Eye of Experience #12:
Understanding the Stall

Stall entry and recovery is one of the most discussed - and cussed - portions of a flight training syllabus. Yet, AVweb's Howard Fried believes that stalls remain one of the most misunderstood aspects of safe and knowledgeable flying. His dissection of stalls, spin entry and maneuvering speed in this Eye of Experience is a must-read for students, instructors and grizzled veterans alike.


Eye Of ExperienceRecognition

F light schools and flight instructors are doing itall wrong. We are teaching our students how to make a stall and recover from it when whatwe should be teaching is stall recognition. Ask 100 pilots what makes an airplane stalland at least 70 of them will tell you it got too slow. The majority of the remainder willtell you that the nose was pitched up too high. And a very few will say the airflow overthe wing separated, so the wing quit flying and an aerodynamic stall resulted. Possiblyone, or even two, will give you the correct answer. An airplane stalls for one reason andone reason only. It has exceeded its critical angle of attack (AOA), period.Thats all she wrote. Exceed that angle and the airplane will stall. Dontexceed it and it cant stall. Very few pilots, and this includes air carrier airplanedrivers as well as general aviation people, really understand AOA, what it really is, andhow it is affected by airplane configuration.

An AOA diagram

A picture is worth a thousand words.

Reduced to its simplest terms, the AOA is the angular difference between where theairplane is pointing and where it is going. An angle, any angle, is formed by theintersection of two lines and the two lines that form the AOA are the mean aerodynamicchord (MAC)* of the wing and the relative wind. Normally, wind isthought of as air in motion, but in this case, it is the motion of the airplane throughthe air that creates what we call “relative wind.” The AOA at which any givenairplane will stall is a built-in, fixed number (usually around 16 to 18 degrees) and whenthat number is exceeded, the airflow over the wing separates and the stall occurs. Thisconcept of gearing our thinking to AOA seems to be particularly difficult to get across tostudents, perhaps because we cant see the two lines that form the angle of attack.


Where pilots, particularly student pilots, are being misled is in thefact that airplane manuals publish “stall speeds.” Somehow the pilot gets it inhis or her head that as long as the airplane is above that speed, then it wontstall. Of course, nothing could be further from the truth. The airplane can and will stallwhen going faster than the published stalling speed, a great deal faster. This is why”accelerated maneuver” stalls are demonstrated and practiced during training.Even so, what many pilots fail to realize, occasionally with rather severe consequences,is that the published stalling speed is valid only under a very narrowly-prescribed set ofcircumstances, including configuration, weight, airplane attitude and others. The onlyeffect that speed has on the stall is in the fact that at a reduced speed a high angle ofattack results. I do wish, therefore, that those involved in aviation education would quitteaching stalls as being related to speed. This approach seems to firmly plant in thestudents head the idea that if he/she just keeps the airplane going above thepublished stalling speed, it simply will not stall, when, of course, it can and will. Ibelieve that more emphasis should be placed on the so-called “acceleratedmaneuver” stall, although this has more to do with weight than speed.

Corporate jets and air carrier airplanes have angle of attack indicators, but we ingeneral aviation have to struggle along without them. Although the trigger for the stallwarning device is really a sort of angle of attack indicator, we still relate the stall tothe factor of speed. I do wish the general aviation fleet was equipped with AOAindicators. Theyre cheap, simple, and they give us really useful information. If ourairplanes were so equipped, it would be a lot easier to teach our students to think of AOArather than speed as being related to stalls.


Even less understood by many pilots is the effect of power on AOA. If anairplanes pitch attitude does not change, an increase in power will always result ina reduction in the angle of attack. Think about this for a moment. The best way tovisualize AOA is to think in terms of the relative wind striking the bottom of the wingrather than crawling over the top of the wing. Visualize yourself on final approachmaintaining a level attitude with reduced power and the airplane is descending. Therelative wind is striking the bottom of the wing at a fairly high angle. Now, withoutchanging the pitch attitude of the airplane, add power. What happens? You are now drivingthe airplane straight ahead, no longer descending, the relative wind has aligned itselfwith the direction of flight and you have zeroed out the angle of attack.

Another AOA diagram

“It is a lot easier and more practical to think of the relative wind striking the bottom of the wing rather than crawling over the top of the wing.”

Now try this: In level flight, at a nice, safe altitude, reduce the power. The airplanewill tend to pitch down, but dont let it do this. With the application of upelevator, maintain level flight. The airplane will slow down and will ultimately stall(the angle of attack has gone beyond the critical point). When the stall occurs, theairplane will want to pitch down. Do not permit it to do this. Hold the same attitude withelevator pressure. Add power. The airplane will want to pitch up. Again do not permit itto do this. Hold the same attitude with elevator pressure. What do you think will happen?The airplane will recover from the stall without ever lowering the nose! Dont get mewrong. Im not advocating this as the way to recover from an inadvertent stall, butmerely using it as a means of demonstrating the effect of power on angle of attack. Torecover from a stall you still want to lower the nose, keep the wings level, and addall available power.


