What happened
On April 10, 1973, an Invicta International Airlines Vickers Vanguard 952, registration G-AXOP, crashed while attempting an instrument approach to Basel-Mulhouse Airport. The aircraft was operating charter flight IM 435 from Bristol. During the approach in heavy snowfall and low visibility, the crew attempted two instrument landing system (ILS) approaches to runway 16.
While navigating through low clouds, the aircraft's position became uncertain. Despite the crew reporting they were established on the glidepath and localizer, air traffic controllers noted the aircraft was actually south of the airport. During a missed approach and subsequent climb, the aircraft struck a wooded ridge in the Jura mountains near the hamlet of Herrenmatt, approximately 16 km south of Basel. The impact caused the aircraft to flip and catch fire. The crash resulted in 104 fatalities among the passengers and 4 fatalities among the crew, with 35 passengers and one flight attendant sustaining injuries.
The investigation
SUST investigators examined the flight path reconstruction, cockpit voice recordings, and the aircraft's navigational equipment. The investigation included specialized flight tests of the ILS equipment and the Intergrad Flight System (IFS) used on the aircraft. Investigators also reviewed the meteorological conditions, which involved heavy snow and low cloud bases.
Technical analysis focused on the radio and navigation aids, specifically the reception of the localizer and glide slope signals. The investigation also looked into the crew's performance, including their use of cross-checks and their response to conflicting navigational information. Evidence from ground witnesses, including a meteorologist who observed the aircraft at a very low altitude, helped confirm the aircraft's final trajectory.
Findings
The investigation concluded that the primary cause of the accident was the loss of orientation during the two instrument approaches. Several contributing factors were identified:
- Inadequate navigation, specifically regarding the improper initiation of the final approach in terms of both altitude and the approach track.
- Confusion between different navigational aids.
- A failure to properly perform cross-checks and comparisons between various navigational aids and instrument displays.
- Technical difficulties, including poor reception of medium-frequency radio beacons and malfunctions in the localizer and glide slope receivers, which significantly hindered the crew's ability to navigate accurately.