I just heard someone said this way. I can understand the meaning but I don't really understand what "out" means in the phrase?
"Fly back out" is basically the reverse of "fly in", but with the added sense that you are returning to the place you came from: "I flew in from London this morning, and I fly back out tomorrow evening" means you are going back to London. Contrast this with "I flew in from London this morning, and I flight out to Tokyo tomorrow evening". As Tokyo is not your origin, you use "fly out" rather than "fly back out".
It's true that "out" is often understood in a relative sense. For example if a migrant travels from Scotland to the US, they are going "out" from Scotland, and coming "in" to the US.
As to "flying", I believe it's become part of common understanding that aircraft usually fly out when taking off, whereas they come "in" to land. This is of course from the point of view of an airport, military base or whatever.
So the "out" in "fly back out" means that after returning to their station (airport, base, carrier), they take off again, hence flying out from wherever they are stationed.