A Loganair Avions de Transport Regional ATR-42-600, registration G-LMSA performing flight LM-436 from Sumburgh,SC to Glasgow,SC (UK), was enroute at FL180 about 20nm north of Aberdeen,SC (UK) when the crew needed to shut the left hand engine (PW127M) down and decided to divert to Aberdeen for a safe landing on runway 34 about 25 minutes later.
The airline stated the aircraft made a precautionary diversion following a technical issue.
On Aug 2nd 2024 the AAIB reported that the aircraft suffered a contained engine failure that however resulted in an engine fire. The fire was extinguished after discharging both fire bottles. The occurrence was rated a serious incident resulting in minor damage and is being investigated.
On Jun 19th 2025 the AAIB released their final bulletin concluding the probable causes of the serious incident were:
The No 1 engine suffered a contained failure. This was due to significant degradation of its No 6 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. It 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 No 1 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.
The AAIB analysed:
Engine
The disassembly of the engine identified that the No 6 bearing had suffered significant degradation resulting in the loss of location of the high-pressure turbine assembly and the damage seen to the turbines and their casings.
The No 6 bearing has been sent for more detailed examination and the results of this work will be used to inform the established airworthiness and reliability processes of the engine manufacturer. If the findings of this work require it, an addendum to this report will be issued.
Motive flow valve
Testing of the motive flow valve confirmed it was operating normally and that, with the valve de-energised, a small flow from outlet to inlet is expected due to the valve design. The leak from the loose fuel return line connector to the HMU was consistent with this flow rate. In normal conditions this connector would be secure, and the fuel would remain contained in the line. In this event as the connector was loose, the fuel leaked into the cowling area and then ignited.
During this event and following the engine failure, the crew followed the relevant procedure, Engine Fire or Severe Mechanical Damage, to shut down the engine.
Operating the fire handle closed the LP fuel valve and thereby shutoff the main fuel supply to the engine. Once the engine was shut down, there was no motive flow from the HMU, and the motive flow valve piston would drop closing off the valve. However, in this de-energised condition, fuel would still flow from the outlet to the inlet of the valve and leak through the loose fuel return line connection due to gravity as the fuel tank is higher than the loose connection.
It was only later in the crew procedures, and after the fire had been extinguished, the crew moved on to the Single Engine Operation checklist. This checklist requires the crew to select the fuel pump push switch off. This selection energised the motive flow valve solenoid causing it to fully close the motive flow valve and stop the flow of fuel from the loosened fuel return line.
The fuel flow would be stopped for the remainder of the flight providing the fuel pump switch remained off.
If the crew had subsequently needed to cross-feed fuel to maintain lateral balance, the fuel pump for the failed engine would need to be selected on to feed fuel from that tank. This selection would de-energise the motive flow valve and allow the leak to recommence. It would stop again when the electrical fuel pump press switch was selected off after the completion of fuel transfer.
The time taken from the fire warning until the electrical fuel pump being selected off was approximately five minutes.
The crew completed the required checklists in a timely and accurate manner and successfully extinguished the fire using both fire bottles at the required intervals. The crew remained unaware of the leaking fuel until after the aircraft was parked. Their prompt completion of the Single Engine Operation checklist ensured the fuel leak in flight was stemmed and prevented a potential escalation of the emergency. Due to the proximity of the diversion airport, fuel transfer was not required.
Later, when the aircraft was parked and electrical power removed, the motive flow valve solenoid became de-energised, and the leak recommenced. Maintenance staff were able to stem the leak by re-tightening the connection.
The motive flow valve will only be fully closed when it is energised following flight crew selection of the electric fuel pump to off. Should the crew need to cross-feed to maintain lateral balance the electric fuel pump would need to be selected on again and then motive flow valve would be no longer shut-off.
Fuel return line
The engine failure was violent as witnessed by the rapid reduction in torque and tears in the elastomer anti-vibration mounts and it is considered possible that the reaction to this sudden reduction in torque could have caused the fuel return line to whip and loosen the connection and for consequent engine vibrations to have loosened it further. There is no secondary locking on the connection and the line leaves the HMU via a 90° elbow which would add leverage to any loosening forces.
It is also possible that the connection was not correctly torqued during installation, but maintenance records indicated it had been and there were no reports of leakage before the event. The operator had also conducted a check of its fleet of similar aircraft and found all the fuel return lines had been correctly installed.
The design of the fuel return line installation complied with the certification standards in place at the time of certification. However, as was seen in this event, loosening of the fuel return line was not identified as a risk as the integrity of the line was assured by the use of fire-resistant materials, the installation and associated maintenance procedures.
