This acceleration is for any change of speed. It doesn’t matter if the object increases or decreases in speed, it is still an acceleration. If you know the acceleration of something, you can find the stopping distance using the following kinematic equation (here’s a derivation if you want).
In this expression, vv1 is the initial speed (20 mph in this calculation) iv2 it would be the final speed, hopefully zero, as it will stop. Thus, with a known acceleration, the stopping distance (Δx) would be:
Now I just need to get a value for the acceleration of a Boeing 747 that stops. Ah, yes! This is not so easy. Of course, large planes stop all the time, usually called “landing”. However, the normal method during a landing will not work here. Typically, a large aircraft like the Boeing 747 uses two things to brake. Not only does it use the wheels, which have brakes, but it also has reverse thrusters. Reverse thrusters are essentially the force of the engines directed backwards (hence the “reverse” part). This backward thrust force, along with the brakes, slows the plane.
For this trick Principles, the 747 will only have brakes, as it is not a fully operational aircraft. So what would be the acceleration if an airplane didn’t use reverse thrusters? Well, congratulations. Here’s what’s called the rejected takeoff test (RTO). For this maneuver, a plane takes off and rises to take off speed. At that point, the pilot is nailed to his brakes (no reverse thrusters) and stops. It is the test of the worst case to make sure the plane’s brakes can withstand extreme cases.