Incident Overview
A turboprop airplane was stationary on a taxiway, awaiting takeoff clearance. The flight crew had feathered the left propeller and was using high-pressure bleed air from the left engine to supply the air conditioning system. While the airplane was stationary, several warning lights illuminated. The crew identified a fire in the vicinity of the left engine. Both engines were shut down, and the airplane was evacuated.
Examination Findings
Examination of the wing structure showed that a titanium flap line, situated between the wing and the flap panel, had failed. The cause of the failure could not be determined. Following the separation, the line broke two electrical wires and struck a bolt end, resulting in ignition of the MIL-5606 hydraulic oil.
Relevant Information
The airplane manufacturer stated that the temperature in the compartment where the fire started should not exceed 70 degrees Celsius. The bleed air leak warning lights were designed to activate at 204 degrees Celsius. The flash point of MIL-5606 hydraulic fluid is 110 degrees Celsius. Flight crews had previously encountered false bleed air leak warnings. The practice of using a feathered engine to power the air conditioners was recommended by the engine manufacturer but was not included in the airplane's flight manual. The Federal Aviation Administration Principal Operations Inspector had approved this procedure.