Chapter 16 - Aeromedical Factors
It is important for a pilot to be aware of the mental and physical standards required for the type of ﬂying done. This chapter provides information on medical certiﬁcation and on a variety of aeromedical factors related to ﬂight activities.
Obtaining a Medical Certiﬁcate
Most pilots must have a valid medical certiﬁcate to exercise the privileges of their airman certiﬁcates. Glider and free balloon pilots are not required to hold a medical certiﬁcate. Sport pilots may hold either a medical certiﬁcate or a valid state driver’s license.
Acquisition of a medical certiﬁcate requires an examination by an aviation medical examiner (AME), a physician with training in aviation medicine designated by the Civil Aerospace Medical Institute (CAMI). There are three classes of medical certificates. The class of certificate needed depends on the type of ﬂying the pilot plans to do.
A third-class medical certiﬁcate is required for a private or recreational pilot certiﬁcate. It is valid for 3 years for those individuals who have not reached the age of 40; otherwise it is valid for 2 years. A commercial pilot certiﬁcate requires at least a second-class medical certiﬁcate, which is valid for 1 year. First-class medical certiﬁcates are required for airline transport pilots, and are valid for 6 months.
The standards are more rigorous for the higher classes of certiﬁcates. A pilot with a higher class medical certiﬁcate has met the requirements for the lower classes as well. Since the required medical class applies only when exercising the privileges of the pilot certiﬁcate for which it is required, a ﬁrst-class medical certiﬁcate would be valid for 1 year if exercising the privileges of a commercial certiﬁcate, and 2 or 3 years, as appropriate, for exercising the privileges of a private or recreational certiﬁcate. The same applies for a second-class medical certiﬁcate. The standards for medical certiﬁcation are contained in Title 14 of the Code of Federal Regulations (14 CFR) part 67 and the requirements for obtaining medical certiﬁcates can be found in 14 CFR part 61.
Students who have physical limitations, such as impaired vision, loss of a limb, or hearing impairment may be issued a medical certificate valid for “student pilot privileges only” while learning to ﬂy. Pilots with disabilities may require special equipment installed in the aircraft, such as hand controls for pilots with paraplegia. Some disabilities necessitate a limitation on the individual’s certiﬁcate; for example, impaired hearing would require the limitation “not valid for ﬂight requiring the use of radio.” When all the knowledge, experience, and proﬁciency requirements have been met and a student can demonstrate the ability to operate the aircraft with the normal level of safety, a “statement of demonstrated ability” (SODA) can be issued. This waiver, or SODA, is valid as long as the physical impairment does not worsen. Contact the local Flight Standards District Ofﬁce (FSDO) for more information on this subject.
Health and Physiological Factors Affecting Pilot Performance
A number of health factors and physiological effects can be linked to ﬂying. Some are minor, while others are important enough to require special attention to ensure safety of ﬂight. In some cases, physiological factors can lead to inﬂight emergencies. Some important medical factors that a pilot should be aware of include hypoxia, hyperventilation, middle ear and sinus problems, spatial disorientation, motion sickness, carbon monoxide (CO) poisoning, stress and fatigue, dehydration, and heatstroke. Other subjects include the effects of alcohol and drugs, anxiety, and excess nitrogen in the blood after scuba diving.
Hypoxia means “reduced oxygen” or “not enough oxygen.” Although any tissue will die if deprived of oxygen long enough, usually the most concern is with getting enough oxygen to the brain, since it is particularly vulnerable to oxygen deprivation. Any reduction in mental function while ﬂying can result in life-threatening errors. Hypoxia can be caused by several factors, including an insufﬁcient supply of oxygen, inadequate transportation of oxygen, or the inability of the body tissues to use oxygen. The forms of hypoxia are based on their causes: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.
Hypoxic hypoxia is a result of insufﬁcient oxygen available to the body as a whole. A blocked airway and drowning are obvious examples of how the lungs can be deprived of oxygen, but the reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots. Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. As the airplane ascends during ﬂight, the percentage of each gas in the atmosphere remains the same, but there are fewer molecules available at the pressure required for them to pass between the membranes in the respiratory system. This decrease in number of oxygen molecules at sufﬁcient pressure can lead to hypoxic hypoxia.
Hypemic hypoxia occurs when the blood is not able to take up and transport a sufﬁcient amount of oxygen to the cells in the body. Hypemic means “not enough blood.” This type of hypoxia is a result of oxygen deﬁciency in the blood, rather than a lack of inhaled oxygen, and can be caused by a variety of factors. It may be due to reduced blood volume (due to severe bleeding), or it may result from certain blood diseases, such as anemia. More often hypemic hypoxia occurs because hemoglobin, the actual blood molecule that transports oxygen, is chemically unable to bind oxygen molecules. The most common form of hypemic hypoxia is CO poisoning. This is explained in greater detail on page 16-11. Hypemic hypoxia can also be caused by the loss of blood due to blood donation. Blood can require several weeks to return to normal following a donation. Although the effects of the blood loss are slight at ground level, there are risks when ﬂying during this time.
Stagnant means “not ﬂowing,” and stagnant hypoxia, or ischemia, results when the oxygen-rich blood in the lungs is not moving, for one reason or another, to the tissues that need it. An arm or leg “going to sleep” because the blood ﬂow has accidentally been shut off is one form of stagnant hypoxia. This kind of hypoxia can also result from shock, the heart failing to pump blood effectively, or a constricted artery. During flight, stagnant hypoxia can occur with excessive acceleration of gravity (Gs). Cold temperatures also can reduce circulation and decrease the blood supplied to extremities.
The inability of the cells to effectively use oxygen is deﬁned as histotoxic hypoxia. “Histo” refers to tissues or cells, and “toxic” means poisonous. In this case, enough oxygen is being transported to the cells that need it, but they are unable to make use of it. This impairment of cellular respiration can be caused by alcohol and other drugs, such as narcotics and poisons. Research has shown that drinking one ounce of alcohol can equate to about an additional 2,000 feet of physiological altitude.
Symptoms of Hypoxia
High-altitude ﬂying can place a pilot in danger of becoming hypoxic. Oxygen starvation causes the brain and other vital organs to become impaired. One noteworthy attribute of the onset of hypoxia is that the ﬁrst symptoms are euphoria and a carefree feeling. With increased oxygen starvation, the extremities become less responsive and ﬂying becomes less coordinated. The symptoms of hypoxia vary with the individual, but common symptoms include:
- Cyanosis (blue ﬁngernails and lips)
- Decreased reaction time
- Impaired judgment
- Visual impairment
- Lightheaded or dizzy sensation
- Tingling in ﬁngers and toes
As hypoxia worsens, the ﬁeld of vision begins to narrow, and instrument interpretation can become difﬁcult. Even with all these symptoms, the effects of hypoxia can cause a pilot to have a false sense of security and be deceived into believing everything is normal. The treatment for hypoxia includes ﬂying at lower altitudes and/or using supplemental oxygen.
