 Acceleration forces operate in different ways upon the human body. Acceleration can affect the body's cardiovascular, pulmonary, and neurological systems. Most general aviation pilots are not exposed to the same high accelerations that aerobatic and military pilots must endure. However, it is important for all pilots to understand the effects of acceleration forces on the human body. Pilots can unexpectedly get into an unusual flight attitude, causing them to experience a rapid onset of acceleration forces. Speed is defined as the rate of motion. It is the distance one travels in a certain amount of time. Speed equals distance over time. Velocity is defined as the rate of motion coupled with the direction of travel. Speed equals speed and direction. Acceleration is defined as a change in velocity per unit of time. It is produced when either speed or direction or both change. Acceleration is measured in feet per second squared or meters per second squared. The Earth produces a form of acceleration known as gravity. This affects everyone and everything on the ground and in the air. Acceleration produced by gravity is represented by a constant value of 32 feet per second squared. As an example, a free falling object will increase its velocity by 32 feet per second for every second it falls. The three types of acceleration are linear, radial, also called centrifugal, and angular. Let's look at what each type of acceleration represents. Linear acceleration reflects a change in speed, either an increase or decrease without a change in direction. Linear acceleration occurs during takeoff, landing, or on level flight with a change in throttle set. Radial or centrifugal acceleration is the result of a change in direction without a change in speed. An example of this type of acceleration is when a pilot performs a sharp turn, pulls out of a dive, or pushes over into a dive. Radial acceleration becomes most relevant to pilots because of the adverse effects it has on the human body. Angular acceleration results from a simultaneous change in both speed and direction. This kind of acceleration takes place during a tight spin. To better understand the effects of acceleration on the human body, it is necessary to discuss in more detail the concept of G-forces. Inertial force is a result of acceleration produced by Earth's gravity. This force, acting upon a mass, is termed 1G. In aviation, we refer to G-forces as forces resulting from acceleration exposure. For example, an airplane capable of pulling seven Gs can expose the pilot to acceleration intensity seven times the force of Earth's gravity. To imagine what a G-force experience is like, envision that your body weight has suddenly increased seven times. Then try to perform ordinary movements like moving your head, arms, or legs. A pilot weighing 150 lbs on Earth would weigh the equivalent of 1,050 lbs at seven Gs. The position of the body relative to the acceleration applied allows us to classify G-forces into three types, transverse, lateral, and vertical. A transverse G-force is also known as GX. It is the force applied to the front or back of the body. A positive transverse G, which is front to back, is typically experienced during takeoff and while increasing airspeed during level flight. A negative transverse G, which is back to front, is experienced during landing, in-flight air braking, and while decreasing airspeed during level flight. Transverse G accelerations affect the body's equilibrium system. Equilibrium originates from the inner ear, causing pitch up and pitch down illusions. These illusions are discussed in detail in the training module on spatial disorientation. A second type of G-force is lateral, or GY. This is where the force is applied to the lateral axis of the body. Lateral Gs occur when a pilot is exposed to acceleration from side to side. Vertical Gs occur during a vertical roll, a rudder roll, an aileron roll, or an uncontrolled flat spin. Third type of G-force is vertical, or GZ. Vertical G-force is applied to the vertical axis of the body. A pilot exposed to this type of acceleration from head to foot experiences positive Gs. In contrast, negative Gs are experienced from the foot to the head. Positive vertical Gs are usually experienced during sharp turns, an inside loop, or while pulling out of a dive. Similarly, you can experience negative vertical Gs during an inverted turn, an inverted loop, or while pushing over into a dive. Negative Gs, whether positive or negative, are potentially the most dangerous type of G-forces because they can result in sudden pilot incapacitation during flight. There are several factors about G-forces that impact an individual's tolerance level. Magnitude of the G-force is the number associated with the amount of G-force applied to the body. For example, a low-magnitude G-force is more tolerable than a high one. General aviation pilots are rarely exposed to positive vertical Gs higher than 2.5, while fighter pilots are routinely exposed to seven or more positive vertical Gs. Duration of exposure is how long