NTSB Identification: LAX91LA079.
The docket is stored on NTSB microfiche number 43048.
Accident occurred Friday, January 25, 1991 in SCOTTSDALE, AZ
Probable Cause Approval Date: 03/31/1993
Aircraft: ROBINSON R-22 HP, registration: N21439
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

THE PILOT, AFTER SEEING THE HELICOPTER'S CLUTCH CAUTION LIGHT ILLUMINATE, HEARD A LOUD 'BANG'. HE LOWERED THE COLLECTIVE AND ENTERED AN AUTOROTATIVE DESCENT. HE TERMINATED LOW TO THE TERRAIN, AND THE AIRCRAFT STRUCK TAIL STINGER FIRST, CAUSING IT TO BOUNCE AND THEN ROLL OVER. A VISUAL INSPECTION OF THE HELICOPTER REVEALED TWO OF THE DRIVE BELTS HAD FAILED, CAUSING A DRIVE TRAIN SEPARATION. THE HELICOPTER MANUFACTURER DISCOVERED THE BELTS FAILED AS A RESULT OF EXCESSIVE WEAR ON THE CLUTCH ASSEMBLY UPPER SHEAVE. THE EXCESSIVE WEAR ON THE SHEAVE WAS CAUSED BY THE BELTS, GRITS, AND SAND. THIS CREATED A SHARP EDGE ON THE SHEAVE WHICH CAUSED THE FAILURE OF THE BELTS.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

THE FAILURE OF THE DRIVE BELTS DUE TO EXCESSIVE WEAR OF THE CLUTCH ASSEMBLY UPPER SHEAVE AND THE PILOT'S INACCURATE FLARE DURING THE TERMINATION OF THE AUTOROTATION. A FACTOR IN THE ACCIDENT WAS THE INADEQUATE DESIGN OF THE CLUTCH ASSEMBLY UPPER SHEAVE.

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