By William E. Dubois · January 26, 2023 · 17 Comments

Megan, a commercial pilot candidate in Arizona, writes: I’ve just been introduced to accelerated stalls and it made me wonder if there is such a thing as a decelerated stall. I checked my training books and couldn’t find anything, and when I checked in with Uncle Google, all I got was stuff about car engines stalling when decelerating. I asked my flight instructor and he said, “I don’t know, but I know who would know…”

Yes, absolutely, 100%, there is a decelerated stall. Or, at least there will be after today, because I think there’s a good case to be made for the term, even if it hasn’t received official recognition just yet.

But, before we dig into that, if you’ll pardon the pile of puns, we need to decelerate and get everyone up to speed on some terminology.

So, without stalling any further, for our non-pilot readers, in aviation, when we talk about stalls, we aren’t generally talking about an engine problem.

While airplane engines, both piston and jet, can stall, when pilots talk about stalls, they are usually talking about the aerodynamics of the aircraft’s lifting surfaces. In airplanes, this is largely the wing. As we’ve covered in the past, wings create lift by using several flavors of black magic, and part of that magic is the flow of air around the wing.

Keeping it simple for today, as long as air flows smoothly over the top of the wing, it generates enough lift to keep the plane levitating. However, if you angle the wing too steeply against the flow of on-coming air, the air above the wing becomes turbulent, breaks away from the wing, and all the magic dissolves. In technical terms, the wing has exceeded its critical angle of attack or AOA. The wing stops “flying” and this is called a stall.

It sounds dramatic, and in some airplanes it can be. But in most general aviation training airplanes, the nose just drops a little bit and the plane starts a descent.

For any of you student pilots worrying about upcoming stall training, it’s really not a big deal, and for perspective, every landing you’ve done to date is actually a stall. After all, a stall is the only way to get the plane to stop flying.

While the easiest way to stall an airplane is to just slow down and start pulling the nose up until the wing’s angle gets too steep, an airplane can actually stall at any flight angle and at any speed. You can be pointed straight at the ground at redline, and if you pull out too quickly you’ll stall, because it’s all about the angle between the oncoming wind and the wing.

Now, despite the fact that a stall has nothing to do with speed, all airplanes have a published stall speed, conveniently marked at the bottom of the green arc on the airspeed indicator, which gets people into trouble, because — as mentioned above — angle makes the stall, not the speed.

So at the commercial pilot level, you need to demonstrate that the airspeed indicator is not to be trusted by performing a maneuver that stalls the airplane at an airspeed well into the green arc.

Without getting too far into the weeds here, I next need to mention that a heavier airplane needs a greater AOA to generate enough lift than a lighter airplane. So while stalls are all about angle, weight plays an indirect part because a heavier airplane — at a given speed — is already closer to its critical AOA than a lighter airplane is at the same speed.

And here’s another secret: Thanks to the magic of aerodynamics, putting an airplane into a steep turn effectively increases its weight. We call this “pulling Gs,” so while pulling Gs, an airplane will stall at a higher airspeed than the published “stall speed,” which is based on gross weight in level flight.

The accelerated stall maneuver is simply stalling an airplane in a steep bank to demonstrate a stall at higher speed than the published “stall speed.”

So what’s being accelerated here? Well, contrary to logic, it’s not the stall speed.

If you look at older flight training materials you’ll find these stalls were originally called “accelerated maneuver stalls.” The word “accelerated” was not referring to stall speed but, rather, the increase in G forces in the maneuver. In fact, the Aussies call them G-stalls, instead of accelerated stalls. Regardless, by definition, any stall that happens at more than 1G is an accelerated stall.

So linguistically speaking, if garden-variety stalls are at 1G, and any stall over 1G is an accelerated stall, then it stands to reason that any stall under 1G should be called a decelerated stall. Right?

Wait a second, you ask, an airplane can stall in negative G flight?

Of course it can. Not that negative G flight for any period of time is common, but Gs, positive or negative — like speed and the angle of the airplane — are irrelevant to stalls. The wing stalls when the angle between the wing and the oncoming wind is too steep.

Now, as a quick detour, people with too much time and too much internet on their hands will tell you that an airplane can’t stall at zero G (such as during aerobatic maneuvers).

That’s nonsense. Of course it can, it just doesn’t matter, because for any brief time that an airplane is at zero G, it is weightless, and doesn’t need lift. So the stall effect would not be apparent. But it would still occur.

