Chapter 17: Aeromedical Factors
A number of health factors and physiological effects can be linked to flying. These include hypoxia, hyperventilation, middle ear and sinus problems, spatial disorientation, motion sickness, carbon monoxide (CO) poisoning, stress and fatigue, dehydration, and heatstroke. Pilots also should be aware of the effects of alcohol and drugs, anxiety, and excess nitrogen in the blood after scuba diving.
Hypoxia causes a reduction in oxygen available to the brain. Any reduction in mental function while flying can result in life-threatening errors.
Hypoxic hypoxia is a result of insufficient oxygen available to the body as a whole. The reduction in partial pressure of oxygen at high altitude will cause this condition. As an aircraft ascends during flight, the proportion of nitrogen to oxygen in the atmosphere remains the same. However, there are fewer molecules available for the respiratory system.
Transportation of dry ice can cause a specific risk, since dry ice sublimates into large quantities of CO2 gas, which rapidly displaces oxygen-containing air. Without proper ventilation, dry ice can rapidly pressurize. Because of these factors, dry ice is only transported by air under specific safety conditions.
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. In this case, it's not a lack of available oxygen, but instead a lack of blood. The most common form of hypemic hypoxia is CO poisoning. Other causes can be blood loss and anemia. Pilots should refrain from flying for a period of time after donating blood.
Stagnant hypoxia, also called "ischemia," results when the oxygen-rich blood in the lungs is not moving. In this case, oxygen and blood are available, but oxygen is not transported by blood to the brain. A common form of stagnant hypoxia is when a hand, arm, or leg goes "to sleep" due to constricted blood flow. Cold temperatures and shock also can cause stagnant hypoxia. In fight, stagnant hypoxia can result from maneuvers with high G-loads, as the excessive acceleration of gravity may prevent blood from flowing to the brain.
Histotoxic hypoxia refers to inability of the cells to effectively use oxygen. Generally, "toxic" refers to a poison. In this case, it also includes alcohol or drugs. Consumption of alcohol can cause mild to severe histotoxic hypoxia. Drinking one ounce of alcohol can equate to an additional 2,000 feet of physiological altitude.
The first symptoms of hypoxia can include euphoria and a carefree feeling. The effects of hypoxia can cause a pilot to have a false sense of security and be deceived into believing everything is normal. Subsequent symptoms can include:
Treatment for hypoxia includes flying at lower altitudes and/ or using supplemental oxygen. The term "time of useful consciousness" describes the maximum time a pilot has to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen. Above 10,000 feet, time of useful consciousness rapidly decreases.
Hyperventilation is the excessive rate and depth of respiration leading to abnormal loss of carbon dioxide from the blood. It seldom incapacitates completely, but, increased breathing rate and anxiety further aggravate the problem.
Pilots should correctly diagnose hypoxia versus hyperventilation, which may seem similar at first. Symptoms include:
Abnormal breathing may case hyperventilation. Breathing normally is the best cure for hyperventilation. In addition to slowing the breathing rate, breathing into a paper bag, talking aloud, or singing helps to overcome hyperventilation.
Middle Ear and Sinus Problems
During climbs and descents, gas 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 gas cannot escape, it can result in ear pain, sinus pain, and temporary loss of hearing.
The middle ear is a small cavity that is closed off from the external ear canal by the eardrum. Pressure differences are equalized by Eustachian tubes, which lead from inside each ear to the back of the throat. The tube open when chewing, yawning, or swallowing.
During a climb, lower external pressure may cause the eardrum to bulge outward. During a descent, higher external pressure can cause the eardrum to bulge inward.
To remedy this often painful condition and restore hearing, pinch the nostrils shut, close the mouth and lips, and blow slowly and gently into the mouth and nose. This will force air through the Eustachian tube into the middle ear.
Flight with a cold, an ear infection, or sore throat can be extremely painful, as well as damaging to the eardrums.
