Transport by air became a very common activity during the second half of the twentieth century. It provides a rapid means of transport over long distances, in relative comfort. Commercial air travel is amongst the safest forms of transportation. Military aircraft have played a very major role in war since the late 1930s. The modern fighter-bomber is a very formidable weapon.
The environment in which aircraft operate differs markedly from that on the ground. The fall in the pressure and temperature of the air which occur with ascent to altitude have major effects upon the body, including expansion of gas in gas-containing cavities, hypoxia (oxygen lack) due to the fall in the partial pressure of the oxygen (PO2) in the air, decompression sickness due to the formation of bubbles of gas in the tissues, cold injury, and hypothermia. The ability of aircraft to execute turns at high speed exposes the occupants to far greater accelerations than are normally encountered in terrestrial life. These accelerative forces produce profound effects upon the cardiovascular and musculo-skeletal systems (see G and G-suit). The additional freedom of motion and abnormal accelerative forces which occur in flight can give rise to misinterpretation of the information provided by the senses, giving rise to spatial disorientation, with potentially dangerous consequences.
Gas expansion
The pressure of the atmosphere falls in an approximately exponential manner with increasing altitude, but the proportions of the major components of the atmosphere — oxygen (20.9%), nitrogen (78.1%), and the rare gases (1%) — however, remain constant up to 300 000 feet. The fall in pressure which occurs on ascent to altitude is transmitted throughout the tissues and gas-containing cavities of the body, namely the middle ear, the sinuses, the lungs, and the gut. The gases in these cavities expand as the pressure falls. If the escape of gas to the atmosphere is hindered, then the cavity will be stretched, and discomfort, pain, and tissue damage may ensue. With normal rates of ascent, the only site in which failure of venting may occur is the intestines, especially if the altitude exceeds 25 000 feet, when it can cause abdominal pain. If the fall of pressure occurs very rapidly the gas in the lungs may not be able to escape and the lungs may be damaged by over-expansion, and gas may enter the circulating blood (gas embolism) with potentially fatal results. On descent from altitude, gas must enter the middle ear and sinuses. The valve-like function of the tube which connects the middle ear cavity to the back of the nose (the pharyngo-tympanic, or eustachian tube) may prevent air re-entering the middle ear cavity on descent, causing pain in the ear and deafness, and, on occasion, rupture of the ear-drum. Voluntary actions, such as swallowing, open the tube in about 50% of healthy individuals. Others find that they must raise the pressure in the mouth and nose in order to force gas into the middle ear. A head cold may make inflation of the middle ear much more difficult.
Hypoxia
The fall in the partial pressure of oxygen (PO2) in the air which occurs on ascent to altitude reduces the PO2 in the tissues — the condition termed ‘hypoxia’. Normal cellular function is impaired when the local PO2 falls below a critical value. The effects of hypoxia are seen first in the central nervous system, especially the higher centres. Thus the time taken to learn a new task is increased at an altitude as low as 5000 feet. Impairment of the performance of well-practised tasks does not occur until the altitude exceeds 10 000-12 000 feet. Subjects seated at rest exhibit virtually no symptoms of hypoxia at altitudes below 15 000 feet. Moderate physical exercise will, however, induce breathlessness at altitudes above 10 000 feet. Breathing air at altitudes between 15 000 and 18 000 feet rapidly produces impaired mental performance, lack of insight, and loss of judgement and self-criticism, leading to euphoria and neuro-muscular uncoordination. The increase in pulmonary ventilation stimulated by the hypoxia removes an excessive amount of carbon dioxide from the body. This hypocapnia causes light-headedness, apprehension, tingling sensations, and muscle spasm in the face and limbs. Acute exposure to altitudes above 18 000-20 000 feet causes gross impairment of mental function and leads in a matter of a few minutes to unconsciousness and convulsions. Prolonged exposure to these or higher altitudes is fatal. The time which elapses between a sudden exposure to breathing air and serious impair-ment of consciousness falls from 3 to 5 min at 25 000 feet to 40 sec at 35 000 feet and to 15 sec at 45 000 feet.
Two methods of preventing the hypoxia induced by ascent to altitude are employed in aviation. The first is to maintain the PO2 in the inspired gas by increasing the concentration of oxygen in the gas breathed, which requires the individual to wear a mask. The second method is to limit the fall of environmental pressure to which the individual is exposed by raising the pressure in the crew and passenger compartments of the aircraft above that of the external environment.
