A split diaphragm in a helicopter engine was the starting point of a chain of failures that led to the death of a Royal Navy serviceman during a training exercise, according to a formal service inquiry published on 11 September 2026.
On 4 September 2024, three crew members were aboard the AW101 Merlin HC4 (tail number ZJ135) from 846 Naval Air Squadron, based at Royal Naval Air Station (RNAS) Yeovilton in Somerset, conducting night deck-landing training with HMS Queen Elizabeth. At approximately 20:49 local time, the aircraft ditched—a deliberate emergency landing on water—roughly 25 nautical miles south of Lyme Regis, Dorset.
Right-hand seat pilot Lieutenant Rhodri Leyshon, who was first publicly named by the Royal Navy on 6 September 2024, sustained serious injuries during the impact and was unable to escape as the aircraft entered the water. He was recovered about two hours after the ditching and pronounced dead in hospital. The other two crew members suffered only minor injuries.
How did the engine failure occur?
The diaphragm split in the number two engine's high-pressure fuel pump assembly at 20:46, according to aviation safety analysis. The rupture allowed excess fuel to flow into the engine, causing it to operate at a much faster speed than normal. However, the underlying cause ran deeper: the diaphragm had been overstressed during a process introduced to fix a separate manufacturing defect, creating a sequence of interconnected failures rather than a single isolated fault.
Why did the crew shut down working engines?
The crew members erroneously concluded that the number one and three engines had failed, likely due to inadequate warning systems and emergency diagnosis procedures. They manually switched off these two engines, which caused the remaining engine to reach its overspeed trip threshold and automatically deactivate. The loss of power from all three engines forced the aircraft to descend into the sea.
What other factors contributed to the outcome?
Investigators found that an incorrect setting on Lieutenant Leyshon's energy-absorbing seat reduced the protection it would have provided during impact. The inquiry also identified broader systemic issues beyond the mechanical failure: deficiencies in cockpit warning systems, emergency diagnosis training, and aircrew decision-making procedures all played a role in the sequence of events that led to the ditching.
What does the inquiry recommend?
The Defence Safety Authority's formal service inquiry panel made 44 recommendations to prevent similar accidents. These include an audit of Safran's UK maintenance and design approvals, a comprehensive review of the Merlin cockpit warning system, and improved aircrew training specifically focused on power-related malfunctions. Air Marshal Alan Gillespie, director general of the Defence Safety Authority, urged all commands to consider these recommendations carefully.
On behalf of the Defence Safety Authority and the wider Defence Aviation community, I offer my sincere condolences to Lt Leyshon's family, friends and colleagues.
The sunken helicopter had been damaged beyond repair and was recovered from the seabed in two major sections on 12 and 23 September 2024.
What happens next?
The next step involves command-level review and implementation of the 44 recommendations issued by the inquiry panel. Organisations across the Royal Navy and wider Defence Aviation community are expected to assess how these recommendations apply to their operations and training protocols.
Key Facts
- The accident occurred on 4 September 2024 during night deck-landing training approximately 25 nautical miles south of Lyme Regis, Dorset
- A diaphragm split in the number two engine's high-pressure fuel pump assembly, causing excess fuel to enter the engine and triggering a cascade of crew errors
- The underlying cause was overstressing of the diaphragm during a repair process for a separate manufacturing defect, revealing a chain of failures in design and maintenance
- The Defence Safety Authority published its formal service inquiry report on 11 September 2026, containing 44 recommendations for improved warning systems, training, and cockpit procedures
- Two crew members survived with minor injuries; Lieutenant Rhodri Leyshon died in hospital after being recovered from the water
