What happened
On June 13, 1970, a Sokol MID aircraft, registration HB-TAI, was performing a private flight from Birrfeld to Triengen. During the takeoff roll on runway 26, the pilot rotated the aircraft at an indicated speed of approximately 110 km/h. Immediately after liftoff, the aircraft began to oscillate heavily around its longitudinal axis. At an altitude of roughly 2 meters, the pilot reduced engine power, causing the aircraft to stall and drop onto its right main landing gear. The impact caused the aircraft to skid and veer approximately 110 degrees to the right. Despite the impact, the pilot, two passengers, and the crew remained uninjured, though the aircraft sustained damage estimated at 30% of its total value.
The investigation
Investigators examined the aircraft's configuration and mechanical condition following the accident. The investigation revealed that the aircraft was in a highly tail-heavy configuration, with the center of gravity positioned at the extreme rear of the allowable range. Furthermore, the airspeed indicator was found to be significantly inaccurate at lower speeds, overstating the speed by approximately 13 km/ and potentially misleading the pilot during the critical takeoff phase.
Structural analysis of the wreckage showed significant damage to the right wing, including a broken main spar and detached skinning. The investigation also noted that several glue joints within the wooden structure were in an inadequate state, likely due to the known embrittlement of the phenol-formaldehyde resin used in the construction of Sokol aircraft. While the pilot had consumed a small amount of wine several hours prior to the flight, medical analysis determined there was no alcohol impairment at the time of the accident.
Findings
- The aircraft was operating in a highly tail-heavy state, which increased its tendency to pitch up.
- The pilot failed to adjust the trim to compensate for the rearward center of gravity, leaving the trim at zero instead of a nose-heavy setting.
- The airspeed indicator provided inaccurate readings in the low-speed range, which may have led to an premature rotation.
- The pilot's decision to reduce power while in an excessive pitch attitude caused the aircraft to stall and drop from a low altitude.