Internal sinus pressure equalizes with external pressure through small openings that connect the sinuses to the nasal passages. Congestion can create a painful sinus block, which occurs most frequently during descent. Pilots with an upper respiratory infection or nasal allergic condition should avoid flying.
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 that work together to ascertain orientation and movement in space:
Most of the time, the three streams give a clear idea of where and how the body is moving. However, conflicting information can lead to disorientation.
The visual system is the primary source of information in visual meteorological conditions (VMC), and visual information usually prevails over false sensations. Without the benefit of external vision, false sensations can cause a pilot to quickly become disoriented.
The vestibular system (in the inner ear) features three semicircular canals are positioned at approximate right angles to each other. Each canal is filled with fluid and has a section full of fine hairs. Deflection of these hairs stimulates nerve impulses, sending messages to the brain, which can be used 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. "Seat of the pants" flying is largely dependent upon these signals, but the body cannot distinguish between acceleration forces due to gravity and those resulting from maneuvering the aircraft. This can lead to sensory illusions.
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. Learning to fly with reference only to flight instruments requires training and awareness.
The leans is caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot. After leveling the wings, the pilot may experience an illusion that the aircraft is banking in the opposite direction. This is the most common illusion during flight. The initial turn often goes unnoticed because a rotational acceleration of two (2) degrees per second or lower is below the detection threshold of the semicircular canals.
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. Any movement of the head may then disturb the fluid, creating a turning or accelerating illusion.
The graveyard spiral is the the illusion of not turning when the airplane is in a prolonged, coordinated, constant-rate turn. During recovery to level flight, the pilot will experience the sensation of turning in the opposite direction. This illusion may cause the pilot to re-enter the graveyard spiral, eventually with catastrophic results if flying in non-visual conditions. In addition, if pilot instinctively pulls back on the controls in an attempt to climb or stop the descent, this may tighten the spiral and increases the loss of altitude. A loss-of-control accident is possible.
Somatogravic illusion gives the pilot the sensation of being in a nose-up attitude when the aircraft is level. It can be caused by rapid acceleration (such as during takeoff), which stimulates the otolith organs in the same way as tilting the head backwards. The pilot's response may place the aircraft in nose-low or dive attitude. Notably, rapid deceleration can have the opposite effect, causing the pilot to pitch nose-up or stall the aircraft.
Inversion illusion is the sensation of tumbling backwards, which can be caused by an abrupt change from climb to straight-and-level flight.
Elevator illusion conveys a climb or descent sensation, which can be caused by an upward/downward vertical acceleration, such as in an updraft or downdraft. The disoriented pilot may push the aircraft into a nose-low or nose-high attitude.
Visual illusions are especially hazardous, since pilots rely on vision for critical information.
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 a false horizon when attempting to align the aircraft with the actual horizon. The disoriented pilots as a result may place the aircraft in a dangerous attitude.
Autokinesis occurs when flying in the dark. A stationary light may appear to move if it is stared at for a prolonged period of time, which may cause the pilot to attempt to align the aircraft with the perceived moving light, potentially causing him/her to lose control of the aircraft.
Demonstration of Spatial Disorientation
False sensations in maneuvers are an effective demonstration of disorientation. In teaching this, pilots can understand the susceptibility of the human system to spatial disorientation, and that bodily sensations are frequently false. Students also will have fewer instances of disorientation, and they will transition to relying on flight instruments for assessing true aircraft attitude. All maneuvers are done with the pilot's eyes closed:
Pilots should understand the causes of vestibular and visual illusions and remain constantly alert for them. Training and proficiency are essential when flying in marginal visibility or where a visible horizon is not evident (such as flight over open water at night). Adverse weather conditions and dusk/darkness also require instrument proficiency. Sudden head movements should be avoided, and the IMSAFE checklist should be conducted before each flight.
Vision is the most important of the five senses for safe flight. Terrain features and atmospheric conditions can create optical illusions, most often when landing.
