Fit to Fly

Philip Belsham, FRCS, Consultant in Accident and Emergency Medicine to a leading London hospital, looks at the hostile environment in which the modern military pilot operates, in the first of a series about aviation medicine.

THE HUMAN ANIMAL, Homo sapiens, has, over the millennia of his evolution, become superbly adapted to his environment. Although his leas are not as fast as the Cheetah, his strength not that of the bear, his eyesight not as sharp as the cat, and his sense of smell not that of the dog, he has still become monster of the whole land surface of the globe. His undoubted success is due to the supreme power of his brain, allied to remarkable manual dexterity. However, at the end of the day, Man’s body is adapted to life on the surface of the land and in a modest range of temperatures. His excursions into more alien environments are only possible by dint of his ability to create vehicles in which he can control his immediate environment.

The atmosphere above the surface of our planet becomes increasingly hostile to Man as we ascend in it, and the development of flight (in all its forms) has demanded a parallel understanding of human physiology and pathology. This series of articles explores our investigation, knowledge and manipulation of the hostile environment of altitude and flight.

The Environment

The atmosphere in which we live, and take for granted, is the most unusual (even unique) in the known universe and comprises a very stable mix of gases: inert nitrogen (N2; approx 79%) and essential oxygen (O2; approx 21%) with small, but important, contributions from others, such as carbon dioxide (CO2), ozone and water vapour.

All the cells in our bodies need oxygen to ‘burn’ fuel (derived from food and body reserves), and produce CO2, in much the same way that an engine burns hydrocarbon fuel in air producing that ozone depleting, planet warming CO2. Plants, on the other hand work in a different way using sunlight and CO2 to produce energy and O2. The common result of all this ‘burning’ is the conversion of one energy form to another, more readily usable by the organism. The importance to all animals of oxygen is immediately apparent and our systems are optimised to make use of this gas on the planet surface, rather than at great altitude.

Atmospheric Pressure

The weight of atmosphere pressing down on all of us, at sea level, is about 15lb/sq inch, or enough to support a column of mercury 760mm high (760mm Hg). Pressure can be measured in many units and this does make life more confusing, especially as aviation uses one and medicine another (see table 1).

Table 1: Pressure Measurement Units:

1 bar = 1,000 millibars = 105 newton/m2 = 1.00 kg/cm2 1 Pascal = 1 newton/m2 and 1 Kilo Pascal (kPa) = 10 hectoPoscal (hPa) = 1,000 Pascal therefore 1 millibar = 1 hPa; in UK aviation and meteorology we use millibars; on the continent hPa (fortunately numerically identical)

100 mm Hg = 13.33 kPa = 3.93 inches Hg; medicine still uses mm Hg for pressure measurements, the USA uses inches of mercury for aviation and meteorology!

As we ascend in the atmosphere the ‘weight’ of gas above us declines and so the atmospheric pressure falls, in a smooth curve, to the emptiness of space where there is, to all intents and purposes, no atmosphere. The rate of fall is initially about 1 millibar per 30ft.

This falling atmospheric pressure poses the most serious problems for man both because of the direct pressure effects and the secondary effects on oxygen pressure. Below sea level, the pressure rises much more rapidly (water is much denser than air) increasing by 1 atmosphere every 33 feet or 10 metres.

Because of day to day air pressure variation, and changes with temperature, etc, all these variables are converted to a ‘standard atmosphere’ (see table 2) in order to allow direct comparison, in different locations and times, of similar studies.

Table 2: The Standard Atmosphere:

760 mm Hg = 29.89 in Hg = 1,013.25 millibars or hPa = 101.325 kPa = 15.085 lb/in2 = 1.01325 kg/cm2.

Sudden exposure to low atmospheric pressure, as might occur with a structural failure in a pressurised aircraft, allows the formation of bubbles of nitrogen gas in the blood and tissues. At sea level, nitrogen dissolves in the blood and if we suddenly ascend (as in a sudden depressurisation) the gas, which was dissolved at sea level pressure, comes out of solution at the lower pressure. The bubbles thereby formed block the tiny blood vessels throughout the body leading to the syndrome known as ‘decompression sickness’ — exactly the same mechanism occurs with divers who suddenly shoot to the surface. The symptoms can be very varied and depend on the body system most affected (see table 3).

