What happened
On August 30, 2019, a formation of 13 historic De Havilland aircraft arrived at St. Stephan airfield in Switzerland for a 25th-anniversary celebration. The group, which included the DH60G III Moth Major, registration G-ACGZ, had flown from Thun as part of a coordinated arrival. To facilitate the landing of these vintage aircraft, which lack wheel brakes and primarily utilize taildraggers, the pilots planned to land on the paved runway 14 using a technique where the tail remains airborne as long as possible before the tailwheel contacts the ground.
During the landing sequence, the formation encountered unexpected wind conditions. While the pilots had prepared for light, variable winds, a significant tailwind component developed. As the G-ACGZ touched down, the pilot experienced a loss of directional control. Approximately 300 meters past the runway threshold, the aircraft began a rapid rotation around its vertical axis. The lateral forces generated during this uncontrolled turn caused the rear main landing gear struts to tear away from the fuselage, resulting in the gear collapsing and the left wing scraping the ground. The aircraft eventually came to a stop in the grass, approximately 30 meters from another aircraft that had also experienced a similar excursion.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft's flight data, the local meteorological conditions, and the mechanical state of the aircraft. Investigators reviewed navigation logs from the G-ACGZ, which showed that the ground speed had reduced from 70 knots to 65 knots just prior to touchdown. The investigation also looked into the wind patterns at the time, noting that valley and mountain winds can create sudden, opposing wind directions at different ends of the runway.
Findings
- The primary cause of the accident was the presence of a significant tailwind component during the landing roll.
- The aircraft's taildragger configuration, specifically the use of a metal tailspring rather than a steerable tailwheel, provided insufficient directional stability on the hard runway when subjected to crosswind and tailwind forces.
- As the aircraft rotated, the dynamic forces exceeded the structural limits of the landing gear attachments, causing the rear struts to fail.
- There were no pre-existing technical defects found in the aircraft's systems or airframe prior to the event.
- The incident was part of a series of similar excursions by other aircraft in the formation, including G-ANFM and G-ANKZ, all of which struggled with directional control due to the same wind conditions.