A collision happens when a part of the aircraft contacts the ground surface. When the part is made of rubber and can rotate this usually just fine and preferred over all other parts of the aircraft

When the part passes through the ground a force is generated which is proportional to the depth below the ground (in meters) times the factor K (N/m) plus the relative velocity (in meters per second) times the factor D (N per m/s). The force acts on the rigidbody assigned in the Body attribute.

A critical crash with any part that is not a wheel can cause the aircraft to be reset, when the designer of the aircraft, us, flags the part as important and sets the CrashOnContact property to true.

            // GA size
            <[wheel][NoseWheelHull][]
                <[string8][Body][NoseGear]>
                <[tmvector3d][R0][ 0.0 0.0 0.0 ]>
                <[float64][Radius][0.2]>
                <[float64][Width][0.14]>
                <[float64][RimRadius][0.07]>
                <[float64][Inflation][213737.56]>
                <[float64][K][400000.0]>
                <[float64][D][10000.0]>
            >

            // airliner size
            <[wheel][FrontWheelHullLeft][]
                <[string8][Body][NoseGearLower]>
                <[tmvector3d][R0][ 0.0 0.0 0.0 ]>
                <[float64][Radius][0.3815]>
                <[float64][RimRadius][0.2222]>
                <[float64][Width][0.243]>
                <[float64][Inflation][1372057.24]>
                <[float64][K][1000000.0]>
                <[float64][D][100000.0]>
            >

            <[collisionhull][CollisionFuselage][]
                <[string8][Body][Fuselage]>
                <[string8][Geometry][Fuselage]>
                <[float64][K][65000000.0]>
                <[float64][D][520000.0]>
                <[bool][CrashOnContact][true]>
                <[float64][StaticFrictionCoefficient][1.0]>
                <[float64][KineticFrictionCoefficient][0.8]>
            >

A spherical object at position R0 with a set Radius that repels the Body from the ground. Used for small skids or a tail gear without a wheel.

            <[collisionsphere][LeftWingWheel][]
                <[string8][Body][LeftWing]>
                <[tmvector3d][R0][ -0.011895 6.741012 0.065757 ]>
                <[float64][Radius][0.05]>
                <[float64][K][150000.0]>
                <[float64][D][9000.0]>
            >

When the geometry contacts the ground a part is damaged, e.g. when the left side propeller disk hits the ground the left side propeller is set to damaged.

            <[collisionblade][CollisionPropellerLeft][]
                <[string8][Body][Fuselage]>
                <[string8][Geometry][LeftPropellerDisc]>
                <[string8][DamageOnCrash][LeftPropeller.Damage]>
            >