All pilots are susceptible to the effects of oxygen starvation, regardless of physical endurance or acclimatization. When ﬂying at high altitudes, it is paramount that oxygen be used to avoid the effects of hypoxia. The term “time of useful consciousness” describes the maximum time the pilot has to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen. As altitude increases above 10,000 feet, the symptoms of hypoxia increase in severity, and the time of useful consciousness rapidly decreases. [figure 16-1]
figure 16-1. Time of useful consciousness.
Since symptoms of hypoxia can be different for each individual, the ability to recognize hypoxia can be greatly improved by experiencing and witnessing the effects of it during an altitude chamber “flight.” The Federal Aviation Administration (FAA) provides this opportunity through aviation physiology training, which is conducted at the FAA CAMI and at many military facilities across the United States. For information about the FAA’s one-day physiological training course with altitude chamber and vertigo demonstrations, visit the FAA web site: www.faa.gov/pilots/training/airman_education/aerospace_physiology/index.cfm.
Hyperventilation is the excessive rate and depth of respiration leading to abnormal loss of carbon dioxide from the blood. This condition occurs more often among pilots than is generally recognized. It seldom incapacitates completely, but it causes disturbing symptoms that can alarm the uninformed pilot. In such cases, increased breathing rate and anxiety further aggravate the problem. Hyperventilation can lead to unconsciousness due to the respiratory system’s overriding mechanism to regain control of breathing.
Pilots encountering an unexpected stressful situation may subconsciously increase their breathing rate. If ﬂying at higher altitudes, either with or without oxygen, a pilot may have a tendency to breathe more rapidly than normal, which often leads to hyperventilation.
Since many of the symptoms of hyperventilation are similar to those of hypoxia, it is important to correctly diagnose and treat the proper condition. If using supplemental oxygen, check the equipment and ﬂow rate to ensure the symptoms are not hypoxia related. Common symptoms of hyperventilation include:
- Visual impairment
- Lightheaded or dizzy sensation
- Tingling sensations
- Hot and cold sensations
- Muscle spasms
The treatment for hyperventilation involves restoring the proper carbon dioxide level in the body. Breathing normally is both the best prevention and the best cure for hyperventilation. In addition to slowing the breathing rate, breathing into a paper bag or talking aloud helps to overcome hyperventilation. Recovery is usually rapid once the breathing rate is returned to normal.
Middle Ear and Sinus Problems
During climbs and descents, the free gas formerly present in various body cavities expands due to a difference between the pressure of the air outside the body and that of the air inside the body. If the escape of the expanded gas is impeded, pressure builds up within the cavity and pain is experienced. Trapped gas expansion accounts for ear pain and sinus pain, as well as a temporary reduction in the ability to hear.
The middle ear is a small cavity located in the bone of the skull. It is closed off from the external ear canal by the eardrum. Normally, pressure differences between the middle ear and the outside world are equalized by a tube leading from inside each ear to the back of the throat on each side, called the Eustachian tube. These tubes are usually closed, but open during chewing, yawning, or swallowing to equalize pressure. Even a slight difference between external pressure and middle ear pressure can cause discomfort. [figure 16-2]
figure 16-2. The Eustachian tube allows air pressure to equalize in the middle ear.
During a climb, middle ear air pressure may exceed the pressure of the air in the external ear canal, causing the eardrum to bulge outward. Pilots become aware of this pressure change when they experience alternate sensations of “fullness” and “clearing.” During descent, the reverse happens. While the pressure of the air in the external ear canal increases, the middle ear cavity, which equalized with the lower pressure at altitude, is at lower pressure than the external ear canal. This results in the higher outside pressure, causing the eardrum to bulge inward.
This condition can be more difﬁcult to relieve due to the fact that the partial vacuum tends to constrict the walls of the Eustachian tube. To remedy this often painful condition, which also causes a temporary reduction in hearing sensitivity, pinch the nostrils shut, close the mouth and lips, and blow slowly and gently in the mouth and nose.
This procedure forces air through the Eustachian tube into the middle ear. It may not be possible to equalize the pressure in the ears if a pilot has a cold, an ear infection, or sore throat. A ﬂight in this condition can be extremely painful, as well as damaging to the eardrums. If experiencing minor congestion, nose drops or nasal sprays may reduce the risk of a painful ear blockage. Before using any medication, check with an AME to ensure that it will not affect the ability to ﬂy.
In a similar way, air pressure in the sinuses equalizes with the pressure in the flight deck through small openings that connect the sinuses to the nasal passages. An upper respiratory infection, such as a cold or sinusitis, or a nasal allergic condition can produce enough congestion around an opening to slow equalization. As the difference in pressure between the sinuses and the ﬂight deck increases, congestion may plug the opening. This “sinus block” occurs most frequently during descent. Slow descent rates can reduce the associated pain. A sinus block can occur in the frontal sinuses, located above each eyebrow, or in the maxillary sinuses, located in each upper cheek. It will usually produce excruciating pain over the sinus area. A maxillary sinus block can also make the upper teeth ache. Bloody mucus may discharge from the nasal passages.
Sinus block can be avoided by not ﬂying with an upper respiratory infection or nasal allergic condition. Adequate protection is usually not provided by decongestant sprays or drops to reduce congestion around the sinus openings. Oral decongestants have side effects that can impair pilot performance. If a sinus block does not clear shortly after landing, a physician should be consulted.
Spatial Disorientation and Illusions
Spatial disorientation specifically refers to the lack of orientation with regard to the position, attitude, or movement of the airplane in space. The body uses three integrated systems working together to ascertain orientation and movement in space.
- Vestibular system—organs found in the inner ear that sense position by the way we are balanced.
- Somatosensory system—nerves in the skin, muscles, and joints, which, along with hearing, sense position based on gravity, feeling, and sound.
- Visual system—eyes, which sense position based on what is seen.
All this information comes together in the brain and, most of the time, the three streams of information agree, giving a clear idea of where and how the body is moving. Flying can sometimes cause these systems to supply conﬂicting information to the brain, which can lead to disorientation. During ﬂight in visual meteorological conditions (VMC), the eyes are the major orientation source and usually prevail over false sensations from other sensory systems. When these visual cues are removed, as they are in instrument meteorological conditions (IMC), false sensations can cause a pilot to quickly become disoriented.
