effects of temperature extremes on
The human body is adapted to a narrow
temperature range; it cannot function normally in hot and cold
temperature extremes. Exposure to such extremes in the aviation
environment impairs the efficiency of aircrews and adds to other
stresses such as hypoxia and fatigue. Extreme climates can cause
uncomfortable or unbearable cockpit conditions. Likewise, atmospheric
temperature or altitude changes, aircraft interior ventilation and
heating, and protective equipment can also create temperature extremes.
At times, aircrew members may have
thought that the temperature inside their aircraft resembled that of a
flying oven. Private aviation usually takes place at the relatively
low altitudes that are associated with extremely high temperatures and
humidity. Heat can seriously hamper mission requirements to accomplish
During high speed flight, the aircraft structure is heated by
friction between its surface and the air and by the rise in
temperature caused by air compression in the front of the aircraft.
Insulation in the cockpit and cabin air ductwork can reduce the
effects of kinetic heating.
Solar radiant heat is the primary heat-stress problem in
aircraft; the large expanses of glass or Plexiglas™ produce the
greenhouse effect. This effect is caused by the differing transmission
characteristics for radiation of differing wavelengths; thermal energy
can become trapped within the cockpit. The temperatures in cockpits of
aircraft parked on airfield ramps may be 50 to 60 degrees Fahrenheit
higher than those in hangars because of the radiation of solar heating
through transparent surfaces. This radiation, in turn, heats the
interior objects of the cockpit. These heated objects then reradiate
the waves at frequencies that cannot penetrate the glass or Plexiglas™
outward. Therefore, heat accumulates within the cockpit and becomes a
significant stress factor at altitudes below 10,000 feet.
electrical heat loads and
With the development of new high-performance aircraft, the
electrical heat load in the cockpit increases as more and improved
avionics equipment is fitted into these aircraft. The greater the
temperature in the cockpit, the greater the possibility of degraded
Comfortable limits in the cockpit are 68 to 72 degrees
Fahrenheit and 25 to 50 percent relative humidity. To maintain these
temperatures and this humidity range, aircraft must have extra heating
and cooling equipment. This equipment is expensive in both performance
and cost. (A rule of thumb is that one pound of extra load requires
nine pounds of structure and fuel to fly it.)
The body maintains its heat balance with several mechanisms.
These are radiation, conduction, convection, and evaporation.
Radiation involves the transfer of heat from an object of
intense heat to an object of lower temperature through space by
radiant energy. The rate of heat transfer depends mainly on the
difference in temperature between the objects. If the temperature of
the body is higher than the temperature of the surrounding objects, a
greater quantity of heat is radiated away from the body than is
radiated to the body.
Conduction is the transfer of heat between objects, in
contact at different temperatures, from heated molecules (body) to
cooler molecules of adjacent objects. The proximity of these objects
will determine the overall rate of conduction.
Convection is the transfer of heat from the body in liquids
or gases in which molecules are free to move. During body-heat loss,
the movement of air molecules is produced when the body heats the
surrounding air; the heated air expands and rises because it is
displaced by denser, cooler air. Respiration, which contributes to the
regulation of body temperature, is a type of convection.
Evaporative heat loss involves the changing of a substance
from its liquid state (sweat) to its gaseous state. When water on the
surface of the body evaporates, heat is lost. Evaporation is the most
common and usually the most easily explained form of heat loss.
Radiation, convection, and conduction all suffer one major
disadvantage in cooling the body; they become less effective as
temperature increases. When the temperature of the air and nearby
objects exceeds skin temperature, the body actually gains heat. This
gain may be dangerous to the aviator.
When the temperature increases to about 82 to 84 degrees
Fahrenheit, sweat production increases abruptly to offset the loss of
body cooling through radiation, convection, and conduction. By the
time the temperature reaches 95 degrees Fahrenheit, sweat evaporation
accounts for nearly all heat loss.
