I do not think they make you, but you should check with the airline.
We used ours on the way to Minneapolis this summer with DS2.5 I feel like he was much more comfortable in his carseat and did not fuss. However, the way the carseat fit in the airplane made his feet touch the back of the seat of the person in front of us. Everytime DS moved, he would hit the seat. It really bothered us.
On the way back, we did not use the carseat. Solved the problem of hitting the seat in front of us, but then I do not think he felt secured so he kept fussing and sliding out of the seatbelt.