a reasonable operating temperature that will prolong
NOTE: To convert Fahrenheit to Celsius
the life of various components. In most cases,
(centigrade), use 5/9(F32).
equipment cooling is provided by teeing off the ducting
For example, 4°F is converted as:
from the cabin/cockpit system. On other aircraft, a
5/9(4 32) = 5/9 X 36 = 20ºC.
separate cooling system may be used primarily for
Celsius to Fahrenheit is converted
using 9/5°C + 32.
Q6-1. What is the atmospheric pressure at sea
For example, 55°C is converted as:
9/5(55) + 32 = 99 + 32 = 67°F.
Q6-2. As an aircraft ascends to higher altitude, the
The aircraft manufacturer considers these varia-
decrease in atmospheric pressure may affect
tions in outside air temperature and atmospheric pres-
flight personnel. What is the most noticeable
sure when designing the aircraft.
Q6-3. The atmospheric pressure above 35,000 feet
is extremely low. This condition may cause
Pressurization and air-conditioning of aircraft are
what effect on the human body?
necessary at high altitudes. With operational ceilings
now in excess of 50,000 feet, flight personnel, and in
some cases aircraft components, are supplied with an
artificial means of maintaining a reasonable pressure
around the entire body and/or equipment. This is done
LEARNING OBJECTIVE: Recognize the
by sealing off the entire cabin/cockpit and any
need for environmental control systems (ECS).
equipment area that may require pressurization and
The environmental control systems of most aircraft
maintaining an inside air pressure equivalent to that at
substantially lower altitudes. This is known as a pres-
ment cooling, defogging, windshield washing and rain
surized cabin, cockpit, or compartment, as applicable.
In addition to pressurizing them, the cabin, cockpit,
Coverage in this chapter is limited to air cycle
and some compartments are also air-conditioned, if the
air-conditioning and pressurization. There are five
aircraft is to fly at high speeds. This requirement is
requirements necessary for the successful functioning
partly due to the difference in temperatures at various
of a pressurization and air-conditioning system:
altitudes and also to aerodynamic heating. For example,
an aircraft flying at supersonic speeds at an altitude of
· The cabin must be designed to withstand the
35,000 feet may generate a temperature on its skin of
necessary pressure differential. This is primarily an
200°F, and twice that temperature at altitudes near sea
airframe engineering and manufacturing problem.
· There must be a means of limiting the maximum
In addition to aerodynamic heating, other factors
pressure differential to which walls will be subjected.
affecting cabin/cockpit temperatures are engine heat,
This is provided by the cabin safety valve.
heat from the sun (solar heat), heat from electrical units,
· The aircraft must have an adequate supply of
and heat from the body. Through research and test, it
was determined that the average total temperature of
compressed air. This is provided through the
these five heat sources will raise cabin/cockpit
temperature to approximately 190°F (88°C). Through
compressor or supercharger is used on aircraft having
experiments it was determined that the maximum
reciprocating engines. On all jet aircraft, the air is taken
temperature that a person can withstand and maintain
directly from the compressor section of the jet engine.
efficiency for extended periods is 80°F (27°C);
This is generally referred to as bleed air.
therefore, air-conditioning of the cabin/cockpit area is
· There must be a means of cooling the bleed air
just as essential as pressurization. Under low-speed
before it enters the cabin. This is provided by an aircraft
operating conditions at low temperature, cabin/cockpit
heating may be required.
· There must be a means of controlling the cabin
The proper operation of much of today's aircraft
pressure. This is provided by the cabin pressure
electronic equipment is also dependent on maintaining