Incident Overview
A pilot reported encountering icing conditions during an approach to Spokane. Upon arrival, the aircraft remained at Spokane for 30 to 40 minutes, during which freezing drizzle and below-freezing temperatures existed both prior to and during the subsequent departure.
Contributing Factors
The pilot did not utilize the manually operated alternate air system aboard the aircraft. As a result, cold, moist air was entrained into the subfreezing filter chambers. The gradual blockage of air from ice in the filters resulted in a loss of power on both engines.
Forced Landing
With sufficient power available, the pilot chose to execute an intentional forced landing. During the landing and ground slide, the aircraft struck an airport perimeter fence.
Conclusion
The incident highlights the importance of following procedures for alternate air systems in icing conditions. The aircraft sustained damage from the fence strike during the forced landing.