The force required to stop on an inclined plane is proportional to the sine of the angle. At angles less than 10 degrees (~17% slope, far steeper than the FAA

The force required to stop on an inclined plane is proportional to the sine of the angle. At angles less than 10 degrees (~17% slope, far steeper than the FAA would allow for an airport), the small angle approximation of sine is within 0.5% of linear.