What happened
During a personnel transport operation at Linköping/Saab airport, a Saab AB aircraft experienced a runway excursion while departing from runway 11. After lining up on the runway, the crew performed a standard daily propeller overspeed test. Following this procedure, the flight crew proceeded immediately into the takeoff roll.
During the initial phase of the takeoff run, the aircraft experienced a sudden yaw to the left. The aircraft departed the runway surface near two diagonal military taxiways, traveling onto the area north of the runway. The aircraft eventually returned to the runway surface, where it decelerated and came to a halt in the middle of the runway. The incident resulted in no injuries, though a runway edge light north of the runway sustained damage and required repair.
The investigation
The Swedish Accident Investigation Authority (SHK) analyzed data from the flight data recorders (DFDR and QAR). While the recorders provided relevant flight parameters, the investigation noted that nose wheel steering angle and brake pressure were not captured by these systems. Technical inspections of the nose wheel steering mechanism revealed no malfunctions prior to or during the event.
Investigators also reviewed the cockpit procedures and the pilot's actions. The investigation established that the takeoff was initiated from partial power, which reduced the window of time available to correct the aircraft's path. Additionally, the investigation considered the possibility that the nose wheel angle was not aligned with the runway heading at the moment of brake release, potentially due to vibrations experienced during the preceding propeller overspeed test.
Findings
- The primary cause of the excursion was a failure to activate the nose wheel steering tiller during the start of the takeoff roll.
- The takeoff was initiated from partial power, which led to an abrupt course change and left the crew with very little time to recover the aircraft's heading.
- A contributing factor may have been the pilot's recent transition between two different aircraft types that utilize the nose wheel tiller in different operational manners.
- It is possible that vibrations from the propeller overspeed test caused the nose wheel angle to deviate from the runway heading prior to the takeoff run.