What happens when an airplane stalls and why do pilots practice it?
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?
Stall was an unfortunate choice of words for an engine that suddenly quits since the aerodynamic stall in aviation means something very different and isn't related to the aircraft engine at all1.
To a non-pilot, an aerodynamic 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, inadvertent 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 corrected using muscle 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.
You can also stall from overspeed (exceeding the critical mach number) and from flying too high.
@Aron True, but I'm not sure that it adds value when describing a stall to a non-pilot though. :)
What's "uneventful" to one, is "terrifying" to another, and "awesome" to others.
@Aron I thought that you can stall from underspeed (well below the minimum required speed of flight) and from flying too high (and getting into the "coffin corner"). But overspeeding with a subsonic aircraft is associated with "crashing into the air", so to say, and the aircraft breaking apart, but not with a stall. Am I wrong?
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.
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.
There's another reason to practice stalling: Landing an airplane is equivalent to stalling it -- slowing it down so it stops flying -- when the wheels are on the runway or the floats are a centimeter above the water.
@OllieJones Not to nitpick, but that's not always true. Large transport aircraft tend to be "flown onto the runway" and at no point should the airplane experience an imminent stall. Even small aircraft pilots often prefer to land slightly above stall speed to achieve a smooth landing, unless they are landing on a short runway and are using short-field technique. I would say "slow flight" practice is far more applicable to landing than stalls, because it teaches the student how to control the airplane in slow flight, as if on final approach.
"the pilot is able to use it to force the nose down. This makes the airplane go faster" -- a stall can happen in any attitude and at any airspeed. I've inadvertantly stalled a T6 upsidedown at the top of a loop because I didn't pull enough Gs to get over the top. Needless to say, my muscle memory didn't help, and I had to consciously pull the stick back to break the stall. I have also done a few accelerated stalls with the nose 20 degrees below the horizon by trying to pull out of the dive too hard during spin recovery
@rbp Well, you are still forcing the nose down, but in referebce to the relative wind instead of the horizon. Again, for the layperson that this is targeted to, there isn't a need to explain the corner cases. :)
@reirab: What separates is flow _above_ the wing and if there was vacuum below the separated flow, the wing would work extremely well. Alas below the separated flow is stagnant air at ambient pressure and that makes the wing not work.
@landroni: Air speed is highest over the wing, so the first effect of approaching speed of sound (actually occurs at about 70% of it with straight wings) is forming shock wave above the wing and associated loss of lift _and_ shift of lift aft, which together cause the aircraft to pitch down ("mach tuck"). This pitch down causes the aircraft to accelerate further. Subsonic aircraft may not have sufficient control authority to pull out and eventually overspeeds to the point of structural failure or hitting the ground. It's why supersonic aircraft usually have all-moving elevator.