Pilots should anticipate the possibility of visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions. The altimeter and VASI should be used on approach, as well as the Visual Descent Point (VDP) found on many non-precision instrument approach charts.
Additional Risk Factors
Motion sickness is caused by the brain receiving conflicting messages about the state of the body. Opening fresh air vents, focusing on objects outside the airplane, and avoiding unnecessary head movements may help alleviate some of the discomfort. Medications are not recommended, as they may cause drowsiness.
Carbon Monoxide (CO) is a colorless and odorless gas produced by all internal combustion engines. When pilots are exposed, CO prevents the hemoglobin from carrying oxygen to the cells, resulting in hypemic hypoxia. Symptoms of CO poisoning include headache, blurred vision, dizziness, drowsiness, and/or loss of muscle power. Prolonged exposure can be fatal.
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. An exhaust gas odor should suggest that CO is present. It also can present a risk in smaller amounts. Disposable, inexpensive CO detectors are widely available.
Stress causes an increase in adrenaline, metabolism, blood sugar, heart rate, respiration, blood pressure, and perspiration. Acute stress involves an immediate threat that is perceived as danger. Chronic stress, due to psychological pressures, causes individual performance to fall sharply.
Fatigue causes lowered attention and concentration, impaired coordination, and decreased ability to communicate. Acute fatigue is short term and normal, requiring only rest for recovery.
Skill fatigue is a type of acute fatigue that results in workflow timing disruptions, as well as disruption of the perceptual field.
Chronic fatigue is caused by continuously high levels of stress. It usually requires treatment by a physician.
Pilots should avoid exposure to chemicals commonly found in aircraft fluids. Hydraulic fluid, engine oil, and fuel have specific risks associated with eye and/or skin contact, as well as inhalation and ingestion.
Dehydration is a critical loss of water from the body, commonly caused by hot airplanes, wind, humidity, and diuretic drinks (coffee, tea, alcohol, and caffeinated soft drinks). The first noticeable effect of dehydration is fatigue. Additional signs include headache, fatigue, cramps, sleepiness, and dizziness. High temperatures and altitudes increase the risk of dehydration. Pilots should drink two to four quarts of water every 24 hours.
Heatstroke is caused by the inability of the body to control its temperature. Water should be carried and used at frequent intervals on any long flight, whether the pilot is thirsty or not.
The influence of alcohol drastically reduces the chances of completing a flight without incident. Impairments in vision and hearing can occur from consuming one drink. When combined with altitude, the alcohol from two drinks may have the same effect as three or four drinks. Title 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 aircraft.
Title 14 CFR part 61, section 61.53 prohibits acting as pilot-in-command or in any other capacity as a required pilot flight crew-member, while that person is aware of a medical condition that would invalidate their medical certificate, or is taking medication or receiving other treatment that would invalidate their medical certificate.
Title 14 CFR part 91, section 91.17 prohibits the use of any drug that affects a pilot in any way contrary to safety. The safest rule is not to fly as a crew-member while taking any medication, unless approved to do so by the FAA.
Altitude-Induced Decompression Sickness (DCS)
Nitrogen is an inert gas normally stored throughout the human body (tissues and fluids) in physical solution. Exposure to decreased barometric pressures — such as when flying in an unpressurized aircraft — can cause nitrogen and other inert gasses to come out of physical solution and form bubbles. Decompression Sickness (DCS) is characterized by a variety of symptoms, most commonly joint pain, also known as as "the bends."
Altitude-induced DCS requires immediate attention, starting with supplemental oxygen and emergency descent to the first suitable airport. A physician specialized in aviation or hyperbaric medicine may be necessary, and a a hyperbaric chamber may be used for recovery.
Note that symptoms of DCS can occur after return to ground level, and may not be present during flight.
Scuba diving subjects the body to increased pressure, which allows more nitrogen to dissolve in body tissues and fluids. 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.
