What happened
On December 21, 1976, a Cessna 421 B, registration HB-LGM, was conducting an IFR flight from Ajaccio to Zurich when it struck the ground 1,700 meters short of the runway threshold at Zurich Airport. The aircraft was on an ILS approach to runway 14 during periods of extremely poor visibility, characterized by heavy fog and a runway visual range of only 350 to 400 meters.
Upon impact, the aircraft's landing gear was lost, and the propellers struck the ground, causing the aircraft to skid and eventually come to rest on its side. While the two occupants sustained minor injuries, the aircraft was destroyed.
The investigation
The investigation established that the pilot had been informed of weather conditions at Zurich that were below the required minimums for a Category I ILS approach. Despite this, the pilot proceeded with the approach, likely due to an assumption that actual visibility would be better than reported.
During the approach, the pilot attempted to manage the descent manually after the autopilot disconnected. The investigation found that the pilot was maintaining a speed of 180 knots, significantly higher than the recommended 105–115 knots for intercepting the localizer. Furthermore, the pilot's decision to extend flaps to 30 degrees during the final stages of the approach caused a significant deviation from the glidepath. The investigation also noted that an open alternate static air valve caused the altimeter to read 60 feet higher than the actual altitude.
Findings
- The primary cause of the accident was a series of pilot errors, specifically the decision to continue an ILS approach in very poor weather conditions without a stabilized aircraft.
- The aircraft was not stabilized at the Outer Marker, as the pilot was struggling with excessive speed and vertical deviations.
- The pilot failed to initiate a timely and decisive go-around after the aircraft dropped significantly below the glidepath and the decision height.
- The pilot's limited IFR experience and lack of familiarity with the specific automated flight systems of the Cessna 421 B contributed to the loss of control.
- Distractions occurred when the pilot repeatedly looked outside for approach lights that were not yet visible, leading to a failure to monitor the instruments effectively.