Complex sentence: He joined the Royal Air Force at the outbreak of WWII, receiving a Distinguished Flying Cross for flying 37 missions during the war including one in which the turret gun he operated was badly damaged by flak and he was left hanging from the plane's fuselage.
Split into simpler sentences: He joined the Royal Air Force at the outbreak of WWII, receiving a Distinguished Flying Cross for flying 37 missions during the war. On one mission, the turret gun he operated was badly damaged by flak and he was left injured hanging from the plane's fuselage.