the national ministry of transport. The term ministry of transport is also sometimes applied to the departments or other government agencies administering transport in nations who do not employ ministers. The framework regulations from the ministry of transport are largely based on ICAO SARPs, PANS, SUPPs and any regional regulations (such as EU regulations).

The national regulator will publish detailed regulations, which are adapted by the air navigation service providers, as well as airlines, pilots, air traffic controllers, maintainence companies etc. etc.

The service providers, commonly referred to as ANSPs, are the organisations actually providing air navigation services, such as communication, navigation and surveillance equipment, meteorological services, air traffic management (including ATC), aeronautical information and search and rescue. Most ANSPs operate within a single nation, but there are also multinational ANSPs, a few examples are ASCENA (Africa and Madagascar), COCESNA (Central America), EUROCONTROL (Benelux) and NUAC (Sweden and Denmark).

The service providers work together with the national regulators to develop detailed procedures, but do not (usually) have any regulatory authority.


How does airspace work with the navy? Let's say some ships from the US Navy are in the middle of the Pacific Ocean.

Is there anything to stop another country, say the Chinese or the Russians flying around them in circles, either to watch what they're doing or just to piss them off and provoke a reaction?

I mean technically it would be international airspace?

There are a few factors and it may also depend on the relations of the nations.

If the aircraft is unarmed and simply surveilling it may be covered under the open skys treaty. This is largely about overflying other nations territories but presumably applies to over flying marine areas as well.

If the aircraft is armed and potentially making a move to attack the fleet may respond by intercepting. This is more or less the plot of Top Gun.

If the aircraft hails from a friendly nation, they may attempt to contact them and may interrogate their IFF system


Why do turbofans go faster than turboprops? Can someone explain why turboprops are less effective in terms of thrust than turbofans? But at the same time, why turboprops burn less fuel? For example, the Bombardier Q400/De Havilland Dash 8 has a fuel burn of 2.79L/seat/100km, whilst the 737 MAX 8 with the same number of engines burns 2.93L/seat/100km, but the Bombardier has a max. cruising speed of 360 knots and the Boeing plane can go up to 526-527 knots, even though it's heavier.

To generate a given thrust, a turbofan accelerates less air but faster than a turboprop. This means that turbofans can fly faster. However thermodynamically it is more efficient to accelerate a greater mass of air slower, so turboprops are more efficient and that translates into lower fuel burn.


Is the FF (Fuel Flow) the fuel consumption index? I am creating a program to calculate the fuel needed to fly and I need to know the fuel cost of the plane. In the boeing 737-800 (PMDG), on the display that shows the plane's systems, there is the FF (Fuel Flow) field. I think that is the fuel consumption index.

However, using a stopwatch, the value there appears to be incorrect. Advancing the throttle to keep the FF at 5, about 10 pounds was spent in 8 seconds. I expected 10 pounds to be spent in 2 seconds (5 pounds * 2 seconds = 10 pounds).
My question is: does the FF indicate the plane's fuel consumption? If so, does it indicate pounds per second? If not, how can I know the fuel consumption?

The FF value can show either the fuel flow or the fuel used, depending on the position of a switch on the center forward panel. If it shows the fuel flow rate, then the value shown is in thousands of kilograms or pounds per hour per engine.

1 FUEL FLOW Switch (spring-loaded to RATE)
RATE – displays fuel flow to engine.
USED –

displays fuel used since last reset
after 10 seconds, display automatically reverts to fuel flow.

RESET –

resets fuel used to zero
displays fuel used for 1 second, decreases to zero, then displays fuel flow.

2 Fuel Flow (FF) Readout (digital)
[Option - kgs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (kilograms per hour x 1000).
[Option - lbs] Displayed (white) – fuel flow to engine with FUEL FLOW switch in RATE
position (pounds per hour x 1000).
3 Fuel Used Readout (digital)
Displayed (white) – when FUEL FLOW switch moved to USED or RESET.

(Boeing 737 NG FCOM 7.11.13 - Engines, APU - Over/Under-Displays)


Why don't we simulate similarity parameters simultaneously in the same wind tunnel? As 'Fundamentals of Aerodynamics' by JD Anderson suggest

Today, for the most part, we do not attempt to simulate all the
parameters simultaneously; rather, Mach number simulation is achieved
in one wind tunnel, and Reynolds number simulation in another tunnel.

Is it done for accuracy? Or Does it need an extra big wind tunnel to do that?

It would be great if we could match all parameters at once. But that would require a tunnel large enough to accommodate the full size airplane and powerful enough to match the actual flight speed. The reality is that we cannot in most cases.
We have some answers here on windtunnel testing. They should cover the basics:

How are wind tunnels used with scale models?
What are some specific benefits of cryogenic wind tunnels?
How is windspeed scaled in wind-tunnel tests?

The most important similarity numbers are the Reynolds number and the Mach number. Both grow linearly with flow speed, so it helps to match flow speed. In case of the Reynolds number, the other parts of it are the length of the flow path and the viscosity of the fluid. If we use air, which is plentiful and free, we also need to match the actual size so that the Reynolds number matches the original. The Mach number also depends on the speed of sound in the fluid, which in turn depends on temperature.
It is possible to match both Reynolds and Mach number in super cold tunnels using a pressurised gas like nitrogen. This lowers the required gas speed so the tunnel needs less power to run and produces realistic results with smaller models. The expense of building and operating such a tunnel is immense, though.


Why are there so many 7700 squawks? I installed an app some months ago, which gives a push message when an aircraft squawks 7700. The first time it happened, I thought the plane was about to crash, but fortunately nothing happened.

However, within the last months, I got a lot of notifications, maybe a few a week or so. I'm wondering, why there are so many emergency alerts?

I've read about possible reasons for 7700 squawks and they seem to be manifold. Isn't the purpose of an emergency call to be used only in really critical scenarios? How is an aircraft going down to be differentiated from one carrying a sick passenger? Are there any plans in the industry to change the current system?

7700 is a "general emergency" squawk. It tells ATC that there is "a problem" of some sort with a particular plane. And I agree that this "general umbrella" type of squawk is a good idea. I have several disagreements with the FAA, but this is a good one. Several thoughts:

the specific nature of the emergency is only for the PIC and ATC. Nobody else needs to be informed, they can't help anyway.
oftentimes lost comm is part of the problem. So they can't talk, but you still want to show ATC that you have an emergency. You could squawk 7600 for lost comm, but if you have some truly serious, like an engine failure (I personally don't consider lost comm anything even remotely near "truly serious"), you can inform ATC with 7700 that you have a more serious problem than just lost comm. They'll notice the "lost comm" aspect of this situation on their own, by not being able to talk to you. Now they know you have something more serious, because you don't show 7600.
If you really want to use different squawk codes for different types of emergencies, it would merely add to the already large amount of numbers and codes pilots and controllers have to learn by heart. In an emergency where time might be of the essence, trying to remember abstract information like this can be difficult.

"The first time it happened, I thought the plane was about to crash"

Why did you think that? 7700 is not some "we're gonna die" type of squawk. It's a general "umbrella type" of squawk for all sorts of emergencies. An intelligent pilot will squawk 7700 for any "small" or "big" type of emergency (and who is supposed to be the judge on "big" vs. "small"?).

"How is an aircraft going down to be differentiated from one carrying a sick passenger?"

