“After dinner, the weather being warm, we went into the garden and drank thea, under the shade of some apple trees…he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion’d by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself….”₁

…and so the mysterious force of gravity was revealed to Sir Isaac Newton and opened for human contemplation. As fighter aircraft came along, Newton’s law of gravitational force was soon exploited for tactical advantage.

The Force Of Gravity

Gravity is a force of acceleration. This means that it acts on objects to change their rates of velocity. All objects exert a gravitational force over one another and this force is unique because it can act over very large distances. On and near the planet Earth, the gravitational force of our planet is so great due to Earth’s large mass that all other gravitational forces are essentially negligible. This force has been calculated to be approximately 9.82 m/s², and is often called ‘g’, as you likely recall from high school physics class.

It is important to note that in accordance with Newton’s Second Law of Motion, F = ma, gravitational force is intimately tied to an object’s mass and varies in direct proportion to this value. For example, gravity on the moon (a much less massive object than earth) is only 1.62 m/s². Gravitational force is the reason why objects drop to the surface of the earth, and is also the force that an aircraft’s airfoils must contend with to create lift. When the lift of an aircraft is greater than the force of gravity, controlled flight becomes possible as the Wright Brothers demonstrated to the world in 1903.

G-Force Lingo & Notation

The human body, much like the rest of life on earth, has adapted to a terrestrial life in which we are always exposed to the gravitational force of Earth (g). For simplicity, let’s call this standard gravitational force of earth (9.82 m/s²) 1G. During powered flight, however, it is possible to experience both more or less than this 1G constant. Magnitudes of this value are expressed numerically and therefore “pulling 3 G’s” is equivalent to experiencing 3 times the normal gravitational force. A person who weighs 150 lbs at 1 G will actually weigh 450 lbs at 3 G’s – no kidding!

As you may expect, our body’s physiology will both be affected by and respond to this novel variability in G forces. When an aircraft is traveling towards the earth and exerting thrust in that path of motion, it is accelerating at that rate plus 1G (9.82 m/s²). When the same aircraft is accelerating away from the Earth’s surface, the sum of accelerative forces will be the difference from the thrust and 1G.

Newton’s First Law of Motion explains why the occupant of the aircraft will attempt to remain in motion at a constant direction and velocity during changes in direction and accelerative forces during flight.  Though they will be prevented from doing so by the seat restraints. These safety restraints will exert an equal (almost) and opposite force on the occupant’s body as Newton’s Third Law of Motion predicts. A detailed discussion on the physics behind G-forces and the gas laws of aerospace physiology can be found elsewhere. But it is important to note that on Earth we are always under 1G of force, but that in-flight vertical accelerations increase or decrease this value depending on the direction.

G-forces act on the human body in different axes (or directions).  These are usually described as the x, y, z axes.  Each has a positive (+) or negative (-) direction. When standing upright, the force of gravity acts along the longitudinal or G_z axis parallel to the spinal cord. +G_z acts downward in the same direction as Earth’s gravity.  Negative G_z‘s act in a direction opposite to gravity. Common notation identifies the axis acting through the front and back of the body as G_x and the axis acting laterally as G_y. These different axes correspond to yaw (G_z), roll (G_x), and pitch (G_y) of the aircraft.

So, how do these forces affect the body’s ability to function?  The most relevant axis to consider is G_z. This is due to both the frequent experience of G’s transmitted along this axis in flight and also it’s significantly greater physiologic effects. Acceleration in the G_x axis is more commonly experienced by astronauts during shuttle launch.  G_y accelerations are less relevant, but are gaining more attention due to newer generation fighter jets with multi-directional thrust engines like the F-22 and SU-35. For simplicity, the term ‘G’ is often applied only to forces in the Gz axis.

Human Physiology In Response To Gravity

The circulatory system is most significantly affected by increased G-forces during flight. Even at 1G, blood pressure in an upright person is highest in the lower extremities (the legs) and lowest intracerebrally (in the cranium) due to gravity. Because our bodies have adapted in a 1G environment, we have built in mechanisms to compensate for this discrepancy. Experiencing higher magnitudes of gravity presents unique problems to circulatory regulation.

At larger +G forces, this physiologic phenomenon is magnified and a larger discrepancy of blood pressures between cranium and the lower body occurs. At some point, intracranial perfusion cannot be maintained and significant cerebral hypoxia (no blood = no oxygen) follows. The end result is unconsciousness. In the world of aviation this is called a G-LOC, aka G-induced loss of consciousness, and remains a significant cause of loss of aircraft and pilot in both military fighter aviation and civilian acrobatic aviation. Throughout the 1990’s, for example, the USAF lost approximately one aircraft per year due to G-LOC.

