What happened
The aircraft was operating in icing conditions while approaching Spokane International Airport. Upon arrival, the airplane remained on the ground for approximately 30 to 40 minutes. During this period, freezing drizzle and below-freezing temperatures persisted both prior to and during the subsequent departure phase.
During the takeoff roll or initial climb, the pilot failed to utilize the manually operated alternate air system available on the aircraft. This oversight allowed cold, moist air to be drawn into the subfreezing filter chambers of the engine induction systems. The moisture within this air froze, causing a gradual blockage of airflow through the filters.
The ice accumulation restricted enough intake air to cause a complete loss of power in both engines. Despite having sufficient fuel available for continued flight, the pilot was forced to execute an intentional forced landing due to the mechanical failure. During the landing sequence, the aircraft struck an airport perimeter fence and subsequently slid along the ground before coming to rest.
The investigation
The investigation focused on the mechanical state of the engines and the operational history leading up to the power loss. Examination confirmed that the loss of engine power was directly caused by ice blockage in the air intake filters. The root cause was identified as the failure to engage the alternate air system, which is designed to bypass iced filters with warmer air from the engine compartment.
Findings
Contributing factors included the environmental conditions and pilot decision-making. The presence of freezing drizzle and sub-freezing temperatures created a high risk for induction icing. The critical error was the omission of a standard anti-icing procedure (using alternate air) during known icing conditions, which led to the simultaneous failure of both engines.