Background of the ProblemOne of the major stresses associated with flying high performance aircraft is acceleration (G). The letter 'G' represents the acceleration being experienced. When we are at rest on Earth, we are experiencing 1G. This is because of the gravitational attraction Earth possesses. During acceleration, the amount of G forces we experience increases. The amount of G forces that can be experienced while flying is typically much higher than normally sustained on Earth due to the inherent design of aircraft. A common example of G forces is when a bucket of water is swung in a circle. G forces are what keep the water in the bucket and prevent it from spilling.
Humans are not designed to tolerate environments outside of the typical 1G habitat for long. During periods of G forces greater than 1, the potential to experience a G-induced Loss of Consciousness (G-LOC) increases. A G-LOC occurs when the brain and eyes are deprived of blood.
When experiencing positive G forces (typically in excess of 3Gs, depending on the onset rate) the blood in a human's upper body is pulled away from the head (Newman, 2002). When the heart is unable to provide the brain and eyes with blood, and their short supply of oxygen and glucose is diminished, G-LOC occurs (Watson, 1990).
G-LOC was first reported in 1918, and loosely defined as fainting in the air. After this event, a small amount of research was conducted to study this phenomenon. As early as the 1920s, aircraft were restricted on their turn rates to help prevent G-LOC. In the early 1930s, the Royal Air Force came to the conclusion that 4Gs was the human threshold (Balldin, n.d.). Since then, much research and training has occurred in hopes of preventing pilots from experiencing G-LOC. The necessity for so much training and research comes from the fact that aircraft today are capable of sustaining over 9G's and able to produce an onset rate of over 15G's per second (Newman, 2002).
The History and Background of G-LOCG-LOC has been identified as a risk to humans virtually as long as aircraft have been developed. Possibly one of the first articles that focused on G-LOC was published in 1919. This article discussed a symptom of "fainting in the air" in different aircraft, including the Sopwith Camel and DeHaviland (Watson, 1990). The first report of "fainting in the air" has been documented as occurring sometime in 1914 (Patten, 2003). The article accurately described the problems associated with G-LOC and was fairly accurate in the assessment that G-LOC occurred when 4.5 - 4.6 Gs was reached (Watson, 1990).
Contestants of the Pulitzer races in the 1920s also had complained of suffering from G-LOC related symptoms (Watson, 1990). Jimmy Doolittle, a graduate student at the Massachusetts Institute of Technology, conducted research following this finding. He conducted his research while stationed at McCook Field and concluded that the average Gs humans could temporarily sustain were around 4.5. Doolittle also identified that G-LOC was a direct result of cerebral circulation (Patten, 2003). Around this time frame, it was also first discovered that straining maneuvers could help improve a person's tolerance to G- forces (Watson, 1990).
Aircraft of the 1930s were still lacking in their ability to sustain high Gs, mainly due to the technological limitations imposed that resulted in little engine thrust and elevated aerodynamic drag. One technique that was used in these aircraft that did result in serious complaints of G- symptoms was dive-bombing. The high rates of acceleration that occurred combined with the need to quickly pull up at the end of the maneuver resulted in higher than normal Gs. Many pilots reported that they would black out during this time. Because of this, the Royal Air Force came to the conclusion that 4 Gs was the limit of human tolerance to acceleration. The US Navy also implemented an "acceleration belt" to be used prior to dive-bombing. The belt was wrapped around the pilot's abdomen and inflated via an external pump. It was suggested that this device had minimal impact on the pilots G- tolerance (Balldin, n.d.).
Another important research device that first began to be used strictly for aviation purposes during this time frame was centrifuges. Centrifuges are machines capable of producing acceleration by suspending a capsule on the outside of a 20 - 30ft. control arm. The capsules rotation is controlled by a motor; an increase in rotation is relative to the increase in G- forces. Germany and the United States were the first countries to begin using centrifuges for aviation research (Balldin, n.d.). Research on a human's tolerance to Gs can be done more efficiently and safely using a centrifuge. Because of this many records have been set using them. In 1958 a Navy Officer named Carter Collin maintained 20 Gs for 54 seconds. Later during this time period, a researcher maintained alertness throughout a 25 second long run to 32Gs while encased in an aluminum shell filled with water (Wolverton, 2007). In response to the increased performance of aircraft in the 1970s and 1980s, most centrifuges were modified to maintain higher Gs (some as high as 12 Gs) and also to achieve higher G- onset rates (Balldin, n.d.).
