What happened
On 29 September 1998, a De Havilland Canada DHC-8-311, registration G-BRYS, was performing a scheduled public transport flight from Manchester to Aberdeen. During the approach to Runway 34, the crew attempted to intercept the ILS using the Flight Management System (FMS). While the aircraft successfully reached the inbound heading, a critical error occurred in the cockpit's navigation settings.
The commander had failed to set the correct inbound course on the HSI, leaving the course selector 90 degrees offset from the actual ILSB inbound QDM. When the autopilot engaged the localiser capture mode, the flight control system reacted to this massive course error by initiating a steep right bank of nearly 30 degrees. This maneuver sent the aircraft drifting far to the east of the runway centerline.
As the aircraft drifted, it moved toward the city centre, passing within 650 metres laterally and only 82 feet vertically from a radio transmitter mast. The deviation was so significant that the crew experienced Ground Proximity Warning System (GPWS) 'TERRAIN TERRAIN' alerts. Realizing the aircraft was off track, the crew initiated a go-around. However, during the climb, the aircraft's altitude fluctuated, climbing to 3,800 feet before descending back through the cleared altitude of 3,000 feet.
The investigation
The AAIB examined flight data from the Flight Data Recorder (FDR) and radar information from Aberdeen. The investigation focused on the interaction between the autopilot and the incorrectly set course selector. Investigators also analyzed the weather conditions, which included mist and a surface inversion, and considered whether wake turbulence from a preceding Boeing 757 could have caused the deviation. The investigation also reviewed the crew's use of the FMS and the cockpit's manual flying intentions.
Findings
- The primary cause of the deviation was that the course selector was set 90° removed from the inbound ILS course, causing the autopilot to command an extreme bank to correct the perceived error.
- The crew's decision to re-engage the autopilot during the descent, after initially intending to fly manually, contributed to the automated response to the incorrect heading setting.
- The commander did not activate the specific go-around mode during the missed approach, leaving the aircraft in speed mode, which led to altitude fluctuations.
- The aircraft's track took it dangerously close to a radio transmitter mast located in the Cummings Park district.
- The investigation ruled out wake vortex from a previous flight as the cause of the roll.