Redout is a visual phenomenon associated with negative G-forces where a person's visual field turns red as a result of a sudden rush of blood to the head. A classic example of a situation where this can occur is in stunt flying where a pilot takes a sudden steep dive, subjecting his body to negative G-forces. This can be very dangerous and pilots must take care to protect their bodies from acceleration stress. Positive G-forces, as when people climb and speed up suddenly, can cause the opposite problem, a flow of blood away from the head, leading to graying of the visual field, confusion, and other problems.
Physiologically, during a redout the blood vessels inside the eyes expand, creating hazy, reddish vision. If they expand far enough, they can rupture. The patient can develop retinal damage and free-flowing blood inside the head may also lead to hemorrhagic stroke, where brain cells start to die because of pressure buildups inside the skull. During a redout, the patient may lose consciousness or become confused and sluggish as tissues inside the brain start to experience damage.
Pilots in training for situations where they will encounter G-forces go through a number of rigorous tests to make sure they can handle the high stress associated with things like stunt flying, piloting military aircraft, and riding in space ships and shuttles. This includes simulated conditions on the ground where pilots are put in an environment with very high acceleration stress to see how their bodies respond. Safety gear helps to prevent some injuries, and patients must regularly undergo health exams to make sure they are still fit for duty in environments where high G-forces will be present.
If a pilot starts to notice redout while flying, it may be necessary to slow down and adjust the flight plan to prevent damage to the eyes and brain. Pilots usually maintain constant communication with ground control, as well as other pilots in the air, and can alert people to the fact that there is a problem and they may need medical aid on the ground. An ophthalmologist can check the pilot's eyes for signs of injuries and a neurological assessment may also be necessary to check for any brain injuries.
In addition to redout, other distortions of the visual field can happen while flying. The vision may white or gray out as blood flows away from the head and towards the feet, a warning that the pilot's brain is not getting enough blood. Tunnel vision can also occur. Pilots who notice anything strange with their vision should report it so they can receive a medical evaluation.