A “G” is a multiple of the gravitational pull of Earth. When a bucket full of water is spun around in a horizontal circle on a rope, Gs are what keep the water from spilling out. The water is being accelerated toward the center of rotation hard enough to generate a counter (centrifugal) force that keeps it in the bucket.
The same phenomenon affects the blood nourishing a fighter pilot’s brain and vision when he maneuvers in tight turns in air combat. Not only do the Gs affect his blood supply, they also have an impact on every fluid, muscle, bone, and other tissue in his body. At nine Gs, a 200- pound pilot weighs the equivalent of 1,800 pounds, and he feels every ounce.
Over the last few years, media reports of pilot deaths resulting from G-induced loss of consciousness (G-LOC, or “G-Lock”) in high-performance fighters have underscored the current realities of life for the professional fighter pilot. The pilot who aspires to fly such fighters as the F-16 must first understand that this plane is a different proposition than the fighters of the preceding generation. In planes such as the F-4, pulling really high Gs is done infrequently and carefully. Because of the lack of limiters, it is possible to bend an F-4 seriously if it is stressed beyond design limits.
In a modern fighter, the presence of digital, fly-by-wire flight controls permits the use of “relaxed stability criteria.” This means that you can build a basically unstable aircraft, keep it under control with computers, and use it to pull Gs like nothing seen before. This is what is meant by “high-agility aircraft.”
Grave Risks for the Untrained
These design advances allow incorporation of G limiters, which prevent airframe damage and give the pilot the freedom to reach a maneuver limit of a bit over nine Gs in about one and a half seconds. In the past, any pilot who had not been properly trained to resist these effects was at grave risk.
This is why the training of the new-generation fighter pilot affects every aspect of his life, not just those having to do with the stick, throttle, and rudder.
Tolerance of high, rapid-onset Gs varies widely from individual to individual and from day to day for each individual. The eight primary factors that determine acceleration tolerance are anatomy and physiology, body orientation with respect to the G vector, magnitude of Gs, duration of Gs, rate of change of Gs, proficiency in performance of self-protection maneuvers, protective equipment, and illnesses or medications.
Additional factors make the current generation of fighter jocks resemble athletes. These include body type, physical fitness, age, blood pressure, fatigue or rest status, diet and nutrition, and dehydration.
In aviation medicine, the effects of acceleration on human physiology are separated according to the body axis through which the acceleration acts. When flight surgeons talk about Gs, they are talking about the inertial effects of the acceleration. Three physiological axes are defined, and ” + ” and ” – ” signs are used to denote the direction of the inertial force in a given axis. These axes and their directions are defined as follows: +Gz, or headward acceleration (“eyeballs down”); -Gz, or footward acceleration (“eyeballs up”); +Gy, or lateral acceleration (“eyeballs left”); -Gy, or lateral acceleration (“eyeballs right”); +Gx, or transverse acceleration (“eyeballs in”); and -Gx, or transverse acceleration (“eyeballs out”).
Thus, inside loops produce +Gz, outside loops produce -Gz, sideslips produce +Gy or -Gy, abrupt forward acceleration in the aircraft’s longitudinal axis produces +Gx, and abrupt deceleration produces -Gx.
Human tolerance of acceleration is lowest in the +Gz direction. The vertical distance from the aorta to the retina of the eye is, on the average, 350 millimeters (almost fourteen inches). When one is sitting in a chair in normal one -G gravity, one’s heart must pump a column of fluid up to the eyes and brain. Pulling +2Gz makes that fluid column twice as heavy (or twice as high, depending on how you choose to look at it). The heart must greatly increase its pressure output in order to keep the eyes and brain perfused with blood. At +5Gz, the heart must pump with even greater pressure in order to keep one conscious.
For the average relaxed and unprotected man subjected to gradually increasing acceleration in the +Gz direction, dimming vision starts at three to three and a half Gs. Loss of peripheral vision occurs at three and a half to four and a half Gs.
The Collapse of Vision
When dealing with acceleration, however, “tolerance” is a tricky word to define. Different kinds of tolerance endpoints are used. Symptoms of slowly applied +Gz acceleration are primarily visual, aside from sensations of increasing body heaviness. The earliest symptom is loss of peripheral vision, which becomes worse as the stress is sustained. The vision eventually collapses to tunnel vision, accompanied by graying or dimming of vision, followed by blackout, followed by unconsciousness.
