What happened
On 11 September 2021, a Boeing 737-800, registration G-FDZF, was performing a commercial passenger flight from Palma de Majorca to Aberdeen when it was instructed to break off its approach. The maneuver was necessitated by the need to prioritize a search and rescue helicopter. During the manually flown go-around, the aircraft initially climbed but subsequently began an unintended descent. The aircraft reached a minimum altitude of 1,780 ft amsl (1,565 ft agl) with a peak descent rate of 3,100 fpm. During this 57-second descent, the airspeed accelerated to 286 kt, significantly exceeding the crew's selected speed of 200 kt. The descent was only halted after the tower controller alerted the crew, prompting them to re-establish the climb.
The investigation
The AAIB investigation utilized flight data monitoring (FDM) and radar recordings to reconstruct the event. Investigators examined the behavior of the Autopilot and Flight Director System (AFDS) and the autothrollle. The analysis revealed that because the go-around was initiated above 2,000 ft radio altitude, a single press of the TO/GA switches caused the autothrottle to advance thrust toward the full go-around N1 limit, a behavior unexpected by the crew.
Further examination of the FDM data showed that as the aircraft approached the target altitude, the reduction in thrust and the retraction of flaps (from 15 to 5, and then to 1) caused pitch changes that were not adequately managed through manual trimming. The investigation also looked into the possibility of a somatogravic illusion, but concluded that the primary issue was the aircraft being out of trim. Additionally, the investigation found that the ATC radar descent rate monitor failed to alert the controller because the radar data processor's lack of granularity prevented the 2,500 fpm threshold from being triggered.
Findings
- The crew experienced a high workload due to simultaneous ATC heading instructions and the dynamic nature of the manual go-around.
- The unexpected large increase in thrust at an altitude above 2,000 ft radio altitude caused a significant pitch-up, which the crew countered with nose-down inputs, leaving the aircraft out of trim.
- The retraction of flaps during the descent further altered the aircraft's pitch attitude.
- Both pilots had experienced significant periods of reduced flying due to the COVID-19 pandemic, which may have eroded their capacity to manage the complex situation.
- The aircraft's Terrain Awareness and Warning System did not activate during the descent.