Revealing mistake: The B17 that crash lands in the beginning appears quite realistic, until during a head on shot when you can see daylight through the clear nose from a window in the back of the plane. This shows it to be model as in the real plane bulkheads would block this from happening.
Revealing mistake: When C-Cup's nose gets clipped and the guy falls out, it is most obviously a doll/dummy falling out.
Answer: That would be a flare being fired from the aircraft. Yellow ones meant the aircraft had sustained serious damage, although that was often self-evident, and a red one meant that there was a seriously injured crewman on board - prompting first aid teams to be ready to meet the aircraft once it had landed. Their was also an unofficial green flare that meant someone had completed their tour of duty.
Farmersboy