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. Divers should use these figures as actual flight altitudes and not cabin-pressure altitudes, since unexpected decompression could lead to a medical emergency.
Vision in Flight
The human eye includes an aperture, a lens, a mechanism for focusing, and a surface for registering images. The retina contains light sensitive cells that convert light energy into electrical impulses that travel through nerves to the brain.
The two kinds of light-sensitive cells in the eyes are rods and cones.
Cones are are concentrated toward the center of the field of vision at the back of the retina, and they are responsible for all color vision.
Rods detect movement and provide vision in dim light. They are unable to discern color. Large amounts of light overwhelm rods, and they required time to "reset" and adapt to the dark again.
Photopic vision provides the capability for seeing color and resolving fine detail. It is experienced during daylight or under brightly illuminated lights.
Mesopic vision is achieved by a combination of rods and cones and is experienced at dawn, dusk, and during full moonlight. Visual acuity decreases and color perception changes.
Scotopic vision is experienced under low-light levels and the cones become ineffective, resulting in poor resolution of detail. Visual acuity decreases dramatically, color perception is lost, and a night blind spot in the central field of view appears.
The area where the optic nerve connects to the retina in the back of each eye is known as the optic disk. There is a total absence of cones and rods in this area, and thus a central blind spot. This typically is not a problem, since an object cannot be in the blind spot of both eyes at the same time. However, if the field of vision of one eye is obstructed, an object can fall in the blind spot.
While looking at the above image:
You can try this with your right eye covered by staring at the black X with your left eye. As you move closer, the airplane will disappear.
Empty-field myopia occurs when flying above the clouds or in a haze layer. Without external points of focus, the eyes seek a comfortable focal distance (10-30 feet). This can mean "looking without seeing." Focusing on distant light sources helps prevent the onset of empty-field myopia.
Pilots trained in night-flying techniques can become comfortable and proficient. Dark-adapted rods are the primary receptors for night vision, while also responsible for most peripheral vision. Because of the central location of cones in the eye, a night blind spot in the center of the field of vision can develop. To see an object clearly at night, pilots should looking 5° to 10° off center of the object.
Dark adaptation is the adjustment of the human eye to a dark environment, which requires time. Dark adaptation will take longer if moving from a bright area to a dark area, compared to moving from a dim area to a dark area. Cones adapt rapidly in changes to light, whereas rods can take approximately 30 minutes to fully adapt to darkness. Bright light undermines night adaptation, requiring the adaptation process to be repeated.
Scanning techniques enable the identification of objects at night. Pilots should look right-left or left-right, scanning at the greatest distance an object can be perceived (top) and move inward toward the position of the aircraft (bottom). For each stop, an area approximately 30° wide should be scanned. The duration of each stop is based on the degree of detail that is required, but no stop should last longer than 2 to 3 seconds. When moving from one viewing point to the next, pilots should overlap the previous field of view by 10°.
Off-center viewing requires an object be viewed by looking 10° above, below, or to either side of the object, so that the peripheral vision can maintain contact with an object. With off-center viewing, the images of an object viewed longer than two to three seconds will disappear. Avoid viewing an object for longer than two or three seconds. The peripheral field of vision will continue to pick up the object when the eyes are shifted from one off-center point to another.
Night Vision Protection
If a night flight is scheduled, pilots and crew members should wear sunglasses when exposed to bright sunlight. This increases the rate of dark adaptation at night and improves night visual sensitivity.
Lack of oxygen to the rods (hypoxia) significantly reduces their sensitivity. Clear vision requires significant oxygen, especially at night. Without supplemental oxygen, the pilot's night vision declines at pressure altitudes above 4,000 feet. For the pilot suffering the effects of hypoxic hypoxia, a simple descent to a lower altitude may not be sufficient to reestablish vision, and visual acuity may not be regained for over an hour.
If a high-intensity lighting area is encountered during flight, turn the aircraft away and fly in the periphery of the lighted area. When possible, plan a night route to avoid direct flight over built-up, brightly lit areas.
