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ACCELERATION
© Col (Dr) Pierre Erasmus
ACCELERATION
• Introduction
• Aircrew can be insulated from most environmental
stresses of flight such as heat, hypoxia and noise.
• However this is not true for acceleration.
• As a/c become more agile the worse there acceleration
stresses.
• There is unfortunately no anti-gravity device.
• G-suits are not anti-gravity devices but just alleviates
the effects.
• With new aircraft operating in excess of 8G the human
is operating at the limit of his performance.
• Of particular concern is the potential of for G-induced
loss of consciousness or G-LOC
ACCELERATION
• Forces due to acceleration are vector quantities having
proportion both of magnitude and direction.
• Acceleration occur whenever there is a change in
velocity, direction of motion at uniform velocity.
• Forward acceleration causes a backwards acting
inertial force.
• Centripetal force will be sensed as the resultant
centrifugal force. The effect on the body will depend
markedly upon its orientation relative to the force
vector.
ACCELERATION
• EFFECT OF ACCELERATION DEPENDS ON:
• Magnitude: Measured in units of G, this being the
ratio of the applied force to the standard acceleration
of gravity g, or 9,81m/s². Thus, 5G is equal to an
acceleration of 49,05m/s² and the weight of the body is
increased fivefold.
• Direction: The vector direction of acceleration forces is
defined according to a three-coordinate system based
upon the long axis of the body.
• Duration: Classified into:
– a. Impact acceleration acting for a second or less.
– b. Sustained acceleration acting for a second or
more.
ACCELERATION
• NOMENCLATURE FOR VECTORS OF ACCELERATION AND
INERTIAL FORCE
• Direction
Description
Standard
Vernacular
• Upward s
Positive G
+GZ
Eyeballs down
• Downwards
Negative G
-GZ
Eyeballs up
• Forwards
Transverse
+GX
Eyeballs in
•
supine G
• Backwards
Transverse
-GX
Eyeballs out
•
prone G
• To right
Left lateral G
+GY
Eyeballs left
• To left
Right lateral G -GY
Eyeballs right
Supine G
• Restraint: In a crash the occupant will continue along
his initial velocity vector unless acted upon by a force.
This can be done in a controlled manner through the
use of a restraint harness or uncontrolled by him
striking surrounding structures.
• Site of action: Sustained acceleration affects all parts
of the body but in the dynamic situation of impact
differential forces occur.
• Rate of onset: If the onset time of an impact force is
comparable to the natural undamped frequency of the
injury mechanism then the system will be excited and
overshoot can occur.
ACCELERATION
• Physiology of sustained +GZ acceleration
• Effects similar to those of moving from the lying to
standing posture.
• Primary effect is on the cardiovascular system in the
form of increase in hydrostatic pressure gradients.
• 1 GZ BP at brain level is 22 mmHg lower than at heart
level. At 5 GZ BP at brain level will fall to 10 mmHg.
This effect is immediate and inevitable.
• The heart and diaphragm fall and the heart to brain
distance increases.
ACCELERATION
• Hydrostatic increase in intravascular pressure below
heart level causes vascular engorgement, especially the
veins in legs and abdomen, which causes a decrease in
the circulating blood volume and decreased venous
return.
• Raised intravascular and transmural pressure causes
extravasation of fluid into the tissues with a further
slow but progressive loss in circulating volume.
• The initial fall in BP causes stimulation of the carotid
baroreceptors and an increase in heart rate and
peripheral resistance with a little restoration of BP at
head level.
ACCELERATION
• Local circulatory demands acts in opposition to the
generalised vasoconstrictor response with a
redistribution of the available cardiac output with the
myocardium and brain receiving disproportionately
more blood flow.
• Owing to the intraocular tension which must be
overcome to permit retinal perfusion, the blood supply
to the eye fails at about 1GZ lower acceleration than
that to the brain.
• Gradual onset will lead to greyout with tunneling up to
blackout and finally G-LOC. With rapid onset retinal
Oxygen allows vision to be maintained for 4-5 seconds.
• G-LOC is frequently followed by fitting, with flailing of
the arms and a period of relative incapacitation.
ACCELERATION
• The gradient of transpulmonary pressure stems from
the weight of the lung tissue so that at +5GZ the
gradient increases fivefold to about 1cm water per cm
vertical distance. Thus for a lung 30cm tall there will
be 30cm water difference in transpulmonary pressure
from apex to base with gross changes in regional
alveolar pressure and regional alveolar ventilation with
the following:
– The terminal airways close off with ventilation of
the overperfused area ceasing.
– Absorption of the trapped gas is delayed by the
presence of Nitrogen but when 100% Oxygen has
been breathed the entire gas volume will be rapidly
absorbed and the lung collapses, called acceleration
atelectasis.
ACCELERATION
• If maintained, +GZ leads to vasovagal syncope. This
is caused by the continued loss of circulatory blood
volume with eventual breakdown of the cardiovascular
compensatory mechanism causing a bradycardia, fall
in peripheral vascular resistance and hypotension with
loss of consciousness.
ACCELERATION
• Tolerance to +GZ acceleration
• Anti-G suit: Counter pressure acts on capacity vessels
to decrease their transmural pressure and so reduce the
pooling of blood and extravasation of fluid.
• Anti-G straining manoeuvre: Combination of
generalised muscle tensing, straining,and a forced
expiratory effort.
• Positive pressure breathing: The application of positive
pressure through the aircraft breathing regulator.
Reduces muscle fatigue.
ACCELERATION
• Physical conditioning: Modest weight training of all
muscle groups increases the time for which high-G can
be tolerated.
• Centrifuge training: An effective way of teaching an
efficient AGSM and so raise G-tolerance.
• Posture: Crouching forward and raising the rudder
pedals assist G-tolerance by reducing the vertical
heart-brain distance and improving venous return
from the legs.
ACCELERATION
• G-LOC
ACCELERATION
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Risk factors
Intercurrent illness.
Heat Stress.
Lay off.
Inadequate diet.
Fatigue.
Lack of sleep.
Excessive alcohol.
Hyperventilation.