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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Pressurized Aircraft

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




The following terms will aid in understanding the operating
principles of pressurization and air conditioning systems:
• Aircraft altitude—the actual height above sea level at
which the aircraft is flying
• Ambient temperature—the temperature in the area
immediately surrounding the aircraft
• Ambient pressure—the pressure in the area immediately
surrounding the aircraft
• Cabin altitude—cabin pressure in terms of equivalent
altitude above sea level
• Differential pressure—the difference in pressure
between the pressure acting on one side of a wall and
the pressure acting on the other side of the wall. In
aircraft air-conditioning and pressurizing systems, it is
the difference between cabin pressure and atmospheric

Standard atmospheric pressure chart.
Figure 6-41. Standard atmospheric pressure chart.

The cabin pressure control system provides cabin pressure
regulation, pressure relief, vacuum relief, and the means
for selecting the desired cabin altitude in the isobaric and
differential range. In addition, dumping of the cabin pressure
is a function of the pressure control system. A cabin pressure
regulator, an outflow valve, and a safety valve are used to
accomplish these functions.

The cabin pressure regulator controls cabin pressure to a
selected value in the isobaric range and limits cabin pressure
to a preset differential value in the differential range. When an
aircraft reaches the altitude at which the difference between
the pressure inside and outside the cabin is equal to the
highest differential pressure for which the fuselage structure
is designed, a further increase in aircraft altitude will result in
a corresponding increase in cabin altitude. Differential control
is used to prevent the maximum differential pressure, for
which the fuselage was designed, from being exceeded. This
differential pressure is determined by the structural strength of
the cabin and often by the relationship of the cabin size to the
probable areas of rupture, such as window areas and doors.

The cabin air pressure safety valve is a combination
pressure relief, vacuum relief, and dump valve. The pressure
relief valve prevents cabin pressure from exceeding a
predetermined differential pressure above ambient pressure.
The vacuum relief prevents ambient pressure from exceeding
cabin pressure by allowing external air to enter the cabin
when ambient pressure exceeds cabin pressure. The flight
deck control switch actuates the dump valve. When this
switch is positioned to ram, a solenoid valve opens, causing
the valve to dump cabin air to atmosphere.

The degree of pressurization and the operating altitude of
the aircraft are limited by several critical design factors.
Primarily, the fuselage is designed to withstand a particular
maximum cabin differential pressure.

Several instruments are used in conjunction with the
pressurization controller. The cabin differential pressure gauge
indicates the difference between inside and outside pressure.
This gauge should be monitored to assure that the cabin does
not exceed the maximum allowable differential pressure. A
cabin altimeter is also provided as a check on the performance
of the system. In some cases, these two instruments are
combined into one. A third instrument indicates the cabin rate
of climb or descent. A cabin rate-of-climb instrument and a
cabin altimeter are illustrated in Figure 6-42.

Decompression is defined as the inability of the aircraft's
pressurization system to maintain its designed pressure
differential. This can be caused by a malfunction in the
pressurization system or structural damage to the aircraft.