Introduction
On 4 March 1994, a Piper PA-18-180M (HB-PAR) and a Piper PA-18-150 (HB-PKI) were involved in a ground collision at the Croix-de-Coeur mountain landing strip in Valais, Switzerland. The accident occurred during a mountain flight instructor course organised by the Federal Office of Civil Aviation (BAZL). No occupants were injured, but both aircraft sustained substantial damage.
Background
The course took place from 23 February to 5 March 1994 in Sion. On 4 March, four aircraft—three PA-18-180 and one PA-18-150—departed Sion around 1230 local time for landing practice in the Mont-Blanc massif. They landed at Megève altiport at about 1320 and subsequently performed landings on the Leschaux, Talèfre, and Argentière glaciers, where deep snow conditions were encountered.
Events Leading to the Accident
During the return flight, the pilot of HB-PAR found he could not retract the hydraulically operated skis. After radio consultation with the course leader, it was decided that HB-PAR and HB-PKI would make an intermediate landing at the Croix-de-Coeur mountain strip to attempt a repair. HB-PKI landed first, touching down on the right side of the prepared landing area. The pilot taxied to the upper right corner, about 5 to 10 metres from the lateral markers, and shut down the engine. The surface had been prepared by ski piste machines; according to the HB-PKI copilot, the snow was slightly softened by the sun, with ski tracks 1 to 2 cm deep, and the surface was ideal for gliding.
Accident Sequence
The pilot of HB-PAR overflew the strip, noted its prepared condition and the parked aircraft, and decided to land on the left side to eventually park nearby. Influenced by the earlier deep-snow landings, he approached slightly faster and longer than normal. He touched down approximately 180 metres from the beginning of the landing area, which had a total length of about 270 to 280 metres. The touchdown point was near the rounded transition between a steep initial slope and the final flat section. After about 20 metres of straight ground slide, the aircraft deviated 20° to the right and began moving in a gentle right turn towards the stationary HB-PKI. The pilot applied left rudder, but could not correct the trajectory sufficiently. Seeing the collision as inevitable, he shut off the engine using the mixture control and magnetos. The impact was violent: the engine of HB-PAR struck the left wing tip of HB-PKI, causing HB-PKI to pivot and its tailplane to penetrate the fuselage of HB-PAR.
Investigation
A video recording from one of the other aircraft captured the entire sequence. Analysis of the video and other evidence established that the pilot was properly licensed and experienced, the aircraft were airworthy and within weight and balance limits, and the weather was good with calm wind. The landing area was well prepared. The video showed that the rudder remained neutral for about 20 metres after touchdown, then a right rudder input caused the aircraft to turn right. No corrective left rudder was visible until just before the collision.
Probable Cause
The accident was attributed to three factors: the pilot's inappropriate application of right rudder after touchdown in snow; a landing that was too long, leaving insufficient distance to stop; and a misjudgment of the runway surface condition, as the pilot expected deep snow similar to previous landings.