Piper PA-31-350 N3BT accident near Portland, Oregon

Probable cause

An in-flight engine fire due to the fatigue fracture of an exhaust pipe that resulted from a failure of the pipe's slip joint.

NTSB analysis

The pilot reported that shortly after departure, and about 1,000 feet above ground level, she noticed that the left engine’s manifold pressure dropped about 6 inches below the right engine’s indications. The engine was surging, and she thought that the turbocharger had stopped operating. The left cylinder head temperature was reading 0 degrees, but all other gauges were normal. No smoke or flames were noticed as the pilot returned to the airport for landing without declaring an emergency or shutting down the engine. During taxi to the ramp, the engine lost power. Postflight examination revealed a broken exhaust pipe on the right side of the exhaust system, with evidence of a fire in the accessory section and a burned-through section of skin. Metallurgical examination disclosed that the aft exhaust pipe flange had cracked and fractured due to fatigue. Dimensional measurements of the flange wall thickness were within design limits. The hardness was as expected for the specified type 321 steel, indicating that bending stresses acting on the flange exceeded its design capability. The cracking occurred due to excessive stresses that developed from thermal expansion of the exhaust line and insufficient sliding action at the slip joints within the exhaust pipe system. Bending of the flange and subsequent fatigue cracking indicated that the slip joints had likely not been working properly for some time.

Text from the NTSB. Open the NTSB docket for WPR09LA013 for the full record.