Description    

On 23 July 2024, an ATR 42-600 (G-LMSA) operated by Loganair on a scheduled domestic passenger flight from Sumburgh to Glasgow as LM436 was in cruise at FL 180 when a contained left engine failure occurred. After shutdown, this was followed by a fuel fire within the same engine nacelle. A MAYDAY was declared, the corresponding emergency checklist actions were successfully performed and the flight diverted to Aberdeen. On the way, the captain determined that the declared emergency status could be downgraded to a ‘PAN’ and the flight landed just under half an hour after the engine failure.

Investigation 

A serious incident Investigation was carried out by the UK Air Accident Investigation Branch (AAIB). The cockpit voice recorder (CVR) and the flight data recorder (FDR) were removed from the aircraft and their data were successfully downloaded. Data from the quick-access recorder (QAR) were also available.

It was noted that the 40-year-old captain had a total of 2,700 hours flying experience, which included 210 hours on type. Corresponding experience information for the first officer was not recorded.

What Happened

The crew was operating their fourth sector (leg) of the day, and the captain was acting as pilot flying (PF). The captain reported that he heard, just over a half hour after takeoff, a sudden noise he described as a “big thump” or a “muffled bang,” and the aircraft yawed to the left. FDR data showed that the left engine torque had reduced from 90% to zero within one second as the interstage turbine temperature (ITT) rose and the aircraft began to sideslip to the left. A Master Warning and an ‘ENG1 OVER LIMIT’ message was annunciated almost immediately, and the Master Warning and an ENG 1 FIRE Warning followed 11 seconds later. The ‘ENG 1 FIRE OR SEVERE MECHANICAL DAMAGE’ checklist was actioned with both fire bottles discharged before the engine fire indication ceased.

In accordance with the checklist requirement to land as soon as possible, the first officer transmitted a MAYDAY whilst the captain began a descent. The captain alerted the cabin crew to the engine failure and informed them that he intended to divert to Aberdeen, which was approximately 24 nm southwest of the aircraft’s current position. Meanwhile, the first officer began to run the ‘SINGLE ENG OPERATION’ Checklist. Given the short distance to Aberdeen, it was decided the there was no need to crossfeed fuel and on contact with Aberdeen, the captain downgraded the emergency to a PAN and radar vectoring to runway 34 was commenced.

During the positioning to final approach, FUEL FEED LO PR annunciations for the remaining right engine appeared but since the remaining engine was running normally, no action was taken. The landing on runway 34 occurred 27 minutes after the engine had failed. Once the right engine had been shut down, a ground engineer reported to the crew that fuel was leaking from the vicinity of the failed left engine.

Why It Happened

Disassembly and an initial examination of the failed left engine (a Pratt & Whitney Canada PW127M), which had been installed on the aircraft only five months prior to its failure - found that its number six bearing had been subject to significant degradation. This had led to its high-pressure turbine assembly moving out of position, which had caused significant damage to both the turbines and their casings. The number six bearing has been sent for a more detailed examination by the engine manufacturer under the proxy supervision of the Canadian Transportation Safety Board Safety Board. Findings will be considered by the AAIB in the context of the established airworthiness and reliability processes of the engine manufacturer. If the findings of this work require it, an addendum to this otherwise final report will be issued.

Regarding the fuel leak from the left engine, it was noted that fuel is delivered to the engine via an electric fuel pump for engine starts. However, the engine feed jet pump takes over once sufficient fuel pressure is provided by the engine driven hydro-mechanical unit (HMU) via the motive flow valve, and the electric fuel pump is automatically shut down. Operation of the fire handle automatically closes the low pressure (LP) fuel valve and shuts off the fuel supply to the engine involved. However, the left engine fuel return line, which had been installed at the same time as the engine was fitted to the aircraft, was found to be loose and was leaking at its connection to the HMU. Post flight, the leak was stopped by re-tightening the connection and it was noted that “with the valve de-energised, a small flow from outlet to inlet was consistent with that observed and was to be expected due to the valve design." The observed leak rate was consistent with this flow rate and in flight, the leaked fuel would have been ignited by hot surface contact. This connection would normally have been secure and the transit fuel would have remained within the line. The aircraft manufacturer advised that they were not aware of any other in-service cases of the fuel return line becoming loose.

In flight, once the first officer began to run the Single Engine Operation checklist, the action to select the fuel pump push switch off would have stopped the flow of fuel into the return line. If fuel cross-feed had subsequently been necessary to maintain lateral balance, the failed engine’s fuel pump would have had to be switched back on, which would have restarted the leak for the duration of the required fuel transfer.

The narrative conclusion of the investigation was as follows:

The contained failure of the left engine was due to significant degradation of its number six main bearing which resulted in the rotating high-pressure turbine being no longer correctly located. This allowed the rotating parts to contact adjacent parts causing significant damage to the turbine stages.

There was an under-cowling fire which was due to leaking fuel igniting on hot engine parts and this was detected and extinguished by the crew using onboard systems - both fire bottles were discharged.

The fuel leak was from the connection between the HMU and the fuel return line which had become loose. The reason for this could have been either that the connection had not been tightened correctly when it was installed or that it had become loose due to the forces of the engine failure ‘whipping’ the fuel return line. Maintenance records, interviews with the maintenance staff, and indicator markings appeared to confirm that it had been tightened correctly and the aircraft manufacturer was not aware of any other cases where this connection had become loose.

Due to the fuel system design, specifically that of the motive flow valve, fuel continued to leak from the loosened fuel return line after the crew, as part of the Engine fire or Severe Mechanical Damage checklist, operated the fire handle to shut down the engine. It continued to leak until the crew selected the left engine electrical fuel pump off as part of the Single Engine Operation checklist, as it was this selection to off that fully closed the motive flow valve. The fuel leak would have recommenced if the crew had needed to cross feed fuel to maintain the lateral balance of the aircraft as this process requires the electrical fuel pump to be selected on, thereby allowing the motive flow valve solenoid operated needle valve to open which would then allow fuel to leak through it from the feeder tank and out of the loose connection.

Safety Action

Although the aircraft met certification requirements in place at the time of certification, and there have been no other similar events in the history of the aircraft, ATR has commenced a safety review to see if any possible safety action which would further reduce the likelihood of leakage from the fuel return line is feasible. They anticipate completing this review by the end of 2025.

The Final Report was published on 19 June 2025. No safety recommendations were made.

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