What happened
On June 19, 2007, a Dyn'Aero MCR 01 ULC, registration HB-WAK, was performing a glider tow operation near Corgémont, Switzerland. The aircraft was towing an Alexander Schleicher ASK 21 glider, registration HB-1624. During the climb, the glider pilot noted a lower rate of climb compared to previous experiences with more powerful towplanes.
As the flight reached an altitude of approximately 813 meters, the glider pilot encountered turbulence and focused on checking the airspeed following a radio request from the towplane pilot. During this period, the glider pilot lost visual contact with the towplane. In response to the loss of visual contact, the glider pilot immediately released the tow rope. Simultaneously, the towplane pilot experienced a sudden pitch-down maneuver, with the aircraft nose dropping toward the ground. During the subsequent recovery attempt, the HB-WAK struck the treetops, resulting in grave damage to the aircraft, including punctures to the wing undersides and landing gear damage. Both aircraft landed safely at Courtelary aerodrome with no injuries to the two crew members.
The investigation
The investigation examined the flight parameters recorded by the Flarm system and testimonies from the flight club members. Investigators looked into the technical condition of both aircraft, the meteorological conditions, and the specific operational characteristics of the "Ecolight" category of aircraft being tested for towing duties at the time.
Findings
- The investigation found no technical defects in either the HB-WAK or the HB-1624.
- The glider pilot lost visual contact with the towplane because they were distracted by monitoring airspeed during a period of turbulence.
- The tow formation had exceeded the upper vertical limit of the convenience box, which is the designated safe flight envelope for the tow formation.
- A key contributing factor was that the tow formation becomes significantly more unstable and sensitive to external influences, such as turbulence, when the towplane is lighter than the glider being towed.
- At the time of the accident, the specific operational risks associated with using lightweight Ecolight aircraft for towing were not yet fully understood by the pilots involved.
Safety action
Following the accident, findings from subsequent comparison flights were integrated into the basic training and recurrent training of flight instructors by the Federal Office of Civil Aviation (OFAC).