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When Croft (Peter Firth) and Captain Sinclair (Christopher Plummer) inspect an S.E. 5 aircraft, the latter says, "We had pups before." "We are damn lucky to get these." The "pups" is a reference to the Sopwith Pup fighter aircraft. It entered service in autumn 1916 and had been mostly withdrawn from service by the time the movie takes place, in October 1917.
In World War I, attacks on balloons were dangerous because machine guns and anti-aircraft guns formed a protective circle beneath the balloons. Therefore, pilots dove at a steep angle when they attacked. They did not attack horizontally.
This movie is an adaptation of the play "Journey's End" by R.C. Sherriff, written just after World War I. The setting is transplanted from the trenches to an airfield, but most of the characters and some incidents are almost unchanged.
At this point in his career Peter Firth was often likened to the decade-older Malcolm McDowell, especially when he appeared in Equus (1977).
The scene with the balloon observer jumping out with a parachute is re-used footage from The Blue Max (1966).
The red German biplane is a Finnish "Valmet Viima". It first flew in 1935.