The challenges that acrobatic flight imposes on pilots' bodies

Our species is acclimatized to a world under the yoke of constant gravity - in this case, an omnipresent force of acceleration born of the Earth's attraction (the unit of Earth's gravity, g, is 9.81 ^m/s2). However, there are circumstances in which our bodies are subjected to a force stronger than classic terrestrial gravity... Here again, it's a question of acceleration.

Stéphane Perrey, University of Montpellier

In aeronautics and the automotive industry, specialists refer to G (for Gravitational), or load factor, as the unit of acceleration. And its effects can be formidable.

As children learning to walk, we soon discover that one false step will eventually result in a painful impact with the ground due, precisely, to gravity. When we board an airplane, albeit without crashing this time, everything we've learned about gravity and become accustomed to changes abruptly. Just look at Pete "Maverick" Mitchell's latest aerial circumnavigation in the latest Top Gun for proof.

Flying involves overcoming gravity to rise into the air, and speed is essential. Any aeronautical maneuver can therefore expose our bodies to considerable acceleration, with significant repercussions on the cardiovascular, cerebral and joint systems. Some aircraft are capable of speeds of up to 12G, with acceleration rates in excess of 15 G/s!

How many Gs do we have to put up with on a daily basis?

Such figures are, of course, extremes. Standing still on the ground, the acceleration felt is 1G. All is well. At 2G, for example when taking a 60-degree banked turn, we already feel a moderate compression on our seat, a difficulty in moving. A person weighing 80 kg on Earth (considering this to be a situation equal to 1G) will feel as if he or she weighs 160 kg if subjected to 2G. From 8-9 G, it's impossible to move your limbs, with the exception of your extremities.

In fact, there are three main types of G in three axes of space. We can experience lateral Gs (Gy) during a turn, resulting from the centrifugal acceleration that pushes us outwards. For horizontal acceleration or deceleration, we speak of Gx. Finally, Gz occurs when the aircraft descends, or following a steep climb. We are particularly sensitive to these accelerations in the vertical axis (Gz), i.e. from head to toe, since this is where we feel the force of the Earth's gravity, necessary to maintain our balance.

To complicate matters further, for all three axes, both positive and negative Gs are possible... Whether turning a car or flying an aircraft vertically, a resistance to movement - the force of inertia- is added to the actual weight due to gravity to give the aircraft's "apparent" weight in flight. When the apparent weight in motion is greater than the actual weight, the load factor is greater than +1G. On the other hand, if the aircraft is flying on its back, for example, the load factor is expressed as a negative -G.

To calculate the Gs to which they are subjected, airplane pilots, who are particularly exposed, are equipped with three-axis accelerometers: this enables them to know in real time what they are experiencing.

How our bodies deal with gravity in normal times

During flight, aircraft pilots are subject to a wide variety of physiological effects due to the combination of acceleration and gravity. These effects are inherent to the inertial forces generated by acceleration, and apply to all the body's organs, especially the cardiovascular system: the heart (the pump), the vessels (the circuit) and the blood (the fluid).

Blood circulation transports oxygen, which is essential for the proper functioning of organs. The brain is particularly demanding in this area, both in terms of consumption (it's greedy) and the regularity of its supply. It doesn't like jolts, surpluses or shortages!

On Earth, there's a complex mechanism for controlling and adapting all the machinery that ensures regular, well-oxygenated blood flow at a constant rate to the brain, whether at rest or under stress: this is cerebral self-regulation. Any variation in blood pressure is therefore of no consequence. But there are limits to this fine balance... Accelerating into a turn, braking or, a fortiori, practicing aerobatics will greatly disrupt it.

The ability to maintain cerebral blood flow, resilient to repeated exposure to increased load factors, is therefore a critical issue for pilots who step outside normal everyday conditions.

When our physiological adaptations are no longer sufficient

The risks were identified, albeit poorly explained, over a century ago. In 1918, the first acceleration-induced disorder was experienced during the Schneider Cup air race, when a sharp turn had to be taken. First described as "malaise in the air", it is now known as "G-LOC", or "G-induced loss of consciousness", and is characterized by confusion and impaired judgment following a temporary abolition of cerebral circulation. This condition occurs at +4.5-6G in trained pilots.

