Fortunately, airplanes are designed so that even during a stall the tail is still effective 2 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. I don't think that this is important when describing it to a layperson though.
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. 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. 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! 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. The stalls that pilots practice are aerodynamic stalls not engine stalls. It happens when the critical angle of attack is exceeded. 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. 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 the angle constant.
As to why pilots practice it, nothing to add to what flyingfisch said. 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. An "engine stall" would be called just that.
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 AF 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. How difficult a plane can recover from a stall is usually based on the plane itself. For instance, planes with stick shakers can be quite hard to recover from the stall. This is because the shaking stick helps to alert the pilot when a stall is imminent. Furthermore, the characteristics of a stall also depend on the way the plane is loaded.
This is because the center of gravity of an aircraft must be sufficiently ahead. Spin is a worse version of a stall as the plane spirals down. A stall can develop into a spin when the plane turns sideward at the wrong time. However, the mechanics of a spin can be quite complex. Furthermore, recovery from a spin may be more difficult based on the plane.
Recovery from a spin needs good efficiency from the tail surfaces of the plane and recovery. Recovery typically involves using the rudder for stopping the spinning motion while adding to the elevator to help break the stall. However, the wing may block the airflow to the tail. Furthermore, if the center of gravity of the plane is far back, recovery can be more difficult. Pilots can also be disoriented by the dizzying effects of the spin and then apply the wrong corrections.
Although the plane may be well-designed and loaded with allowable range, recovery needs to be executed flawlessly as the height loss during a spin can be very much. The FAA mandates that private pilots do stall training before they can fly. The stall training helps the pilots easily recognize an impending stall to take corrective measures before a true stall or spin can even occur.
I know the main reason you are reading this article is finding out if stalling can pose a significant risk to passengers during a flight.
The best planes are now equipped with high-tech autopilots and pilots with extensive training to help avoid the danger of stalling in a flight. However, a stall can be dangerous if the crew is not alerted on time or get incorrect feedback. From small single-engine rotary airplanes to massive twin- or four-engine commercial jets, stalling is a problem to which all airplanes are susceptible.
During flight, an unexpected stall can pose a significant threat to the airplane and its passengers. But the good news is that most airplanes have safety systems in place to control and eliminate stalls. A common misconception is that stalls are attributed to a mechanical problem in an airplane.
In cars, trucks and other ground-based automobiles, an engine stall is, in fact, a mechanical problem. Rather, airplanes experience stalls when the angle at which they enter the wind current is greater than the critical angle of attack. As a result, the airplane will drop, thereby reducing its altitude, until the angle of attack is correctly adjusted.
With that said, the pitch of an airplane can also affect whether airspeed will cause a stall. An airplane gaining altitude at a high pitch may stall at a lower airspeed than an airplane flying horizontally at a flat pitch.
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