Many factors affect the evaporation process. Some of these
Availability of drinking water.
Relative humidity above 50 percent.
Environmental temperature above 82 degrees Fahrenheit.
Relative humidity is the factor that most limits evaporation;
at a relative humidity of 100 percent, no heat is lost by this
mechanism. Although the body continues to sweat, it loses only a tiny
amount of heat. For example, a person can function all day at a
temperature of 115 degrees Fahrenheit and a relative humidity of 10
percent if given enough water and salt. If the relative humidity rises
to 80 percent at the same temperature, that same person may be
incapacitated within 30 minutes.
The body will undergo certain physiological changes to
counteract heat stress. To get heat from the inner body core to the
surface where it can be lost to the surroundings, blood flow to the
skin (cutaneous circulation) increases tremendously. Blood flow to
other organs, such as the kidneys and liver, is reduced, and the heart
rate is increased so that the body can maintain an adequate blood
pressure. As the heat builds up, receptors in the skin, brain, and
neuromuscular system are stimulated to increase sweat production.
Normal heavy sweating produces one pint to one quart of sweat per
hour; heat-stress conditions, however, can result in 3 to 4 quarts
being produced. If a person does not replace this sweat loss by
drinking liquids, the body rapidly dehydrates, the rate of sweat
production drops, and the body temperature increases, causing further
Individuals vary in their response to heat stress. Some
serious reactions are heat cramps, heat exhaustion, and heatstroke.
Factors that influence the physiological responses to heat stress
include the amount of work that individuals perform and their physical
condition as well as their ability to adapt to the environment. Old
age, excessive alcohol ingestion, lack of sleep, obesity, or previous
heatstroke can also diminish tolerance to heat stress. A previous
episode of heatstroke can predispose an individual to repeated
Heat stress not only causes general physiological changes
but also results in performance impairment. Even a slight increase in
body temperature impairs an individual’s ability to perform complex
tasks such as those required to fly an aircraft safely. A body
temperature of 101 degrees Fahrenheit roughly doubles an aviator’s
error rate. Generally, increases in body temperature have the
following effects on an aviator:
Error rates increase.
Short-term memory becomes less reliable.
Perceptual and motor skills slow, and the capacity to perform
aviation tasks decreases.
heat stress prevention
By taking certain preventive measures, personnel can avoid
heat stress. They can reduce their workload, replace lost water and
salt, adapt to the environment, and wear protective clothing.
replace water and salt
The human body cannot adjust to a decreased intake of water.
People must replace water that is lost through sweating to avoid heat
injury. The body normally absorbs water at the rate of 1.2 to 1.5
quarts per hour. A reasonable limit for the total consumption for a
12-hour workday is from 12 to 15 quarts. Therefore, additional water
intake is required. Individuals should drink one quart per hour for
severe heat-stress conditions or one pint per hour for moderate stress
conditions. Executing activities at night can minimize water loss.
Salt loss is high in personnel who either have not adapted to
the environment or have adapted but are subjected to strenuous
activity under heat stress. Replenishing this salt is important.
Normally, adding a little more salt to food during preparation is
enough to replenish the salt level. If larger amounts are required,
the flight surgeon should be consulted.
adapt to the environment
Adaptation is essential to prevent heat injury. An
individual who has not adapted to the environment is more susceptible
to heat injury and disability; work performance will also decrease. A
good plan of adaptation is based on a gradual increase in physical
stress rather than a mere subjection of personnel to heat. A minimum
of two weeks should be allowed for normal, healthy individuals to
adapt; those who are less physically fit may require more time.
Acclimation to heat can be attained in 4 to 5 days. Full heat
acclimation takes from 7 to 14 days with two to three hours per day of
carefully supervised exercise in the heat.
wear protective clothing
In direct sunlight, an individual should wear loose clothing
for adequate ventilation and evaporative cooling. In a hot
environment, clothing protects an individual from solar radiation but
reduces the loss of body heat from convection and conduction. Dark-coloured
clothing absorbs more radiant heat while light-coloured clothing
reflects it. To help reduce the heat load to the head, individuals
should wear headgear to shade their head.
