What happened
On 11 January 2018, a DHC-8-402 Dash 8, registration G-ECOE, was performing a commercial passenger flight from Glasgow to Belfast City Airport. During the turnaround, a revised load sheet caused a slight delay, prompting the crew to attempt to save time by configuring flight director modes before pushback was complete. The commander set the autopilot modes to go-around, heading, and altitude select, but did so before entering the cleared altitude of 3,000 ft.
During the subsequent takeoff from Belfast City Airport, the aircraft climbed normally until reaching the acceleration altitude. At approximately 1,350 ft, the autopilot was engaged. Immediately following engagement, the aircraft began a progressive nose-down pitch. The descent accelerated, reaching a maximum rate of 4,300 ft/min, which triggered both "don't sink" and "pull up" warnings from the EGPWS. The commander promptly disconnected the autopilot and recovered the aircraft, which reached a minimum altitude of 928 ft before returning to a stable climb.
The investigation
The AAIB investigation examined flight data from the CVR and FDR, as well as the crew's actions and the aircraft's automation logic. It was established that because the altitude was not set before the modes were selected, the system's logic caused the "altitude select" mode to transition to "altitude hold" mode, with a target altitude of 0 ft amsl (the airfield elevation).
The investigation also found that the crew was managing a high workload due to a delayed engine start during the runway backtrack and the pressure of an approaching aircraft. While the commander noticed a discrepancy in the flight mode annunciator during the line-up checks, the crew failed to notice that the "altitude hold" mode was active instead of the intended "altitude select" mode.
Findings
- The crew selected autopilot flight director modes prior to entering the cleared departure altitude, which caused the system to default to a target altitude of 0 ft.
- The incorrect autopilot altitude mode was active when the autopilot was engaged, causing the aircraft to descend toward a target altitude of 0 ft.
- High flight deck activity and a sense of urgency during the takeoff roll prevented the crew from identifying the incorrect mode settings.
- The EGPWS provided timely warnings that allowed the pilot to intervene and prevent a collision with the ground.