Although a car seat on an airplane is not required, it is strongly recommended. Children up to 24 months of age can sit on their parents lap and ride free of charge. This is very ironic, considering that the agencies that come up with the requirements for aircraft safety mandate that all items in a cabin must be battened down for takeoff, landing and when there is turbulence. A child sitting on a parent’s lap is certainly not battened down.
If you do decide to use a car seat on a plane, it will cost you the price of an extra seat, but many times, depending on the airline and the flight times, you can ask if there is a discount for a child in a car seat. It’s hard to put a price on your child’s safety.
The aviation agencies of the world recommend that children under 20 lbs. ride in a car seat that is rear facing. Children from 20 to 40 lbs can ride forward facing, and children over 40 lbs can use the plane’s seat belt without a car seat.
There are also some lap belts that you can purchase that are proven to protect a child sitting on a parent’s lap.  They have weight and age restrictions, so if you decide to purchase one of these devices, make sure you are selecting the correct one.