When the airplane contacts the ground during landings, its vertical speed is instantly reduced to zero. Unless provision is made to slow this vertical speed and cushion the impact of touchdown, the force of contact with the ground may be so great as to cause structural damage to the airplane.
The purpose of pneumatic tires, rubber or oleo shock absorbers, and other such devices is, in part, to cushion the impact and to increase the time in which the airplane's vertical descent is stopped. The importance of this cushion may be understood from the computation that a 6 inch free fall on landing is equal, roughly, to a 340 foot per minute descent. Within a fraction of a second the airplane must be slowed from this rate of vertical descent to zero, without damage.
During this time, the landing gear together with some aid from the lift of the wings must supply whatever force is needed to counteract the force of the airplane's inertia and weight (Fig. 10-5).
The lift decreases rapidly, however, as the airplane's forward speed is decreased, and the force on the landing gear increases as the shock struts and tires are compressed by the impact of touchdown. When the descent stops, the lift will be practically zero, leaving the landing gear alone to carry both the airplane's weight and inertia force. The load imposed at the instant of touchdown may easily be three or four times the actual weight of the airplane depending on the severity of contact.