Forces of 3-4 G during Spiral Dives and up to 7 G in Acro

3-axis accelerometer data during an acro sequence by Jim Nougarolles.

At the peaks of his infinity tumbles, our acro pilot Jim Nougarolles transiently reached approximately 7 G. However, more usefully for the rest of us regular pilots, we found that a consistent spiral dive of -10 m.s^-1 descent rate led to sustained acceleration forces of approximately 3-4 G in multiple axes and required three times as much oxygen and energy as normal flight.

Our findings were similar to the G forces measured in an investigation by a group of Dutch pilots in 2008. Bear in mind that acceleration forces as low as 2.6 G in a single axis have been known to cause loss of consciousness and G forces in multiple axes are even harder on the body.

Key Messages

Spiral dives generate sufficient G force to cause loss of consciousness in some individuals. Everyone’s G tolerance will vary during the course of their flying career.

Be aware of the factors that can reduce G tolerance, including hypoxia, low blood sugar, infection, dehydration and time away from flying.
Learn techniques to improve blood flow to the brain during high G manoeuvres.
The additional oxygen requirements of spiral dives may mean that lower energy manoeuvres like big ears are a safer and more sustainable way to descend from extremes of altitude.

What Happens to Your Body Under G Load

Positive G force, the kind experienced when banking hard into a spiral or pulling through the bottom of a manoeuvre, forces blood away from your head and into your lower extremities and abdomen. The immediate consequence is a drop in cerebral perfusion pressure — less oxygenated blood reaching the brain. At around 2 to 3 G, most pilots begin to notice a greyout: colour drains from vision, peripheral sight narrows, and a heavy pressure settles on the chest and shoulders. If the load is sustained or increases further, greyout progresses to tunnel vision and then blackout. At 4 to 5 G, loss of consciousness can occur within seconds unless countermeasures are applied, and at 7 G, even a well-practised straining manoeuvre may only buy a second or two of extra awareness.

What makes paragliding and paramotoring different from fixed-wing aerobatics is that the G vector is rarely steady or aligned with a single axis. In a spiral dive, the pilot is being pulled outward and downward while the wing rotates around them, creating a continuously changing direction of acceleration. The body must cope with lateral, vertical and rotational components simultaneously. Multi-axis G loading is more physiologically stressful than the same peak value in a single axis because the cardiovascular reflexes that normally compensate for blood pooling are confused by the conflicting signals. Even relatively modest sustained loads of 3 to 4 G measured in Jim Nougarolles’ spiral dives are enough to cause significant visual and cognitive impairment in many individuals, and brief peaks of 7 G in an infinity tumble can exceed the tolerance of all but the most conditioned pilots.

The timeline from first symptom to incapacitation is brutally short. Once greyout begins, you may have only 2 to 5 seconds to unload the wing or apply a countermeasure before vision greys out completely. Unconsciousness can follow within another 3 to 5 seconds without immediate action. The recovery period after G-LOC is equally disorienting: pilots typically experience 10 to 30 seconds of confusion, amnesia of the event, and sometimes involuntary muscle jerks. In a paraglider, that is more than enough time for the wing to enter a stable spiral, a cascade, or a fully developed autorotation from which the pilot cannot recover without active input.

Recognising the Early Warning Signs

Your first and most reliable instrument for G tolerance is not on your flight deck — it is your own vision and the sensation of pressure building in your body. Greyout begins as a loss of colour saturation, often described as the world turning grey or sepia, and then progresses to a narrowing of the visual field from the outside edges inward. Tunnel vision is the classic threshold symptom: you can still see what is directly ahead, but everything around that central point has disappeared. If you notice your visual field shrinking during a spiral, that is the moment to ease off the brake or exit the manoeuvre, not after you have already lost peripheral awareness.

Other early indicators include a heavy, compressed feeling in the chest and diaphragm, difficulty lifting your arms, tingling in the fingers, and a sudden sense of cognitive slowness — simple decisions take longer, and your scan of instruments or terrain becomes hesitant. Some pilots report a metallic taste or a rushing sound in the ears just before greyout. These symptoms can vary from flight to flight and are heavily influenced by the factors already listed in the Key Messages: dehydration, low blood sugar, hypoxia, infection, and time away from flying. The same spiral that you tolerated easily last month may produce greyout much sooner after a poor night’s sleep or a missed meal.

