History of Flight
On January 29, 2021, at about 10:49 Pacific Standard Time, a Boeing 767-300, registration N363CM, operated by Aerotransportes Mas de Carga, S.A. de C.V. (Mas Air Cargo) as flight MMA6853, experienced a right (No. 2) engine fire during initial climb after takeoff from Los Angeles International Airport (LAX), California. The flight was a 14 Code of Federal Regulations Part 129 cargo flight destined for Mexico City International Airport, Mexico.
The flightcrew reported feeling vibrations as the landing gear retracted. A second gear cycle was attempted, but the vibration continued. Soon after, a strong burning odor was detected, and a right engine fire warning appeared on the Engine Indicating and Crew Alerting System (EICAS). The flightcrew declared an emergency, performed the Quick Reference Handbook engine fire procedures—including shutting down the affected engine and discharging one fire suppression bottle—and diverted to Ontario International Airport (ONT), Ontario, California. An uneventful single-engine overweight landing was executed with no reported injuries to any crew members.
On-Scene Examination
A Powerplant Group formed, including GE, Boeing, the FAA, the NTSB, and Mas Air Cargo. On-scene assistance was provided by Jett Pro Line Maintenance and ONT operations center personnel. The NTSB did not travel to ONT; instead, initial pictures and descriptions were provided by Jett Pro and ONT staff. Boeing field representatives arrived on January 30, 2021, to photograph and document damage. A GE representative later arrived to assist in engine removal for shipment to GE’s Evendale, Ohio facility for examination.
The airplane examination revealed thermal distress and fire damage inside the right engine core cowl and thrust reverser. The outer portion of the turbine exhaust nozzle was missing, as was the aft portion of the inner sleeve; the forward portion remained attached. The right engine strut/pylon outer skirt exhibited gouging and scratch marks. The airplane had impact marks on the underside of the right wing and flaps, vertical and horizontal stabilizers, and right cargo door, though some appeared pre-existing.
No signs of engine uncontainment were found, but there was low-grade thermal distress: sooting, melted cushion clamps, fire detector loop isolators, and wire sheathing. Fan blades showed no damage and rotated freely; the low-pressure turbine (LPT) rotated with the fan. Through the exhaust, damage was observed: LPT stage 5 blade roots were installed but all blades had transverse fractures, impact damage, tears, gouges, and missing material. Outer shroud segments had gouging and heavy rub. Nozzle segments had trailing edge impact damage, and the turbine rear frame had cracks, tears, and puncture holes, but no evidence of LPT debris penetrating the skin. The oil tank was low, and debris was found on the magnetic chip detector. A fractured oil supply line was observed behind the integrated drive generator.
Detailed Engine Examination
From February 22–26, 2021, the Powerplant Group examined the engine (serial number 695440) at GE Evendale. Pre-disassembly borescope inspection found: no damage to the combustor; all high-pressure turbine stage 1 blades present but with thermal distress consistent with mid-span core burn-through; first event-related turbine damage on LPT stage 1 blade leading edges; and damage throughout all five LPT stages.
Each LPT stage exhibited varying degrees of: impact damage, material loss, and thermal distress on blades and nozzles (except stage 1 nozzles, which had only minor thermal distress); blade roots installed but all airfoils fractured transversely; gouged outer shroud segments with honeycomb worn to backing strip; disk rub marks; interstage air seal knife edge contact rub and material loss; stationary honeycomb trenching and gouging; and evidence of rotating/stationary contact.
Notable: one stage 5 nozzle segment (No. 3, part number 9367M85P10, serial number HCM21194) was missing two adjacent airfoils (Nos. 5 and 6). No other nozzle segments were missing airfoils. All LPT blades were fractured transversely at various lengths, but none at the platform; all roots remained in the disk. The LPT case had outward impacts, bulges, tears, and a small hole near stage 4 blades, but no penetrations by debris.
Metallurgical Examination
GE evaluated LPT hardware, turbine rear frame, and the fractured oil supply line at its metallurgical laboratory. Boeing evaluated turbine exhaust sleeve and plug at its laboratory. GE found that cracks in the turbine rear frame and exhaust were due to overstress with no pre-existing defects. The oil supply tube fracture exhibited plastic deformation and planar fracture consistent with secondary high-amplitude fatigue/cyclic tensile loading. LPT blade fracture surfaces showed no fatigue but features consistent with overload. However, fracture surfaces of airfoils Nos. 5 and 6 on stage 5 nozzle segment No. 3 showed features consistent with high-cycle fatigue growth, with cracks propagating from leading and trailing edges. No material anomalies were found in the base material except intergranular oxidation in interpretable areas.
The stage 5 nozzle segment had undergone extensive overhaul repair in 2014 and had accumulated 10,232 hours and 4,068 cycles since then. Due to impact damage at the fracture locations on airfoils Nos. 5 and 6, no determination could be made whether a repair had been performed there, so repair conformity was not assessed.
Boeing found that all fractures in the turbine exhaust sleeve were ductile separation, consistent with a single event, with no other anomalies.
Rotor Deflection Analysis
GE performed a rotor imbalance and deflection analysis to estimate radial deflection for each LPT stage based on documented hardware condition. The analysis did not provide exact vibration levels due to unknowns and the vibration monitoring system's maximum of 5 cockpit units. However, it gave a general assessment of expected deflection as the engine decelerated. Results showed expected radial deflection several times greater than nominal blade running clearances at takeoff for each stage. The observed gouging and heavy honeycomb wear were consistent with loss of radial clearance predicted by the analysis. The analysis indicated that LPT stage 5 would experience the most deflection, occurring not at takeoff/climb rotor speed but as it decelerated from damage accumulation and the pilot's engine shutdown.