This August 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Piper Super Cub Injuries: 2 Minor. Location: Billings, Mont. Aircraft damage: Substantial.
What reportedly happened: The pilot, who had entered the pattern and was given clearance to land on runway 28L, reported that he heard the controller tell an inbound helicopter to follow him. However, a review of a recording of the tower communications revealed that the controller instructed the helicopter to pass behind the airplane, and the helicopter was cleared to land on taxiway H in the opposite direction of the airplane. The controller subsequently told the Super Cub pilot that there was a helicopter on his left side and that it would be landing on the taxiway adjacent to the runway. He did not acknowledge the transmission.
He 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. 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°, stalled, and hit the ground. A yellow paint transfer mark was found 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 hitting 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.
The weather at the time of the accident included a direct crosswind from the left of about six knots.
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.
Probable cause: The pilot’s inadequate compensation for the wind conditions and failure to maintain lateral and directional control of the airplane during landing.
For more information: NTSB.gov. NTSB Identification: WPR10LA393