History of Flight
On August 30, 2013, at approximately 2110 local time, a Martinair Cargo MD-11F, registration PH-MCW, powered by three Pratt & Whitney PW4462 engines, experienced an uncontained engine failure during takeoff at Borinquen International Airport (BQN) in Aguadilla, Puerto Rico. The flight crew reported hearing a loud bang and noted a decrease in No. 1 engine fan speed (N1). The No. 1 engine fire warning activated, and the captain discharged the No. 1 fire bottle. The airplane was operating as Martinair flight 1156, a scheduled cargo service from BQN to London Stansted Airport, England. There was minor damage to the airplane, and no injuries were reported.
A post-incident inspection found that the No. 1 engine low-pressure turbine (LPT)-to-turbine exhaust case (TEC) flange (P-flange) was open between 5 and 7 o'clock. Inspection through the engine exhaust revealed near-platform LPT stage 4, 5, and 6 blade airfoil fractures and extensive LPT vane and seal damage. A large amount of internal turbine component debris was recovered from the runway and from the No. 1 nacelle. The engine was removed and shipped with the debris to a repair facility for investigation.
Aircraft Damage
The left wing sustained numerous small dents and punctures, including two 3-inch long punctures. Similar damage was found on the main landing gear assembly. The No. 1 engine nacelle had five holes axially in line with the case flange separation. The largest nacelle penetration was an 8-inch by 5-inch hole through the inboard thrust reverser door.
Recorders
The airplane was equipped with a Loral/Fairchild F1000 flight data recorder (FDR) and two dual-channel electronic engine controls (EECs) with non-volatile fault memory. FDR data showed a sudden decrease in No. 1 engine EPR about 11 seconds into the takeoff roll, followed by a steady decrease in airplane longitudinal acceleration. The No. 1 engine fire warning activated 13 seconds after the EPR drop. Maximum recorded ground speed was 17 knots. The No. 1 engine EEC fault memory had stored 192 faults over the last 76 flights/78 ground legs. Both EEC channels recorded one fault during the incident: a surge flag at 9,600 RPM low pressure rotor speed (N1). Review of fault data from previous flights found no fault records pertinent to the failure.
Fire
Airport fire and rescue personnel responding to the aircraft determined that no fire was present. Subsequent inspections of the No. 1 nacelle and engine found no evidence of undercowl fire. Soot found on the engine rear mount, TEC lower left surface, and deposited in an aftward pattern from one of the cowling penetrations was consistent with a release of combustion products through the engine breach.
Engine Examination
External examination of the engine found that the P-flange attachment hardware was missing from 62 of 132 bolt holes (bolt holes 40 through 102). There was local case distortion and the flange was separated up to two inches. External component discrepancies were minor except for the No. 4 bearing oil pressure tube, which was dented and deformed where it spans the P-flange. Inspection of recovered debris found 71 bolt segments consistent with P-flange attachment hardware (36 shanks with bolt heads and 35 shanks with nuts installed).
Engine borescope inspections found no mechanical failure upstream of the second LPT stage (S4). Disassembly of the LPT section exposed advanced corrosion attack at the S4 vanes with extensive internal corrosion damage (cracking and material breakouts) and visible corrosion products. The lower left quadrant displayed the most severe damage, and the inner diameter attachments of four vane clusters in this quadrant had disengaged and displaced aft into the S4 blade path. The aft edges and honeycombs of the displaced clusters were scored consistent with rotational contact with the S4 blade leading edges during operation.
All S4 blades were fractured off close to the platform. An approximately 0.25-inch deep groove was machined into the leading edges of all S4 blades; undamaged areas of several blade fracture surfaces displayed fatigue features just aft of the consumed leading edge area. All S5 and S6 turbine blades displayed near-platform separations, and undamaged blade fracture surfaces exhibited coarse and grainy appearances. Four S4 vane clusters and over half of the S5 and S6 vane clusters were liberated and missing from the engine. The remaining LPT gas path components and the LPT case, TEC, and exhaust nozzle internal surfaces were severely battered and scored consistent with impacts from liberated hardware exiting the engine through the exhaust.
Tests and Research
Metallurgy
Metallurgical examination of four exemplar S4 vane clusters found significant corrosion/oxidation consumption of vane airfoil wall thicknesses. Chemical analysis of the corrosion attack fronts revealed sulfur-rich particles, indicative of sulfidation as the corrosion mechanism. EDS and WDS spectra and elemental maps collected at multiple locations along the attack fronts confirmed a consistent sulfur presence and the presence of chromium. Inspection of the displaced S4 vane clusters found that corrosion-induced material loss resulting in loss of structural stiffness had allowed the clusters to disengage at the inner attachment.
Four exemplar S4 blade fracture surfaces displayed machining-type leading edge damage typical of rotor contact with static hardware during operation. Metallurgical examination of the fracture surfaces immediately aft of the consumed leading edges revealed high cycle fatigue crack propagation leading to airfoil overload fracture.
Additional Information
LPT Service History
The incident LPT was manufactured in 1990 and underwent overhaul in 2003. It was placed in storage at 23,536 hours and 10,112 cycles since new, 5,245 hours and 2,203 cycles since overhaul. In 2006, it was removed from storage and installed on the incident engine. Module-level inspections were performed during scheduled engine shop visits in 2008 and 2009. At the time of the incident, the LPT/engine had accumulated 14,406 hours and 2,765 cycles since the 2009 shop visit.
The S4 LPT vane clusters installed in the LPT (part number 51N674-01) were affected by Federal Aviation Administration (FAA) airworthiness directive (AD) 2012-14-09; however, the engine was within the specified compliance.
AD 2012-14-09
On October 5, 2012, the FAA published AD 2012-14-09 in response to reports of Pratt & Whitney PW4000 94-inch and 100-inch LPT S3 and S4 vane fractures that caused uncontained engine failure, LPT case puncture, and multiple in-flight shutdowns. The AD requires the assembly of the LPT S4 rotor by alternating heavy and light blades and by balancing blades of similar weights 180 degrees across the rotor; the dimensional examination of LPT S3 vane clusters; the removal from service of S4 vane clusters with certain casting identifications. It also requires that heavy and light blades are alternated and blades of similar weights are balanced 180 degrees across when assembling the HPT S2 at HPT overhaul.