DETROIT LAKE, OR, USA
N70NW
BELL UH-1B
THE AIRCRAFT STRUCK TREES WHILE ATTEMPTING AN AUTOROTATION AFTER AN ENGINE FAILURE. THE PILOT HAD PUNCHED OFF HIS EXTERNAL LOAD AFTER THE POWER LOSS, BUT WAS UNABLE TO SUCCESSFULLY ACCOMPLISH THE AUTOROTATION DUE TO INSUFFICIENT ALTITUDE AND UNSUITABLE TERRAIN. THE ENGINE FAILURE WAS THE RESULT OF THE FATIGUE FRACTURE OF BLADE NUMBER 19 OF THE SECOND STAGE TURBINE ROTOR ASSEMBLY.
On September 24, 1993, at approximately 0930 Pacific daylight time, a Bell UH-1B, N70NW, sustained substantial damage when it collided with terrain after an apparent loss of engine power, while engaged in aerial logging operations. The commercial pilot, who was the sole occupant of the aircraft, was seriously injured. Visual meteorological conditions prevailed at the time of the accident. The pilot reported that he was approaching the drop pad at between 200 and 250 feet above ground level with a 3000 pound load on a 150 foot long line. In a telephone conversation, he stated that he heard a high pitch sound, the rotor low rpm light illuminated, and he noted a loss of power. He said he punched the load, told the crew to clear the area, and initiated an autorotation to the pad. In his written statement, he noted that he heard a loud screeching, grinding noise when he lost power. he said that he intended to set down on the landing (pad) and made a radio call to clear the landing of landing crew. As there were longs on the landing, he decided to set down on the road a couple hundred feet away. The aircraft struck a tree during the descent and landed hard. The engine was disassembled for inspection. The second stage turbine rotor assembly, P/N P1-140-205-06Y was removed and shipped to the NTSB materials laboratory for inspection and testing. Visual examination of the turbine disk noted that ten of the 62 blades were broken through the airfoil section. Two of the ten were fractured close to the blade platforms near the disk rim. Additionally, the forward bearing shaft of the rotor was separated near the rotor disc. The metallurgist's report is attached separately.
FAILURE OF A SECOND STAGE TURBINE WHEEL TOO LOW FOR A SUCCESSFUL AUTOROTATION. FACTORS INCLUDE A TREE IN THE APPROACH PATH.
Source: NTSB Aviation Accident Database
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