The 12-day flight across the world in a 60-year-old plane

"It's a massive adventure," says Vicky Auld.

Deputy chief pilot for the British Antarctic Survey (BAS), Auld is preparing for the organisation's annual autumn ferry flight from Canada to Antarctica. Months in the planning, this will be an epic 55-hour journey over 12 days, covering some 13,700km (8,500 miles) as the crow flies. This year, Auld will be piloting BAS' relatively luxurious and spacious four-engine de Havilland Dash-7, but most of her pilot colleagues will be heading south in 40-year-old twin-engine aircraft with unpressurised cabins.

Designed in the 1960s and still in production today, de Havilland Twin Otters have been described by some as the Land Rover of the skies. "They're overengineered, adaptable and rugged, designed for bush flying," says Auld. "Whether it's on skis, floats or big tundra tyres, with their short take-off and landing you can get into places that you just couldn't think about with other aircraft."

In Antarctica, BAS uses Twin Otters for everything from airborne scientific research to delivering fuel, supplies and field parties to remote locations. "It's unique," Auld says. "I can land at places where no-one has ever landed before."

Although they are fitted with modern turboprop engines, Twin Otters have certainly got a vintage look about them with clearly visible struts, wires and rivets. "You could say it is fly-by-wire, but the wires connect directly from the control column to the control surfaces on the wings and tail," says Dan Beeden, BAS aircraft operations manager who is responsible for ferry flight logistics.

We are sitting in the cockpit of "Ice Cold Katy", named after one of the UK's original Antarctic research planes (a single-engine Auster). The gleaming red aircraft is parked on the apron at the Imperial War Museum Duxford and doesn't look out of place in its position alongside a World War Two-era B-17 Flying Fortress. BAS uses the museum's airfield as their summer base as it's close to their Cambridge headquarters and has well-equipped maintenance facilities.

Although Antarctica is our focus, we also do a lot of work in other parts of the world – Dan Beeden

The cockpit is cramped and narrow, fitted with a mix of old and new technology. The control column is similar in appearance to those inside many of the museum pieces around us, and the main engine controls are hefty levers hanging from the ceiling. But most of the dials have been replaced by modern "glass cockpit" electronic screens, and the aircraft has also been upgraded with weather radar and an autopilot. And while that might make this Twin Otter easier to fly, it doesn't make the flights any more comfortable.

"In terms of toilet facilities, we don't really have any," Beeden explains. "There is just a tube at the back of the aircraft." Neither is there a galley, air conditioning or enough room to even stand up. "We do have cabin heaters that will keep the cockpit nice and warm, and the top of the aeroplane is painted black which allows snow to melt more easily, but that means it can get very hot here in summer."

Several airlines use Twin Otters in remote parts of the world – in the UK, for instance, Loganair operate them to connect Scottish islands – and in a commercial configuration the main cabin can hold up to 20 passengers. In the case of Ice Cold Katy, half this space is currently taken up with a giant additional fuel tank for her forthcoming flight across the Atlantic. The aircraft will gain a further extra tank in Canada for the journey to Antarctica. But before we get onto the perils of the ferry flight itself, there is an obvious question to put to Beeden: why do they need to make these ferry flights at all?

"I think people are surprised because the aeroplane says British Antarctic Survey on the side," says Beeden. "Although Antarctica is our focus, we also do a lot of work in other parts of the world." In recent years the Twin Otters have supported research in Iceland, Greenland, Bolivia and Brazil.

Maintaining the aircraft in Antarctica during the winter is off the table because of the limited facilities at BAS's Rothera research station – the main base of operations on the Antarctic Peninsula. "The hangar in Antarctica can only accommodate four aircraft and we've got a total of five," Beeden explains. "Also, the hangar is not heated so, during the Antarctic winter if the aircraft were there, you would have people having to work in an unheated hangar in perpetual darkness."

Every year then, the fleet ends up in Canada – which turns out to be the best country to go to for all your Twin Otter needs. "They are built in Canada, there's a lot of them operated in Canada, and therefore there is a big infrastructure of companies who can provide that expert maintenance and engineering support for the aircraft," says Beeden.

But whereas a flight in a large commercial aircraft from North to South America would be relatively straightforward – and could even be done these days in a single hop – the Twin Otter flights typically involve flying into at least 12 different airports from Canada down to Chile. For an aircraft that flies at only 150 knots (around 170 mph/274km/h) with a normal operating ceiling of 10,000ft (3km), the journey presents Beeden with a unique set of challenges.

"There are just lots of plates spinning all the time, and you're constantly looking at what is the best thing to do, or in some cases, what is the least bad thing to do," he says.

As a new pilot you have 12 days ahead of you where you fly into a new airport every day, with new approaches, new departures, new taxiways – Vicky Auld

"You have to take account of obvious things like how far can the aeroplane fly in one go and what contingencies do you have to have in place for a diversion," he adds. "There are also some countries in Central and South America that we will not go to now because the security and the political situation is just too volatile."

Each aircraft is flown by a single pilot, but they are always accompanied by an engineer or another member of BAS staff, such as Beeden, for extra safety. Auld has been flying the ferry flights for more than 10 years and still finds them exciting. "But I do remember feeling incredibly apprehensive the first time I was going to do it," she admits.

"As a new pilot you have 12 days ahead of you where you fly into a new airport every day, with new approaches, new departures, new taxiways," she says. "The Americans talk really fast at you. In Central America you can hardly understand [air traffic control], but then on top of that, you're in a Twin Otter unpressurised flying for eight hours with a massive amount of fuel in two fuel tanks behind you."

These are certainly riskier flights than your average long haul and are specially certified accordingly. "We're taking off with a weight at take-off that if we lose an engine we will basically be going down," says Auld. "We train for these events in a simulator and, if we can't turn around, the other person on board will be able to dump all the fuel."

They also have to contend with weather – at 10,000ft Twin Otters tend to go through it, rather than over it.

"You could be taking off on icy runways in snowstorms, then once you're into the Gulf of Mexico you're into hurricane season, so you're watching out for hurricanes and tropical storm warnings before you fly," says Auld. "You usually see a pretty decent thunderstorm or two to avoid on the way…then when you start going down the western side of the Andes, there's quite often fog." Finally, when they cross the Andes to Punta Arenas they have to cope with strong headwinds and need to climb to 18,000ft (5.4km), and wear oxygen masks to avoid passing out.

But the most potentially dangerous part of the journey comes on the final day and the 600-mile (966km) stretch of sea between the tip of South America and the Antarctic continent. The crew wear bulky immersion suits and life jackets in case they need to ditch in the Southern Ocean – notoriously, the roughest waters in the world – and, along the way, there is what is known as a "point of safe return".

"There is a certain point beyond which you don't really have any other options – you're just carrying on straight to Rothera and you deal with what emergencies you have on the way as best you can."

Every year the months of maintenance, planning, preparation and training pay off when the Twin Otters touch down on the runway at Rothera.

"It's just the best and most challenging flying you can do," says Auld, smiling. "But then once you get to Antarctica you've got more challenges ahead of you!"

-BBC