On June 20, 2006, a Hughes 369D (registration N88SE) was involved in an aviation accident near Salt Lake City, UT. Investigators recorded the probable cause as: A loss of rotor rpm for undetermined reasons, resulting in a hard landing on top of the commercial building over which the helicopter was working. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While placing an air conditioning unit on a commercial building, a helicopter experienced rotor droop, prompting an autorotation and emergency landing on the roof, resulting in a rollover.
Incident Overview
While positioning an air conditioning unit on top of a commercial building, the helicopter pilot heard the main rotor pitch droop and observed a decrease in rotor rpm. The pilot confirmed the throttle was in the full flight position, moved the load away from personnel on the roof, and lowered the collective. The low rotor audio warning sounded as rotor rpm continued to decline, prompting the pilot to activate the load release switch and execute an autorotational emergency landing onto the roof.
Landing and Rollover
The helicopter made a hard touchdown, bounced, touched down again, and then balanced on the right skid before very slowly rolling onto its right side. The load being placed weighed less than one-half of the helicopter's lifting capability under the ambient conditions.
Post-Incident Examination
Neither an engine and airframe inspection nor a test run of the engine were able to establish any anomalies or the initiating factor in the rotor droop event.