Trivia: The idea of helicopters was not necessarily new in WW2, but effective designs were not made until after the war; there is evidence from some literary sources (one of them being a book named "German Secret Weapons of WW2" or something like that) that the Germans did indeed conduct experiments and designs on this vehicle type.
Trivia: When Mary goes undercover in the castle, her name is Maria Schenck. They are the middle names of Col Stauffenberg, the German officer played by Tom Cruise in Valkyrie, who tried to kill Hitler.
Answer: The way I interpret it is that "following" the raid on Nuremberg, the plane was then on its way to Egypt when it was shot down by the Germans, prompting the mission to rescue Carnaby.
raywest ★
The "saturation bombing of Nuremberg" was a separate raid which should have lured all the fighters away from the Austrian border. The "General's" plane, on its way to Crete, was shot down by a roving Messerschmitt.