Robinson R44 II N440SA accident near Cheyenne, Wyoming

Probable cause

The pilot's improper preflight performance planning, which resulted in a hard landing due to low rotor rpm while operating near maximum gross weight in high density altitude conditions.

NTSB analysis

The commercial helicopter pilot and two passengers were conducting a pipeline inspection flight. After picking up the passengers and prior to conducting the inspection, the pilot landed the helicopter at an airport to obtain fuel. During approach for landing, the passengers heard the low rotor rpm warning horn activate, but the helicopter landed normally. The pilot fueled the helicopter, and during the subsequent departure, the low rotor rpm warning horn activated. The pilot was able to "reposition" the helicopter and depart without incident. After completing a portion of the flight, the helicopter returned to a pipeline terminal near the airport for landing. The pilot stated that, during the off-airport landing, the main rotor rpm began to decay, so he lowered the collective and tried to fly "into clean air;" however, the helicopter was too low, and he realized it was going to hit the ground. The pilot increased collective and leveled the helicopter to cushion the impact. The helicopter impacted the ground from an altitude of about 15 ft and slid about 10 ft before coming to rest.

The helicopter was operating about 50 lbs under its maximum gross weight, and the density altitude around the time of the accident was about 8,000 ft. Given these conditions, the helicopter's out-of-ground-effect hover ceiling was about 4,400 ft; its in-ground-effect hover ceiling was about 7,700 ft. Therefore, the helicopter would not have been able to hover out of ground effect, but could hover in ground effect. Operating under such conditions could lead to a settling-with-power or low rotor rpm situation if the helicopter became too slow while out of ground effect. It could not be determined whether the pilot conducted preflight performance planning; however, the repeated activation of the low rotor rpm warning suggests that he did not adequately adjust the helicopter's takeoff and landing profiles to account for the flight conditions present at the time of the accident.

Text from the NTSB. Open the NTSB docket for CEN15LA387 for the full record.