What is an aircraft stall?

A stall is the result of an aircraft losing lift and starting to lose altitude.

For an aircraft, a stall is one of the most potentially serious events, much more so than the turbulence that so frightens passengers. Potentially only, because a stall only becomes critical if it is not identified.

To explain what a stall is, and how to avoid or remedy it, we first need to understand how an aircraft flies.

This article is of course intended for the general public, and experts will forgive certain simplifications.

How does an airplane fly?

When an aircraft flies, air flows over both the underside of the wing (pressure surface) and the top of the wing (pressure surface). The airflow sticks to the wing’s profile, creating a pressure difference (lower above and higher below), the result of which is a kind of suction that pulls the aircraft upwards: lift .

So, contrary to popular belief, the airplane doesn’t rest on its wings, but is sucked upwards by them. Nor is the pressure due to the difference in speed between the airflows, just to the fact that they follow the wing’s profile.

What is stall?

The angle at which the air strikes the wing is called the angle of attack or angle of incidence. When this angle is too steep, 15° to 20°, the airflow is lifted off the upper surface, creating a more or less abrupt drop in lift, at which point the wing “stalls”.

Source : http://wwwsam.brooks.af.mil/af/files/fsguide/HTML/Graphics/fig_28-11.gif by Interiot

What happens is that the two airflows don’t meet at the wingtip, which prevents them from generating lift. This phenomenon is clearly visible in the photo below.

Source : http://www.dlr.de/100Jahre/DesktopDefault.aspx/tabid-3300/5149_read-7460/gallery-1/gallery_read-Image.37.2921 by wikimedia

To avoid a stall, pilots must not fly below a certain speed, called the stall speed. In fact, a decrease in speed requires an increase in angle of attack in order to maintain altitude. But it’s not airspeed that causes a plane to stall, it’s the angle of attack it provokes.

This airspeed depends on a number of factors, and therefore varies according to the aircraft, its load, the phase of flight, etc.

What situations cause stalling?

As we said, an aircraft stalls if and only if the angle of attack is too high.

This can happen on take-off when the climb rate is too high for the airspeed, and conversely on landing when the airspeed is too low and the pilot tries to pitch up to delay the landing.

In flight, it’s the same thing: when the pilot continues to pitch up as speed decreases. This happens when he tries to preserve altitude at the expense of speed.

How to get out of a stall?

If the pilot does nothing, the aircraft will gradually pitch downwards, reducing its angle of incidence, increasing its speed and the pilot can quickly regain control.

The only mistake he can make is to try to limit the loss of altitude and pitch the aircraft up again before it has regained sufficient lift.

In theory, if the pilot does nothing, the stall will be easily recovered.

In theory, if the pilot does nothing, the stall will be easily remedied.

So, in principle, it’s very easy to get out of a stall, provided you’ve identified it!

It’s a situation I’ve personally experienced and tested in a simulator, with the excellent Flightsensations team.

Starting from a certain altitude, I pitched the plane up to maximum, it climbed and then stalled. It then went into a steep descent and I let it regain lift and speed until I was able to recover it, exactly at the altitude where I had started the climb.

Well, it may sound simple, but without an instructor in the right-hand seat to “coach” me, I’d probably have made a big mistake and done the wrong thing.

Can a stall be fatal?

It’s easy to understand that dropping out can have dramatic consequences, but in reality it (almost) never happens.

Firstly, because it’s a situation that pilots are trained to deal with very early on. Any amateur pilot can quickly learn to identify and deal with a stall. It’s a bit like parallel parking when you’re learning to drive: it’s the basics.

Secondly, because aircraft have reached a level of sophistication which means that they are capable of avoiding risky situations on their own, and in any event the pilot will be alerted by alarms.

In fact, the real risk is when the pilot fails, for one reason or another, to identify a stall situation and undertakes the opposite maneuvers to those required, in the good-faith belief that he has another problem, usually a problem with altitude.

This is what happened on the tragic flight AF447.

The pilots were fooled by their instruments, undertook the wrong maneuvers, and it also became apparent that although modern aircraft were theoretically incapable of stalling, pilots were less trained to handle the situation than amateur pilots flying less sophisticated aircraft. Nowadays, even if an airliner is no longer supposed to stall, pilots are still regularly trained to identify and manage a stall.

This is one of only two cases of fatal stall on a cruise, along with West Caribbean flight 708 in 2015.

We could also talk about the Northwest flight, but there were no passengers on board, and the stall was not the cause of the plane’s loss. The pilots managed to regain control of their aircraft, but then mismanaged their priorities: they concentrated on restarting the engines rather than considering landing as soon as possible.

But if you want to see the consequences of irresponsible actions on the part of a crew, I recommend this video.

Bottom line

A stall is very simple to counteract, but can have dramatic consequences if the crew doesn’t recognize the situation. In fact, it’s only happened twice, so you can travel easy.

Image : aircraft wing by BABAROGA via Shutterstock

Bertrand Duperrin
Bertrand Duperrinhttp://www.duperrin.com
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.

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