Mcdonnell Douglas Helicopter 369E N473E accident near Cottonwood, Arizona

Probable cause

The pilot’s low-speed, high-power maneuvering in high density altitude and near maximum gross weight, which resulted in an uncommanded right yaw due to a loss of tail rotor effectiveness and a subsequent hard landing.

NTSB analysis

The sightseeing helicopter pilot made a slow, high-powered maneuver near a canyon tour site with three passengers on board in light gusting wind conditions. As the pilot tried to fly away from the area, an uncommanded right yaw occurred. The pilot applied full left pedal control, but the helicopter maintained its right yaw. Shortly after, the low rotor rpm audio warning sounded, and the temperature caution light flashed. The pilot performed a low-level autorotation, and the right yaw stopped, but the helicopter landed hard and bounced. The left skid separated, and the helicopter slid forward, balanced on the right skid. The pilot then departed back to the operator’s base airport and made an emergency landing on one skid, at which time the passengers safely disembarked. Shortly after, the pilot relocated the helicopter to an open grass area at the airport to shut down the helicopter, after which the helicopter came to rest on its side and the drivetrain sustained substantial damage.

The pilot reported a possible tail control rigging issue; however, postaccident examination of the 4-bladed tail rotor assembly and the airframe revealed no preimpact mechanical anomalies. Flight control and drivetrain continuity were confirmed from the cockpit to all main and tail rotor flight control surfaces.

Conditions reported by the pilot showed that he was operating at a high density altitude with a high gross weight while the helicopter was in a low-speed, high-powered, descending maneuver. As such, the pilot’s maneuvering was in an environment and flight profile that was conducive to the onset of loss of tail rotor effectiveness (LTE). According to the manufacturer and FAA Advisory Circular (AC) 90-95, the 4-bladed tail rotor assembly configuration is susceptible to LTE while operating with a low airspeed (typically below translational speed) with high power setting, and near or above maximum gross weight. It is likely that the pilot flew a flight profile that resulted in an uncommanded rapid yaw due to LTE, which was only mitigated by a low-altitude autorotation.

Text from the NTSB. Open the NTSB docket for WPR24LA243 for the full record.