Answer:
Zero (assuming the airplane is moving in straight direction)
Explanation:
Acceleration is equal to the rate of change in velocity divided by the time taken for the change in velocity to occur:
[tex]a=\frac{\Delta v}{\Delta t}[/tex]
where
[tex]\Delta v[/tex] is the change in velocity
[tex]\Delta t[/tex] is the time taken
In this problem, the airplane is moving at constant speed, so its change in velocity is zero (if the airplane is moving in a straight direction*), therefore [tex]\Delta v=0[/tex] and the acceleration is zero: [tex]a=0[/tex].
* If the airplane is changing direction, than the acceleration is non-zero, because it means that the velocity (which consists of both magnitude and direction) is changing.