G-force, or gravitational force equivalent, measures the acceleration an object or person experiences relative to Earth’s gravity. In flight, “10G” signifies an acceleration ten times this force. This means a pilot experiencing 10G feels a force equivalent to ten times their body weight pushing on them.
Understanding G-Force
In aviation, G-forces become apparent during maneuvers that involve changes in an aircraft’s speed or direction. This can happen during turns, climbs, descents, or rapid acceleration and deceleration.
Pilots experience G-forces along different axes. Positive G-forces (+Gz) push a pilot down into their seat, occurring during maneuvers like pulling up from a dive or in a steep turn. Conversely, negative G-forces (-Gz) pull a pilot out of their seat, which can occur during inverted flight or push-over maneuvers. Lateral G-forces (Gx, Gy) are experienced side-to-side or front-to-back, such as during aileron rolls or catapult launches.
How High G-Forces Affect the Body
High G-forces significantly impact the human body, primarily due to blood displacement. Under increasing positive G-forces, blood is pulled away from the head towards the lower extremities. This reduction in blood flow to the brain and eyes can lead to visual disturbances: tunnel vision (peripheral vision narrows), greyout (loss of color vision), and blackout (complete loss of sight while conscious).
If positive G-forces continue to rise, the lack of blood to the brain can result in G-induced Loss Of Consciousness (G-LOC). G-LOC is a state where awareness is lost due to a reduction in cerebral blood circulation, and it can occur within seconds, typically around 5-6 Gs for an untrained person, though some can tolerate up to 9 Gs. Recovery from G-LOC usually involves disorientation after regaining consciousness.
Negative G-forces, while less common in sustained flight, cause blood to rush towards the head. This can lead to symptoms like congestion, throbbing pains, and in severe cases, redout, where the visual field appears reddish due to increased pressure in the eyes. Prolonged exposure to high negative G-forces can cause retinal damage or hemorrhagic stroke.
Aircraft Design and G-Loads
Aircraft are designed with specific G-load limits, also known as load factors, to ensure structural integrity during flight. Exceeding these limits can compromise the aircraft’s structure, leading to bending, loosening of bolts, or complete structural failure like wings folding or tails snapping off.
Military fighter jets are engineered to withstand higher G-forces than commercial or general aviation aircraft due to operational requirements, such as rapid turns and combat maneuvers. For example, while civilian aircraft might be limited to around +3.8 Gs, fighter jets routinely sustain 9 Gs. Engineers account for these forces using strong, lightweight materials and advanced structural designs that distribute stress effectively across the airframe.
Strategies for Managing G-Forces
Pilots employ strategies and use specialized equipment to manage the effects of extreme G-forces. A primary tool is the G-suit, an anti-gravity garment. This suit has inflatable bladders that automatically pressurize during high-G maneuvers, exerting pressure on the abdomen and legs. This prevents blood from pooling in the lower body, helping maintain adequate blood flow to the brain and prevent G-LOC.
Pilots also use specific breathing techniques, such as the Anti-G Straining Maneuver (AGSM). This maneuver involves forcefully tensing muscles in the legs, abdomen, and chest combined with a controlled breathing pattern, helping increase blood pressure and push blood back toward the upper body and brain. A well-executed AGSM can provide an additional 3 to 4 Gs of protection. Physical training, including resistance training focusing on the core and lower body, also enhances G-tolerance and helps sustain these maneuvers.