What happened
On March 6, 1970, a Handley Page HP 137 Jetstream, registration D-INAH, was operating a non-scheduled commercial flight from Munich to Samedan. The flight initially proceeded under instrument flight rules before transitioning to visual flight rules.
At approximately 14:56 local time, the crew issued a distress call reporting an engine fire. The pilot also indicated that the landing gear was malfunctioning and announced an intention to attempt an emergency landing on the snow. During the descent, witnesses observed flames emanating from the left engine and noted that the propeller was rotating very slowly.
Shortly after, the aircraft entered a longitudinal roll and crashed into a snow-covered meadow near a forest, approximately 3 km from the threshold of runway 21 at Samedan. The impact destroyed the aircraft, severed high-voltage power lines, and struck a tree. All 11 people on board, including 2 crew members and 9 passengers (6 of whom were children), were killed.
The investigation
The investigation, conducted by a joint team from Switzerland and Germany, focused on the mechanical failure of the left engine. Investigators examined the wreckage, which was recovered from the snow, and analyzed the engine components.
Technical scrutiny revealed that the first-stage turbine wheel of the left engine had burst. The resulting debris penetrated the engine casing and the horizontal fire bulkhead, severing fuel and hydraulic lines. The investigation also looked into the functionality of the fire suppression systems and the status of the fuel and hydraulic shut-off valves. Furthermore, the team examined the electrical system, specifically the main fuse for the fire extinguishing circuit, and the structural integrity of the engine mounts following the impact.
Findings
- The primary cause of the accident was the bursting of the first-stage turbine wheel in the left engine.
- This failure caused a loss of aircraft controllability due to structural damage to the engine mounts and a subsequent fuel tank explosion.
- The fire was fueled by continuous leakage of fuel and hydraulic oil from lines severed by the turbine debris.
- The fire warning system was rendered inoperable by the initial turbine failure, making the fire difficult for the crew to detect.
- A defect in the fire extinguishing system prevented the crew from suppressing the flames; specifically, the main fuse for the extinguishing circuit had failed.
- The fuel and hydraulic shut-off valves remained open, which allowed the fire to persist and feed on leaking fluids.
- The loss of hydraulic pressure caused the nose landing gear to remain in an intermediate position between up and down.