This phenomenon is generated under the influence of G-force, which can clearly age the faces of pilots but can also make them appear younger in just a few seconds.
From the face of a young girl, suddenly transforming into an old woman and then returning to the original. The effects of G-force on the human face can be seen on the muscles and in the temporary changes in appearance, but they actually do much more to our bodies.
Acceleration is measured in units of gravitational force, abbreviated in English as G-force (gravity). Normally, on the ground, people experience a gravitational force of 1G. When flying in loops, dives, or performing vertical rolls, the maximum G-force that pilots may experience can reach 7.5 to 8G, and when ejecting from a seat, it can even exceed 10G, surpassing the tolerance limits of even the fittest and best-trained pilots. During the emergency parachute ejection of a jet pilot, the phenomenon of unconsciousness (blackout) often occurs, although the time that pilots endure high G-forces is only about 1 to 2 seconds. This is also why many fighter pilots flying over the ocean in emergencies face significant dangers when experiencing this temporary blackout.
Gravity is an accelerating force. This means it acts on objects to change their velocity. All objects exert gravitational force on each other, and this force is unique because it can act over vast distances. On and near the Earth, our planet’s gravitational force is strong due to its large mass, making all other gravitational forces essentially negligible. This force is calculated to be approximately 9.82 m/(s^2), commonly referred to as ‘g’, as you might remember from high school physics.
It is important to note that according to Newton’s second law of motion, F = ma, gravitational force is intrinsically linked to the mass of an object and changes proportionally with that value. For example, the gravitational force on the Moon (a much smaller object compared to Earth) is only 1.62 m/(s^2). Gravity is why objects fall to the surface of the Earth, and it is also the force that aircraft wings must overcome to generate lift. When the lift force of an aircraft exceeds gravitational force, controlled flight becomes possible, as demonstrated by the Wright brothers to the world in 1903.
The physiology of the human body is affected and reacts to changes in G-force.
The human body, like the rest of life on Earth, has adapted to life on land, where we are always exposed to Earth’s gravitational force (g). For simplicity, let’s call this standard gravitational force of Earth (9.82 m/s^2) as 1G. However, during flights, pilots may encounter situations with more or less than this constant 1G.
Accordingly, their physiology will be affected and respond to changes in G-force. When an aircraft is flying toward Earth and exerts thrust in that direction, it accelerates at that speed plus 1G (9.82 m/s^2). When the same aircraft accelerates away from the Earth’s surface, the total forces of acceleration will be the difference between the thrust and 1G.
Consequently, G-force acts on the human body along different axes (or directions). These are typically described as x, y, z axes. Each has a positive (+) or negative (-) direction, where the positive direction is downward along the direction of Earth’s gravity 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, encountered during aileron rolls (an aerial maneuver in which the aircraft performs a 360° rotation). Pilots who perform aerobatics often experience this type of G-force and can still control the aircraft safely and accurately. Gz is the gravitational force acting along the vertical axis of an object – parallel to the spine.
Pilots who perform aerobatics often experience this type of G-force.
So, how do these forces affect the body’s operational capability? The axis most relevant to this is Gz, as the G-force transmitted along this axis occurs frequently during flight and has a significantly greater physiological impact. Acceleration along the Gx axis is often experienced by astronauts during shuttle launches. Gy acceleration is less relevant but is gaining more attention due to the development of newer generation fighter aircraft with thrust-vectoring engines like the F-22 and F-35. For simplicity, the term ‘G’ in the remainder of this article will generally refer to forces along the Gz axis.
Acceleration along the Gx axis is often experienced by astronauts during shuttle launches.
The circulatory system is significantly affected by increased G-force during flight. Even at 1G, blood pressure in humans under stress will be highest in the lower limbs (legs) and lowest in the brain (cranial cavity) due to gravity. Because our bodies have adapted to a 1G environment, we have built mechanisms to compensate for this difference. As G-force increases, this physiological phenomenon is amplified, and the blood pressure difference between the cranial cavity and the lower part of the body becomes greater, leading to significant cerebral hypoxia (no blood = no oxygen). The final result is unconsciousness.
The circulatory system is significantly affected by increased G-force during flight.
In aviation, this is referred to as G-LOC, or G-induced loss of consciousness, and it remains a leading cause of loss of control in both military fighter aviation and civilian aerobatics. For instance, throughout the 1990s, the U.S. Air Force lost about one aircraft per year due to G-LOC.
In addition to the effects on circulation, increased G-force also disrupts respiration by shifting blood to the bases of the lungs, collapsing small air sacs (called alveoli), and creating a mismatch in ventilation/blood pumping due to air remaining in the upper parts 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 known as petechiae caused by ruptured capillaries. This commonly occurs in areas dependent on gravity in the body and is referred to as Geasles.
Other less serious effects of G-force include musculoskeletal pain.
To somewhat mitigate the effects of G-force, pilots often wear specialized protective suits as well as oxygen masks. Additionally, they must regularly undergo high G-force training to prepare for potential unconsciousness during flights.