These symptoms are caused by decreasing blood pressure at the level of the eyes. Because of this decrease in effective pressure, the heart is unable to fill the arteries in the retina, and, as this process goes on, the eyes cease to be perfused at all, and one is then “blacked out.” The pressure of fluid within is still enough to perfuse the brain, or at least some of it. For this reason, it is possible to black out and still be conscious. If the stress is continued, unconsciousness will result.
G-LOC typically starts with fixation of the gaze. Then the eyes roll up and to one side, and complete muscular collapse follows. There is a period of absolute incapacitation, and during the latter stages of this period the pilot may experience dreams and have convulsions. This is followed by a period of relative incapacitation in which the individual is nominally conscious but not capable of purposeful action. This process takes as long as thirty seconds, during which no one is flying the aircraft.
Human tolerance to rapid-onset-rate (ROR) acceleration is less than tolerance to gradual-onset-rate (GOR) acceleration. During a GOR acceleration (a less than one G per second increase in Gs), tolerance is increased by cardiovascular compensatory reflexes. Sensors in the circulatory system detect changes in flow and blood pressure and initiate changes that can raise the blood pressure. This process requires about ten seconds to develop fully and increases tolerance about one G.
In the extremely rapid buildup of Gs in modern fighters, there is no opportunity for these reflex actions to develop. The unwary pilot may lose consciousness abruptly as soon as oxygen in his brain is used up. This reserve of the brain maintains consciousness for three to five seconds, irrespective of the onset rate.
The brain’s blood oxygen reserve is responsible for the anecdotes you hear around the bar that so-and-so pulled twelve Gs without blacking out. Sure he did, but not for long.
Through training and experience, pilots of high-performance aircraft learn to fine-tune their G tolerance by observing the changes in their peripheral vision as it progresses toward tunnel vision, grayout, and blackout.
It is common for pilots to add and unload Gs to maintain a maneuver short of actual blackout. When performing an ROR run in a “snap” maneuver to a sustained high-G level, there is a real danger of an abrupt G-LOC without any warning symptoms because there is no time for cardiovascular compensation and the individual is running on the brain’s blood oxygen reserve. The anti-G straining maneuver (AGSM) can, to a degree, alleviate a rapid drop in pressure and is critical in these maneuvers.
Straining for Four Gs
The AGSM consists of a deep intake of breath, followed by breath-holding and grunting for about three seconds, followed by an explosive exhalation and repetition of the process. This act increases the pressure inside the lungs and chest and, in effect, supercharges the blood pressure on the “inlet” side of the heart. With this technique, a well-trained pilot can raise his blood pressure around four Gs’ worth.
For their baseline protection, military pilots have the anti-G suit and anti-G valve. The anti-G suit is a cutaway trouser-like garment containing air bladders over the abdomen, thighs, and calves. In accordance with the level of Gs on the aircraft, these bladders are pressurized by the anti-G valve, which admits compressed air into the bladders as necessary. The anti-G suit has the effect of increasing resistance to the pooling of blood in the legs and the abdomen and provides one to two Gs of protection, depending on its design and the type of valve being used. This level of protection, combined with basic human tolerance, is sufficient for about six Gs of maneuvering-not high enough for the F-16. Additional protective measures must be used, and the AGSM can add the necessary tolerance margin needed to fly the F-16.
The important word here is “training.” As the saying goes, “Pulling Gs makes you good at pulling Gs.” It has been repeatedly observed that a long layoff from pulling Gs reduces one’s ability to tolerate that stress. Frequent exposure to high, sustained Gs probably causes increased reactivity in the cardiovascular system, or perhaps frequent exposure begets better straining technique.
Riding the Centrifuge
When the magnitude of the GLOC problem became apparent in the early 1980s, the aeromedical community designed and implemented a program to train pilots to reach an informed awareness of how Gs affect their physiology and to perform the AGSM effectively. This program was first implemented on the centrifuges in the research facilities at the USAF School of Aerospace Medicine at Brooks AFB, Tex., and, to a lesser extent, on the centrifuge at the Armstrong Aerospace Medical Research Laboratory at Wright-Patterson AFB, Ohio.
