FAA Brochure - Acceleration in Aviation (G-Force)

Acceleration

in Aviation:

G-Force

OK-24-1154 9/18/2024If the pilot does not back off the control pressure,

loss of consciousness will ensue in short order

because the blood does not flow through the brain.

To survive in an aviation environment, pilots must

respect the laws of acceleration.

OK….What Does This Mean to Me?

Any aircraft, civilian or military, can expose the

pilot, crew, and passengers to forces in excess

of 1 G. During steep turns and unusual attitude

recovery, civil aviation pilots can experience high G

forces that may take them by surprise unless they

are prepared. Subsequently, all aviators need to

understand what makes their body more resistant

to the effects of G acceleration. Conversely,

aviators need to understand those conditions

that will make their body more susceptible to

the effects of G forces. The bottom line is that G

tolerance for each individual aviator may fluctuate

from day to day, and this can lead to disastrous

consequences in flight. This is one of the reasons

that military pilots do a “G warm-up” maneuver

prior to flying high-performance aircraft. It allows

them to assess their own body and how well they

will be able to tolerate the high-G environment.

The Bad Things

G tolerance is degraded as a result of alcohol,

fatigue, and dehydration, which are often

associated with a social event. With the “Big

Three” above, the aviator may experience severe

symptoms of G exposure at much less than the

customary level. Lack of physical conditioning and

a sedentary lifestyle can also degrade G tolerance

and increase the aviator’s susceptibility. Also, once

again, smoking and flying do not mix. Individuals

who smoke have diminished performance at

high altitude and high-G environments. Flying

is difficult, do not make it harder by needlessly

abusing your body.

The Good Things

Most civilian aircraft are not equipped to handle

G-protective clothing a “G suit.” However, there are other things that can be done to enhance aviator

performance in the high-G environment. A well-

rested, hydrated, and fit aviator will physically be

able to withstand higher G forces. When an aviator

is well hydrated, there is more circulating volume

in the blood stream, and it is easier for the heart to

keep the brain perfused with oxygenated blood.

A regular program of conditioning that includes a

mix of aerobic exercise coupled with resistance

weight training will increase an aviator’s resistance

to the effects of Gs. (All exercise programs should be

physician-approved prior to the initiation of training.)

Training that is only aerobic in nature (jogging and

swimming, for example) may lower the aviator’s

heart rate to the point where is does not accelerate

fast enough to compensate for the effects of Gs.

The combination of aerobic and resistance weight

training enhances the response to G forces and adds

a layer of protection that is not present in aerobic

conditioning alone.

The Anti-G Straining Maneuvers

Another technique for dealing with G forces is

the anti-G straining maneuver. This is a physical

technique where the aviator pushes air out of the

lungs against a closed glottis, while simultaneously

contracting the muscles in the calves, thighs, and

shoulders. This resistance inhibits the blood from

flowing away from the brain, and it simultaneously

increases the pressure in the carotid arteries. This

maneuver is practiced by military pilots, and it can

increase G-tolerance. To avoid injury, however, it

should not be attempted at 1 G with 100% effort.

Respect

Aviators need to respect G acceleration just as they

respect other aspects of flight. Proper flight planning will

take a number of things into account, such as weather,

fuel, distance, and time. A smart aviator will also include

consideration of the G forces for the aircraft and all aboard

when it comes to flight planning. A healthy respect, training,

and planning will help to avoid possible encounters with the

ground.

Provided by

Aerospace Medical Education Division, AAM-400

To obtain copies of this brochure online:

https://www.faa.gov/pilots/safety/

pilotsafetybrochures/

or contact:

Federal Aviation Administration

Civil Aerospace Medical Institute

AAM-400

P.O. Box 25082

Oklahoma City, OK 73125

(405) 954-4831

Acceleration in Aviation: G-Force

Human beings are adapted to live and survive within the

ever-present, accelerative force of gravity. While on earth,

this is a constant, and we live and function with it from

the day we are born until the day we die. As an infant

learning to walk, we learn very quickly that a misstep

will ultimately lead to a painful gravity-induced incident

with the ground that we call “a fall.” As we develop

and start to solve problems, we learn that a cookie jar

falling off the counter will accelerate all the way to floor

with shattering results. Many hours of our youth are

spent determining the results of gravity on spherical

objects of various shapes and sizes to our advantage

in competition. We became accustomed to gravity at

the standard 1 “G-force.” When we pilot an aircraft, all

that we have learned about gravity and have become

comfortable with suddenly changes. Flight in its purest

definition is overcoming gravity to ascend through the

air. Just as when we were learning to walk, a primary

goal of every flight should be to avoid painful, gravity-

induced incidents with the ground. These encounters are

called aircraft accidents and mishaps, and they can be

destructive, even fatal.

