Why can't boosters and vests be used on board?

Aircraft seats are different from motor vehicle seats, so some auto products work differently and fit differently in them. Crash tests have shown that car safety vests and booster seats with shields may not protect a child in an aircraft seat. Therefore, they are not allowed during take-off and landing, even though some have labels saying they are certified for aircraft use.

• A vest with a rigid plate behind the child allows too much forward movement.
  
• Some aircraft seats have backs that fold forward. In a crash, a child in a shield-booster could be crushed against the shield.
• A belt-positioning booster seat that uses a lap and shoulder belt in a car cannot be used, since in an airplane there is no shoulder belt for upper-body restraint. If the booster seat has an internal harness and your child weighs less than 40 pounds, you can use it that way on aircraft. For a heavier child, use the lap belt alone.