As the heart is in the thorax, in an upright position (standing or sitting), the vascularization of the brain, positioned above it, requires the blood flow to fight against its own weight (hydrostatic pressure) to rise from one to the other. In the presence of +Gz, the force of inertia oriented on the head-foot axis will add to the hydrostatic force and aggravate the situation by opposing the movement of blood from the heart to the head.

Above +3Gz maintained for more than ten seconds, our self-regulatory mechanisms are overwhelmed, with the immediate consequence of reduced vision and mental performance. This can result in visual disturbances such as "grey haze" (from 3-4.5G, due to reduced blood flow to the retina and peripheral vision) and "black haze" (from 4.5-6G, with blood flow stopped).

Negative accelerations (-Gz) trigger the opposite adaptation mechanisms to those of +Gz, accompanied by more unpleasant sensations and greater perceived fatigue.

But the essential problem lies in the rapid succession of -G and +G at high values ("push-pull" effect), as in aerobatics, which is particularly poorly tolerated. This is due to the disruption of our adaptive mechanisms and our greater sensitivity to the phenomena of veiling and/or loss of consciousness that can occur as early as +2Gz.

Identify the limits...

If the cardiovascular system's response doesn't keep pace with the onset of Gs, the pilot's performance will be degraded to the point of unconsciousness. To avoid this dangerous outcome, studies have helped us better understand the limits of our adaptive capacities and develop techniques to overcome them.

The establishment of +Gz-time tolerance curves has enabled us to compare asymptomatic and symptomatic individuals. The upper limit of these curves, marked by loss of consciousness (LOC-G), is an essential factor in our physiological response to acceleration.

It has been shown that if the increase in acceleration is gradual, visual symptoms precede cerebral symptoms. However, for accelerations above +7Gz reached rapidly, loss of consciousness is not preceded by warning signs. This is because, if the acceleration rate is sufficiently low, cardiovascular reflexes can at least partially compensate for changes in circulation. The tolerance threshold is thus increased.

Generally speaking, it has also been found that everyone's sensitivity to these effects is variable and can be modified with practice. Several factors can influence acceleration tolerance.

If the heat is not too great, a well-rested, hydrated and physically fit pilot will be able to tolerate +5Gz. This is because the volume of blood circulating in the body is greater and more available, making it easier for the cardiovascular system to keep the brain perfused with oxygenated blood.

... To overcome them: expert pilot training

Expert pilots also use musculo-respiratory movements: head tucked into shoulders, leaning forward to reduce the height of the hydrostatic column, contraction of abdominal muscles and lower limbs to slow blood flow, intrathoracic overpressure by expelling air, or closed glottis with the diaphragm and neck muscles tightly contracted.

A regular physical training program including a mix of endurance and strength exercises also increases the rider's tolerance to the effects of Gs. Important factors to consider are core strength and aerobic capacity. Any aerobic endurance activity (even in apnea or at altitude) is good for the cardiovascular system.

Trunk-strengthening exercises (sheathing, push-ups, pull-ups, sit-ups) and, above all, those that strengthen neck muscles are a must: high Gs mean that the head weighs more than normal, and with a helmet, that's a lot of weight to bear. Pilots of the fastest, most agile aircraft have to keep a constant eye on their external landmarks, and modify their head position as they manoeuvre.

Aerobatics is responsible for the onset and/or aggravation of spinal pain. Muscular reinforcement to cope with repeated high accelerations is essential for these pilots, who are considered to be high-level sportsmen and women, flying in extreme environments.

A number of tools can also improve individual tolerance to acceleration. Developed early on during the world wars, anti-G pants, by applying counter-pressure to the lower body in response to accelerations, help to ensure sufficient venous return. However, these devices only treat +Gz and are unsuitable for aerobatic aircraft due to their weight.

Other innovative devices are being developed by research centers and companies in the sector. For example, EuroMov Digital-Health in Motion and Semaxone are developing algorithms and sensors to measurecerebral oxygenation in real time, in order to anticipate changes in pilots' tolerance to acceleration.

The author would like to thank Mr. Jacky Montmain, University Professor (IMT Mines Alès, EuroMov Digital Health in Motion), for his review; Mr. Gérard Dray, University Professor (IMT Mines Alès, EuroMov Digital Health in Motion), for his review; and Mr. Guilhem Belda, engineer (CEO Semaxone), for his input and review.

Stéphane Perrey, PR, Director of the Research EuroMov Digital Health in Motion Unit, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.