On January 25, 1991, a Robinson R-22 HP (registration N21439) operated by Arizona Wing & Rotor, Inc. was involved in an aviation accident near Scottsdale, AZ. Investigators recorded the probable cause as: The drive belts failed due to excessive wear on the clutch assembly upper sheave, caused by belts, grits, and sand, which created a sharp edge leading to belt failure. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 2 related events involving the same aircraft type or operator are linked below.
After the clutch caution light illuminated and a loud bang was heard, the pilot entered an autorotative descent. The helicopter struck tail stinger first, bounced, and rolled over. Inspection revealed two failed drive belts due to excessive wear on the clutch assembly upper sheave caused by belts, grits, and sand.
Event Sequence
The pilot observed the helicopter's clutch caution light illuminate and subsequently heard a loud bang. In response, the pilot lowered the collective and initiated an autorotative descent. During the descent, the helicopter terminated low to the terrain. The aircraft struck the ground tail stinger first, causing it to bounce and then roll over.
Damage and Inspection
A visual inspection of the helicopter revealed that two of the drive belts had failed, resulting in a drive train separation. Further examination by the helicopter manufacturer identified the cause of the belt failure.
Cause Determination
The manufacturer discovered that the belts failed as a result of excessive wear on the clutch assembly upper sheave. This wear was caused by the belts themselves, as well as grits and sand, which created a sharp edge on the sheave. The sharp edge subsequently led to the failure of the belts.
Probable cause
The drive belts failed due to excessive wear on the clutch assembly upper sheave, caused by belts, grits, and sand, which created a sharp edge leading to belt failure.