Piper PA18A N2660A accident near Billings, Montana

Probable cause

The pilot's inadequate compensation for the wind conditions and failure to maintain lateral and directional control of the airplane during landing.

NTSB analysis

As the airplane approached the airport, the pilot was given clearance to land on runway 28L. The pilot reported that he also heard the tower controller tell an inbound helicopter to follow him. Review of a recording of the tower communications revealed that the controller actually instructed the helicopter to pass behind the airplane and cleared the helicopter to land on taxiway H in the opposite direction of the airplane. (Taxiway H was south of and parallel to runway 28L with a taxiway centerline to runway centerline distance of about 200 feet.) About 1 1/2 minutes later, the tower controller told the airplane pilot that there was a helicopter on his left side and that it would be landing on the taxiway adjacent to the runway. The airplane pilot did not acknowledge the transmission.

The airplane pilot said that he was planning to fly the airplane low over the runway and land long to expedite his ground taxi. As the airplane pilot flew low over the approach end of the runway, he was surprised to see the helicopter flying towards him. He thought it was flying over the edge of his runway and that its rotor wash would impact his airplane. However, radar data from an antenna located on the airport indicated that the two aircraft were horizontally separated by about 300 feet when they passed each other at a point about 885 feet from the approach end of the runway. Additionally, the helicopter pilot reported that his landing on taxiway H was accomplished about 300 feet from the airplane.

The airplane pilot stated that just as he was passing the helicopter, his left wing tip was violently slammed down. The airplane veered right about 30 to 40 degrees, stalled, and impacted the ground. A witness observed the airplane's left wing strike the runway before the airplane veered right off the runway and over the grass. Airport operations personnel located a yellow paint transfer mark on the runway surface about 1,616 feet from the approach end of the runway. The location and paint color of the transfer mark were consistent with the airplane's left wing tip impacting the runway when the airplane's fuselage was over the runway's right edge. Radar data indicated that, at the time the airplane produced the mark, the helicopter was more than 1,350 feet away.

At the time, the airplane was landing with a direct crosswind of about 6 knots from the left.

The FAA Aeronautical Information Manual states that "Pilots of small aircraft should avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover." Since this helicopter's main rotor disc diameter was about 33.5 feet, the recommended distance to maintain from small airplanes in order to prevent downwash encounters was about 100 feet. Additionally, a representative from the helicopter manufacturer's safety department said that a helicopter descending during air taxiing and with a forward speed of about 58 mph, the helicopter's ground speed when it passed the airplane, would produce significantly less downwash relative to what it would produce when static hovering or hover taxiing.

Text from the NTSB. Open the NTSB docket for WPR10LA393 for the full record.