In addition to circulatory effects, increased +G_z disrupts respiration by shifting blood to the lung bases, which collapses the small sacs of air (called alveoli) and creates a general ventilation/perfusion mismatch as air remains in the upper lung where there is little blood flow. As +G_z forces increase less blood flow combined with poorly oxygenated blood compound the cerebral (brain) hypoxia described above. Other less serious effects of large G forces are musculoskeletal pain (usually confined to the back and neck) and small punctate bruises called petechiae from overwhelmed capillaries that rupture.  This usually occurs in gravity-dependent areas of the body and are affectionately known as G-measles, or Geasles.

The G-LOC

As stated above, the most significant physiologic effect from G-forces are related to tissue ischemia (insufficient blood flow), specifically intracerebral (brain) ischemia. Because of the high sensitivity that the eye’s retina has to hypoxia, symptoms are usually first experienced visually.

As the retinal blood pressure decreases below the eye’s globe pressure (usually 10-21 mm Hg), blood flow begins to cease to the light sensing receptors in the retina, first affecting perfusion (and therefore vision) farthest from the optic disc  with progression of central vision loss. Therefore visual symptoms in response to increased G’s usually progresses from increasing tunnel vision to ‘graying out’ to full ‘black out’ – a phenomenon in which a person retains consciousness, but full retinal ischemia causes absolute blindness.

The final submission to G-forces produces a G-induced loss of consciouness (aka G-LOC), which is usually divided into a relative and absolute component. Absolute incapacitation is the period of time when the aircrew member is physically unconscious and averages about 12 seconds. Relative incapacitation is the period in which the consciousness has been regained, but the person is confused and remains unable to perform simple tasks (obviously cannot fly an aircraft). This period averages about 15 seconds. Upon regaining cerebral blood flow, the G-LOC victim usually experiences myoclonic convulsions (a seizure-like episode often called the ‘funky chicken’) and oftentimes full amnesia of the event is experienced.

Many militaries train their aircrew about G-forces and the anti-G straining maneuver (AGSM) using a centrifuge. See the video below for an anthology of centrifuge-induced G-LOC’s. The AGSM has two components – isometric muscle contraction and a particular respiratory sequence, which attempts to maximize cerebral blood flow and cardiac output, while maintaining an adequate level of oxygenation. A separate post written for pilots describing theAGSM technique with recommendations on how to improve G-tolerance can be found here.

When the AGSM is combined with an inflatable G-suit, one’s tolerance to high G’s increases markedly. Studies performed in the 1940s and 1950s by the U.S. DoD found that without any strain or G-suit, average G’s prior to G-LOC was dependent on the rate of G onset. G-LOC occurred at an average of 5.4 G’s at 1 G/sec rate and 4.5 G’s at 2 G/sec rate. An effective AGSM is thought to increase one’s G tolerance by 3 full G’s.

Most legacy G-suits, like the CSU 13B/P used by the USAF and CSU 15 A/P used by the USN/USMC, can increase G tolerance for an additional 1 to 1.5 G’s.  Newer G-suits such as the ATAGS provides even greater protection. When using this important life support equipment, a modern fighter pilot can be expected to remain conscious and continue to fly tactically at up to +9G_z. For a demonstration of G-LOC and AGSM (though in this case, inadequate AGSM), see the youtube video below.

Pilot’s AGSM Training in the Centrifuge with G-LOC

Transient Forces

It should also be noted that any discussion of gravitational and acceleration forces in the field of aviation would not be complete without mention of transient forces. The above discussion focused almost exclusively on sustained G-forces, but during a crash or ejection sequence transient accelerative forces act upon the human body, usually with high risk for blunt traumatic injury. The topic of transient forces of acceleration is in a separate post.

References

Stukeley, William. Memoirs of Sir Isaac Newton’s Life. 1752.
Fundamentals of Aerospace Medicine (4 Ed.)– April 16, 2008 by Jeffrey R. Davis MD MS, Robert Johnson MD MPH MBA, Jan Stepanek MD MPH, Jennifer A. Fogarty PhD

Adapted from original content on GoFlightMedicine by Capt Rocky “APOLLO” Jedick.  Part 1 of 2.