Table 3: Manifestations and Treatment of Decompression Sickness:

The Bends — bubbles in the joints cause agonising pain and are the commonest problem.

The Creeps — bubbles in the skin cause pain, tingling, rashes and itching.

The Chokes — chest pain and constriction are very serious symptoms.

The Staggers — if bubbles form within the brain they may lead to varied symptoms similar to strokes (limb paralysis, etc.), eye problems, coma and even severe shock.

The treatment of decompression sickness is recompression as rapidly as possible — in an aircraft this usually takes the form of an emergency descent to low altitude where the increased pressure will force the bubbles back into solution. Along with recompression, pure oxygen needs to be administered as this gas is much more soluble than nitrogen, and the sickness itself may result in a lack of oxygen supply to the tissues (by blocking the tiny blood vessels). Although severe decompression sickness is rare in aviation (aircraft can usually descend rapidly before serious damage results) it is occasionally necessary to administer hyperbaric treatment in a special chamber where the pressure can be increased above atmospheric. These chambers are usually found at Naval stations as divers are the usual patients.

Gas contained within any body cavity will expand as the pressure falls on ascent — and if this can’t be vented there may be a problem. The middle ear contains air which normally flows up and down the Eustachian tube, connecting the ear to the throat. With inflammation in the throat, such as a common cold or flu, this lube may become partially blocked and, although air can usually escape on ascent, it cannot re-enter on descent thereby allowing the eardrum to implode.

Air bubbles within the teeth, underneath fillings, expand on ascent and may cause severe toothache. Air within the gut expands and can cause agonising colic and even collapse.

The prevention of pressure related problems is by cabin pressurisation — because this imposes considerable cost, in terms of weight, on the aircraft structure, most commercial aircraft are not pressurised to sea level but rather somewhere between 5,000 and 8,000 feet (1,524m/2,438m). Concorde, flying far higher than most aircraft, has to be correspondingly stronger to withstand the greater pressure differential across its hull, and steps are taken to minimise the risk of sudden depressurisation (small windows, etc.).


As we ascend in the atmosphere, the atmospheric pressure falls, but, not only does the total pressure fall, more importantly that proportion of the pressure exerted by O2 also declines and herein lies the major danger to Man.

The ‘partial pressure’ of oxygen (pO2) at sea level is about 160mm Hg (21% of the dry atmospheric pressure) and it is this pressure that forces oxygen into the blood whence it is delivered to all parts of the body. Oxygen is carried in the blood mainly by a complex molecule, haemoglobin, which is bright red when saturated with O2 and blue when the O2 is removed. It is a particular characteristic of haemoglobin that it gives up O2 as the partial pressure falls (usually at the tissues, where it is needed) in a non-linear fashion. As the partial pressure falls with increasing altitude, the amount of O2 carried by the blood also falls leading to a lack of O2 at the tissues — hypoxia. This oxygen starvation may be manifest by the subtle, but characteristic, symptoms and signs taught to all aircrew (see table 4).

Table 4: Symptoms and Signs of Hypoxia: mental changes

— unco-ordination and loss of judgement & memory

— euphoria overall, similar to drunkenness physical changes

— tunnel vision

— hyperventilation (excessive breathing) producing further mental changes

— convulsions

— unconsciousness

All these symptoms occur insidiously and the airman may be quite unaware of what is happening, and unable to co-ordinate his response even if he were aware. The rapidity with which they develop depends on the severity of hypoxia, i.e. the partial pressure of O2 (the altitude).

The most severe and dramatic incident would occur with catastrophic decompression to altitudes above 40,000ft (12,192m) as might occur in military flying or supersonic transports such as Concorde. Such a decompression is immediately obvious to the aircrew who can take remedial action in the form of donning pressure breathing oxygen masks, and initiation of emergency descent. The more dangerous forms of hypoxia occur insidiously when the induced mental deterioration may preclude recognition or action.

Military aircrew everywhere use pressurised oxygen delivered through tight fitting masks — at very high altitudes (not really the province of the RAF until Eurofighter 2000 comes into service) the O2 has to be delivered at pressures higher than atmospheric. Such pressures would burst the lungs without the ‘pressure-breathing waistcoat’ (worn with the anti-g suit), and suitable regulators. Military aircraft are seldom pressurised to the same level as civilian because of the weight cost.


