Plane & Pilot
Tuesday, March 11, 2014

The Stall Warning System

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

A fog bank/stratus layer had moved inland toward the airport from the Pacific Ocean. This type of weather, often called a "marine layer," was typical for that time of year, and was located just southwest of the airport. Two witnesses, one of who was a pilot, reported that the climb angle after takeoff appeared "steep." Both saw the airplane begin a very rapid left roll when it was approximately 500 feet above the departure end of runway 20. The airplane appeared to roll until it was "nearly inverted," and the nose "dropped," so that it was pointing toward the ground. Fire and smoke were then seen shortly after it disappeared behind trees.

The pilot was a co-owner of the airplane. His flight logbook couldn't be located, but his partner in the airplane provided copies that indicated the pilot had about 151 hours of flight experience, with all but 15 of those hours in the accident airplane.

The airplane's annual inspection was current. The stall warning system consisted of a wing-mounted sensor vane and switch assembly wired to an electric horn in the cockpit. The volume output of the horn was dependent on the voltage supplied. Full normal volume was expected at 12 volts. A drop in the voltage, for example due to circuit resistance, would cause the audio output of the horn to drop.

Maintenance records revealed that a new stall warning horn switch was installed about eight years before the accident when the airplane had a total time in service of about 3,442 hours. About four years before the accident, there was an effort to troubleshoot and repair an inoperative stall warning horn. About three years before the accident, at about 3,878 hours, a technician replaced a broken wire to the stall warning system.

The airplane co-owner told investigators that he had never determined the accuracy of the stall warning system, that the horn volume was low, and that the horn was inaudible to a pilot wearing a noise-canceling headset. Two headsets were found in the wreckage. The co-owner told investigators that he and the pilot "...both considered the stall warning system to be totally inadequate and ineffective—totally useless. The stall warning horn could never be heard in flight over engine noise and the muffling effect of headphones."

The co-owner said he never heard the stall warning in flight, and the accident pilot told him "...that he could barely hear the horn if he removed his headset." The co-owner stated that at the pilot's "insistence," the day before the accident, he took the airplane to a service center to address the system performance, and that the "center's conclusion was that the audio level could not be increased with current FAA approved parts." The co-owner told investigators that he had never stalled the airplane or verified the accuracy of the stall warning system. He knew that the pilot had intentionally approached stalls with the airplane during his flight training, but the co-owner did not know whether the pilot had ever tried to check the accuracy of the system.


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