JUNEAU, AK, USA
N99676
BELL UH-1B
WHILE ENGAGED IN A LOG SLINGING OPERATION OVER MOUNTAINOUS, DENSELY FORESTED TERRAIN THE ROTOR CRAFT SUFFERED A TOTAL POWER LOSS FORCING THE PILOT TO EXECUTE AN AUTOROTATIVE LANDING INTO HIGH CONIFER TREES. DISASSEMBLY OF THE ENGINE REVEALED EVIDENCE THAT THE SPACER BETWEEN THE NO. 5 COMPRESSOR AND THE IMPELLER HAD NOT BEEN POSITION SEATED DURING THE LAST OVERHAUL AND THAT DURING THE ACCIDENT FLIGHT THE SPACER SLIPPED INTO ITS PROPER POSITION THUS RELEASING THE COMPRESSIVE TORQUE LOAD UNDER WHICH THE ASSEMBLY HAD BEEN SUFJECTED DURING BUILDIP. THE COMPRESSOR ASSEMBLY SUBSEQUENTLY BECAME UNCOUPLED FORCING THE IMPELLER FORWARD INTO ITS MAGNESIUM HOUSING. THIS ACTION RESULTED IN AN OVERTEMPERATURE CONDITION AND BREAKUP WITHIN THE TURBINE ASSEMBLY BROUGHT ON BY THE COMBUSTION OF MAGNESIUM PARTICLES.
Source: NTSB Aviation Accident Database
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