It isn't, because it doesn't matter. The people that are familiar with the situation (PIC and ATC) are in communication, unless lost comm, in which case the controller who can't glean more info from the 7700 squawk can't help anyway, and nobody else needs to be informed because they can't help anyway. Squawk codes aren't meant for desktop people like you who download some app and watch.

"Would you recommend handling this issue differently than it is done now, if you were in the position to?"

No. There are good reasons to have an "umbrella" type of squawk for all types of emergencies.


Does the country of issue of an EASA medical certificate matter? I am choosing the country where to do my initial for the EASA Class 1 medical. The choice is to either do it in the UK (let's ignore Brexit for now) or in Italy. In both countries I would get (if deemed fit) an EASA Class 1.

My question is: to fly and train in Europe, how important is the country of issue?
If I pass the medical, I am planning to go to an FTO in the UK.

I discovered that it's probably better to do the medical in the UK as the country of issue of your licenses has to be the same as the country of issue of your medical. This way, I avoid the conversion.


What are the effects of crosswind on glide angle? When gliding what are the effects of a crosswind on my glide path angle?

If I have head-wind, the minimum flight path angle (usually called gamma) will be lower than the one without crosswind and with an attitude with a larger lift coefficient  Cl than lift coefficient of max efficiency with no wind?

On the other hand, if I have tailwind gamma will be greater and the Cl will be lower than Cl of max efficiency. Briefly if I have tailwind without engine I can glide much longer, is it right?

Wind affects your flight path angle by affecting how far you travel while you're descending.
With no thrust from the engine an airplane is falling out of the sky at a roughly constant rate of descent. The wind direction affects the groundspeed, and thus the distance covered between when the engine failed & when the aircraft hits the ground.

This is more directly observable from outside the airplane.
Consider these 4 scenarios on a post-it note: The airplane has lost its engine, and the pilot is flying at best-glide airspeed while holding a constant magnetic heading (making no wind corrections):

No Wind
The aircraft descends along the no-wind glide path (about 45 degrees in my drawing), and travels in a straight line over the ground (the "ground track") while doing so.

Headwind
The aircraft descends along a steeper glide path (about 60 degrees in my drawing), because while it's still descending at the same rate the headwind is slowing its progress over the ground. Since it's a direct headwind (right on the nose) the ground track is still straight.

Tailwind
The aircraft descends along a shallower glide path (about 30 degrees in my drawing), because while it's still descending at the same rate it has a tailwind, which is making it move faster over the ground. Since it's a direct tailwind the ground track is still straight here too.

Crosswind
The aircraft descends along the no-wind glide path (because it has neither a headwind nor a tailwind), but the crosswind pushes the ground track (in this case into the inconveniently-placed tree).

Your endurance (time aloft) will be similarly unaffected: If you've set the aircraft up at its optimum glide angle-of-attack (best glide airspeed) the descent rate is pretty much the same regardless of whether you have a headwind, tailwind, or crosswind. You will fall the same distance in the same amount of time, it's just a matter of how far you travel while you're falling (which determines where you land).
You can test this for yourself in an airplane by doing engine-out drills: Line up with a headwind, pull the throttle out, and pitch for the aircraft's best glide speed. Note your vertical speed (rate of descent), and then repeat the test in the opposite direction (tailwind) or at 90 degrees to your original course (direct crosswind): The vertical speed will be effectively the same.


How beneficial are flaps when landing with a tailwind? I understand that flaps are devices used to increase the amount of lift and drag. When landing with a tailwind your groundspeed will be higher. Therefore, flaps would help reduce this groundspeed, logically allowing for a lower touchdown speed.

Should flaps always be applied with a tailwind? Are there reasons not to use flaps when landing with a tailwind?

Forget about tailwind/headwind for flap use.  Flaps allow landing with a lower airspeed.  And you always want a lower airspeed.  The wind itself is irrelevant except that your groundspeed will be higher or lower depending on where the wind is, and which you always want to be slow as possible.  On light aircraft you want to avoid tailwind components over 5kt. Jets are usually certified, takeoff and landing performance wise, with a max tailwind component of 10-15 kt. But in any case you are still using normal landing flap to have the minimum airspeed, giving the minimum groundspeed for a particular condition.

The time when you may want to land with less than landing flap is when the winds are strong and very gusty (and you will be landing into wind if you aren't crazy) and you want to have a bit more energy with a bit less drag (so the aircraft conserves more energy with its inertia) during the landing flare to cope with sudden small airspeed variations from gusts, which can drop you on the runway hard before you can react if you have the flaps all the way out.


Why would my electric turn coordinator suddenly stop working? The electric Turn Coordinator in our Cardinal stopped working and we would like to understand the cause.
It simply doesn't move at all, it remain fix.
The red flag is not on, I have double checked the circuit breakers as well.
What could be the cause to that?

The motor for the gyro in a Turn Coordinator runs off fixed frequency ac power. For this there is a circuit board in the unit that includes a solid state static inverter to convert the airplane's 12 vdc to ac (110 v at 400 hZ IIRC).  When they just die like that it's usually because the inverter or something else on the circuit board has gone south.


How much snow can cover a runway while still allowing commercial airliners to land safely? In regards to this question I thought about how much snow is shown on the runway and the exits in the movie.

So normally a commercial airport will have a fleet of snow plowers or heaters to keep runways, taxiways and aprons mostly clear of snow. But what is the safety limit on when the airport would have to close down due to too much snow coverage on the runway (not due to heavy snowfall, but due to the runway being covered completely in snow).

I think once the markers are no longer visible you would have a severe problem navigating on the ground or once snow piles up so high that the engines would take in a lot of snow it might also be a problem. But what are the actual limits during which one could safely land an aircraft on a snowy runway? And has it ever happend that someone landed on a runway covered in snow anyways?

There is no set legal limit (ultimately it is up to the PIC to decide) but condition reports are available to pilots at many airports to assist in assessing takeoff and landing performance.   The FAA has recently created a standardized way of reporting runway conditions through their TALPA (takeoff and landing performance assessment) initiative.

TALPA is an improved way of assessing runway conditions, based on contaminant type
and depth, which provides an aircraft operator effective means to anticipate airplane
braking performance.

The RCAM (shown below) is the tool airport operators use to report a runway surface assessment when contaminants such as water, ice, snow, slush, and others are present on runways, taxiways, and aprons.   To find an updated RCAM look in Chapter 5 of Advisory Circular 150/5200-30, Airport Field
Condition Assessments and Winter Operation Safety.

The RwyCC is the runway condition code -- as you can see a code of 0 yields minimal to non-existant braking and/or directional control.


How is the fuselage of the Boeing 787 produced? I am particularly interested in the Boeing 787 Fuselage because it is one of the first aircraft to carry such a high percentage of Carbon Fibre Reinforced Polymer (CFRP).

Can someone explain how the fuselage is made? I know the basics of making
CFRP which include fibre being wrapped onto in mandrel in different orientations then heating in an autoclave to cure.

However, I also know that the whole fuselage is not produced in one piece. Pieces of the fuselage are produced then joined together.

I would like to know the joining process works.
Since it is not fabricated as a single piece there are bound to be stresses on the joints. Will it effect/endanger the passengers?
What loads can it take since the joints will cause a drop in overall pressure (what is the maximum pressure if possible). I don't think its being welded together but not sure the reason.
Are the same methods used to join the wings?
Last but not least, are there problems due to the joints before and if so, what has been done to rectify these problems?