What Goes Up Must Come Down

The force of gravity on earth causes a constant

acceleration of 32 feet-per-second squared. An object

in freefall will accelerate at an ever-increasing speed

toward earth until it impacts the earth or reaches

terminal velocity the point at which the force of

aerodynamic drag acting on the object overcomes the

force of acceleration induced by gravity. Acceleration

is described in units of the force called “Gs.” A pilot

in a steep turn may experience forces of acceleration

equivalent to many times the force of gravity. This

is especially true in military fighter jets and high

performance, aerobatic aircraft where the acceleration

forces may be as high as 9 Gs. Air race pilots in a

tight pylon turn also experience high G-forces, but the

important thing to remember is that any aircraft operated

in a maximum performance profile will subject the pilot

to acceleration that is greater than the 1G acceleration

encountered on the ground. Pilots need to understand

this in to successfully master flying.Types of Acceleration

There are three types of acceleration. These types are

Linear, Radial, and Angular Acceleration.

Linear Acceleration - reflects a change of speed

in a straight line. This type of acceleration occurs during

take-off, landing, or in level flight when a throttle setting

is changed.

Radial Acceleration - is the result of a change in

direction such as when a pilot performs a sharp turn,

pushes over into a dive, or pulls out of a dive.

Angular Acceleration - results from a simultaneous

change in both speed and direction, which happens in

spins and climbing turns.

G Forces

During flight, a pilot may experience a combination

of these accelerations as a result of input to the flight

controls. These accelerations induce G-forces on the

body that may be described as Gx, Gy, and Gz.

+Gx is described as force acting on the body from chest

to back; +Gx is experienced, for example, during the

take-off roll as the throttle is advanced. This is the force

that pushes the pilot back into the seat as the aircraft

accelerates. –Gx is described as force from back to

chest, and it is encountered during landing as the throttle

is closed. This force pushes the pilot forward into the

shoulder strap.

Naval pilots flying from aircraft carriers feel the extremes

of this type of G force. During a catapult launch, the

aircraft accelerates to 160-plus mph in just under two

seconds. During landing, the aircraft will decelerate to

a complete stop in just a few feet. Carrier pilots have

adapted and successfully functioned with these extreme

Gs for decades.Gy is a lateral force that acts from shoulder to

shoulder, and it is encountered during aileron rolls.

Aerobatic pilots routinely encounter this type of G

force and can still safely and precisely maneuver

their aircraft.

Gz is a gravitational force that is applied to the

vertical axis of the body. If it is experienced from

head to foot, it is termed (positive) +Gz. This

happens when a pilot pulls out of a dive or pulls into

an inside loop. –Gz (negative) travels from foot to

head, and it is experienced when a pilot pushes over

into a dive.

Physiological Effects of

High G Forces

Everyone’s response to hypoxia varies. Unless, as

Human beings are adapted for life at 1 G on the

surface of the earth. In the aviation environment,

any maneuver has the potential to expose the

human body to more than 1+ Gz of acceleration

force. This can be particularly hazardous for pilots

in the Gz axis. This is a G force that acts from head

to toe in the case of +Gz and from toe to head in

the case of –Gz. As an aircraft enters into a high-

speed, coordinated turn or begins the pullout from a

steep dive, the pilot experiences +Gz. The heart and

cardiovascular system must respond quickly to G

acceleration to keep blood flowing to the brain and

maintain consciousness. Physiological response to

+Gz causes the heart to beat harder and faster with

an increased vascular tone to keep the blood flowing

“northward” toward the head. If the physiologic

response of the heart and vascular system does not

keep pace with the rapid onset of the G forces, pilot

performance will be degraded to the point where

unconsciousness and inability to pilot the aircraft

may ensue.

One of the first indications of impending disaster

may be a progressive loss of vision as the aircraft

enters the maneuver. The eyes are extremely

sensitive to low blood flow, and if the vascular

system cannot keep up with the onset of Gs, the

retina will not be supplied with adequate blood.

As arterial pressure in the eye falls below that needed for the retina, the pilot may notice a loss

of peripheral vision (tunnel vision), which may

then be followed by progressive degradation to

a smaller visual field (gun barrel vision), which

in turn may be quickly followed by Gray Out and

Blackout of the visual fields. If the rapid onset of G

forces continues, the end result may be G-induced

loss of consciousness (GLOC). In this condition,

unless the aircraft has sufficient altitude for the

pilot to back off the Gs and recover vision and/

or consciousness, the result can be tragic. This

has been the cause of far too many military and

civilian aviation fatalities.

The symptoms that result from high G exposure

are dependent on the rate of onset of the

acceleration. When the onset is gradual (about 0.1

G per sec.), visual symptoms precede GLOC. If the

onset is rapid (1 G per second or more), GLOC can

occur without visual warning.

While the effects of +Gz can be profound, the

human body is even less well equipped to handle

–Gz, which is described as a foot-to-head force

and is encountered when a pilot pushes over into

a dive or enters an outside loop. Under –Gz, the

blood is prevented from flowing back down the

jugular veins into the heart, but the arterial blood

flow to the head is enhanced. Once again, the

retina of the eye is extremely sensitive, and the

visual effect is a loss of vision due to “Red Out.”