Re: Lemmings !
Whilst I agree that the on-board avionics systems would be kept separate from the in-flight entertainment system; your assertion that someone could not "reprogram" an executable is incorrect. If we're presuming it was already running then you could cause something like a buffer overflow which would provide an entry point for your own code. Alternatively if you knew what you were looking for and could elevate access then you could patch an executable (which is how cracked games work). Would they have protection against these attacks? I hope so. Will they have every scenario covered? Probably not.