Let's write down some variables:
c: speed of light (electromagnetic radiation is light); c = 3*10^8 m/s
All we need to calculate the time it takes for the signal to go to and from the airplane is the speed of the transmission, which is c.
The distance the signal needs to travel is double the distance between the transmitter and the airplane, so 12km * 2 = 24 km = 24000 m
At the speed of light, the transmission takes 24000/c seconds to return to the transmitter.
24000/c = 24000/(3*10^8) = 8*10^-5 seconds = 0.00008 seconds
