It was December 17, 1917 when Lieutenant James Sandy and Sergeant Henry Hughes climbed into their R.E.8 reconnaissance aircraft and took to the skies over the Western Front near Armentières. The weather was bad and there were patches of snow on the ground, but the pair had a job to do.

It's a story Dr Aaron Pegram, a senior historian at the Australian War Memorial, knows well.

"Sandy and Hughes were members of No. 3 Squadron Australian Flying Corps, which carried out photo-reconnaissance and artillery spotting sorties in support of the newly-formed Australian Corps in late 1917," he said.

"Equipped with the ungainly twin-seater reconnaissance aircraft, the Royal Aircraft Factory R.E.8 (or 'Harry Tate' as it was known to airmen and ground crews of the time), they performed a very important role in flying over no man's land between British and German lines, observing the fall of artillery, making sure rounds were landing where they should be, and photographing the German positions so they could be used to create trench maps.

"This sort of work was exceptionally dangerous. The R.E.8 was a slower, less nimble aircraft than the single-seater fighters, or "scouts" as they were called, and was not built for aerial combat.

"Its job was to fly over German lines, turn around, and fly back the other way - and then repeat the process until they had completed mapping or observing the corps front ... it was a crucial role in the air war during the First World War.

"The pilot had one forward-firing Vickers machine-gun, and the observer in the back was equipped with two ring-mounted Lewis guns to defend the aircraft from the rear, but the job of these airmen was to avoid getting into scraps with the Germans and to monitor what was happening on the ground."

Sandy, 32, was one of No 3. Squadron's more experienced airmen, while his observer, Hughes, who was four years younger, was making his first operational flight. The pair was flying over the battlefield, ranging an eight-inch howitzer for the 151 British Siege Battery, when they were attacked by six German Albatros D.Va scouts.

They were heavily outnumbered, and the fight should have been over quickly. Watched by hundreds of men on the ground, Sandy and Hughes managed to defend themselves for several minutes. With Hughes manning a Lewis machine-gun, and Sandy controlling the forward-firing Vickers machine-gun, they managed to bring down the German aircraft, forcing it to land in front of one of the 21^st Battalion's dugouts. Australian troops on the ground quickly scrambled out of the dugout and the German pilot was captured.

Seeing that Sandy and Hughes were still outnumbered and in trouble, two other R.E.8s from the squadron came to their aid. The two observation planes managed to cover each other and combine their defensive fire. As a third R.E.8 flew to join the firefight, the German formation broke off and flew back to their lines. One of the Australian pilots then flew close to Sandy and Hughes, and, seeing that their aircraft was cruising normally, assumed the pair was going to continue with their artillery work.

Amanda New, an assistant curator at the memorial, picks up the story.

"All appeared to be well," she said.

"However by 6pm, the men still hadn't returned to base. Despite No. 3 Squadron sending out numerous telephone messages, no trace of the men or their aircraft could be found. It wasn't until 24 hours later that the mystery of their disappearance was solved.

"A telegram was received from No. 12 Stationary Hospital at Saint Pol, stating that the bodies of Sandy and Hughes had been found in a crashed R.E.8 in a neighbouring field.

"In what must have been the last seconds of the fight, a bullet fired from behind the R.E.8 struck Hughes in the left lung and then passed on to strike Sandy in the base of the skull, killing them both instantly. The R.E.8, with its flight controls apparently in a neutral position and the throttle high, settled into a series of wide left-hand circles until the petrol ran out and the engine stopped."

Without engine power, the R.E.8 came down in a steep glide and landed, unseen, about 80km from the scene of the air battle. Those who examined the scene believe it would have taken about two hours for the R.E.8 to complete its remarkable flight. There were no further injuries to Sandy and Hughes. The two men were buried in the Saint Pol Communal Cemetery Extension.

While they were missing, Sandy was recommended for the Military Cross, and Hughes for the Distinguished Conduct Medal - but the recommendations could not be posthumously approved.

The German Albatros D.Va they forced down was salvaged, and is now part of the national collection at the memorial. Its pilot, Leutnant Rudolf Clauss of Jasta 29 was a relatively new pilot who had been four times decorated for his actions on the ground as an artilleryman and as a pilot. Wounded in the leg, he was taken prisoner by the Australians who dressed his wounds before moving him to the rear.

His flying boots were left behind while the Australians were tending to his injuries, and are now in the memorial's collection, as is and a memorial plaque honouring Sandy.

Sandy was considered "one of the mainstays" of the squadron and his story was told in a Last Post Ceremony on the 100th anniversary of the R.E.8's remarkable flight.

"Manned aerial flight was in its infancy during this time, however the technology and its application took tremendous leaps and bounds during the First World War," Dr Pegram said.

"Soldiers of the Great War were still coming to terms with these relatively new machines and a completely revolutionary way of engaging the enemy from the air.

"For a single bullet to enter a moving aircraft and kill both the pilot and the observer, Sandy and Hughes were incredibly unlucky. So too was the German pilot, Clauss, whose aircraft was put out of action by a single bullet that punctured his fuel tank and ended up in his thigh.

"But that was the nature of the air war over the Western Front. Airmen from both sides during this period took enormous risks every time they took to the skies."