What happened
On 7 April 2022, a Boeing 767-322ER, registration N670UA, operating a scheduled commercial flight from Dulles International Airport to Zurich, declared an emergency while over the south coast of Ireland. The flight crew reported a significant drop in oil pressure in the No. 1 engine, necessitating a diversion to Shannon Airport.
Following the emergency landing, the aircraft taxied toward its parking stand under the escort of the Airport Fire and Rescue Service. During this maneuver, the fire service observed a fire at the left-hand undercarriage bogie. The aircraft was ordered to stop, and fire-retended foam was deployed. Due to the airflow from the running right-hand engine and the APU, the foam was blown back, creating a cloud that the fire service initially mistook for smoke, leading to a brief request for an emergency evacuation. The fire was quickly extinguished, and after the brakes cooled for 60 minutes, the passengers and crew disembarked without injury.
The investigation
The AAIU investigation focused on two distinct issues: the engine oil loss and the undercarriage fire. Investigators examined the engine's lubrication system and the maintenance history of the Magnetic Chip Detector (MCD) probes. The investigation also reviewed the condition of the landing gear and the presence of any flammable substances near the brake units.
Findings
Regarding the engine failure, the investigation established that the No. 1 engine experienced a near-total loss of oil. This was caused by the migration of the No. 4 bearing Magnetic Chip Detector probe from its housing, which left the valve in a partially open state and allowed oil to leak. This failure was linked to a recent maintenance task performed in the United States, where the probe was not properly installed or locked. Notably, the maintenance was performed by a single individual, bypassing the operator's required 'dual set of eyes' verification process. Additionally, the absence of red alignment paint on the probe and housing, which was permitted by the operator's specific procedures, may have contributed to the improper installation.
Regarding the undercarriage fire, investigators found no permanent damage to the landing gear. The fire was likely caused by the heat from the brake units igniting excess lubrication grease or residual oil from the leaking engine that had accumulated on the undercarriage.