The vestibular system in the inner ear allows the pilot to sense movement and determine orientation in the surrounding environment. In both the left and right inner ear, three semicircular canals are positioned at approximate right angles to each other. [figure 16-3] Each canal is ﬁlled with ﬂuid and has a section full of ﬁne hairs. Acceleration of the inner ear in any direction causes the tiny hairs to deﬂect, which in turn stimulates nerve impulses, sending messages to the brain. The vestibular nerve transmits the impulses from the utricle, saccule, and semicircular canals to the brain to interpret motion.
figure 16-3. The semicircular canals lie in three planes and sense motions of roll, pitch, and yaw.
The somatosensory system sends signals from the skin, joints, and muscles to the brain that are interpreted in relation to the Earth’s gravitational pull. These signals determine posture. Inputs from each movement update the body’s position to the brain on a constant basis. “Seat of the pants” ﬂying is largely dependent upon these signals. Used in conjunction with visual and vestibular clues, these sensations can be fairly reliable. However, the body cannot distinguish between acceleration forces due to gravity and those resulting from maneuvering the aircraft, which can lead to sensory illusions and false impressions of an aircraft’s orientation and movement.
Under normal flight conditions, when there is a visual reference to the horizon and ground, the sensory system in the inner ear helps to identify the pitch, roll, and yaw movements of the aircraft. When visual contact with the horizon is lost, the vestibular system becomes unreliable. Without visual references outside the aircraft, there are many situations in which combinations of normal motions and forces create convincing illusions that are difﬁcult to overcome.
Prevention is usually the best remedy for spatial disorientation. Unless a pilot has many hours of training in instrument ﬂight, ﬂight should be avoided in reduced visibility or at night when the horizon is not visible. A pilot can reduce susceptibility to disorienting illusions through training and awareness, and learning to rely totally on ﬂight instruments.
A condition called the leans can result when a banked attitude, to the left for example, may be entered too slowly to set in motion the ﬂuid in the “roll” semicircular tubes. [figure 16-4] An abrupt correction of this attitude sets the ﬂuid in motion, creating the illusion of a banked attitude to the right. The disoriented pilot may make the error of rolling the aircraft into the original left banked attitude, or if level ﬂight is maintained, will feel compelled to lean in the perceived vertical plane until this illusion subsides.
figure 16-4. Human sensation of angular acceleration.
The coriolis illusion occurs when a pilot has been in a turn long enough for the ﬂuid in the ear canal to move at the same speed as the canal. A movement of the head in a different plane, such as looking at something in a different part of the ﬂight deck, may set the ﬂuid moving and create the illusion of turning or accelerating on an entirely different axis. This action causes the pilot to think the aircraft is doing a maneuver that it is not. The disoriented pilot may maneuver the aircraft into a dangerous attitude in an attempt to correct the aircraft’s perceived attitude.
For this reason, it is important that pilots develop an instrument cross-check or scan that involves minimal head movement. Take care when retrieving charts and other objects in the ﬂight deck—if something is dropped, retrieve it with minimal head movement and be alert for the coriolis illusion.
As in other illusions, a pilot in a prolonged coordinated, constant-rate turn, will have the illusion of not turning. During the recovery to level ﬂight, the pilot will experience the sensation of turning in the opposite direction. The disoriented pilot may return the aircraft to its original turn. Because an aircraft tends to lose altitude in turns unless the pilot compensates for the loss in lift, the pilot may notice a loss of altitude. The absence of any sensation of turning creates the illusion of being in a level descent. The pilot may pull back on the controls in an attempt to climb or stop the descent. This action tightens the spiral and increases the loss of altitude; this illusion is referred to as a graveyard spiral. [figure 16-5] At some point, this could lead to a loss of aircraft control.
figure 16-5. Graveyard spiral.
A rapid acceleration, such as experienced during takeoff, stimulates the otolith organs in the same way as tilting the head backwards. This action creates the somatogravic illusion of being in a nose-up attitude, especially in situations without good visual references. The disoriented pilot may push the aircraft into a nose-low or dive attitude. A rapid deceleration by quick reduction of the throttle(s) can have the opposite effect, with the disoriented pilot pulling the aircraft into a nose-up or stall attitude.
An abrupt change from climb to straight-and-level ﬂight can stimulate the otolith organs enough to create the illusion of tumbling backwards, or inversion illusion. The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.
An abrupt upward vertical acceleration, as can occur in an updraft, can stimulate the otolith organs to create the illusion of being in a climb. This is called elevator illusion. The disoriented pilot may push the aircraft into a nose-low attitude. An abrupt downward vertical acceleration, usually in a downdraft, has the opposite effect, with the disoriented pilot pulling the aircraft into a nose-up attitude.
Visual illusions are especially hazardous because pilots rely on their eyes for correct information. Two illusions that lead to spatial disorientation, false horizon and autokinesis, are concerned with only the visual system.
A sloping cloud formation, an obscured horizon, an aurora borealis, a dark scene spread with ground lights and stars, and certain geometric patterns of ground lights can provide inaccurate visual information, or false horizon, for aligning the aircraft correctly with the actual horizon. The disoriented pilot may place the aircraft in a dangerous attitude.
In the dark, a stationary light will appear to move about when stared at for many seconds. The disoriented pilot could lose control of the aircraft in attempting to align it with the false movements of this light, called autokinesis.
The postural system sends signals from the skin, joints, and muscles to the brain that are interpreted in relation to the Earth’s gravitational pull. These signals determine posture. Inputs from each movement update the body’s position to the brain on a constant basis. “Seat of the pants” ﬂying is largely dependent upon these signals. Used in conjunction with visual and vestibular clues, these sensations can be fairly reliable. However, because of the forces acting upon the body in certain ﬂight situations, many false sensations can occur due to acceleration forces overpowering gravity. [figure 16-6] These situations include uncoordinated turns, climbing turns, and turbulence.
figure 16-6. Sensations from centrifugal force.
Demonstration of Spatial Disorientation
There are a number of controlled aircraft maneuvers a pilot can perform to experiment with spatial disorientation. While each maneuver will normally create a speciﬁc illusion, any false sensation is an effective demonstration of disorientation. Thus, even if there is no sensation during any of these maneuvers, the absence of sensation is still an effective demonstration because it illustrates the inability to detect bank or roll. There are several objectives in demonstrating these various maneuvers.
- They teach pilots to understand the susceptibility of the human system to spatial disorientation.
- They demonstrate that judgments of aircraft attitude based on bodily sensations are frequently false.
- They help decrease the occurrence and degree of disorientation through a better understanding of the relationship between aircraft motion, head movements, and resulting disorientation.
- They help instill a greater conﬁdence in relying on ﬂight instruments for assessing true aircraft attitude.
A pilot should not attempt any of these maneuvers at low altitudes, or in the absence of an instructor pilot or an appropriate safety pilot.
Climbing While Accelerating
With the pilot’s eyes closed, the instructor pilot maintains approach airspeed in a straight-and-level attitude for several seconds, then accelerates while maintaining straight-and-level attitude. The usual illusion during this maneuver, without visual references, is that the aircraft is climbing.
