Is it safe to use this car seat on an airplane, and are there any restrictions?

This child restraint is certified for aircraft use only when used with the 5-point harness system; it cannot be used in booster mode on an aircraft. The FAA recommends rear-facing for children under 9 kg (20 lbs), forward-facing for children 9–18 kg (20–40 lbs), and the aircraft seat belt for children over 18 kg (40 lbs). Check with your airline before departure to confirm the restraint meets their requirements. (Page 17, 63)