What happened
On 8 March 2000, a BN2B-26 Islander, registration G-BLDV, was operating an air ambulance flight from Glasgow to Stornoway. The flight was following the A1D airway, which traverses the Grampian Mountains. During the climb, the pilot encountered significant mountain wave activity, which prevented the aircraft from reaching the requested flight level of 10,500 feet.
As the flight progressed through icing conditions, the pilot utilized the aircraft's pneumatic de-icing boots and propeller de-icing systems. However, at 10:12 UTC, while flying at 6,500 feet, the pilot observed a sudden and rapid buildup of ice on the aircraft's wheels, struts, and tyres. The accumulation was so significant that the aircraft was unable to maintain its altitude. The pilot declared an emergency and requested radar vectors toward the coast.
Following the controller's suggestion to turn west, the pilot descended to 70 knots while maintaining full engine power. Approximately three nautical miles from the coast, the aircraft entered clear air, causing the ice to melt rapidly. The pilot subsequently elected to continue the flight to Stornoway, eventually landing safely at 11:26 UTC.
The investigation
The investigation examined the aircraft's de-icing systems, the meteorological conditions, and the flight route. The BN2B-26 Islander was found to be fully serviceable, with all electrical and pneumatic de-icing equipment functioning correctly. The investigation also reviewed the meteorological forecast, which had indicated moderate icing and turbulence, as well as the potential for mountain waves.
Findings
- The aircraft was flying in an area of complex low pressure with a strong westerly airstream.
- The flight path over the Grampian Mountains subjected the aircraft to significant mountain wave activity, which likely increased the water density in the clouds and the risk of icing.
- While the aircraft's de-icing systems were operational, they were unable to prevent the rapid accumulation of ice on the non-heated components, such as the landing gear and struts.
- An alternative route via airway N 573 D, which follows the western coast and avoids higher terrain, was available and would have likely reduced the impact of standing wave activity and associated icing.