What happened
On September 28, 2018, a private Yak-52, registration SP-YGI, was performing a local flight at Grenchen Airport (LSZG) when it failed to stop before the end of the runway. The aircraft, which had recently undergone maintenance, was being operated by a single pilot under Visual Flight Rules (VFR).
During the landing on runway 24, the pilot reported that the aircraft experienced a strong leftward veer immediately after touchdown. While attempting to decelerate, the pilot found the braking effectiveness insufficient to prevent the aircraft from exiting the paved surface. The aircraft traveled approximately 80 meters past the runway threshold, crossed a cantonal road, and came to a halt in an adjacent field. A vehicle on the road had stopped in time to avoid a collision. Following the excursion, the pilot attempted to taxi the aircraft back toward the airport without communicating with Air Traffic Control, which resulted in the left main tire being forced off its rim due to the soft terrain.
The investigation
SUST examined the landing sequence using pilot statements, eyewitness accounts, radar data, radio communications, and airport webcam footage. The investigation noted that the aircraft's touchdown speed was significantly higher than the recommended 120 km/h, with webcam footage showing a ground speed of approximately 145 km/h at a point 340 meters before the runway end.
Investigators also analyzed brake marks on the runway. The marks revealed that the braking action was asymmetrical, with the left wheel's braking track being significantly longer than the right. While the technical cause for this asymmetry could not be definitively determined because the pneumatic brake lines were severed during the excursion, a leak in the right-side brake line was considered a possibility.
Findings
- The primary cause of the excursion was a late touchdown at an excessive speed.
- The pilot's approach speed of 170 km/h exceeded the Pilot's Operating Handbook (POH) recommendation of 160 km/h.
- Asymmetrical braking caused the aircraft to veer left, complicating the deceleration process.
- The pilot's decision to taxi back across the road without coordination with Air Traffic Control increased the risk to road users and caused additional damage to the aircraft.