This is long but wonderful, an article on the Senses of a Pilot. Even though it is oriented toward flying, all the information is generally useful.

Your Senses and Fatigue



The physical condition of the pilot will affect the way decisions are made. As accident findings show, it is the pilot who needs the most preventive instruction. The pilot is often trained to know more about other things related to flying than about himself. The physiology and humanness of the pilot make him subject to many failings.

The Eye
Of all our senses, vision is our most reliable sense. However, in conjunction with our other senses it does disrupt and affect the way we interpret what we think we are seeing. The eye's sense of up, down, left, right and level can come in conflict with these senses and cause vertigo.

Your one inch eyeball is more capable than any camera to make lens changes that focus and re-focus light arriving through the cornea, pupil and lens on the rear interior of the eyeball called the retina. All these adjustments are muscular. The functions of the eye are various; you have a central vision, a peripheral vision, coordination and balance information, and the sleep clock.

The convex lens inverts the image on the retina which consists of multiple layers (10) of sensors that make the conversion from light to electrical stimuli applied to both cones and rods. The cones electrically respond to both light and color input. The brighter the light the better the cone response. Cones exist mostly in the central fovea area and around the outer region. Rods are only light sensitive but are able to react to very little light but with no color. Rods are not at the fovea and are most numerous in an arc 20 degrees from it. Looking directly at an object at night in poor light will not allow you see it. The eyes see a 160° spherical arc with only a 5° cone of focus. The cone of 10° cannot see better than 20/100. We can only see three colors, red, green and blue. The brain mixes these to 'see' all other colors.

The eye has two lenses, one fixed and one variable. The outer part of the eye 'window' called the cornea is the fixed lens. Muscles that change its shape by making it thinner or thicker adjust the interior lens To focus as needed. There is not surgical correction for the effect age has on the ability of the eyes to focus.

Light adaptation occurs 90% in the first half-hour but will continue for an hour with the rods. Rods can detect low light levels down to that of a full moon. You cannot see using the center 5 degrees of focus. This adaptation can be destroyed in an instant by a bright light. Ability to keep one eye closed when using a light in the cockpit is a handy skill.
You have motion sensors in the ears and neck that stabilize the eyes to see in one place when you move your head sideways. These stabilizers do not work up and down. You cannot see while moving your eyes. All you will get is a gray blur.

There is a type of nearsighted deficiency that is common at night. Detection of this requires a special test that is worthwhile if continuous night flying is a project. Night vision depends on rhodopsin, commonly called visual purple. The production of rhodopsin in your body is strictly an individual matter dependent on genetics, health, diet, and age. Know your limitations.
Oxygen and the lack of it dramatically affect night vision. Altitude causes lose of retina efficiency from 5% to 40% from 4000' up to 16,000'. Smoking can add 7000' to these altitudes. Night vision is also subject to dramatic illusions. Bright lights and reflections will not only destroy your night adaptation but will give false impressions of proximity, speed, attitude, and changes.


The Eye reacts to flight conditions.

--The smoker will have reduced vision, specifically peripheral vision. A smoker is more likely to miss seeing an aircraft approaching the side
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--The pilot who is under stress will tend to fixate on one thing. This may be the runway or the altimeter. Cockpit myopia will destroy the very importance of a variable scan and focus.

--Certain background conditions make it difficult to impossible to separate an aircraft from the background clutter of clouds, sun glare, or rooftops.

--Haze makes it very difficult to judge distance. This is especially true at night. The military is presently experimenting with using light to camouflage aircraft. Certain placements and intensities of lights on an aircraft in daylight can make an airplane harder to see.

--The eye can see only when it stops moving. Stopped the eye will focus on a 10 degree arc. To make a focused scan move your eyes in 10-degree jumps. To make a full scan move your head. Peripheral vision detects movement. Use peripheral vision at night for things you want to locate.

--Binocular vision is good for measuring distance up to about 20 feet. Beyond 20 feet we must depend on shadows, relative size of similar objects, overlapping, or other clues. Clouds are particularly difficult to judge for distance because there are no references.

