Pilot And Crash Dynamics

A pilot needs some understanding of the mechanics of crash injuries if he is to make the wisest decision in a forced landing situation that looks grim at best. The following discussion is intended to give this understanding, without getting involved in the medical and engineering aspects of the subject.

Crash injuries, like aircraft damage, are the result of the violence generated by sudden stoppage and fall into two broad categories.

Contact injuries, resulting from forceful contact between occupants and environmental structures. This is the most common form of injury during forward decelerations, when the occupants do not use an adequate restraint system (seat belt and shoulder harness). Injuries caused by loose objects in the cockpit/cabin area also fall into this category.

Decelerative injuries. Although all contact injuries involve a deceleration process, the term decelerative injuries is generally used to indicate bodily damage resulting solely from loads directly applied through the occupant's seat and restraint system. They affect the body internally, and one of the characteristic forms is spinal injury during vertical decelerations (excessive positive G). Internal injuries caused by seat belt impact in the lower abdomen may occur during severe forward decelerations, especially when the seat belt is not properly installed or used. (Note: The seat belt should cross the hips at about a 45-degree angle, and the buckle should be worn as low as possible so that decelerative loads are applied to the hip bones and not the soft abdominal area.)

Injuries resulting from post-crash complications form a separate category. In the event of fire or during ditching, fuselage distortion and final aircraft attitude may interfere with the timely evacuation of the wreckage. Although this hazard can be controlled to some extent by the design of fuel systems and emergency exits, it is often the pilot's landing technique and his knowledge that govern the post-crash survival aspects.

The violence of the stopping force, expressed in Gs, depends on speed and stopping distance. The total energy of motion crash energy is a function of ground speed and varies with the square of the velocity. For example, and assuming a 20-knot wind, an aircraft with a 60-knot stalling speed could be landed with a ground speed of 40 or 80 knots, depending on landing direction. Under normal conditions, the downwind landing would require four times as much roll-out distance as a landing into the wind, assuming similar braking action. In a crash situation, the same 4 to 1 relationship holds true for the total crash energy.

Speed in itself is not a killer. The danger lies in how it is dissipated. A common misconception in this respect is that it takes hundreds of feet of obstacle-free terrain to make a survivable crash landing. Theoretically, it would take only 20 feet to stop a 20 -G deceleration, if the stopping force could be applied uniformly over this distance. The same uniform deceleration (20 Gs) would bring an aircraft to a stop from 60 knots in a distance of about 2.5 meters. The arresting gear of aircraft carriers and runway barriers shows how this concept can be applied under controlled conditions.

The problem in some crash landings is that the deceleration process is not uniform. Every time the aircraft strikes an obstacle or digs a gouge mark, a peak deceleration occurs, and it is during these peaks that injury exposure is at its greatest. It should be pointed out, however, that as far as impact survival is concerned, only the forces transmitted to the occupant's area (cockpit/cabin) are critical. The dispensable structure (nose section, wings, main rotor, etc.) should be used (sacrificed) as an energy-absorbing buffer between the point of impact and the cockpit/cabin structure.

Pilots should look at the cockpit/cabin enclosure protective container and try to keep this container reasonably intact by instinctively avoiding direct impact against it. Accident experience and full-scale experimentation show that a reasonably intact cockpit/cabin structure generally means that the impact conditions were survivable, as far as deceleration is concerned. As long as a pilot can avoid collapse or excessive deformation of the protective container, he meets the first requirement for impact survival.

Disregard for this basic law of physics kills thousands of car drivers every year in front-end collisions. Even when using a seat belt, the driv-er's upper torso and head maintain momentum with respect to his rapidly slowing down car interior, resulting in a sledge hammer-like impact against the steering wheel, instrument panel, or windshield. The obvious conclusion is that the car or aircraft occupant needs adequate restraint--which always includes a shoulder harness--since he has to slow down at the same rate as his environment. This basic requirement for impact survival in any type of vehicular crash is illustrated by the following example.

During the rollout after an emergency landing, an aircraft runs nose-first into a solid obstacle at 20 mph, crushing the nose section and shortening it by 25 centimeters. Assuming the deceleration is uniform, a 25-centimeter stopping distance for the cockpit behind the nose results in a mean deceleration of 13.6 Gs. The pilot who is not using his shoulder harness jackknifes over his seat belt, striking his head on the instrument panel. Assuming that the panel stopped by the time he reaches it, the impact velocity of his head will be 20 mph. Assuming that the panel crushes to a depth of 2.5 centimeters, the effective stopping distance of the pilot's head will result in a head impact of approximately 146 Gs, or 12 times that of the overall cockpit deceleration. This could easily be a fatal blow, depending on the shape and hardness of the head impact area, and whether or not a crash helmet is worn, In addition to understanding the reaction of aircraft structures to crash loads, pilots must have general knowledge of the reaction and tolerance of the human body under these conditions. G-loads imposed by crash-type decelerations and those imposed by flight maneuvers differ in their effects on the body. Flight loads are of long enough duration to affect the blood circulation, for which the body has very limited tolerance. Unconsciousness may occur at about 4-6 Gs. Impact loads are measured in fractions of a second and impose a mechanical shock for which the body has a rather high tolerance--about 20-25 Gs during decelerations perpendicular to the spine when restrained by a seat belt and a shoulder harness. With a seat belt only, this tolerance to forward deceleration drops below 25 Gs. Actually, the human body can take more punishment than the aircraft structures, as long as pilots manage to maintain a semblance of integrity in the occupiable