Accident Overview
On August 10, 2010, at approximately 0818 mountain daylight time, a Piper PA-18A, registration N2660A, operated by 172 LLC of Park City, Montana, sustained substantial damage during an impact with terrain following a loss of aircraft control while landing at Billings Logan International Airport (BIL), Billings, Montana. The private pilot and a passenger received minor injuries. The flight was conducted under 14 Code of Federal Regulations Part 91 as a personal cross-country trip that had departed from Laurel, Montana, about 15 minutes prior. Visual meteorological conditions prevailed, and no flight plan had been filed.
Flight Details
The pilot reported that the BIL tower cleared him to land on runway 28L. He noted that the tower advised an inbound helicopter to “follow the cub,” and the helicopter pilot acknowledged with “traffic in sight.” The pilot intended to fly low down the runway to exit at the runway 25 intersection. As he was touching down, he saw the helicopter flying toward him on the left edge of the runway, not over the parallel taxiway. Just as he passed the helicopter, his left wing tip was “violently slammed down.” He had flaps fully extended, which he believed magnified the effect of rotor wash. He likely added power to correct, causing the airplane to “break right” and encounter more turbulence, resulting in a loss of control to the right. The airplane then struck the ground at a steep angle.
The passenger, a student pilot on intercom, stated that as the airplane approached runway 28L, the pilot lined up left of the centerline. He first saw the helicopter as they passed over the runway end, thought it was over the left wing. The left wing dipped hard but did not touch the runway, then the airplane “jumped” to about 15 feet above ground level (agl) as if lifted by rotor wash. The left wing rose, the airplane veered right, then dropped steeply with left wing and nose impacting the ground simultaneously.
Witness Accounts
A witness at the approach end of runway 28L observed the accident airplane flare over the runway end and fly at 10–15 feet agl without touching down. He saw a helicopter flying east over taxiway Hotel, parallel to and south of runway 28L. At about the midpoint of the runway, the helicopter leveled at 10–15 feet agl. Shortly after the aircraft passed each other, the airplane made an abrupt hard left roll, its left wing tip striking the runway. The airplane appeared to recover briefly, rocking, then stalled, rolled left, and struck the ground from about 20 feet agl. The left wing and propeller spinner hit simultaneously; the airplane cartwheeled to the right wing before the tail settled, coming to rest upright about 180 degrees from its original heading.
The helicopter pilot stated that the tower cleared him to land on taxiway Hotel and hover taxi to parking. He was established eastbound over taxiway Hotel at about 100 feet agl when he saw an airplane westbound on runway 28L. He continued descent to taxiway Hotel and hover taxied to parking, stating that at no point did the helicopter hover over or around the airplane, and its landing to a hover was about 300 feet from the airplane. A mechanic passenger in the helicopter confirmed they executed a right turn over the west corner of runway 10R and descended over taxiway Hotel, never overflying any aircraft.
Radar and Physical Evidence
Review of BIL radio communications revealed that the tower instructed the helicopter to “pass behind [the airplane], you are cleared to land Hotel taxiway, taxi to parking.” When the airplane turned onto final for runway 28L, the tower advised the pilot that “off your left you'll see a helicopter, he'll be landing [on] Hotel taxiway.” The airplane pilot did not acknowledge.
BIL’s Airport Surveillance Radar (ASR) recorded aircraft positions every 4.8 seconds. Data showed the airplane landing west on runway 28L and the helicopter landing east on taxiway Hotel. The tracks were parallel with lateral separation of about 300 feet. The aircraft passed one another about 885 feet from the approach end of runway 28R. At that moment, the helicopter’s groundspeed was about 58 mph, the airplane’s about 50 mph. The last radar hit from the airplane showed the helicopter more than 1,350 feet away.
Photographs of the runway surface revealed a yellow paint transfer matching the accident airplane’s color at coordinates 45:48:25.33 N, 108:33:17.42 W, about 1,616 feet from the runway 28L threshold and 18 feet from the right edge. Radar data placed the transfer 731 feet beyond the passing point and 170 feet beyond the last radar hit. The airplane’s wingspan was 35 feet 2 inches, so the left wing paint transfer implies the fuselage was over the right runway edge. The main wreckage rested at 45:48:27.21 N, 108:33:19.17 W, about 1,808 feet from the threshold and 111 feet north of the runway edge. The landing gear collapsed, the fuselage twisted, the engine displaced aft damaging the firewall and instrument panel, and the bottom of the rudder and right elevator were bent and wrinkled.
The helicopter involved was a Eurocopter Deutschland GMBH EC135P1 with a main rotor diameter of 33.46 feet. The Federal Aviation Administration's 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,” which would be 101 feet for this helicopter. A representative from the helicopter manufacturer’s safety department noted that a helicopter descending during air taxi with a forward speed of about 58 mph produces significantly less downwash than when static hovering or hover taxiing.
Weather and Runway Information
At 0826, reported weather at BIL was: wind 190 degrees at 6 knots, visibility 10 statute miles, clear skies, temperature 61°F, dew point 55°F, altimeter 29.95 inches of mercury. This indicated a direct left crosswind for landing on runway 28L. Runway 10R/28L is 3,801 feet long and 75 feet wide, paralleled on its south side by taxiway H (44 feet wide). Aerial photographs showed the distance between runway centerline and taxiway centerline was 200 feet.