Flight deck lighting should be kept as low as possible. During an adjustment period after departure, night vision should continue to improve until optimum night adaptation is achieved. When it is necessary to read maps, charts, and checklists, use a dim white (or red) flashlight and avoid shining it in your or any other crew-member's eyes.
When an airport has pilot-controlled lighting, the airfield lighting should be reduced to the lowest usable intensity. Position the aircraft at a part of the airfield where the least amount of lighting exists. Select approach and departure routes that avoid bright illumination.
Night Flying and Stress
Drugs, exhaustion, poor physical conditioning, alcohol, and tobacco are all risk factors during any phase of flight, especially at night. Missing or postponing meals can cause low blood sugar, which impairs night flight performance. A lack of vitamin A impairs night vision (although high quantities of vitamin A do not increase night vision).
Distance Estimation and Depth Perception
Motion parallax refers to the apparent motion of stationary objects. When looking perpendicular from the flight path, near objects can appear to move backward, past, or opposite the path of motion, while far objects can seem to move in the direction of motion or remain fixed. The rate of apparent movement depends on the distance the observer is from the object.
An object may appear to have a different geometric perspective when viewed at varying distances and from different angles. In the linear perspective, parallel lines, such as runway lights, power lines and railroad tracks, tend to converge as distance from the observer increases. In apparent foreshortening, the true shape of an object or a terrain feature appears elliptical when viewed from a distance. With vertical position in the field, objects or terrain features farther away from the observer appear higher on the horizon than those closer to the observer.
As distance increases, the clarity of an object, its detail, its texture, and the shadow cast by are visual cues for its distance. However, at night these distinctions in the aerial perspective may become blurry. Additionally, shadows depend on the position of light sources. If a shadow of an object is cast toward the observer, the object is closer than the light source is to the observer.
Binocular cues depend upon the slightly different viewing angle of each eye toward an object. In the flight environment, most distances are so great that binocular cues are of little or no value. In addition, binocular cues operate on a more subconscious level than monocular cues and are performed automatically.
Night Vision Illusions
Autokinesis is caused by staring at a single point of light against a dark background for more than a few seconds. The light will appear to move on its own in about 8 to 10 seconds. This illusion can be eliminated or reduced by visual scanning, by increasing the number of lights, or by varying the light intensity.
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.
A reversible perspective illusion may make an aircraft appear to be moving away from a second aircraft when it is, in fact, approaching a second aircraft. This illusion often occurs when an aircraft is flying parallel to another's course. To determine the direction of flight, pilots should observe aircraft lights and their relative position to the horizon. If the intensity of the lights increases, the aircraft is approaching. If the lights dim, the aircraft is moving away.
A size-distance illusion results from viewing a source of light that is increasing or decreasing in brightness (luminance). Pilots may interpret the light as approaching or retreating.
Fascination, also called "fixation," occurs when pilots ignore orientation cues and fix their attention on a goal or an object, such as when they are concentrating on aircraft instruments, or when attempting to land. This can be especially dangerous at night because aircraft ground-closure rates are difficult to determine.
Flicker vertigo is caused by a light flickering at a rate between 4 and 20 cycles per second, which can produce nausea, vomiting, and vertigo. Convulsions and unconsciousness are possible. Proper scanning techniques at night can prevent flicker vertigo.
Night Landing Illusions
Above featureless terrain at night, there is a natural tendency to fly a lower`than-normal approach. Rain, haze, or a dark runway environment can also cause low approaches. Regularly spaced lights along a road or highway can appear to be runway lights
Have a plan for night flight before departure. Study the area, and know how to navigate your way through areas that may pose a problem at night. Always consider safer alternatives rather than hope things will work out by taking a chance.
Enhanced Night Vision Systems
A Synthetic Vision System (SVS) is an electronic means to display a synthetic vision image of the external scene topography, which enhances pilot awareness of spatial position relative to important features in all visibility conditions.
