For the first hour after they crossed the coast, the men flew in sunshine, which sparkled on the icebergs floating far below.

However, things soon started go wrong. A bank of sea-fog wrapped itself around them, leaving Brown unable to calculate their position relative to the sun.

This forced him to rely on dead reckoning, a much less precise method which could see them blown off course without realising their error until it was too late.

Leaning forward, he began to send a message on their wireless transmitter, only to watch helplessly as a small part of the machine snapped suddenly and broke away. A piece of the exhaust had also fallen off, making the engines so deafening the men’s voices could not be heard.

Brown wrote in his notebook - “wireless generator smashed” - and showed it to Alcock instead. From now on they had no way of telling anyone they were still alive.

Brown made navigational calculations in his notebooks while flying Brown made navigational calculations in his notebooks while flying

After several hours of flying Alcock took the plane up, hoping to break free of the mass of cloud around them so Brown could calculate their position.

However, there was another layer of cloud above it.

This, at least, was thin enough for Brown to see Vega and the Pole Star shining. Using only these stars as a guide, he was able to fix their position.

“The level of navigational skills shown was extraordinary,” says Kevin Glynn, a filmmaker who has made a documentary about the flight.

“What Brown had done was to work out his calculations in advance. He did the maths on land, then tried to get confirmation he was right while in the air.

“If you want to get from A to B you plot your route, then when you are on the way you want to know where you are to see if you are getting it right.

“He was doing complex calculus in his head.”

Brown had calculated they had flown 850 nautical miles, short of halfway.

He sat in silence as around them the cloud formed monstrous, unreal shapes in the darkness.

As dawn broke, the wind picked up. The Vimy shuddered. Then it stalled.

Spinning down inside the cloud, unable to see the horizon, the men lost all sense of balance. Brown later recalled thoughts chasing each other through his mind. If they survived a water landing, they could drown. The odds were against a ship being close enough to rescue them.

Bursting out of the cloud, the sea seemed to be standing sideways to them, so close they could almost reach out and touch it. Brown could taste the salt spray on his lips.

But Alcock was not going to be beaten. To him, this race was the culmination of a decade’s preparation, since he first moved to Brooklands.

A quick glance at the horizon and he regained his balance, centralised the steering wheel, opened the throttles and the big Rolls-Royce engines thundered into life again.

The plane climbed up, away from the ocean. Now sleet and hail slashed across them, soaking them and covering the plane in ice.

After a few hours of this, hunkered down in the cockpit, Brown turned to check the petrol-flow gauge behind them, and realised it was clogged with snow.

He stood up in the cockpit and leaned over to clear it. The wind rushed at him, cold and violent.

Meanwhile, Alcock kept staring forward, wrestling with the plane to keep it level, unable to lift his hands from the controls for even a moment.

Still flying blind and trapped in silence with a broken wireless, Brown craved communication with the outside world. “The complete absence of such contact made it seem that nobody cared a dam [sic] about us,” Brown later wrote.

He roused himself to sort out some breakfast and was stowing away the remains of their meal when Alcock grabbed his shoulder. Brown turned and, following his friend’s outstretched hand, saw two specks of green beneath them – small islands, off the coast of Ireland.

The navigator put away his chart and calculations. His heart was light. His task was over.

The men were relieved to see the Irish coastline The men were relieved to see the Irish coastline

Flying low, they crossed the coast at 08:25 BST and circled, looking for a landing site. They had to be first on the ground to claim the £10,000 prize.

Near Clifden, County Galway, Alcock saw what looked like a flat meadow and brought the Vimy down, its wheels seeming to glide over the surface.

Brown indulged himself in thinking that their nervous flight had ended in a perfect landing. Then, with a squelch, the plane sank axle-deep, pivoting over its wheels, the nose crashing forward.

This was not a meadow. The first non-stop flight over the Atlantic ended face first in Derrygimla peat bog.

A permanent memorial lies on the bog where Alcock and Brown landed A permanent memorial lies on the bog where Alcock and Brown landed

People, some still in their pyjamas, ran towards them and found Alcock sitting in the cockpit throwing out its contents and asking, “Who’ll have an orange, direct from Newfoundland? Who’ll have some cigarettes?”

“Anybody hurt?” somebody asked the airmen. “No.” Brown had a bloody nose and mouth but nothing worse.

“Where are you from?” said another. “America,” Alcock replied. The crowd laughed, refusing to believe it.

Nervously, the two men asked about the rival crew, led by Admiral Kerr. Had they landed?

Their questions drew blank faces. Brown’s fortune teller had been right. The Atlantic had been crossed - all 1,890 miles of it - in 15 hours and 57 minutes.

Alcock and Brown were first.