--Empty field myopia is caused by haze or fog conditions that give the eyes nothing to focus on at a distance. The eye defaults to a distance of about 20 feet. To break this you must focus momentarily inside the cockpit and then outside. This process must be repeated frequently since there is no way of knowing when the default distance has kicked in.

--Red is a poor warning color. Green and yellow are much better.

--Red light in the cockpit should be avoided since it reduces the ability of the eyes to accommodate to reading. Military now uses blue cockpit lighting.

Light color test
On failing a day test you may take a night test and even if you pass your license will be restricted by a note. It is possible to improve red/green discrimination by wearing ruby colored contact lenses.

Vision

Seeing is a practiced art. The eye sees images that may or may not be transmitted to the brain. The fovea of the retina is the region of sharpest vision. This is only one degree of conical field. By taping a quarter to a window and backing off slightly more than four feet you have measured your fovea vision. Our eye can move and see a vertical arc of 135 degrees, 60 above level and 75 below. Your horizontal field is 160 degrees total with 100 of these to the side and 60 across the nose.

With the best vision in the one-degree fovea cone, at 10 degrees your ability to see is only 1/10 as good. In poor visibility your foveal focus fades in less than 90 seconds to a point just in front of the aircraft. To avoid this it is important that you constantly re-focus your fovea on the most distant object available.

The eye has some functional peculiarities that are of particular importance to the pilot. The eye can see only about a 15° arc of space at one time. The eye cannot see while moving. The cone sensors of the eye are centered in the back of the eye. Cones give us color and sharp detail focus in bright light conditions. The rods of the eye are what we use to see in poor light conditions. The rods are to the sides of the eye and to best utilize them we must look slightly to the side of where we actually wish to see. A single flash of white light can destroy the ability of rods to function for over half an hour. The older you are the more time required to adapt to darkness. Red light is still the best light to use if you wish to retain night sight even though it is lacking in color and detail. The preferred color of the Military is now blue.

Objects that are farther away look smaller since they use less of your visual field. We know this because the brain must convert the two-dimensional vision field into three dimensions. This is possible because we have two eyes, which give us stereoscopic views. This binocular ability is good only for about twenty feet. Beyond twenty feet we must make use of other abilities. We have acquired a concept of perspective. This makes objects viewed appear to be near or far according to the vanishing lines of perspective. Also, objects that are near or far appear to move in opposite directions as we move. This can be shown by selecting an object across the street and another on your side of the street. As you move your head the objects move in opposite directions. The next element of vision has to do with your knowledge of relative size. We know the size of people, cars, most buildings and many airplanes. Their size in relation to other known objects makes it possible to judge distance.

Proprioception
Proprioception is what we feel in our muscles. The most sensitive areas are in the neck and knees. Our muscle senses tell us the position of the body.

The vestibular sense is related to our hearing. The vestibular organ gives us balance. It has three semicircular canals which at 90deg; each with the others. Additionally, there are two otoliths, nearby. Otoliths are linear accelerometers or motion detectors. We sense gravity through our otoliths. These senses have threshold levels below which they are not able to detect any sensations. Of the two, the otoliths are the more sensitive.

Our senses can be dangerously fooled but the vestibular sense is the one most susceptible. Even in VFR conditions we can get the 'leans' when the fluids of the semicircular canals fail to warn us of a slow turn or if a turn continues long enough the hairs in the fluid stabilize and give no sensory indications.

A takeoff or climb can, if not countered by noting airspeed and the AI, be felt as an extreme raising of the nose which if countered by lowering the nose can result in an CFIT accident. Reliance on your instruments is most important when you do not have visual cues.

Hearing
Humans detect from 16 to 20,000 Hz but speak between 250 and 2,0000 Hz. The sounds of most G.A. aircraft exceed 90 dB (decibels). Such sound energy is capable of actually breaking the hairs that connect to the cells in the cochlea of the inner ear. These hairs and cells are not replaceable.

