Plane & Pilot
Tuesday, March 11, 2014

The Stall Warning System

An aircraft’s stall warning system doesn’t always receive the attention it deserves

Whether it's a comparatively simple system (a leading-edge vane operates a switch to complete an electrical circuit and sound a horn or illuminate a bulb), or a complex system (which generates signals to activate a stick shaker), a properly operating stall warning system can prevent you from having a really bad day.

Back in the dark ages when I was learning to fly in a Cherokee 140, the routine was for someone to turn on the master switch and watch to see if the stall warning red light illuminated when the person doing the walkaround flicked the vane protruding from the left-wing leading edge. When the bulb did light, it produced a reassuring feeling. No thought was given to the possibility that the test had caused the bulb to reach the end of its life, and it wouldn't work when really needed. It was easier to check the stall warning systems on newer aircraft in the FBO's rental fleet because they used horns, and reasonably good hearing substituted for having a second person looking at the panel bulb. On instructional flights, we'd make careful note of the airspeeds and attitudes at which the stall warning would illuminate or sound.

The lack of effective stall warning systems figured in two recent NTSB accident reports. In one, there was an electrical issue. In the other, investigators looked at repairs, accuracy and the use of headsets.

American AA-1
The flight instructor and student were killed in the crash of a Grumman American AA-1 near Llano, Texas. Visual meteorological conditions prevailed for the Part 91 instructional flight, which operated without a flight plan. The flight originated from the Llano Municipal Airport (AQO).

The airplane was owned by the student pilot. The airport manager told investigators that he saw the student pull the airplane from the hangar and add fuel. The manager said the flight remained in the traffic pattern for touch-and-goes. He said he radioed the instructor that a jet was scheduled to be inbound to the airport. The instructor acknowledged. There was no additional radio traffic involving the accident airplane, and the manager said he thought they had departed the traffic pattern. He wasn't concerned about the whereabouts of the airplane until about three hours later when the instructor's next student showed up.

The airport manager contacted several nearby airports, but couldn't find the airplane. A local pilot then took off to search for the missing airplane. After only a few minutes in the air, the pilot spotted the airplane's wreckage southeast of the runway. It was in a lightly wooded area with cactus plants, approximately one mile southeast of the airport.


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