B763, en-route, south of Cork Ireland, 2022
B763, en-route, south of Cork Ireland, 2022
On 6 April 2022, a Boeing 767-300 lost left engine oil pressure whilst eastbound and passing south of Cork and diverted to Shannon after declaring an emergency on account of intended engine shutdown. During the subsequent taxi in, a fire was observed and extinguished and the aircraft towed to the terminal after an initial fire service request for evacuation had been withdrawn. An engine oil leak from a chip detector which had been routinely inspected by a company engineer prior to departure but not reinstalled correctly was found to have caused the leak and thus the loss of oil pressure.
Description
On 6 April 2022, a Pratt & Whitney PW4060-engined Boeing 767-300 (N670UA) being operated by United Airlines on a scheduled overnight international passenger flight from Washington Dulles to Zurich with an augmenting crew member on board and in the cruise at FL 360 declared an emergency and diverted to Shannon when the crew were obliged to shut down the left engine due to loss of oil pressure. The diversion and night VMC approach and landing were uneventful but an apparent landing gear fire started when taxiing in. After this had been quickly confirmed as out, the aircraft initially remained stopped as a precaution for brake cooling purposes before being declared safe to move. It was then towed to its assigned parking position for a normal disembarkation.
Investigation
A Serious Incident Investigation was carried out by the Irish AAIU. Relevant data were obtained from the DFDR and the QAR but as the CVR was not isolated after the event, its data was overwritten. Recorded ATC data was also available.
It was noted that the 58 year-old Captain had a total of 24,500 hours flying experience which included 6,710 hours on type. The First Officer had total of 4,532 hours flying experience with the operator which included 3,581 hours on type and the International Relief Officer had a total of 3,180 hours flying experience with the operator which included 2,895 hours on type.
What Happened
It was established that the flight had initially climbed to FL 340 where it remained for over two hours before climbing to FL 360 which was reached after a little over 4 hours airborne. DFDR data showed that having recorded a constant left engine oil quantity for over an hour between approximately three and four hours after takeoff, the quantity then initially fluctuated before and during the climb to FL 360 and then began to steadily decrease. Around 5½ hours after takeoff, the recorded left engine oil quantity indication reached zero which, according to the OEM, actually meant that “there could be up to 8.5 L remaining in the tank”. Approximately 23 minutes later, the left engine oil pressure then began to fall quite quickly from 194psi which after 13 minutes initially triggered an intermittent low pressure warning at 138psi. After a further 10 minutes, continuous low pressure warning was annunciated at 69psi.
At this point, the aircraft was still at FL 360 and passing approximately 20nm south of the Irish coast. An emergency was declared to ATC requesting a diversion to Shannon because they were about to shut down an engine due to low oil pressure. This was approved and as descent was commenced, the left engine was then shut down in accordance with the applicable procedure with the aircraft passing FL 295 and some 52 nm from Shannon. Twenty minutes later a landing was made there on the 3,059 metre-long runway 24 with the airport RFFS in attendance.
The aircraft initially made a 180° turn at the end of the landing runway in order to reach an exit taxiway but as it did so, a brake temperature warning was annunciated. This was considered a possible consequence of the higher-than-normal braking energy required with reverse thrust only available from the right engine.
Initially the turn was completed and the backtrack was commenced but once the left turn onto the taxiway leading to the assigned parking position had been completed, the accompanying RFFS team, who were not in direct contact with the aircraft, called the GND controller to get the aircraft to stop immediately as its left main gear was on fire. When the aircraft then stopped, the RFFS applied fire-retardant foam which they subsequently reported had resulted in the visibility in the area reducing to zero as prevailing wind blew the efflux from the right engine and the APU towards their vehicles. The RFFS subsequently called the GND controller to say that “the foam being blown back seemed to be smoke” and therefore requested that the crew “evacuate the aircraft on the right-hand side” which was relayed as requested. When no immediate acknowledgement was received, the message was repeated and acknowledged. The controller then asked the RFFS to confirm that a fire had been observed and was told they could see fire on the left-hand side. However, “moments later, when the visibility had improved”, the RFFS asked the controller to cancel the evacuation request because the fire had been extinguished and this was relayed and acknowledged.
It was subsequently found that a brake temperature warning had been activated as the aircraft turned at the end of the runway. Only after the fire had been extinguished had the flight crew advised ATC that they had been subject to a procedural requirement for the aircraft to remain stationary for 60 minutes after landing to allow the brakes to cool. On being informed of this, the RFFS then monitored the aircraft with thermal imaging cameras whilst all occupants remained on board. Once the 60 minutes had elapsed, and with RFFS consent, the aircraft was towed to its assigned parking position for a normal disembarkation.
Why It Happened
An examination of the left hand engine found that one of the four Magnetic Chip Detectors (MCD) fitted to the lubrication and scavenge oil pump at the base of the engine to collect any magnetic particles present in the engine oil scavenge circuits (see the first illustration below), was found below its housing in an upright position in the engine cowling with evidence of oil in the surrounding area (see the second illustration below).
