What happened
On 20 April 2024, a Dornier D328-100, registration D-CSUE, was performing a charter flight from Stuttgart to Zurich under instrument flight rules. While executing an RNP approach to runway 34, the aircraft was cleared to descend to an intermediate altitude of 5000 ft. As the aircraft passed through this altitude with the autopilot engaged, the rate of descent suddenly and sharply increased.
The rapid descent triggered both the ground-based Minimum Safe Altitude Warning (MSAW) and the aircraft's onboard Enhanced Ground Proximity Warning System (EGPWS). The flight crew responded by disconnecting the autopilot and initiating a go-around. During the event, the aircraft's radio altimeter recorded a minimum height of only 300 ft above the terrain near the Pfannenstiel area.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined several data sources, including the flight data recorder, cockpit voice recorder, radar recordings, and radio communications. The investigation also included simulator tests to replicate the flight conditions. Investigators focused on the technical performance of the aircraft's vertical navigation systems and the crew's response to the sudden change in flight path.
Findings
- The primary cause of the sudden descent was a software anomaly within the Primus 2000 avionics system (specifically the Honeywell Legacy FMZ 2000, version NZ4.8). The internal logic of the Flight Control System was capable of commanding high descent rates under specific conditions.
- The flight crew's intervention to correct the descent was delayed.
- The aircraft was in a clean configuration with landing gear and flaps retracted when the descent rate increased.
- The aircraft passed through the required intermediate altitude of 5000 ft and subsequently dropped to approximately 4600 ft shortly after passing waypoint ZH492.
Safety action
The STSB issued two safety recommendations. The first directs EASA to ensure that the software anomaly is resolved in all D328 aircraft, suggesting the installation of updated Honeywell avionics hardware and software. The second recommendation directs the FAA to ensure that Honeywell updates the Pilot's Manual for the Primus 2000 system to address the high vertical rate issue.