N363CM

HISTORY OF FLIGHT

On January 29, 2021, about 10:49 pacific standard time, a Boeing 767-300, registration number N363CM, operated by Aerotransportes Mas de Carga, S.A. de C.V. as flight MMA6853, and powered by two General Electric (GE) CF6-80C2-B6 turbofan engines, experienced a right (No. 2) engine fire during initial climb right after takeoff from Los Angeles International Airport (LAX), Los Angles, California. The flightcrew reported feeling vibrations in the airplane as the landing gear was cycled up, so a second cycling of the gear was attempted, and the vibration continued. Shortly after that, the flightcrew detected a strong odor consistent with burning and a right engine fire warning message displayed on the engine indicating and crew alerting system (EICAS) display. The flightcrew declared an emergency, performed the Quick Reference Handbook (QRH) engine fire procedures, which included shutting down the affected engine and discharging 1 fire suppression bottle, and diverted to the Ontario International Airport (ONT), Ontario, California for an uneventful single engine overweight landing with no reported injuries to any of the flightcrew members. The incident flight was a 14 Code of Federal Regulations (CFR) Part 129 cargo flight from LAX to the Mexico City International Airport (MEX), Mexico City, Mexico.

ON-SCENE AIRPLANE AND ENGINE DAMAGE ASSESSMENT

A Powerplant Group was formed and comprised of members from GE, Boeing, Federal Aviation Administration (FAA), and the National Transportation Safety Board (NTSB), and Aerotransportes Mas de Carga, S.A. de C.V. (referred to for the remainder of this report as Mas Air Cargo). Assisting in the on-scene part of the investigation were personnel from Jett Pro Line Maintenance stationed at ONT and ONT operations center. The NTSB did not travel to ONT to examine and document the airplane and engine damage. Instead, Jett Pro and ONT operations center personnel provided initial pictures and descriptions of the damage. On January 30, 2021, the day following the event, Boeing field representatives arrived on-site to photo document the damage along with providing written field notes. Several days later, a GE representative arrived on-site to assist in the removal of the engine and securing it into a transportation stand for shipping to the GE Evendale, Ohio facility for examining and disassembly.

The on-scene examination of the airplane revealed thermal distress and fire damage to the inside of the right engine core cowl and thrust reverser. The entire outer portion of the right engine turbine exhaust nozzle was missing as was the aft portion of the inner sleeve; the forward portion of the inner sleeve remained attached to the engine. The right engine strut/pylon outdoor skirt in-line with the aft edge of what remained of the turbine exhaust nozzle exhibited gouging and scratch marks. The airplane exhibited some impact marks and dings on the underside of the right wing and flaps, on the vertical and horizontal stabilizer, and on the right cargo door; however, some of the impacts appeared old and could not be confirmed if all the damage observed was from this incident.

The on-scene examination of the No. 2 engine revealed no signs of engine uncontainments but did show signs of low-grade thermal distress comprised of sooting and melted, consumed, and damaged cushion clamps, fire detector loop isolators, and electrical wire outer sheathing. Looking at the front of the engine, no damage was observed to the fan blades and the fan rotated freely by hand; the low pressure turbine rotated along with the fan. Looking through the engine exhaust: 1) all the low pressure turbine stage 5 blade roots were still installed in the disk and all the blades exhibited a combination of airfoil transverse fractures at various lengths and airfoil impact damage, tears, gouges, and missing airfoil material, 2) all the low pressure turbine stage 5 blade outer shroud segments were still present and exhibited a combination of gouging, missing material, heavy rub, and the honeycomb was worn down to the backing strip, 3) low pressure turbine stage 5 nozzles segments were present and exhibited a combination of trailing edge airfoil impact damage, gouges, tears, and missing material, and 4) the turbine rear frame exhibited a combination of multiple cracks, tears, openings, and punctures holes; no signs of low pressure turbine debris penetrating through the turbine rear frame skin was observed. The oil tank was low of oil and debris was noted on the magnetic chip detector. When the integrated drive generator was removed in preparation for transportation of the engine, a fractured oil supply line that runs from the aft side of accessory gearbox behind the integrated drive generator was observed (Photo 1). The No. 2 engine, engine serial number 695440, was removed and shipped to GE for disassembly and examination.

