As I'm typing I'm aware this is going to sound like such a dumb question, but I've never flown more than 3 hours with a baby in nappies (and luckily he didn't poo in that time). However, if a baby does do a dirty nappy, or if you're on a longer flight, just where do you change them ?!
Plane loos are difficult enough to get in and out of on your own - it'd be impossible to lie a baby down (and would you want to ?) and I've never been aware of specific baby changing areas. Maybe long haul jumbos have them (?) but imagining a charter jet where everyone is crammed in, I can't think how you'd do this !