The aircraft manufacturer confirmed that they were not aware of any events where the motive flow fuel line had become loose and leaked. Interviews with maintenance staff, checks of maintenance documentation and inspection of the connection, confirmed that it was likely this connection had been correctly installed and tightened when the engine was installed in the aircraft in February 2024.
The airline stated the aircraft made a precautionary diversion following a technical issue.
On Aug 2nd 2024 the AAIB reported that the aircraft suffered a contained engine failure that however resulted in an engine fire. The fire was extinguished after discharging both fire bottles. The occurrence was rated a serious incident resulting in minor damage and is being investigated.
On Jun 19th 2025 the AAIB released their final bulletin concluding the probable causes of the serious incident were:
The No 1 engine suffered a contained failure. This was due to significant degradation of its No 6 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. It 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 No 1 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.
The AAIB analysed:
Engine
The disassembly of the engine identified that the No 6 bearing had suffered significant degradation resulting in the loss of location of the high-pressure turbine assembly and the damage seen to the turbines and their casings.
The No 6 bearing has been sent for more detailed examination and the results of this work will be used to inform the established airworthiness and reliability processes of the engine manufacturer. If the findings of this work require it, an addendum to this report will be issued.
Motive flow valve
Testing of the motive flow valve confirmed it was operating normally and that, with the valve de-energised, a small flow from outlet to inlet is expected due to the valve design. The leak from the loose fuel return line connector to the HMU was consistent with this flow rate. In normal conditions this connector would be secure, and the fuel would remain contained in the line. In this event as the connector was loose, the fuel leaked into the cowling area and then ignited.
During this event and following the engine failure, the crew followed the relevant procedure, Engine Fire or Severe Mechanical Damage, to shut down the engine.
Operating the fire handle closed the LP fuel valve and thereby shutoff the main fuel supply to the engine. Once the engine was shut down, there was no motive flow from the HMU, and the motive flow valve piston would drop closing off the valve. However, in this de-energised condition, fuel would still flow from the outlet to the inlet of the valve and leak through the loose fuel return line connection due to gravity as the fuel tank is higher than the loose connection.
It was only later in the crew procedures, and after the fire had been extinguished, the crew moved on to the Single Engine Operation checklist. This checklist requires the crew to select the fuel pump push switch off. This selection energised the motive flow valve solenoid causing it to fully close the motive flow valve and stop the flow of fuel from the loosened fuel return line.
The fuel flow would be stopped for the remainder of the flight providing the fuel pump switch remained off.
If the crew had subsequently needed to cross-feed fuel to maintain lateral balance, the fuel pump for the failed engine would need to be selected on to feed fuel from that tank. This selection would de-energise the motive flow valve and allow the leak to recommence. It would stop again when the electrical fuel pump press switch was selected off after the completion of fuel transfer.
The time taken from the fire warning until the electrical fuel pump being selected off was approximately five minutes.
The crew completed the required checklists in a timely and accurate manner and successfully extinguished the fire using both fire bottles at the required intervals. The crew remained unaware of the leaking fuel until after the aircraft was parked. Their prompt completion of the Single Engine Operation checklist ensured the fuel leak in flight was stemmed and prevented a potential escalation of the emergency. Due to the proximity of the diversion airport, fuel transfer was not required.
Later, when the aircraft was parked and electrical power removed, the motive flow valve solenoid became de-energised, and the leak recommenced. Maintenance staff were able to stem the leak by re-tightening the connection.
The motive flow valve will only be fully closed when it is energised following flight crew selection of the electric fuel pump to off. Should the crew need to cross-feed to maintain lateral balance the electric fuel pump would need to be selected on again and then motive flow valve would be no longer shut-off.
Fuel return line
The engine failure was violent as witnessed by the rapid reduction in torque and tears in the elastomer anti-vibration mounts and it is considered possible that the reaction to this sudden reduction in torque could have caused the fuel return line to whip and loosen the connection and for consequent engine vibrations to have loosened it further. There is no secondary locking on the connection and the line leaves the HMU via a 90° elbow which would add leverage to any loosening forces.
It is also possible that the connection was not correctly torqued during installation, but maintenance records indicated it had been and there were no reports of leakage before the event. The operator had also conducted a check of its fleet of similar aircraft and found all the fuel return lines had been correctly installed.
The design of the fuel return line installation complied with the certification standards in place at the time of certification. However, as was seen in this event, loosening of the fuel return line was not identified as a risk as the integrity of the line was assured by the use of fire-resistant materials, the installation and associated maintenance procedures.
The aircraft manufacturer confirmed that they were not aware of any events where the motive flow fuel line had become loose and leaked. Interviews with maintenance staff, checks of maintenance documentation and inspection of the connection, confirmed that it was likely this connection had been correctly installed and tightened when the engine was installed in the aircraft in February 2024.
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