AVIATION, MEDICAL ASPECTS OF. Flying may be regarded from the points of view : (a) of the passenger and (b) of the pilot.
The Passenger.—As a general rule passenger flying consists in what is called "straight flying." An opinion as to the safety of flying must not be formed from the number of accidents which occur in military flying. Such flying is totally different from civil ian flying, involving more risk. This point is emphasized because the enjoyment of flying as passenger depends greatly upon whether the passenger experiences a feeling of security or other wise. Above 5oof t. sense of contact with the earth is lost and, in some people this induces a feeling of anxiety not allayed until they land. For such, travel is best accomplished by other means.
The other chief factor which determines whether flying is enjoyable or not is the state of the atmosphere. If the weather is "bumpy" a passenger may become "air-sick," more especially when travelling in an enclosed cabin. As with sea-sickness many people anticipate being "air-sick" and in this frame of mind the malady is prone to occur. Some people also are afraid of the effects of altitude. At the heights of average aerial travel, i.e., up to 5,000 or even I o,000f t. nothing beyond a slight deepening of breathing, and some exhilaration is noticed. The effects of altitude are not felt by most people until 12,00o-15,000f t. Civilian passenger machines do not ascend high enough to induce anything akin to "mountain-sickness," neither are the average altitudes reached sufficient to induce harm in people who, though suffering from lung or heart ailments, are able to pursue an average every-day life on the ground.
First, he must be possessed of those qualities which enable him efficiently to manipulate his machine. Simple flying calls for certain co-ordinated limb movements which are initiated as the result of sensory impressions. Of such impressions those of vision are the most important. A blindfolded pilot cannot fly a machine with safety, although such has been stated to be the case. Tests, however, undertaken under dual control quickly prove the contrary. In fog and cloud flying a pilot has to rely upon the information obtained from instruments by the use of his eyes. The same is true in a large measure of night flying, although here a certain amount of visual information is generally also available from external sources (horizon, stars, etc.). The experienced pilot derives an increasing amount of information from the nerves of "deep" sensation— viz., the "feel" of the control column, rudder bar and seat, and as a result can more or less automatically initiate the appropriate co-ordinated movements necessary for the ac curate control of his machine.
Except to the experienced pilot, tactile sensations play but little part in the act of flying, although information is derived from the "feel" of the wind and varying air currents upon the face. Audi tory sensations also play but little part in the art of flying, although good hearing is advantageous and necessary for other considerations, e.g., the appreciation of the "note" of the engine, the reception of wireless and so forth. Delicately co-ordinated movements of arm and leg are necessary for the accurate control of an aeroplane. Some individuals are incapable of achieving this delicacy and are consequently heavy handed or heavy footed or both. Other individuals are incapable of combining arm and leg movements with sufficient accuracy owing to an inability to perform successfully two relatively simple movements at the same time. Although such incapacity falls within the prov ince of the instructor to correct, the examination of the re sponses by means of a special apparatus for the purpose is of great value in certain cases.
Effects of Altitude.—The effects of altitude call for especial consideration from the point of view of the pilot. In addition to the effects of diminution of oxygen supply, the effects of extreme cold and the actual diminution of the air pressure have to be considered. With regard to the last, the idea that diminution of pressure in any way affects the body as a whole is quite erroneous. The main effects of diminution of pressure in itself are due to the expansion of the air enclosed within the middle ear. This tends to expand as the pressure is reduced, but the pressure of such air is, generally regarded, automatically adjusted by swallow ing. Therefore, only subjects suffering from catarrh of the tubes leading to the middle ear are likely to suffer any inconvenience from this cause. The same is true when atmospheric pressure is " again increased on coming down from average heights (i,000 2,000f t.) . The movement of swallowing again automatically ad justs the pressure. When descending from greater heights the increased pressure on the ear drum through the outer ear is easily neutralized within the middle ear by inflation brought about by holding the nose and gently blowing up the ear drums by the movements of forced expiration, a device well known to all pilots. It has been falsely supposed that at great heights there is a danger of trouble arising in the body from the release of gases into the blood owing to the diminution of pressure, such as takes place in the diver when he rises to the surface after prolonged immersion at the increased pressure of great depths, or in the caisson worker emerging from work in compressed air chambers. This is not the case. It takes a diminution of at least one atmosphere to cause such bubble formation and that suddenly, whereas an aviator is seldom exposed to a diminution of pressure of more than half an atmosphere (9,000ft. approximately) and then but gradually owing to the relatively slow climbing powers of the aeroplane. There is no danger of an "air disease" corresponding to caisson disease or diver's palsy.
