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A smile from Cdr Chris Knowles before his final flight
The Merlin returns to Albion at sunset
The trials team with their Merlin 1

Extensive trials extend range – or payload – of Royal Marines’ Merlins

Published: 28 Jul 2023

The wings of the Royal Marines can fly further – or carry more commandos into action – after extensive trials pushing their Merlin helicopters to the limit.

Extensive trials with commando assault ship HMS Albion over the past few weeks around the UK – in good weather and bad – mean Merlins could carry up to a dozen extra troops or, with extra fuel aboard, fly for another two hours, which mean operating much deeper into hostile territory if necessary.

The Merlin has been the ‘battlewagon’ of the Commando Helicopter Force since 2016, ferrying up to two dozen Royal Marines from ship to shore and around the battlefield, wherever the green berets deploy around the world. The helicopters also carry supplies, ammunition, guns, even vehicles where needed.

For nearly a decade, the ‘handbook’ for safe Merlin operations – such as weight limits, wind speeds, humidity, weather conditions – known as Ship Helicopter Operating Limits or SHOL have largely been based on data gathered using the helicopter’s predecessor, the venerable Sea King: older, smaller, lighter.

With the Royal Marines transitioning to new Littoral Response Groups – operating in northern Europe and the Middle East region – with an increased focus on raiding operations, the decision was taken to squeeze as much as possible out of the Merlin Mk4 the Yeovilton-based helicopter force operates.

Tests years in the planning – which drew upon expertise in the MOD’s Defence Equipment and Support organisation, science/tech firm QinetiQ, the Air and Space Warfare Centre Rotary Wing Test and Evaluation Squadron (RWTES), 846 Naval Air Squadron – were arranged on HMS Albion as she returned from her spring deployment to the Baltic.

It resulted in taking the Merlin into unexplored areas of the flight envelope and culminated with a series of aviation firsts.

Both the ship and the aircraft were fitted with bespoke instrumentation, allowing flight test engineers to see in real time the aircraft’s performance and handling characteristics.

The Merlin was also fitted with ballast which was incrementally increased to artificially increase the aircraft’s operating weight.

As the aircraft weight increases the amount of power required increases. Additionally, for certain wind conditions, the handling pilot will encounter turbulence caused by the ship’s superstructure.

The combination of power required and turbulence encountered increase the pilot’s workload and makes landings and departures more challenging – exacerbated in rough seas where the ship’s flight deck is pitching and rolling.


It is the trials team’s job to safely define the edges of the flight envelope such that front-line aircrew can safely recover the aircraft to the ship without excessive workload whilst remaining within the helicopter’s engine and transmission limits.

After ten days of testing, 350 deck landings were achieved including into wind landings ‘heavy’ across a range of headings relative to the ship’s position and aft facing landings, offering a 360-degree SHOL clearance.

And running landings – rolling to a stop on the deck rather than dropping down vertically – will provide a very effective profile for heavy, limited-power recoveries in low winds and taxi and castor data will allow the helicopter to reposition safely on deck upon landing.

Lieutenant Commander Tom Lofthouse, the detachment commander for the trials – and also the last person to land on Albion’s deck in a Commando Sea King - hailed the trials as “a game-changer for amphibious operations in constrained waters such as the Norwegian Fjords. We are delighted to be able to increase the capability of the Commando Helicopter Force and to support the growth of Littoral Readiness Groups by enabling more Royal Marines to fly further.”

As well as impacting future Merlin helicopter operations from Albion or her sister HMS Bulwark, the data from the trials is being pored over by experts to see whether the lessons learned can be transferred to other ships – and the submarine-hunting version of the Merlin, the Mk2 – to increase its range and lifting power too.

Lt Cdr Lofthouse continued: “The trial was the culmination of years of planning and preparation. Bringing lots of teams together to achieve success. From instrumentation to aircraft to ship to analysis, each has to be spot on to safely define the edges of the handling and performance envelope for each condition.

“Its success is down to the incredible individuals involved across a large number of teams.”

The trials also brought down the curtain on the extensive Royal Navy career of Commander Chris Knowles, Commanding Officer of Rotary Wing Test and Evaluation Squadron.

“My last flight involved test points at 16 tonnes corrected lift for Merlin, aft-facing landings and was preceded by a compassionate passenger move into HMS Raleigh to get a sailor to his wife’s hospital bed. I can’t think of a better way to finish,” he said.



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