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Friday, 02 December 2011 14:58

G-LOC Part 1

Written by  Snap&Roll / Dr. Carme Reguant
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Ejemplo de un piloto en G-LOC a solo 5,5 Gs

G-LOC is known as G-force Loss Of Consciousness caused by spending a period of time in acceleration or G's.

Our brain is the most important organ of our body, and besides also being that which pays the gas bills to fly, it is the most essential component within the aerobatic plane. One might expect that it works when everything else has failed, but for that to be true, it would have to have an STBY energy storage system and completely be independent of other organs and tissues.

Unfortunately, the reality is that our brains only have a reserve of energy to operate for 5 seconds without a continuous flow of oxygen and glucose (sugar), delivered by the blood and continuously pumped to the brain from the heart – the CBF (Cerebral Blood Flow). Stop the CBF for 5 seconds, and the brain instantly turns off completely (G-LOC). Although the CBF is fully restored immediately, the brain needs <15 seconds to boot-it again (faster than Windows!), <15 seconds to act (we're up to 30 seconds), 3 minutes to do anything considered useful, and <30 minutes before we can fly an instrument approach in an acceptable manner. A plane descending in upright position during aerobatic maneuvers, say at a speed of 120 kts, only needs 30 seconds to reach the ground from a height of 6000 feet. As we can see, this is enough time for us to generate panic in the judges and our observers.

G-LOC is not just a product of our modern age that only happens with fighter planes maneuvering at high speeds and supporting more than +12 G's. G-LOC has been around since man has tried to venture beyond straight and level flight. In the twenties, they described the problem that caused G-LOC in pilots flying military aircraft Sopwith Camel, Sopwith Triplane and DeHaviland. Over the years, 20 severe experiments have shown that a pilot can easily go into G-LOC working only between 4.5 to 6 G.

Circulación arterial del celebro

How does it happen?

Our human body is designed to naturally withstand gravity at +1 Gz (Gz is defined as the acceleration to our bodies when in an upright position) and + Gz increases occasionally when we run, jump or fall. By nature, we are not designed to withstand the prolonged and high accelerations that occur during aerobatic maneuvers in an aircraft. For this reason, we have to train on a regular basis, with heavy physical exertion and occasionally using mechanical aids. Even so, this will only marginally increase our tolerance for Gz.

According to Dr. Carme Reguant, Aviation Medical Examiner at Chiron hospital in Barcelona and Sabadell Airport (Spain):

"The loss of consciousness occurs when the pilot is subjected to a sustained positive longitudinal acceleration displacing most of the volume of blood to the lower extremities, reducing the circulation of blood (and blood pressure) in the brain, an organ very sensitive to lack of oxygen supplied by blood circulation (with insufficient blood pressure to facilitate an exchange of gases from the bloodstream to the nerve tissue)."

Our brain and eyes need oxygen and glucose to function properly, having virtually no reserves of oxygen or sugar. A constant supply of these nutrients within the blood flow through the carotid arteries (major arteries that supply oxygenated blood to the brain), is necessary for these two organs to work perfectly.

As we have seen, G-LOC occurs when there is a decrease in blood flow in the brain when the magnitude of the + G's exceeds a certain value reaching the threshold area of G-LOC. But before we get to the point where neurons stop functioning in the absence of oxygen and sugar, there is a period of 3-5 seconds in which the first symptoms begin to show. These symptoms are known as gray vision (gray-out) and black vision (black-out). These symptoms precede the G-LOC, but this does not imply that having them necessarily produces G-LOC.

Symptoms of the G's:

- G's positive + Gz:

The doctor tells us, "One of the first effects on the human organism under sustained + Gz acceleration (G forces from head-feet) before reaching the loss of consciousness, are the effects at the level of vision.

With +4 Gz,+4.5 Gz, tubular vision occurs, consisting of a concentric reduction of the visual field with the central vision blurred.

Beginning at 10s at 4.5 Gz, GRAY vision or GREY OUT appears: tubular vision loses "color" and everything is seen in shades of gray.

Fases de la visión gris

If you experience gray vision, you are not very far from the loss of consciousness, and this can be corrected by relaxing the pressure on the flight stick reducing accelerations and facilitating the flow of blood to the upper body.

At +5 Gz, +5.5 Gz, BLACK vision or BLACK OUT occurs: complete loss of vision although you can hear and are conscious.