Climbing While Turning
With the pilot’s eyes still closed and the aircraft in a straight-and-level attitude, the instructor pilot now executes, with a relatively slow entry, a well coordinated turn of about 1.5 positive G (approximately 50° bank) for 90°. While in the turn, without outside visual references and under the effect of the slight positive G, the usual illusion produced is that of a climb. Upon sensing the climb, the pilot should immediately open the eyes to see that a slowly established, coordinated turn produces the same sensation as a climb.
Diving While Turning
Repeating the previous procedure, except the pilot’s eyes should be kept closed until recovery from the turn is approximately one-half completed, can create the illusion of diving while turning.
Tilting to Right or Left
While in a straight-and-level attitude, with the pilot’s eyes closed, the instructor pilot executes a moderate or slight skid to the left with wings level. This creates the illusion of the body being tilted to the right.
Reversal of Motion
This illusion can be demonstrated in any of the three planes of motion. While straight and level, with the pilot’s eyes closed, the instructor pilot smoothly and positively rolls the aircraft to approximately 45° bank attitude while maintaining heading and pitch attitude. This creates the illusion of a strong sense of rotation in the opposite direction. After this illusion is noted, the pilot should open his or her eyes and observe that the aircraft is in a banked attitude.
Diving or Rolling Beyond the Vertical Plane
This maneuver may produce extreme disorientation. While in straight-and-level ﬂight, the pilot should sit normally, either with eyes closed or gaze lowered to the ﬂoor. The instructor pilot starts a positive, coordinated roll toward a 30° or 40° angle of bank. As this is in progress, the pilot tilts his or her head forward, looks to the right or left, then immediately returns his or her head to an upright position. The instructor pilot should time the maneuver so the roll is stopped as the pilot returns his or her head upright. An intense disorientation is usually produced by this maneuver, and the pilot experiences the sensation of falling downward into the direction of the roll.
In the descriptions of these maneuvers, the instructor pilot is doing the ﬂying, but having the pilot do the ﬂying can also be a very effective demonstration. The pilot should close his or her eyes and tilt the head to one side. The instructor pilot tells the pilot what control inputs to perform. The pilot then attempts to establish the correct attitude or control input with eyes closed and head tilted. While it is clear the pilot has no idea of the actual attitude, he or she will react to what the senses are saying. After a short time, the pilot will become disoriented and the instructor pilot then tells the pilot to look up and recover. The beneﬁt of this exercise is that the pilot experiences the disorientation while ﬂying the aircraft.
Coping with Spatial Disorientation
To prevent illusions and their potentially disastrous consequences, pilots can:
- Understand the causes of these illusions and remain constantly alert for them. Take the opportunity to experience spatial disorientation illusions in a device such as a Barany chair, a Vertigon, or a Virtual Reality Spatial Disorientation Demonstrator.
- Always obtain and understand preflight weather brieﬁngs.
- Before ﬂying in marginal visibility (less than 3 miles) or where a visible horizon is not evident, such as ﬂight over open water during the night, obtain training and maintain proﬁciency in airplane control by reference to instruments.
- Do not continue ﬂight into adverse weather conditions or into dusk or darkness unless proﬁcient in the use of ﬂight instruments. If intending to ﬂy at night, maintain night-ﬂight currency and proﬁciency. Include cross-country and local operations at various airﬁelds.
- Ensure that when outside visual references are used, they are reliable, ﬁxed points on the Earth’s surface.
- Avoid sudden head movement, particularly during takeoffs, turns, and approaches to landing.
- Be physically tuned for ﬂight into reduced visibility. That is, ensure proper rest, adequate diet, and, if ﬂying at night, allow for night adaptation Remember that illness, medication, alcohol, fatigue, sleep loss, and mild hypoxia are likely to increase susceptibility to spatial disorientation.
- Most importantly, become proﬁcient in the use of flight instruments and rely upon them. Trust the instruments and disregard your sensory perceptions.
The sensations that lead to illusions during instrument ﬂight conditions are normal perceptions experienced by pilots. These undesirable sensations cannot be completely prevented, but through training and awareness, pilots can ignore or suppress them by developing absolute reliance on the flight instruments. As pilots gain proficiency in instrument ﬂying, they become less susceptible to these illusions and their effects.
Of the senses, vision is the most important for safe ﬂight. However, various terrain features and atmospheric conditions can create optical illusions. These illusions are primarily associated with landing. Since pilots must transition from reliance on instruments to visual cues outside the ﬂight deck for landing at the end of an instrument approach, it is imperative they be aware of the potential problems associated with these illusions, and take appropriate corrective action. The major illusions leading to landing errors are described below.
Runway Width Illusion
A narrower-than-usual runway can create an illusion the aircraft is at a higher altitude than it actually is, especially when runway length-to-width relationships are comparable. [figure 16-7] The pilot who does not recognize this illusion will ﬂy a lower approach, with the risk of striking objects along the approach path or landing short. A wider-than-usual runway can have the opposite effect, with the risk of the pilot leveling out the aircraft high and landing hard, or overshooting the runway.
figure 16-7. Runway illusions.
Runway and Terrain Slopes Illusion
An upsloping runway, upsloping terrain, or both, can create an illusion that the aircraft is at a higher altitude than it actually is. [figure 16-7] The pilot who does not recognize this illusion will ﬂy a lower approach. Downsloping runways and downsloping approach terrain can have the opposite effect.
Featureless Terrain Illusion
An absence of surrounding ground features, as in an overwater approach, over darkened areas, or terrain made featureless by snow, can create an illusion the aircraft is at a higher altitude than it actually is. This illusion, sometimes referred to as the “black hole approach,” causes pilots to ﬂy a lower approach than is desired.
Rain on the windscreen can create an illusion of being at a higher altitude due to the horizon appearing lower than it is. This can result in the pilot ﬂying a lower approach.
Atmospheric haze can create an illusion of being at a greater distance and height from the runway. As a result, the pilot will have a tendency to be low on the approach. Conversely, extremely clear air (clear bright conditions of a high attitude airport) can give the pilot the illusion of being closer than he or she actually is, resulting in a high approach, which may result in an overshoot or go around. The diffusion of light due to water particles on the windshield can adversely affect depth perception. The lights and terrain features normally used to gauge height during landing become less effective for the pilot.
Flying into fog can create an illusion of pitching up. Pilots who do not recognize this illusion will often steepen the approach quite abruptly.
Ground Lighting Illusions
Lights along a straight path, such as a road or lights on moving trains, can be mistaken for runway and approach lights. Bright runway and approach lighting systems, especially where few lights illuminate the surrounding terrain, may create the illusion of less distance to the runway. The pilot who does not recognize this illusion will often ﬂy a higher approach.
