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When a non-pilot hears the word stall, it brings to mind what happens when a car stalls - the engine quits. It seems like that would be a dangerous scenario in an airplane.

From a non-pilot perspective, what happens when an airplane stalls and why is it important for pilots to practice it?

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10 Answers 10

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Stall was an unfortunate choice of words for an engine that quits suddenly since the aerodynamic stall means something different1.

To a non-pilot, a stall can best be described as the situation where there is not enough air flowing over the wings to create the amount of lift needed to hold up the airplane.

The main reason that student pilots practice stalls is to learn the telltale signs that occur just before it happens and to make the recovery procedure automatic. If pilots can recognize an impending stall, they can take corrective action to either avoid the stall altogether or to recover as quickly as possible.

Outside of training, stalls typically only occur shortly before landing and after takeoff, when the pilot gets distracted while already at a slow speed. In both of these situations the airplane is very close to the ground, immediately requiring the correct action from the pilot in order to avoid a crash. This needs to be instinctive and done from memory so that it is accomplished as rapidly as possible.

The next logical question is usually: How does a pilot fix an airplane that has stalled?

Fortunately, airplanes are designed so that even during a stall the tail is still effective2 and the pilot is able to use it to force the nose down. This makes the airplane go faster, since it is pointed down towards the ground, and gets more air moving over the wing which allows it to create enough lift for the airplane to start flying again. During practice it is usually pretty uneventful, but when it happens at a low altitude there may not be enough time to regain flying speed before the airplane crashes.

For more information, AOPA has a great Safety Publication targeting flight instructors called Why we teach slow flight and stalls which is available on their website.


1 However, the sail on a sailboat can also "stall" when there isn't enough wind and since they have been around since 3,000 BC I guess that technically this usage of the word applies to both situations.

2 There are some stalls in particular airplane designs known as deep stalls that can be unrecoverable. I don't think that this is important when describing it to a layperson though.

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You can also stall from overspeed (exceeding the critical mach number) and from flying too high. –  Aron Jan 30 at 2:03
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@Aron True, but I'm not sure that it adds value when describing a stall to a non-pilot though. :) –  Lnafziger Jan 30 at 2:46
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What's "uneventful" to one, is "terrifying" to another, and "awesome" to others. –  Dax Fohl Jan 30 at 3:59
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landroni: While your aircraft breaking apart may happen first with many subsonic craft, wings (and control surfaces) that are designed purely for subsonic flight can stall even if they do hold together in supersonic flight (or, more accurately, as Aron said, in flight above the critical Mach number.) The shock wave separates the airflow, potentially pushing it away from the wing/control surfaces. Wings and control surfaces aren't effective in a vacuum for obvious reasons. –  reirab Jan 30 at 16:01
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An interesting addon to the second footnote is that this happens to (almost?) all fixed wing aircraft operated aft of their design cg envelopes. It's one of the easiest ways to see that you should pay respect to the weight and balance chart. Unfortunately skydiving operations tend to stage a demonstration every now and then, which is tragic but does provide an object lesson. –  Doug McClean Jan 31 at 9:41

An engine stall and an aerodynamic stall are completely different. In aviation, an engine stall is referred to as an engine failure, and an aerodynamic stall is simply referred to as a stall.

For Nerds

An aerodynamic stall happens when the wing stops producing lift because the Angle of Attack is too high. This is usually, but not always, caused by pulling back on the stick without adjusting power appropriately. Various factors including the weight of the aircraft, flaps, and icing can change the angle of attack the aircraft stalls at.

For the rest of us

A stall happens when the wing no longer creates lift. This happens when the speed of the air going over the wing decreases too much. Basically, this is why the advice one mother gave her son during World War I, 'Fly low and slow, I don't want you to get hurt!' makes very little sense aerodynamically speaking.

Why pilots practice stalls

Pilots practice stalls to learn the warning signs that they are entering a stall, and to practice recovering from a stall should they ever end up in one.

