What happened
During a standard instrument arrival into Adelaide Airport, a Fokker aircraft was operating using an automated flight mode controlled by the flight management system. As the aircraft approached the KERRS waypoint, the automatic flight system transitioned into altitude hold mode. However, the aircraft continued to descend despite this mode change, though it did successfully meet the 5,000 ft minimum altitude restriction at that specific waypoint.
Following the KERRS waypoint, the flight management system reverted to descent mode. The system failed to capture the pre-selected altitude of 3,800 ft, which served as the minimum safe altitude for the segment between KERRS and the GULLY waypoint. This failure caused the aircraft to descend approximately 480 ft below the required safety limit.
The investigation
The investigation focused on why the flight management system failed to level the aircraft at the programmed altitude. Investigators consulted with both the aircraft manufacturer, Fokker, and the manufacturer of the flight management computer, Honeywell. Despite these consultations, the specific reason why the automatic flight system did not capture the selected altitude could not be determined.
Findings
- The flight crew was actively monitoring the aircraft instruments during the approach.
- Upon detecting the unauthorized descent below the selected altitude, the crew disconnected the automatic flight system and manually initiated a climb.
- The aircraft passed the GULLY waypoint and intercepted the lateral localiser track while the crew was performing corrective actions.
- During the period of altitude deviation, the aircraft was transitioning through cloud layers, allowing the crew to visually confirm terrain clearance.
- The crew successfully reconnected the autopilot and completed the instrument landing system approach.
- The flight management system failed to capture the selected altitude for reasons that remained undetermined.