The white flag is the classic sign of requesting surrender, ceasefire or parlay in military contexts, yet it is unfortunately rather unwieldy to take with one on a fighter patrol, or use during one.
Conditions on the sky would often make winning the engagement or disengaging impossible. Onboard radios were only coming in slowly and using one would require knowing the opponent's frequency --- as well as an opponent who had a radio, too!
This answer in Aviation.SE suggests there's no universal signal for surrender. Was there a widely used signal in use during WWI? If not, what were the common ways of indicating surrender during that period?