Robinson Helicopter Company R44 II N2153S accident near Mammoth, California

Probable cause

Dynamic rollover of the helicopter during a precautionary landing.

NTSB analysis

The pilot departed on the final leg of a helicopter cross-country flight. About 30 minutes after departure he began to experience stomach cramps. He landed the helicopter in a level meadow on the slopes of a mountain, at an altitude close to the helicopter's maximum Hover Out of Ground Effect altitude ceiling. The pilot left the engine running, locked the flight controls, and instructed the passenger to hold the controls steady. He departed the helicopter, reentered, confirmed all systems were in order, and lifted it to a low hover. Cognizant of the helicopter's performance limitations at altitude, he departed toward descending terrain. During the initial low-altitude takeoff phase, and prior to reaching translational lift, the pilot observed an auxiliary fuel pump warning light. He elected to discontinue the departure and land. During the landing, the skid made contact with the ground and vegetation and the helicopter experienced a dynamic rollover. The pilot did not recall a change in engine tone after receiving the warning, but did recollect a loss of rotor speed indication prior to impact. During the rollover sequence, the occupants were engulfed in fuel, which then ignited causing serious injury to both.

The helicopter remnants, ground scars, and the pilot's recollection of the accident sequence indicated relatively low impact forces during the accident. Arcing signatures on an electrical wire located next to both fuel tanks could indicate a potential ignition source for the fuel; however, thermal damage to the helicopter structure, and the location of the fuel tanks above the engine compartment, precluded a definitive confirmation of the ignition source. The helicopter's manufacturer had issued a service letter recommending installation of fuel tank bladders in an effort to reduce the likelihood of a postaccident fuel leak. This service letter had not been complied with; however, compliance was not required per Federal Aviation Administration regulations.

The engine and fuel system sustained considerable thermal damage, and as such a complete examination was not possible. Examination of the engine and fuel system remnants did not result in any findings that would have indicated a loss of engine power or the observed auxiliary fuel pump warning. The helicopter's manufacturer issued a safety alert recommending restricting high-temperature and high-altitude operations due to the potential for development of fuel vapor within the fuel system. While illumination of the auxiliary fuel pump warning was an indication of the possible formation of fuel vapor, the investigation was unable to conclusively determine the reason for the auxiliary fuel pump warning.

Text from the NTSB. Open the NTSB docket for WPR10LA458 for the full record.