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Air Combat Manoeuvres and +Gz Forces

During air combat manoeuvres* or while practicing basic fighter manoeuvres**, the pilots are exposed to the commonest type of acceleration encountered in aviation – +Gz. Interestingly, the aircraft are capable of pulling higher Gs and sustain it longer than an unprotected human being can tolerate. The effects of +Gz on the circulatory system, and in turn the blood flow, are the most important in turn determining the human tolerance to +Gz exposure.

Various effects of exposure to +Gz are:-

  • Apparent increase in body weight
  • Heaviness of limbs
  • Difficulty in getting up from the seat
  • Internal organs in the thoracic cage and the abdomen are pushed downwards
  • Disturbance in the mechanics of breathing
  • Drop in the hydrostatic pressure above the level of heart, compromising the blood supply to the brain and the eyes
  • Substantial increase in hydrostatic pressure below the level of heart,
  • Pooling of blood in lower limbs and reduction in venous return
  • Reduced cardiac output, affecting efficient blood flow
  • Visual symtoms: Greyout, Blackout
  • A-LOC: Almost loss of consciousness
  • G-LOC: G-induced loss of consciousness

Hydrostatic Pressure of the Blood column determines occurrence of Grey-/Black-out and GLOC

It is known that the hydrostatic pressure of a column of fluid depends upon the height of column (h), density of fluid (d) and acceleration to which it is exposed (G), i.e.,

Hydrostatic pressure = h x d x G

This pressure under acceleration is directly proportional to the values of ‘G’ provided ‘h’ and ‘d’ are constant. The positive acceleration (+Gz) increases the weight of the blood column above and below the heart resulting in reduced vascular pressure above the heart and increased vascular pressure below the level of heart.

Say, for a column of blood having a height of 30 cm at 1 ‘G’ the pressure exerted is approximately 22 mmHg. Thus, if the Mean Arterial pressure at the level of heart is 100 mm Hg, it will be reduced to 78 mm Hg at the level of brain at 1 ‘G’ and at 4.5 ‘G’, the reduction will be nearly 99 mm of Hg, thus resulting in only 1mm Hg, or near zero, of mean arterial pressure for the supply of blood to the brain. On the other hand, the arterial pressure levels below the heart are increased concomitantly. This excessive increase in the pressure results in pooling of blood below the level of heart. The net result is reduction in the circulating volume of blood in the head and neck area, particularly affecting the vision and leading to ‘Grey out’ and ‘Black out’ and compromised supply to the brain resulting in G-induced loss of consciousness (G-LOC).

Schematic diagram of Mean Arterial Pressure at the Hydrostatic Indifference Point i.e. the Heart, as compared to the Brain at 1G and 5G

Visual Symptoms of +Gz

Visual symptoms of Grey-out or Peripheral Light loss (PLL) also referred to as ‘tunneling or veiling of vision’ and Black-out or Central Light Loss (CLL) are due to disturbances in the circulation of the inner-most layer of eye, the retina. These changes may precede cerebral circulatory disturbances, as the blood flowing into the retinal arteries  have to overcome an Intra-Ocular pressure of about 20 mm Hg. As the mean arterial pressure in the head and neck drops down to near Intra-Ocular pressure level, retinal arteries start collapsing at the periphery and later even the central retinal artery collapses resulting in grey out and black out, respectively. While visual symptoms occur, the blood may still be flowing into the brain, if the acceleration onset rates are gradual.

However, within 6-12 seconds of the onset of acceleration forces of combat manoeuvres, reflex changes, triggered by the blood starved brain, occur in the circulatory system. These result in vasoconstriction, increased force of contraction of heart and increased heart rate, in turn trying to maintain blood supply to the brain. If the blood supply to the brain falls below critical levels, the aircrew may have ALOC or GLOC.

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Reference

1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

* Air Combat Manoeuvers

** Basic Fighter Manoeuvers

Acknowledgement.  Image courtesy Wikimedia Commons 

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