Complex sentence: The RAF pilots carried out their final operational flights in November, 1941; by that time, the immediate crisis on the front had passed and the British handed their aircraft and equipment over to the Soviet Air Force and returned to Britain.
Split into simpler sentences: The RAF pilots carried out their final operational flights in November 1941 by which time the immediate crisis on the front had passed. At that point, they handed their aircraft and equipment over to the Soviet Air Force and returned to Britain.