What happened
On August 26, 1972, a flight involving a tow operation departed from Bad Ragaz airfield. The aircraft, an MBB BO-209 Monsun with registration HB-UEK, was performing a glider tow with a Blanik L-13 (registration HB-1069) carrying a pilot and a passenger.
During the flight toward the Vilan mountain range, a significant altitude discrepancy developed between the two aircraft. The glider pilot observed the tow plane descending below his line of sight. Initially suspecting a downduring, the glider pilot maintained his position. However, as the tow plane continued to drop and a sudden, sharp tension was applied to the tow rope, the glider pilot released the cable. Following the release, the HB-UEK entered an uncontrolled flight attitude at a low altitude and struck a wooded slope near Pligugg at approximately 270 meters above ground level. The pilot of the tow aircraft was killed, and the aircraft was destroyed.
The investigation
Investigators examined the wreckage and the flight profiles of both aircraft. The wreckage of the HB-UK showed a heavily compressed nose and a destroyed cockpit, with the aircraft having impacted the slope vertically. The investigation also included flight test reconstructions using the same aircraft type to determine the effects of sudden cable tension.
Testing revealed that if a glider releases the cable while significantly higher than the tug, the resulting sudden tension can cause the MBB BO-209 Monsun to lose between 150 and 200 meters of altitude almost instantly. The investigation also looked into potential engine issues, noting that while the engine showed some signs of fuel system contamination, there was no conclusive evidence of a mechanical failure that would have caused the descent.
Findings
- A large altitude difference existed between the glider and the tow plane shortly before the accident.
- The sudden tension of the tow rope acted as a primary trigger for the aircraft's unstable attitude.
- The pilot of the tow aircraft had relatively limited experience in towing operations.
- The investigation could not definitively determine why the large altitude gap and the subsequent sharp cable pull occurred, though possibilities included a sudden maneuver by the glider pilot or a momentary reduction in power by the tow pilot.