The pressure cabin
The cabins of all modern civil transport and military combat aircraft are pressurized with air supplied by the engines. The flow of air through the cabin is determined principally by the requirements for ventilation (removal of carbon dioxide and body odours) and thermal comfort. The differential pressure between the pressure cabin and the environment is controlled by the cabin air outlet valves. In passenger-carrying aircraft, the degree of pressurization of the cabin is determined by the requirements to prevent significant hypoxia at altitude and damage to the middle ear on descent. Present international requirements allow the pressure in the cabins of these aircraft to be reduced to the equivalent of 8000 feet. Breathing air at this altitude, however, impairs the ability of aircrew to respond to a new task, which may be significant in an emergency situation. There is also evidence which suggests that this degree of hypoxia when combined with sitting for several hours can produce deterioration of the condition of individuals suffering from certain cardio-respiratory diseases. In practice, therefore, the cabin altitudes of many passenger-carrying aircraft do not exceed 6000 feet.
The pressure cabins of military aircraft normally employ a smaller pressure difference between inside and outside (‘low differential pressure’) in order to minimize the weight of the cabin and to reduce the consequences of a failure of its structure. The crew of these aircraft breathe oxygen-enriched gas throughout the flight. The maximum cabin altitude in a combat aircraft is determined by considerations of the time available in the event of a failure of the oxygen supply and reversion to breathing air, and the incidence of decompression sickness. Typical maximum cabin altitudes lie between 18 000 and 22 500 feet.
Pressurization of the cabin introduces the possibility of decompression of the cabin in flight due to a failure of the structure, of the air supply to the cabin, or of the air outlet valves. A major structural failure may well be associated with break-up of the aircraft. The fall of pressure produced by a more limited failure, such as the loss of a window or door, may be less catastrophic, although individuals close to the defect may be blown out of the aircraft. The major hazard is hypoxia. It is likely that only a small proportion of passengers would succeed in using the drop-down oxygen masks. The life-saving measure in the event of a decompression is immediate rapid descent of the aircraft to low altitude. It is essential, therefore, to prevent hypoxia in the flight deck crew, by the correct use of efficient oxygen delivery equipment.
Pressure breathing
Breathing 100% oxygen at an altitude of 40 000 feet produces a PO2 in the lung gas equal to that produced by breathing air at 10 000 feet. The PO2 in the lung gas can be maintained at this value at higher altitudes by breathing 100% oxygen at a raised pressure — a procedure termed positive pressure breathing. Breathing oxygen at pressures above 30 mm Hg requires a counterbalancing pressure to be applied to the external surface of the trunk to aid breathing and prevent over-distension of the lungs. At higher breathing pressures (above 50 mm Hg) counter-pressure must also be applied to the lower limbs to minimize the circulatory disturbances induced by the high pressure in the chest. Several types of partial pressure suits based upon these principles are used to provide emergency short-duration protection against hypoxia at altitudes between 40 000 and 80 000 feet. Longer duration protection against the effects of exposure to altitudes above 40 000 feet requires the use of a full pressure suit: this is essentially a personal pressure cabin which applies counter-pressure to the whole body and can thus protect against both hypoxia and decompression sickness.
Spatial disorientation
Nearly all aircrew experience illusory sensations of the attitude or motion of their aircraft, or fail to detect changes in the orientation of the aircraft, at some time during their careers. These incidents are due principally to the limitations of the sensory mechanisms of the body. False perceptions of orientation may give rise to errors in the control of an aircraft which can, in turn, cause an accident. Disorientation has been implicated in about 10% of all civil airline accidents and in about 20% of military fixed wing aircraft accidents. The principal sources of information which provide the perception of the spatial orientation of the body are the eyes, the vestibular system of the inner ear, and sensory endings in the skin, joints, muscles, and ligaments. Vision is of prime importance to spatial orientation both on the ground and in flight. The vestibular apparatus of the inner ear, and the sensory receptors in skin muscle and joints, provide information which ensures balance and spatial orientation on the surface of the earth even when the eyes are closed. The vestibular apparatus frequently provides erroneous information in flight because the magnitude and time course of the motions to which the pilot is exposed are atypical and outside the normal dynamic range of this system.
