Pilot Debrief

BELL 205 near Tehachapi, CA — 2011-09-04

Final reportWPR11GA431
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Date
2011-09-04
Location
Tehachapi, CA, USA
Aircraft
BELL 205
Registration
N205WW
Category
Helicopter
Highest injury
Minor
Fatalities
0
Phase of flight
Climb

Probable cause

The pilot's inability to adequately execute an emergency autorotation due to the flight's low altitude during external load operations, which resulted in a hard landing. Contributing to the accident was a torsionally overstressed tachometer shaft, which sent erroneous engine rpm readings to the cockpit.

NTSB narrative

During an external load operation to drop water on a fire, the helicopter was about 100 feet above ground level at 10 knots when the engine rpm light illuminated and the low rotor rpm horn sounded. The emergency procedures section of the flight manual states that in the event of an engine failure or low rpm, a red light will illuminate and an audio signal will sound when the audio switch is in the AUDIO position. The flight manual instructs the pilot to immediately execute an autorotative descent. The pilot released the water, and made a left-pedal turn to exit the canyon and move away from the fire. He checked his engine rotor rpm gauge and saw that the needles had split: the rotor needle was at the 4-5 o'clock position, and the engine needle was at the 6-o'clock position, which he stated indicated maximum rpm. He maneuvered to establish an autorotation into a landing zone. The helicopter sustained substantial damage to the airframe and tail boom as the result of a hard landing, which collapsed the landing skid. Postaccident examination discovered that the N2 tachometer drive shaft was sheered as a result of torsional overstress. The N2 tachometer drive delivers engine rpm readings to the cockpit engine tachometer; failure of the N2 tachometer drive would send erroneous engine rpm readings to the cockpit. Accordingly, the pilot's instruments indicated that there was an engine overspeed, but the warning lights and audio were indicating a low power condition. The pilot elected to perform an autorotative landing in accordance with the flight manual instructions for a low rotor rpm.

Analysis

Primary failure mode
Engine power loss
First missed decision gate
Pilot could have executed autorotation sooner after engine rpm warning.

NTSB coding

Evidence available

  • Photos
  • 11 docket documents
View NTSB final reportView NTSB docket

Docket documents11

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