History of the Flight
The pilot was conducting his fifth parachute lift of the day in G-LEAP, a BN2T Islander with two Allison 250-B17C turboprop engines. Weather conditions were similar to previous lifts, with about four oktas of cloud cover organised in lines with clear air in between. While climbing to position overhead the airfield for a parachute drop, the pilot commenced a wide turn that took the aircraft through the side of a cloud at approximately FL080.
Weather Conditions
The outside air temperature was about 0°C. As the aircraft penetrated the cloud, the pilot reached down to select engine anti-ice on. Before he could do so, the aircraft was enveloped in what the pilot described as a “sleet storm”. Almost immediately, both engines ran down. The pilot noted decreasing engine torque and fuel flow, a rise in Turbine Gas Temperature (TGT) of about 40°C from the climb setting of 700°C, and decreasing turbine rpm (N1).
Pilot Actions
The pilot established the aircraft in a glide at 120 kt and completed the turn towards Swansea Airport. The aircraft descended out of the cloud at about FL070, and the pilot selected the igniters ON and the power levers to idle. Both engines relit immediately. The pilot left both engines at idle for stabilisation while descending a further 500 ft, then slowly increased power with no anomalies. The parachuting lift was aborted, and the aircraft returned to Swansea without further incident.
Met Office Aftercast
The Met Office provided an aftercast indicating a showery north-westerly flow with good visibility and scattered to broken cloud layers between 6,000 and 10,000 ft. The freezing level was estimated at around 6,500 ft. The cloud layers were thick in places and capable of causing significant, moderate engine and airframe icing. Forecast charts issued were consistent with actual conditions.
Power Plant Anti-Icing System
The Turbine Islander Flight Manual describes the system as an electrically heated engine air intake and application of compressor bleed air. The air intake, compressor bleed, and propeller de-icing systems are operated by a single lever for each engine on the lower part of the centre pedestal. The Flight Manual states: “At outside air temperatures of less than 5 deg C, in conditions where visible moisture exists, select Power Plant Anti-Icing ON for both engines and ensure that these services remain on during flight in such conditions.” It also cautions that the formation of intake ice may cause rapid power loss.
Flight Manual Emergency Procedures
Section 3 of the Flight Manual includes a procedure for failure of both engines en-route, providing immediate action for a quick restart and a longer procedure if unsuccessful. The recommended gliding speed is 75 kt IAS. Due to thermal changes, the gas producer section may lock up after inflight shutdown for a temporary period of approximately one to ten minutes, during which air starts may be attempted but are not guaranteed.
Previous Occurrence
In August 2007, a GAF Nomad N22B (N6302W) equipped with two Allison 250-B17 engines was engaged on parachuting operations. At 8,500 ft, the aircraft entered icing conditions with engine anti-ice selected off, and the left engine ran down before the pilot could select anti-icing on. The engine could not be restarted, and the aircraft was damaged in the subsequent single-engine landing.
Conclusion
Power plant anti-icing was not selected on before the aircraft entered engine icing conditions, resulting in the failure of both engines. In-flight restart is not assured on this engine type between one and ten minutes after shutdown. Once the aircraft had exited engine icing conditions, both engines were relit and operated normally.