What are the physiological consequences of the speed and accelerations on board such an aircraft? How do pilots prepare? Explanations.
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Our species is acclimatized to a world placed under the yoke of constant gravity – in this case, an omnipresent accelerating force born of the terrestrial attraction (the unit of terrestrial gravity, denoted g, equal to 9.81 m/s2). There are however circumstances where our body is subjected to stronger than the traditional terrestrial gravity… It is there still a matter of acceleration.
In aeronautics or in the automobile, specialists refer to the G (for Gravitational), or load factor, as a unit of acceleration. And its effects can be dreadful. As children learning to walk, we very quickly discover that one misstep will eventually result in a painful impact with the ground due, precisely, to gravity. When we get on a plane, this time short of crashing, everything we've learned about gravity and what we're used to suddenly changes. One only has to see Pete “Maverick” Mitchell's latest aerial convolutions in the latest Top Gun to be convinced.
Flight is indeed about overcoming gravity to rise through the air, and speed is essential. Any aeronautical maneuver can therefore expose our body to significant accelerations, with significant repercussions on the cardiovascular, cerebral and joint levels. Some aircraft are thus capable of reaching 12G, with acceleration climb speeds of over 15 G/s!
How many Gs do we experience on a daily basis?
Such figures are of course extremes. While remaining motionless on the ground, the acceleration felt is 1G. Everything is fine. At 2G, for example when taking a 60 degree inclined turn, we already have a feeling of moderate compression on our seat, a difficulty in moving. A person weighing 80 kg, or a weight of 784.8 N on Earth (considering it to be a situation equal to 1G), will feel like they weigh 1569.6 N if they experience 2G (the kg being the unit of mass, the Newton that of weight). From 8-9G, it is impossible to mobilize its limbs, except for the extremities.
In fact, there are three major types of G present in three axes of space. We can experience lateral Gs (Gy) when turning as a result of centrifugal acceleration pushing us outward. For a horizontal acceleration or deceleration, we speak of Gx. Finally, Gz occurs during a descent of the aircraft or following a sudden climb. We are more particularly sensitive to these accelerations undergone in the vertical axis (Gz), that is to say from head to toe, since it is there that we feel the force of the earth's gravity necessary to maintain its balance.
To further complicate the situation, for all three axes, both positive and negative G's are possible… Whether it's turning for a car or vertical for an airplane, a resistance to movement, the force of inertia, is added to the actual weight due to gravity to give the “apparent” weight of the aircraft 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 upside down, for example, the load factor is expressed as a negative, – G.
To calculate the G to which they are subjected, aircraft pilots, particularly exposed, are equipped with a three-axis accelerometer so they can know in real time what they are going through.
How our body normally manages gravity
The airplane pilot is indeed subjected in flight to a wide variety of physiological effects due to the combinations of acceleration and gravity. They are inherent in the forces of inertia generated by accelerations and apply to all the organs of the body, and in particular to the cardiovascular system: the heart (the pump), the vessels (the circuit), the blood (the fluid).
However, blood circulation ensures the transport of oxygen, which is essential for the proper functioning of the organs. The brain is particularly demanding in this area, both in terms of consumption (it is greedy) and the regularity of its supply. He doesn't like jerks, excesses, or misses!
On Earth, there is a complex mechanism of control and adaptation of all the machinery which ensures regular and well oxygenated blood circulation at a constant flow to the brain, whether it is at rest or in full effort: it is the cerebral self-regulation. Any variation in blood pressure is thus without consequence. But this beautiful balance has its limits… Acceleration in turns, braking or a fortiori the practice of aerobatics will disturb it greatly. The ability to maintain a cerebral blood supply, resilient in the face of repeated exposures to increased load factors, is therefore a critical issue for pilots outside normal everyday conditions.
When our adaptations physiological are no longer enough
The risks were identified, though poorly explained, more than a century ago. In 1918, the first disturbance induced by acceleration was thus felt during the air race of the Schneider Cup where a sharp turn had to be taken. First described as “unconsciousness in the air”, it is now known as “G-induced loss of consciousness”, or G-LOC, and results in confusion and impaired judgment at the following a temporary abolition of cerebral circulation. A state that occurs from +4.5-6G in the trained pilot.
