Answer: Zero
Step-by-step explanation:
The acceleration of the plane is given by:

where
is the change in velocity
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,
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.