What happened
On 28 January 2000, a BN2A, Mk 111-2 Trislander, registration G-WEAC, was conducting a cargo flight from Edinburgh to Belfast. During the climb to a planned cruising level of 4,500 feet, the aircraft entered cloud at 2,500 feet. The pilot noted ice accumulation on the external temperature probe and observed ice building up on the wing leading edges.
Despite activating the pneumatic de-icing boots, the aircraft's rate of climb steadily declined. By 4,300 feet, the climb had ceased entirely, and the aircraft began a slow descent. The pilot requested a lower cruising altitude of 4,000 feet due to the icing, but air traffic control could not grant the request because 4,500 feet was the minimum level required for terrain clearance.
As the aircraft continued to descend through 2,000 feet, the pilot requested a diversion to Prestwick. The aircraft continued to lose altitude, eventually reaching 900 feet while still in cloud. The pilot eventually performed a visual approach and landed safely at Prestwick.
The investigation
Investigators examined the aircraft's de-icing systems and the meteorological conditions. The BN2A, Mk 111-2 Trislander was equipped with several de-icing features, including heated windshields and pneumatic boots on the wings and tail. The investigation found that while the boots were functional, they were ineffective at removing the accumulated ice from the leading edges.
The investigation also reviewed the pilot's workload and the air traffic control response. Although the aircraft was operating under a single-pilot exemption, the pilot managed the workload without becoming overwhelmed, though monitoring the wing edges required looking behind the cockpit, which likely contributed to observed heading deviations. Controllers at the Scottish Area Control Centre monitored the descent and provided necessary assistance, though the investigation noted that the Distress and Diversified Cell was not formally alerted via the standard transponder code procedure.
Findings
- The primary cause of the descent was the accumulation of airframe icing that the pneumatic de-icing boots could not dislodge.
- The aircraft's performance was severely degraded by the weight and aerodynamic impact of the ice.
- The pilot's need to visually monitor the wing leading edges for ice accumulation likely compromised instrument flying precision.
- Meteorological analysis indicated that while standing waves were present, the primary factor was the icing encounter rather than vertical air movement.