SEA06LA124

While placing an air conditioning unit on top of a commercial building, the pilot heard the main rotor pitch droop, and confirmed that the rotor rpm was beginning to decrease. He then confirmed the throttle was open in the full flight position, moved the load away from the personnel on the roof, and lowered the collective. The low rotor audio sounded, and the rotor rpm continued to decrease, so the pilot hit the load release switch and autorotated to an emergency landing on top of the roof. After a hard touchdown, the aircraft bounced, touched down again, and then balanced on the right skid before very slowly rolling onto its right side. The load he was placing on the roof weighed less than one-half of what the helicopter was capable of lifting under the ambient conditions, and neither an engine/airframe inspection nor a test run of the engine were able to establish any anomalies or the initiating factor in the rotor droop event.

On June 20, 2006, approximately 1030, mountain daylight time, a Hughes 369D helicopter, N88SE, rolled over onto its side after autorotating to a hard landing to the top of a commercial building in Salt Lake City, Utah. The airline transport pilot, who was the sole occupant, was not injured, but the aircraft, which is owned by Pathfinder Leasing LLC, sustained substantial damage. The 14 CFR Part 133 long-line external load flight, which initially departed Bountiful, Utah, about 0840, was being operated in visual meteorological conditions.

According to the pilot, who had already lifted 20 of 24 air conditioning units into place on top of the 30 foot tall building, just before placing the twenty-first unit on top of the building, he heard what he believed was a main rotor pitch droop. He then quickly looked at the main rotor rpm on the tachometer, which confirmed that the rotor rpm was beginning to decrease. He then confirmed the throttle was open in the full flight position, moved the load away from the personnel on the roof, and lowered the collective. The low rotor audio sounded, and the rotor rpm continued to decrease, so the pilot hit the load release switch and autorotated to an emergency landing on top of the roof. After touchdown, the aircraft bounced, touched down again, and then balanced on the right skid before very slowly rolling onto its right side.

According to the pilot's calculations, the ambient conditions on the day of the accident allowed for loads up to 1,000 pounds, and it was determined that the load he was lifting was no greater than 425 pounds.

A post-accident inspection of the airframe and engine did not reveal any malfunction or anomalies that would have contributed to a reduction in engine or rotor rpm. The engine was shipped to the Rolls-Royce inspection/test run facility in Indianapolis, Indiana, where it underwent an FAA-supervised test run. During all portions of the test run regime, the engine performed at or above all required specifications, and at full throttle produced power 11.9 percent above required output.

At the end of the investigative process, no clear determination of the source of the rotor droop event could be established.