What happened
On January 18, 1971, an Iljus0chin IL-18 D (registration LZ-BED) operated by Balkan Bulgarian Airlines was performing a flight from Paris-Le Bourget to Sofia. Although the original flight plan did not include a stop in Zurich, the pilot decided to divert to Zurich to pick up ten additional passengers after receiving favorable weather reports via radio.
During the instrument approach to runway 16, the aircraft encountered significantly reduced visibility. The final weather report received by the crew indicated a runway visual range (RVR) of 900 meters and vertical visibility of only 180 feet. Approximately 900 meters before the threshold of the runway, the aircraft struck the ground. The impact caused the left wing to separate, and the aircraft caught fire before sliding 600 meters into a ditch, where it overturned. While two individuals were rescued alive by a passing security patrol, the remaining 46 fatalities (including all crew members and most passengers) were recovered from the wreckage.
The investigation
The investigation examined the flight path, the aircraft's technical condition, and the weather conditions at the time of the accident. Investigators reviewed the flight data recorder and analyzed the cockpit communications, which were found to be calm and professional. The investigation also looked into the ground-based navigation aids and the accuracy of the weather information provided by the tower.
While the aircraft was found to be airworthy and the crew was properly trained, the investigation noted difficulties in obtaining certain technical data from the manufacturer in the USSR. However, the investigators were able to establish that the aircraft's engines and propellers were functioning normally at the time of impact and that no foul play was involved.
Findings
- The aircraft undershot the minimum allowable altitude required for instrument flight conditions.
- The crew initiated a go-around too late for operational reasons that could not be specifically determined.
- The approach was observed by witnesses to be lower and faster than typical approaches, with the aircraft flying at a significant bank angle.
- There was a lack of updated weather information regarding deteriorating conditions being passed from approach control to the aircraft.
- The investigation could not rule out that engine power mismanagement or turbulence from a preceding aircraft might have contributed to the descent, though neither factor alone was sufficient to explain the crash.