What happened
On 1 December 2021, a Boeing 737-800, registration G-JZHL, was preparing for a commercial passenger flight from Kuusamo Airport, Finland. Due to snow on the ramp, the crew performed a pre-takeoff engine run-up to clear ice from the engines, accelerating the engines to 70% N1 while holding the brakes.
During this procedure, the aircraft began to slide and yaw because the co-pilot applied insufficient brake pressure. This movement startled the co-pilot, causing him to focus on steering the aircraft and inadvertently failing to press the TOGA button to set the required 89% takeoff thrust. Simultaneously, the commander was distracted by a radio transmission from the Flight Information Service Officer.
As the aircraft accelerated down the runway, the crew did not notice the thrust was incorrectly set. The aircraft became airborne with only 400 m of runway remaining. The lack of power was not identified until the aircraft reached 250 ft agl, at which point the commander manually advanced the thrust levers to the correct setting. There were no injuries to the 6 crew members on board.
The investigation
The AAIB investigation, conducted on behalf of the Finnish authorities, examined flight data recorder information which confirmed the engines were at 70% N1 during the takeoff roll. The investigation established that the brake pressure applied during the run-up was significantly lower than the 1,000-2,000 psi required to counteract the engine power.
Manufacturer performance modelling indicated that had an engine failure occurred near V1, the aircraft would have likely overrun the end of the runway. The investigation also noted that the commander's attention was diverted by an ATC-related query regarding the departure turn direction.
Findings
- The takeoff thrust was insufficient because the TOGA button was not pressed.
- The co-pilot failed to press the button because he was startled by the aircraft's movement caused by insufficient brake pressure.
- The commander failed to verify the thrust setting due to distraction from a radio call.
- Existing procedural barriers failed to detect the error because both pilots were preoccupied with other tasks.