Average relaxed ‘G’ tolerance of combat aircrew varies between 4 to 5G, although the range may be 3 to 8G. Yet, one must note there are large individual variations in G tolerance and even in the same individual, the tolerance may vary on different days or different times of the day. It is here that the combat ready aircrew must remember those factors that lower one’s G tolerance, and must take precautions against the same.
There are several factors that could lower the G tolerance. The combat pilots are advised to consider those factors which could compromise their G tolerance, while preparing for a mission. The factors to consider are:-
- Hypoglycemia, i.e. lack of adequate glucose in the blood, say, due to missed meal before a sortie.
- Heat stress reduces tolerance to +Gz due to reduced effective circulatory volume caused by sweating and peripheral vasodilatation.
- Alcohol ingestion is known to reduce +Gz tolerance.
- Hyperventilation markedly reduces tolerance to +Gz due to accentuation of reduced cerebral blood flow caused by washing off of Carbon dioxide.
- Gastric filling or filling the stomach to prevent the descent of the diaphragm, in turn that of heart, thus maintaining heart-brain distance.
- Hypoxia, i.e. lack of adequate Oxygen in the blood reduces +Gz tolerance.
- Fatigue due to any cause, e.g., excessive flying, sleep deprivation, stress, etc. will reduce +Gz tolerance.
- Psychological factors like motivation, fear etc. may affect an individual’s tolerance to +Gz.
- Minor sickness, including mild rise in body temperature due to intercurrent infections reduce +Gz tolerance.
- Break in combat flying, for even a one-week can reduce +Gz tolerance.
Protective measures against +Gz to enhance one’s +Gz tolerance are:-
- Avoid those factors that reduce ‘G’ tolerance, as enumerated above.
- Prevention of blood pooling in lower parts of body and enhancement of venous return.
- Reduction of height of hydrostatic column above heart i.e. reduced heart to brain distance.
Fifth generation combat aircraft demand optimal performance from its Aircrew
Various means to achieve protection against +Gz are:-
- Anti-G System.This consists of anti-G valve and anti-G suit.
- Anti-G valve. The conventional Anti-G valve is a mechanical device. It consists of spring loaded weights adjusted such that at 2 G the weights are pushed down to allow flow of air or oxygen (as per aircraft system) to inflate the anti-G suit worn by the combat aircrew. Conventionally pressure in the suit builds at a rate of 1 psi/G at low setting and at 1.5 psi/G at high setting. Once the acceleration washes off, the springs pull the weight up closing the inlet of the anti-G valve, allowing the suit to deflate.
- High flow ready pressure (HFRP) valve is an improvement over the conventional anti-G valve. This provides a ready pressure of 0.15 to 0.2 psi to AGS even at 1G, Thus 60% of the suit volume fills up with ready pressure, and along with higher flow rates during manoeuvres, there is rapid inflation of the suit to maximal pressure. HFRP valve is meant to match the high onset rate (6-7G/sec) capability of modern generation combat aircraft. It is found that the anti-G suits supplied by such valves can give a protection of 1.5 – 2.0G.
- Anti-G suit. Conventional anti-G suit strategically compresses the covered area to prevent pooling and extravasation of blood. It is pneumatically inflated. Typical suit is worn like a trouser, cut-away at crotch and knees to allow ease of mobility and reduce heat load. An outer non-stretch layer contains 5 bladders inside – one bladder covers the abdomen and one each are for both thighs and calves. The fitment is done with the help of lacing-cords, and the suit is tightened around abdomen and legs with the help of slide fasteners. The bladder system inflates through a flexible hose, connected to the personal equipment connector mounted in aircraft, integrating it with the aircraft’s anti-G valve. Ideally it should be worn directly over the skin, but most aircrew wear it over their flight suit. A well fitting anti-G suit provides a protection of 1 to 1.5 G.
- Full coverage anti-G trousers (FAGT) are now available, which cover almost 90% of the lower body compared to nearly 60% by the conventional anti-G suit. Enhanced area of coverage allows better protection by these trousers, ranging between 2 – 2.5G
- Atlantis Warrior and Libelle were two full body, fluid filled, suits designed for G protection. However without wider acceptance amongst the aircrew, both have not been operationally deployed.
- Physiological measures
- Anti-G Straining Manoeuvre (AGSM) is the single most cost effective means of increasing G-tolerance. AGSM has two components muscle tensing and breathing out against closed glottis (L-1 manoeuvre) or semi closed glottis (M-1 manoeuvre). Timing of the breathing component, i.e., straining is vital in AGSM. The air exchange (breathing out and in) should be sharp, akin to a gasp, lasting for just about half a second, thereafter holding the breath in the lungs and straining against the closed glottis for the next 3 seconds. A well-performed AGSM can increase a pilot’s G-tolerance up to 4G. It is advisable that the training for AGSM be undertaken in the safety of the human centrifuge, as is practiced by various air forces the world over.
- Crouching forward can reduce heart-brain vertical distance and thus increase G tolerance. Crouching is something that the rookie pilots perform inadvertently when not indoctrinated in AGSM. However there is a fear of head dropping down, besides likelihood of compromised vision outside the cockpit.
- Physical Training. Combat aircrew must undertake judicious mix of aerobic and anaerobic training. The aerobic exercises of moderate intensity is aimed at improving the cardiovascular fitness, while the specifically designed weight training of the lower limbs, torso and neck muscles results in an improved ability of the pilot to perform effective AGSM under during the high-G conditions of air combat manoeuvers.
- Positive Pressure breathing for protection against G (PBG). PBG increases +Gz tolerance by increasing the intrathoracic pressure, in turn transmitted to great vessels in the chest, thus aiding the cardiac pump pressure. This requires use of chest counter pressure garment (vest) along with an anti-G suit and an anti-G system capable of delivering gradually increasing positive pressure through tight fitting mask starting from 4G at a rate of 12 mmHg, and increasing up to 60 mm Hg at 9G. PBG is known to enhance G endurance.
Combat aircrew, trained to perform AGSM correctly, and donning snuggly fitting anti-G suit, along with PBG system in aircraft can withstand peak +Gz forces of up to 9G to out-manoeuvre and outperform any enemy during air combat manoeuvres. The efforts now are towards crossing the ‘psychological 9G barrier’ using full coverage suit without the need for fatiguing AGSM…
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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.
Acknowledgement. Image courtesy Wikimedia Commons