A split diaphragm in an engine fuel component, coupled with a misdiagnosed emergency and the shutdown of two working engines, caused the fatal ditching of a Royal Navy Merlin in September 2024, according to a UK Defense Safety Authority (DSA) report published on September 11, 2026.
The AW101 Merlin HC4, ZJ135, of 846 Naval Air Squadron ditched off the Dorset coast at 20:49 local time on September 4, 2024, during night deck landing training with HMS Queen Elizabeth.
Lieutenant Rhodri Leyshon, the handling pilot, was seriously injured on impact and was unable to escape the aircraft. He was later pronounced dead in hospital. Two other crew members survived, sustaining only minor injuries.
Crew mistook runaway engine for two engine failures
At 20:46, the diaphragm split in the pressure drop regulator of the number two engine’s high-pressure fuel pump assembly, causing excess fuel to flow to the engine.
Engines one and three automatically reduced torque to keep rotor speed within limits. However, a red engine failure warning, combined with the falling torque indications on engines one and three, led the crew to conclude that both engines had failed.
The warning does not distinguish between an engine producing excess power and one losing power, and the report found that most aircrew did not understand that limitation.
The crew initially set up for a running landing on the carrier, a course of action that the inquiry panel said would almost certainly have succeeded.
Instead, amid what the panel described as siloed crew actions, the aircraft commander moved the condition switches for engines one and three to OFF, without the confirmation from a second crew member required by procedure.
The number two engine subsequently reached its overspeed limit and shut down. Attempts to restart the other two engines were unsuccessful, leaving the helicopter without power before it ditched.
Safran diaphragm procedure linked to fuel-system failure
The diaphragm had been weakened by a ‘setting-in’ procedure, introduced by Safran Helicopter Engines in November 2020 to work around batches of misshapen diaphragms. Safran supports the Merlin’s RTM322 engines.
According to the inquiry, approval of the procedure did not assess its effect on the pressure drop regulator itself, and no written record of the decision was retained.
Of 226 diaphragms suspected of having undergone the procedure, 117 were examined. Ten, including the two that later failed, showed damage capable of developing into a split.
The first known split occurred on October 31, 2023, on Merlin ZJ998 during ground refueling at Royal Naval Air Station Yeovilton.
Safran’s investigation into that event remained open when ZJ135 was lost, although the panel found both the company’s response and the Merlin Delivery Team’s risk assessment to have been appropriate.
The DSA noted that the setting-in procedure has since been stopped, with all affected diaphragms having been identified and managed.
Training currency and supervision also scrutinized
The inquiry also examined crew currency and squadron supervision.
Although the crew remained within regulatory limits for the sortie, Leyshon was not current for deck landings.
The aircraft commander was not current for night vision device deck landings and had failed to meet hours-based competency requirements in four of the six preceding months.
Supervisory oversight had also been reduced from two people to one.
Inquiry issues 44 recommendations after fatal ditching
The panel made 44 recommendations following the accident.
They include an audit of Safran’s UK maintenance and design approvals, a review of the Merlin cockpit warning system, and improved aircrew training for power-related malfunctions.
Investigators also found that an incorrect setting on Leyshon’s energy-absorbing seat reduced the level of protection it would have provided during the impact.