16-1

Introduction

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.

Aeromedical 

Factors

Chapter 16
16-2

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 firstclass 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

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

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

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 
16-3

Figure 16-1. Time of useful consciousness.

Altitude Time of Useful Consciousness

45,000 feet MSL

40,000 feet MSL

35,000 feet MSL

30,000 feet MSL

28,000 feet MSL

25,000 feet MSL

22,000 feet MSL

20,000 feet MSL

9 to 15 seconds

15 to 20 seconds

30 to 60 seconds

1 to 2 minutes

21⁄2 to 3 minutes

3 to 5 minutes

5 to 10 minutes

30 minutes or more

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 Hypoxia

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.

Histotoxic Hypoxia

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)

• Headache

• Decreased reaction time

• Impaired judgment

• Euphoria

• Visual impairment

• Drowsiness

• Lightheaded or dizzy sensation

• Tingling in fingers and toes

• Numbness

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] 

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

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 
16-4

Figure 16-2. The Eustachian tube allows air pressure to equalize 

in the middle ear.

Opening to Throat

Middle Ear

Outer Ear

Auditory Canal

Eustachian Tube

Eardrum

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

• Unconsciousness

• 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] 

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 
16-5

Figure 16-3. The semicircular canals lie in three planes and sense motions of roll, pitch, and yaw.

YAW

ROLL

YAW

PITCH

PITCH

ROLL

The semicircular tubes are arranged 

at approximately right angles to each

other in the roll, pitch, and yaw axes. Vestibular Nerve

Endolymph Fluid

Ampulla of Semicircular Canal

Hair Cells

Cupola

Semicircular Canals

Otolith Organ

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.

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.
16-6

Figure 16-4. Human sensation of angular acceleration.

Figure 16-5. Graveyard spiral.

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.

Vestibular Illusions

The Leans

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. 

Coriolis Illusion

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.

Graveyard Spiral

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.
16-7

Figure 16-6. Sensations from centrifugal force.

Skid, slip, and uncoordinated turns feel similar.

Pilots feel they are being forced sideways in their seat.

Level Coordinated turn Pull out

Level skid Forward slip Uncoordinated turn

Somatogravic Illusion

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.

Inversion Illusion

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.

Elevator 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

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. 

False Horizon

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. 

Autokinesis

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.

Postural Considerations

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.
16-8

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.

1. They teach pilots to understand the susceptibility of 

the human system to spatial disorientation.

2. They demonstrate that judgments of aircraft attitude 

based on bodily sensations are frequently false.

3. They help decrease the occurrence and degree of 

disorientation through a better understanding of the 

relationship between aircraft motion, head movements, 

and resulting disorientation.

4. 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-andlevel 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 straightand-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:

1. 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.
16-9

will fly a lower approach, with the risk of striking objects 

along the approach path or landing short. A wider-thanusual runway can have the opposite effect, with the risk of 

the pilot leveling out the aircraft high and landing hard, or 

overshooting the runway.

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.

Water Refraction

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.

Haze

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.

Fog

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.

2. Always obtain and understand preflight weather 

briefings.

3. 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. 

4. 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 crosscountry and local operations at various airfields.

5. Ensure that when outside visual references are used, 

they are reliable, fixed points on the Earth’s surface.

6. Avoid sudden head movement, particularly during 

takeoffs, turns, and approaches to landing.

7. 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.

8. 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.

Optical Illusions

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 
16-10

Figure 16-7. Runway illusions.

25

25

No

Normal Approach

rmal Approach

25 25

25 Normal Approach

25 Normal Approach

Downsloping runway Upsloping runway

Downsloping runway Upsloping runway

Narrower runway Wider runway

Narrower runway Wider runway

Normal approach

Approach due to illusion

Runway width illusion

• A narrower-than-usual runway can 

create an illusion that the aircraft 

is higher than it actually is, leading 

to a lower approach.

• A wider-than-usual runway can 

create an illusion that the aircraft is 

lower than it actually is, leading to

a higher approach.

Runway slope illusion

• A downsloping runway can create

the illusion that the aircraft is lower 

than it actually is, leading to a 

higher approach.

• An upsloping runway can create

the illusion that the aircraft is higher

than it actually is, leading to a lower 

approach.
16-11

How To Prevent Landing Errors Due to Optical 

Illusions

To prevent these illusions and their potentially hazardous 

consequences, pilots can:

1. 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.

