Answer: Zero
Explanation:
The acceleration of the plane is given by:
[tex]a=\frac{\Delta v}{\Delta u}[/tex]
where
[tex]\Delta v[/tex] is the change in velocity
[tex]\Delta t[/tex] is the time taken
In this problem, we are told that the airplane is traveling at constant speed, so (assuming it is traveling in a straight line*), its velocity is not changing. Therefore, [tex]\Delta v=0[/tex] and the acceleration is zero.
*if the airplane is changing direction, then its velocity is changing (because velocity is a vector), so the acceleration in this case would not be zero.