What happened
On October 13, 2015, a flight training mission involving an instructor and a student pilot resulted in a runway excursion at Neuchâtel aerodrome (LSGN). The aircraft, a FFA AS202/15-1/16M with registration HB-HFH, was performing a navigation flight when the crew decided to return to Neuchâtel due to changing weather conditions.
As the aircraft approached the airfield, the instructor encountered increasing turbulence caused by the "Joran," a localized downslope windstorm. During the final approach, the instructor increased the approach speed to 80 MPH to manage the wind and applied significant power to counter downdrafts near a treeline. After the initial touchdown, the aircraft bounced. Upon the second contact with the runway, the aircraft was centered laterally but misaligned with the longitudinal axis. At this moment, the left main landing gear collapsed, causing the left wingtip to strike the ground. The aircraft veered off the paved surface and came to rest on the grass. Both occupants were uninjured.
The investigation
Sust examined the aircraft's structural integrity and the environmental conditions at the time of the accident. The investigation focused on the mechanical state of the landing gear and the impact of the meteorological phenomena. Investigators analyzed the metallurgical properties of the left main landing gear's main tube, which is composed of a magnesium, zinc, and zirconium alloy.
Technical analysis of the broken gear leg revealed that a pre-existing crack was present in the main tube prior to the accident. While the exact origin of the crack could not be determined, the high number of landings on the aircraft (over 73,000) suggested that the crack likely developed due to mechanical fatigue. The investigation also reviewed the flight's operational decisions, noting that the instructor's actions regarding speed and configuration were appropriate for the wind conditions.
Findings
- The primary cause of the accident was the runway excursion resulting from the failure of the left main landing gear.
- A fatigue crack had developed in the left landing gear's main tube prior to the flight, which was nearly undetectable during pre-flight inspections.
- The landing involved a bounce and a subsequent touchdown that was misaligned with the runway centerline, which contributed to the propagation of the existing crack.
- The presence of the "Joran" windstorm created turbulent conditions and downdrafts that necessitated power adjustments during the approach.
- The aircraft's high total landing count likely contributed to the mechanical fatigue of the landing gear component.