commands control valve position rather than setting a
Automatic Mode
temperature. If the flight crew is uncomfortable, the
The automatic mode of operation regulates the
valve positions must be changed by moving the selector
cabin or flight station environment at a temperature
bug. The PPI needle should follow the selector bug
setting between 65F (18C) (full cold, AUTO) and
closely (within one needle width) as the bug is moved.
85F (29C) (full hot, AUTO) depending on the setting
The master temperature sensor, duct rate sensor, and
of the selector bug. The face of the selector-indicator
program amplifier inputs are not used in the manual
has reference marks on it at settings of 70F (21C)
mode.
(one dot), 73F (23C) (two dots), and 80F (27C)
Q6-6. The term air-to-air comes from what
(three dots).
principle?
In the automatic mode, the temperature controller
Q6-7. The cabin air cycle cooling system EDC is
receives three inputs and uses them to determine the
mounted on what engine?
proper control valve positions. First, the flight crew
Q6-8. What component balances airflow in case of
uses the selector-indicator to select the desired
back-pressure?
temperature. This tells the controller what the flight
crew wants. The master temperature sensor provides
Q6-9. The water separator removes what per-
the second input to the temperature controller. This
centage of moisture?
device senses the current flight station or cabin ambient
Q6-10.
What component is the heart of the tem-
temperature.
perature control system?
The temperature controller amplifier compares
Q6-11.
When selecting full cold, all of the airflow
the temperature requested by the flight crew
into the air cycle cooling system is being
(selector-indicator) with the actual cabin or flight
cooled by what component?
station temperature (master temperature sensor), and
develops an output called the program voltage. This
Q6-12.
What are the modes of operation for the P-3
signal is a dc voltage command for the three
air cycle cooling system?
servo-amplifiers to drive the airflow control valves. The
selector-indicator needle, called the program position
AIRCRAFT PRESSURIZATION
indicator (PPI), is positioned by the command from the
SYSTEMS
program amplifier. The PPI tells the flight crew what
the system is going to produce, regardless of the
LEARNING OBJECTIVE: Recognize the
position of the selector bug.
purpose and function of an aircraft pres-
surization system to include maintenance and
The duct rate sensor senses the rate of temperature
troubleshooting operations.
change in the air supply duct, then it sends a third signal
that is proportional to this rate of change to the
As aircraft became capable of obtaining altitudes
controller program amplifier. This signal enables
above that at which flight crews could operate ef-
the temperature controller to prevent temperature
ficiently, a need developed for complete environmental
instability when the actual temperature approaches the
systems.
desired temperature. The duct rate sensor is mounted in
Air conditioning could provide the proper tem-
the system supply duct at the control valve blending
perature and supplemental oxygen could provide
location. This is the point where the hot air bypass, the
sufficient breathable air. The one problem was that not
warm turbine bypass, and the cold turbine refrigeration
enough atmospheric pressure exists at high altitude to
unit air discharges are blended. As the three airflow
aid in breathing, and even at lower altitudes the body
control valves move in response to changes in program
must work harder to absorb sufficient oxygen through
voltage or position, the temperature of the air will
the lungs to operate at the same level of efficiency as at
change at a proportional rate.
sea level. This problem was solved by pressurizing the
cockpit/cabin area.
Manual Mode
PRESSURIZATION SYSTEM
The manual mode of operation for the temperature
control system is a backup mode in case the automatic
The area of an aircraft to be pressurized must be
mode fails. In the manual mode, the selector bug
free from all air leaks. This is accomplished by use of
6-12
