Human factors in aviation

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human factors in aviation

These issues are looked into in much greater depth in our aviation medicine section

human factors

A.  general

Elsewhere we have discussed the technical aspects of flying. The reader should now understand how lift is produced by an airfoil to make an airplane fly, the basic construction of an airframe. ail about the operation and care of an aero engine, how to use aircraft communication and navigation radio equipment, how to navigate from A to B, the vagaries of weather, etc. Not too many years ago, it was generally believed that if an individual had a good understanding of all these technical aspects of pilotage, he had acquired the basic prerequisites to be a successful, efficient and safe pilot.

In the last few years, however, it has been learned that a thorough grasp of these subjects, though essential, is not enough. Human factors are a very important part of flight crew training. Human aspects, such as cockpit organization, crew co-ordination, fitness and health, sensory illusions and decision making are as vital to safety in the air as are flying techniques. The relationship of people with machines, the environment and other people is part of the human factor equation.

There is much to understand about the pilot himself and his physical and involuntary reactions to the unnatural environmental conditions of flying. During the Second World War, it was first realized that some airplane losses were due to pilot incapacitation rather than to enemy action. The challenge of explaining these unusual occurrences was taken up and since that time much research has been conducted into such subjects as hypoxia, spatial disorientation, hyperventilation, the bends, impairment due to drugs and alcohol, and mental stress. Startling and sobering information is now available.

Man is essentially a terrestrial creature. His body is equipped to operate at greatest efficiency within relatively narrow limits of atmospheric pressure and, through years of habit, has adapted itself to movement on the ground.

In his quest for adventure and his desire for progress, man has ventured into a foreign environment, the air high above the ground. But these lofty heights are not natural to man. As altitude increases, the body becomes less and less efficient to a point, at sufficient altitude, of incapacitation and unconsciousness. Completely deprived of oxygen, the body dies in 8 minutes. Without ground reference, the senses can play tricks, sometimes fatal tricks.

Airplane accidents are an occurrence that every conscientious pilot is concerned with preventing. Most aircraft accidents are highly preventable. Many of them have one factor in common. They are precipitated by some human failing rather than by a mechanical malfunction. In fact, statistics indicate that human factors are involved in 85% of aircraft accidents. Many of these have been the result of disorientation, physical incapacitation and even the death of the pilot during the flight. Others are the result of poor management of cockpit resources.

It is the intention of this chapter to explain briefly some of these human factors, to help pilots understand and appreciate the capacities and limitations of their own bodies, so that flying might never be a frightening or dangerous undertaking. But instead, the enjoyable and safe and efficient experience ail lovers of airplanes and the airways have always believed it to be.

B.  general health

Since flying an airplane demands that the pilot be alert and in full command of his abilities and reasoning, it is only common sense to expect that. an individual will ensure that he is free of any conditions that would be detrimental to his alertness, his ability to make correct decisions, and his rapid reaction times before seating himself behind the wheel of an airplane.

Certain physical conditions such as serious heart trouble, epilepsy, uncontrolled diabetes, and other medical problems that might cause sudden incapacitation and serious forms of psychiatric illness associated with loss of insight or contact with reality may preclude an individual from being judged medically fit to apply for a license.

Other problems such as acute infections are temporarily disqualifying and will not affect the status of a pilot's license. But they will affect his immediate ability to fly, and he should seek his doctor's advice before returning to the cockpit of his airplane.

In fact, any general discomfort, whether due to colds, indigestion, nausea, worry, lack of sleep or any other bodily weakness, is not conducive to safe flying. Excessive fatigue is perhaps the most insidious of these conditions, resulting in inattentiveness, slow reactions and confused mental processes. Excessive fatigue should be considered a reason for cancelling or postponing a flight.

C.  hypoxia

The advance in aeronautical engineering during the past few years has produced more versatile airplanes capable of flying at much higher altitudes than only a few years ago were considered attainable by the private pilot. At such high altitudes, man is susceptible to one of the most insidious physiological problems. hypoxia. Because hypoxia comes on without warning of any kind, the general rule of oxygen above 10,000 feet ASL by day and above 5000 feet ASL by night is one the wise pilot will practice to avoid the hazard of this debilitating condition. Hypoxia can be defined as a lack of sufficient oxygen in the body cells or tissues.