These complex centrifuges have a rotating arm about twenty feet long, at the end of which is a gondola mounted in at least one set of roll gimbals. These machines generate the G stresses of air combat maneuvers in a supervised, laboratory environment. The most recent addition to the USAF inventory of centrifuges is the one used in the lead-in fighter training (LIFT) program at Holloman AFB, N. M. This machine is dedicated to the enhancement of G tolerance skills in the potential fighter pilot.
The training takes an entire day. It begins with a classroom lecture by a flight surgeon or physiological training officer. Also included in the classroom session is a demonstration of the proper performance of the AGSM. This is most heavily emphasized since it is the cheapest, quickest, and most effective way to raise a pilot’s G tolerance.
Following the classroom sessions, students move to the centrifuge and are individually exposed to a very-gradual-onset-rate centrifuge ride in which Gs build up at the rate of about a tenth of a G every second. In this ride, the pilot does not wear an anti-G suit and, in the early part of the run, is relaxed and not performing the AGSM. The ride continues until a pilot informs the medical monitor that he has lost a good portion of his peripheral vision. At that point, he commences the AGSM, and the level of Gs continues to build until he reaches a point at which he can no longer maintain his peripheral vision while straining. At that point, this initial run ends.
The objective of this run is to permit the pilot to learn how his personal symptoms of impending G-LOC develop and to observe how a properly performed AGSM can help overcome those symptoms to a large degree. The pilot is continuously monitored by closed circuit television and voice communication and coached and encouraged in his AGSM performance.
With the support of a flight surgeon, the pilot reviews his videotapes to understand what he is doing right or wrong. This is especially effective in cases where G-LOC occurs. For a pilot, G-LOC is a highly threatening event and is later viewed with high levels of anger, embarrassment, or denial. Denial is less rare than you’d think; an episode of G-LOC is usually accompanied by amnesia.
ROR in Check Six
After pilots complete the GOR run, they proceed to the ROR exposures. These consist of onset rates of six Gs per second up to plateaus of ten to fifteen seconds duration at levels ranging from +5Gz to +9Gz. Because fighter Pilots spend a lot of time looking over their shoulders, they are given the opportunity in training to take rapid onset runs while in the “check six” position. These ROR, high-G runs are the brass rings of the training program. On successful completion of this program, the pilot is well trained and confident of his ability to cope with eight or nine Gs in his aircraft. The success of this program has been demonstrated by the marked drop over the past three years in the rate of pilot losses to G-LOC.
The Navy has initiated a G-LOC program of this type for Navy and Marine Corps fighter aircrews, constructing a new centrifuge dedicated to training. The advanced Navy machine will be equipped with both roll and pitch gimbal systems, both of which will be powered and under computer control. This machine promises to enhance training in the coming era of supermaneuverable aircraft and G stresses in multiple directions.
Some experts favor recurrent centrifuge training. Maj. Gen. V. A. Ponomarenko, commander of the USSR Air Force Institute of Aerospace Medicine, recently stated that Su-27 and MiG-29 pilots as well as other fighter/attack/reconnaissance pilots in the Soviet forces receive G-currency training. The Soviets claim to have never lost a pilot to G-LOC.
Because exercises that strengthen major muscles also enhance performance of the AGSM, USAF pilots are encouraged to pump iron. At the same time, they are encouraged to rein in their aerobic fitness regimens. The reasons are complex, but there is strong evidence that a high level of aerobic fitness can contribute to decreased G tolerance and increased vulnerability to motion sickness. Pilots are also encouraged to be aware of the impact of diet and lifestyle on G tolerance.
It cannot be said that the new generation of fighter pilots is composed entirely of straitlaced, humorless health fanatics. There are aspects of the “fighter pilot personality” that should not change, and God forbid that anyone should try to alter that. Nevertheless, the current crop has a new awareness of what it takes to be a true professional.
Robert E. van Patten is an assistant clinical professor at Wright State University School of Medicine, Dayton, Ohio. He is a consultant in aerospace medicine, life sciences, information sciences, and accident reconstruction. His most recent article for AIR FORCE Magazine, “Pioneers at High Altitude,” appeared in the April 1991 issue.