Also, as brinky stated in a comment to this answer:

While in flight training or when moving to a different airframe, pilots practice putting the aircraft in a stall and then recovering from it to also develop "muscle-type" memory, because stalls are probably the single most unsettling and unpredictable behavior that all aircraft share, and different airframes may react very differently to stalls. Stalls are unforgiving and must be rectified promptly, due to rapid altitude loss.

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Good, but one more point which is the same that I mentioned to p1l0t on his answer: A minor point that you may want to consider editing: When a wing stalls it is still generating lift (it doesn't come plummeting out of the air at terminal velocity), it just doesn't produce enough lift for the airplane to maintain altitude. –  Lnafziger Jan 30 at 2:28
    
@Lnafziger: ah, ok, will add that –  flyingfisch Jan 30 at 14:25
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I wouldn't say a stall is merely when the wings are not producing enough lift to maintain altitude, though. Any descent requires the wings to produce less lift than what is required to maintain altitude. Aerodynamic stall is entered at the point where smooth airflow separation from the wing has reached the point that further increase in angle-of-attack will produce less lift rather than more. A normal descent is very different and still has smooth airflow over the wings with little or no separation. –  reirab Jan 30 at 16:22
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@reirab: To further add to the confusion, not all descents require the wings to produce less lift than necessary to maintain altitude. A constant rate of descent actually requires roughly the same lift as maintaining altitude. –  Marcks Thomas Feb 1 at 13:42
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@MarcksThomas - Good point. Obviously, the total upward and downward forces have to be balanced to maintain a constant vertical speed. The total lift is dropped below normal in order to enter the descent, but is then increased back to normal once the desired rate of descent is reached in order to maintain that rate. –  reirab Feb 3 at 21:51

The stalls that pilots practice are aerodynamic stalls not engine stalls. It happens when the critical angle of attack is exceeded. [Typically the nose is pitched up too much is what it means]. It results in airflow separation that means that the wing no longer is generating any [significant] lift. As soon as the pilot recovers by letting the nose down and regaining some airspeed they are flying again. We practice these for the purpose of being able to recognize the onset of a stall and being able to recover from an inadvertent stall. This is important because while it is actually kind of fun at altitude, near the ground (like while on approach or departure for example) it can be deadly.

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P.S. there is also something called accelerated stalls but it's a bit more complicated. –  p1l0t Jan 29 at 20:10
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Pilots practice engine failures too :) –  falstro Jan 30 at 8:12
    
SIMULATED engine failures we don't actually shut it off. At least not in a single-engine. –  p1l0t Jan 30 at 17:56
    
@p1l0t and roe - I used to teach full power-off engine failures and restart procedures. But I always made sure that I was at altitude and I was close enough to get back to the airport in case I also ended up unintentionally demonstrating power-off landings. :-| –  Shawn Feb 4 at 18:18
    
I don't see the point in that, the difference with having the motor at idle is minimal but it does provide you with a second chance. Especially with a student at the controls. You're being sarcastic I hope. –  p1l0t Feb 4 at 18:49

When the airplane is at a "regular" angle of attack (angle with the direction of the wind), with the nose more or less forward, the wing works as designed and produces lift.

If the airplane turns the nose straight up while continuing to go forward, it's intuitive that the wings stop producing lift, as they are just vertical walls against the wind at this point. (The plane can still fly in this condition if the engines alone can pull all the weight, but that's only common in fighter and acrobat planes).

It turns out that the transition between these two is relatively sudden. As the angle of attack increases, wing lift goes up and up and up, then suddenly drops sharply as the smooth air flow detaches from the back of the wing. That's the stall. It can also happen when lowering speed while keeping he angle constant.

As to why pilots practice it, nothing to add to what flyingfisch said.

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Stall has to do with the attachment of the boundary layer on the upper surface of the wing. When the airflow separates (video) from the surface it stops generating lift and the wing stalls. Lift is the aerodynamic force that keeps the aircraft airborne. It's the reaction of the air to the mass of the airplane.