The greatest concentration of air molecules is near to the earth's surface. There is progressively less air and therefore less oxygen (per unit volume) as you ascend to higher altitudes. Therefore each breath of air that you breathe at, for example, 15,000 feet ASL has about half the amount of oxygen of a breath taken at sea level.

The most important fact to remember about hypoxia is that the individual is unaware that he is exhibiting symptoms of this condition. The brain centre that would warn him of decreasing efficiency is the first to be affected and the pilot enjoys a misguided sense of well-being. Neither is there any pain, or any other warning signs that tell him that his alertness is deteriorating. The effects of hypoxia progress from euphoria (feeling of well being) to reduced vision, confusion, inability to concentrate, impaired judgment, slowed reflexes to eventual loss of consciousness.

effects on visions at 5000 feet

The retina of the eye is actually an outcropping of the brain and as such is more dependent on an adequate supply of oxygen than any other part of the body. For this reason, the first evidence of hypoxia occurs at 5000 feet in the form of diminished night vision. Instruments and maps are misread; dimly lit ground features are misinterpreted.

above 10,000 feet

It is true that general physical fitness has some bearing on the exact altitude at which the effects of hypoxia will first affect a particular individual. Age, drinking habits, use of drugs, lack of rest, etc.. all increase the susceptibility of the body to this condition. However, the average has been determined at 10,000 feet.

At 10,000 feet, there is a definite but undetectable hypoxia. This altitude is the highest level at which a pilot should consider himself efficient in judgment and ability. However, continuous operation even at this altitude for periods of more than, say, four hours can produce fatigue because of the reduced oxygen supply and a pilot should expect deterioration in concentration, problem solving and efficiency.

At 14,000 feet, lassitude and indifference are appreciable. There is dimming of vision, tremor of hands, clouding of thought and memory and errors in judgment. Cyanosis (blue discolouring of the fingernails) is first noticed.

At 16,000 feet, a pilot becomes disoriented, is belligerent or euphoric and completely lacking in rational judgment. Control of the airplane can be easily lost.

At 18,000 feet, primary shock sets in and the individual loses consciousness.

At higher altitudes, death may result after a prolonged period.

The Air Navigation Orders rule that an aircraft should not be operated for more than 30 minutes between 10,000 feet and 13,000 feet or at ail above 13,000 feet unless oxygen is readily available for each crew member.

D.  stagnant hypoxia

Stagnant hypoxia is a condition in which there is a temporary displacement of blood in the head. It occurs as a result of positive "g" forces (as in an abrupt pull out from a high speed dive), and, can be attributed to the fact that the circulatory system is unable to keep blood pumped to, the head.

E.  prevention of hypoxia

The only way to prevent hypoxia is to take steps against it before its’ onset. Remember the rule: Oxygen above 10,000 feet by day and above 5,000 feet at night.

gases and the body

A.  ozone sickness

Another problem associated with flight at very high altitudes is ozone sickness. Although it has been evident only with flights operating at altitudes of 30,000 feet or more, the advent of general aviation airplanes that operate at subsonic speeds at such levels makes this a problem of which even the private pilot should be aware.

Ozone is a bluish gas that exists in relatively high concentrations in the upper levels of the atmosphere, especially in the tropopause. Because the tropopause fluctuates in its average altitude from season to season, any flight operating above 35,000 feet is likely to come into, contact with ozone at some time.

Although ozone does have a distinctive colour and odour, passengers and flight crew who have experienced ozone sickness have been unaware of the apparently high concentrations of ozone prior to the onset of the symptoms.

The symptoms of ozone sickness are hacking cough, poor night vision, shortness of breath, headache, burning eyes, mouth and nose, mild chest pains, leg cramps, fatigue, drowsiness, nose bleed, nausea and vomiting. The symptoms become more severe with continued exposure and with physical activity but do diminish rapidly when the airplane descends below 30,000 feet.

Some relief from the symptoms can be achieved by breathing through a warm, moist towel. Limiting physical activity to a minimum and breathing pure oxygen are also effective in alleviating the symptoms. 

B.  carbon monoxide

Oxygen is transported throughout the body by combining with the haemoglobin in the blood. However, this vital transportation agent, haemoglobin, has more than 200 times the affinity for carbon monoxide that it has for oxygen. Therefore, even the smallest amounts; of carbon monoxide can seriously interfere with the distribution of oxygen and produce a type of hypoxia, known as anernic hypoxia..

Carbon monoxide is colourless, odourless and tasteless. It is a product of fuel combustion and is found in varying amounts in the exhaust from airplane engines. A defect, crack or hole in the cabin heating system may allow this gas to enter the cockpit of the airplane.

