What happened
On September 25, 2014, a British Aerospace ATP, registration SE-MAF, operated by West Air Sweden AB, was performing a routine cargo flight from Oslo to Trondheim. While cruising at 15,000 feet, the crew noticed a gradual decrease in airspeed. Despite activating the airframe de-icing systems, the aircraft's speed continued to drop.
As the crew prepared to descend to a lower altitude, the aircraft suddenly experienced intense vibrations and severe buffeting. The nose pitched up, and the aircraft entered an uncontrolled left bank. The pilot flying had to apply significant force to push the nose down. During this period, the vibrations were so violent that the crew struggled to read the flight instruments. After approximately 30 seconds of manual intervention and disconnecting the autopilot, the crew regained control as the airspeed increased. The aircraft descended 1,000 feet before stabilizing.
The investigation
The Norwegian Safety Investigation Authority (SHT) examined the flight data, cockpit voice recordings, and meteorological conditions. The investigation focused on the aerodynamic performance of the aircraft in icing conditions and the crew's awareness of minimum operating speeds. Investigators also reviewed the technical status of the de-icing systems and the company's training protocols regarding flight in icing environments.
Findings
- The primary cause of the speed decay was a combination of active icing and mountain waves creating downward air currents.
- The aircraft's speed had dropped to 22 kt below the recommended minimum speed for flight in icing conditions.
- The severe buffeting and loss of control were likely precursors to a stall, which can occur at speeds significantly higher than the standard stall warning (stick shaker) activates when ice is present on the wings.
- A technical fault in the de-icing system for the tail section was identified but determined to have had minimal impact on the primary loss of control.
- There was a significant lack of awareness regarding minimum speeds during the en-route phase of icing, a gap that extended to the operator's training programs and was exacerbated by unclear information in the aircraft's approved manuals.
Safety action
Following the incident, BAE Systems took steps to improve the documentation regarding icing and minimum speeds for the ATP type. The investigation resulted in safety recommendations to ensure that recommended minimum speeds in icing conditions are easily accessible to pilots within the cockpit, specifically through updated speed cards or improved manual clarity.