What is the acceleration of an airplane maintaining a constant speed of 67.7 m/s?

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:

where

is the change in velocity

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 and the acceleration is zero: .

* 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.