This acceleration is for any gear change. It doesn’t matter if the object increases or decreases in speed – it’s still 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).
In this expression, vv1 is the starting speed (20 mph in this calculation) and v2 would be the final speed – hopefully zero, because it will stop. So, with a known acceleration, the stopping distance (Δx) would be:
Now I just need to get a value for accelerating a Boeing 747 that stops. Ah, ha! It’s not that easy. Of course, big planes stop all the time – this is normally called “landing”. However, the normal method during landing will not work here. Usually, a large plane like the Boeing 747 will use two things to slow down. Not only does it use the wheels, which have brakes, but it also has reversed thrusters. Inverted thrusters are essentially the force of the engines pointing backwards (hence the “reverse” part). This backward force, together with the brakes, slows down the aircraft.
For this stunt in Principle, the 747 will have brakes only because it is not a fully functional aircraft. So what would be the acceleration if an aircraft didn’t use reverse thrusters? Well, we’re lucky. This is called a rejected take-off test (RTO). For this maneuver, an aircraft takes off and rises to take off at speed. At that moment, the pilot slams the brakes (without reverse thrust) and stops. It is a test in the worst case to make sure that the plane’s brakes can cope with extreme cases.