There is a YouTube video showing the method for creating the structure:

YouTube: 787 Composite Construction

This video from British Airways shows the joining process:

Fuselage Section Joining Process which you can see is not welded, but riveted together. The sections are lined up and riveted together using joining plates.

  Since it is not fabricated as a single piece there are bound to be stresses on the joints. Will it effect/endanger the passengers?

No, not more than any other conventionally constructed fuselage. The joining plates are metal and the structure around them is a metal bulkhead. The 787 fuselage is not a 100% composite construction.

  Are the same methods used to join the wings?

Yes, if you watch the second linked video in full, you can see the composite wings being bolted to the fuselage structure. The entire wing support structure looks to be a conventional metal structure.

  Last but not least, are there problems due to the joints before and if so, what has been done to rectify these problems?

There are new challenges with any type of new construction technique, here are a few articles that outline the challenges:

NY Times: New Challenges for the Fixers of Boeing’s 787
MIT Technology Review: Boeing's Composite Problem

Boeing has said that the 787 is as easy to repair as traditional aluminum skin aircraft. One case in point is the Ethiopian Airlines aircraft ET-AOP damaged by fire. The procedure to fix it is to cut out the carbon fiber material and put a "cure in place" patch over the hole. In this case other structure had to be replaced or repaired like stringers and bulkheads. This was the first major airframe repair that Boeing had to undertake. ET-AOP took 18 months to repair before being returned to service.


What is the difference between a stall fence and a vortex generator? Both of them prevent the aircraft from stalling, so is there any difference between them? The wing fence doesn't produce a vortex, so how does it work?

The wing fence is more or less what it is called- a fence. For swept wing aircraft, the local flow is not along the direction of flight, as there is a significant spanwise flow from the center towards the tips. It can be seen in the following figure.

Image from fuckyeahfluiddynamics.tumblr.com

The end result is that the boundary layer continues to grow (thicken) in this direction, with outboard regions more prone to stall than inboard ones. Basically, the wing fences stop the spanwise flow, with the result being a new (thinner) boundary layer starts on the other side of the fence.

In case of a vortex generator, the VGs basically create a vortex which re-invigorates the boundary layer by mixing free stream air with that of the boundary layer, thereby preventing separation.


How was the endurance test in the Cessna 172 legal? The flight in the Cessna 172 lasted around 64 days with two pilots switching to fly the plane. To my knowledge both FAA and EASA have rest periods which pilots have to follow. Also the refueling was done by a truck matching the speed and handing over a hose to refuel. How was this flight legal regarding rest periods and refueling maneuver?

Regarding flight time, duty periods, or rest requirements, no current regulations would have specifically imposed limitations on this flight. This flight would have been operated as a private flight under 14 CFR 91 under today's regulations.

The flight, duty, and rest requirements are specified only for certain commercial operations including airline, charter, and fractional. Flight instruction is also generally limited to 8 hrs in a 24 hr period. Paid pilots routinely fly 12-14+ hours in a day, including in certain patrol activities and in flying private jets.

Refueling is not generally restricted by FAA aiation regulations in a way that would specifically impact this operation. As with the flight time concern, the catch all "careless and reckless" regulation might apply first.

For some of the same reasons that the crew flight time was not restricted, many of the maintenance requirements such as 100 hr inspections, would not be required of a private flight. However, the 100 hr seat rail inspection AD would be problematic today.


Is the MD-11's third engine slightly nose up? From my visual inspection, the third engine (the engine on the horizontal stabilizer) is slightly "nose up", especially compared to the engines on the wing. That means, the front part of the engine is lifted slightly. Am I correct? If yes, what is the intention?

Yes, there is a slight tilt to the engine. Engines are heavy, so placing them on the tail shifts the center of mass towards the tail, which is not good for stability. To counter this, aircraft with rear engines often tilt the engines slightly, so the thrust they produce can offset their own weight, helping with stability. It's extremely slight though, as large angles can produce major pitch oscillations with power changes (something known as pitch coupling).


Can commercial pilot get paid for introductory flights with their own customers? Can a pilot with their commercial certificate look for their own customers, rent an airplane, and get paid to do introductory flights?

If yes, where in the FARs does it talk about it?

There are three parts to your question and the answer to all three is no. And there isn’t one specific place in the FARs that says you can’t do this, but reading Parts 91 and 135 along with appropriate ACs and Legal Opinions of the FAA leads most pilots to realize that they can’t do this.
§91.147   Passenger carrying flights for compensation or hire. contains a list of requirements that you must satisfy as well.

If you look for customers it is considered 'holding out' and according to AC 120-12A ‘A carrier becomes a common carrier when it "holds itself out" to the public, or to a segment of the public, as willing to furnish transportation within the limits of its facilities to any person who wants it.’ And if you are a common carrier, then for this type of operation, you must have a Part 135 certificate.

As a commercial pilot, you can be paid to fly someone in their airplane. Lots of commercial pilots do this. However, if you provide the plane then you are acting as a Part 135 operation and you can’t do it.

Getting paid is also not allowed. Since you can‘t do parts one and two of your question, you are acting under your Private Pilot certificate for the flight. At most, you can split the immediate expenses i.e. fuel, landing fees, etc. Even that is a bit dicey since you won’t have a ‘common purpose’ with the person you are flying. In other words, without holding out for customers, you wouldn’t have been making the flight anyway.

Not part of the question, but there are lots of things you can do with a commercial pilot’s license:

Corporate Pilot, Charter Pilot, Contract Pilot (for a private owner), Freight/Cargo Pilot, Ferry Pilot, Banner towing Pilot, Sight-Seeing/Tour Pilot, Agricultural Pilot/Crop Duster, Skydiver Pilot, Photographer Pilot, Airshow Pilot, Demo Pilot (aircraft sales), Medical Evacuation Pilot, Humanitarian/Charity Pilot, News/Traffic Pilot, Pipeline Patrol Pilot, National Defense or Border Patrol Pilot, Firefighting or Forestry Pilot.


How can I avoid floating during short-field landings? I fly a Piper Cherokee PA-28R for my training and have a question about the techniques used for a short field landing.

What is the difference between a normal landing roundout/flare and a short-field landing roundout/flare? Should I begin the roundout and/or idle the power sooner to get minimum floating? Where does the difference in 'float distance' come from?

I did all my training in PA-28's (Warrior and Archer II mainly) and here is my advice)

First off, practice makes perfect in all cases here.

Second, I am not a CFI and you should ALWAYS seek the experience of a CFI to help you learn how to fly properly. Discuss everything with your CFI before trying anything.

The PA-28 is one of the most benign airframes and even in the bumpiest the bumpiest of conditions you can fly it by the numbers every time. Floating on landing is generally caused by (in my experience) 2 things.

Excessive speed on final - Remember you will only land nicely once you have gotten rid of all excess speed. This happens during your round-out and flare to landing. On a short field even a perfect landing will take up more runway if you are overspeed since you need to bleed that speed off.
Ballooning - This is generally a result of over speed and over flaring. Since the PA-28 is a low wing (compared to the high wing of the C172) an over-flare to early will cause significant ballooning as you effectively bounce off the ground effect. A proper timed round out and flare will come in time.

