4-7. The gravitational force (G-force) and the direction in which the body receives that force are important physiological factors that affect the body during acceleration. As shown in Figure 4-4, G-forces can affect the body in three axes: Gx, Gy, and Gz. The physiological effects of prolonged acceleration depend on the direction of the accelerative (centripetal) force and, consequently, on how the inertial force acts upon the body. The inertial (centrifugal) force is always equal to, but opposite, the accelerative force. The inertial force is the most important physiologically. The various G-forces are explained below:

• Positive G, or + Gz, acceleration occurs when the body is accelerated in the headward direction. The inertial force acts in the opposite direction toward the feet, and the body is forced down into the cockpit seat.
• Negative G, or -Gz, acceleration occurs when the body is accelerated footward. The inertial force is toward the head, and the body is lifted out of the cockpit seat.
• Forward transverse G, or +Gx, acceleration occurs when the accelerative force acts across the body in a chest-to-back direction. The G acceleration is experienced during acceleration.
• Backward transverse G, or -Gx, acceleration occurs when the accelerative force acts across the body in a back-to-chest direction. The -Gx acceleration is experienced during deceleration.
• Right- or left-lateral G, or +/-Gy, acceleration occurs when the accelerative force impacts across the body from a shoulder-to-shoulder direction.

4-16. During a maneuver that produces +Gz, the weight of the body increases in direct proportion to the magnitude of the force. For example, a 200-pound person weighs 800 pounds during a 4-G maneuver. Normal activities are greatly curtailed, and the person is pushed down into the seat. The arms and legs feel heavy, the cheeks sag, and the body becomes incapable of free movement. In fact, a pilot cannot escape unassisted from a spinning aircraft if the magnitude of the force exceeds 2 to 3 +Gz. This is the primary reason for the adoption of the pilot’s ejection seat.

4-17. During a +Gz maneuver, the internal organs of the body are pulled downward. The increased weight of the internal organs pulls the diaphragm down, increases the relaxed thoracic volume, and disturbs the mechanics of respiration.

4-18. Comparing the body to a long cylinder helps explain the effects of a +Gz maneuver on the arterial blood pressure. In a seated individual, the heart lies approximately at the junction of the upper and middle thirds of the cylinder. The head and brain (the structures most sensitive to decreased blood pressure) are at the upper end of this vertical cylinder and about 30 centimeters from the heart. When a force of 5 +Gz is exerted on the body, a standing blood column of 30 centimeters exerts a pressure of 120 mm/Hg upon its base. Because this pressure is equal to the normal arterial systolic blood pressure, it exactly balances out the arterial pressure and causes the blood profusion of the brain to cease. Unconsciousness can result when a force of 5 +Gz is applied to the body. Figure 4-6 shows the effects of 1 +Gz to 5 +Gz conditions.

4-20. The effects described above are usually progressive, as shown in Figure 4-6. In relaxed subjects in the human centrifuge, for example, the first symptoms from increased +Gz forces occur at 2.5 to 4 +Gz and involve a graying or dimming of the visual fields. At slightly higher accelerations (4 to 4.5 +Gz), blackout occurs and individuals can no longer see although they remain conscious. The retinal arteries have collapsed, but some blood still flows through the blood vessels of the brain. At 4.5 to 5 +Gz, unconsciousness occurs.

4-21. Blood pools in the lower extremities, and there is a relative loss of blood volume and blood pressure to the brain. Stagnant hypoxia and hypoxic hypoxia, caused by unoxygenated blood from impaired respiration, also occur. Oxygen saturation of the blood can fall from the normal 98 percent to 85 percent during an exposure of 7 +Gz for 45 seconds.

4-22. With the loss of blood pressure and the hypoxic state combined, it may take up to one minute following the end of acceleration for an individual to recover. After regaining consciousness, the crew member may still experience a period of disorientation and loss of memory for some time.

4-43. Although tolerance limits to G-forces are relatively constant from one person to another, certain factors decrease or increase an individual’s tolerance to +Gz. These are the decremental and incremental factors.

4-27. Negative acceleration, inertial force applied from foot to head, causes a sharp rise in arterial and venous pressures at the head level. The increased pressure within the veins outside the cranial cavity may be sufficient to rupture the thin-walled venules (small veins). The intracranial venous pressure also rises, but it is counterbalanced by an accompanying rise in intracranial cerebral spinal-fluid pressure. Therefore, there is little actual danger of intracranial hemorrhage or cerebral vascular damage as long as the skull remains intact. Hemorrhages within the eye present the primary source of damage from -Gz. Distension of the jugular veins and veins of the sinuses and conjunctiva is caused by -Gz.

4-28. Sudden acceleration producing a force of 3 -Gz reaches the limit of human tolerance. When such a force is applied, venous pressure of 100 mm/Hg develops and causes small conjunctival bleeding areas and marked discomfort in the head region.

4-29. During -Gz maneuver, redout may be experienced (Figure 4-7). This phenomenon occurs when the gravitational pull acts on the lower eyelid, causing the lower eyelid to cover the cornea. The constant pull of gravity on the lower eyelids tends to weaken their muscles.

4-32. Individual are more tolerant of forces received in the +/-Gx axis than of those received in the other axes because transverse Gs interfere very little with blood flow. Extreme values of transverse G (12 to 15 +/-G) acting for five seconds or more can displace organs or shift the heart’s position and, thereby, interfere with respiration.

4-33. At levels above 7 +G, breathing becomes harder because of the effect on the chest movement. Some individuals, however, have withstood levels of 20 +G for several seconds with no severe difficulty.

• Degree of intensity of the acceleration, known as the "peak G."
• Duration of the "peak G" and the total time of the deceleration.
• Rate of application or rate of onset of the acceleration, known as the "jolt." The jolt, expressed in feet per second or Gs per second, is the rate of change of acceleration or the rate of onset of accelerative forces.
• Direction or axis of force application that determines whether acceleration or deceleration occurs.

• C—Container. An aircraft must be designed with an effective protective shell around the occupants. Its maximum structural and component weight should be below the occupants to reduce cabin crushing by inertial loading. The airframe should contain crushable material to attenuate crash forces before they are transmitted to the crew members. Fuel cells (tanks) should be of crashworthy design and be protected by the airframe to prevent outside objects from penetrating them.
• R—Restraint Systems. Restraint systems should attenuate crash forces and protect the occupants in all conditions of flight. These systems should be comfortable to wear and not interfere with cockpit duties. The head is the most likely point of injury in an accident sequence; therefore, occupants should use shoulder harnesses to minimize upper-body motion. A failure in any part of the restraint system—seat, seat belt, or anchor points—results in a higher degree of exposure to injury.
• E—Environment. The cockpit and cabin area must be "delethalized" to include adequate equipment restraints for withstanding crash forces.
• E—Energy Absorption. With their energy-absorbing features, aircraft are designed to withstand disruptive forces. Some features are the aircraft undercarriage, landing gear, and seat design that deform during the accident sequence. These modify high-peak G-loads of short duration into more survivable G-loads of longer duration.
• P—Postcrash Protection. Two major postcrash factors must be considered: fire and evacuation. The crashworthy fuel system has drastically reduced the fire hazard in Army aircraft accidents. However, timely evacuation is still desirable. The timeliness in evacuating aircraft occupants who survive an impact is often governed by the adequacy of emergency exits. Other factors that enhance timely evacuation are convenience of location, ease of operation (the UH-1 cargo door window is a prime example), and adequacy of markings.