2. Make frequent reference to the altimeter, especially 

during all approaches, day and night.

3. If possible, conduct aerial visual inspection of 

unfamiliar airports before landing.

4. 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.

5. Utilize the visual descent point (VDP) found on many 

nonprecision instrument approach procedure charts.

6. Recognize that the chances of being involved in an 

approach accident increase when some emergency or 

other activity distracts from usual procedures.

7. 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

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

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 
16-12

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

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.
16-13

Type Beverage Typical Serving Pure Alcohol

(oz) Content (oz)

Table Wine

Light Beer

Aperitif Liquor

Champagne

Vodka

Whiskey

4.0

12.0

1.5

4.0

1.0

1.25

.48

.48

.38

.48

.50

.50

0.01–0.05 average individual appears normal 

(10–50 mg%) 

0.03–0.12* mild euphoria, talkativeness, decreased

(30–120 mg%) inhibitions, decreased attention, impaired 

 judgment, increased reaction time

0.09–0.25 emotional instability, loss of critical

(90–250 mg%) judgment, impairment of memory and 

 comprehension, decreased sensory

 response, mild muscular incoordination

0.18–0.30 confusion, dizziness, exaggerated

(180–300 mg%) emotions (anger, fear, grief) impaired

 visual perception, decreased pain

 sensation, impaired balance, staggering 

 gait, slurred speech, moderate muscular

 incoordination

0.27–0.40 apathy, impaired consciousness, stupor,

(270–400 mg%) significantly decreased response to

 stimulation, severe muscular 

 incoordination, inability to stand or walk, 

 vomiting, incontinence of urine and feces

0.35–0.50 unconsciousness, depressed or 

(350–500 mg%) abolished reflexes, abnormal body

 temperature, coma; possible death from 

 respiratory paralysis (450 mg% or above)

* Legal limit for motor vehicle operation in most states is 0.08

 or 0.10% (80–100 mg of alcohol per dL of blood).

Figure 16-8. Impairement scale with alcohol use.

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

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 
16-14

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.

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.

Drugs

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.

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.
16-15

Figure 16-9. Adverse affects of various drugs. Alcohol Sedative Hypnotics Opiates Stimulants Psychedelics THC

Psychoactive

Drugs

Development

of Tolerance

Prolonged Use of 

Large Amounts

Withdrawal Symptoms

After Prolonged Use

Range of Effects

From To

Beer

Wine

Hard Liquor

Barbiturates:

- Nembutal

- Phenobarbital

- Seconal

Tranquilizers:

- Valium

- Librium

- Quaaludes

Opium

Morphine

Heroin

Codeine

Dilaudid

Percodan

Darvon

Methadone

Dexedrine

Methamphetamine

Diet Pills

Ritalin

Cocaine

Caffeine

LSD

Mescaline

Psilocybin

PCP

Marijuana

Hashish

Relaxation, lowered 

inhibitions, reduced 

intensity of physical 

sensations, digestive 

upsets, body heat loss, 

reduced muscular 

coordination.

Relaxation, lowered 

inhibitions, reduced 

intensity of physical 

sensations, digestive 

upsets, body heat loss, 

reduced muscular 

coordination

Suppression of pain, 

lowered blood pressure 

and respiratory rate, 

constipation, disruption

of menstrual cycle, 

hallucinations, sleep

Increased blood 

pressure and pulse 

rate, appetite loss, 

increased alertness, 

dilated and dried out 

bronchi, restlessness, 

insomnia

Distorted perceptions, 

hallucinations, 

confusion, vomiting

Sedation, euphoria, 

increased appetite, 

altered mental processes

Loss of body control, 

passing out (also 

causing physical 

injuries), susceptibility 

to pneumonia, 

cessation of breathing

Passing out, loss of

body control, stupor,

severe depression of

respiration, possible

death (Effects are 

exaggerated when

used in combination

with alcohol— 

synergistic effect.)

Clammy skin, 

convulsions, 

coma, respiratory 

depression, possible 

death

Paranoid reaction, 

temporary psychosis, 

irritability, convulsions, 

palpitations (not 

generally true for 

caffeine)

Psychosis, 

hallucinations, 

vomiting, anxiety, 

panic, stupor.

