It is two years since the UK sent the first Merlin helicopters nearly 4,000 miles (6,400km) to Afghanistan. It is too far to fly the RAF helicopter. So, how do they get there?
At double the weight of a double-decker bus and standing at roughly the height of a bungalow, Merlin helicopters are not easy to shift half way around the world.
They cannot simply be flown over, says pilot Flt Lt Patrick Hearne. They need too many fuel stops and there are issues with flying over certain countries.
"Physically it's possible, but diplomatically and logistically it's very difficult," he adds.
Merlins play a vital support role in Afghanistan, moving troops, equipment and food or medical supplies. Flt Lt Hearne, who has flown in Afghanistan, says the aircraft is "pushed to its limits".
But before a helicopter even reaches the front line, engineers and air crew face a major battle to get it there - a task that can take more than a week.
'No room for error'
The Merlin begins its journey from Oxfordshire's RAF Benson to Camp Bastion, the main UK base in Afghanistan, under its own steam. It's a quick 10-minute flight to nearby Brize Norton, the RAF's largest base - but there the simplicity ends.
Firstly, says Senior Aircraftsman Tristian Stay - one of the many engineers tasked with preparing the Merlin for the journey - the helicopter needs to be broken down so it can fit on board a C-17 Globemaster aircraft carrier.
Heavier parts, such as the rotor head and blades and the tail section, are removed and the wheels folded up.
The helicopter is fitted with stabilisers - effectively a cradle with small wheels - so it can be inched on to the carrier using a large winch. This delicate process takes up to three hours and half a dozen movement staff and engineers to complete.
"It's a very fine process because the Merlin is a very big helicopter. It pretty much fills the C-17," says Flt Lt Hearne, 33, from Leeds.
"When you bump these things it tends to cost a lot."
Once inside, the helicopter is secured to the floor of the carrier with giant chains.
Wing Cdr David Manning is in charge of the UK's fleet of seven C-17s, the RAF's biggest aircraft, which transport both equipment and personnel around the world to support operations.
"The last thing we want is it moving around in the air. There's no room for error at all," he says.
Merlins are the most challenging package to deliver, he says, when compared to transporting troops, freight or other aircraft, such as a Chinook helicopter.
Though larger than a Merlin, a Chinook can be broken down into smaller pieces.
And Wing Cdr Manning says looking after Merlins is an even more daunting prospect than carrying VIPs, which the C-17 did when Prime Minister David Cameron visited UK troops in Afghanistan last week.
But for his 340 air crew it is all part of the "bread and butter" of the "best job in the world", which can involve rescuing injured personnel and providing operational support to troops, says the 42-year-old pilot, from Chipping Norton.
The flight from Brize Norton to Camp Bastion - including a fuel stop in the Mediterranean or Middle East - takes roughly 12 hours.
But the hard work is far from over on arrival at the camp, where another team of specialist engineers is waiting.
"Taking the helicopter apart in this country is quite an enjoyable job - it's putting it together at the other end that's more stressful," says SAC Stay.
Flt Lt Hearne adds: "If you took a Ferrari to pieces on your kitchen table and transported it half-way round the world and then tried to rebuild it again, it might take a little longer to get it working again.
"Every Merlin helicopter has got hundreds of thousands of moving parts. They all have to work in perfect harmony for it to run smoothly. That takes time."
'Sense of satisfaction'
Once the Merlin has been rebuilt, the testing process begins. Only when crews are sure the helicopter is functioning correctly will it be allowed to fly on missions.
"We enjoy a sense of satisfaction when we know we're getting a helicopter out there to do some good," says SAC Stay, 20, from Cheltenham.
The Merlin's introduction to Afghanistan followed criticism of the previous government for failing to properly support troops. Critics had suggested helicopter shortages contributed to an increased casualty toll by forcing more personnel to travel by land littered with roadside bombs.
Flt Lt Hearne says while the Merlin's role in Afghanistan involves seemingly mundane tasks, such as carrying people, its presence can save lives.
The helicopters face formidable conditions in the hot, dusty, mountainous environment of Afghanistan.
"Any helicopter that goes to Afghanistan is pushed to the absolute limit," he adds.