pilot. Most T-34C G-LOC episodes occur during rapid G-loading of 3 to 5 G's over 2 to 5
second intervals. Pilots can prepare themselves for the physical stress of rapid accelerations and
therefore prevent G-LOC by taking certain precautions:
Learn and utilize the proper Anti-G Straining Maneuver (AGSM), more commonly called
the "HOOK Maneuver." There are two components to the recommended AGSM:
The first component is a continuous and maximum contraction (if necessary) of all
skeletal muscles including the arms, legs, chest, and abdominal muscles. This tensing
of the skeletal muscles restricts blood flow in the G-dependent areas of the body and
thereby assists in the retention of blood in the thoracic region (including the heart)
and the brain.
The second component of the AGSM involves repeated closing of the respiratory
tract at 2.5 to 3.0-second intervals. Its purpose is to counter the downward G force by
expanding the lungs and increasing the chest pressure, thereby forcing blood to flow
from the heart to the brain.
The respiratory tract is an open breathing system which starts at the nose and mouth and ends
deep in the lungs. It can be completely closed off at several different points, the most effective
of which is the glottis. Closing the glottis (which is located behind the "Adam's Apple") yields
the highest increase of chest pressure. The glottis can be closed off by saying the word "HOOK"
and catching it about ¾ of the way through the word ("Hooo-"). This should be done after a deep
inspiration, followed by forcefully closing the glottis as you say "HOOK." Bear down for 2.5 to
3.0 seconds, then rapidly exhale by finishing the word HOOK ("-ka"). This is immediately
followed by the next deep inhalation, repeating the cycle until the G-loading is discontinued.
The exhalation and inhalation phase should last for no more than 0.5 to 1.0 second. Since the
blood pressure falls dramatically during this phase, its duration must be kept to a minimum.
Do not hold your respiratory straining too long (more than five
seconds) since this will prevent the blood from returning to the heart
properly and may result in loss of consciousness.
Anticipate the onset of high G forces whenever possible. Skeletal muscles should be tensed prior
to the onset, coupled with the "HOOK" respiratory cycle as the G-loading increases. Initiating
the AGSM too early can inhibit the body's natural cardiovascular reflex responses, while
beginning too late creates a deficit situation which may be difficult to overcome.
If properly performed, the AGSM should provide adequate protection
against G-LOC while performing the various aerobatic maneuvers. If
you experience difficulty, or are in doubt as to whether or not you are
executing the maneuver correctly, see your squadron flight surgeon or
wing Aeromedical Safety Officer.
INTRODUCTION TO AEROBATICS 10-5