The FAA recommends that a child weighing:
under 20 pounds be placed in a rear-facing CRS.
from 20 to 40 pounds use a forward- facing child restraint. Although the safety technology of forward-facing child restraint systems in aircraft is still developing, current restraints offer dramatic improvements in protection compared to lap-held or unrestrained children.
over 40 pounds may safely use an aircraft seat belt.
Reminders:
Use an approved CRS when traveling to and from the airport, and when you arrive at your destination.
The safest place for a child on an airplane is in an apporved CRS based on the child's weight. While booster seats and harness vests enhance safety in automobiles, they are banned for use during taxi, take-off and landing. Although they are permissible for use during the cruise portion of flight, they do not afford the same level of protection as a hard-shell CRS.
In the United States, supplemental lap restraints, "belly belts," are banned from use in both automobiles and aircraft.