What happened
On 17 March 2022, a Boeing 737-8AS, registration EI-ENF, was taxiing to a gate at Manchester Airport following a routine landing on Runway 2D. During the taxi towards Terminal 3, the flight crew noted that maintaining standard taxi speeds required a thrust of 40% N1, which was higher than their usual operating parameters. Suspecting a flat tyre, the crew continued to the stand.
As the aircraft approached the gate, the crew became aware of a radio transmission requesting fire service assistance for an aircraft with brakes on fire. The crew immediately shut down the engines and followed parking procedures. The fire service arrived and extinguished a fire located around the left main landing gear. Because the fire was suppressed quickly, the crew determined that an emergency evacuation was not required, and the 167 passengers and 6 crew members disembarked without injury.
The investigation
Investigators examined the aircraft's quick access recorder, which confirmed that the approach, landing, and braking were all performed normally, with taxiing speeds around corners remaining below 10 knots. CCTV footage captured the fire igniting as the aircraft turned left from the apron toward the gate.
Post-event inspections by engineering personnel revealed that the inner hub of the left main wheel had fractured. This structural failure caused the wheel to misalign, which in turn damaged the brake piston. This damage resulted in a leak of hydraulic fluid. The heat produced by the misaligned wheel during taxiing was sufficient to ignite the escaping fluid.
Detailed examination of the wheel and brake components by the manufacturer identified multiple fractures in the inboard wheel hub, including a circumferential crack and four radial fractures. While most of the damage was consistent with an overload, investigators found a fatigue crack originating from a corrosion pit on the lead-in chamfer of the inner wheel half bearing bore. Other fatigue cracks stemming from corrosion pits were also present in the bore.
Findings
- The fire was caused by the combustion of hydraulic fluid ignited by heat from a misaligned wheel.
- The wheel misalignment resulted from a fractured left main gear wheel hub.
- The primary cause of the hub failure was a fatigue crack that originated from a corrosion pit located on the bearing bore's lead-in chamfer.
Safety action
- The wheel manufacturer has developed a new ultrasonic inspection technique to detect cracks in the internal bore of the wheel hub.
- The operator has implemented this ultrasonic inspection into the maintenance programme for their fleet of Boeing 737-8AS aircraft.
- The manufacturer has committed to updating the component maintenance manual to include ultrasonic inspections of the inner wheel half hub lead-in chamfers.