As we ascend in the atmosphere, the temperature also falls, at a steady rate of 1,98°C/1,000ft (305m), up to a height of 36,090ft (11,000m) after which it stays constant at minus 56.5°C. Clearly, temperatures of this level are intolerable for any length of time and movement through the air will add to the cooling effect (wind-chill) — protective clothing becomes essential in military aircraft, where loss of environmental control from aircraft damage in action might occur, or where ejection might be anticipated.


The higher in the atmosphere we travel, the less filtering there is of all radiation. High-altitude flight is associated with greater glare and the apparent paradox of a dark sky and light ground and clouds. Problems arise because of the relatively dark interior of the cockpit requiring adaptation of the eyes between looking outside and scanning the instruments. Generally, aircrew wear tinted visors to protect against the effects of glare, including infrared and ultraviolet.

Cosmic radiation is only really a problem in prolonged very high altitude flight, but may be very significant in space.

The Vehicle

G forces

The force acting on everyone, all of the time, is that of gravity (g) which is that force tending to pull him into the earth, and familiar to all as ‘weight’ — in fact, it is not the accelerating force pushing us into the earth of which we are aware, but rather the opposite force resisting that acceleration. When accelerating, say in a car, we are aware of additional forces pushing us back into the seat. It is convenient to describe these forces of acceleration as multiples of the normal gravitational g-force, for example 4g, but, while this describes the magnitude, the direction is also important. By convention, we describe these directions as x, y, and z. It is also important to remember that an acceleration acting on the body is opposed by an equal and opposite inertial force — hence the driver is thrown forwards in a head-on collision by a rearwards acting acceleration.

The tolerance of the human body to acceleration forces depends on the magnitude, the duration, and the plane of action. In practice, short duration accelerations are those of less than one second, usually occurring in an accident, and resulting in injury. Long duration forces are those felt by aircrew in high-performance aircraft, particularly in close combat, and result in changes in the body’s physiology and function and it is these that we will discuss here. It has long been the case that the structural limitations of aircraft were the factor limiting their overall performance, but now aircraft can, and are designed, that can tolerate far greater forces than can the aircrew — whereas engineering was the limiting factor it is now the pilot!

In aviation medicine, it is usually gz forces that are most important, both positive and negative — ‘pulling g’ refers to this z-plane. Pitching up produces headward acceleration of the body with footward inertial forces described as +gz (positive g); while pitching down produces -gz (negative g). The physiological effects on the body of these forces are different.

Effects of +gz

The typical pattern is of a progressive sequence of visual changes starting with darkening and loss of peripheral vision, progressing to complete visual blackout (+5 to +6gz) followed shortly by loss of consciousness. This characteristic sequence is only apparent, however, when the rate of change of g is low — in agile aircraft, such as the F-16, g can change so fast that the pilot gels none of the visual warning signs before he loses consciousness (g-LoC — loss of consciousness). The reason for the symptoms is the reduction in blood Row through the brain and the retina of the eye as blood is forced into the legs by the g force.

In combating the effects of +gz, the aircraft designer and physiologist both have roles to play. The designer can place the seat in an inclined position, which reduces the vertical distance between heart and brain — while this is the route adopted in some aircraft (the F-16 is a good example), it does pose problems of its own. The mainstay of control of +gz symptoms is the anti-g suit, which provides positive pressure (by inflation) around the leas and trunk, countering the increased hydrostatic pressure generated by the g forces. Such suits have come a long way from the early days, and provide a considerable degree of protection — raising tolerance by up to +1.5gz. Combined with this mechanical control, aircrew training is of immense importance — a straining manoeuvre may protect against the visual symptoms for +2gz.

Effects of -gz

So-called ‘negative g’ is less well tolerated by crew and aircraft alike — the head and neck swell with blood and the pilot may experience red misting of the vision. The whole experience is most unpleasant and the anti-g suit has no role in protection — indeed, there is no protection other than avoidance!

Sensory Factors

The special senses of vision, hearing and balance are clearly of enormous importance to a pilot, but the very strangeness of flight stretches the ability of the human brain to assimilate and incorporate the information into the overall awareness of environment and orientation. Much work has been done to expand our understanding of the interpretation of these senses, but the sum of our knowledge is still incomplete.


