Apologies for being a pedant and taking this slightly OT. The chance of more than one person on a plane sharing the same birthday - whatever day that happened to be - is not the same as more than one person sharing a birthday on the specific day of the flight.
23 people are all that is needed for there to be at least 50/50 chance of at least two of the people sharing ANY birthday. 253 people are needed for there to be at least 50/50 chance someone shares the same birthday as you.
en.wikipedia.org
