can i use an ipad on a flight

Indeed you can, by putting it in airplane mode.

I travel London / Bangkok (12.5 hrs) return every month and use my iPad for watching movies I have either downloaded or made, use Keynote to make presentation and Office to catch up on some work.

I find that fully charged before I leave the battery gives me enough power for the flight, assuming 4-5 hours of videos then the remainder on other stuff. You can buy a power lead that plugs into the power socket of your seat's console (depending on the class you are flying) which takes away any worry about running out of power.

Also any apps that do not require an internet connection work OK. So you can play Angry Birds to your heart's delight 🙂

Yes, but only in aeroplane mode. See Settings, General.

If you want to test it, turn on airplane mode and try your apps. See what does/doesn't work. anything internet based will not work. You can watch a downloaded movie but not a streaming one, etc.