There are two important classes of illusion which can arise from the vestibular organs in flight. The first is the perception of linear acceleration, which is sensed by the otolith organs of the vestibular apparatus, which signal the position of the head relative to the gravitational vertical. They also respond to linear accelerations of the head so that when an aircraft accelerates, the pilot has the sensation that the aircraft is rotating nose-up. This ‘somatogravic’ illusion may be so strong, especially in the absence of visual cues in fog or at night, that the pilot pushes the control column forward in an attempt to regain level flight, which may well increase the strength of the illusion. The pilot may then push the control column further forward and rotate the aircraft into a dangerous position — a pattern which has occurred in crashes associated with overshoot from an abandoned approach in poor visibility.
The second class of vestibular illusions is concerned with angular accelerations which are sensed by the fluid-filled semicircular canals of the vestibular apparatus. The commonest form of spatial disorientation is a false sensation of roll attitude. It occurs typically on recovery from a co-ordinated turn to level flight. The pilot enters the turns gradually and smoothly so that the angular velocity in roll is well below the level of detection by the semicircular canals and the pilot feels that the wings of his aircraft are level. If recovery from the turn is made relatively abruptly so that the semicircular canals are stimulated, the pilot now feels that the aircraft is flying one wing low when in fact the wings are level. This false sensation of bank, the leans, can persist for many minutes. In situations where the aircraft performs a prolonged spin the pilot will at first experience a sensation of spinning in the direction of the rotation. This sensation, however, ceases after 7-10 sec. When the spin ceases the pilot feels that he has entered a spin in the opposite direction and this somatogyral illusion may cause him to re-enter the original spin in an attempt to counter the apparent new one. These sensations are very disorientating, and the powerful control which the vestibular system has over the movements of the eyes can also seriously impair vision at the beginning of a spin and on recovery from a spin.
Pilots are taught to recognize conditions (e.g. poor visibility, landing and take off, and particular manoeuvres) which may lead to disorientation, to reject bodily sensations as unreliable, and to rely upon the visual information of aircraft behaviour and orientation provided by flight instruments.
Long-duration flight
Long-distance flight can cause fatigue in aircrew due to excessively long periods of duty, disturbances of sleep, and transmeridian travel. A critical factor in ensuring that excessive fatigue and disturbances of sleep do not occur in long-distance operations is to limit the total duty hours in a given period. Thus aircrew operating worldwide routes are considered able to cope with a total of 50-55 hours in the first 7 days and a total of 75 hours by the end of 14 days. Performance of the flying task by a pilot who is well rested typically increases over the first 5 hours of the duty period, but then falls precipitously over the next few hours, levelling out after 16 hours. Time of day also exerts a marked effect on performance (circadian rhythm). Performance rises during the day and falls during the late evening and overnight, reaching its nadir at about 05.00 in the morning. Very low levels of performance will occur if the fall in performance produced by a long period of duty coincides with the fall of performance produced by circadian rhythmicity early in the morning. Flight schedules for aircrew are designed to avoid such a gross impairment of performance.
Transmeridian flight through a number of time zones introduces the additional complication of the changes in the circadian rhythm, the magnitude of which depend upon the number of time zones crossed, and the speed of adaptation to the new time zone, which varies with the direction of travel. The adaptation phase is associated with disturbances of sleep, appetite, and bowel function, general discomfort, and reduced mental performance (‘jet lag’). The circadian rhythms of the body adapt to a new time zone more rapidly on westbound travel than when travelling towards the east. Typically a flight through 6 time zones in an eastward direction produces disturbed sleep for 3-4 days, with the greatest disturbance occurring on the second night in the new time zone. Worldwide, aircrew flight schedules take account of these disturbances. They are designed to ensure that the aircrew obtain adequate sleep between duty periods. In military operations it may be impossible to ensure that adequate sleep can be taken at the appropriate time of day and it has been shown that the induction of sleep by the controlled use of hypnotic drugs can greatly enhance the maintenance of intense and sustained air operations.
— John Ernsting
Bibliography
- Ernsting, J., Nicolson, A. N., and Rainford, D. J. (1999). Aviation medicine, 3rd edn. Butterworth-Heinemann, Oxford
See also altitude; balance; body clock; decompression sickness; hypoxia; vestibular system.