How To Prevent Landing Errors Due to Optical Illusions
To prevent these illusions and their potentially hazardous consequences, pilots can:
- Anticipate the possibility of visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions. Consult airport diagrams and the Airport/Facility Directory (A/FD) for information on runway slope, terrain, and lighting.
- Make frequent reference to the altimeter, especially during all approaches, day and night.
- If possible, conduct aerial visual inspection of unfamiliar airports before landing.
- Use Visual Approach Slope Indicator (VASI) or Precision Approach Path Indicator (PAPI) systems for a visual reference, or an electronic glideslope, whenever they are available.
- Utilize the visual descent point (VDP) found on many nonprecision instrument approach procedure charts.
- Recognize that the chances of being involved in an approach accident increase when some emergency or other activity distracts from usual procedures.
- Maintain optimum proﬁciency in landing procedures.
In addition to the sensory illusions due to misleading inputs to the vestibular system, a pilot may also encounter various visual illusions during flight. Illusions rank among the most common factors cited as contributing to fatal aviation accidents.
Sloping cloud formations, an obscured horizon, a dark scene spread with ground lights and stars, and certain geometric patterns of ground light can create illusions of not being aligned correctly with the actual horizon. Various surface features and atmospheric conditions encountered in landing can create illusions of being on the wrong approach path. Landing errors due to these illusions can be prevented by anticipating them during approaches, inspecting unfamiliar airports before landing, using electronic glideslope or VASI systems when available, and maintaining proﬁciency in landing procedures.
Motion sickness, or airsickness, is caused by the brain receiving conﬂicting messages about the state of the body. A pilot may experience motion sickness during initial ﬂights, but it generally goes away within the ﬁrst few lessons. Anxiety and stress, which may be experienced at the beginning of ﬂight training, can contribute to motion sickness. Symptoms of motion sickness include general discomfort, nausea, dizziness, paleness, sweating, and vomiting.
It is important to remember that experiencing airsickness is no reﬂection on one’s ability as a pilot. If prone to motion sickness, let the ﬂight instructor know, there are techniques that can be used to overcome this problem. For example, avoid lessons in turbulent conditions until becoming more comfortable in the aircraft, or start with shorter ﬂights and graduate to longer instruction periods. If symptoms of motion sickness are experienced during a lesson, opening fresh air vents, focusing on objects outside the airplane, and avoiding unnecessary head movements may help alleviate some of the discomfort. Although medications like Dramamine can prevent airsickness in passengers, they are not recommended while ﬂying since they can cause drowsiness and other problems.
Carbon Monoxide (CO) Poisoning
CO is a colorless and odorless gas produced by all internal combustion engines. Attaching itself to the hemoglobin in the blood about 200 times more easily than oxygen, CO prevents the hemoglobin from carrying oxygen to the cells, resulting in hypemic hypoxia. The body requires up to 48 hours to dispose of CO. If severe enough, the CO poisoning can result in death. Aircraft heater vents and defrost vents may provide CO a passageway into the cabin, particularly if the engine exhaust system has a leak or is damaged. If a strong odor of exhaust gases is detected, assume that CO is present. However, CO may be present in dangerous amounts even if no exhaust odor is detected. Disposable, inexpensive CO detectors are widely available. In the presence of CO, these detectors change color to alert the pilot of the presence of CO. Some effects of CO poisoning are headache, blurred vision, dizziness, drowsiness, and/or loss of muscle power. Any time a pilot smells exhaust odor, or any time that these symptoms are experienced, immediate corrective actions should be taken. These include turning off the heater, opening fresh air vents and windows, and using supplemental oxygen, if available.
Tobacco smoke also causes CO poisoning. Smoking at sea level can raise the CO concentration in the blood and result in physiological effects similar to ﬂying at 8,000 feet. Besides hypoxia, tobacco causes diseases and physiological debilitation that are medically disqualifying for pilots.
Stress is the body’s response to physical and psychological demands placed upon it. The body’s reaction to stress includes releasing chemical hormones (such as adrenaline) into the blood, and increasing metabolism to provide more energy to the muscles. Blood sugar, heart rate, respiration, blood pressure, and perspiration all increase. The term “stressor” is used to describe an element that causes an individual to experience stress. Examples of stressors include physical stress (noise or vibration), physiological stress (fatigue), and psychological stress (difﬁcult work or personal situations).
Stress falls into two broad categories, acute (short term) and chronic (long term). Acute stress involves an immediate threat that is perceived as danger. This is the type of stress that triggers a “ﬁght or ﬂight” response in an individual, whether the threat is real or imagined. Normally, a healthy person can cope with acute stress and prevent stress overload. However, ongoing acute stress can develop into chronic stress.
Chronic stress can be deﬁned as a level of stress that presents an intolerable burden, exceeds the ability of an individual to cope, and causes individual performance to fall sharply. Unrelenting psychological pressures, such as loneliness, ﬁnancial worries, and relationship or work problems can produce a cumulative level of stress that exceeds a person’s ability to cope with the situation. When stress reaches these levels, performance falls off rapidly. Pilots experiencing this level of stress are not safe and should not exercise their airman privileges. Pilots who suspect they are suffering from chronic stress should consult a physician.
Fatigue is frequently associated with pilot error. Some of the effects of fatigue include degradation of attention and concentration, impaired coordination, and decreased ability to communicate. These factors seriously influence the ability to make effective decisions. Physical fatigue results from sleep loss, exercise, or physical work. Factors such as stress and prolonged performance of cognitive work result in mental fatigue.
Like stress, fatigue falls into two broad categories: acute and chronic. Acute fatigue is short term and is a normal occurrence in everyday living. It is the kind of tiredness people feel after a period of strenuous effort, excitement, or lack of sleep. Rest after exertion and 8 hours of sound sleep ordinarily cures this condition.
A special type of acute fatigue is skill fatigue. This type of fatigue has two main effects on performance:
- Timing disruption—Appearing to perform a task as usual, but the timing of each component is slightly off. This makes the pattern of the operation less smooth, because the pilot performs each component as though it were separate, instead of part of an integrated activity.
- Disruption of the perceptual ﬁeld—Concentrating attention upon movements or objects in the center of vision and neglecting those in the periphery. This is accompanied by loss of accuracy and smoothness in control movements.
Acute fatigue has many causes, but the following are among the most important for the pilot:
- Mild hypoxia (oxygen deﬁciency)
- Physical stress
- Psychological stress and
- Depletion of physical energy resulting from psychological stress
- Sustained psychological stress
Sustained psychological stress accelerates the glandular secretions that prepare the body for quick reactions during an emergency. These secretions make the circulatory and respiratory systems work harder, and the liver releases energy to provide the extra fuel needed for brain and muscle work. When this reserve energy supply is depleted, the body lapses into generalized and severe fatigue.