Photo 1: Fractured Gearbox Oil Line

DETAILED EXAMINATION OF THE NO. 2 ENGINE – ESN 695440

The Powerplant Group comprised of members from GE, FAA, Mas Air Cargo, and the NTSB convened at the GE Evendale facility from February 22-26, 2021, to perform a detailed examination of the incident engine, engine serial number 695440. During the pre-disassembly/induction borescope inspection: 1) no damage was noted to the combustor section, 2) all the high pressure turbine stages 1 blades were present, and no impact damage was noted but many exhibited thermal distress consistent with mid-span core burn-through, 3) first indications of event related turbine damage or distress was to the leading edge of the low pressure turbine stage 1 blades, and 4) damage was observed throughout all five stages of the low pressure turbine rotor.

Each stage of the low pressure turbine exhibited combinations of the following damage to various degrees: 1) significant amount of leading and trailing edge blade and nozzle segment airfoil impact damage, material loss, and thermal distress except for the stage 1 nozzle segments that only exhibited minor thermal distress and minor trailing edge impact damage, 2) all blade roots were installed in their respective disk and all were fractured transversely across the airfoil, 3) blade outer shroud segments were gouged, the honeycomb worn down to the backing strip, and were thermally distressed, 4) disk rub and contact marks, 5) the rotating interstage air seal knife edges contact rub and material loss, 6) stationary honeycomb interstage air seal land trenching, gouging, material loss, and attachment bolt contact rub, and 7) consistent throughout each stage was evidence of contact between rotating and stationary hardware.

With all the impact damage, contact rub, and thermal distress to each of the low pressure turbine stages observed, a few items were of particular interest. One stage 5 nozzle segment, labelled as the No. 3 segment, was missing two complete adjacent airfoils labeled airfoil Nos. 5 and 6; each stage 5 nozzle segment is comprised of 6 individual airfoils (Photo 2). No other nozzle segments from any other low pressure turbine stage were missing airfoils. Although all the low pressure turbine blades were fractured transversely across the airfoil at various lengths, none were fractured at or close to their platform and all the blade roots remained in the disk. The low pressure turbine case exhibited numerous outward impacts, bulges, tears, and a small hole in line with the stage 4 blades, none appeared to be a penetration hole where low pressure turbine debris passed through it.

Photo 2: LPT Stage 5 Nozzle Segment Missing Airfoil

METALLURGICAL EXAMINATION

GE evaluated the material condition of the low pressure turbine hardware, the turbine rear frame, and the fractured oil supply line at their metallurgical laboratory in Evendale, Ohio while Boeing evaluated the material condition of the exhaust hardware, which included the turbine exhaust sleeve and the exhaust plug, at their Boeing Equipment Quality Analysis laboratory in Seattle Washington.

The GE metallurgical analysis found the cracks and tears in the turbine rear frame and the turbine exhaust were due to overstress and no anomalies or pre-existing defects were noted that would have contributed to their condition. The fracture surfaces of the oil supply tube exhibited plastic deformation and a planar fracture consistent with secondary high amplitude fatigue/cyclic tensile loading. None of the low pressure turbine blade fracture surfaces showed signs of fatigue but features consistent with an overload failure. Signs consistent with high cycle fatigue growth were found on the fracture surfaces of airfoil Nos. 5 and 6 of the low pressure turbine stage 5 nozzle segment No. 3; the fatigue cracks propagated from both the leading and trailing edges of the missing airfoils (Photos 3 and 4). In the interpretable areas (not damaged area), GE found no material anomalies in the base material, aside from the areas of intergranular oxidation.

Photo 3: Stage 5 Nozzle Airfoil Fatigue Fractures – Top View – Yellow Arrows Show Fatigue Propagation

Photo 4: Fatigue Fractures – Airfoil View

Low pressure turbine stage 5 nozzle segment No. 3, part number 9367M85P10, serial number HCM21194, had an extensive overhaul repair conducted in 2014 and has been in operational service for 10,232 hours and 4,068 cycles since the overhaul repair was performed. at the time of the event. Due to the leading and trailing edge impact damage on airfoils Nos. 5 and 6 of the low pressure turbine stage 5 nozzle segment No. 3 at the fracture locations, no determination could be positively made on whether a repair had been previously performed in that area, so no assessment of the repair conformity was made.

The Boeing metallurgical analysis of the turbine exhaust sleeve revealed that all the fractures were composed entirely of ductile separation fracture mode, consistent with a single event, an...