Enhanced Vision (EV) or Enhanced Flight Vision System (EFVS) is an electronic means to provide a display of the external scene by use of an imaging sensor, such as a Forward-Looking InfraRed (FLIR) or millimeter wave radar (MMWR).
Commercial Pilot & Flight Instructor Test Questions
Which statement is true regarding alcohol in the human system? Alcohol renders a pilot more susceptible to hypoxia.
If an individual has gone scuba diving which has not required a controlled ascent and will be flying to cabin pressure altitudes of 8,000 feet or less, the recommended waiting time is at least 12 hours.
If an individual has gone scuba diving which has required a controlled ascent and will be flying to cabin pressure altitudes of 8,000 feet or less, the recommended waiting time is at least 24 hours.
During a climb to 18,000 feet, the percentage of oxygen in the atmosphere remains the same.
Which statement regarding hypoxia is true? Belligerence or a false sense of security may be symptoms of hypoxia.
What physical change would most likely occur to occupants of an unpressurized aircraft flying above 15,000 feet without supplemental oxygen? A blue coloration of the lips and fingernails develop along with tunnel vision.
— Blue discoloration of the lips and fingernails is cynanosis.
Hypoxia is the result of decreasing amounts of oxygen as your altitude increases.
How can smoking affect a pilot? Reduces the oxygen-carrying capability of the blood.
— Smoking also reduces night vision by approximately 20%.
Anemic hypoxia has the same result as hypoxic hypoxia, but it is most of the result of a leaking exhaust manifold.
— Anemic hypoxia is also known as hypemic hypoxia, and is caused by contaminated blood. CO poisoning would cause anemic/hypemic hypoxia.
Hyperventilation results in breathing too rapidly, causing a lack of oxygen
— Hyperventilation blows off carbon dioxide (CO2), which the body needs to function.
Rapid or deep breathing while using oxygen can cause hyperventilation
— Hyperventilation is caused by a deficiency of carbon dioxide, which can be brought about by an excess of oxygen.
A person should be able to overcome the symptoms of hyperventilation by slowing the breathing rate and increasing the amount of carbon dioxide in the body.
The advantage of experiencing hypoxia in an altitude chamber is it helps pilts learn to recognize their own symptoms in a controlled environment.
Which procedure is recommended to prevent or overcome spatial disorientation? Rely entirely on the indications of the flight instruments.
— Reducing head and eye movements will help prevent spatial orientation, but not help overcome it.
A rapid acceleration can create the illusion of being in a nose-up attitude.
The illusion that the aircraft is at a higher altitude than it actually is, is produced by upsloping terrain.
What effect does haze have on the ability to see traffic or terrain features during flight? Air traffic or terrain features appear to be farther away than their actual distance.
— Haze causes the eyes to relax, stare without seeing, and focus at a comfortable distance outside the cockpit.
Dark adaptation is impaired by exposure to cabin pressure altitudes above 5,000 feet.
— Dark adaptation is also impaired by exposure to carbon monoxide (CO), vitamin A deficiency, and prolonged exposure to sunlight.
Although not required, supplemental oxygen is recommended for use when flying at night above 5,000 feet.
What suggestion could you make to students who are experiencing motion sickness? Tell the students to avoid unnecessary head movement and to keep their eyes on a point outside the aircraft.
Motion sickness is caused by continued stimulation of the tiny portion of the inner ear which controls sense of balance..
Which technique should a student be taught to scan for traffic to the right and left during straight-and-level flight? Systematically focus on different segments of the sky for short intervals.
One aid for increasing night vision effectiveness would be to force the eyes to view off center.
What is an effective way to prevent a collision in the traffic pattern? Maintain the proper traffic pattern altitude and continually scan the area.
The most effective technique to use for detecting other aircraft at night is to avoid staring directly at the point where another aircraft is suspected to be flying.
— Off-center viewing is most effective for night vision.
Most midair collisions occur during clear days.