Around the time frame that the Second World War took place, many important discoveries were made that helped to better understand and prevent G-LOC. Research was made that identified the importance of pilot health and experience in preventing G- related symptoms. It was also discovered that a pilot who flies and experiences Gs on a more constant basis will have a higher tolerance than a pilot who is less practiced. The most important finding that came from this time period was the invention of the G-suit. It has been suggested that the combination of the G-suit along with straining maneuvers was sufficient enough for reducing the risk that Gs placed on pilots. Because of this, little research was conducted after the 1950s, until aircraft capable of higher onset rates and higher G- forces became available in the 1970s (Watson, 1990).
Physiological Effects of G- ForcesThe human body is designed to maintain life in a 1 G environment. As a pilot begins to increase their number of G- forces, they will essentially feel their weight begin to increase (Watson, 1990). If a pilot weighing 200 pounds is pulling 9 Gs, they will feel approximately 1,800 pounds of force being applied onto their body (Patten, 1991). Modern aircraft are capable of going from 1 to 9 Gs in less than one second. The more Gs a pilot subjects him or herself to the more blood is forced out of the upper body (including the brain) and into the lower part of your body (Newman, 2002).
During periods of increased Gs, movement of the head and arms are restricted and a sensation of being pushed into your seat occurs (Watson, 1990). At 2.5 Gs it is difficult to raise yourself from your seat. The ability to raise your limbs is severely limited over 3 Gs. Movement is virtually impossible over 8 Gs, however if a pilots arm and hand is supported effectively, they are still able to operate a control stick and the associated buttons installed on it up to 12 Gs (Balldin, n.d.).
When a human being is standing, the amount of blood present in the lower body is proportionally more than in the upper body. This is because of the gravitational effect of the Earth, even while remaining stationary in a 1G environment. Any increase in the number of Gs being sustained will amplify this effect. Typically between 3 to 4 Gs, a person will begin to lose their visual acuity (Newman, 2002). Symptoms during this period include loss of peripheral vision, followed by tunnel vision as the stress increases or is sustained (Patten, 1991). These symptoms are commonly referred to as grey-out (Newman, 2002).
Normally between 4 and 4.5 Gs all visually acuity is lost. This period is known as black-out. Pilots are still able to fly the aircraft during this time, including being able to speak and hear. Pilots are typically trained that when black-out occurs they are nearing their tolerance for Gs (Newman, 2002). The loss of visual acuity can directly be related to the loss of blood pressure at the eyes (Patten, 1991). The average eye-level blood pressure for a person at rest is approximately 85 millimeters of mercury (mmHg). Every 1G increase reduces that blood pressure by about 20 mmHg (Whinnery, n.d.).
This helps to understand how visual ability is lost near the 4G threshold. As the heart is unable to pump blood to the eyes (due to being unable to overcome the increased force of gravity) the arteries in the eyes remain unfilled. Soon the eyes cease to be perfused and black-out occurs (Patten, 1991).
The average person will experience a complete G- induced loss of consciousness between 4.5 and 5.5 Gs, although G-LOC has been reported to have happened while maintaining as low as 2 Gs. During these stress levels, the brain and eyes are deprived of blood flow which is critical in maintaining the crucial levels of glucose and oxygen (Watson, 1990). G-LOC can first be manifested as a fixation or stare in the visual system. After this, the eyes will typically roll back into the head. Complete loss of muscular ability ensues shortly thereafter. At this point, the pilot is completely incapacitated and no one is flying the aircraft (Patten, 1991). This period lasts approximately 15 seconds and is known as the "absolute incapacitation period" (Newman, 2002).
During the absolute incapacitation period, the pilot's head will drop to his or her chest (Balldin, n.d.). Occasionally this is also accompanied by flailing movements that will also include the arms (Watson, 1990). During G-LOC, the pilot is unable to maintain their grip on the control stick, and the aircraft will typically return back to a 1G setting (Newman, 2002). If the pilot were to sustain these G- forces and remain unconscious, brain damage could result (Watson, 1990). The symptoms and syndromes of G-LOC are all part of a protective mechanism the human body puts in place with the ultimate goal of protecting the brain. It should be noted that only rarely does the loss of bowel or bladder control during G-LOC occur and that this should not be considered a symptom of G-LOC (Whinnery, n.d.).