To paraphrase Gertrude Stein, a stall is a stall, is a stall. Anytime the critical AOAis exceeded the airplane will stall; dont exceed that critical AOA and it cantstall. However, over the years, we have put a bunch of fancy names on the stall series fortraining purposes, all of which are really meaningless if we can get our students to thinkin terms of angle of attack. It is a lot easier and more practical to think of therelative wind striking the bottom of the wing rather than crawling over the top of thewing. When I was trained, we had what we called the approach to a stall, now known as theimminent stall. We had the stall out of a climbing turn, which we call today thetakeoff-and-departure stall, and the stall out of a gliding turn which today is called theapproach-to-landing stall. Back then, the accelerated maneuver stall was called aloaded-up stall or a high-speed stall. The bottom line is still, anytime the criticalangle of attack is exceeded, the airplane will stall.

Rarely taught anymore, but extremely useful, is the “delayed recovery” stall.It is accomplished like this: At a good, safe altitude and after carefully clearing thearea to determine that there is no traffic around, the power is reduced and the pitchincreased until the airplane stalls. The stick (or yoke) is held fully back against thestop. When the stall break occurs, the wings must be kept level, and the nosepointed straight with the rudder. The airplane will pitch down, recover itself, pitch downand recover itself all the way to the ground (if permitted). This exercise is a greatconfidence builder.

Maneuvering Speed

It is not marked on the airspeed indicator (although it may be placarded on the panel)but one of the most important speeds for the pilot to know and be aware of at all times ismaneuvering speed (Va). The definition of maneuvering speed is the fastest speed at whichan abrupt full control deflection will not engender structural damage. By “abruptcontrol deflection” is meant all the controls, but it is the elevator on which theemphasis is placed. Visualize this situation: You are charging through the air above Vaand you suddenly reach out and give the stick (or yoke) a hard yank, right to the stop.What happens? The airplane will attempt to stand on its tail and go straight up. But theforward momentum, opposing the attempt to go vertical, will exert so much force that thewings will bend or break. And we all know that airplanes dont fly very well when thewings fall off!

Now let’s cruise along at or below maneuvering speed. Again, brutally haul back on theyoke (or stick) right to the stop. What happens this time? The airplane will zoom up untilit runs out of poop and then stall. In other words, maneuvering speed (or below) is thespeed at which an airplane will stall before it bends or breaks. What would you rather do,recover from a stall, or try to fly an airplane from which the wings have just departed?Simple question, simple answer.

I knew an old instructor who taught maneuvering speed by comparing it to an automobiledriving down a road and encountering a rough railroad track crossing the road. If thedriver fails to slow down, he/she might very well damage the car as it bounces over thetracks. But if the car is slowed to a moderate speed before crossing the rough tracks, itwill ride right over them taking the bumps in stride. This is why we instantly go tomaneuvering speed when we encounter rough air. In heavy chop with vertical gusts, the aircan be striking the underside of the wing with considerable force. If this happens whilethe airplane is progressing at a very high speed (above maneuvering speed), this forcecould bend or break the wings. However, if such an updraft is encountered at or below Va,the airplane will have exceeded its critical AOA and stall instead of bending or breaking.In this case, the stall is so brief, so transitory, that recovery technique is notrequired. If you have ever been flying along in light to moderate turbulence in anairplane with an aural stall warning device, you have heard it occasionally going beep,beep, beep What it is telling you is that the relative wind has momentarily struck thebottom of the wing at an angle above the critical AOA. Did you apply recovery technique?No. It wasnt necessary because the airplane flew right out before a stall couldfully develop.


For an airplane to spin, two elements must be present: a) it must bestalled and, b) a yaw moment must be introduced. It follows, therefore, that if anairplane is not permitted to stall, it cant spin. And if we recognize an incipientstall before it happens, we can prevent it from happening, and we have a whole bunch ofcues to make us aware of this situation. This brings us back to the angle of attack. If weare constantly aware of the difference between where were going and where werepointing, we have a pretty fair grasp of the angle at which the wing is meeting therelative wind.

As far as the second element in the formation of the spin is concerned, the yaw isalmost invariably the result of misusing the rudder. On a tight turn from base to final,we have increased the load factor on the wing drastically, thus increasing the speed atwhich the critical angle of attack will be reached. (There we go again, thinking aboutspeed.) Simultaneously, we are holding a lot of back pressure on the elevator control, andif we have not properly coordinated rudder with the bank, we are inviting a disastrousspin, because we are low and close to the ground, with little or no space for recovery.And this is a situation we must seek to avoid at all costs.

*On a commercial flight test I once had an applicant tell me when asked about the MAC, “That’s a great big hamburger!”

Usual Boilerplate

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