If lever pressure is not relaxed, you can enter the loss of consciousness within one second.

A +6, +6.5 Gz, you have loss of consciousness or G-LOC.

All these are physiological effects in vision due to loss of pressure in the central retinal artery as a result of displacement of the blood volume towards the foot which causes a decrease in blood flow (and drop in blood pressure) at the top of the body (head).

Other effects that occur in the body due to + Gz are:

-Sensation of increased body weight.

-Limited mobility.

-Possibility of vertebral fractures.

-Great facial distortion

-Increased heart rate and breathing.

-Possibility of occurrence of bleeding and bruising of scrotum ...

Although positive G are the only ones responsible for producing G-LOC, Dr. Reguant reminds us of the importance of negative Gs, symptoms and consequences:

- G's negative-Gz:

-Gz forces are least tolerated by the human body and we will find them in the dive, reverse loop, or significant turbulence. From-3G appears incapacitation. Physically, there is blood volume displacement at the head causing an increase in blood volume, vasodilation, increased pressure in cerebral arteries and in the cerebrospinal fluid.

During aerobatic maneuvers, it is common to go from – Gz to + Gz or vice versa, which significantly increases the effort required of the pilot to tolerate the change, since there has not been time for the launch of cardiovascular reflexes to redistribute blood volume and increase arterial pressure. A situation that favors the emergence of G-LOC.

A -1 Gz: feeling flushed in the face.

A-1Gz,-2.5Gz: vision begins to turn red.

A -2.5 Gz, -3 Gz Vision RED or RED OUT appears, which could be explained by the vision through the lower eyelid that covers the cornea or by ocular hemorrhages.

At more than -3 Gz incapacitation occurs.

Other negative accelerations symptoms are mental confusion, headache, decreased heart rate and respiratory dilation of blood vessels of the upper body and increased cerebrospinal fluid pressure.

The threshold of G-LOC:

Virtually all studies on G-LOC have been applied to military and military aircraft, but in 1988, after a fatal accident of a pilot flying a Bellanca Dechatlon occurred during an acrobatic training because of G-LOC, the "Bureau of Air Safety Investigation" of the "Royal Australian AirForce” decided it was necessary to do an intensive study on the effects of G's and their intensities, as experienced by light aircraft during aerobatic flight.

These studies, performed on normal and medically healthy people, revealed that gray and black vision as we have seen, some symptoms can be very useful in determining an impending G-LOC. But if the person is subjected to moderate accelerations maintained beyond the period of physiological reserves of the body (3-5 seconds), instant unconsciousness can be caused if the G-LOC threshold is exceeded during this time period. In this case, the loss of consciousness occurs without visual symptoms of gray and black vision.

On the other hand, we can tolerate high values of Gs (> 7) without showing symptoms of vision and of G-LOC if the time of exposure to these Gz is between 3 and 5 seconds.

Figura 1: Umbral del G-LOC

As shown in figure above, the threshold of G-LOC varies slightly with time: the possibilities of G-LOC are lower at the beginning of the period when the pressure of the heart is changing to offset the effects of the G's. At 4-5 seconds, if the body is still subjected to accelerations, we enter the threshold of G-LOC.

For example, for the average person, the body must be subject to a value of 4.5 Gs for a period of 5 seconds before they experience gray vision, black vision, and a second longer for them to lose consciousness entering G-LOC.

Once the threshold has been reached, there is a period of functional incapacitation preceding the G-LOC that lasts about 15 seconds and you need about 30 seconds to completely recover after the loss of consciousness. At the same time, it was revealed that the pilots experienced total amnesia, explaining why this phenomenon has been frequently unrecognized.

RAF, USN, and RAAF pilots also revealed that G-LOC is not only applicable to high-performance aircraft. They noted that a high proportion of G-LOC incidents were at values of 4Gs or lower and occurred in low performance, training aircraft.

In the video we see several cases of G-LOC. It is worth mentioning that the boy at 2:00, after losing consciousness and having his head and body knocked around, believes is seated correctly.

The "U.S. Naval Aerospace Medical Research Laboratory" suggested to the "Bureau" that in this type of aircraft, the greatest danger occurs in performing maneuvers when the subject is under negative Gz just before exposure to high positive Gz. Negative Gz maintained for some seconds causes an overpressure at the top of the heart, slowing the heart beat (bradycardia) and reducing drastically the tolerance for positive Gz exposure immediately after negative Gz.

 
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