WALLA WALLA, WA, USA
N39SD
BELL UH-1B
THE PILOT STATED THAT DURING AN EXTERNAL LOAD OPERATION, JUST AFTER THE LOAD CLEARED THE GROUND THE HELICOPTER SHUTTERED AND MADE A LOUD SCRAPING NOISE FOLLOWED BY LOW ROTOR RPM. THE PILOT RELEASED THE LOAD AND MANEUVERED FOR AN EMERGENCY LANDING. DURING THE MANEUVER, THE HELICOPTER COLLIDED WITH TREES. DURING THE POST CRASH INSPECTION, IT WAS FOUND THAT THE FREEWHEELING ONE-WAY CLUTCH ASSEMBLY HAD FAILED IN FLIGHT. THE CLUTCH ASSEMBLY APPEARED TO BE WORN AND THE INNER CLUTCH RACE DISPLAYED SIGNS OF BRINELLING.
On April 22, 1995, approximately 0950 Pacific daylight time, a Bell UH-1B, N39SD, experienced a loss of engine power during an external load operation and collided with trees on steep mountainous terrain approximately 10 miles southeast of Walla Walla, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was substantially damaged and the commercial pilot, the sole occupant, was seriously injured. The flight originated from a remote job site and was operating in the local area. The president of Blue Mountain Helicopters reported that during the external load operation, the flight was at the mid point of the one hour cycle. The pilot positioned the helicopter for a turn of logs. The ground crew hooked the load and the pilot started up with the load. The crew reported that the logs had just cleared the ground when the load was released. The crew stated that the helicopter's engine "immediately got quiet and the blades began getting slower." The pilot released the long line and nosed the helicopter down hill. The helicopter travelled approximately 300 feet before colliding with trees. The pilot reported that he was in a hover over the load and had just picked it off the ground when the helicopter "shuddered and made a loud scraping noise like that of a chain hoist slipping." The pilot noticed a low rotor rpm and he immediately bottomed the collective to maintain rotor rpm. The helicopter yawed slightly to the right and the pilot guided the helicopter to an opening in the trees and pulled full collective to stop before colliding with the trees. The operator reported that the long line was 150 feet in length. The load weighed approximately 2,400 pounds and consisted of two logs measuring 16 feet and 22 feet. During the engine and airframe inspection, it was found that the freewheeling one-way clutch assembly failed in flight when the sprags rolled over and portions of sprags were ground off by the clutch races. This failure resulted in the loss of engine power that was transmitted to the main rotor transmission. The one-way clutch appeared to be worn. The inner clutch race displayed signs of brinelling.
TOTAL FAILURE OF THE FREEWHEELING SPRAG UNIT. A FACTOR TO THE ACCIDENT WAS: SUITABLE TERRAIN WAS NOT AVAILABLE FOR LANDING.
Source: NTSB Aviation Accident Database
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