What happened
On 3 May 2006, a Boeing 737-800, registration TC-AAP, was preparing for a charter flight from Skellelyfteå, Sweden, to Antalya, Turkey. During the takeoff preparation, the aircraft taxied to the end of runway 10, performing a 180-degree turn to align with the runway centerline.
After coming to a stop, the commander manually increased engine power to approximately 40% on both engines. At this point, the Auto Throttle System (ATS) was activated to increase power to takeoff levels. As the aircraft began to accelerate, it experienced an uncontrolled leftward movement. Despite attempts to use nosewheel steering and wheel brakes, the aircraft drifted toward the left edge of the runway. The nosewheel eventually left the paved surface, plowing a furrow in the ground before the aircraft came to a halt. There were no injuries among the 194 people on board, and the aircraft sustained no visible damage.
The investigation
The Swedish Accident Investigation Authority (SHK) examined flight data recorder (FDR) and cockpit voice recorder (CVR) data, as well as runway markings and aircraft components. The investigation focused on the engine power settings, the timing of the ATS activation, and the crew's response to the directional deviation. Investigators also reviewed the aircraft's technical systems, including the nosewheel steering and braking mechanisms, and found no mechanical failures.
Findings
- The primary cause of the incident was the activation of the ATS while the aircraft was not aligned with the runway and while engine power was asymmetrical.
- At the moment of ATS engagement, the right engine was producing approximately 10% more power than the left engine. Because engine acceleration is faster at higher power settings, this thrust imbalance widened significantly during the takeoff roll, reaching a difference of over 54% before the crew could intervene.
- The pilots failed to correct the engine power imbalance in a timely manner.
- The application of the wheel brakes was performed too late and with insufficient pressure to prevent the excursion.
- The pilots were unable to counteract the leftward yaw using only the nosewheel steering.
- A buzzing or rumbling noise heard by the crew was attributed to the nosewheel skidding on the runway surface.
- Potential issues with cockpit resource management and possible fatigue were noted as contributing factors.