What happened
On February 21, 1970, a Swissair Convair 3/A "Coronado", registration HB-ICD, was operating flight SR 330 from Zurich to Tel Aviv. Shortly after departing Zurich, the crew reported a cabin pressure disturbance and initiated an emergency return to the airport. During the descent, the crew reported a suspected explosion in the rear cargo compartment, followed by reports of fire and smoke filling the cabin.
As the aircraft attempted to navigate back to Zurich, the crew reported failures in the onboard navigation systems and the electrical system. The intense fire, fueled by the burning cargo and aircraft materials, produced dense, toxic smoke that severely obscured visibility for the pilots. The aircraft entered an uncontrolled flight state, deviating from its assigned course. At approximately 12:34 GMT, the aircraft impacted a forest near Würenlingen at a speed of roughly 422 knots. All 47 occupants, including 9 crew members and 38 passengers, were killed.
The investigation
The investigation established that the primary cause of the accident was an explosive device located in the rear cargo hold. Forensic analysis of the wreckage revealed that the explosion occurred in a package that had been sent from Munich via Zurich. Investigators found evidence of extreme temperatures, exceeding 530°C, in the area of the explosion.
Technical examinations of the wreckage showed that the explosion triggered an intense fire that spread from the cargo hold into the passenger cabin. The investigation also noted that the smoke produced by the burning materials was so thick that it likely prevented the crew from reading instruments or maintaining visual flight references. Furthermore, the investigation identified a modified altimeter near the center of the explosion, which may have served as the trigger mechanism. Evidence suggested a possible link to a similar explosion involving an Austrian Airlines aircraft on the same day.
Findings
- The crash was caused by an intense fire triggered by a bomb attack in the rear cargo compartment.
- The explosion caused the failure of navigation and electrical systems, leading to the loss of autopilot and flight control difficulties.
- Dense, toxic smoke filled the cabin and cockpit, significantly impairing the crew's ability to monitor instruments and navigate.
- The fire was too intense to be suppressed by the onboard firefighting equipment.