Incident Overview
During descent after the aircraft exited light icing conditions, the left engine lost power and power fluctuations were noted on the right engine. The flightcrew declared an emergency, diverted to the nearest airport, and performed an uneventful landing. Indications of engine ice ingestion were present, but no engine damage was found.
Ground Runup Findings
During subsequent ground runups, the right engine operated normally. The left engine ran normally except that its auto-ignition and overspeed protection systems were inoperative due to an open fuse in the Digital Electronic Control Unit (DECU). The open fuse would have prevented an automatic relight had a flameout occurred.
Fleet History and Manufacturer Actions
The Saab 340 fleet has a known history of power losses associated with in-flight icing, dating back to 1985. The manufacturer, Saab, determined that ice or slush could accumulate on the rear wall of the inlet protective device (commonly referred to as the bird catcher) and shed from the splitter lip into the engine airflow path. As a preventive measure, Saab developed an auto-ignition system to provide automatic relighting of the engine. Additionally, Saab was testing an improved heating system to prevent ice or slush accumulation on the splitter lip.
Regulatory Requirements and Flight Tests
Title 14 CFR Part 25 requires that airplanes be designed to prevent the accumulation of engine inlet ice that would adversely affect engine operation. During flight tests with the left engine inoperative and the propeller synchrophaser engaged, multiple power fluctuations of the right engine occurred. Instructions to turn the synchrophaser off were not included in the engine failure checklist.