After the approximate 15 seconds of absolute incapacitation period, the pilot will enter into a stage known as the "relative incapacitation period". The relative incapacitation period lasts approximately 10 - 15 seconds during which the pilot is technically conscious, but not capable of decided action (Newman, 2002). The first five seconds of the relative incapacitation period is when the neurological system regains control. The remaining time (typically about 10 seconds) is spent reorienting to the pilots environment (Whinnery, n.d.). The relative incapacitation period is usually accompanied by short periods of confusion, disorientation, fear, anxiety and embarrassment (Watson, 1990). The pilot's ability to recognize their environment and to regain control of their aircraft signifies the ending point of the relative incapacitation period and the G-LOC episode in general (Newman, 2002). The average G-LOC total incapacitation period lasts about 30 seconds, but has been reported to last as long as 3 minutes (Watson, 1990). Unfortunately, this does not always happen prior to ground impact (Newman, 2002).
Often following a G-LOC incident, a pilot will have no recollection of what happened (Watson, 1990). Short dreams are often experienced by pilots during a G-LOC. It has been suggested that the purpose of these dreams is to make the pilot aware that a loss of consciousness has occurred. When a G-LOC goes unnoticed by a pilot, their recognition of the importance of the incident as well as gaining a better understanding of their G- tolerance is loss (Whinnery, n.d.).
The G-LOC symptoms described thus far are all based on the assumption that the onset rate of G- forces is 2Gs per second or less. At these lower onset rates, the eyes are affected prior to the brain. When exposed to higher onset rates, the loss of blood flow to the upper body will be sudden and prompt. This causes both the eyes and brain to stop functioning at close to the same time, giving the pilot no warning of the impending loss of consciousness. The Eurofighter aircraft is capable of onset rates of 15Gs per second. These high onset rates can only be tolerated for a short amount of time. The brain normally has a reserve supply of oxygen that it uses for occasions such as these, however these supplies will only last about 3 - 5 seconds. Once the oxygen is used up, G-LOC will occur (Newman, 2002).
Recent research has also suggested that another effect of G- forces is a symptom known as almost loss of consciousness (A-LOC). During episodes of A-LOC, some impairment of the brain occurs but the pilot remains technically conscious. Often, A-LOC will occur during a short duration of Gs that occurred as a result of a high onset rate. Symptoms of A-LOC often include the loss of speech or temporary incapacitation (Newman, 2002).
Increasing Acceleration ToleranceBoth military and civilian aircraft are capable of sustaining and providing G- onset rates that could lead to a G-LOC. Many discoveries and innovations have been made that help pilots cope with these G- forces. In some applications, the consideration of aircraft design, implemented equipment, and pilot physiology will allow a human to sustain G- forces well beyond the normal threshold (Whinnery, n.d.).
Recent technological advancements in aircraft control, specifically fly-by-wire flight controls, allow the aircraft to limit the amount of Gs sustained (Patten, 1991). It has also been demonstrated that an aircraft's seat position has an impact over how many Gs a person can sustain. For example, the F-16's seat is reclined at an angle of 30 degrees. This 30 degree seating position has been reported to add about 1Gs worth of additional protection to the average human body. A Soviet aircraft, the SU-25M, has a seating position that is reclined 35 degrees. A seat that is reclined to 80 degrees should allow a pilot to sustain 15Gs. However, this seating position would not be of practical use due to the limited field of vision it would give the pilot (Watson, 1990). A pilot able to fly his aircraft in the prone position would be the most beneficial arrangement in relation to sustaining high amounts of G- forces (Balldin, n.d.).
The most common protection device used by military pilots is the G-suit. A G-suit is fundamentally a pair of pants containing air bladders. The pilot wears these pants, typically over a standard flight suit, when he is flying a high performance aircraft. A hose that emanates from the G-suit is plugged into a valve on the aircraft. During periods of increased Gs, the valve opens and fills the air bladder inside the G-suit with air. These bladders help to squeeze the legs and abdomen of the pilot, helping to retain blood in the upper portion of the body (Watson, 1990). The G-suit can help to add 1 - 2G levels of protection to the human body (Patten, 1991). Most modern G-suit systems also include a pressure breathing option that forces air into the pilot's lungs. It is suggested that this pressure breathing capability helps to provide increased G- protection (Whinnery, n.d.). It has also been found that breathing 100% oxygen during periods of high Gs can help to increase G- tolerance (Watson, 1990).
A pilot's level of physical fitness, health and physical attributes also have a direct correlation to the number of Gs he or she is able to sustain. Flying with a sickness can greatly reduce the bodies' ability to tolerate G- forces. Not only does an illness typically reduce your strength, fevers can often dilate blood vessels which will allow an increased effect of blood pooling under Gs.
Dehydration also has negative effects on G- tolerance. When dehydrated, the body possesses a smaller volume of blood. This allows a greater amount of it to be taken to your lower extremities and away from the brain (Newman, 2002). Being dehydrated is often a direct result from consumption of alcohol or caffeine (Harradine, 1999). It has been found that alcohol can reduce a pilots G- tolerance by up to 0.5 Gs. The typical hangover symptoms of a hangover from alcohol including headache, fatigue and digestion problems can last up to 48 hours. This time period typically exceeds the normal legislation regarding how long a pilot must wait before flying after consuming alcohol (Newman, 1999).