The location of MCDs at the base of the engine (typical installation). [Reproduced from the Official Report]
The left engine No 4 bearing MCD as found. [Reproduced from the Official Report]
Having received and examined the liberated MCD, the engine manufacturer outlined a possible sequence of events accompanied by its reasoning and an illustration of the MCD design (see below).
An annotated diagram of the MCD involved adapted from maintenance documentation. [Reproduced from the Official Report]
- The MCD probe was installed but not locked due to lack of spring force from the self-closing valve caused by debris, or because the MCD probe locking pins were on the crests of the locking slots (i.e. not in the locked position), with the Engine Manufacturer considering the former ‘most likely’. (The supporting evidence was debris embedded in housing i.e. debris was moving relative to the housing.)
- Engine vibration resulted in the MCD probe migrating within and then being liberated from the housing. (The supporting evidence was distress on the probe’s primary o-ring and the elapsed time between MCD inspection and in-flight shutdown.)
- Debris in the valve caused it to remain partially open and permit oil leakage, once the probe was liberated. (Supporting evidence: DFDR data showed a progressively reducing left engine oil quantity and pressure during the event flight.)
They also noted that they were “unaware of any other oil loss events following the complete liberation of a MCD”.
The Investigation noted that United Airlines required an inspection of all the engine type MCDs to be carried out every 850 aircraft operating hours. This required the removal of each MCD probe, its inspection for contamination and an examination of the packing (o-ring) on the probe followed by re-installation. The flight involved was the first since the MCD inspection task had been carried out prior to departure from Washington and the required documentation for completion of the check with no findings was available. It was also noted that the AMM MCD probe installation procedure included a physical check to ensure correct installation by pushing in on the probe to verify that spring pressure pushes the probe back into the locking slot and to then pull the MCD probe to confirm it is locked. It was further noted that installation also requires a visual check of the alignment marks. However, although MCD checks such as the one involved were supposed to be carried out by a technician and then signed off by an Inspector, the one prior to this flight had been carried out and signed off only by an Inspector.
As context for United Airlines’ maintenance procedure relative to MCD inspection, it was noted that the FAA had granted the operator permission to make minor changes to the AMM and other instructions for continued airworthiness “based on operational experience and engineering judgment” which affected the way MCD inspections were carried out. The detail of the differences was relatively minor but not insignificant in terms of human factors issues.
The reported landing gear fire could not be definitely confirmed. Although the airport CCTV recording included “an amber-coloured flash at the aircraft’s left main landing gear” visible for approximately 15 seconds, no definite evidence of a fire was found when the landing gear was subsequently inspected. However, excess lubrication grease was found at several of the landing gear lubrication points and it was therefore considered at least possible that heat from the brake units had ignited either this or oil residue from the left engine.
The Probable Cause of the oil loss was formally documented as “migration of the No. 4 bearing Magnetic Chip Detector probe from its housing on the oil pump assembly on the left engine, followed by a loss of oil through the valve in the Magnetic Chip Detector housing, which remained partially open, due either to debris in the valve or as a result of a partially migrated probe”.
Five Contributory Factors were also identified as follows:
- The MCD probe was not in the installed and locked position.
- The Operator’s MCD maintenance procedures were not followed when maintenance was performed on the MCD probe prior to the occurrence flight.
- There was no red paint in the alignment grooves on the grip of the MCD probe or on the MCD housing (although) this was permitted by the Operator’s MCD inspection procedures.
- The presence of debris within the MCD housing.
- The MCD probe’s o-ring (primary seal) and Teflon® (secondary) seal were damaged.
Safety Action taken as a result of the Investigation findings was noted as having included the following:
United Airlines added a requirement to their AMM MCD Check procedure to “make sure that the red marks on the grip are aligned with the red marks on the MCD housing” and reminded its technicians and inspectors that “the red paint in the valve and probe alignment grooves may have diminished” and directed that in this case, it must be restored using suitable paint unless this is not immediately available in which case “it is acceptable to release for up to 20 flight hours without touchup”. The AMM was also amended to direct that “the alignment grooves must be used to ensure correct installation alignment".
Pratt and Whitney amended their Engine Manufacturer’s Maintenance Manual content on the Inspection of the Magnetic Chip Detectors to add a previously omitted reference to the need to inspect the non-replaceable Teflon® seal for damage and corrected an image of the MCD which was not representative of the MCDs fitted to the subject aircraft. A new requirement to inspect the inner surface of the valve body for packing debris and a corresponding illustration were also added, as was an illustration showing an example of debris within a valve with replacement of the MCD assembly mandated if debris is found. They also advised Boeing of this action and Boeing stated that corresponding amendments to the AMM would be made during the April 2024 revision cycle.
The Final Report was published on 29 February 2024. In view of action taken, it was considered that no Safety Recommendations were necessary.