What happened
On June 19, 2007, an Ecolight-category 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 formation passed near the Les Carolines quarry, the glider pilot noticed a potential increase in airspeed and checked the instruments. During this period of distraction, the glider pilot lost visual contact with the towplane. Simultaneously, the towplane pilot experienced a sudden pitch-down maneuver, with the aircraft''s nose pointing almost vertically toward the ground.
In response to the loss of visual contact, the glider pilot immediately released the tow rope. During the recovery attempt, the HB-WAK struck the treetops, causing significant damage to the underside of the wings and the landing gear. Both aircraft subsequently landed safely at Courtelary aerodrome without any injuries to the two crew members.
The investigation
The investigation focused on the flight dynamics of the tow formation and the specific characteristics of the Ecolight aircraft category. Investigators examined flight data from a Flarm system and interviewed the pilots involved.
At the time of the accident, the use of Ecolight aircraft for glider towing in Switzerland was in a testing and certification phase. The investigation looked into the differences in handling between the lightweight HB-WAK and more conventional, heavier towplanes like the Robin DR 300. The investigators also reviewed the flight limitations documented in the aircraft's manual and cockpit placards regarding recommended tow speeds and weight limits.
Findings
- The primary cause of the accident was a loss of control of the towplane caused by the glider pilot exceeding the upper vertical limit of the designated "convenience box."
- The glider pilot lost visual contact with the towplane while focused on monitoring airspeed.
- The tow formation was more unstable due to the lightweight nature of the MCR 01 ULC, which made the tow string more sensitive to external influences like turbulence.
- The pilots were not fully aware of the specific operational risks associated with using a much lighter aircraft for towing a heavier glider.
- No technical failures were found in either the HB-WAK or the HB-1624.
Safety action
Following the incident, findings from subsequent comparative flight tests were integrated into the basic training and recurrent training programs for flight instructors by the Federal Office of Civil Aviation (OFAC).