Bell 47G 4A N1396X accident near Gilbert, Iowa

Probable cause

The total loss of engine power due to fuel starvation because the available fuel shifted within the tanks due to an abrupt nose-low maneuver to avoid an obstacle, which resulted in a hard forced landing.

NTSB analysis

The commercial pilot was conducting an aerial application flight. The pilot reported that, while about 150 ft above ground level, he rapidly pitched the nose up to avoid an obstacle and then pitched it nose down to a steep descent angle. When the helicopter entered the nose-low attitude, the engine experienced a total loss of power. The pilot subsequently made a forced landing to a corn field. The pilot reported that he thought the loss of engine power might have been due to the nose-low attitude, which allowed the fuel to shift in the tanks and starve the engine of fuel. He also reported that he never flew the helicopter with less than one quarter of the fuel tanks full (about 15.25 gallons). The helicopter's type certificate holder also noted that, if the helicopter was low on fuel and then put into an abrupt nose-low attitude, the engine could be starved of fuel.

During the postaccident examination, 11 to 12 gallons of fuel were drained from the tanks. An engine test run was conducted with no abnormalities noted. Based on the evidence, it is likely that the engine was starved of available fuel due to the rapid and steep descent, which shifted the fuel in the tanks.

Text from the NTSB. Open the NTSB docket for CEN15LA322 for the full record.