All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The time for Fajr prayer for those travelling by plane starts with the emergence of the real Fajr. The Fajr time of the destination city or the one from which they departed does not count. So, if the Fajr time started while they are in the plane, then they are obliged to offer the prayer. However, if they could reach their destination before the time of Fajr ends (i.e. the sun rising), they should wait. But if they fear that the time will end before they are dropped off, then they must offer the prayer in the plane as according to what is possible.
Therefore, you did well if you offered the prayer in the plane. Otherwise, it is considered an aspect of negligence that implies repentance and seeking forgiveness and then you must make it up at once.
Allaah Knows best.