WPR14LA133 · Peter W Stevens BEARHAWK PATROL

The pilot reported that, during the landing roll, the airplane slowed to about 10 to 15 mph as it came abeam four helicopters on a taxiway. The pilot further reported that the airplane's left side encountered a sudden blast of air and that the left wing lifted; he attributed the blast of air to rotor wash from the helicopters. The airplane spun hard to the right, exited the runway into a dirt area, and then stopped facing 180 degrees in the opposite direction. The left wing tip struck the ground during the excursion and sustained substantial damage. The pilot reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation.

The four pilots in the helicopters on the taxiway were conducting engine health indicator test checks using 30 percent power with two engines operating and 60 percent power with one engine operating, which was about half the power needed to hover. Federal Aviation Administration guidance advised pilots of small aircraft operating around helicopters to avoid operating within about three times the diameter of the helicopters' main rotor blades. The distance between the airplane and the helicopters when the airplane passed them abeam was calculated to be about 400 ft, which was over six diameters away; therefore, helicopter rotor wash likely did not contribute to the pilot's loss of directional control.

Verbatim NTSB analysis and probable cause from the NTSB dataset