The most dangerous aspect of these early warnings is that they are easy to dismiss. In the middle of a committed spiral, the instinct to keep pulling or to enjoy the descent can override the recognition that your brain is being starved of oxygen. Experienced aerobatic pilots learn to treat the first sign of greyout as an automatic go-around: unload the wing, return to level flight, and reassess. The difference between a safe spiral and a G-LOC incident is usually not peak G, but how quickly the pilot recognises the symptoms and reduces the load.

Practical Ways to Improve Your G Tolerance

There is no magic pill that raises your G threshold, but there are several proven techniques that buy you valuable extra seconds of consciousness under load. The most effective is the anti-G straining manoeuvre, or AGSM, used by military and aerobatic pilots. It has two components: a forceful contraction of the muscles in your legs, abdomen and chest to squeeze blood back toward the brain, and a rapid breathing cycle — a short, sharp exhale against a partially closed glottis (as if straining to lift a heavy weight) every 3 to 4 seconds, followed by a quick inhale. In a paragliding harness, this is more difficult than in a rigid seat, but even a partial contraction of the core and thigh muscles while tightening your grip on the brakes can reduce blood pooling and delay greyout by a few critical seconds.

Physical conditioning plays a supporting role. Strong core and leg muscles make the straining manoeuvre more effective, while good cardiovascular fitness improves your body’s ability to regulate blood pressure under stress. Avoid flying when dehydrated, hungry, or recovering from illness — a blood alcohol level that is technically legal for driving can still halve your G tolerance, and the effects of a cold or mild fever are similar. Caffeine and nicotine constrict blood vessels and can actually help slightly in the short term, but the rebound vasodilation and dehydration are not worth the trade-off for regular flying. Time away from the sport is one of the strongest predictors of reduced tolerance; pilots returning after a winter layoff should treat their first high-G manoeuvres as if they were new to them.

Practise the AGSM on the ground first, then in straight-and-level flight, before you ever need it in a spiral.
Stay hydrated before and during long flights — even 2% dehydration measurably lowers orthostatic tolerance.
Eat a small, balanced meal 1-2 hours before flying, avoiding heavy or greasy food that diverts blood to digestion.
Be honest about your current state. Illness, poor sleep and stress all reduce G tolerance silently.
After a break from flying, build back up gradually. Do not jump straight into a 7 G infinity tumble on your first day back.

Multi-Axis G and the Spiral Dive Problem

Single-axis G tolerance figures from ejection seat tests or centrifuge studies do not translate cleanly to a rotating paraglider. In a fixed-wing pull-up, the G vector acts primarily head-to-toe, and countermeasures like the AGSM are designed around that predictable direction. In a spiral dive, the pilot’s body is constantly changing orientation relative to the ground while centrifugal force pushes outward and gravity pulls downward. The combined acceleration vector can sweep through the torso, head and chest within a fraction of a second. This multi-axis loading defeats the normal baroreceptor reflexes that would otherwise raise heart rate and constrict peripheral vessels to maintain brain perfusion. The result is that a sustained spiral producing only 3 to 4 G on a single accelerometer axis may feel and incapacitate like a much higher load.

Data from Jim Nougarolles’ flights showed a consistent spiral dive with a descent rate of around -10 m/s generated approximately 3-4 G across multiple axes. That is already above the 2.6 G level at which loss of consciousness has been documented in single-axis studies, and it was sustained for the duration of the spiral rather than being a momentary peak. The fact that many recreational pilots routinely use spiral dives as a descent technique without incident does not mean they are immune; it means their individual tolerance on that particular day was sufficient. The margin is often smaller than pilots assume, and it disappears quickly with hypoxia at altitude, dehydration on a long cross-country, or the accumulated fatigue of a multi-hour flight.

For pilots who need a reliable descent method, especially from extreme altitude where oxygen saturation is already low, lower-energy manoeuvres such as big ears with speed bar, or B-line stalls on suitable wings, place far less G loading on the body. They come with their own handling trade-offs but avoid the combined cardiovascular and neurological stress of a spiral. If you do choose to spiral, exit at the first sign of greyout, unwind gradually rather than snapping out of the rotation, and be aware that the G tolerance you had at sea level is not the same at 4,000 metres with borderline hypoxia.

This is a summary of one of our key findings from Phase I of the Free Flight Physiology Project. For a full write up of our methods and results, please see our our Cross Country Magazine article or our scientific paper in High Altitude Medicine and Biology.