By Donald Talleur
Last month we looked at the g-loading and its effects on the airplane. This month we finish with a look at the effects of g-loading on the pilot.
The effect of centrifugal g’s on the pilot is of most interest since studies
by the United States Air Force have shown that pilots can withstand enormous
linear accelerations and large decelerations as well. The extenuating factor in
all of this is the duration of the g-loading and the rate of the change of
velocity.
Linear accelerations just don’t pack the same “punch” as the centrifugal g since
the rate of change in forward velocity is limited by power output during
acceleration and by drag during deceleration. For these reasons we’ll focus on
the effect of the centrifugal g on the pilot.
While the average pilot can handle a relatively large amount of g’s for a short
duration, it is the sustained g from prolonged acceleration that causes
problems. For example, since a 180-pound pilot effectively weighs 540 pounds
during sustained three g flight, movement not only becomes difficult, but may be
impossible.
For this reason, aircraft that are susceptible to high g-loading during a
departure from controlled flight are generally equipped with ejection seats so
the pilot has a chance of escaping.
The pilot’s vestibular system is also highly susceptible to g-loading. The
g-receptors of the inner ear are critical to determining spatial orientation,
especially in the absence of visual cues. High g-loading disturbs the accurate
sensing of position and can lead to severe disorientation under limited visual
conditions such as flight in the clouds.
Once again, we see a good reason for limiting the angle of bank during actual
instrument flight. The eyes are also affected negatively during accelerated
flight. A decrease in blood circulation to the eye under positive g-loading is
thought to be responsible for most of the visual acuity loss.
However, the displacement of the eye’s lens in the direction of the g-force
vector (generally the direction opposite of motion) may also serve to reduce
visual acuity. Under sustained g-loading exceeding 3.5 gs, for the average
pilot, reduce peripheral vision is common as well.
G increases beyond 3.5, for most pilots, leads to a condition called grey-out.
During grey-out, the pilot is still conscious of their surroundings but will
experience a general graying of the visual field. With further increase in
g-loading, black out may occur and, if sustained, unconsciousness may follow.
Consciousness resumes quickly after the aircraft is “unloaded” but several
seconds to minutes of disorientation may occur afterwards.
The increase in g-loading has the most pronounced effect on the cardiovascular
system. The blood supply is transported under well established pressures for
normal bodily function. Clearly, under high g-loading, abnormal pressures exist
and interfere with the transportation of blood to critical organs. Most critical
of these organs is the brain.
The brain’s mechanism for lack of oxygen is generally unconsciousness and so we
see this as a natural result of sustained high g loading. A mere sustained 4.5
to 5 g’s is all that is necessary for the average pilot to experience a blackout
and unconsciousness.
High negative g-loading also produces numerous physiological problems, but due
to the extreme discomfort at relatively low negative g’s, pilots have difficulty
sustaining the negative g long enough to cause these problems.
A phenomenon similar to black out, but physiologically quite different, occurs
with sustained negative g-loading. The condition referred to as red-out
(supposed as the pooling of blood in the eyes) has been reported during
sustained negative g’s.
However, due to the design of the eye, it is more likely that the lower eyelid,
which is not designed to hold itself down, partially covers the eye during
negative acceleration and hence gives the appearance of a red and reduced visual
acuity.
The pressure of blood congesting in the brain during negative acceleration
quickly becomes unbearable and we therefore see negative 3 g’s as the
approximate limit of human tolerance. Although the brain does not usually suffer
adverse consequences of such g-loading, the pain suffered by the pilot usually
leads to quick unloading of the aircraft. Usually, this is accomplished by
simply letting go of the control column since the aircraft will not maintain
sustained negative g’s on its own.
Tolerance of g’s is somewhat variable between pilots and several factors combine
to predict ones ability to withstand g’s. However, good physical condition does
not necessarily lead to high g tolerance as one might expect.
Athletic conditioning may lead to relatively low blood pressures which in turn
lead to decreased tolerance to g loading in the head-to-foot direction. Repeated
exposure to acceleration also leads to a loss of g tolerance.
There are a few ways to stave off the affects of g-loading however. Only one of
these can be effectively employed in the light aircraft; the straining manoeuvre.
This procedure is quite effective and can increase g tolerance by up to 2 gs.
We won’t go into detail about this procedure here, but suffice to say that
training in the proper technique is required or the pilot can unintentionally
inflict physiological damage to him or herself.
In advanced aircraft, seated posture is adjusted to maximize both visibility and
g tolerance. The general goal of adjusting seat posture is to reduce the
vertical distance between the heart and the eyes since vision is the first to
fail during the g-loaded manoeuvre.
Anti-g suits are also worn, particularly by military pilots, in order to further
increase g tolerance. Depending on the style suit, g-tolerance is increased by
one to two additional gs. The combination of all these methods of increasing
g-tolerance allows the pilot to handle g-loading well above three to four gs for
extended periods and loading as high as nine to 10 g’s for a very short period
of time.
Regardless of the protection for the pilot, g-loading in excess of 10 g’s cannot
be sustained for more than about three seconds. At these high levels,
circulation to the brain is cutoff and oxygen within the cells will last no more
than approximately three seconds.
The pilot must unload quickly to restore oxygen to the brain and cells or else g
tolerance will fall substantially to the point where even small g-loads will
impose adverse symptoms.
Now you know just about everything you would ever want to know about g-loading
effects on the airplane and pilot. G’s are a normal part of flying and
understanding what they are and their effects will help you keep both the
aircraft and your body safe during flight.
Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Aviation Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 100 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.
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