Acute fatigue can be prevented by proper diet and adequate rest and sleep. A well-balanced diet prevents the body from needing to consume its own tissues as an energy source. Adequate rest maintains the body’s store of vital energy.
Chronic fatigue, extending over a long period of time, usually has psychological roots, although an underlying disease is sometimes responsible. Continuous high stress levels produce chronic fatigue. Chronic fatigue is not relieved by proper diet and adequate rest and sleep, and usually requires treatment by a physician. An individual may experience this condition in the form of weakness, tiredness, palpitations of the heart, breathlessness, headaches, or irritability. Sometimes chronic fatigue even creates stomach or intestinal problems and generalized aches and pains throughout the body. When the condition becomes serious enough, it leads to emotional illness.
If suffering from acute fatigue, stay on the ground. If fatigue occurs in the ﬂight deck, no amount of training or experience can overcome the detrimental effects. Getting adequate rest is the only way to prevent fatigue from occurring. Avoid ﬂying without a full night’s rest, after working excessive hours, or after an especially exhausting or stressful day. Pilots who suspect they are suffering from chronic fatigue should consult a physician.
Dehydration and Heatstroke
Dehydration is the term given to a critical loss of water from the body. Causes of dehydration are hot ﬂight decks and ﬂight lines, wind, humidity, and diuretic drinks—coffee, tea, alcohol, and caffeinated soft drinks. Some common signs of dehydration are headache, fatigue, cramps, sleepiness, and dizziness.
The ﬁrst noticeable effect of dehydration is fatigue, which in turn makes top physical and mental performance difﬁcult, if not impossible. Flying for long periods in hot summer temperatures or at high altitudes increases the susceptibility to dehydration because these conditions tend to increase the rate of water loss from the body.
To help prevent dehydration, drink two to four quarts of water every 24 hours. Since each person is physiologically different, this is only a guide. Most people are aware of the eight-glasses-a-day guide: If each glass of water is eight ounces, this equates to 64 ounces, which is two quarts. If this ﬂuid is not replaced, fatigue progresses to dizziness, weakness, nausea, tingling of hands and feet, abdominal cramps, and extreme thirst.
The key for pilots is to be continually aware of their condition. Most people become thirsty with a 1.5 quart deﬁcit, or a loss of 2 percent of total body weight. This level of dehydration triggers the “thirst mechanism.” The problem is that the thirst mechanism arrives too late and is turned off too easily. A small amount of ﬂuid in the mouth will turn this mechanism off and the replacement of needed body ﬂuid is delayed.
Other steps to prevent dehydration include:
- Carrying a container in order to measure daily water intake.
- Staying ahead—not relying on the thirst sensation as an alarm. If plain water is offensive, add some sport drink ﬂavoring to make it more acceptable.
- Limiting daily intake of caffeine and alcohol (both are diuretics and stimulate increased production of urine).
Heatstroke is a condition caused by any inability of the body to control its temperature. Onset of this condition may be recognized by the symptoms of dehydration, but also has been known to be recognized only by complete collapse.
To prevent these symptoms, it is recommended that an ample supply of water be carried and used at frequent intervals on any long ﬂight, whether thirsty or not. The body normally absorbs water at the rate of 1.2 to 1.5 quarts per hour. Individuals should drink one quart per hour for severe heat stress conditions or one pint per hour for moderate stress conditions. If the aircraft has a canopy or roof window, wearing light-colored, porous clothing and a hat will help provide protection from the sun. Keeping the ﬂight deck well ventilated aids in dissipating excess heat.
Alcohol impairs the efficiency of the human body. [figure 16-8] Studies have proved that drinking and performance deterioration are closely linked. Pilots must make hundreds of decisions, some of them time-critical, during the course of a ﬂight. The safe outcome of any ﬂight depends on the ability to make the correct decisions and take the appropriate actions during routine occurrences, as well as abnormal situations. The inﬂuence of alcohol drastically reduces the chances of completing a ﬂight without incident. Even in small amounts, alcohol can impair judgment, decrease sense of responsibility, affect coordination, constrict visual ﬁeld, diminish memory, reduce reasoning power, and lower attention span. As little as one ounce of alcohol can decrease the speed and strength of muscular reﬂexes, lessen the efﬁciency of eye movements while reading, and increase the frequency at which errors are committed. Impairments in vision and hearing occur at alcohol blood levels due to as little as one drink.
figure 16-8. Impairement scale with alcohol use.
The alcohol consumed in beer and mixed drinks is ethyl alcohol, a central nervous system depressant. From a medical point of view, it acts on the body much like a general anesthetic. The “dose” is generally much lower and more slowly consumed in the case of alcohol, but the basic effects on the human body are similar. Alcohol is easily and quickly absorbed by the digestive tract. The bloodstream absorbs about 80 to 90 percent of the alcohol in a drink within 30 minutes when ingested on an empty stomach. The body requires about 3 hours to rid itself of all the alcohol contained in one mixed drink or one beer.
While experiencing a hangover, a pilot is still under the inﬂuence of alcohol. Although a pilot may think he or she is functioning normally, motor and mental response impairment is still present. Considerable amounts of alcohol can remain in the body for over 16 hours, so pilots should be cautious about ﬂying too soon after drinking.
Altitude multiplies the effects of alcohol on the brain. When combined with altitude, the alcohol from two drinks may have the same effect as three or four drinks. Alcohol interferes with the brain’s ability to utilize oxygen, producing a form of histotoxic hypoxia. The effects are rapid because alcohol passes quickly into the bloodstream. In addition, the brain is a highly vascular organ that is immediately sensitive to changes in the blood’s composition. For a pilot, the lower oxygen availability at altitude and the lower capability of the brain to use what oxygen is there, add up to a deadly combination.
Intoxication is determined by the amount of alcohol in the bloodstream. This is usually measured as a percentage by weight in the blood. 14 CFR part 91 requires that blood alcohol level be less than .04 percent and that 8 hours pass between drinking alcohol and piloting an airplane. A pilot with a blood alcohol level of .04 percent or greater after 8 hours cannot ﬂy until the blood alcohol falls below that amount. Even though blood alcohol may be well below .04 percent, a pilot cannot ﬂy sooner than 8 hours after drinking alcohol. Although the regulations are quite speciﬁc, it is a good idea to be more conservative than the regulations.
