History of Flight
On February 13, 2012, a FedEx McDonnell Douglas (Boeing) MD10-30, registration N304FE, equipped with three General Electric CF6-50 turbofan engines, encountered a No. 1 (left) engine stall, high vibration, and high exhaust gas temperature during the initial takeoff roll at about 60 knots at Portland International Airport (PDX), Portland, Oregon. The flight crew performed a rejected take-off and pulled the fire handle. There were no reports of the fire extinguishing bottles having been discharged. The airplane taxied back to the gate, and a normal deplaning was conducted. No injuries were reported. According to FedEx, visual examination of the airplane revealed no damage or penetration of the cowlings and no damage to the airplane. The incident flight was a regularly scheduled domestic cargo flight operated under 14 CFR Part 121 from Portland to Oakland, California.
Engine Damage Examination
The engine was removed and shipped to the FedEx maintenance facility in Memphis, Tennessee, for initial visual examination. A Powerplant group comprising members from GE, FedEx, and the National Transportation Safety Board (NTSB) convened on February 23, 2012. Examination revealed no signs of an undercowl fire or fluid leaks. When the fan was rotated, the low pressure turbine (LPT) rotor did not rotate with it; when an attempt was made to rotate just the LPT rotor, it was seized. Three penetration holes and tears at the 4:00, 6:00, and 11:30 o'clock positions were observed in the LPT case, and a single hole at the 1:00 o'clock position was found in the turbine rear frame.
The engine was subsequently shipped to the GE Celma facility in Petrópolis, Rio de Janeiro, Brazil, for a detailed teardown and examination, which took place from March 13 to March 20, 2012. Disassembly revealed that the fan mid shaft, the center vent tube, and the high pressure compressor air duct were all fractured. The fan mid shaft was fractured about 18.5 inches aft of the forward end into two sections. The aft section exhibited a spiral crack measuring about 69.5 inches in total length (about 3½ revolutions around the shaft), extending from the torsional fracture face aft about 20.5 inches axially. Over its entire length, the crack appearance changed from tight near the torsional fracture to wide, gapped, jagged, and offset in the middle, then tight again near the aft end. The center vent tube was fractured into two pieces and was twisted and collapsed onto itself at the fracture. The forward parts of the fan mid shaft and center vent tube remained attached to the fan forward shaft, while the aft portions remained within the engine core. The high pressure compressor air duct was fractured into two major pieces (with two other smaller pieces broken out), with the forward part attached to the stage 2 high pressure compressor disk and the remaining aft parts resting on the fan mid shaft. The high pressure compressor air duct exhibited circumferential wear, damage, and heat discoloration.
All LPT blades exhibited tip fractures, leading and trailing edge impact damage, missing material, and some heat discoloration. All stages 2, 3, and 4 LPT stator vanes exhibited outboard and inboard leading edge contact rub consistent with contact from the LPT blade tips. All main line bearings were intact and in good condition except for the No. 7 bearing, which was oil-wetted but seized. Five roller elements of the No. 7 bearing were missing; the remaining roller elements were heat discolored black and mostly flattened. The No. 7 bearing cage was damaged and heat discolored but intact, and the inner race aft edge was damaged with rolled material.
Metallurgical Findings
Metallurgical examination of the fan mid shaft parent material composition and hardness measurements showed consistency with required material and proper heat treatment. The inner diameter of the fan mid shaft was coated with dry soot-like debris and some oil deposits but no residual liquid. Localized areas of corrosion pitting, scaling, and loss of SermeTel® anti-corrosion coating were observed throughout the inner diameter. Some pits measured up to approximately 0.040 inches deep, and some pit diameters up to approximately 0.067 inches.
The spiral fracture was angled 45° to the longitudinal axis, consistent with torsional influence upon crack initiation and propagation, and was entirely intergranular. Initiation comprised numerous discolored thumbnail-shaped cracks from the inner diameter surface, some of which had progressed through the shaft thickness. These through cracks were torsionally oriented, consistent with primary stress direction, and originated in areas with corrosion pitting and coating deterioration. The cracks were branched and intergranular, consistent with stress corrosion cracking mechanisms typical of high strength steel. Chemical analysis of the fracture surface revealed no causative species such as chlorides or sulfides within the cracks; however, trace amounts of chloride were found around some corrosion pits.
Debris found within the corrosion pits and along the inner diameter surface of the fan mid shaft and on the outer surface of the center vent tube was positively identified as an ester-based engine oil residue, typical of oil used in turbine engines. GE concluded that the stress corrosion cracking observed on the fan mid shaft was caused primarily due to the breakdown of a synthetic oil product inside the fan mid shaft during both storage and engine operation. The exact source or mechanism by which the oil entered the dry cavity between the fan mid shaft and the center vent tube is unknown.
Corrective Actions
In response to the failure, GE issued Service Bulletin (SB) 72-1323 on August 31, 2012, providing instructions for a one-time borescope inspection of the fan mid shaft inner diameter to detect signs of corrosion pitting. The SB requested that all inspection results be reported to GE Product Support Engineering. In the January 2013 issue of the CF6 fleet highlights, a monthly publication sent to all CF6 customers, an article was published discussing this fan mid shaft failure event (no operator specified) and the rationale for performing SB 72-1323.