On January 5, 1984, a Hiller UH-12E (registration N4027K) was involved in an aviation accident near Mapleton, OR. Investigators recorded the probable cause as: Failure of the spur adapter gearshaft due to fatigue originating at an area of heavy wear on the load side of the forward splines. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced a loud bang and power loss while approaching a spray area. The pilot performed an autorotative landing that ended with the aircraft rolling onto its back on a steep slope. Post-accident inspection revealed fatigue failure of a spur adapter gearshaft.
Incident Overview
While approaching a spraying area at a speed of 20 to 25 miles per hour and an altitude of about 100 feet above ground level, the helicopter was in level flight. The pilot suddenly heard a loud bang and observed a loss of engine power. He immediately initiated an autorotative landing procedure.
Landing and Rollover
During touchdown on hilly terrain, the helicopter rolled to the right and came to rest inverted on a steep slope.
Engine Teardown Findings
A subsequent teardown of the engine revealed that a spur adapter gearshaft (part number 6890482) and a compressor coupling adapter (part number 6871472) had failed. Detailed examination determined that the failure of the spur adapter gearshaft was caused by fatigue. The fatigue crack originated at an area of heavy wear located on the load side of the forward splines.
Probable cause
Failure of the spur adapter gearshaft due to fatigue originating at an area of heavy wear on the load side of the forward splines.