What happened
On 23 September 2007, a Boeing 737-300, registration G-THOF, was performing a routine passenger flight from Faro, Portugal, to Bournemouth Airport. During the instrument landing approach to Runway 26, the aircraft's autothrottle disengaged without any command from the crew or any identified system fault. Because the thrust levers had remained at idle, the aircraft began to decelerate rapidly.
The crew did not notice the disengagement, and the airspeed dropped to 20 kt below the required approach speed. When the commander initiated a go-around, the application of full thrust combined with the existing stabilizer trim position caused the aircraft to pitch up aggressively. The aircraft reached a maximum pitch of 44 degrees and the airspeed fell to 82 kt, resulting in a stall. The crew eventually regained control and successfully completed a subsequent landing without further incident.
The investigation
The AAIB investigation examined flight data from the Quick Access Recorder and reviewed the crew's training and the operator's safety management systems. The investigation found that the flight crew were properly licensed, medically fit, and trained according to regulations. No mechanical defects were found on G-THOF that would have contributed to the event, and the aircraft's mass and center of gravity were within limits.
However, the investigation highlighted issues within the operator's safety reporting process. The initial Air Safety Report filed by the commander did not accurately reflect the severity of the event, and the incident was not identified as a mandatory occurrence until much later. A delay in reviewing flight data meant that the aircraft and crew continued to operate for several days before the seriousness of the autothrottle disengagement was fully understood.
Findings
- The primary cause of the stall was the aircraft decelerating below commanded speeds while the engines remained at idle thrust, which led to a pitch-up moment that overwhelmed the elevator authority.
- The crew failed to recognize that the autothrottle had disengaged.
- The autothrollle warning system, while functioning as designed, failed to alert the crew to the disengagement.
- The crew did not follow the appropriate recovery procedures for an approach to stall.
- The aircraft's airspeed decay was masked by the nature of the constant deceleration approach.
- The wording of the go-around drill in the Quick Reference Handbook did not instruct pilots to use forward trim to manage pitch.
Safety action
Following the investigation, the AAIB issued three safety recommendations:
- A study into the effectiveness of the Boeing 737-300/400/500 autothrottle warning system.
- A review by the European Aviation Safety Agency regarding certification standards to ensure crew alerts for autoflight disengagements are sufficient.
- A clarification of the Quick Reference Handbook wording to ensure pilots are aware that forward trim may be necessary during stall recovery maneuvers.