The just-sit-there-and-don’t-change-anything solution to loss of airspeed indication is a pretty good one, at least at climbing or cruising speed. For approach and landing, we need a way not just to maintain speed, but also to set it at a desired value. Ideally, we would fall back on our old and much-ignored friend the angle-of-attack indicator. Angle of attack is not of much use for managing speed in cruise, because small changes correspond to large speed differences. At approach speed and below, however, an instrument displaying angle of attack is really superior to an airspeed indicator, because it automatically compensates for weight and bank angle. In fact, airspeed is a surrogate for angle of attack, and not the reverse. I have used an angle of attack indicator — a Safe Flight SC-150 — for 18 years and 2,500 hours; for approach and landing I consider it primary and the airspeed indicator a backup. One of the greatest wonderments of my 50 years of flying is the failure of a display of angle of attack to become a part of every airplane’s instrument panel.
Angle of attack indicators usually consist of some sort of green-yellow-red color scheme rather than absolute numbers. The SC-150’s edgewise dial has four reference points: climb, approach, slow approach and stall warning. But in a pinch it would need only one: approach. And that suggests that in case of loss of airspeed indication, you could use the attitude indicator instead, provided that it has pitch angle markings and that you know the angle of the flight path.
The angle of attack is, to a pretty close approximation, the difference between the pitch attitude of the airplane and the flight path through the air. The angle of attack of a clean wing at approach speed is around six degrees. If the airplane is descending on a three-degree glidepath, such as a VASI or ILS, and its pitch attitude is three degrees nose up, then its angle of attack is somewhere around six degrees — most likely a degree or two more, because the wing is probably set on the fuselage at a small positive incidence.
One way to make an approach without an airspeed indicator is to use the knob on the attitude indicator to adjust the miniature airplane to a position two or three degrees below the horizon while in level cruising flight. Slow down by holding the miniature airplane on the horizon while maintaining altitude. Upon intersecting the glidepath, leave the flaps up, trim to keep the miniature airplane on the horizon bar and maintain the glidepath with power.
It would not be a bad idea to try this once or twice when the airspeed indicator is working, just to see what the result looks and feels like.
The important thing to remember is that the attitude indicator alone is not a substitute for an angle-of-attack indicator. It is only in combination with a known flight path angle, provided by the VASI or ILS, that it can tell you — very roughly — what you need to know.
Send reader mail to: email@example.com or P.O. Box 8500, Winter Park, FL 32789.