January 29th, 2021. A routine cargo flight, MMA6853, a Boeing 767-300 operated by Aerotransportes Mas de Carga, lifts off from Los Angeles International Airport, bound for Mexico City. Onboard, the flight crew, experienced professionals, guide the heavy jet into the California sky.
But something is wrong.
Almost immediately after the landing gear is retracted, a vibration begins. A strange, unsettling tremor running through the airframe. The crew cycles the gear again, hoping it’s a simple mechanical anomaly, perhaps a gear door not quite settled. The vibration persists. And then, a new sensation… an odor. A distinct, acrid smell of something burning.
Moments later, the chilling confirmation flashes on the cockpit displays: a right engine fire warning. Emergency declared. The Quick Reference Handbook is pulled, the familiar, urgent steps initiated. Shutting down the affected engine. Discharging the fire suppression bottle. The roar of one engine falls silent, replaced by the focused hum of the other. They turn, diverting towards Ontario International Airport for an emergency landing.
The landing, remarkably, is uneventful. An overweight landing, certainly, but controlled, bringing the damaged aircraft safely to a stop. No injuries to the crew. The immediate crisis averted. But the question remains: what happened?
The investigation began immediately. A Powerplant Group, a collaboration of experts from GE, Boeing, the FAA, the NTSB, and the operator, Mas Air Cargo, was formed. Initial assessments on the ramp at Ontario revealed the grim reality of the engine’s condition. Thermal distress, soot, and fire damage coated the inside of the right engine’s core cowl and thrust reverser. The entire outer portion of the turbine exhaust nozzle was gone, the inner sleeve mangled. Gouges scarred the engine strut and pylon.
Looking into the engine’s exhaust, the sheer extent of internal destruction became apparent. All the low pressure turbine stage 5 blade roots were still in place, but the blades themselves… fractured. Impact damage, tears, gouges, missing material. The outer shroud segments, meant to contain the blades, were heavily rubbed, the honeycomb worn down to the backing strip. The stage 5 nozzle segments, just upstream, showed similar impact damage. The turbine rear frame, the engine’s structural backbone in that section, was riddled with cracks and holes.
Crucially, when the integrated drive generator was removed, a fractured oil supply line was discovered, its break consistent with severe stress. This line, investigators realized, would have sprayed flammable oil onto the hot engine cases. The source of the fire.
The engine, serial number 695440, was carefully removed and shipped to GE’s facility in Evendale, Ohio for a meticulous disassembly and examination. This was where the true story of the failure began to unravel.
During the detailed inspection, damage was found throughout all five stages of the low pressure turbine. Blades fractured transversely. Shrouds gouged. Disks rubbed. Evidence of contact, violent contact, between rotating and stationary components. But one item stood out.
A single low pressure turbine stage 5 nozzle segment, designated No. 3, was missing two adjacent airfoils. Two of its six vanes were simply gone. Metallurgical analysis confirmed the grim suspicion: these airfoils hadn’t just broken off. They had fractured due to high cycle fatigue. Tiny cracks, growing over time, from both the leading and trailing edges, until the material could no longer withstand the forces.
These liberated airfoils, investigators determined, were the initiating event. Traveling downstream in the airflow, they slammed into the stage 5 blades directly behind them. The impact shattered the blades, creating an immediate, severe imbalance in the low pressure turbine rotor.
Flight data recorder information confirmed this. Shortly after takeoff, during the climb, the low pressure turbine vibration began to climb. Within seconds, it hit the maximum recorded value of 5 cockpit units. The actual vibration was likely far higher, but the system couldn’t measure it.
GE engineers performed a rotor imbalance and deflection analysis. While they couldn’t pinpoint the exact vibration levels at any given moment, their analysis showed that the anticipated radial deflection of the low pressure turbine rotor, based on the observed damage, was several times greater than the normal running clearances between the blades and their shrouds. This explained the heavy gouging and wear found throughout the turbine stages. The blades were flailing, contacting the static structure.
The analysis also predicted that the stage 5, where the initial failure occurred, would experience the most deflection, and that this deflection would be greatest not at high takeoff speed, but as the engine decelerated following the accumulation of damage and the crew’s shutdown action.
The progressive failure of the low pressure rotor, its imbalance worsening as more blades fractured and material was lost, reached a critical point. The immense vibrational loads, far exceeding the engine’s design limits, caused the oil supply tube to fracture. Oil sprayed onto the superheated components within the engine nacelle, igniting a fire.
The National Transportation Safety Board’s determination was clear, a chain of events set in motion by a microscopic flaw. The probable cause of this incident was the fatigue fracture and liberation of two airfoils from a low pressure turbine stage 5 nozzle segment. This initial failure precipitated a cascade of destruction within the engine, leading to the catastrophic imbalance of the low pressure turbine rotor. The resulting extreme vibration loads ultimately fractured the oil supply tube, allowing oil to ignite and cause the undercowl fire.
This incident, while resulting in only minor damage to the aircraft itself and no injuries, serves as a stark reminder of the critical importance of every single component within a complex machine like a jet engine. A tiny fatigue crack, unseen, unheard, growing over thousands of hours of service, can, in an instant, trigger a cascade of failures, turning a routine flight into an emergency, and highlighting the continuous need for rigorous inspection, maintenance, and the relentless pursuit of understanding the subtle vulnerabilities within powerful systems.