a person has been exposed to a G-force. Short duration is more tolerable than long duration. Using three positive vertical Gs for several seconds is not difficult for the average individual, but having to endure it for over 15 seconds becomes physically challenging. Rate of acceleration, known as the G-onset rate, is measured in units of G per second. A slow or gradual G-onset rate is more tolerable than a high G-onset rate. Duration exposure from 0 to 7 positive vertical Gs at a rate of 1 tenth of 1 G per second is more tolerable than a rate of 1 G per second. Direction of force is defined by the axis of the body where the G-force is applied. By determining the direction of the G-force, you can determine the G-type. Exposure to vertical positive Gs can result in sudden inflight incapacitation. Exposure to transverse Gs can cause breathing difficulties. Exposure to lateral Gs can cause problems in maintaining yoke or stick control of the aircraft. Heart to eye distance. The greater the vertical distance between the eyes and the heart, the lower the individual tolerance of exposure to positive vertical Gs. In other words, the greater the distance between the heart and eyes, the more difficult it is for the heart to pump enough blood to the brain and eyes during exposure to positive vertical Gs. Furthermore, a pilot seated in an upright position has a lower tolerance to positive vertical Gs than a pilot seated in a reclined position. Keep in mind that self-imposed stress, such as fatigue, dehydration, and alcohol consumption, as well as cardiovascular and pulmonary medical problems, may decrease individual tolerance to G-forces. Remember, you are exposed to a G-force from your head to your feet when you perform a sharp turn or an inside loop. Most symptoms associated with exposure to positive vertical Gs are related to the pooling of blood in the abdomen and extremities. This results in an inability of the blood to reach critical organs, such as the brain and eyes. When blood pools into your lower body and extremities, the blood supply to your brain decreases, which then leads to oxygen deprivation. Since the eyes are an extension of the brain and are vulnerable to the lack of oxygen, eyes are most likely to be affected by the exposure of positive vertical Gs. If the initial magnitude of the G-force reaches about 4.1 positive vertical Gs, a condition known as greyout will occur. If you do not stop the ongoing flight maneuver at this point, within a few seconds you will experience a total blackout. If you still do not stop the ongoing maneuver and the initial magnitude of the G-force becomes about 5.4 positive vertical Gs, within a few seconds you can experience G-induced loss of consciousness. This is also known as G-lock, which can result in losing control of the aircraft. Even if you recover from G-lock, there is a period of amnesia and confusion that may occur for about 20 to 30 seconds. This can also result in impaired aircraft control, which can be critical if you are at low altitude. Progression of symptoms from greyout to blackout and then to G-lock can occur very quickly depending upon the G-onset rate. Having a high rate of 7 Gs per second, one can experience G-lock in about 4 to 7 seconds. Today's very maneuverable high-performance aircraft can achieve very high G-onset rates. Unfortunately, this can result in sudden G-lock without any preceding visual symptoms. Other effects of exposure to positive vertical Gs include breathing difficulties, heartbeat abnormalities, motion sickness, muscular fatigue and abdominal arm, leg and neck pain. Military and civilian aerobatic pilots sometimes use anti-G suits, anti-G straining maneuvers and a change in posture to increase or prolong positive vertical G-tolerance. An anti-G suit consists of a series of air bladders attached tightly around the legs and the abdomen. These bladders automatically inflate during exposure to positive Gs. When inflated, the abdomen and legs are squeezed and the blood in the upper body is prevented from pooling in the legs. This enhances the blood flow to the brain. When the air bladder over the stomach is inflated, it also helps lift the diaphragm, which in turn elevates the heart and decreases its distance to the brain and eyes. An important point to remember is that the closer the heart is to the brain, the less the heart has to work to keep blood pumping to the brain. This explains why short-statured pilots have greater tolerance to positive vertical G-forces than taller pilots. The anti-G straining maneuver consists of special breathing and muscle tensing techniques. Using these techniques helps improve blood flow to the heart and brain during positive vertical G exposure. The anti-G straining maneuver involves forcefully exhaling against a completely closed glottis, the structure that closes the windpipe from the esophagus, and at the same time contracting the muscles of the legs, arm and abdomen. The cyclic breathing technique must be used that involves a rapidly inhaled lungful of air. You must forcefully exhale against