WPR14LA133

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.

On March 8, 2014, about 1330 mountain daylight time, a Peter W Stevens Bearhawk Patrol, N123WW, had a runway excursion during the landing roll at South Valley Regional Airport, Salt Lake City, Utah. The owner/pilot was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot was not injured; the airplane sustained substantial damage to the left wing. The cross-country personal flight departed Bountiful, Utah, at 1315. Visual meteorological conditions (VMC) prevailed, and no flight plan had been filed. The pilot reported that he landed on runway 34. He observed five Blackhawk helicopters hovering in formation on taxiway Bravo. The airplane had slowed to 10-15 miles per hour as it came abeam the helicopters. The pilot said that the left side of the airplane encountered a sudden blast of air, and the left wing lifted. The airplane spun hard to the right, exited the runway into a dirt area, and stopped facing 180 degrees in the opposite direction. The left wing tip struck the ground during the excursion, and sustained substantial damage. The pilot attributed the wind gust to the rotorwash from the helicopters. A Federal Aviation Administration (FAA) inspector determined that there were four helicopters on taxiway Bravo that were doing engine Health Indicator Test (HIT) checks. They were using 30 percent power with two engines operating, and 60 percent with one engine operating, which was about 1/2 the power needed to hover. The inspector used scaled airport construction diagrams to calculate that the distance from the helicopters to the airplane's location when it passed abeam was about 400 feet. The FAA Aeronautical Information Manual Section 7-3-7 and Advisory Circular AC 90-23G paragraph 10 stated that if a helicopter was in a stationary hover near the surface, the main rotors generated downwash producing high velocity outwash vortices to a distance of approximately three times the diameter of the rotor. They advised pilots of small aircraft to avoid operating within that distance. The diameter of the Blackhawk's main rotor blades was 53 feet 8 inches; three diameters computed to 176 feet. Wind reported at the nearest recording station was 360 degrees at 5 knots.