Boosters aren't allowed on planes no matter what. If you think about it, all boosters are designed to be used only with combo lap/shoulder belts (thus, you can never use a booster in a car that has only lap belts, either), and all airplanes have is a lap belt. If you are using a convertible carseat/booster, and have it in carseat mode with the straps still in use, you can use it that way. If you have one of the bigger convertible models (like some of the Britax ones) that allow use of the 5 point harness up to 60 or 80 lbs, you'd want to check with the airline about the width of the seat, and compare that to your model to make sure it will fit.