Pilot performance can be seriously degraded by both prescription and over-the-counter medications, as well as by the medical conditions for which they are taken. Many medications, such as tranquilizers, sedatives, strong pain relievers, and cough suppressants have primary effects that may impair judgment, memory, alertness, coordination, vision, and the ability to make calculations. [figure 16-9] Others, such as antihistamines, blood pressure drugs, muscle relaxants, and agents to control diarrhea and motion sickness have side effects that may impair the same critical functions. Any medication that depresses the nervous system, such as a sedative, tranquilizer, or antihistamine, can make a pilot more susceptible to hypoxia.
figure 16-9. Adverse affects of various drugs.
Painkillers are grouped into two broad categories: analgesics and anesthetics. Analgesics are drugs that reduce pain, while anesthetics are drugs that deaden pain or cause loss of consciousness.
Over-the-counter analgesics, such as acetylsalicylic acid (aspirin), acetaminophen (Tylenol), and ibuprofen (Advil) have few side effects when taken in the correct dosage. Although some people are allergic to certain analgesics or may suffer from stomach irritation, ﬂying usually is not restricted when taking these drugs. However, ﬂying is almost always precluded while using prescription analgesics, such as drugs containing propoxyphene (e.g., Darvon), oxycodone (e.g., Percodan), meperidine (e.g., Demerol), and codeine since these drugs are known to cause side effects such as mental confusion, dizziness, headaches, nausea, and vision problems.
Anesthetic drugs are commonly used for dental and surgical procedures. Most local anesthetics used for minor dental and outpatient procedures wear off within a relatively short period of time. The anesthetic itself may not limit ﬂying as much as the actual procedure and subsequent pain.
Stimulants are drugs that excite the central nervous system and produce an increase in alertness and activity. Amphetamines, caffeine, and nicotine are all forms of stimulants. Common uses of these drugs include appetite suppression, fatigue reduction, and mood elevation. Some of these drugs may cause a stimulant reaction, even though this reaction is not their primary function. In some cases, stimulants can produce anxiety and mood swings, both of which are dangerous when ﬂying.
Depressants are drugs that reduce the body’s functioning in many areas. These drugs lower blood pressure, reduce mental processing, and slow motor and reaction responses. There are several types of drugs that can cause a depressing effect on the body, including tranquilizers, motion sickness medication, some types of stomach medication, decongestants, and antihistamines. The most common depressant is alcohol.
Some drugs that are classiﬁed as neither stimulants nor depressants have adverse effects on ﬂying. For example, some antibiotics can produce dangerous side effects, such as balance disorders, hearing loss, nausea, and vomiting. While many antibiotics are safe for use while ﬂying, the infection requiring the antibiotic may prohibit ﬂying. In addition, unless speciﬁcally prescribed by a physician, do not take more than one drug at a time, and never mix drugs with alcohol, because the effects are often unpredictable.
The dangers of illegal drugs also are well documented. Certain illegal drugs can have hallucinatory effects that occur days or weeks after the drug is taken. Obviously, these drugs have no place in the aviation community.
14 CFR prohibits pilots from performing crewmember duties while using any medication that affects the body in any way contrary to safety. The safest rule is not to ﬂy as a crewmember while taking any medication, unless approved to do so by the FAA. If there is any doubt regarding the effects of any medication, consult an AME before ﬂying.
Altitude-Induced Decompression Sickness (DCS)
Decompression sickness (DCS) describes a condition characterized by a variety of symptoms resulting from exposure to low barometric pressures that cause inert gases (mainly nitrogen), normally dissolved in body ﬂuids and tissues, to come out of physical solution and form bubbles. Nitrogen is an inert gas normally stored throughout the human body (tissues and ﬂuids) in physical solution. When the body is exposed to decreased barometric pressures (as in ﬂying an unpressurized aircraft to altitude, or during a rapid decompression), the nitrogen dissolved in the body comes out of solution. If the nitrogen is forced to leave the solution too rapidly, bubbles form in different areas of the body, causing a variety of signs and symptoms. The most common symptom is joint pain, which is known as “the bends.” [figure 16-10]
figure 16-10. Signs and symptoms of altitude decompression sickness.
What to do when altitude-induced DCS occurs:
- Put on oxygen mask immediately and switch the regulator to 100 percent oxygen.
- Begin an emergency descent and land as soon as possible. Even if the symptoms disappear during descent, land and seek medical evaluation while continuing to breathe oxygen.
- If one of the symptoms is joint pain, keep the affected area still; do not try to work pain out by moving the joint around.
- Upon landing seek medical assistance from an FAA medical ofﬁcer, AME, military ﬂight surgeon, or a hyperbaric medicine specialist. Be aware that a physician not specialized in aviation or hypobaric medicine may not be familiar with this type of medical problem.
- Deﬁnitive medical treatment may involve the use of a hyperbaric chamber operated by specially trained personnel.
- Delayed signs and symptoms of altitude-induced DCS can occur after return to ground level regardless of presence during ﬂight.
DCS After Scuba Diving
Scuba diving subjects the body to increased pressure, which allows more nitrogen to dissolve in body tissues and ﬂuids. [figure 16-11] The reduction of atmospheric pressure that accompanies ﬂying can produce physical problems for scuba divers. A pilot or passenger who intends to ﬂy after scuba diving should allow the body sufﬁcient time to rid itself of excess nitrogen absorbed during diving. If not, DCS due to evolved gas can occur during exposure to low altitude and create a serious inﬂight emergency.
figure 16-11. To avoid the bends, scuba divers must not fly for specific time periods following dives.
The recommended waiting time before going to flight altitudes of up to 8,000 feet is at least 12 hours after diving that does not require controlled ascent (nondecompression stop diving), and at least 24 hours after diving that does require controlled ascent (decompression stop diving). The waiting time before going to ﬂight altitudes above 8,000 feet should be at least 24 hours after any scuba dive. These recommended altitudes are actual ﬂight altitudes above mean sea level (AMSL) and not pressurized cabin altitudes. This takes into consideration the risk of decompression of the aircraft during ﬂight.
Vision in Flight
Of all the senses, vision is the most important for safe ﬂight. Most of the things perceived while ﬂying are visual or heavily supplemented by vision. As remarkable and vital as it is, vision is subject to limitations, such as illusions and blind spots. The more a pilot understands about the eyes and how they function, the easier it is to use vision effectively and compensate for potential problems.
The eye functions much like a camera. Its structure includes an aperture, a lens, a mechanism for focusing, and a surface for registering images. Light enters through the cornea at the front of the eyeball, travels through the lens, and falls on the retina. The retina contains light sensitive cells that convert light energy into electrical impulses that travel through nerves to the brain. The brain interprets the electrical signals to form images. There are two kinds of light-sensitive cells in the eyes: rods and cones. [figure 16-12]
figure 16-12. The human eye.
The cones are responsible for all color vision, from appreciating a glorious sunset to discerning the subtle shades in a ﬁne painting. Cones are present throughout the retina, but are concentrated toward the center of the ﬁeld of vision at the back of the retina. There is a small pit called the fovea where almost all the light sensing cells are cones. This is the area where most “looking” occurs (the center of the visual ﬁeld where detail, color sensitivity, and resolution are highest).