One thing you can do to mitigate this (and I found this helped as a student pilot for me) is to fly a longer final leg (and thus a longer downwind). When I was learning to fly a stable approach it took time for me to get the settings in and really nail the approach. A longer final will help you settle the plane into place as a student and really nail a maneuver like this. Generally tight patterns are taught but if its safer and better for you to fly a longer final and nail the maneuver, do it.

In the PA-28's you can (and should) be right on 65 kts when doing a soft field. Practice descending at a constant airspeed at altitude with your instructor. 500ft/min right on 65Kts with full flaps, then come back into the pattern and use what you learned to come in right as you should.

Cut the power a bit earlier. Generally you are most likely taught to cut the power over the numbers and touch down at the markers. In a short field situation you can cut the power a bit earlier, pitch down to keep you speed up, and touch down right on the numbers. You should have no issue getting the plane down in under 1000Ft.

Train at an actual short field (or as short as you are allowed to fly). My flight school generally had us going between 5000ft airports with 2 runways (we were based at KPNE). Crosswinds were never really an issue and runway length left plenty of margin for error. My instructor made it a point to take me to 3000 X 60 fields to really get practice in (the shortest the fligh schools insurance would cover). Depending on where you are training out of this may not be an issue.

One more general habit of new pilots is over correcting and getting your self into an increasingly corrective state. This happens both laterally and vertically but in your case the vertical over correction is going to be the killer. If you are comming in and notice that you are low you add power, pitch up a bit and now you are too high. You pull power and pitch back down but this builds up some speed. Now you find your self over correcting all over the place to get back to a stabilized aproach. This is where the long final will help. You need to really get the plane on that 65Kt mark, trim it off, leave the power alone and fly it in. If you get bumped by a gust or a thermal wait a second and see if the termal settles then and only then should you come in with a small correction.


How should these translated takeoff and climb settings be interpreted? I am having difficulties understanding this transcript. It is originally not in English, so I was wondering if anyone could help me to make sense of this and also help me  with the correct wording in English. The training cap originally says

  after the ground roll (four engine turboprop) : 12 degrees nose-up; 1500 feet; decrease power/ reduce engine thrust power (?); climb speed (?); flaps up.

I know that usually at around 1000 feet agl the pitch is lowered so I imagine that was an order to lower the pitch to 12 degrees nose up or a confirmation of that. Does that make sense? Would one lower it after the final phase of the initial climb?

Then there is the mention to decrease power. Would power be reduced at height?

I've found a document from a flight school that says power is reduced to remain in the traffic pattern, but that was after 2500 feet. And I found a skybraero page that says that after 1500 ft aal one would "reduce thrust to climb power". Would reducing thrust make more sense than reducing power? I thought they both meant the same thing. How are they different?

At the end, when (I think) he says climb speed. I found nothing about climb speed specifically... since this is said after the final phase of the initial climb out (that goes up to 1500 ft), when climb to cruise phase starts, would it make more sense if that read climb to cruise speed/ cruise climb speed (Vcc)?

12 degrees nose up

Since it says this comes after the ground roll, it sounds like this is the pitch you rotate to when you reach rotation speed. You then hold this pitch for the initial climb out.

  1500 feet; decrease power/reduce engine thrust power

In English, generally "power" is used on propeller aircraft, and "thrust" used on jet aircraft. Other languages might not translate exactly. On a turboprop airplane, you would have power levers, which you use to select a power setting. At this point, you would decrease the power setting to a climb/continuous setting.

  climb speed

This just means you will fly whatever speed you plan in climbing at. This would depend on the aircraft, and the flight. Especially larger, higher performance aircraft will have some standard "climb speed" that is used during climb, to balance ground speed and rate of climb. "Cruise climb" may be used to describe a slower climb where emphasis is placed on a higher ground speed. In smaller aircraft there may be less performance margin. This wording may also be leaving it open, as on some flights you may want best rate/angle of climb, others would use a regular climb speed.


What are the forces acting on an air particle along curved streamline(wing upper surface)? What forces act at air particle at curved streamline (at first 1/4 upper wing surface) , looking from inertial and non-inertial frame of reference? (show free body diagram)

Pressure gradient normal to the local flow direction balance the centrifugal force associated with the curvature of flow..
Which Centrifugal force,inertial or reactive?

In the inertial frame of the free airstream, the pressure force causes the particle to accelerate inwards. Centrifugal and centripetal forces do not play a direct role; there is no space station wall for centrifugal force to be exerted on, nor to exert a reactive centripetal force. The pressure force acts in the role of the centripetal force shown in the astronaut diagrams. The balancing reactive force is instead another pressure force, which acts on the airfoil where it is known as lift.

In the rotating frame of the air particle, inertial centrifugal force does come into play. The particle must by definition remain stationary within its reference frame, so all forces on it must now balance. The pressure force is thus balanced by the inertial centrifugal force. The airfoil lift no longer plays a direct role within this reference frame (having helped to create it).


What are the differences between PC and professional simulators? It looks like there are a differences between the models used in the schools and the simulators you can run in a PC .

I'm curious to know the differences and price of both type of simulators.

I thought that the one in the schools were just simple PC simulators.

What simulator can I use to practice basic helicopter stuff?

The main difference is in the flight dynamics model. A PC simulator can be realistic, but who is to say that it is? A professional simulator must show to the authorities (EASA,FAA etc) that the modelled behaviour matches flight data.

There are several authorisation levels, from very basic to zero-flight-time. Even for the basic devices, the authorities want to make sure that only correct cues are generated and that there never will be any negative training habits formed by the simulator user.

PC-based programs can teach students where the buttons and indicators are, what to push, which instrument to monitor far what etc. Some of them are accredited by the authorities, more info in answers to this question.


Why do the N1 and N2 fans have different shafts? This question here contains a cutaway of a jet engine (which I'm putting right....here)

Credit: K. Aainsqatsi, Wikipedia

After reading the answer, and looking over the chart, I noticed that the shafts for the N1 and N2 fans are completely separate (something I'd never noticed before.)

What purpose does that serve?  Do they always move freely from each other, or can they be locked?

Several reasons:

The fan, LPC/T, and HPC/T spools all operate at different speeds for optimal efficiency.  A hard link between them affects the speeds at which the other spools can operate, reducing efficiency.

Separate spools reduced the rotational inertia of each, reducing 'spool-up' lag, and allowing them quicker response times to throttle inputs.

The spools can be contra rotating, reducing the effective moment about the engine casing and mount, making the engine lighter.


Does the FAA maintain pilot records from before 1998? I have held a PPL with an instrument rating since 1999, and I am desperate to continue, but I lost my logbook.

Is there any way to get records from the FAA about my total flying time?

I contacted my flight school, but they say they don't have a record of me, so they asked me to start over from 0 hours.

What can I do?

If you took a checkride the FAA has your 8710 (form you would have filled out) which will have your flight times and can be used as proof of time.  Follow these steps to get your records.

You can search the airman database.  My father's and grandfather's records are in there from the 1970s and 1980s.  This just lists certificates though, no times.

You can also request a replacement certificate.  If you are a PPL then they will send you one.  You'll need to do this anyway to get the new plastic card.  Also, no times...just a certificate...which prove that you have at least the minimum flight time required to hold the certificate.

Good luck.


