Inherit the Skies by Janet Tanner

Alicia Morse - rich, spoiled and ruthless. Sarah Thomas - daring, independant and lovely.
She has grown up with Alicia, but on the wrong side of the stairs. She has made name for herself, parachuting with a display team, but it's no longer enough. For she, too, is in love with Adam, a dare-devil pioneer in the exciting new world of powered flight.