What happened
On November 14, 1990, Alitalia flight AZ 404, an DC-9-32 registered I-ATJA, was performing an instrument approach to Zurich Airport. After departing from Milan-Linate, the aircraft was cleared by Zurich Approach to descend to 4,000 feet to intercept the ILS runway 14. While intercepting the localizer at approximately 14 nautical miles from the touchdown point, the aircraft began descending at a constant rate. However, the crew failed to maintain the prescribed altitude of 4,000 feet until reaching the final approach fix at 8 nautical miles. Instead, the aircraft descended significantly below the required altitude, eventually striking terrain at Stadlerberg near Weiach.
The investigation
The investigation focused on the aircraft's navigation systems, specifically the VHF NAV No. 1 receiver and the Ground Proximity Warning System (GPWS). Investigators examined the functionality of the King KNR 6030 glide path receiver and the NAV switching unit (NSU). The analysis included a review of the cockpit voice recorder (CVR) and digital flight data recorder (DFDR) to reconstruct the flight path and cockpit interactions. The inquiry also scrutinized the air traffic controller's monitoring of the aircraft's altitude and the crew's adherence to standard operating procedures during the approach and a subsequent aborted landing attempt.
Findings
Several critical factors contributed to the accident. A primary cause was a false indication from the onboard VHF NAV No. 1 system, which provided inaccurate guidance. This was compounded by a likely erroneous altimeter reading by the pilot in command. Furthermore, the GPWS failed to provide a cockpit alert during the descent. The investigation also identified significant human factors, including the crew's failure to recognize the possibility of a navigation error without a flag alarm, inadequate error analysis by the pilots, and a lack of effective cooperation between the two pilots. The pilot in command also interrupted a go-around procedure that had been initiated by the first officer. Additionally, the air traffic controller failed to intervene when the aircraft exceeded the prescribed altitude before the final approach fix.
Safety action
The investigation led to several recommendations, including the discontinuation of use for navigation receivers whose output signals are not monitored and the removal of certain types of barometric altimeters with drum pointers. Recommendations were also made to ensure GPWS functionality remains operational despite navigation receiver failures and to improve the installation of obstacle lighting on Stadlerberg.