Two phenomena result in force on the landing gear. The first is the normal force exerted by the ground; the second is the friction on the tires. Either way, the tire must contact the ground for the force to be generated. Thus, the simulator tests whether the extreme point of the fully extended landing gear is below the ground. If it is, the tire is touching the ground, with the strut and tire compressed.
The landing gear model in this section is mostly a simplified version of the model found in Reference 19. Each gear is considered separately in calculating landing gear force.