What happened
On October 22, 2005, a Bücker 131 Jungmann, registration HB-UUO, was conducting a VFR aerobatic training flight at Grenchen Airport. The flight was part of a student pilot's advanced training to acquire an aerobatic rating. Following several successful training maneuvers, the pilot attempted to land on the grass runway 25L. To manage excess altitude, the pilot performed a prolonged forward slip.
As the aircraft approached the runway threshold, it was in a high angle of attack with low airspeed. Although both occupants increased engine power in an attempt to execute a go-around, the aircraft struck the grass runway with a high sink rate. The impact caused the landing gear to collapse, and the aircraft slid along the fuselage for several meters. Both occupants escaped the aircraft without injury, but the aircraft was heavily damaged.
The investigation
The investigation examined the technical condition of the aircraft, the flight profiles, and the experience levels of the crew. The aircraft was found to be airworthy, with its last maintenance check completed shortly before the accident. The weight and center of gravity were within prescribed limits.
Investigators noted that while the instructor was a highly experienced specialist in this aircraft type, the student pilot was relatively new to tailwheel aircraft without landing flaps. The investigation also looked into the aerodynamic effects of the forward slip maneuver, specifically how the slip can lead to unreliable airspeed indications and increased drag, making the transition back to level flight critical near the ground.
Findings
- The primary cause of the accident was the collapse of the landing gear due to a high sink rate.
- This high sink rate was driven by a large angle of attack and low airspeed at the conclusion of the forward slip.
- The recovery from the forward slip was performed too late to prevent the impact.
- The instructor's intervention in the controls occurred too late to avert the accident.