Plane & Pilot
Tuesday, March 11, 2014

The Stall Warning System

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

The stall warning horn was located in the wreckage. It was the one originally installed at the time of airplane manufacture. The wiring remained attached to the switch. When 12 volts were applied to the horn, it sounded a steady tone as designed. The volume wasn't measured. Both the stall warning vane/switch assembly and the section of the left wing in which it was mounted were essentially undamaged. Examination of the assembly revealed that it wasn't the original device installed at manufacture.

Investigators compared the installation as found in the wreckage with the manufacturer's installation drawing. The comparison revealed differences, but the investigators were unable to determine the accuracy of the system since this can only be verified in flight.
The stall warning horn could never be heard in
flight over engine noise and the muffling effect
of headphones.
In 2007, the FAA had published Information for Operators #07001 designed to alert "...operators, directors of operations, chief pilots and flightcrew members who may be using noise-canceling headsets of the potential for misdetection of audible alarms and other environmental sounds." The FAA cautioned that when flightcrew members were using active noise reduction headsets "...electronic attenuation of important environmental sounds and alarms may occur." The FAA suggested testing noise-canceling headsets to find whether they're compatible with aircraft sounds and aural alarms, and to discontinue use of any headsets which prevent the pilot from hearing alarms.

The NTSB determined that the probable cause of this accident was the pilot's decision to take off toward a nearby low cloud layer and the subsequent turn, stall, and spin during the pilot's attempt to avoid the cloud layer. Contributing to the accident was the pilot's failure to avoid the stall. His ability to avoid the stall was hindered by an inaudible stall warning system of questionable accuracy.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, N.Y. 10602-0831.


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