If your bird is used to being in a carrier for that long, then go for it if you absolutely have to. Astro (CAG) can be in a carrier for 9 hours and is okay. An airplane in my opinion is not worse than a car. Cars get stressful because birds can't balance well while the car is moving. An airplane is only stressful to board. Once you are on it it should be fine. Also cockatiels are small so you should be able to find a reasonably comfortable carrier.