Actually, very little stopping distance is required if the speed  can be dissipated uniformly; that is, if the deceleration forces  can be spread evenly over the available distance. This concept  is designed into the arresting gear of aircraft carriers that  provides a nearly constant stopping force from the moment  of hookup. Avoiding forcible contact with interior structure is a matter  of seat and body security. Unless the occupant decelerates  at the same rate as the surrounding structure, no benefit is  realized from its relative intactness. The occupant is brought  to a stop violently in the form of a secondary collision. The typical light airplane is designed to provide protection  in crash landings that expose the occupants to nine times  the acceleration of gravity (9G) in a forward direction.  Assuming a uniform 9G deceleration, at 50 mph the required  stopping distance is about 9.4 feet. While at 100 mph, the  stopping distance is about 37.6 feet—about four times as  great.  [Figure 17-2]  Although these figures are based on  an ideal deceleration process, it is interesting to note what  The overall severity of a deceleration process is governed  by speed (groundspeed) and stopping distance. The most  critical of these is speed; doubling the groundspeed means  quadrupling the total destructive energy and vice versa.  Even a small change in groundspeed at touchdown—be it as  a result of wind or pilot technique—affects the outcome of  a controlled crash. It is important that the actual touchdown  during an emergency landing be made at the lowest possible  controllable airspeed, using all available aerodynamic devices. Most pilots instinctively—and correctly—look for the largest  available flat and open field for an emergency landing.  Figure 17-2.  Stopping distance vs. groundspeed. Figure 17-1.  Using vegetation to absorb energy. 17-3 10 20 30 9G deceleration 37.6 feet 9.4 feet 50 mph 100 mph Dispensable airplane structure is not the only available energy  absorbing medium in an emergency situation. Vegetation,  trees, and even manmade structures may be used for this  purpose. Cultivated fields with dense crops, such as mature  corn and grain, are almost as effective in bringing an airplane  to a stop with repairable damage as an emergency arresting  device on a runway.  [Figure 17-1]  Brush and small trees  provide considerable cushioning and braking effect without  destroying the airplane. When dealing with natural and  manmade obstacles with greater strength than the dispensable  airplane structure, the pilot must plan the touchdown in such  a manner that only nonessential structure is “used up” in the  principal slowing-down process.