What happened
On August 2, 1969, a K 7 "Rhönadler" glider, registration HB-769, was being towed from Sarnen-Kägiswil airfield by a Piper J3C, registration HB-ODH, into the Melchtal region. As the aircraft entered the valley, the flight conditions shifted from rising air to descending air and heavy turbulence.
While flying along the western valley flank, the tow formation experienced significant sink rates and rope slack. The tow pilot performed a steep turn near the village of Melchtal to head back down the valley. During this maneuver, the forward weak link of the hemp tow rope snapped. At the time of the break, the glider was flying at a very low altitude, with some witnesses estimating the height at approximately 400 meters above the valley floor.
Following the separation, the pilot of the K 7 "Rhönadler" attempted an emergency landing on a meadow on the right side of the valley. However, the glider remained attached to the trailing tow rope. The rope likely snagged on high-voltage power lines or the ground, creating significant drag. This resistance caused the glider to pitch nose-down and roll into a steep dive, eventually striking the valley slope and flipping onto its back. The impact resulted in two serious injuries and the total destruction of the aircraft.
The investigation
The investigation examined the mechanical condition of the aircraft and the tow equipment. The wreckage of the K 7 "Rhönadler" showed no evidence of pre-existing structural defects. The TOST-type release mechanism was found to be functional, though investigators noted that the required travel to release the rope increases as rope tension decreases.
The 60-meter hemp tow rope featured weak links with a breaking strength of 540 kg. The investigation confirmed that the forward weak link had failed during the flight. Additionally, the investigation looked into the flight path and the environmental conditions, including the presence of high-voltage lines near the crash site.
Findings
- The primary cause of the accident was the failure of the emergency landing attempt while the glider was still attached to the tow rope.
- The tow pilot's assessment of the weather was overly optimistic, leading to an excessively low flight altitude within the valley.
- The glider was flying at a low airspeed (approximately 70-80 km/h) when the rope broke, which may not have provided sufficient energy reserves to overcome the additional drag of the trailing rope.
- The trailing rope likely interacted with ground obstacles or power lines, creating a braking effect that induced the fatal loss of control.