What happened
On 4 March 2024, a Boeing 737-8K5, registration G-FDZS, was performing a commercial passenger flight from Bristol Airport to Las Palmas, Gran Canaria. During the takeoff roll on Runway 09, the aircraft's autothrottle system disengaged immediately after the crew selected the takeoff/go-around mode.
Following this disengagement, the crew manually set the thrust to 84.5% N1, which was notably lower than the required 92.8% N1. The crew did not identify this discrepancy during the takeoff roll, despite standard operating procedures requiring thrust checks at 60 and 80 knots. Consequently, the aircraft reached rotation only 260 metres from the runway end and crossed the threshold at a height of approximately 10 feet, passing over a nearby road at an altitude of less than 100 feet.
The investigation
The AAIB examined flight data recorder information and the aircraft's maintenance logs. The investigation established that the autothrottle disengaged due to a fault in the autothrottle servo motor (ASM) for the number 1 engine. This specific component was an older model known to be susceptible to uncommanded disconnections during the takeoff mode engagement.
Data from the Multipurpose Control and Display Unit (MCDU) revealed that 11 faults had been logged during the flight, five of which were uncommanded autothrottle disconnections. The investigation also reviewed the manufacturer's guidance, noting that while a service bulletin exists to retrofit aircraft with more robust, newer-model servos, G-FDZS was still equipped with the older version.
Findings
- The aircraft operated with insufficient thrust for the calculated takeoff performance.
- An uncommanded autothrottle disconnection occurred due to a fault in the engine 1 servo motor.
- The crew failed to notice the incorrect thrust setting despite established checking procedures.
- The aircraft's takeoff performance was significantly degraded, leading to a very close margin at the runway end.