As the heart is in the thorax, in a vertical position (standing or sitting), the vascularization of the brain, positioned above it, forces 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-to-feet axis will add to the hydrostatic force and aggravate the situation by opposing the movement of blood from the heart to the head.
< p>Beyond + 3Gz maintained for more than ten seconds, our self-regulation mechanisms are overwhelmed with the immediate consequence of a drop in vision and mental performance. This can result in visual disturbances such as “grey haze” (from 3-4.5G, due to decreased blood flow to the retina and peripheral vision) and “black haze” (from 4.5 -6G, with cessation of blood flow). Negative accelerations (-Gz) cause opposite adaptation mechanisms to those of +Gz, accompanied by a more unpleasant feeling and greater perceived fatigue.
But the main problem lies in the rapid succession of – G and + G at high values (“push pull” effect, or pitch-up), as in aerobatics, which is particularly poorly tolerated. This stems from the disruption of our adaptation mechanisms and our greater sensitivity to veiling and/or loss of consciousness phenomena that can occur from + 2Gz.
Identify the limits…
If the response of the cardiovascular system does not keep pace with the appearance of the Gs, the pilot's performance will be degraded to the point of causing loss of consciousness. To avoid this dangerous extremity, studies have helped to better identify the limits of our adaptive capacities and to develop techniques to overcome them. The establishment of tolerance curves + Gz-time allowed comparison of asymptomatic and symptomatic individuals. The upper limit of these curves, marked by the loss of consciousness (LOC-G), is an essential factor in our physiological response to accelerations.
It appeared that if the increase in acceleration is gradual, the visual symptoms precede the cerebral symptoms. However, for accelerations greater than +7Gz reached quickly, loss of consciousness is not preceded by warning signs. Indeed, if the rate of rise in acceleration is sufficiently low, the cardiovascular reflexes can, at least partially, compensate for the modifications of the circulation. The tolerance threshold is thus increased. In general, it has also been found that everyone's sensitivity to these effects is variable and can be modified with practice. Several factors can affect tolerance to accelerations.
If the heat is not too much, 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 more important and available: it is then easier for the cardiovascular system to keep the brain perfused with oxygenated blood.
… to overcome them: expert pilot training
Expert pilots use in addition musculo-respiratory movements: head tucked into the shoulders leaning forward to reduce the height of the hydrostatic column, contraction of the abdominal muscles and lower limbs to slow the flow of blood, intrathoracic overpressure by expelling the air or closed glottis with greatly contracted diaphragm and neck muscles. jpg” alt=”What fighter pilots' bodies go through” />
A regular physical training program that includes a mix of endurance and strength exercises also increases the pilot's tolerance to G effects. Important factors to consider are core strength and aerobic capacity. Any aerobic endurance activity (even snorkeling or altitude) is good for the cardiovascular system.
Core-strengthening exercises (core-strengthening, push-ups, pull-ups, sit-ups) and especially those that strengthen the neck muscles are a must: high G's make the head weigh more than normal, and with a helmet, that's a lot of weight to bear. Pilots of the fastest and most agile aircraft must constantly monitor their outward landmarks and alter their head position as they maneuver.
Aerobatics is responsible for the onset and/or aggravation of spinal pain. Muscular reinforcement to cope with repeated strong accelerations is essential for these pilots, considered as high-level athletes who, moreover, evolve in extreme environments. Several tools can also improve individual tolerance to accelerations. Developed very early during the World Wars, the anti-G pants, by applying counter-pressure to the lower part of the body in response to accelerations, helps ensure sufficient venous return. However, these devices only deal with + Gz and are inappropriate in aerobatic aircraft due to their weight.
Other innovative devices are under development in research centers and companies in the sector. This is the case of the work carried out by EuroMov Digital-Health in Motion and the company Semaxone, which are developing algorithms and sensors to measure cerebral oxygenation live to anticipate the evolution of pilots' tolerance to accelerations.
*Stéphane Perreyis PR, Director of the EuroMov Digital Health in Motion Research Unit, at the University of Montpellier.