Could the Olympus 593 have been windmill-restarted during supersonic flight? The Rolls-Royce/SNECMA Olympus 593 (which powered the Concorde) used a complex intake-ramp system to improve the engine's efficiency and prevent an engine shutdown or failure during supersonic flight from tearing the aircraft apart; it did the latter by dumping almost all of the intake air overboard via a set of spill doors, allowing it to bypass the inoperative engine and thereby avoid the considerable drag generated therein.

On the other hand, a windmilling engine restart (one of only two midair relight methods possible for the Concorde, as it had no APU - the other being the use of bleed air from a still-operating engine - and the only one available in the event of a quadruple engine failure)1 requires that ambient air be forced through the engine in order to get it spinning, which would seem to necessitate closing the spill doors and not dumping the intake air overboard.

Could the Concorde's engines have been windmill-lit during supersonic flight, or would the aircraft have had to have been slowed to subsonic speeds prior to attempting a windmill restart?

1: While admittedly an unlikely scenario, there are multiple possible ways for this to happen other than fuel exhaustion; possible examples would be if a Concorde inadvertently flew through a volcanic ash cloud, or suffered fuel contamination, or experienced an upset at high altitude.

Yes they could be re-lit without cross-bleed (all engine flame-out above M1.2).

Regarding lack of APU, Concorde has emergency power generation based on hydraulic power (which will be operational above M1.1 due to windmilling and is used to power some or all the fuel pumps for the re-start). (If you're wondering about slower speeds, Concorde would use RAT.)

Regarding the spill-doors, they do not dump all the air out. The automatic operation is a function of inlet pressure (among other parameters). They will open to reduce pressure build-up (and may also open at low power setting above M1.3), but air will still enter the engine and exit the back end.

The all engine flame-out above M1.2 procedure (first 2 pages attached) is simple in nature and does not contain any notes on manual spill door operation. To summarize it:

Page 1:

Fuel transfer pumps will be configured for fwd transfer;
Hydraulic system will be configured for emergency power generation;
Avionics cooling will be configured (anticipating loss of air conditioning);
If no automatic re-light, then thrust levers to idle to permit manual re-light.

Page 2:

HP [fuel] valves (which feed the engines) to be selected off (so not to drown the engines);
Auto ignition off;
Select re-light mode;
Wait 30 seconds from turning off the HP valves (for fuel drainage);
Open HP valves and wait 20 seconds for a re-light.

And below is the optimum re-light envelope.

Source: Concorde FCOM


How can I find aircraft and airline listing data for planespotting? I vaguely remember when younger a spotter I knew having a book listing all airlines and all aircrafts within the airline at time of print. Is this data available now in book or online format? I'm looking to get back into spotting.

You can find much of this sort of information at http://www.airfleets.net

For example, searching for British Airways lists their fleet, and clicking the number of active for a model (eg A318) will list the tail number and other interesting information. From there you can click a specific airplane for all the information along with a picture.


Why do military jets seem to always take off using the afterburner? I was wondering. At different places I heard that civilian airplanes often (or even most of the time?) take off at less than 100% thrust.

For example a question that tackles this is here:
Is it possible that derated thrust takeoffs are safer than normal takeoffs?

This makes a lot of sense to me.

However, whenever I see videos of fighter jets taking off (from normal runways, not from carriers), they always seem to use the afterburner. But surely they must have plenty of power to get to take-off speed without them, so doesn't this waste large amounts of fuel unneccessarily? What are the considerations for doing it?

Or is this just a "filter bubble" impression I got, because it just looks cooler and whenever those planes are filmed they know that they are and thus do it for the show?

Military aircraft takeoff with afterburner when it is safer to do so with an exception- carrier pilots always light their cans while taking off (and also when the pilot wants to show off, but lets leave it at that).
The main consideration for using after burner in operational land based aircraft is the available runway length. When the aircraft is loaded heavily with ordnance, the runway required may be more depending on other factors (like OAT etc). In this case, it is better to go for afterburner takeoff. For example, Air Force Instruction 11- 2F- 16V3, F- 16 Operations Procedures specifically states:

3.6 Takeoff
3.6.6 Make an afterburner takeoff anytime the computed MIL power takeoff roll exceeds 50 percent of the available runway.

The same is true for other aircraft like the F-15 too:

3.7 Takeoff
3.7.2 ... For single ship takeoffs, if the  single ship computed  military power takeoff distance exceeds one-half of the available runway, takeoff using afterburner.

Another reason the pilots do it is to check the systems- it is better to check the afterburners on ground rather than finding any problems (like intermittent afterburner) in air. Of course in case of naval aircraft, the afterburners are on till the aircraft clears the ship.
Also, as @Jörg  pointed out, people upload videos that look 'cool'- hence the impression that they always use afterburners rather than the boring military power takeoff.


What was the Trim Problem on the Brabazon's First Flight? In this video, starting at 18:15, the copilot on the Bristol Brabazon's first flight recounts that the aircraft became airborne a little sooner than its pilots expected, and that despite the captain holding the yoke fully forwards, the nose continued to rise. The copilot responded by retarding the throttles and then advancing them again, which cleared up the problem. He does not mention any adjustment to the trim, though he states it was fully forwards prior to his action. He also mentions that an adjustment was made to the elevator to fix the problem for subsequent flights.

I am curious as to what caused this problem, and in particular, why it did not recur when the copilot reapplied power. I realize that the copilot may have omitted some key details, such as the aircraft accelerating before the reapplication of power (this occurred at 200 - 300 feet, and the film of the take-off appears to end before it has reached that altitude.) I am also aware, from other reading, that the Brabazon was intended to have a complex (at least for the time) hydraulic control system that was intended to smooth its response to turbulence, that never really worked properly, but it would seem an unnecessary risk to have it engaged for the first flight.

The one thing I have read about this aircraft years ago, is that it had numerous problems in the flight controls, all of them due to the flight controls cables, whose rigging was unstable. I remember having read that it was a  banana (???) effect to the fuselage.I can’t tell  whether it was distortions in the XY plan on in the XZ plan or something else

Reducing the power would it have modified this banana effect?
I can’t tell.


So fast, it melts the ice off? Approximately, how fast would an aircraft have to travel through freezing air to melt the ice off the wing, propellers, or rotor blades?

As already done in this answer, we can calculate the True Air Temperature (TAT) that the aircraft's skin will feel from the Static Air Temperature (SAT) and the Mach number:
$$ \frac{\text{TAT}}{\text{SAT}} = 1 + \frac{\gamma - 1}{2} M^2 = 1 + \frac{1}{5} M^2 $$
(using $ \gamma = 1.4 $ for air). For different SATs the curves then look like this:

I added a dashed line at a TAT of 10°C because this is the limit defined for icing conditions. At higher TATs, the ice should definitely melt (although it could already start melting before). We can now obtain the intersection Mach number as a function of SAT by inverting the formula above:
$$ M = \sqrt{5 \left( \frac{\text{TAT}}{\text{SAT}} - 1 \right)} $$
The resulting curve looks like this:

As you can see, a Mach number between ~0.4 and ~1.3 will be required to melt ice, depending on the SAT.


How did pilots maintain the Vomit Comet's parabolic trajectory? How did the Vomit Comet maintain its free-fall trajectory?  Careful trimming?  Custom autopilot software?  Or just good stick and rudder technique?
Wikipedia, citing references but without substantiation, suggests manual control of at least the throttle. To use an automotive analogy, a trained pilot could correct for which direction the fuzzy dice are hanging from the rear view mirror, by playing the throttle and elevator and spoilers.  But even a legend like Bob Hoover might lose precision doing that dozens of times in a row.