With PCP: Aggressive 

behavior, catatonia, 

convulsions, coma, 

high blood pressure

Distorted perception, 

anxiety, panic

Moderate

Moderate

High

High

High

Moderate

Liver damage, ulcers, 

chronic diarrhea, 

amnesia, vomiting, 

brain damage, internal 

bleeding, debilitation

Amnesia, confusion,

drowsiness, personality 

changes

Depressed sexual drive,

lethargy, general

physical debilitation, 

infections, hepatitis

Psychosis, insomnia, 

paranoia, nervous

system damage (not

generally true for 

caffeine)

Psychosis, continued 

hallucinations, mental 

disruption

Amotivation (loss of

drive)

Convulsions, shakes, 

hallucinations, loss of memory, 

uncontrolled muscular spasms, 

psychosis

Watery eyes, runny nose, 

severe back pains, stomach 

cramps, sleeplessness, 

nausea, diarrhea, sweating, 

muscle spasms

Severe depression, both 

physical and mental (not true

for caffeine)

Occasional flashback 

phenomena, depression

No true withdrawal symptoms

except possible depression 

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.
16-16

Figure 16-10. Signs and symptoms of altitude decompression sickness.

DCS Type Bubble Location Signs & Symptoms (Clinical Manifestations)

BENDS

NEUROLOGIC

Manifestations

CHOKES

SKIN BENDS

Mostly large joints

of the body (elbows,

shoulders, hip, wrists,

knees, ankles)

Brain

Spinal Cord

Peripheral Nerves

Lungs

Skin

• Localized deep pain, ranging from mild (a “niggle”) to excruciating–sometimes a dull 

ache, but rarely a sharp pain

• Active and passive motion of the joint aggravating the pain

• Pain occurring at altitude, during the descent, or many hours later

• Confusion or memory loss

• Headache

• Spots in visual field (scotoma), tunnel vision, double vision (diplopia), or blurry vision

• Unexplained extreme fatigue or behavior changes

• Seizures, dizziness, vertigo, nausea, vomiting, and unconsciousness

• Abnormal sensations such as burning, stinging, and tingling around the lower chest

and back

• Symptoms spreading from the feet up and possibly accompanied by ascending

 weakness or paralysis

• Girdling abdominal or chest pain

• Urinary and rectal incontinence

• Abnormal sensations, such as numbness, burning, stinging and tingling (paresthesia)

• Muscle weakness or twitching

• Burning deep chest pain (under the sternum)

• Pain aggravated by breathing

• Shortness of breath (dyspnea)

• Dry constant cough

• Itching usually around the ears, face, neck, arms, and upper torso

• Sensation of tiny insects crawling over the skin

• Mottled or marbled skin usually around the shoulders, upper chest, and abdomen,

accompanied by itching

• Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema)

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]

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 
16-17

Figure 16-11. To avoid the bends, scuba divers must not fly for 

specific time periods following dives.

Figure 16-12. The human eye.

The rods and

cones (film) of 

the retina are 

the receptors 

which record

the image and

transmit it 

through the 

optic nerve to 

the brain for 

interpretation.

Rods and

Cones

Fovea

(All Cones)

Lens

Retina

Optic Nerve

Iris

The pupil (aperture) is the opening at

the center of the iris. The size of the 

pupil is adjusted to control the amount

of light entering the eye.

PUPIL

Light passes through the cornea (the 

transparent window on the front of the 

eye) and then through the lens to 

focus on the retina.

CORNEA

Rod Concentration

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.

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]

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 
16-18

Figure 16-13. The eye’s blind spot.

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.

Empty-Field Myopia

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.

Night Vision

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.

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.
16-19

Figure 16-14. Night blind spot.

Cones active

Rods active

Night blind spot

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

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.

False Horizon

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]

Night Landing Illusions

Landing illusions occur in many forms. Above featureless 

terrain at night, there is a natural tendency to fly a lowerthan-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-thannormal 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.
16-20

Figure 16-15. At night, the horizon may be hard to discern due to dark terrain and misleading light patterns on the ground.

Apparent horizon Actual horizon

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 handeye coordination is necessary (e.g., IFR), or if a smoker or 

not perfectly healthy. 

Chapter Summary

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.