The UK Ministry of Defence has published the final inquiry into the fatal Royal Navy Merlin HC4 ditching off Dorset, concluding that a fuel-system failure and a sequence of mistaken cockpit actions led to the loss of all three engines on the AW101 operating with HMS Queen Elizabeth. The helicopter, tail number ZJ135 from 846 Naval Air Squadron, ditched during night deck landing training in September 2024, killing one service person and injuring two others.
The report identified a split diaphragm in the pressure drop regulator of engine two’s high-pressure fuel pump assembly as the initiating fault. Excess fuel flow made that engine run abnormally fast, while a red engine-failure warning and falling torque indications on engines one and three led the crew to believe those engines had failed. They manually shut down engines one and three, and engine two then reached overspeed protection and also switched off.
The inquiry criticised the warning system for not clearly distinguishing between an engine producing excess power and one losing power, and found that most aircrew did not understand that limitation. It also highlighted siloed crew actions and issued 44 recommendations, including changes to cockpit warning procedures, maintenance oversight and training for power-related emergencies.