Back to decelerated stalls: While the term is not in the Pilot’s Handbook of Aeronautical Knowledge or the Airplane Flying Handbook (or anywhere else other than in your head and mine), I for one will champion the use of “decelerated stall” going forward, ‘cause I love it.

But if accelerated stalls happen at higher airspeeds than non-accelerated stalls, will decelerated stalls happen at lower airspeeds than non-accelerated airspeeds?

Ah, I wish that were so, it would be so perfect. But no. Remember that horrible Vg diagram that confused you as a student pilot?

Take a look at the stall speeds shown for 2G and -2G, and you’ll see that the negative G stall occurs at quite a bit higher airspeed than the positive G stall.

But that’s not a concern for our proposed use of “decelerated” stalls, because the acceleration (or deceleration) isn’t about the stall speeds — it’s about the G forces acting on the airplane.

William E. Dubois is a NAFI Master Ground Instructor, commercial pilot, two-time National Champion air racer, a World Speed Record Holder, and a FAASTeam Representative.

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From a practical teaching point of view, Gabriel Braun’s lengthy and enlightening discussion points out an important point: “Just don’t pull back to try to keep your nose up, and you will not increase the AoA and you will not stall.”

Said in a way my students can understand: simply “unload the wing” and you “break the stall”.

What a great article, but now my head hurts!

“Accelerated” refers to the the airplane itself. In order to maintain a circular motion (a constant radius turn), the plane is subjected to a constant centripetal force, causing it to continually accelerate centripetally (towards the center of the circle). A “decelerating” airplane is one that is exiting a turn and trending towards straight flight. A plane in straight flight has zero acceleration.

“Decelerating” further will subject the plane to an opposing centripetal force, causing it to turn to the opposite direction. But in practice we call this “acceleration in the opposite direction” and “turning in the opposite direction” rather than “deceleration”.

All of the above assumes constant forward velocity and constant altitude flying.

Big loss opportunity here. If I had to use the term “decelerated stall” for something, I would use it for stalls at leads than 1G. This is very important and not well understood by many pilots, and not well taught in general. First of all, stall is ONLY a matter of AoA. At any combination of speed, bang angle, pitch angle and load factor that you exceed the AoA, you will stall. And by the way AoA is a matter almost exclusively of elevator deflection (+ stabilizer angle for airplane that trim via a movable stabilizer). The thing is that at a fixed AoA let’s say just below stall, the wing will generate more lift at higher speeds, and since the load factor (what we call G’s) is by definition lift divided by weight (actual fixed airplane’s rest weight, that is force due to gravity, not the apparent weight due to acceleration) then there will be only one speed (for a given weight) where the lift produced just before stall is equal to the weight, hence we have 1G, an our 1G stall speed, the one marked in airspeed indicators. If you are doing more than 1G, you will still stall at the same AoA but, because you are faster, your lift will be greater than your weight and we have a load factor greater than 1, or more than 1G, what we call an accelerated stall. Now, banking is not accelerating and accelerating is not banking. You can be very close to the stall speed, bank your plane 60 degrees to make a steep turn, And NOT stall. Remember what I said before? About AoA and elevator? Just don’t pull back to try to keep your nose up, and you will not increase the AoA and you will not stall. Your nose will go down and descend though, not because if the stall but because your are not pulling up which is what you would need to do to keep the nose from goin down in a turn. If you pull back to keep the nose up, you are increasing the AoA and will likely stall not because of the bank, but because you are pulling back. Remember the relationship between AoA and elevator that I mentioned above. But you don’t need to be in a turn to pull back, do you? If you pull back when you were flying straight and level, you are increasing the AoA too, and if you started with an AoA that was already close to stall, you will stall. That would count as an accelerated stall too. Now comes the interesting part., let me introduce it with a riddle that many pilots fail. You are flying straight and level 10 knots above the stall speed, there is no wind, and you are hit by a 20-knot sustained tailwind. Your airspeed will instantly fall to 10 knots BELOW the stall speed. WILL you stall? Typical answer is yes of course. The sudden tailwind put your airspeed well below stall so there is nothing you can do to prevent the stall, just execute a prompt stall recovery. We’ll, if you payed attention to what I’ve written so far should enable you to discover that that cannot be true. The famous riddle saying “you can stall at any speed” has a very important an very overlooked corollary: You can NOT-stall at any speed. Even slower than the stall speed. So haw can you not-stall if suddenly hit by a tailwind that puts your airspeed well below the official stall speed? Easy. Do nothing. Remember that stall is a matter of AoA only? And AoA is mostly a matter of elevator deflection? When hit by this sudden tailwind your airspeed has instantly changed but your AoA has not. Of course, with the same AoA than before and a quite slower airspeed your wing will be producing a lift that is quite less than the weight, you are at less than 1G. Positive Gs, but less than 1 (say for example 0.8 G). Because the lift is less than the weight te plane will descend and the nose will go down. No stalls involved. No pilot input involved. It is YOU, the pilot, who tries to prevent the nose from going down, who actively stalls the plane when you pull back. To prevent the nose from falling, causing it to to fall even more in the stall and risking a loss of lateral control which is the real killer in stall accidents. Remember the relationship elevator – AoA . In the same way than in the turn discussed earlier where it was not the bank but the pull back which stalked the plane. Again, stall is AoA, and AoA is mostly elevator. Final word: you can see in the V-n diagram that the stall speed varies with load factor, and that you have a range of stall speeds between 0 it’s at 0 G and the highest speed Va (maneuver speed) where the load factor reaches the design load factor, the stall speed at 1G is just one point in that curve. An important one, don’t get me wrong, because we spend most of the time in an airplane flying at 1G. But in no way the only one that matters. Accelerated stall typically refers to those points in the diagram between 1G and Va, so if anything I would call decelerated stall to those points between 0G and 1G. But the real important thing is that we need to improve our understanding of stalls and the AoA-elevator relationship. In practice. Unintentional stalks (and subsequent loss of lateral control) is still one of the main killers in general aviation, but also in airlines. Coltan, Air France, Yeti just 1 week ago…