While the cones and their associated nerves are well suited to detecting ﬁne detail and color in high light levels, the rods are better able to detect movement and provide vision in dim light. The rods are unable to discern color but are very sensitive at low light levels. The trouble with rods is that a large amount of light overwhelms them, and they take a long time to “reset” and adapt to the dark again. There are so many cones in the fovea that the very center of the visual ﬁeld hardly has virtually no rods at all. So in low light, the middle of the visual ﬁeld is not very sensitive, but farther from the fovea, the rods are more numerous and provide the major portion of night vision.
The area where the optic nerve enters the eyeball has no rods or cones, leaving a blind spot in the ﬁeld of vision. Normally, each eye compensates for the other’s blind spot. figure 16-13 provides a dramatic example of the eye’s blind spot. Cover the right eye and hold this page at arm’s length. Focus the left eye on the X on the right side of the windshield and notice what happens to the airplane while slowly bringing the page closer to the eye.
figure 16-13. The eye’s blind spot.
Empty-ﬁeld myopia is a condition that usually occurs when ﬂying above the clouds or in a haze layer that provides nothing speciﬁc to focus on outside the aircraft. This causes the eyes to relax and seek a comfortable focal distance which may range from 10 to 30 feet. For the pilot, this means looking without seeing, which is dangerous. Searching out and focusing on distant light sources, no matter how dim, helps prevent the onset of empty-ﬁeld myopia.
It is estimated that once fully adapted to darkness, the rods are 10,000 times more sensitive to light than the cones, making them the primary receptors for night vision. Since the cones are concentrated near the fovea, the rods are also responsible for much of the peripheral vision. The concentration of cones in the fovea can make a night blind spot in the center of the ﬁeld of vision. To see an object clearly at night, the pilot must expose the rods to the image. This can be done by looking 5° to 10° off center of the object to be seen. This can be tried in a dim light in a darkened room. When looking directly at the light, it dims or disappears altogether. When looking slightly off center, it becomes clearer and brighter.
Refer to figure 16-14. When looking directly at an object, the image is focused mainly on the fovea, where detail is best seen. At night, the ability to see an object in the center of the visual ﬁeld is reduced as the cones lose much of their sensitivity and the rods become more sensitive. Looking off center can help compensate for this night blind spot. Along with the loss of sharpness (acuity) and color at night, depth perception and judgment of size may be lost.
figure 16-14. Night blind spot.
While the cones adapt rapidly to changes in light intensities, the rods take much longer. Walking from bright sunlight into a dark movie theater is an example of this dark adaptation period experience. The rods can take approximately 30 minutes to fully adapt to darkness. A bright light, however, can completely destroy night adaptation, leaving night vision severely compromised while the adaptation process is repeated.
Hypoxia also affects vision. Sharp clear vision, (with the best being equal to 20-20 vision) requires signiﬁcant oxygen especially at night. As altitude increases, the available oxygen decreases, degrading night vision. Compounding the problem is fatigue, which minimizes physiological well being. Adding fatigue to high altitude exposure is a recipe for disaster. In fact, if ﬂying at night at an altitude of 12,000 feet, the pilot may actually see elements of his or her normal vision missing or not in focus. Missing visual elements resemble the missing pixels in a digital image while unfocused vision is dim and washed out.
For the pilot suffering the effects of hypoxic hypoxia, a simple descent to a lower altitude may not be sufﬁcient to reestablish vision. For example, a climb from 8,000 feet to 12,000 feet for 30 minutes does not mean a descent to 8,000 feet will rectify the problem. Visual acuity may not be regained for over an hour. Thus, it is important to remember, altitude and fatigue have a profound effect on a pilot’s ability to see.
Several things can be done to keep the eyes adapted to darkness. The ﬁrst is obvious: avoid bright lights before and during ﬂight. For 30 minutes before a night ﬂight, avoid any bright light sources, such as headlights, landing lights, strobe lights, or ﬂashlights. If a bright light is encountered, close one eye to keep it light sensitive. This allows the use of that eye to see again when the light is gone.
Red ﬂight deck lighting also helps preserve night vision, but red light severely distorts some colors and completely washes out the color red. This makes reading an aeronautical chart difﬁcult. A dim white light or a carefully directed ﬂashlight can enhance night reading ability. While ﬂying at night, keep the instrument panel and interior lights turned up no higher than necessary. This helps to see outside references more easily. If the eyes become blurry, blinking more frequently often helps.
Diet and general physical health have an impact on how well a pilot can see in the dark. Deﬁciencies in vitamins A and C have been shown to reduce night acuity. Other factors, such as CO poisoning, smoking, alcohol, certain drugs, and a lack of oxygen also can greatly decrease night vision.
Night Vision Illusions
There are many different types of visual illusions that commonly occur at night. Anticipating and staying aware of them is usually the best way to avoid them.
Autokinesis is caused by staring at a single point of light against a dark background for more than a few seconds. After a few moments, the light appears to move on its own. To prevent this illusion, focus the eyes on objects at varying distances and avoid ﬁxating on one target. Be sure to maintain a normal scan pattern.
A false horizon can occur when the natural horizon is obscured or not readily apparent. It can be generated by confusing bright stars and city lights. It can also occur while ﬂying toward the shore of an ocean or a large lake. Because of the relative darkness of the water, the lights along the shoreline can be mistaken for stars in the sky. [figure 16-15]
figure 16-15. At night, the horizon may be hard to discern due to dark terrain and misleading light patterns on the ground.
Night Landing Illusions
Landing illusions occur in many forms. Above featureless terrain at night, there is a natural tendency to ﬂy a lower-than-normal approach. Elements that cause any type of visual obscurities, such as rain, haze, or a dark runway environment can also cause low approaches. Bright lights, steep surrounding terrain, and a wide runway can produce the illusion of being too low, with a tendency to ﬂy a higher-than-normal approach. A set of regularly spaced lights along a road or highway can appear to be runway lights. Pilots have even mistaken the lights on moving trains as runway or approach lights. Bright runway or approach lighting systems can create the illusion that the airplane is closer to the runway, especially where few lights illuminate the surrounding terrain.
Pilots who are ﬂying at night should strongly consider oxygen supplementation at altitudes and times not required by the FAA, especially at night, when critical judgement and hand-eye coordination is necessary (e.g., IFR), or if a smoker or not perfectly healthy.
This chapter provides an introduction to aeromedical factors relating to ﬂight activities. More detailed information on the subjects discussed in this chapter is available in the Aeronautical Information Manual (AIM) and online at www.faa.gov/pilots/safety/pilotsafetybrochures.