The NASA Vomit Comet was flown manually.
From The dynamics of parabolic flight: flight characteristics and passenger percepts

The control inputs required by the pilots of a parabolic flight aircraft are relatively simple, although precision is required to make the 0 g phase as close to freefall as possible, and care is required not to exceed the load limits of the aircraft which at times flies near its maximum rated speed. The pilots modulate lift L with the elevators and wings, which indirectly changes attitude θ, and thrust T with the engines (Figure 5).

....The pitch angle of the aircraft is not directly controlled by the pilots, but is a secondary result of other control inputs. During level flight the wings produce an upward lift, while the tail-mounted elevators produce a small downward component of lift in order to maintain a desired pitch angle. When flight conditions change, such as when the pilots deflect the elevators down during freefall, the relative forces from these two surfaces change. Since their moment arms are different, this causes a shift in the center of lift so that it no longer coincides with the aircraft center of gravity, resulting in a torque that causes a pitch rotation of the aircraft.

..At approximately 225 KT IAS (360 KT TAS, 185 m/s, Mach 0.61), when the aircraft is pitched nose-up 45°, the pilots commence the 0 g parabola. They push forward on the control yoke ("push over") to lower the angle of attack of the wings, which reduces wing lift, and simultaneously reduce power to a level just sufficient to overcome drag.

...After 25 seconds, at the end of the parabola, when the nose is pitched down 45° and airspeed is close to 350 KT IAS, the pilots pull up (pull back on the control yoke) and increase thrust to change the aircraft’s downward velocity into upward velocity, and restart the cycle.


How does one calculate the taper ratio of a compound wing? For tapered wings, Taper Ratio = tip chord/root chord.
Is this formula still valid for compound wings or do I take the taper ratio for each section and take an average?

Source: https://www.grc.nasa.gov/www/k-12/airplane/Images/area.gif

Taper Ratio is a simple geometrical metric that is valid for rectangles and trapezoids. If the leading edge and/or trailing edge of the wing is bent or jagged, Taper Ratio does not have a very useful meaning anymore. The Taper Ratio of the Concord wing in the OP diagram is zero.

For the compound wing in the OP diagram, the situation is a bit ambiguous as well. Is the wing root the projection of the leading edge into the fuselage, and is the extension at the root just an aerodynamic streak that uses the structural members of the main wing?

The definition of Mean Aerodynamic Cord uses wing root and wing tip as well, and is subject to the same complications. There are some pretty intricate graphs for wings wing a compound planform, this one is from Torenbeek.

If you would want to use Taper Ratio with a compound planform wing, the best average would be a weighted one, where for each of the wing planform segments the Taper Ratio and the area is determined, resulting in average TR per area.


What is this three-bladed vintage aircraft? We found this poster at IKEA, EDELVIK Poster 103.776.29.

Allposters.com has another poster, which includes the same aircraft, but 2 in formation, possibly the same original photo/artist?

That is a North American T-28 Trojan, a postwar era military aircraft used for basic flight training an the Navy and Marine Corps.

They still remain popular with warbird collectors and enthusiasts.


How can the 777X's longer wing reduce vortices? I saw the following on this web page:

  The purpose of the longer wing is to reduce drag caused by vortices, or wake turbulence, that form at an aircraft's wing tips. The less drag, the greater the fuel efficiency, and the more cheaply the aircraft can be operated.

How is this correct? How can a longer wing reduce vortices?

So it can fit into standard size airport gates and taxiways.

One reason the A380 didn't see wider adoption was it exceeded the maximum wingspan that most airports are set up for, requiring airports to build special gates and widen taxiways to accommodate it... which is expensive so very few did that.

Even the airports that did make the A380 mods found that an A380 flight couldn't be rescheduled to any available gate, as they only modified a few berths for the wider wingspan.

By folding the wingtips, the 777-X gets the efficiency of a longer wing and wingtips, but doesn't require a wider than normal gate space and taxiway. It can use any airport that can handle commercial traffic, without requiring expensive changes to the airport.


Why do some AFBs have planes parked at 45 degrees to others? I'm assuming this is just a matter of available space but it seems you could get 4 across all aligned but maybe there is another reason. The image is from McGuire AFB but I've seen this on other AFBs.

McGuire AFB being an Air Mobility Command (AMC) base, accounts for the very large C-5 Galaxy when it visits. It may not seem like it, but the 68 m (220 ft) wide C-5 when parked at a 45-degree angle saves ~20 m (65 ft) -- approx. one-third of its wingspan.
The reference used by the USAF is AFMAN 32-1084 (PDF), from which:

2.1.5.4.1. Parking Dimensions. Table 2.9 presents the minimum geometric criteria for fixed-wing apron design. When designing new aprons for AMC bases hosting C- 5, C-17, KC-10, and KC-135 aircraft, provide 15.3 m (50 ft) of wingtip separation.
EXCEPTION: When you are rehabilitating an existing apron, provide the maximum wingtip separation the existing apron size allows (up to 15.3 m [50 ft] but not less than 7.7 m [25 ft]). This additional separation is both desirable and permitted.

By applying that to the C-5 and adding the clearance to the taxiway, those angled spots are a perfect fit:

The measurement above (~83 m) shows two half-span C-5s (68 m) plus the clearance (15.3 m), with superimposed C-5s via pinterest.com

For the other examples you've stated in a comment: Beale AFB and Albuquerque Airport, Tinker AFB, and March AFB -- in that order:

The parking spots are between two taxiways, so it's the same reason as the main example. It also makes parking and taxiing out easier.

Those by the hangar are for ease of taxiing out instead of a hard left and potential for collision.

Plenty of space. I couldn't find a 45-degree parking.

(All imagery is via Google Earth)


Why does the F-16 have a fixed stabilizer section inboard of the stabilators? So, we have a F-16 chilling on the ramp, showing us its tailfeathers:

Interestingly enough though, inboard of the all-flying stabilator sections, there is a fixed stabilizer section (this is quite clearly visible on the right due to what the stab is doing on that side).  This isn't universal on stabilator-equipped aircraft though, or even fighters, as the F-22 appears to have its all-flying horizontal tail hinged at the fuselage joints:

and the trimmable stabilizers on airliners are hinged at the fuselage-stabilizer joint as well:

So, why is it that the F-16 doesn't follow this pattern? Is there some aerodynamic design reason for that?

The F-16 has an all moving tail plane all right. What you are referring to as the 'fixed part' is actually the fuselage portion which houses the air brakes. It can be seen clearly in the following photograph.

Belgian AF stabilizer; image from designer.home.xs4all.nl


How should the VFR Cruising Altitudes (FAR 91.159) be applied over rising terrain? Imagine flying at 4500' indicated altitude, 4500' AGL, heading 270, directly towards a steep mountain ridge that tops out at 4000 feet.  Imagine the mountain being almost like a flat wall.

When can I change altitudes to >4500' and still be in compliance with 91.159?  Do I have to wait until I get close to the ridge when I'll be under "3,000 feet above the surface"?  Can I interpret "above the surface" in a more liberal way?

  § 91.159 VFR cruising altitude or flight level.

  Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by ATC:

  (a) When operating below 18,000 feet MSL and -

  (1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude 500 feet (such as 3,500, 5,500, or 7,500); or

  (2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude 500 feet (such as 4,500, 6,500, or 8,500).

  (b) When operating above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC.

First, you said you're cruising at 4500 indicated, but then state that you are 4500 AGL. Indicated altitude is usually MSL, so this means the terrain is at 0 ft MSL until the base of the cliff, which then shoots up to 4000 MSL.

So as you travel westbound at 4500 MSL, you are in compliance with 91.159. The moment that the distance between your aircraft traveling at 4500 MSL and the ground is 3000 feet or less, then 91.159 no longer applies, and you can fly any altitude you need in order to maneuver around or over obstructions at or below 3,000 AGL.

Based on your scenario no action is required, you can just keep cruising along and fly over the cliff at 4500 MSL and 500 AGL over the "flat wall" mountain.

However, your question would be more interesting (and I'm guessing this is what you meant) if you had said that the flat wall was 5000 ft, because technically if the ground is at sea level up to the flat wall, then you are still more than 3000 AGL up until the moment your aircraft reaches the wall and 91.159 would still apply. The answer to this is that you would have to climb to the next westbound VFR altitude of 6500, and you could do it whenever its needed if you're on your own navigation, or when instructed by ATC if you are on an assigned altitude, or at anytime to avoid crashing into the mythical "flat wall" mountain.

The key is that this rule applies only during "level cruising flight" so when you are climbing or descending 91.159 does not apply, otherwise, VFR aircraft could never get to the upper VFR cruising altitudes.


What does "square the base" mean? I was flying the other day and heard tower say to another aircraft, "square the base". What does it mean, and when might a controller say this?

The controller wants the base to have two distinct 90° turns, from downwind to base, and base to final. And not one continuous turn from downwind to final. And no shortcuts to be taken to shorten the downwind.

It means the base will be slightly longer, and the downwind more parallel to the runway.

The controller is buying time (delaying the arrival) to accommodate someone else.

Top image is square-the-base, as opposed to the tight circular pattern—which experienced pilots tend to prefer1. (Images above are a mere representation.)

Note: exact answer will depend on the plane's exact location in the pattern.

It's one of those terms you won't find in a manual, like "hold our code".

It's a shorter time extension than "extend downwind leg":

Further reading: studentpilot.com, AOPA, 1 humorous AVweb video.


Is there a good systematic approach to flying an 800 foot pattern? I have a systematic approach to flying a 1000 foot pattern:  abeam the threshold, start a 500 fpm descent; turn when 300 feet below pattern altitude; etc.

Does anyone have a systematic approach to flying 800 foot patterns?  I'm looking for something more definite than "start a descent, don't get too low, turn on the 45, etc".

The trick of putting the runway half way down your wing strut visually, or a third to a half way down the wing on a low wing works great, because it automatically allows for any altitude agl being flown. As far as the rest of the pattern goes, just fly it with the same power settings and airspeed you would have in a1000 foot pattern. The base leg will get turned maybe a tiny sooner than the usual 45 degrees behind you trick, since you are starting out 200 feet lower (so you don't want to go as far away) I think it's more important that you just fly the pattern no differently...just make sure you're not out wide on downwind. The trick to consistent landings is consistent patterns and approaches. And that only comes from making sure your first circuit around is by the numbers. (i.e. airspeeds and power settings were exactly right and exactly at the proper place when they happened) Adjust accordingly after that first one, but really quiz yourself and make sure you are being consistent in the first place, before you start changing up how you do things.


What are the aircraft requirements to legally offer commercial sightseeing flights in a C-172? When I retire in a couple years I want to fly my 172 and make some money. Even if I just break even that's ok.  My CFI certificate has expired and I don't want the hassle of having it re-instated. I do have a commercial pilot license.

What would it take to give "scenic airplane rides" legally (so I have insurance). I read far 91.147 but it says nothing about the airplane itself.

It needs to be airworthy, as per 91.403, which is defined to include compliance with all ADs:

  (c) No person may operate an aircraft for which a manufacturer's
  maintenance manual or instructions for continued airworthiness has
  been issued that contains an airworthiness limitations section unless
  the mandatory replacement times, inspection intervals, and related
  procedures specified in that section or alternative inspection
  intervals and related procedures set forth in an operations
  specification approved by the Administrator under part 121 or 135 of
  this chapter or in accordance with an inspection program approved
  under § 91.409(e) have been complied with.

It needs a 100-hour, as per 91.409:

  (b) Except as provided in paragraph (c) of this section, no person may
  operate an aircraft carrying any person (other than a crewmember) for
  hire ... unless within the preceding 100
  hours of time in service the aircraft has received an annual or
  100-hour inspection and been approved for return to service in
  accordance with part 43 of this chapter or has received an inspection
  for the issuance of an airworthiness certificate in accordance with
  part 21 of this chapter. The 100-hour limitation may be exceeded by
  not more than 10 hours while en route to reach a place where the
  inspection can be done. The excess time used to reach a place where
  the inspection can be done must be included in computing the next 100
  hours of time in service.
  (source: 14 CFR Part 91.409).

Part 43 Appendix D specifies the items which need to be checked in the 100-hour inspection.

And it needs to have all the other necessary mechanic's inspections, including ELT, transponder/pitot static system, etc., and pilot checks, like VOR accuracy.


What is the reason of having an Angle of Incidence on an airplane? I know that the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the fuselage. I know that it doesn't change in flight because it is fixed.

And usually, there is a positive angle of incidence on airplanes, which means that the wing kinda looks like this:

credits: Naval Aviation Schools Command aerostudentguide(dot)pdf

My question is, why is there a need to incline the wing like that? Why is there a need for Angle of Incidence?

Strictly speaking, there is no need to set a particular angle of incidence- the wing will decide that for itself depending on the conditions (speed, weight, altitude etc.). What we are deciding is the The mounting angle, which is set for various reasons and is not variable in flight (except for some rare cases) including:

Mostly this is set so that the fuselage is (nearly) horizontal during cruise. This is especially important for airliners- DC-10, especially flew at a pronounced nose high attitude requiring the cabin crew to walk uphill.
Setting the wing at an incidence helps improve the visibility- this is important especially for carrier aircraft, where the pilot requires good visibility and also higher angle of attack (for keeping t/o and approach speeds low).
Setting the wing at an angle helps in keeping the drag low for the given lift. Having the wings at an angle and fuselage horizontal means that the drag is minimized, while the wing has the required angle of attack.

Though the angle of incidence is usually fixed, it can be varied in response to specific requirements. A good example is the Vought F-8 Crusader, which allowed the wings to be pivoted 7° out of the fuselage on takeoff and landing, resulting in a high angle of attack -- reducing the approach and take-off speed -- while keeping the fuselage level and giving the pilot a good forward field of view.

F-8 Crusader with the wing pivoted up during landing. By USN - U.S. DefenseImagery photo VIRIN: DN-SC-88-06695, Public Domain, Link

Another example is the Martin XB-51 which had a variable incidence wing to reduce the takeoff run.


Why do aircraft on a junk yard have blind windows (e.g. German "Landshut")? Today I saw a picture of the “Landshut” (named after a German city, pronounced lund's hoot, not Land shut). The aircraft was hijacked in 1977 and now rusts at the airport of Fortaleza (Brazil) since 2008. German enthusiasts wanted to transfer the plane back to Germany as it somehow and sadly became part of German history, and the German government intends to do so as well.

