A Republic of China Air Force F-16A during a demonstration flight at Gangshan Air Force Base

Redout Explained: What Negative G Does to a Pilot

by Connor Kerr | Oct 5, 2026 | Militärische Luftfahrt | 0 comments

Most people have heard of blacking out in a fighter jet. Pull hard, the G builds, blood drains from the head, the world goes grey and then dark. But there is an evil twin that gets far less attention, and pilots respect it just as much. Push the stick forward hard instead, and the blood goes the other way. Your face swells, your eyes bulge and, in extreme cases, the world turns red.

That is a redout, the effect of negative G. Here is what it is, why the human body is so bad at handling it, and why aerobatic and fighter pilots treat it with real caution.

Kurzinfo

What it is: Reddening of vision caused by negative G driving blood towards the head
Opposite of: Greyout and blackout under positive G
Typical human limit: About -2 to -3 G, compared with roughly +5 G for an untrained person
Likely cause of the red: The blood-filled lower eyelid being pulled up into the field of vision
Risks: Burst blood vessels in the eyes or brain; retinal damage and hemorrhagic stroke in extreme cases
Where it happens: Outside loops, pushovers and inverted manoeuvres in aerobatic and military flying

Positive and negative G

G is simply acceleration measured against Earth's gravity. Sitting still, you feel 1 G pressing you into your seat. When a pilot pulls back on the stick in a tight turn or a loop, the aircraft accelerates towards the top of the pilot's head, and the body feels heavier: this is positive G. Blood is forced down towards the legs.

Push the stick forward sharply, fly an outside loop or hold the aircraft inverted while pushing, and the acceleration points the other way. The pilot is thrown up against the harness, loose objects float upwards and blood is forced towards the head. That is negative G.

A clear physics explainer on positive, zero and negative G.

Why the body hates it

The human body is surprisingly good at coping with positive G. With training, an anti-G suit and a straining manoeuvre, fighter pilots routinely handle up to 9 G. Negative G is a different story. According to the standard figures, human tolerance for negative G is typically only in the -2 to -3 G range. There is no suit that helps much, and no straining technique that pushes blood back down.

As blood pressure in the head rises, the face swells and feels hot, the eyes feel as though they are being pushed out, and the head pounds. At higher levels, vision takes on a red tint. The most widely accepted explanation is not blood flooding the eye itself, but the blood-laden lower eyelid being pulled up into the field of view by the negative G.

A Pitts S-1C Special aerobatic biplane — The Pitts Special, a classic aerobatic biplane built to take hard positive and negative G. Photo via Wikimedia Commons (CC0).

The real dangers

Redout is more than unpleasant. Raised pressure in the head can make small blood vessels in the eyes or brain swell or burst. In extreme cases the risks include retinal damage and even a hemorrhagic stroke.

There is also a subtler danger. Aerospace medicine researchers have studied what they call the push-pull effect: a spell of negative G just before a hard pull to positive G reduces the body's tolerance to that positive G. The blood vessels that should constrict to protect the brain have relaxed during the push, so when the pull comes, blood drains away faster than expected. A pilot who would normally cope comfortably with a given G load can grey out or lose consciousness. It is one reason instructors teach students to be wary of pushing and then pulling hard in quick succession.

Where pilots meet it

Aerobatic pilots meet negative G more than anyone. Outside loops, sustained inverted flight and many competition figures involve hard pushes, and unlimited aerobatic aircraft are built to take heavy negative loads. Fighter pilots encounter it in pushovers, when diving away from a threat, or in manoeuvres that unload the aircraft quickly. Aircraft are limited too: the F-16, for example, is rated to +9 G but only -3 G.

Even passengers can get a mild taste. The floaty, stomach-in-the-mouth feeling as an airliner noses over at the top of a climb, or on a roller coaster cresting a hill, is reduced or slightly negative G. It is fun in small doses. At -3 G it is anything but.

A Royal Netherlands Air Force F-16 rolling during a flying display — A Dutch F-16 rolls through its display routine, smoke trailing from the wingtips. Display pilots move between positive and negative G throughout a routine. Photo via Wikimedia Commons, CC BY 2.0.

Living with the red

There is no real trick to beating negative G. Pilots build some familiarity through regular exposure, keep the pushes short, and plan sequences so that a hard push is not immediately followed by a hard pull. The best defence is respect: the body is built for 1 G standing upright, and it will let you know very quickly when you turn that upside down.

What sustained negative G looks like on a person: one of the most-watched clips of an aerobatic flight on YouTube.

Inverted flight is where many pilots first meet negative G for real.

It takes practice and a well-set harness to stay comfortable upside down.

Sources: Wikipedia (Redout, G-force, G-LOC), European Journal of Applied Physiology and Indian Journal of Aerospace Medicine research on the push-pull effect

Häufig gestellte Fragen

What is a redout?

A redout is a reddening of a pilot's vision caused by negative G, which forces blood towards the head. It is the opposite of a greyout or blackout under positive G. The red tint is thought to come from the blood-filled lower eyelid being pulled up into the field of vision.

How much negative G can a human tolerate?

Human tolerance for negative G is typically only in the range of -2 to -3 G, far lower than for positive G. An untrained person can tolerate roughly +5 G, and trained fighter pilots with G-suits can handle up to about +9 G.

Is negative G dangerous?

Yes. Negative G raises blood pressure in the head and can burst small blood vessels in the eyes or brain. In extreme cases it can cause retinal damage or a hemorrhagic stroke. It also reduces tolerance to positive G that follows immediately afterwards, known as the push-pull effect.

What is the push-pull effect?

The push-pull effect is a reduction in positive G tolerance when a pilot experiences negative G just before pulling positive G. Blood vessels relax during the push, so blood drains from the head faster during the pull, making a greyout or loss of consciousness more likely than normal.

Why can a G-suit not prevent redout?

An anti-G suit works by squeezing the legs and abdomen to stop blood pooling in the lower body under positive G. Under negative G the problem is blood moving towards the head, which a suit around the legs cannot prevent.

Where do pilots experience negative G?

Pilots experience negative G when they push the stick forward hard, fly outside loops, or push while inverted. Aerobatic pilots meet it most often in competition figures, and fighter pilots meet it in pushovers and when unloading the aircraft quickly. Passengers feel a mild version when an aircraft noses over.