January 1, 1996
By Barry Schiff
Year after year, stall/spin accidents occur with alarming regularity and are responsible for almost half of all fatal accidents.
One reason for this is that the very training designed to prevent such accidents actually contributes to them.
When a student is taught stall entry and recovery, he usually is introduced to the apparent need for a nose-high attitude. These empirical lessons teach the misguided notion that an airplane stalls only when the nose is pointed skyward. Then — usually at some later time — the pilot is told and expected to believe that a stall can occur with the airplane in any attitude. Some convincing logic and basic aerodynamic theory compel him to accept this abstract notion.
Most pilots understand that stalls can and do occur with the airplane in a nose-low attitude, but seldom are they shown situations in which stalls occur with the nose below the horizon.
For similar reasons, pilots conclude early in their training that stalls occur only at low airspeed. They are taught that stall speeds do increase during turns, pull-ups, and turbulence, but seldom is this demonstrated. The information seems more useful when taking a written or practical examination than it does to assist in reducing the likelihood of a stall/spin accident.
Finally, we learn that a stall cannot occur unless the wing is flown at too large an angle of attack. Although accurate, this knowledge does little to help prevent the inadvertent stall, because a pilot cannot easily determine angle of attack at any given time (especially during maneuvering flight). This led some safety experts to conclude that an angle-of-attack indicator might be a solution to the problem. But this instrument never achieved popularity in general aviation (even though it is standard equipment in many military and air carrier aircraft). One reason is that the less- expensive stall-warning indicator is considered equally effective.
Unfortunately, conventional stall training and stall-warning systems do not appear to have had a significant effect on training general aviation pilots to avoid stall/spin accidents.
There is one instrument that would be most useful in stall prevention: a stall-speed indicator incorporated into an airspeed indicator (ASI). Imagine, for example, an ASI with two movable needles. One would be a conventional airspeed needle. The other, however, would indicate the actual stall speed of the airplane at any given time and configuration (as determined by a small computer). In this way, a pilot would always be aware of his actual airspeed margin above stall simply by observing the distance between the two needles. As the stall margin narrowed, the separation would decrease. At the moment of stall, the needles would overlap.
During a power-off dive, for example, the airspeed needle would be well into the green arc and the stall-speed needle would rest at VS1, the flaps-up, power-off stall speed. If flaps and landing gear are extended, the stall speed needle would move to the lower stall speed, VS0. But if power was added, the indicator's computer would reduce indicated stall speed further because of the effect that propwash over the wing has in reducing stall speed.
Similarly, the unit's computer would take information from a sensitive G-meter to provide the constantly changing stall speed that occurs during turbulent and maneuvering flight. The indicated stall speed would vary from 0 knots at 0 Gs to the higher stall speeds associated with turbulence, turns, and high-G pull-ups, or a combination of these factors.
To compensate for the effect of aircraft gross weight and center of gravity on stall speed, the pilot might be required to enter actual gross weight and CG into the computer prior to departure. This alone would be a worthwhile feature because pilots often do not perform these important computations. During flight, the decrease in gross weight would be fed from a fuel-flow indicator to the stall-speed computer. In this way, indicated stall speed would always be predicated on actual weight and balance.
It also would be desirable to include elevator position, because this too has an effect on stall speed. When the elevator is moved up, for example, the download (negative lift) on the tail increases. This means that the wing is required to produce additional lift and vice versa. Since elevator position alters the lift requirement of the wing, the effect is much like varying aircraft gross weight, which obviously affects stall speed.
Because pilots frequently refer to their airspeed indicators, such a stall-proximity indicator would always be in their scan. They could see their margin of stall protection at a glance.
Sound revolutionary? It is not. Such a device was suggested 10 years ago by Jerry A. Brown, an air carrier inspector for the FAA. But his brainchild was never accepted, possibly because the required technology was too expensive. Today such a device would be relatively inexpensive, and it is needed as much now as ever.
It is about time that we quit stalling around.
Safety and Education
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