the closed glottis to increase internal chest pressure. This compresses the heart and blood vessels in the chest cavity and provides an artificial pumping action that increases the blood flow to the brain. As a result, blood flow to the brain is maintained during positive vertical G exposure. This breathing technique must be completed once every 3 to 5 seconds. If the breathing cycle is too fast, less than every 3 seconds, then internal chest pressure cannot be maintained. Fatigue, hyperventilation, and G loss of consciousness can occur. If the breathing cycle is too slow, more than every 5 seconds, then chest pressure remains too high and the return of blood to the heart is restricted. This results in reduced blood flow to the brain. The simultaneous tensing of leg and abdominal muscles with the anti-G stranding maneuver helps reduce the pooling of blood in the lower extremities and facilitates the return of blood to the heart so that it can be pumped to the brain. The anti-G stranding maneuver is physically fatigued because it requires the use of body muscles. You should prepare yourself for this activity with a well-rounded physical fitness program that incorporates weight training to help the strengthening of contracting muscles. While aerobic exercise produces a healthy low blood pressure, excessive aerobic exercise may reduce the blood pressure to a level that will actually decrease an individual's tolerance to G forces. Refer to the physical fitness training module for a well-rounded physical fitness program. In-flight postural modification is another technique that increases tolerance to positive vertical Gs. Astronauts sit in a reclined position lying on their backs during a shuttle launch. The F-16 aircraft has a 30-degree tilt back seat to reduce the effects of positive vertical Gs acting on the body during in-flight maneuvering. Aerobatic pilots fly airplanes with seats that tilt back. They also have rudder pedals move forward and upwards to elevate the position of the legs and feet in relation to the upper body. All of these seating positions allow astronauts, fighter pilots and aerobatic pilots to be positioned in a way that decreases the vertical distance from the heart to the brain, thereby helping to maintain adequate blood supply. Human tolerance to negative vertical Gs is lower when compared to positive Gs. Most symptoms associated with exposure to negative vertical Gs are the result of increased blood flow to the upper body. The blood is pulled from your lower body into your chest, upper extremities and head. The symptoms are similar to those experienced when your body is in an upside-down position, but the severity of such symptoms is much greater. The most common symptoms of exposure to negative vertical Gs include sensation of weightlessness, sensation of congestion, a fullness of the head and face, swelling of the face, headache, bleeding from the small blood vessels in the white parts of the eye, eye discomfort or pain, and mental confusion. The term redout is sometimes used to describe what a person sees in a negative vertical G environment. This has nothing to do with blood in the eye. Although there is not much research on this topic, most physiologists believe that the lower eyelids cover the eyes during the negative vertical G and what you see is the light through the eyelids with a red tint. In summary, a G-force affects the body's cardiovascular, pulmonary and neurological systems. The basic concepts of speed, velocity and acceleration can help you to better understand G-forces. The three types of acceleration are linear, a change in speed but not direction, radial, a change in direction but not in speed, and angular, meaning there is a simultaneous change in speed and direction. There are three types of G-forces that have a significant impact on the body. Transverse Gs, defined as the acceleration force applied along the front-to-back axis of the body, lateral Gs, defined as the acceleration force applied along the side-to-side axis of the body, and vertical Gs, defined as the acceleration force applied along the head-to-foot axis of the body. There are several factors about G-forces that have important implications for individual tolerance limits, including the magnitude of the G-force, the greater the magnitude, the lower the tolerance, duration of exposure, the longer the exposure, the lower the tolerance, rate of acceleration, the higher the onset rate, the lower the tolerance, direction of the force, defined by the axis of the body where the G-force is applied. Heart-to-eye distance, the shorter the distance between the heart and brain, the greater the tolerance to positive vertical Gs, a pilot seated in a reclined position has a greater tolerance to positive Gs than a pilot in an upright position. While most general aviation pilots will not be exposed to high G-forces, it is important for all pilots to understand the cause and effects of G-forces on the human body. Having a better understanding of acceleration in aviation can make for a safer, more responsive flight.