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. During the takeoff roll on Runway 09, the crew selected the Takeoff/Go-Around (TOGA) switch, which triggered an uncommand to disconnect the autothrottle (A/T).
Although the pilot monitoring attempted to re-engage the system, the autothrottle remained in an inactive mode and failed to advance the thrust levers to the required N1 setting. Consequently, the aircraft departed the runway with only 84.5% N1, significantly lower than the calculated 92.8% N1. Due to this thrust deficit, the aircraft's performance was compromised, and rotation occurred only 260 metres from the end of the runway. The aircraft crossed the runway threshold at a height of approximately 10 ft. The crew eventually increased power at 450 ft above the airfield, and the flight proceeded to its destination without further incident.
The investigation
The AAIB investigation examined flight data recorder (FDR) information and maintenance logs from the Multipurpose Control and Display Unit (MCDU). The FDR confirmed the thrust discrepancy and the low altitude at the runway end. Maintenance records revealed that 11 faults had been logged for the autothrottle during the flight, five of which were uncommanded disconnections.
The investigation also looked into the crew's cockpit environment. At the time, the flight was a line training sector, with a training captain in the right seat and a new commander in the left seat. The investigators noted that the crew's recent experience had been primarily on the B737-MAX, which features different gauge positioning and different required thrust settings, potentially affecting their visual scan and recognition of the thrust error.
Findings
- The primary cause of the uncommanded autothrottle disconnection was likely insufficient voltage supplied to the autothrottle servo motor (ASM), a known issue with the older revision of the ASM part fitted to G-FDZS.
- The crew failed to identify that the thrust was not set to the required level during the 80-knot check, as the pilot flying was focused on maintaining the centerline and the pilot monitoring likely observed the reference N1 without verifying the actual N1.
- The autothrottle remained in an armed but inactive mode, meaning it did not control the thrust lever servos as the crew expected.
- The manufacturer's guidelines suggest that an uncommanded autothrottle disconnection during takeoff should be treated as a system failure, warranting a rejected takeoff (RTO).