Susceptibility to carbon monoxide increases with altitude. At higher altitudes, the body has difficulty getting enough oxygen because of decreased pressure. The additional problem of carbon monoxide could make the situation critical.

Early symptoms of CO poisoning are feelings of sluggishness and warmness. Intense headache, throbbing in the temples, ringing in the ears, dizziness and dimming of vision follow as exposure increases. Eventually vomiting, convulsions, coma and death result.

Although CO poisoning is a type of hypoxia, it is unlike altitude hypoxia in that it is not immediately remedied by the use of oxygen or by descent to lower altitudes.

If a pilot notices exhaust fumes or experiences any of the symptoms associated with CO poisoning, he should shut off the cabin heater, open a fresh air source immediately, avoid smoking, use 100% oxygen if it is available and land at the first opportunity and ensure that ail effects of CO are gone before continuing the flight. It may take several days to rid the body of carbon monoxide. In some cases, it may be wise to consult a doctor. 

C.  cigarettes

Cigarette smoke contains a minute amount of carbon monoxide. It has been estimated that a heavy smoker will lower his ceiling by more than 4000 feet. Just 3 cigarettes smoked at sea level will raise the physiological altitude to 8000 feet. Because the carbon monoxide in the cigarette smoke is absorbed by the haemoglobin, its oxygen absorbing qualities are reduced to about the same degree as they would be reduced by the decrease in atmospheric pressure at 8000 feet ASL.

The carbon monoxide from cigarettes has detrimental effects not only on the smoker but on the non-smoker as well. After prolonged exposure to, an increased level of carbon monoxide such as that produced within a confined area such as a cockpit by people smoking, symptoms such as respiratory discomfort, headaches, eye irritation can affect the non-smoker.

Cigarette smoking has also been declared as hazardous to health, contributing to hypertension and chronic lung disorders such as bronchitis and emphysema. It has been linked to lung cancer and coronary heart disease. 

D.  hyperventilation

Hyperventilation, or over-breathing, is an increase in respiration that upsets; the natural balance of oxygen and carbon dioxide in the system, usually as a result of emotional tension or anxiety. Under conditions of emotional stress, fright or pain, a person may unconsciously increase his rate of breathing, thus expelling more carbon dioxide than is being produced by muscular activity. The result is a deficiency of carbon dioxide in the blood.

The most common symptoms are dizziness, tingling of the toes and fingers, hot and cold sensations, nausea and sleepiness. Unconsciousness may result if the breathing rate is not corrected.

The remedy for hyperventilation is a conscious effort to slow down the rate of breathing and to hold the breath intermittently, to allow the carbon dioxide to build up to a normal level. Sometimes, the proper balance of carbon dioxide can be more quickly restored by breathing into a paper bag, that is, by re-breathing the expelled carbon dioxide.

Hyperventilation is sometimes associated with hypoxia. A pilot, f lying at high altitude, may think that he can counteract the effects of hypoxia by taking more rapid breaths. Hyperventilation does not help you get more oxygen. It only increases the emission of carbon dioxide.

E.  decompression sickness

trapped gases

During and descent, gases trapped in certain body cavities expand or contract. The inability to pass this gas may cause abdominal pain, toothache or pain in ears or sinuses.

Ear Block

The ear is composed of three sections. The outer ear is the auditory canal and ends at the eardrum. The middle ear is a cavity surrounded by bones of the skull and is filled with air. The Eustachian tube connects the middle ear to the throat. The inner ear is used for hearing and certain equilibrium senses.

As one ascends or descends, air must escape or be replenished through the Eustachian tube to equalize the pressure in the middle ear cavity with that of the atmosphere. If air is trapped in the middle ear, the eardrum stretches to absorb the higher pressure. The result is pain and sometimes temporary deafness.

During climbs, there is little problem since excess air escapes through the tube easily. However, during descents, when pressure in the middle ear must be increased. The Eustachian tubes do not open readily. The pilot and his passengers must consciously make an effort to swallow or yawn to stimulate the muscular action of the tubes. Sometimes it is advisable to use the valsalva technique, that is, to close the mouth, hold the nose and blow gently. This action forces air up the Eustachian tubes. Children may suffer severe pain because of ear blocks during descents. They should be repeatedly reminded to swallow or yawn. Small babies are incapable of voluntarily adjusting the pressure in the middle ear and should be given a bottle to suck during descents.

