United Boeing 767 Engine Failure Over the Atlantic

On March 28, 2022, a United Airlines Boeing 767-300, registration N675UA, encountered a serious in-flight emergency while performing flight UA-134 from Newark, New Jersey, USA to Zurich, Switzerland. The aircraft, carrying 123 passengers and crew members, was cruising at Flight Level 370 (approximately 37,000 feet) over the Atlantic Ocean, about 570 nautical miles west of Shannon, Ireland, when the crew reported a failure in the right-hand engine, a Pratt & Whitney PW4060.

Sequence of Events:

Initial Incident:

At approximately 570 nautical miles west of Shannon, Ireland, while cruising at FL370, the flight crew detected a failure or damage in the right-hand engine. The failure, characterized by unusual noises and vibrations, triggered a series of alarms on the flight deck. The crew immediately assessed the situation and decided to reduce altitude, beginning a drift-down maneuver to FL250 (approximately 25,000 feet) to stabilize the aircraft and conserve fuel. Later, they descended further to FL200 (about 20,000 feet) to maintain safe flight levels for single-engine operation.

Decision to Divert:

Recognizing the seriousness of the engine failure and its potential impact on the aircraft’s ability to complete the transatlantic flight, the crew made the decision to divert to Shannon Airport (SNN) in Ireland, the nearest suitable airport for an emergency landing. Throughout the descent and diversion, the crew maintained continuous communication with air traffic control (ATC) and followed standard operating procedures for engine failure over oceanic routes.

The crew advised ATC that they would not require any inspection on the runway or taxiway upon landing and planned to taxi the aircraft off the runway to the apron immediately after touchdown. Emergency services at Shannon Airport were placed on standby in preparation for the aircraft’s arrival.

Safe Landing at Shannon:

After descending to lower altitudes and navigating towards Shannon, the aircraft approached runway 06 for landing. The crew executed a safe landing on the runway approximately 95 minutes after initially descending from FL370. Following the touchdown, the aircraft taxied off the runway without incident and proceeded to the apron, where emergency services and maintenance personnel were ready to inspect the aircraft.

Investigation and Preliminary Findings:

Irish AAIU Report:

The Irish Air Accident Investigation Unit (AAIU) launched an investigation into the incident to determine the cause of the engine failure and the extent of the damage. Initial inspections revealed that a fan blade had separated from the right-hand engine, causing substantial damage. Additionally, fragments of the broken fan blade were found embedded in the right-hand elevator, a crucial component of the aircraft’s horizontal stabilizer.

Detailed Analysis by the Engine Manufacturer:

The engine manufacturer, Pratt & Whitney, conducted a thorough examination of the failed engine and provided a detailed analysis of the sequence of events leading to the engine failure:

1. No.1 Bearing Distress: The investigation suggested that the No.1 bearing experienced distress due to spalling (a form of surface damage) in both the outer and likely inner race of the bearing. The exact cause of the spalling could not be determined from the available evidence.
2. Fracture of Oil Supply Tube: The No.1 oil supply tube, which lubricates the bearing, was found to have fractured due to high cycle fatigue. This fracture was attributed to vibrations occurring in its first fundamental frequency range, likely caused by bearing distress.
3. Inadequate Lubrication: Due to the fracture in the oil supply tube, the No.1 bearing did not receive adequate lubrication, leading to oil starvation and subsequent overheating of the bearing components.
4. Bearing Oil Starvation and Overheating: The lack of lubrication caused severe wear on the raceway and balls of the bearing, further deteriorating the component’s condition.
5. Aft Axial Shift of Low-Pressure Shaft: The oil starvation and overheating resulted in an aft axial shift of the low-pressure shaft within the engine, leading to rotor clashing—a condition where rotating components come into contact, generating excessive heat and vibration.
6. Multiple Engine Surges: The rotor clash and associated damage caused multiple engine surges, a rapid increase in pressure within the engine that can lead to further mechanical failure.
7. Impact of Released Turbine Blade: A turbine blade was released during the surging events and impacted the aircraft’s horizontal stabilizer, specifically the right-hand elevator. This resulted in structural damage to the stabilizer, with fragments of the blade embedded within it.
8. In-Flight Engine Shutdown: Following the engine surges and damage, the flight crew executed an in-flight shutdown of the right-hand engine, following standard procedures for handling such failures.

Visual Evidence:

A photograph taken by the AAIU during the post-landing inspection showed significant impact damage to the right-hand horizontal stabilizer caused by a fragment of the released turbine blade. This evidence corroborated the sequence of events leading to the engine failure.

Final Report and Ongoing Measures:

On May 8th, 2024, the AAIU released their final factual report on the incident. Although the report did not provide definitive conclusions, it included the findings from the engine manufacturer’s analysis, offering a clear sequence of events leading up to the engine failure.

Safety and Industry Implications:

This incident underscores the critical importance of engine maintenance and monitoring, particularly for high-cycle components such as bearings and oil supply tubes. It highlights how a failure in one component can cascade into multiple issues, leading to severe engine damage and compromising flight safety.

The investigation findings are expected to prompt further scrutiny of the design and maintenance practices for the Pratt & Whitney PW4060 engines. These findings may lead to revisions in maintenance procedures, more frequent inspections of high-stress components, and the potential redesign of vulnerable parts to prevent similar failures in the future.

The incident also demonstrates the proficiency of the flight crew in managing an emergency situation over a remote part of the Atlantic Ocean, making appropriate decisions to ensure the safety of all passengers and crew aboard the flight. The successful diversion and landing at Shannon exemplify the importance of adherence to standard operating procedures and effective communication with air traffic control during emergency scenarios.