What happened
While the Saab 340 was in the cruise phase of flight, the aircraft encountered icing conditions that caused both engines to experience momentary power fluctuations. Following these events, the flight crew initiated a descent. Once the aircraft descended, the power fluctuations ceased, allowing the flight to proceed to its intended destination.
The investigation
An evaluation of information from the flight data recorder provided details regarding the nature of the engine instabilities. The left engine underwent four Type II power fluctuation events, characterized by compressor stalls. The right engine experienced one Type I event, involving partial or full quenching of the engine's combustor flame, as well as one Type II event.
Research into the Saab 340 fleet regarding these fluctuations has been ongoing since its 1985 U.S. certification. Investigations determined that ice accumulates on the engine inlet splitter lip and subsequently breaks off into the engine, which can result in a flame-out. To mitigate this, an auto-ignition system was developed to facilitate engine re-lighting.
Findings
Although aircraft certification rules require that no engine inlet ice accumulation occurs that would adversely affect operation, no redesign of the engine inlet has been identified that would prevent ice from forming on the splitter lip. In 2000, the FAA maintained that the risk associated with these power fluctuation events remained acceptable. The agency further determined that the risk of a dual engine power loss—stemming from simultaneous icing conditions and the failure of both auto-ignition systems—was acceptably low. Additionally, while the risks of inappropriate crew response to power fluctuations combined with an in-flight shutdown were quantitatively indeterminate, they were deemed qualitatively acceptable by FAA certification pilots and human factors specialists.