If we define stall as “ exceeding a given angle of attack,” we have created a binary universe as related to an airfoil. Either we are above that angle of attack (stalled) or we are below that angle of attack(not stalled). At zero G , by our own definition, we are not stalled and can’t be. We may be falling like a briefcase, but we aren’t stalled because we haven’t exceeded that given angle of attack which we defined as stalled. Can you crash unstalled? Of course. Airplanes do funny things if they’re put in a situation of zero airspeed and zero angle of attack because they are not “flying” but are then the same as any other object we may throw in the air. It eventually falls towards earth, heaviest end first

Nice to hear a fighter pilot call it straight. Article way out of line saying stall can occur at any flight angle. Most of us probably know what you didn’t mean, but Ross spelled it out far more precisely. Jim Denike

Well I’ve owned a few planes and flown more and a parked airplane pulling zero Gs never stalls.

A parked airplane is actually “pulling” 1G.

The diagram indicates you don’t want to do more then -1.75 Gs. Beyond that is structural damage and pretty quickly failure, which is a really bad stall. Just saying;-)

From an Engineer’s, and Pilot, point of view:

The term “G” is not really a force but an acceleration equal to one time the gravity on earth (which again is an acceleration). That is roughly equal to 9.806 m/s^2 or 32.1740 f/s^2 depending where you are on this planet (or off). The trick part is that the “acceleration” the aircraft feels is a combination of both gravity and any change in direction or speed which will impart another acceleration vector which will add, vectorially, to that of earth gravity.

In physics, the word acceleration is used to describe motion which changes (both speed and direction). It is common to use acceleration to mean speeding up and the word deceleration to mean slowing down. Strictly speaking, however, acceleration describes both types of motion.

So the effect of acceleration on a straight and level stall caused by a change in direction (or speed to a lesser degree) is an accelerated stall. Anything other than a 1G straight and level stall is an accelerated stall and the term decelerated stall should not be used.

So this is my argument of why the term is called “Accelerated Stall” and nothing else. I bet some Physicist came up with that term :).

Negative and positive stalls above 1G are both accelerated stalls! but, that squishy area between negative 1 and positive 1 G could qualify for a new descriptor such as “decelerated stall” Overall, I personally use and prefer using “G Stall.”

Really great article. Thank you.

A stall (at more than 1G) occurring at the bottom of a loop is also an example of an Accelerated Stall. Would a stall (at less than 1G) occurring at the top of a loop be a ‘Decelerated Stall’?

What I think I understood was not always correct… thanks for the enlightenment.

How about power on or power off stall, no need to try to reinvent the wheel.

I couldn’t agree more. I thing discussions like this come from bored pilots.

I do like the Aussie’s title of G-Stall. That title will work in both the positive G environment and -G environment.

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