The effects of flying at great altitudes were observed as the result of the high flying which became necessary during the World War. In the air the chief among these effects were diminution of judgment, drowsiness, breathlessness and muscular weakness, fol lowed by great bodily fatigue. The effects of high altitudes upon judgment are insidious and constitute for the aviator a subtle danger. There is also a great slackening of morale and loss of offensive spirit. Pilots sometimes complain of headaches which at times come on while in the air, but more usually after landing. In flying the effect of increasing altitude is in the first place a deepening of the respiration in order to secure the oxygen neces sary to maintain the bodily functions. At the same time the heart quickens, and thus is established the beginning of a "vicious circle." For an increase in the rate of the heart-beat means an increase in the amount of work done by the heart, and this in creased work entails an increased oxygen consumption, the supply of which is diminishing; thus each factor reacts unfavourably upon the other. At high altitudes, therefore, all the devices to render the respiration and circulation efficient are called into play to meet the changing conditions, so that with prolonged and repeated stress, a breakdown of the respiratory and circulatory mechanisms, involving also the nervous system, is to be antici pated unless appropriate measures be taken to mitigate the ill effects. It is found that with the provision of oxygen apparatus on high flying machines these symptoms are greatly alleviated. The administration of oxygen tends to preserve an efficient slow pulse and a good arterial pressure, keep off the onset of distress ful breathing, mitigate any ill effect due to excessive deep breath ing and increase the power for nervous concentration and muscu lar work.
For endurance also, apart from high flying, it has been found that a pilot must be fit as regards his respiratory and circulatory mechanism. The examination of officers taken off flying through "flying strain" showed that their respiratory capacity was very much diminished, due chiefly to ineffective working of the expira tory side of the respiratory rhythm. As regards the circulation, the fit 'pilot is possessed of a regular, fairly slow pulse which is not greatly quickened by exercise and speedily returns to its normal rate. The pulse of the man unfit for flying, or unfit to learn to fly, is unduly quickened by exercise and takes considerable time to return to normal. Effective medical selection of the pilot is based upon the following considerations : (a) A careful personal history of the candidate; (b) An eye examination; (c) An ear, nose and throat examination; (d) A general medical and surgical examination, including special physiological tests.
In respect of personal history especial attention is directed to a history of ailments or disabilities which may have impaired nervous stability, respiratory and circulatory efficiency, or ma nipulative power.
In regard to the eye examination, experience and statistical data show that a high degree of visual acuity is called for in flying. The standard now demanded is 6/6 with both eyes sep arately at loft. under conditions equivalent to "standard illumi nation" or better. To this rule there are two exceptions only in Great Britain. The first is that candidates whose physical tests demonstrate exceptional fitness may be accepted with a visual acuity of 6/9 (all letters and each eye separately), on the recom mendation of the ophthalmic specialist. The second exception is that no candidate, whatever his visual acuity, may be passed into the service, who reveals a manifest hypermetropia (that is hyper metropia revealed without the use of a mydriatic) of +2.00 D. Sph. or over. The importance of good visual judgment is brought out by the following considerations : Immediately the aeroplane leaves the ground the pilot loses the one quantity in the judgment of distance to which he has always been accustomed—viz., contact with the ground. In the air all verticals become more or less fore-shortened and, therefore, the size of known objects will be varied from those of previous experience. Further, shadows of objects will be viewed from a totally different aspect, and the speed at which the pilot is travelling is greater than any he has ever experienced. It may be taken as generally true that the faster a machine flies in the air, the faster is its landing speed. The speed of landing is often the one trouble for a pilot with doubtful visual judgment. Normal fields of vision in both eyes are necessary for flying because : (a) The pilot requires to obtain the widest possible view of machines and other objects in his proximity—this is especially important in formation flying; (b) He depends mainly upon the vision of the peripheral fields for judgment of the pace of his machine in relation to laterally placed stationary objects; (c) The grey horizon (false horizon) at night is best perceived by the rod elements of the peripheral fields.
Normal colour vision is essential for piloting duties as coloured flares and lights are employed as signals in formation flying and for night navigation and landing. The risks involved through error of colour perception are obvious. The ear, nose and throat examination is conducted with a view to ensuring adequate acuity of hearing for the operation of wireless and freedom from any abnormal or unhealthy condition of the ears themselves, or of the upper respiratory passages which might tend to interfere with the safe handling of aircraft. It has to be remembered that conditions of but slight importance on the ground frequently become ag gravated in flying. In cases where there is a history of giddiness, faintness, swing or train sickness, severe sea-sickness or sensi tivity to uneven movement, rotation tests are employed. The method of test now used in Great Britain is to take the pulse rate and arterial pressure before and after rotation. In subjects liable to vertigo, the pulse rate and blood pressure have been found to rise, on the other hand in those liable to fainting, the blood pressure, especially as maintained between the beats of the heart, falls. In good pilots there is practically no alteration of pulse rate or blood pressure.
From the medical point of view certain protective measures are essential under certain circumstances. Such, for example, are the administration of oxygen and the provision of adequate cloth ing when high cold flights have to be undertaken. For fog, cloud and night flying, a valuable safety measure is the use of a control indicator which indicates whether the machine is on an even keel and whether it is turning correctly with the right angle of bank, since the human body is not endowed with any mechanism adequate to inform the pilot of his position in space. It has been thought by many authorities that the vestibular mechanism of the internal ear is adequate for this purpose, but such is not the case, and without some such instrument as a control indicator a pilot is liable to get upside down in fog and cloud. The machines of the Imperial Airways are, in consequence, fitted with the Reid control indicator.
The popularity of private flying is greatly on the increase, and the medical examination necessary to obtain a private licence is not severe. Generally speaking, any man or woman with good vision, without or with glasses, and adequate hearing, who is of average soundness in wind and limb is able to pass the medical examination necessary for private piloting. (M. F.)