On November 7, 1987, a Hiller UH-12B (registration N922TE) operated by Unlimited Directions,inc. was involved in an aviation accident near Newnan, GA. Investigators recorded the probable cause as: The failure of the tail rotor 90-degree gearbox mounting flange due to extensive preexistent fatigue cracking resulting from excessive vibration. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During approach at about 75 ft AGL, a helicopter's tail rotor 90-degree gearbox mounting flange fractured, causing gearbox separation and subsequent rollover during landing.
Incident Overview
During the approach to landing, while the helicopter was at approximately 75 feet above ground level, the tail rotor 90-degree gearbox mounting flange fractured. This led to the separation of the gearbox from the helicopter. The pilot initiated an autorotation in response to the failure. However, during the landing, the helicopter rolled over.
Metallurgical Analysis
A metallurgical analysis of the failed flange was conducted. The examination indicated that the flange failed as a result of extensive preexisting fatigue cracking. The cracking was determined to be the result of excessive vibration. No further details regarding the specific nature of the vibration or the duration of the cracking were provided in the source.
Conclusion
The accident was caused by the fracture of the tail rotor 90-degree gearbox mounting flange due to preexisting fatigue cracking from excessive vibration, leading to gearbox separation and a rollover during the subsequent autorotation landing.
Probable cause
The failure of the tail rotor 90-degree gearbox mounting flange due to extensive preexistent fatigue cracking resulting from excessive vibration.