Here's some basic relativity for you.
Imagine two aircraft travelling away from each other at a significant velocity.
One aircraft sends a message at the speed of light (a radio transmission?) to the other.
This message takes a finite time to travel between one aircraft and the other.
From the point of view of BOTH aircraft the messages either approach or recede from them at the speed of light (c)
From the point of view of a observer on the aircraft sending the message, the message leaves them at c relative to them, and must travel slightly further than the distance between the aircraft (since the other is receeding).
From the point of view of an observer on the second aircraft, the message is sent, with a relative speed of c and needs to travel the distance the aircraft are apart, while the other aircraft receeds from the point at which the message was sent.
Thus, the time taken for the message to travel beween the two aircraft differs depending on which aircraft you're on.
A rather more interesting (if potentially useless) question is what would an imaginary observer riding a photon traveling between the aircraft observe?
Of course this is special relativity, and does not take into acount the effect of acceleration. Gravity is the same as acceleration (as far as relativity is concerned).
A similar effect means that if you look very closely at the receeding aircraft, it is shorter by just enough that a person in the tail shining a torch to the cockpit will measure c to be exactly the same, even though the beam is travelling at c with respect to an external (or any other) observer.
Of course they will report the same about your aircraft.