Continuity mistake: The film is supposed to take place in 1917, but several of the german soldiers our heroes encounter are armed with Bergman M18 sub-machineguns which didn't appear until a year later. To make things even worse all the guns comes with a magazine-type that didn't appear until the mid-thirties.
Continuity mistake: During one of the dog-fight scenes, the snow on the ground disappears and comes back several times.
Factual error: The aircraft are definitely post-WWI. Biggles is flying a De Havilland Tiger Moth, which appeared in 1930s. The German fighter is probably the PT-23 Stearman.