What happened
While descending after exiting light icing conditions, the crew of a Saab 340 experienced a loss of power in the left engine. During this period, power fluctuations were also noted in the right engine. The flightcrew declared an emergency and diverted to the nearest airport, where they completed an uneventful landing.
The investigation
Post-flight inspections showed indications of engine ice ingestion, though no physical damage to the engines was found. During subsequent ground runups, the right engine operated normally. While the left engine also performed normally, investigators discovered that the auto-ignition and overspeed protection systems were inoperative because of an open fuse within the Digital Electronic Control Unit (DECU). This open fuse would have been unable to facilitate an automatic relight following a flameout.
Further testing revealed that when the left engine was inoperative and the propeller synchrophaser was engaged, multiple power fluctuations occurred in the right engine. The existing engine failure checklist did not include instructions to turn the synchrophaser off during such an event.
Findings
Investigation into the aircraft type revealed a history of power losses related to in-flight icing dating back to 19ser5. Saab had previously determined that ice or slush could accumulate on the rear wall of the inlet protective device (bird catcher) and subsequently shed from the splitter lip into the engine airflow path. To mitigate this, Saab developed an auto-ignition system for automatic relighting and was testing an improved heating system to prevent accumulation on the splitter lip.