Your inner ear
--Determines balance, orientation and equilibrium
--About pencil eraser size
--A 3 axis gyro with each canal at right angles to the others.
Canals are filled with fluid the flow of which affects bunches of sensory hairs (cilia) that are capable of sensing only rate changes in fluid acceleration. A steady change will cease to be sensed. Any gentle smooth turn or change below the threshold of inner ear perception will not be detected. If visual reference is obscured then disorientation can/will occur.

Head cold
The head has eight sinuses paired across the face and extending back to the spinal column. Each cavity is filled with air. They all open to the nose. The air pressure of the cavities is constantly being equalized with the outside atmosphere. Additionally, the eardrum vibrates as sound moves the outside air. On the other side of the membrane the air is equalized by way of the Eustachian tube. For proper hearing the air on either side of the membrane must be equalized. Never take a decongestant that contains antihistamines at altitude.

During ascent the air easily leaves the Eustachian tube, it is during descent that a collapsed tube can cause excruciating pain due to the unequal pressures. It is necessary to use a small muscle in the back of the throat to hold the Eustachian tubes open. A 5000' difference in the air pressures can rupture the drum.

You can open your Eustachian tubes by using the Valsava maneuver. Pinch your nostrils, close your mouth and try to exhale through the nose. This should clear your nose. You could try the Frenzel method. This requires you to move your jaw as far forward as you can. Swallowing at the same time will improve your chances of clearing the tube.

Ear Block
Earblock or sinus blockages can cause differential air pressures to exist between cavities of the skull and the exterior. If it is not possible to equalize these pressures by slowing or removing the pressure changes severe pain results. Do not fly if you suspect such a condition exists or above 8,000' within 24 hours of scuba diving. Gum chewing and jaw movement are preventive. The Valsalva maneuver consists of opening the mouth wide with the jaw wide, as though yawning. Do this over and over because opening the mouth helps open the Eustachian tubes. Next, pinch your nose closed, shutting the mouth, and blow gently as through your nose.

How to Reduce Ear Pressure Pain

Blow air into your nose while holding the nostrils closed. Pinch low on your nostrils and blow gently. Be sure that your nose balloons out over your fingers. (Otherwise, blowing lifts your palate, blocking the Eustachian tube that you're trying to de-congest.) If pressure persists in one ear, turn your head, putting that ear forward (this relaxes the throat muscles that may be constricting the Eustachian tube) and blow. Gum chewing and jaw movement are preventive. The Valsalva maneuver consists of opening the mouth wide with the jaw wide, as though yawning. Do this over and over because opening the mouth helps open the Eustachian tubes.

Fatigue

Acute fatigue occurs when a long period passes with a lack of sleep. Chronic fatigue occurs when several acute fatigue periods occur without adequate recovery time between. While some fatigue is related to lack of sleep, not all is. Fatigue can result from inadequate nutrition and over exertion. More information needs to be obtained on fatigue distinguished from sleep as a factor in accidents.

Some factors of physical condition are controllable and some are not. Acute fatigue occurs when a long period passes with a lack of sleep. Chronic fatigue occurs when several acute fatigue periods occur without adequate recovery time between. Stress is the result of events causing preoccupation, reducing external awareness, and making activities subject to distraction. Stress causes the taking of risks that would otherwise be unacceptable. The mental/physical condition resulting from fatigue and stress may cause the pilot to make unwise decisions.
There are many causes of fatigue: lack of sleep, hypoxia, noise, time zone factors, temperature extremes, dehydration, stress and more. When you are fatigued you are more irritable and easily annoyed, you will suffer for lapses in short term memory, your attention will fixate to the exclusion of all else, your performance skills will decrease and you will be unaware of any impairment.
The causes of fatigue are primarily lack of recent sleep or a chronic sleep deficiency. Additional fatigue arises from our physiological reaction to noise and vibration, illness, hunger, caffeine "down time", unresolved stress, hypoxia, dehydration, errors in judgment and extended mental and physical demands.
We are far more likely to recognize the impairment of fatigue in others than in ourselves. There is no test for fatigue, but the start of fatigue is the gateway to all the "causes" of accidents. The usual reaction time that a pilot has under normal conditions may be tripled. One-second becomes three, three-seconds becomes nine. This extension of time has nothing to do with training and everything to do with sleep. Degraded performance can kill.

