THE SIX SIGNS OF AN IMPENDING STALL
We've all heard about the six signs of an impending stall that have been drilled into us by some flight instructors. They are extremely important. In case you have forgotten them they are:
1. Excessive back stick pressure
2. Nose high attitude
3. Low airspeed
5. Mushy controls
6. Shudder or buffetting
I do not believe each of these signs is of equal importance. Futhermore, some of these signs are causes while others are results. To truely identify what causes a stall (and therefore how to avoid one) we must separate the cause signs from the results signs.
The number 1 sign, "Excessive back stick pressure", is a cause sign but is very hard to get across to a student. What is excessive? 1 pound, 2 pounds, 10 pounds? What about the trim on the plane? If it is trimmed wrong could that cause excessive back stick pressure without being close to a stall? In other words excessive back stick pressure could exist and one is not close or getting close to a stall. Therefore, I pretty much discount this cause when I am teaching stall awareness/avoidance. It can confuse the student more than help.
The number 2 sign, "Nose high attitude" is a cause of stalls and will always be there when a sailplane stalls. It is unmistakable and it is easy to teach. Couple the teaching of this sign with the hazards of skidding the sailplane and, if followed, a student will never stall a sailplane. This sign is by far the most important in teaching stall awareness/avoidance. If students learn to keep the nose below the horizon they are well on their way to never inadvertently stalling.
The number 3 sign, "low airspeed", goes hand in hand with "Nose high attitude" and is a good indicator for students. However, it is really a result of " nose high attitude". With students it is the main result sign that will help a student avoid stalls. That is, if a student keeps his airspeed well above the published stall speed he will not and, as a matter of fact, can not stall the sailplane. The old adage that a plane can stall at any attitude at any airspeed is, in the real world, simply not correct (unless the student jerks back on the stick as fast and as hard as he can and students just don't do that).
The number 4 sign, "Quietness" is another result sign and is not a cause of stalls. Different sailplanes have different levels of noise. I certainly do not want students to first recognize the beginning of a stall by quietness. It is much too late at this stage.
The number 5 sign, "Mushy Controls", is another result sign of impending stalls. Similar to "Quietness" it is a sign I never want my students to get to. If they pay attention to "nose high attitude" and "airspeed" this number 5 will not happen. Remember, we teach stall awareness/avoidance.
The number 6 sign, "shutter or buffeting", is also a result sign not a cause sign. Furthermore, this sign occurs in other situations. For instance, upon release from the tow plane I have had students get hit by the prop wash of the towplane and push forward on the stick thinking they are stalling. NOT!! On the other hand some sailplane just stall without a hint of a shutter or buffet. So number six sign is not one of the more important one.
In summary, two signs stand head and shoulders above the rest to be used for stall awareness/avoidance. They are:
NOSE HIGH ATTITUDE (a cause)
LOW AIRSPEED (a result)
There is however, one other point that needs to be reemphasized. SKIDDING TURNS drastically increase the stall speed of a sailplane. The more you skid a turn the LESS nose high attitude it takes to stall and this stall will occur at a higher airspeed.