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 witness later came forward and told investigators he was flying a helicopter on the day of the accident and approached the Llano airport. He noted that there were several airplanes in the traffic pattern, and he saw a flash of a wing and an airplane quickly departed controlled flight toward the ground. He added that he flew near the area, and since he didn't see any wreckage, dismissed the sighting as someone flying a radio-controlled model.

Another witness reported that he and several other people were working near the airport. He said he saw an airplane on the downwind leg of the traffic pattern in a very high angle of attack.

The flight instructor held commercial and instructor certificates for airplane, single-engine and multi-engine land, and instrument airplane. Investigators didn't have his logbook, but found he reported having 8,577 hours as of his last FAA medical exam.

The student pilot didn't hold a student pilot or medical certificate. The student applied for a medical certificate, but it wasn't issued pending additional information. At the time of the application, the student reported a total of 15 flight hours.

The 1969 American AA-1 airplane had two seats and fixed landing gear. Its annual inspection was current. The airplane's stall warning switch, located in the leading edge of the wing, was removed for testing. A meter was used to check for electrical continuity. When the switch was activated, no flow of electricity was observed. The screws used to connect the wiring to the switch were removed. A small amount of corrosion was observed under the terminal ends of the wires and switch contact points. The wiring and screws were reassembled, the test was repeated. This time, when the switch was activated, electrical continuity was confirmed on the meter. A mechanic's work order dated shortly before the accident contained the notation: "adjusted stall warning." Investigators said that, according to a family member of the student pilot, the student had told them a couple days before the accident that "the stall warning still was not working right." He reportedly told family members that the flight instructor would disable the stall warning system for each flight.

The NTSB determined that the probable cause of this accident was the flight instructor's delayed remedial action and inadequate supervision during practice traffic pattern work. Contributing to the accident was the flight instructor's use of a sedating medication on the day of the accident and the airplane's high angle of attack at a low altitude during the traffic pattern work, which prevented recovery during an aerodynamic stall.

Mooney M-20F
The pilot and all three passengers were killed when a Mooney M-20F crashed into a parking lot and struck a building shortly after takeoff from Watsonville Municipal Airport (WVI), Watsonville, Calif. Visual meteorological conditions prevailed for the personal flight. During a weather briefing with Lockheed Martin Flight Services, the pilot said the destination would be Pine Mountain Lake Airport (E45), Groveland, Calif.



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