(http://hessenschau.de/panorama/landshut-104~_t-1488105510615_v-16to9.jpg)
(Source)
I noticed the windows are blind. In an answer to an other question here I can also see a “retired” plane and Peter K. mentions the blind windows in a comment. This gave me the impression that all “retired” planes have blind windows.
Do the windows get blind simply because the planes are left on their own for years and wind, sun, etc. make them blind or are they made blind on purpose? If so, which purpose?

Addendum
After reading the good answers, I reviewed the video I already linked. This screenshot:

(Source: tagesthemen from Feb 23, 2017, 22:15h (German TV news magazine))
shows the cabin of the “Landshut” and it's clearly visible that the interior is removed and the windows are blanked from the inside and covered with grey foil. I just wanted to share this image.

taken from Business jet news

  For outside storage, cover windows and recognition/strobe lights with barrier material and pay attention to atmospheric conditions, as humidity will accelerate corrosion. Make sure all control locks or control column gust locks are installed to prevent movement of flight control surfaces.
  (emphasis mine)

One reason to protect the inside fabrics, plastics etc from the effects of discolouration from sunlight, as well as inhibit biological growth. Also without the coverings the temperature inside the aircraft will be significantly higher on sunny days see Desert Usa

  [without protection] the interior could quickly heat up to 200 degrees Fahrenheit during hot summer days


When do you contact ATC when departing from a non-towered airport into class D controlled airspace? I had this question posed to me the other day and I wanted to make sure I responded correctly.

When do you contact ATC when departing from a non-towered airport into class D controlled airspace?

This is the situation where the uncontrolled airport sits under the Class D shelf.

Class D is usually a cylinder so there's no shelf to be underneath; I'm assuming you're asking about departing from an uncontrolled airport that's actually in class D, like this one (N96):

Because class D goes to the surface I'd try to contact ATC on the ground before departure if possible, either on a regular frequency or via an RCO. And it would be worth checking the Chart Supplement for any local rules.

Otherwise, 14 CFR 91.129 says you should contact ATC as soon as you can after departure:

  (2) Departing flight. Each person—

  [...]

  (ii) From a satellite airport without an operating control tower, must
  establish and maintain two-way radio communications with the ATC
  facility having jurisdiction over the Class D airspace area as soon as
  practicable after departing.

There are some special cases like KDTS where you can't takeoff without contacting ATC first, but it's in a special rules area.


What is the lifespan of an airline engine? As per subject, what is the lifespan of a jet engine? Do they have a fixed hours which they can be used and regardless of the state have to be exchanged or are they serviced and maintained until they are deemed not good for use?

If they do have a lifespan, I suppose also that different types of airliner engines and models will have different lifespans.

In general, the engine life is measured in terms of two things:

Flight hours
Flight cycles

A takeoff and a landing counts as a flight cycle.

The most important concept in engine life is the Time Between Overhaul(TBO), which is basically the manufacturer recommended time period after which the engine is stripped down,checked thoroughly and required parts replaced. After overhaul, the engine is usually cleared till the next TBO.

An overhauled engine is theoretically as good as a new one and comes with the same life (TBO) and warranty, if applicable. Airline engines (for example the Rolls Royce Trent series) usually have TBOs of over 15000 hours. The record for maximum time for an engine on wing (i.e. use in aircraft before removal for overhaul) is well over 40,000 hours.

The life of an engine is given by the manufacturer and are different for different engines. However, the engine life depends on a number of parameters and is usually different for various engine components.

The rotating components have life limits which are less than the engine and are replaced periodically, mostly during the TBO.
For example, the turbine blades are replaced after a set number of turbine cycles and/or after a set number of hours, which are lesser than the others.
In addition, most rotating components (compressors, turbines) are periodically subjected to checks, and are replaced if found damaged.
In case of abnormal operations, some parts are required to be replaced. For example, if one engine is required to give excessive thrust (for example in case of One Engine Inoperative), the engine is subjected to thorough checks similar to TBO.

Airline engines are usually serviced (Overhauled at TBOs) and maintained till they are deemed not good to use or not economical to (service/repair and) operate anymore.

Most of the engine manufacturers employ condition based health monitoring programs, which helps them to plan when to service and repair the engine. As a result, the engine life varies based on its condition.


Do logged hours with a Sport Pilot instructor count towards a private pilot certificate? I've my Sport Pilot licence and I've got logged hours with a sport pilot instructor that isn't a private pilot instructor.

Can I use my logged instruction hours in order to upgrade to private pilot?

No.  Sport pilot instructors are not authorized instructors with respect to private pilot certification.

See 61.109(a).

The FAA has issued a legal interpretation on the matter.  See the Kern Ink interpretation


What is this red canister on the tail of the YF-22? On the tail section of the YF-22 further back in the image below, there is a red canister. What is that canister for? I suppose it is for some flight testing purposes from what I can see.

I can't pinpoint the image source.

The canister contains a small parachute and either a spring or an explosive charge to kick the chute out when it is needed. It is a precaution for spin flight tests, that's why it is called a spin chute. See here for a video of the spin chute of the F-35.

I guess now I should explain how a spin chute works.

When an aircraft spins, it will rotate around a vertical axis which is somewhere between slightly ahead of the aircraft (regular spin) or near the leading edge (flat spin). The pitch angle $\Theta$ is nose down and the angle of attack $\alpha$ is high (around 45° in a regular spin, up to 90° in a flat spin), so the airflow over most of the wing and horizontal tail is separated. But the rotation, which is a mix of rolling and yawing, will induce an angle of attack variation over span, so a part of one wing will operate in the normal angle of attack range, where lift is high and drag is low.

Now we have a receding wing with separated flow, at high angle of attack, with low lift and high drag so the resulting air force vector R (green, below) points mostly into the direction of local flow (which is up). The other wing has partially attached flow, a lower angle of attack, high lift and moderate drag, so the resulting air force vector is almost perpendicular to the local flow, pointing up and forward. This difference in lift and drag propels the rotation.

Speeds and resulting force on spinning wing sections. $\omega_z$ is the rotation speed around the vertical axis, y is the local wing station and the green v shows the local flow direction.

Without the rotation the aircraft would immediately pitch nose down, pick up speed and can be pulled out of the resulting dive. With the rotation, however, we get an inertial pitch moment due to the fuselage masses. All parts of the aircraft rotate with the same yaw rate, and the centrifugal force from this yawing motion grows linearly with distance from the spin axis. This difference in centrifugal force along the lengthwise coordinate of the aircraft adds a powerful nose-up moment which in some cases cannot be overcome by the tail surfaces - remember, they have less effectivity in separated flow. Flat spins are almost impossible to escape from.

This can be tested in spin tunnels, but Murphy's Law is very relevant to spin tests. So it is a good precaution to add a spin chute for the first tests: If the aircraft cannot end the spin with control surface deflections, it is in a trap from which it can only escape with something that will add a strong nose-down moment. In the chaotic airflow behind a rapidly spinning airplane. That is why the spin chute is not bolted to the skin of the airplane, but sits in an elevated position hanging out at the back. From there it will not be caught by the tail surfaces but will work as intended in all conceivable situations (save an inverted spin ...)