Painful ear block generally occurs; as a result of too rapid descent. If the pilot or his passengers are unable to relieve the pain of ear block by the methods described, it may be necessary to climb to altitude again and make the descent more gradually.

After a flight in which 100 per cent oxygen has been used, the valsalva procedure should be used several times to ventilate the middle ear and thus reduce the possibility of pain occurring later in the day.

Sinus Block

The sinuses are air filled, bony cavities connected with the nose by means of one or more small openings. If these openings are obstructed by swelling of the mucous membrane lining of the sinuses (as during a cold), equalization of the pressure is difficult. Pain in the cheek bones on either side of the nose, or in the upper jaw, or above the eyes, will result.

The valsalva procedure will relieve sinus pain

For both ear and sinus block, the prudent use of nasal inhalants such as Benzedrex, Afrin, Neosynephrine may be helpful.

A nasal inhalant containing antihistamine, however, should not be used for the reasons stated in the section on drugs below.


Toothaches may occur at altitude due to abscesses, imperfect fillings, inadequately filled root canals. Anyone who suffers from toothache at altitude should see his dentist. However, the pain caused by a sinus block can be mistaken for toothache. If air is able to enter below a filling, the filling may well be blown out as the pilot reaches higher altitude.

Gastrointestinal Pain

Gas pains are caused by the expansion of gas within the digestive tract during ascent into the reduced pressure at altitude. Relief from pain may be accomplished by descent from altitude.

The Common Cold

Don't fly with a common cold. A cold that is a mere discomfort on the ground can become a serious menace to a pilot and his passengers in the air.

Tiredness, irritability, drowsiness and pain are ail symptoms of a cold and work together to make a pilot unsafe in the air. More insidious, however, is the effect a cold may have on the sinuses and on the middle and inner ear. Swollen lymph tissue and mucous membranes may block the sinuses causing disabling pain and pressure vertigo during descent from altitude. Infection of the inner ear, that is a common symptom of a cold, can also produce severe vertigo. The tissue around the nasal end of the Eustachian tube will quite likely be swollen and middle ear problems associated, under normal conditions (see above), with descent from altitude will be severely aggravated. A perforated eardrum is a possible result. Although a perforated eardrum usually heals quickly, in some cases there is permanent hearing impairment or prolonged infection of the middle ear.

Cold remedies do not prevent symptoms. They usually only bring on other problems, drowsiness being the most common.

evolved gases

Nitrogen, always present in body fluids, comes out of solution and forms bubbles if the pressure on the body drops sufficiently as it does during ascent into the higher altitudes. Overweight persons are more susceptible to evolved gas decompression sickness as fatty tissue contains more nitrogen.

Bends is characterized by pain in and around the joints and can become progressively worse, during ascent to higher altitudes.

Chokes are pains in the chest caused by blocking of the smaller pulmonary blood vessels by innumerable small bubbles. In severe cases, there is a sensation of suffocation.

Paresthesia or Creeps is another decompression sickness with symptoms of tingling, itching, cold and warm sensations.

Central nervous system disturbances include visual disturbances, headache and, more rarely, paralysis and sensory disturbances.

Decompression sickness is unpredictable. One of the outcomes may be shock, characterized by faintness, dizziness, nausea, pallor, sweating and even loss of consciousness. Usually the symptoms disappear when a return to the ground is made. However, the symptoms may continue and special treatment (recompression) may be needed.

Decompression sickness, caused by evolved gas, is rare below 20,000 feet. The best defence against this painful problem is a pressurized cabin. Some protection against it can be achieved by breathing 100% oxygen for an hour before ascending to altitudes above 20,000 feet. This action washes the nitrogen out of the blood. Oxygen does not come out of solution or form bubbles. Refrain also from drinking carbonated beverages or eating gas producing foods.

Scuba Diving and Flying

A person that flies in an airplane immediately after engaging in the sport of scuba diving risks severe decompression sickness at much lower altitudes than this problem would normally be expected. The scuba diver uses compressed air in his breathing tanks to counteract the greater pressure of the water on his body. At a depth of 30 feet, his body absorbs twice as much nitrogen as it would on the ground. Ascending to 8000 feet ASL could bring on incapacitating bends. A good rule, if you have dived to a depth below 30 feet, is not to, fly for 24 hours to permit the nitrogen content of the body to return to, normal.

vision and environmental factors

A.  vision

Good vision is of primary importance in flying, in judgment of distance, depth perception, reading of maps and instruments and should, therefore, be scrupulously protected.

