On July 31, 1992, a Boeing A75 (registration N62552) operated by Perdue, Harold F. was involved in an aviation accident near Cibolo, TX. Investigators recorded the probable cause as: Separation of the crankshaft due to fatigue originating in the threads at a position normally in line with the aft face of the thrust nut; the thrust nut also failed in fatigue. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a test flight to check vibration levels after propeller blade angle realignment, the propeller separated, causing left wing collapse and forced landing. Investigation found fatigue failure of the crankshaft and thrust nut.
Overview
A commercial pilot was troubleshooting a vibration in his airplane that had persisted for the previous 10 flight hours. After the blade angles on the propeller were realigned, the pilot flew the airplane to assess the vibration level.
Sequence of Events
While on a left downwind leg, the propeller separated from the engine. The separation severed a wing strut and a portion of the left wing. With both of the biplane's left wings collapsed, the airplane entered a left vertical spiral and impacted in an open pasture.
Investigation Findings
Examination revealed that the separation of the crankshaft resulted from fatigue that originated in the threads at a position normally in line with the aft face of the thrust nut. The thrust nut itself also exhibited fatigue failure. The configuration of the nut differed from the configuration indicated in the engineering drawing; however, it was not established whether this discrepancy was a contributing factor to the failure. There was no evidence of over-torque during installation.
Probable cause
Separation of the crankshaft due to fatigue originating in the threads at a position normally in line with the aft face of the thrust nut; the thrust nut also failed in fatigue.
What happened in the 31 Jul 1992 Boeing A75 accident near Cibolo, TX?
During a test flight to check vibration levels after propeller blade angle realignment, the propeller separated, causing left wing collapse and forced landing. Investigation found fatigue failure of the crankshaft and thrust nut.
What aircraft was involved and where did it happen?
The accident on 31 Jul 1992 involved a Boeing A75, registration N62552, operated by Perdue, Harold F., at Cibolo, TX.
What was the probable cause of the accident?
Separation of the crankshaft due to fatigue originating in the threads at a position normally in line with the aft face of the thrust nut; the thrust nut also failed in fatigue.
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