hi, i flew with 15 week old baby from uk to singapore. depends on the time you fly. we flew off in the afternoon, and brought her sleeping bag and sleepsuit etc with us. on the plane and in the airport, she was so excited she just wouldn't sleep initially, then crashed for 4 hours, arriving in singapore at about 4pm. by then she was so tired, we put her to bed around 6pm, due to unfamiliar surroundings, missing daddy etc, screamed till 2am (sorry if you don't want to hear this!) and then slept till 8am. in a few days, it just righted itself, but we kept the bedtime routine just as it always was e.g. book, bath, sleeping bag, feed, bed.
coming back this way we left in the middle of the night, she was already asleep so we woke her up, again really excited, but crashed in the plane - advice to you is make sure you sleep when they do! arrived back 16 hours later at 1pm, absolutely exhausted, slept at 4pm, and we kept her in bed with us for one night and one night only, and she slept till the next day 7am. didn't take long after that.
sorry for long post! just sharing my experience, as i was very afraid of this as well. i guess my point is, keep to a strict routine, let them nap as necessary, but not too close to the time you want them to sleep. and if need be put them in bed with you.. unfamiliar surroundings can be very disconcerting. hope that helps.