MARMATH, ND, USA
N8617J
ENSTROM F-28C
THE PILOT REPORTED THAT WHILE HE WAS IN FLIGHT AT ABOUT 75 TO 80 FT AGL, THE ENGINE LOST POWER. DURING AN AUTOROTATIVE LANDING IN AN OPEN PASTURE, A SKID DUG INTO THE GROUND AND THE HELICOPTER ROLLED OVER. A SUBSEQUENT ENGINE TEARDOWN REVEALED EVIDENCE THAT THE ENGINE HAD BEEN OPERATED IN AN OVERSPEED CONDITION OVER A PROLONGED PERIOD OF TIME. THERE WAS EVIDENCE OF HIGH VALVE SEATING VELOCITIES AND BOUNCING, THE UPPER SPRING SEATS HAD BEEN STRIKING THE TOPS OF THE VALVE GUIDES, FATIGUE FAILURE OF THE KEEPERS HAD OCCURRED, AND SUBSEQUENTLY, A VALVE FAILED FROM FATIGUE.
Source: NTSB Aviation Accident Database
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