Investigation Summary
The accident was rooted in a design flaw of the King KNR-6030 NAV receiver. A short circuit in the unit's output stage prevented the glide slope signal from reaching the cockpit instruments. Because the failure occurred after the internal monitoring point, the system failed to display a warning flag. On the DC-9's analog instruments, a lack of signal caused the needle to center, falsely indicating the plane was 'on glide slope.' This malfunction also suppressed the Ground Proximity Warning System (GPWS), which relied on the same faulty NAV data. During the approach, the first officer correctly identified the discrepancy and initiated a go-around. However, the captain, believing his own centered needle and misinterpreting his drum-pointer altimeter, overrode the maneuver. At the moment the captain vetoed the go-around and leveled the aircraft, they were approximately 1,400 feet below the correct glide path. By the time the aircraft struck trees on the Stadlerberg mountainside at 5.2 nautical miles from the runway, it was roughly 1,000 feet below the nominal glide path. The impact generated an asymmetric lift force that caused the plane to roll to the right, eventually slamming into the mountain in a nearly inverted position.
Final Conclusions
The Swiss Federal Aircraft Accidents Inquiry Board concluded the primary causes were the failure of the No. 1 NAV unit without a warning flag and the crew's inadequate failure analysis. Contributing factors included the captain's authoritative rejection of the first officer's go-around, the lack of obstruction lighting on Stadlerberg Mountain, and the known difficulty of reading drum-pointer altimeters. The investigation also noted that the approach controller failed to notice the aircraft descending below its cleared altitude of 4,000 feet before the final approach point. Crucially, Zurich Airport was not equipped with a Minimum Safe Altitude Warning (MSAW) system at the time, which would have alerted the controller to the aircraft's dangerous proximity to terrain.
Video Analysis
Controlled Flight into Terrain: Alitalia Flight 404 — An analysis of the 1990 crash near Zurich caused by a faulty ILS receiver and a breakdown in crew resource management. The video examines how the captain overruled a go-around despite conflicting instrument readings.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
King KNR-6030 NAV Receiver
The navigation unit that failed to provide a warning flag during a short circuit, leading to a false 'on-course' indication.
Ground Proximity Warning System (GPWS)
The safety system failed to alert the crew because it received the same false 'on glide slope' data as the captain's instruments.
Drum-pointer Altimeter
An analog flight instrument that combined a needle and a rotating drum, which investigators found contributed to altitude misreadings.
Interesting Facts
- 01The NAV receiver failure was 'silent' because the short circuit occurred after the point where the system monitored for errors.
- 02The GPWS did not sound because it was designed to inhibit 'sink rate' warnings if the aircraft appeared to be on the glide slope.
- 03The aircraft was 1,400 feet below the glide path at the time of impact.
- 04The captain was using a drum-pointer altimeter, which was notoriously difficult to read and prone to 1,000-foot errors.
- 05The first officer's NAV receiver (No. 2) was functioning correctly and showed the aircraft was too low.
Safety Actions & Advisories
NAV Receiver Redesign
Authorities mandated that navigation receivers must display a failure flag if a signal is lost or interrupted, regardless of where the failure occurs in the circuitry.
Go-around Policy
Airlines reinforced the rule that if any pilot calls for a go-around, the maneuver must be executed immediately without debate.
GPWS Logic Update
Recommendations were made to ensure GPWS systems do not rely solely on potentially faulty ILS data to suppress terrain warnings.