The body responds to lack of sleep and disturbance of the circadian rhythm by showing fatigue. Symptoms of fatigue are sleepiness, irritability, depression, apathy, and emotional isolation. Loss of appetite, slurred speech, visual fixation, impaired perception, decreased alertness, channeled thinking, lack of concentration, slowed reactions, requiring greater stimulus, impaired short-term memory, poor judgment, loss of accuracy, error accumulation, neglect of tasks, erratic performance, and increasing reliance on habitual behaviors.
High altitude, noise, vibration, G-forces, heat, and dryness can aggravate all of the above symptoms. Physical factors such as fitness, diet, hydration, weight, rugs, alcohol, medication, caffeine, and tobacco are pre-disposition factors.

Fatigue can be overcome by sleep. The time of sleep is not so important for recovery of lost sleep time as is the depth of sleep. Older people sleep less and less deeply. Older people are more easily disrupted from sleep.

Fatigue is a treacherous hazard to safe flying since it is not apparent to person and will lead to accumulation of errors and eventual accident. Person cannot respond to events in a safe and thoughtful manner. Any fatigue high enough to interfere with decision-making will be greatly influenced by illusions.
Fatigue misconceptions

--Rest is as good as sleep
--You never know just how tired you are.
--I have flown this tired before
--My motivation will solve the problem of fatigue
--I can fly well even if tired
--I don't need much sleep.
--Coffee is all I need
--Pills will cure fatigue
--One cure works for everyone.

What To Expect from Fatigue
--Reduced ability to concentrate
--Reduce awareness of deviations
--Reduced vigilance
--Reduced comprehension of ATC instructions
--Fatigue increases if you are "doing nothing"
--Increased temper excursions
--Increase in 'know-better' mistakes
--Increased rationalization of errors

Vibration Fatigue

A sustained mechanical oscillatory disturbance as while flying a helicopter can cause vibration fatigue. The body reacts by tension of the muscles, the muscles get tired, fatigue sets in. Manual agility, dexterity and precision are reduced. Concentration weakens and flight judgment is impaired.

Subtle Fatigue

This problem often begins with a distraction that causes fixation on an instrument or occurrence. Complex flight operations are the first skills to deteriorate.
--Knowing where you are becomes a problem
--Heading excursions take place
--Instruments are ignored
--Attention and vision fixates
--External references begin to fade from consciousness
--Seat posture relaxes
--Silence prevails
--Writing becomes less linear
--Movements decrease and slow
--Clearances cannot be copied in total
--Eye/hand skills begin to fail
--Pilot accepts what exists as O.K. without checking
--Bad judgment prevails

Sleep

The most common cause of diminished alertness and proficiency is lack of sleep. This condition is said to affect 30% of the U.S. population. This may be due to an actual loss of sleep or a change in a sleep pattern called the circadian rhythm. Pilots tend to neglect their need for sleep. The need for sleep is a defining limit to pilot mental capability. You must have sleep or your mind will fail. Once beyond the limit pilot performance deteriorates and can become irrational. Sleep is a restorative and can be both stored and deprived within limits set by the biological clock of the individual. As you grow older you will need less sleep. Jet lag sleep patterns are worse when flying from west to east. Accident rates climb precipitously when your body begins demanding sleep. The average American gets about one hour too little sleep each night.

Sleep serves as a restorative to both body and mind. Every day we resent our 25-hour biological clock. This extra hour makes it easier for us to stay up late than to get up early. Your energy cycles and you are susceptible to daydreaming or drowsiness at the lowest part.

