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| CHAPTER 12. Night Operations
Basic Safety Concepts A pilot who is faced with an emergency landing in terrain that makes extensive aircraft damage inevitable should keep in mind that the avoidance of crash injuries is largely a matter of:
The advantage of sacrificing dispensable structure is demonstrated daily on the highways. A head-on car impact against a tree at 20 miles per hour (mph) is less hazardous for a properly restrained driver than a similar impact against the driver’s door. Statistics indicate that the extent of crushable structure between the occupants and the principal point of impact on the aircraft has a direct bearing on the severity of the transmitted crash forces and, therefore, on survivability. Compared to an airplane, the WSC aircraft has less structure to absorb the impact and is moving slower, but the same principles apply. Avoiding forcible contact with the front tube, cowling, dashboard, or outside structure is a matter of seat and body security with the use of seatbelts. Unless the occupant decelerates at the same rate as the surrounding structure, no benefi t is realized from its relative intactness. The occupant is brought to a stop violently in the form of a secondary collision. Dispensable aircraft structure is not the only available energyabsorbing medium in an emergency situation. Vegetation, trees, and even manmade structures may be used for this purpose. Cultivated fi elds with dense crops, such as mature corn and grain, are almost as effective in bringing an aircraft to a stop with repairable damage as an emergency arresting device on a runway. [Figure 13-2] Brush and small trees provide considerable cushioning and braking effect without destroying the aircraft. When dealing with natural and manmade obstacles with greater strength than the dispensable aircraft structure, the pilot must plan the touchdown in such a manner that only nonessential structure is “used up” in the principal slowing down process.
It should be noted that examples presented here are not to be practiced because these situations are hazardous and can damage the WSC and injure occupants. These examples are shown for informational purposes, in case similar situations arise in the future. 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 quadruples the total destructive energy and vice versa. Even a small change in groundspeed at touchdown, resulting from 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 means. Most pilots instinctively—and correctly—look for the largest available fl at and open fi eld for an emergency landing. 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 on aircraft carriers, and provides a nearly constant stopping force from the moment of hookup. For example, assuming a uniform 2 G deceleration while landing into a headwind with a 25 mph groundspeed, the stopping distance is about 10.5 feet; in a downwind landing at 50 mph groundspeed, the required stopping distance is 42 feet—about four times as great. [Figure 13-3] Although these fi gures are based on an ideal deceleration process, it is interesting to note what can be accomplished in an effectively used short stopping distance. Additionally, landing uphill reduces the stopping distance and landing downhill increases the stopping distance. Understanding the need for a fi rm but uniform deceleration process in very poor terrain enables the pilot to select touchdown conditions that spread the breakup of dispensable structure over a short distance, thereby reducing the peak deceleration of the fl ight deck area. A careful consideration must be made considering wind, slope, and terrain.
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