Gently massage behind the babies ears and hum or quietly sing Baby, Mine (from Disney's Dumbo). It's because of the change in pressure due to the change in altitude. They're literally in physical pain when they're screaming and crying like that. More sensitive to everything than adults or even older children. My son calmed right down when I did that and he was age 2 when he first got on a plane. He snuggled up with me and his blanky and stuffed animals. Fell right to sleep a few minutes later.