Even though the plane is moving horizontally, the only thing that matters for figuring out how long it’ll take the whee to fall are vertical forces.
In this case the only vertical force is gravity, which has an acceleration of 9.8m/s^2. We are also given height (distance) and starting velocity (0 m/s). We need to solve for time. The equation that incorporates these variables is:
d = vt + ½ at^2
500 = 0t + ½ (9.8)t^2
500 = 4.9t^2
t^2 = 102
t = 10.1 seconds
