What happened
On December 23, 2020, a private flight involving a Kitfox S4, registration HB-YIW, concluded with a loss of control during the landing phase at Birrfeld Airport (LSZF). The pilot had completed a two-hour local flight under Visual Flight Rules (VFR). During the approach to runway 26, the pilot did not utilize flaps, following the aircraft flight manual instructions.
As the aircraft performed a three-point landing on the paved runway, it encountered turbulent southwest winds. Due to the inherent directional instability of tailwheel-type aircraft, the strong left crosswind caused the aircraft to drift. As the plane taxied near taxiway Charlie, the wind lifted the left wing, causing the right wingtip to strike the runway surface. The aircraft subsequently exited the runway perimeter, traveling approximately 20 meters onto the grass area between the runway and a nearby fuel station before coming to a halt. The incident resulted in no injuries to the pilot, but the aircraft sustained light damage, including a torn right wingtip and abrasions on the right aileron.
The investigation
SUST examined the meteorological conditions and operational procedures surrounding the event. Investigators noted that while the pilot had reviewed weather forecasts and NOTAMs, the official METAR and TAF reports for nearby airports like Zurich and Grenchen did not indicate gusts of 20 knots, as the reporting threshold for gusts in these forecasts is 25 knots. The investigation also reviewed the aircraft's technical status, finding no mechanical failures contributing to the event. The investigation focused on the interaction between the wind conditions and the specific handling characteristics of the tailwheel configuration.
Findings
- The primary cause of the incident was a loss of control during landing caused by significant crosswind components.
- The measured wind at the time of landing was 10 knots from 200 degrees, with gusts reaching 20 knots, which approached the manufacturer's demonstrated maximum crosswind limit of approximately 15 knots.
- The directional instability inherent to tailwheel aircraft, where the center of gravity is located behind the main gear, was exacerbated by the strong left crosswind.
- Official weather forecasts did not trigger wind warnings because the gusts had not yet reached the 25-knot threshold required for inclusion in standard aeronautical reports.