What happened
During the approach phase of flight, N500AK, a turboprop aircraft, encountered icing conditions before beginning its Instrument Landing System (ILS) approach. Radar data indicated that as the aircraft approached the outer marker, it experienced a reduction in speed consistent with a loss of power or a significant power reduction. Following this loss of performance, the aircraft entered a steep descent and crashed.
The investigation
Post-accident examination revealed that the engines were not operating at the time of impact and that the propellers had been feathered. Investigators found no evidence of pre-impact component failure or malfunctions involving the engines, propellers, or the anti-ice system. However, evidence indicated that the engine inlet anti-ice annunciator lights and the Stability Augmentation System (SAS) fault warning light were illuminated during the impact.
Findings
The investigation noted that the engine manufacturer had previously reported instances of engine flameouts in other aircraft operating in icing conditions, sometimes occurring after descending into warmer air. The flight manual provided specific procedures for encountering icing if the anti-ice system was initially off, which included selecting continuous ignition and activating engine and propeller heat sequentially while engaging SAS heat.