The length of time one can sustain high Gs can be increased by up to 53% by participating in an aggressive physical training program (Watson, 1990). Today's military pilots are usually required to follow strict exercise programs. These programs usually focus mainly on strength training, and rely moderately on aerobic training. This is because of the finding that people who posses a high level of aerobic fitness actually are less tolerant to G- forces. Anaerobic training such as lifting weights is more helpful to pilots. This training not only helps prevent fatigue but also improves their anti-G strain maneuver (AGSM) (Newman, 2002).
There are a few other factors that can help a pilot tolerate Gs better. Shorter pilots have been found to have an increased tolerance to Gs compared to taller pilots. Also, people with higher blood pressure levels can sustain Gs better than those with lower blood pressure levels (Whinnery, n.d.). It is important for a pilot to maintain his lifestyle in a manner that does not place him or her in danger of being any more susceptible to G-LOC.
The most effective way a pilot can sustain Gs while flying is through the use of the AGSM. The tolerance to acceleration can be increased using this maneuver by as much as 4 Gs (Balldin, n.d.). There are multiple AGSM's, the most common being the Valsalva, M-1 and L-1 methods (Watson, 1990). An AGSM consists of two basic steps. The first is the contraction of muscles in the abdomen and legs. The second is a specific breathing technique (Balldin, n.d.). The breathing practice can be described as taking a deep breath and holding it for three seconds. Upon expiration of the three seconds the air should be expelled violently and a quick gasp or inhalation should be made. This should be repeated until the level of Gs is reduced. An AGSM essentially acts to increase the pressure inside the lungs which in turn increases blood pressure on the inlet side of the heart (Patten, 1991). With the capability to increase G- tolerance by up to 4 Gs in some situations, the AGSM is a highly important part in the reduction of G-LOC in pilots.
Another way of helping pilots cope with Gs is to ensure that they remain current in pulling them. A persons G- tolerance will decrease with the amount of time they spend away from flying aircraft and experiencing Gs. For this reason, it is suggested that pilots who haven't flown for an extended period of time ease into pulling Gs again (Watson, 1990).
Quite possibly the most important part of reducing loss of aircraft and human life due to G-LOC is the training that a pilot receives on the risks of G- forces. Training of USAF pilots typically begins in the classroom. Usually a flight surgeon or physiological training officer will give a detailed lecture about the risks of G- forces and how to prevent them. The AGSM is also first taught in a classroom setting. The AGSM is heavily emphasized since it is a cheap, quick and easy way to significantly increase G- tolerance. Following classroom training, students are moved into the centrifuge. Here they are exposed to various levels of G- forces in order to get acclimated to the sensation and become familiar with their natural tolerance. After this, they are encouraged to practice and perfect their AGSM in the centrifuge as well. Since pilots are often looking behind them while dog-fighting, they are also given the opportunity to experience Gs and practice the AGSM while in the "check six" position. A pilot graduates from this training upon successful demonstration of the AGSM and by showing the capability to withstand 8 - 9 Gs. The Soviet Air Force has claimed to have never lost a pilot due to G-LOC. According to Major General Ponomarenko, the commander of the USSR Air Force Institute of Aerospace Medicine, this has a high relation to the fact that Russian pilots participate in a recurring G-currency training class (Patten, 1991). No detailed information could be obtained as to the recurring training US Air Force pilots receive in regard to G- awareness and prevention, if any at all.
An Overview of Recent G-LOC Related StudiesMany studies have been recently conducted to help understand the effects of Gs on humans. These studies have directly led to many interesting discoveries and an increased knowledge in the realm of G-LOC. An example of a recent discovery that occurred in 2005 was that caffeine can help to increase G- tolerance. This study was carried out using Rhesus monkeys that were injected with a caffeine solvent and accelerated using a centrifuge (Bonneau, 2005).
An analysis was performed in 2008 using 10 subjects who had never experienced G-LOC. Multiple uses of a centrifuge allowed the researchers to compile information from 35 instances of G-LOC that had occurred from the 10 subjects. From this data, they were able to compile the mean times it took for the subjects to experience the various phases of G-LOC. They found that the average G level required to accomplish G-LOC was 6.73 seconds. They also determined that the mean absolute incapacitation period was 13.9 seconds, with a mean relative incapacitation period of 15.1 seconds (Reis, 2008).
