What happened
On August 25, 2017, a pilot operating a Maule M7-235, registration HB-KDM, was taxiing at Grenchen Airport (LSZG) for a private flight to Winterthur. After completing maintenance, the pilot arrived at the airport and prepared to taxi via grass taxiway C toward runway 24. After waiting at the holding point for clearance from air traffic control, the pilot began moving forward.
As the aircraft proceeded, the pilot observed various taxiway markings and the parallel paved taxiway D to the left. Under the impression that taxiway C continued straight and remained parallel to taxiway D, the pilot maintained a straight course. This led to a sudden impact when the right main landing gear struck the taxiway sign for taxiway D. The force of the collision caused the aircraft to pitch forward, resulting in the propeller tips striking the grass. The aircraft sustained heavy damage to the propeller and the right landing gear was compressed. The pilot sustained light injuries.
The investigation
SUST examined the layout of the airfield and the visibility of the taxiway markings. The investigation established that while taxiway D continues straight, taxiway C actually curves to the right to meet runway 24 at a right angle. The sign for taxiway D was positioned in accordance with ICAO standards to maintain required separation from the runway. The investigation also noted that the grass taxiway is primarily used by local pilots and that the grass is regularly maintained to ensure visibility of the taxiway boundaries.
Findings
The investigation identified that the pilot failed to recognize the right-hand curve in taxiway C. This error was attributed to several contributing factors:
- The inherently poorer visibility of grass taxiway boundaries compared to paved surfaces.
- The restricted forward visibility characteristic of a taildragger aircraft.
- The visual presence of the parallel, paved taxiway D, which created the illusion that the grass path continued straight.
Safety action
The investigation highlighted that reviewing aerodrome information (AD INFO) is critical during flight preparation. Specifically, pilots should re-verify the intended taxi route on the aerodrome chart after any significant stops or delays, such as waiting for runway clearance, to ensure the path remains correctly identified.