Pilots; are exposed to higher light levels than is the average person. Very high light levels prevail at altitude because the atmosphere is less; dense. In addition, light is reflected back at the pilot by cloud tops. This light contains more of the damaging blue and ultra-violet wavelengths than are encountered on the surface of the earth. Prolonged exposure can cause damage to the eye and especially to the lens. Sunglasses should, therefore, be worn to, provide protection against these dangers and to prevent eyestrain.

Instrument panels should be dull grey or black, to harmonize with the black instruments, so that the eye does not have to adjust its lens opening constantly as the line of vision moves from the dark instruments to a light coloured panel.

When flying into, the sun, the eyes are so dazzled by the brightness that they cannot adjust quickly to the shaded instrument panel. This situation causes eyestrain and is fatiguing to the pilot. Sunglasses help to minimize the problem.

Atmospheric obscuring phenomena such as haze, smoke and fog have an effect on the distance the normal eye can see. The ability of the eye to maintain a distance focus is weakened. Distant objects are not outlined sharply against the horizon and after a short lapse of time the eye, having no distance point to fix on, has difficulty maintaining a focus at a distance of more than a mile or two, (a condition known as empty field myopia). As a result, scanning for other aircraft becomes difficult and requires special effort on the part of the pilot. With the pilot's focal range reduced, the span of time in which to perceive the danger and take evasive action is considerably shortened. Pilots must learn to recognize the limitations of the human eye under varying weather conditions and realize that the see and avoid maxim has limitations under some atmospheric conditions.

The Anatomical Blind Spot

The area where the optic nerve connects to the retina in the back of each eye is known as the optic disk. There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot. Under normal binocular vision conditions this is not a problem, because an object cannot be in the blind spot of both eyes at the same time. On the other hand, where the field of vision of one eye is obstructed by an object (windshield post), a visual target (another aircraft) could fall in the blind spot of the other eye and remain undetected.

In order to find the blind spot of the right eye, it is necessary to close the left eye, focus the right eye on a single point, and see if anything vanishes from vision some 20 degrees right of this point. The following diagram has a set of characters on the left hand side, and black circle on the right. Keeping your head motionless, with the right eye about 3 or 4 times as far from the page as the length of the red line, look at each character in turn, until the black circle vanishes.

With increasing age, the blind spot enlarges. You may find that the black circle disappears when several of the characters are looked at. The size and shape of the blind spot can be found if a large enough grid of characters is used.

The same test can be done for the left eye. Close the right eye, and look at each character until the black circle disappears.

Note that when the black circle vanishes, you see only a white background where the circle was. What happens if the background colour is different? Say, yellow.

The blind spot appears as yellow. This is interesting, because it means that, although my eye can't detect anything in the blind spot, something knows that it is surrounded by yellow, and has guessed that what is in the blind spot is probably yellow. Smart!

How smart? If a thick horizontal line is drawn through the blind spot, what happens then?

The answer, it seems, is that if the line passes right through the blind spot, whatever is making shrewd guesses about colours is also able to work out that a line going in one side and coming out the other probably continues through the middle. The black circle disappears, but the line remains.

So what happens when a pen or pencil is pushed into the blind spot? It seems that as the tip enters the blind spot, the pencil appears truncated, as if it were vanishing into something (which, after all, it is). But when the tip emerges at the other side, the visual processing system fills in the missing part between. The following animation mimics pushing a pencil into the blind spot.

The first conclusion drawn from this little experiment is that, although each eye has a blind spot, some sort of intelligence is used to give this area not only a likely colour, but also to fill in lines that pass through the blind spot - rather than just have a fuzzy grey area. The net result is that, with one eye closed, it isn't immediately obvious where the blind spot is, because it has been given a suitable colour, and even pattern, based on what is adjacent to it.

The second conclusion drawn is that what we see is not just what has appeared on the retina, but is an image that has been reprocessed, tidied up. And if the human visual cortex is able to tidy up the blind spot, then it may well be that the same is being done for the entire visual field - that what we get to 'see' is not what appears on the retina, like a photograph, but instead something which has a whole bunch of special effects added.

If so, then we can't trust our eyes. We're being given doctored information, massaged figures. The world that we see is not something out there, but a world that we invent. The world I see is my idea.

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