Accident Summary
On an approach to landing, aircraft N500AK encountered icing conditions prior to commencing an ILS approach. Radar data indicated that before reaching the outer marker, the aircraft decelerated in a manner consistent with a reduction or partial loss of power. Subsequently, the aircraft entered a steep descent and impacted the ground. All four occupants were killed.
Investigation Findings
Examination of the wreckage revealed that both engines were not operating at the time of impact, and the propellers were feathered. No pre-impact failures or malfunctions were found in the engines, propellers, or the anti-ice system. Evidence showed that the engine inlet anti-ice annunciator lights and the stability augmentation system (SAS) fault warning light were illuminated during impact.
The engine manufacturer reported that flameouts had occurred in other aircraft during or after operation in icing conditions, sometimes following a descent into warmer air. The flight manual specified that if icing is encountered with the anti-ice system off, the pilot should select continuous ignition, then activate engine and propeller heat one engine at a time (ensuring the first engine is operating satisfactorily before selecting the second), and engage the SAS heat.
Probable Cause
The official probable cause is: Failure of the pilot to follow procedures concerning use of the engine inlet anti-ice system and/or continuous ignition while operating in icing conditions, which resulted in probable ice ingestion and loss of engine power; and the pilot's failure to maintain sufficient airspeed while coping with the engine problem, which resulted in a stall. Factors related to the accident were: darkness, icing conditions, and engine inlet (nacelle) ice.