As mentioned earlier, A-LOC is a somewhat new discovery and phenomena. A test conducted on nine individuals using a centrifuge in Pennsylvania brought them to the threshold of unconsciousness. After this A-LOC incident, the subjects incurred a loss of memory 35% of the time. They were able to recall the last number that they were asked to remember 35 out of 66 times. Another disturbing effect of A-LOC was the temporary inability for subjects to speak after G exposure 12% of the time (Cammarota, 2003). This study helped researchers to understand that even undergoing A-LOC can play quite a negative role in the operation of an aircraft.
A survey conducted with 65 members of the Royal Australian Air Force (RAAF) found that 98% had experienced a visual or cognitive disorder, which includes G-LOC and A-LOC, during their career. Twenty nine percent reported a complete blackout; the most common experience was grey out. Thirty four pilots admitted to have experienced an episode of A-LOC in which the most common symptom was an abnormal sensation in the limbs. Nine percent of the pilots reported have experienced a complete G-LOC event. The RAAF conducts regular G- education classes but does not use centrifuge training (Newman, 2005).
There have been 29 aircraft lost in the USAF due to G related mishaps between 1982 and 2002. Of these 29 aircraft losses, a 79% fatality rate is observed. G- related mishaps in the USAF from 1998 to 2003 have remained at approximately 30.2 per year. All of these mishaps and incidents have occurred despite the high level of training USAF pilots receive. This training includes classroom lectures, centrifuge qualification, the fighter aircrew conditioning test and proper demonstration of the AGSM. A program that was conducted at Luke AFB Arizona attempted to identify high-risk F-16 pilots during their training phase and assist them in raising their G- tolerance and discipline levels. Pilots were identified and admitted to the program based on previous scores they achieved relating to sustaining Gs. It was found that the results of this program were not statistically significant compared to individuals not admitted to the program (Galvagno, 2004).
In 2004, a study was conducted using data compiled by the USAF from 1982-2002. The authors of this research came to some interesting conclusions. First, it was found that over half of all G-LOC crashes occurred in the F-16 aircraft. Second, a conclusion was reached that formal G- awareness training had a greater impact on reducing G-LOC crashes than the implementation of a pressure breathing for Gs (PBG) system did (the PBG system was introduced into the F-16 in the early 1990's and provided a constant flow of air during periods of higher Gs). Lastly, a crash rate (stating Gs as the cause) of 12.8 mishaps per million flight hours occurred between 1982 and 1984. This rate was observed prior to the USAF implementing their G centrifuge training. After G centrifuge training was introduced, the rate fell to 2.3 (Binder, 2004). These facts are helpful in recognizing the importance of training regarding G-LOC and G- awareness.
Another study was performed that analyzed the same data from the USAF during the years of 1982-2002. This research found that crash rates due to G-LOC were highest in single-seat fighter aircraft, and that a crash emanating from G-LOC has never occurred in an aircraft occupied by two or more pilots. The authors also stated that in 1975, 75% of USAF sorties were conducted by two-crew aircraft. By 1982, single crew aircraft sorties exceeded 50%. Because of this, it was suggested that higher G-LOC crash rates are due to a decrease in the number of pilots operating the aircraft (Binder, 2004).
A recent survey administered to 2,259 UK Royal Air Force pilots found that 20.1% had lost consciousness sometime during flight. This study focused not only on aircraft fighter pilots, but also less high performance pilots including rotor craft (Ford, 2006). A case study conducted in 2005 found that since 1993, the F-16 has had a predominantly increasing rate of G-LOC mishaps. This is fairly odd considering that the other aircraft studied (A-10 and F-15) have shown a steady decrease in mishaps directly related to G-LOC. Thirty one percent of all F-16 class-A mishaps (resulting in the loss of an aircraft or life, or over $1 million in damage) occurred from a direct result of a G-LOC. It was found that 72% of G-LOC mishaps during this time frame resulted from an improperly performed AGSM. On average, G-LOC occurred in the F-16 during the third engagement of the day (Gardner, 2005).
ConclusionG- forces have been playing a negative effect on pilots since the early 1900's. In the 1930's, the human tolerance was identified to be approximately 4 Gs. Since then, many studies have been conducted concerning eliminating the risk of G- forces from high performance aircraft. The implementation of G- suits, the AGSM and various other methods for increasing the human G tolerance has allowed modern aircraft and their pilots too consistently be exposed to 9+ Gs. Despite this, G-LOC still plagues the pilot community and poses a serious risk. Increased research, training and education are still ongoing in hopes of removing the menacing possibility of G-LOC all together.
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