This acceleration concerns any change of speed. It does not matter whether the object is increasing or decreasing in speed, it is always an acceleration. If you know the acceleration for something, you can find the stopping distance using the following kinematic equation (here is a derivation if you want it).
In this expression, v1 is the starting speed (20 mph in this calculation) and v2 would be the final speed – hopefully zero since it will stop. Thus, with a known acceleration, the stopping distance (Δx) would be:
All I have to do now is get a value for the acceleration of a Boeing 747. Ah ha! It is not so simple. Of course, large aircraft stop all the time – this is normally called “landing”. However, the normal landing method will not work here. Usually a large airplane like the Boeing 747 uses two things to slow down. It not only uses the wheels, which have brakes, but it also has reverse thrusters. The reverse thrusters are essentially the force of the motors directed backwards (hence the “reverse” part). This backward thrust, combined with the brakes, slows the aircraft down.
For this waterfall in Principle, the 747 will only have brakes because it is not a fully functional aircraft. So what would the acceleration be if an airplane did not use reverse thrusters? Well we are in luck. This is called a rejected take-off test (RTO). For this maneuver, an airplane starts up and gets up to take off. At this point the pilot slammed on his brakes (no reverse thrusters) and stopped. It’s a worst-case test to make sure the plane’s brakes can handle extreme cases.