Flight Events
The aircraft had been parked on the ramp overnight, uncovered, in continuous rain. Takeoff was conducted in heavy rain. After reaching cruising altitude at flight level 250, the crew experienced power interruptions in both engines. The flight conditions included instrument meteorological conditions, an outside air temperature of -20 degrees Celsius, and light rime icing. Additionally, all airspeed indications decreased to zero. The flight crew initiated a descent and declared an emergency. The aircraft was landed safely at its destination.
Investigation Findings
The investigation determined that engine power interruptions had occurred in light icing conditions since the aircraft was introduced into revenue service. Airworthiness directives were issued, first prohibiting revenue flights and later restricting flights in icing conditions unless the aircraft was equipped with an auto ignition system. The manufacturer found that ice or slush could accumulate on the engine inlet particle separator, break off, and enter the engine, resulting in a flameout. This accumulation was not visible to the pilots and was not alleviated by engine anti-ice systems. An auto ignition system was subsequently added that could restart an engine within two seconds.
Certification Implications
Aircraft certification rules require that no accumulation of ice on engine inlets occurs that would adversely affect engine operation. The findings from this event highlighted a deficiency in meeting that requirement under certain conditions.