Williams Helicopter Corp UH-1H N502AW accident near Grand Canyon, Arizona

Probable cause

The loss of main rotor power due to a main rotor transmission failure from a lack of lubrication.

NTSB analysis

According to the pilot, the helicopter was about 500 ft above ground level (agl) when he saw the master caution warning light illuminate; he then saw that the low transmission oil pressure light was also illuminated. He immediately maneuvered away from power lines and set up an approach to land. While descending, the helicopter yawed left and right, and, about 10 ft agl with the airspeed almost at 0, the main rotor speed slowed and then stopped, and the helicopter subsequently fell to the ground. The pilot reported that the engine continued to operate for about 15 seconds after impact.

An on-scene examination revealed that both main rotor blades remained intact with no leading edge damage. Examination of the helicopter revealed a band of discoloration around the circumference of the transmission case near the main rotor reduction gear assembly; localized paint peeling was also noted. There were no oil stains or residue on the transmission case.

During the subsequent transmission disassembly, the mast was found coated in metal debris along its length, and the mast bearing was found coated in oil. Molten metal pieces were found in the upper planetary gears, ring gear, and sun gear, consistent with insufficient lubrication. The lower planetary gears sustained significant damage from the molten metal pieces. There was no damage to the transmission oil pump, oil was present in the oil pump cavity, and some debris and scoring were evident on the housing.

Examination of the passages within the transmission oil manifold revealed medium-to-high density particles in several of the passages, but none of the passages were blocked; particles were also found in the chip detector. The particles were identified as being generally composed of metals commonly used in gears, housings, and bushings. The damage to the transmission indicated that it likely seized as a result of an overtemperature condition due to a lack of lubrication. Due to the severity of the damage, the reason for the lack of lubrication could not be determined.

Text from the NTSB. Open the NTSB docket for WPR12LA039 for the full record.