This phenomenon, caused by the influence of G-force, makes pilots’ faces visibly age, but can also make them look younger for just a few seconds.
From the face of a young girl, it suddenly transforms into that of an old woman, and then back to its original state. The effects of G-forces on the human face are mostly visible in the muscles and temporary changes in appearance, but they actually do much more than that to our bodies.
Acceleration is measured in units of gravitational force, abbreviated as G-force (gravity) . Normally, on the ground, people experience a gravitational force of 1G. When maneuvering, diving, or performing a tight loop, the upper limit of the G-force a pilot may experience reaches 7.5 to 8G, and during ejection, it can even exceed 10G, surpassing the tolerance limit of a physically fit and well-trained pilot. During a pilot’s emergency ejection, unconsciousness (fainting) often occurs, even though the pilot’s overload duration is only about 1-2 seconds. This is also why many fighter pilots flying over the sea who need to eject in an emergency face significant danger when experiencing temporary unconsciousness.
Gravity is an accelerating force . This means it acts on objects to change their velocity. All objects exert a gravitational force on each other, and this force is unique because it can act over very large distances. On and near planet Earth, our planet’s gravitational force is so strong due to Earth’s massive mass that all other gravitational forces are essentially negligible. This force has been calculated to be approximately 9.82 m/s², and is commonly referred to as ‘g’, as you may recall from high school physics.
It is important to note that according to Newton’s second law of motion, F = ma, gravity is tied to the mass of an object and changes proportionally to this value. For example, the gravitational force on the moon (a much smaller object than Earth) is only 1.62 m/s². Gravity is the reason why objects fall to the Earth’s surface, and it is also the force that an airplane wing must resist to generate lift. When the lift of an airplane is greater than gravity, controlled flight becomes possible, as the Wright brothers demonstrated to the world in 1903.
The human body’s physiology will be affected and react to changes in G-force.
The human body, like the rest of life on Earth, has adapted to life on land, where we are constantly exposed to the Earth’s gravitational force (g). For simplicity, let’s call this standard Earth gravitational force (9.82 m/s²) 1G. However, during pilot flights, they may encounter situations with more or less than this constant 1G.
Accordingly, their body’s physiology will be affected and react to changes in the G-force. When an airplane is flying toward the Earth and exerting thrust on its path, it is accelerating at that speed plus 1G (9.82 m/s²). When the same airplane accelerates away from the Earth’s surface, the total acceleration forces will be the difference between the thrust and 1G.
Accordingly, the G-force acts on the human body along different axes (or directions). These are often described as the x, y, and z axes. Each has a positive (+) or negative (-) direction, where the positive direction is downward, in the same direction as the Earth’s gravitational pull, and vice versa.
Gx is described as the force acting on the body from the chest to the back; Gy is a force acting from one shoulder to the other, and it is encountered during an aileron roll (an aerial maneuver in which the aircraft performs a 360° rotation). Aerobatic pilots frequently encounter this type of G-force and are still able to control the aircraft safely and accurately. Gz is the gravitational force acting on the vertical axis of an object – parallel to the spinal cord.
Aerobatic pilots frequently encounter this type of G-force.
So, how do these forces affect the body’s ability to function? The axis most relevant is Gz, because the G-force transmitted along this axis occurs frequently during flight and also has a significantly greater physiological impact. Acceleration along the Gx axis is often experienced by astronauts during space shuttle launches. Acceleration Gy is less relevant, but it is also receiving more attention due to the development of newer generation fighter aircraft with multi-directional thrust engines such as the F-22 and F-35. For simplicity, the term ‘G’ in the remainder of this article will generally only apply to forces along the Gz axis.
The acceleration along the axis Gx is commonly experienced by astronauts during space shuttle launches.
The circulatory system is most significantly affected by the increased G-force during flight. Even at 1G, blood pressure in a person under stress is highest in the lower extremities (legs) and lowest in the brain (cranial pressure) due to gravity. Because our bodies have adapted to a 1G environment, we have built mechanisms to compensate for this difference. And as the G-force increases, this physiological phenomenon is amplified, and the blood pressure difference between the brain and the lower part of the body becomes greater, leading to significant cerebral hypoxia (no blood = no oxygen). The ultimate result is unconsciousness.
The circulatory system is most significantly affected by the increased G-force during flight.
In aviation, this is known as G-LOC , or loss of consciousness due to G-, and to this day it remains a major cause of loss of control in both military fighter aircraft and civilian aerobatic aircraft. For example, throughout the 1990s, the U.S. Air Force lost approximately one aircraft per year due to G-LOC.
In addition to affecting circulation, increased G-forces disrupt respiration by diverting blood to the lower lungs, causing the collapse of small air sacs (called alveoli), and creating a general ventilation/pumping mismatch because air remains in the upper part of the lungs, where there is very little blood flow.
Other less serious effects of G-force include musculoskeletal pain (usually limited to the back and neck) and small bruises called petechiae, caused by broken capillaries. These often occur in gravity-dependent areas of the body and are called geasles.
Other less serious effects of G-force include musculoskeletal pain.
To mitigate the effects of G-force, pilots often wear special protective suits and oxygen masks. Additionally, they frequently undergo high-G-force acceleration training to prepare for the possibility of losing consciousness during flight.
