Incident Overview
On October 14, 2008, at about 0805 Pacific daylight time, a Piper PA-31-350, registration N3BT, operating under 14 CFR Part 135 for Ameriflight, Inc., experienced an in-flight fire in the left engine shortly after departure from Portland International Airport, Portland, Oregon. The cross-country cargo flight had departed Portland around 0800 with a planned destination of Pendleton, Oregon. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed. The certificated commercial pilot was not injured; the airplane sustained substantial damage to the left rear spar.
Pilot Report
The pilot reported that shortly after departure, at about 1,000 feet above ground level, she noticed the left engine’s manifold pressure dropped about 6 inches below the right engine’s indications. The engine was surging, and she thought the turbocharger had failed. The left cylinder head temperature read 0 degrees, but all other gauges were normal. The pilot requested to return for landing without declaring an emergency, as she did not believe she had one. The engine was not shut down in flight, and no flames or smoke were observed. After an uneventful landing on runway 21, the left engine lost power during taxi to the ramp.
Post-Flight Inspection
The operator reported that a post-flight inspection revealed copious oil streaming aft of the left engine. Two holes were burnt in the lower skin of the left flap aft of the cowling exit area, a hole burnt in the exterior skin of the left outboard main gear door, and evidence of fire in the accessory area inside the left cowling. No oil showed on the left engine dipstick. A head pipe on the left engine (leading from the cylinders to the waste gate) had broken off at the flange where it attaches to the waste gate with a V-clamp, allowing raw exhaust flames to enter the accessory area.
Examination of Fractured Components
A Safety Board Materials Laboratory specialist examined the broken pipe. The FAA inspector and the FAA Small Airplane Directorate identified four other fractured exhausts on other airplanes of the same make and model, some manufactured by different companies using different production methods. The specialist examined these pieces to determine commonalities, labeling them as samples A, B, C, and unknown. Some were from left sides of engines, others from the right side.
The exhaust pipes from the accident airplane and airplanes A, B, and C consisted of two or three pipe sections. The aft ends had a flanged end clamped to a cross-shaped transition piece below the turbocharger. The forward end of each section had an expanded diameter forming a slip joint. The aft flanges on all submitted pipe components were either cracked or fractured in the transition radius between the skirt and the flange face. Some fractures were on the forward side, others on the aft side.
Fatigue Fracture Features
The specialist observed light gray metallic deposits on the forward side of the aft pipe section in the inner radius of the bend of the exhaust pipe from the accident airplane. The fracture was generally in a plane perpendicular to the flange surface with step features separating flat fractures, consistent with fatigue initiating from multiple origins. Post-fracture damage, oxidation, and deposits obliterated fine features. Several branching cracks emanated from the fracture toward the radial direction on the flange face at upper and lower sides. The crack initiated at the forward side and propagated toward the aft side.
Exhaust pipes from airplanes A, B, and C exhibited obtuse fracture angles. The unknown airplane’s pipe fracture was also consistent with fatigue from multiple origins.
Dimensional and Material Analysis
The specialist measured wall thickness at a location on the flange face adjacent to the transition radius. In each case, flange thickness exceeded the minimum specified on the manufacturer's engineering drawing. The transition radius between flange skirt and flange face was measured on forward and aft sides for four pipes. Generally, forward-side radii were larger than aft-side. Averaging forward and aft measurements, the accident airplane and airplane B were within drawing specifications; airplanes A and C had slightly greater radii than specified.
Internal surfaces of exposed slip joints at forward ends were examined. Intact slip joints were rigid and could not be separated by hand. Internal surfaces from the accident airplane, A, and B were a mix of gray, brown, and orange; scrapings felt smooth and slippery. Airplane C’s deposits were darker gray to black, with a sticky, gummy texture. Scanning electron microscope examination of scrapings showed peaks for stainless steel and combustion deposits. Samples from the accident airplane, A, and B indicated a significant copper peak, common in antiseize compounds like Loctite C5-A. Airplane C showed greater distortion and generally no copper peak.
Inner diameters of exposed female ends of slip joints were measured vertically and horizontally. All vertical measurements and two horizontal measurements exceeded drawing specifications; one horizontal measurement was within specification, and one was less. Hardness measurements on the back face of the flange from the accident airplane averaged 85 HRB, typical of annealed Type 321 stainless steel tube.