What aircraft was involved and where did it happen?

The accident on 2024-07-23 involved a ATR 42-500, registration G-LMSA, at North-east of Aberdeen International Airport, GB.

What was the probable cause of the accident?

The primary cause of the engine failure was the degradation of the No 6 main bearing, leading to internal turbine damage. The subsequent fire was caused by fuel escaping from a loosened return line fitting and igniting on hot engine parts.

Engine Failure and Under-Cowling Fire on ATR 42-500

A commercial flight to Glasgow experienced a significant engine failure and subsequent fire due to a fuel leak caused by a loosened return line.

What happened

On 23 July 2024, an ATR 42-500 (registration G-LMSA) was performing a scheduled passenger flight from Sumburgh to Glasgow. While cruising at FL180 near Aberdeen, the crew heard a loud impact, followed by a sudden left yaw. The No 1 engine's torque dropped to nearly zero, and interstage turbine temperatures rose rapidly.

Shortly after the initial mechanical failure, a fire warning was triggered inside the No 1 engine cowlings. The crew immediately initiated emergency procedures, which included discharging both fire bottles to extinguish the flames. The pilot declared a MAYDAY and diverted to Aberdeen International Airport, landing safely without further incident. During the approach, the crew noted a low-pressure fuel warning on the No 2 engine, but prioritized the landing. Upon arrival, ground engineers discovered fuel leaking from the No 1 engine area.

The investigation

Investigators examined the engine and the aircraft's fuel system. The inspection of the No 1 engine revealed significant damage to the turbine assemblies. Detailed disassembly of the engine, conducted with the supervision of TSB Canada, identified that the No 6 main bearing had suffered severe degradation. This caused the high-pressure turbine assembly to lose its proper position, leading to contact with surrounding components.

Regarding the fire, investigators found that the fuel return line had become loose at the HMU outlet. The investigation established that while the fire handle had been pulled to shut off the primary fuel supply, fuel continued to leak through the motive flow valve due to its specific design. The leak only ceased once the crew performed the Single Engine Operation checklist, which involved turning off the electrical fuel pump and energizing the motive flow valve solenoid to fully close the valve.

Findings

The No 1 engine suffered a contained failure caused by the degradation of the No 6 main bearing, which led to the loss of turbine assembly alignment.
An under-cowling fire occurred because fuel leaked from a loosened connection and ignited on hot engine components.
The fuel return line connection became loose, potentially due to the mechanical forces of the engine failure causing the line to 'whip,' or due to improper torque during installation.
The fuel leak persisted despite the engine shutdown because the motive flow valve design allowed gravity-fed leakage until the electrical fuel pump was switched off.

Safety action

The aircraft manufacturer has initiated a safety review to identify potential actions to reduce the risk of fuel leakage from the fuel return line, with a target completion date by the end of 2025.