Now, lightheartedly, to make a super simple analogy, let's say that the equivalent of lift when you walk is the upward reaction of the floor to the mass of your body (which is actually true); imagine also that you are walking and suddenly a trapdoor opens under your feet (airflow separation). As soon as you feel that you're falling you instinctively open your arms trying to grab something while your eyes are looking around for something to hold on, say a handle or the edges of the trapdoor. This is the situation that one could say that you have stalled. Your normal walking was interrupted and now you're falling.

Pilots train in order to recognize the symptoms and learn where the "handles" are -i.e. necessary actions to take- when this situation occurs.

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"Stall" in aviation generally means an aerodynamic stall. An "engine stall" would be called just that.

There is an excellent understandable writeup of an Airbus crash from a stall, Air France flight 447, on the UK's Daily Telegraph.

A wing provides lift to a plane as a direct result of the air flowing over the surface. A stationary plane falls - without airspeed, the wings cannot provide lift. A stall occurs when the airspeed falls too low, and the lift provided by the wings cannot maintain flight.

Airspeed can fall too low for several reasons; for AF447 it was the most common case, that the plane's angle of attack was raised too high, and the engines could not provide enough thrust to keep the plane flying above its stall speed. In this scenario, memory of stall practices should kick in and the pilot should point the nose down to regain airspeed, allowing the wings to provide more lift.

Interestingly, the article suggests that the Airbus controls take commands rather than being a modifiable reflection of the current state of the aircraft, so once the stick was released it wasn't apparent that the plane was trying to climb to the other pilot. Eventually airspeed dropped so low that the airspeed sensors themselves switched off, silencing the stall warning. When the pilots realised the mistake and pointed the nose back down, the increasing airspeed re-enabled the airspeed sensors, which restarted the stall warning, perhaps confusing the crew further. The captain realised just before impact: "10 degrees of pitch..."

Practising of scenarios is to prepare crew so that in the event of some danger or failure, they do not panic but respond appropriately and correct the problem.

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A stall is when the plane exceeds the linear and predictable. If it hits the air too steep, the flow over the wings collapses into chaos and the behaviour is not always predictable.

Pilots practice to develop the skills to recover into the linear range of flight

See also and look up the number of people who have died after stalling a "laminar flow" wing (the Gee-Bee racer comes to mind here).

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A stall can be observed visually on an aircraft with a flexible cloth wing, such as a hang glider or micro-light. To land a hang glider, one deliberately triggers a stall which spreads out across the wing surface, and you can see the fabric going limp.

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In paragliding, the pilot stalls the wing on purpose sometimes in order to recover the glider from some situation - when the lines are messed up or there's something else.

It is similar to rebooting your computer.

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Basically stall just means it temporarily stops working, in a car its the engine, in a plane its the wing. An aerodynamic stall happens when the wing fails to provide lift (the plane is falling). This is pretty common, a plane could drop a couple hundred feet without any passengers noticing (the roller coaster feeling is probably a stall). Pilots practice this for multiple different reasons, but the main one is so if the plane ever stalled the pilot can correct it before it becomes a problem.

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That rollercoaster feeling you describe is an interpretation of accelerations you experience, not a stall. –  casey Jan 30 at 1:44
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Aerodynamic stalls are COMMON in airliners? :P –  shortstheory Jan 30 at 3:09
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The roller-coaster stuff are turbulence and wind-shear, not stalls. A stall in an airliner is a very, very, serious condition (due to aircraft design, such as swept wing). –  falstro Jan 30 at 8:17
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Roe is correct. In airliners (or any other type of airplane, for that matter,) the sudden drops (or sudden accelerations in any direction) that you feel are turbulence, not aerodynamic stall. It's quite literally wind briefly pushing your aircraft in some direction. –  reirab Jan 30 at 16:33

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