What happened
A scheduled flight operated by NextJet AB, involving a BAe ATP with registration SE-LLO, departed Hemavan Tärnaby Airport for Vilhelmina with 19 passengers and four crew members. During the ILS approach to runway 28 in Vilhelmina, the aircraft encountered snow and approximately 5 mm of slush. While the pilot used the lowest reported friction coefficient for performance calculations, the aircraft experienced a significant deviation during the landing roll.
Immediately after touchdown, the aircraft drifted toward the right side of the runway. The right-side wheels left the paved surface, traveling 155 metres outside the runway edge before the crew could steer the plane back to the centerline. During this excursion, the aircraft struck a runway edge light, which was dislodged by the wheels. Although the crew initially reported the excursion to air traffic control, a subsequent change in the crew's perception of the event led them to believe the aircraft had not actually left the runway surface.
The investigation
The investigation focused on two distinct phases of the occurrence. First, investigators analyzed flight data recorder (FDR) information to determine the cause of the lateral deviation. Second, they examined the crew's reporting process and the subsequent inspection of the aircraft. While the pilot performed a self-inspection and found no visible damage, a later inspection revealed that the runway light had been thrown upward by the wheels, striking the underside of the right wing flap. This impact caused cracks and structural damage to the flap, meaning the aircraft was not airworthy for its subsequent flight to Stockholm Arlanda.
Findings
- The primary cause of the runway excursion was asymmetrical reverse thrust, where the right engine produced notably higher thrust than the left engine, creating an uncorrectable yawing moment.
- Braking effectiveness was likely lower than the reported friction coefficients suggested.
- The crew's assessment of the incident was flawed, as they prioritized continuing the flight over a thorough investigation of the excursion.
- There were significant failures in the operator's safety management systems regarding the oversight of maintenance inspections.
Safety action
To prevent similar occurrences, the following recommendations were made to EASA:
- Implement generic performance corrections for aircraft operating on surfaces contaminated with water or slush.
- Review the reliability of reported friction coefficients from airports, suggesting that measured values be flagged as unreliable under specific environmental conditions.