The human eye is a remarkable organ. The centre of the visual field is most sensitive to colour and detail, while the periphery is better adapted to detecting movement. The eye automatically centres on the subject and this factor is important for aircrew because we cannot use our vision for scanning in the same way that a video camera can. When a pilot scans the sky, the eye tends to move in a series of jumps and a great deal of training goes into improving the completeness of this scan. With nothing to focus on, it is very difficult to detect another object in the sky unless the peripheral vision picks up a movement. An aircraft on a collision course is on a constant bearing (relative to the observer) and will not therefore move in the sky (although it will grow in size) — it is therefore extremely difficult to detect until perhaps too late.

The need for the brain to have a horizon with which to relate also imposes problems in flight — many times a pilot has chosen to believe his senses rather than his instruments, with fatal results. Many air accident reports have commented on this problem, and anyone who has flown will appreciate the difficulties in weather conditions where there is no natural horizon. This was a noticeable problem with helicopter pilots in the Vietnam War who were very much ‘seat of the pants’ and visual flight pilots — in cloud they were liable to become totally disorientated as they did not have the training, at that time, to rely on instruments.

Particular note has been made in the press, in recent years, of ‘low luminance myopia’ — in the dark, with nothing on which they can focus in the distance, the eyes tend to focus at a neutral distance of about 3.3ft (1 m) and may therefore miss a target in the distance. This may also be true in daylight with an empty featureless sky at high altitude. Compounding the problem, at least at night, is the needed for the eyes to make maximal use of the peripheral field which is far more sensitive in the dark.

Balance organs

The inner ear, deep within the bones of the skull, is not only concerned with hearing, but also with balance and can detect both angular and linear accelerations. It provides the brain with extraordinarily accurate information on orientation and movement, but this is tailored for life on Earth. In flight, the aircraft may impose attitudes and accelerations which serve to confuse the organs of balance. Balance, however, is not just comprised of the middle ear input, but vision and awareness of limb position are all involved. It is easy to see how disorientation can occur, but the mechanisms may be extremely complex and involve all the special senses.


Military aircraft are notoriously noisy, while, at the same time, a great deal of important sensory input is through the ear (audible threat warning sirens, etc.). It is clearly vital that all verbal communication (very often all that is available) be unambiguous and this may be difficult in a noisy environment — reduction of overall noise levels by engineering means (insulation, quieter engines, smoother airflow, headphones which filter unwanted sound, etc.) can also be combined with training to optimise the intelligibility of human speech.

Very high sound levels may be not only painful, but also disorientating and distracting, and rotary wing aircraft have a particular problem with regard to noise and vibration. Even relatively low levels of noise may degrade the performance of a pilot having an enormous work load and diverse, conflicting sensory input. Vibration, while not strictly influencing hearing, has very detrimental effects on several body systems, not least vision and the organs of balance.

Sensation and proprioception

Proprioception is that sense of spatial awareness of the whole body and its component parts — shut your eyes and you can still perform complex actions with hands or legs, and still know exactly where they are in space. While flight does not directly interfere with these senses, the information they pass to the brain may cause a conflict with other senses, resulting in disorientation in its broadest sense.

It may be that, as the eyes and ears are fully occupied with instrumentation and communication, use can be made of sensation as another source of input to the brain for warnings, etc — indeed, some aircraft are fitted with a system that artificially shakes the control column when a stall is imminent.

Psychological factors

This area is, perhaps, the most difficult to appreciate and manipulate, yet is probably one of the most important in day-to-day flying. From routine interpretation (or misinterpretation) of basic flight instrumentation to disorientation in abnormal circumstances or weather the psychological interaction of man with the environment and his machines is fundamental to our success. As machines, particularly computers, get cleverer and faster, designers are ever tempted to task the machine with flying the aircraft and the pilot with monitoring the machine. In reality, Man is much better suited to flying, and the machine to monitoring. Ultimately, a machine’s ability to fly depends entirely on what it has been ‘taught’ by its programmer, whereas a human pilot can use intuition and inventiveness to get out of a situation he has never before experienced. Having said this, there are many situations where the human brain can be deceived and an understanding of the reasons and their prevention is essential to safe flight. Clearly, these psychological factors are intertwined with more purely physical ones, and, in particular, the euphoria and confusion encountered with hypoxia.

Man’s dalliance with flight is fragile and dangerous and we need to very clearly understand the physical and psychological needs of the human body before venturing safely into ever more dangerous environments and situations.

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