This acceleration is for any change in speed. It doesn’t matter if the object is speeding up or slowing down – it’s still an acceleration. If you know the acceleration of something, you can find the stopping distance using the following kinematic equation (here is a derivation, if you like).
In this expression, v1 is the initial speed (20 mph in this calculation) and vtwo 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 stopping Boeing 747. Ah ha! This is not so easy. Of course, large aircraft stop all the time – this is often called a “landing”. However, the normal method during a landing will not work here. Typically, a large aircraft like the Boeing 747 uses two things to slow down. It uses not only the wheels, which have brakes, but also the reverse thrusters. Reverse thrusters are essentially the force of the engines directed backwards (hence, the “reverse” part). This backward thrust, along with the brakes, slows down the aircraft.
For this feat in Principle, the 747 will only have brakes, as it is not a fully functional aircraft. So, what would be the acceleration if an aircraft did not use reverse thrusters? Well, we’re in luck. Here’s something called a rejected takeoff test (RTO). For this maneuver, an aircraft departs and rises to take-off speed. At that point, the pilot steps on the brakes (without reverse thrusters) and stops. It is a worst-case test to ensure that the plane’s brakes can handle extreme cases.