Accident rates rise in the afternoon and become significant at night. Postponing sleep causes a sleep deficit that as it increases an accident becomes more likely. Jet lag is a type of sleep deficit. A sleep deficit can best be resolved by going to bed early, not by sleeping late. A large deficit cannot be made up in one night. 21% of aircraft accidents cite sleep deficiency as a factor.
Sleep begins when you stop knowing what is happening around you. You have stopped getting external information. The circadian rhythm makes you feel the most sleepy between three and five in the early morning and mid-afternoon. Your performance will suffer most when you have missed your normal sleep period and are staying awake for an extended time. You will be slow to recognize flight problems, slow to react to the situation, and will have difficulty selecting the best option when you are sleep deprived.

When drowsiness occurs, you cease to monitor the instruments. You will tend to fixate and drift off mentally. We go into a mental autopilot not thinking of what we are doing. This is the lowest level of alertness. The next level of alertness is one in which you are in constant search-and-scan, seeing what you are looking at, hearing what is said and asking question. This is the "flying" mode from pre-flight to shutdown. This gradual deterioration of alertness is best observed in watching others. It can creep up on you and influence your flying without your even noticing. Your alertness rises again when you have located a problem. You focus on it and prepare to execute a solution. This might occur when required to make a crosswind landing. The highest level of alertness is when adrenaline begins to flow and survival becomes a factor.

Naps can be planned or unplanned, the brain has signals that it sends to the body. The more sleep is needed the more brain signals are sent to the body. Most people get about two hours less sleep daily than they need. Over a few days this sleep deficit must be corrected. Deeper sleep is one way. Longer sleep periods are another. Naps are still another. Avoid caffeine after 5 p.m. A cola has the caffeine equivalent of a half-cup of coffee. Sleeping pills and caffeine are addictive.

Sleep suggestions
--Do not exercise before bedtime.
--Avoid food or drink before bedtime.
--Avoid sleeping pills
--Take naps of less than an hour.
--Do cockpit exercises.

Meniere's Disease

Meniere's disease usually occurs over age 40. Exact cause is in doubt but seems related to fluids in inner ear. Initial symptoms affect hearing but may be followed over a period of time by sweating, nausea, blurry vision and fatigue. Vertigo strikes and continues as intermittent lifelong condition. Affects personality. Occurrence and severity unpredictable.

Nutrition

A study of helicopter accidents has shown that three times as many pilot error accidents occurred during the lunch hours as at other times. Considering the time it takes for the body to benefit from eating, a good breakfast is highly recommended.

Food when converted into glucose is the source of brain energy. Glucose cannot be stored. As blood sugar it requires constant renewal. If glucose is not renewed the body and the mind shows evidence of fatigue, mental confusion, faintness, headache, memory loss, dizziness, vision problem, cold hands and feet.

Reduced blood pressure, tension, depression and hunger are all symptoms of hypoglycemia caused by the lack of a balanced meal for over five hours. Ten hours without food will severely affect decision-making ability, alertness, coordination, and perception. Skipping breakfast causes fasting hypoglycemia. All hypoglycemia types can be aggravated by other physiological factors.

Altitude can incapacitate a pilot through dehydration. Increase your fluid intake prior to and during flight. What you eat is just as important as just eating. Reactive hypoglycemia can cause lack of consciousness. This is a reaction to the doughnut/candy bar meal. The student pilot who does not eat because of possible airsickness is endangering himself if flying solo. High sugar meals cause the pancreas to create excess insulin. Insulin allows the body to use sugar. Too much insulin and deplete sugar to such a low level as to incapacitate the body and mind. Adding caffeine, alcohol and nicotine acerbates the problem. Flying should be preceded by a balanced meal. Neuronutriments are the vitamins and minerals that the body can change into neurotransmitters. Trace minerals such as potassium, zinc, iron, and chromium are essential to control the body's sugar burning process. The more balanced our meals the better will be our mental functioning and memory.

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