The pilot, more than any other crew member, must guard
against heat stress. When speed and altitude permit, the pilot should
open a window or canopy and direct the cool air entering the aircraft
to his head and neck area to reduce heat build-up.
continue to replace
Fluid intake during flight helps prevent dehydration and
makes up for profuse sweating. Crew members should be encouraged to
drink fluids as conditions permit, especially in anticipation of
periods of physical exertion.
cold effects in the
Although heat stress causes pilots the most
significant problems, they cannot overlook the physiological effects
of cold on the body. Because pilots operate in all types of
environments, they must understand how the body reacts to
Many factors influence the incidence of cold injury. Individuals
under 17 or over 40 years of age seem to have a predisposition to
suffer cold injury as do those who have previously suffered from it.
Fatigue level, organizational discipline, individual training and
experience, and physiological factors all affect the tendency of
individuals to experience cold injury. Nutrition, activity, and the
ingestion of certain drugs and medications also influence the
incidence of cold injury.
types and treatment of cold injury
Hypothermia, trench foot (immersion foot), and frostbite are
three types of cold injury that may affect aviators. A cold injury may
be either superficial or deep.
Superficial cold injury usually can be detected by numbness,
tingling, or pins-and-needles sensations. By acting on these signs and
symptoms, individuals often can avoid further injury simply by
loosening boots or other clothing and by exercising to improve
circulation. In more serious cases involving deep cold injury, people
may not be aware of a problem until the affected part feels like a
stump or a block of wood.
Outward signs of cold injury include discoloration of the
skin at the site of the injury. In light-skinned persons, the skin
first reddens and then becomes pale or waxy white; in dark-skinned
persons, the skin looks grey. An injured foot or hand feels cold to
the touch. Swelling may also indicate deep injury. Soldiers should
work in pairs—buddy teams—to check each other for signs of
discoloration and other symptoms. Leaders should also be alert for
signs of cold injuries.
First aid for cold injuries depends on whether the injury is
superficial or deep. A superficial cold injury can be adequately
treated by warming the affected part with body heat. This warming can
be done by covering cheeks with hands, placing hands under armpits, or
placing feet under the clothing of a buddy and next to his abdomen.
The injured part should not be massaged, exposed to a fire or
stove, rubbed with snow, slapped, chafed, or soaked in cold water.
Individuals should avoid walking when they have cold-injured feet.
Deep cold injury (frostbite) is very serious and requires more
aggressive first aid to avoid or to minimize the loss of parts of the
fingers, toes, hands, or feet. The sequence for treating cold injuries
depends on whether the condition is life threatening. That is,
removing the casualty from the cold is the priority.
Some general measures can be taken to prevent all types of
cold injury. Individuals can—
Keep their body dry.
Limit exposure to the cold.
Avoid wearing wet clothing.
Monitor the windchill factor.
Keep activity below the perspiration level.
Avoid the direct contact of bare skin and cold metal.
Use the buddy system to check for early signs of cold injury.
Wear several layers of loose-fitting clothing to increase
insulation and cold-weather headgear to prevent loss of body heat.
Avoid alcohol intake because it dilates surface blood vessels;
this dilation initially causes the body to feel warmer but, because
of heat loss, actually chills it.
The windchill chart in Table 6-1 gives
the time limits for exposure to the cold before individuals experience
injury. This chart correlates wind velocities and ambient air
temperatures and shows the resulting temperatures from the windchill
factor. The same data apply when wet boots or wet clothing is worn or
flesh is exposed. This chart also indicates the level below which
frostbite becomes a real hazard. Trench foot, or immersion foot, can
occur at any temperature shown on the chart, given the right
combination of wind velocity and ambient air temperature.
Table 6-1. Windchill Temperatures