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Human beings are adapted for life at 1 G
on the surface of the earth. In the aviation
environment, any maneuver has the potential
to expose the human body to more than 1+ Gz
of acceleration force. This can be particularly
hazardous for pilots in the Gz axis. This is a G
force that acts from head to toe in the case of
+Gz and from toe to head in the case of –Gz. As
an aircraft enters into a high-speed, coordinated
turn or begins the pullout from a steep dive, the pilot experiences +Gz. The
heart and cardiovascular system must respond quickly to G acceleration to
keep blood flowing to the brain and maintain consciousness. Physiological
response to +Gz causes the heart to beat harder and faster with an increased
vascular tone to keep the blood flowing “northward” toward the head. If the
physiologic response of the heart and vascular system does not keep pace
with the rapid onset of the G forces, pilot performance will be degraded
to the point where unconsciousness and inability to pilot the aircraft may
ensue.
One of the first indications of impending disaster may be a
progressive loss of vision as the aircraft enters the maneuver.
The eyes are extremely sensitive to low blood flow, and
if the vascular system cannot keep up with the onset of
Gs, the retina will not be supplied with adequate blood.
As arterial pressure in the eye falls below that needed for
the retina, the pilot may notice a loss of peripheral vision
(tunnel vision), which may then be followed by progressive
degradation to a smaller visual field (gun barrel vision),
which in turn may be quickly followed by Gray Out and Blackout of the
visual fields. If the rapid onset of G forces continues, the end result may
be G-induced loss of consciousness (GLOC). In this condition, unless the
aircraft has sufficient altitude for the pilot to back off the Gs and recover
vision and/or consciousness, the result can be tragic. This has been the cause
of far too many military and civilian aviation fatalities.
The symptoms that result from high G exposure are
dependent on the rate of onset of the acceleration.
When the onset is gradual (about 0.1 G per
sec.), visual symptoms precede GLOC. If the
onset is rapid (1 G per second or more), GLOC
can occur without visual warning.
While the effects of +Gz can be profound, the
human body is even less well equipped to handle
–Gz, which is described as a foot-to-head force
and is encountered when a pilot pushes over into
a dive or enters an outside loop. Under –Gz,
the blood is prevented from flowing back down
the jugular veins into the heart, but the arterial
blood flow to the head is enhanced. Once again,
the retina of the eye is extremely sensitive, and the visual effect is a loss of
vision due to “Red Out.” If the pilot does not back off the control pressure,
loss of consciousness will ensue in short order because the blood does not
flow through the brain. To survive in an aviation environment, pilots must
respect the laws of acceleration.
OK….What Does This
Mean to Me?
Any aircraft, civilian or military, can expose the pilot, crew, and passengers
to forces in excess of 1 G. During steep turns and unusual attitude
recovery, civil aviation pilots can experience high G forces that may take
them by surprise unless they are prepared. Subsequently, all aviators need
to understand what makes their body more resistant to the effects of G
acceleration. Conversely, aviators need to understand those conditions that
will make their body more susceptible to the effects of G forces. The bottom
line is that G tolerance for each individual aviator may fluctuate from day
to day, and this can lead to disastrous consequences in flight. This is one of
the reasons that military pilots do a “G warm-up” maneuver prior to flying
high-performance aircraft—it allows them to assess their own body and how
well they will be able to tolerate the high-G environment.
The Bad Things
G tolerance is degraded as a result of alcohol,
fatigue, and dehydration, which are often
associated with a social event. With the “Big
Three” above, the aviator may experience
severe symptoms of G exposure at much less
than the customary level. Lack of physical
conditioning and a sedentary lifestyle can also
degrade G tolerance and increase the aviator’s
susceptibility. Also, once again, smoking and
flying do not mix. Individuals who smoke have
diminished performance at high altitude and
high-G environments. Flying is difficult—do not make it harder by
needlessly abusing your body.
　
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