Figure 7-40. High performance airplane pressurization system.
CODE
Ambient air
Compressor discharge air
Pressurization air
Pre-heated ambient air
Conditioned pressurization air
Pressurized cabin
Air scoops
Turbocharger
compressor
section
Heat shroud
Heat exchanger
Forward
air outlets
Cabin heat
valve
Floor level outlets
Safety/dump valve
Flow control
venturi
To cabin altitude
controller
Outflow valve
Figure 7-41. Standard atmospheric pressure chart.
Altitude (ft) Pressure (psi)
Atmosphere pressure
Sea level
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
22,000
24,000
26,000
28,000
30,000
14.7
13.7
12.7
11.8
10.9
10.1
9.4
8.6
8.0
7.3
6.8
6.2
5.7
5.2
4.8
4.4
At an altitude of 28,000
feet, standard atmo-
spheric pressure is 4.8
psi. By adding this
pressure to the cabin
pressure differential of
6.1 psi difference (psid),
a total air pressure of
10.9 psi is obtained.
The altitude at which the
standard air pressure is
equal to 10.9 psi can be
found at 8,000 feet.
a device called an outflow valve. By regulating the air exit,
the outflow valve allows for a constant inflow of air to the
pressurized area. [Figure 7-40]
A cabin pressurization system typically maintains a cabin
pressure altitude of approximately 8,000 feet at the maximum
designed cruising altitude of an aircraft. This prevents rapid
changes of cabin altitude that may be uncomfortable or cause
injury to passengers and crew. In addition, the pressurization
system permits a reasonably fast exchange of air from
the inside to the outside of the cabin. This is necessary to
eliminate odors and to remove stale air. [Figure 7-41]
Pressurization of the aircraft cabin is necessary in order to
protect occupants against hypoxia. Within a pressurized
cabin, occupants can be transported comfortably and safely
for long periods of time, particularly if the cabin altitude
is maintained at 8,000 feet or below, where the use of
oxygen equipment is not required. The flight crew in this
type of aircraft must be aware of the danger of accidental
loss of cabin pressure and be prepared to deal with such an
emergency whenever it occurs.
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 pressure.
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
Figure 7-42. Cabin pressurization instruments.
Cabin rate-of-climb indicator Cabin/differential pressure indicator
4
6
I
0
2
2I
4
CABIN CLIMB
THOUSAND FT PER MIN.5
.5
0
5
10
15
20
25
30
35 0
1
2
34
5
6
PSI
CABIN ALT
DIFF
PRESS
1000 Feet
Cabin pressure
altitude indicator
(thousands of feet)
Maximum cabin
differential pressure
limit
Cabin differential
pressure indicator
(pounds per square
inch differential)
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 into the 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 7-42.
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.
Physiologically, decompressions fall into the following two
categories:
• Explosive decompression—a change in cabin pressure
faster than the lungs can decompress, possibly
resulting in lung damage. Normally, the time required
to release air from the lungs without restrictions, such
as masks, is 0.2 seconds. Most authorities consider any
decompression that occurs in less than 0.5 seconds to
be explosive and potentially dangerous.
• Rapid decompression—a change in cabin pressure in
which the lungs decompress faster than the cabin.
During an explosive decompression, there may be noise,
and one may feel dazed for a moment. The cabin air fills
with fog, dust, or flying debris. Fog occurs due to the rapid
drop in temperature and the change of relative humidity.
Normally, the ears clear automatically. Air rushes from the
mouth and nose due to the escape of air from the lungs and
may be noticed by some individuals.
Rapid decompression decreases the period of useful
consciousness because oxygen in the lungs is exhaled rapidly,
reducing pressure on the body. This decreases the partial
pressure of oxygen in the blood and reduces the pilot’s
effective performance time by one-third to one-fourth its
normal time. For this reason, an oxygen mask should be
worn when flying at very high altitudes (35,000 feet or
higher). It is recommended that the crewmembers select the
100 percent oxygen setting on the oxygen regulator at high
altitude if the aircraft is equipped with a demand or pressure
demand oxygen system.
Figure 7-43. Oxygen system regulator.
The primary danger of decompression is hypoxia. Quick,
proper utilization of oxygen equipment is necessary to avoid
unconsciousness. Another potential danger that pilots, crew,
and passengers face during high altitude decompressions is
evolved gas decompression sickness. This occurs when the
pressure on the body drops sufficiently, nitrogen comes out
of solution, and forms bubbles inside the person that can have
adverse effects on some body tissues.
Decompression caused by structural damage to the aircraft
presents another type of danger to pilots, crew, and
passengers––being tossed or blown out of the aircraft if
they are located near openings. Individuals near openings
should wear safety harnesses or seatbelts at all times when
the aircraft is pressurized and they are seated. Structural
damage also has the potential to expose them to wind blasts
and extremely cold temperatures.
Rapid descent from altitude is necessary in order to minimize
these problems. Automatic visual and aural warning systems
are included in the equipment of all pressurized aircraft.
Oxygen Systems
Crew and passengers use oxygen systems, in conjunction
with pressurization systems, to prevent hypoxia. Regulations
require, at a minimum, flight crews have and use supplemental
oxygen after 30 minutes exposure to cabin pressure altitudes
between 12,500 and 14,000 feet. Use of supplemental
oxygen is required immediately upon exposure to cabin
pressure altitudes above 14,000 feet. Every aircraft occupant,
above 15,000 feet cabin pressure altitude, must have
supplemental oxygen. However, based on a person’s physical
characteristics and condition, a person may feel the effects
of oxygen deprivation at much lower altitudes. Some people
flying above 10,000 feet during the day may experience
disorientation due to the lack of adequate oxygen. At night,
especially when fatigued, these effects may occur as low
as 5,000 feet. Therefore, for optimum protection, pilots are
encouraged to use supplemental oxygen above 10,000 feet
cabin altitude during the day and above 5,000 feet at night.
Most high altitude aircraft come equipped with some type
of fixed oxygen installation. If the aircraft does not have
a fixed installation, portable oxygen equipment must be
readily accessible during flight. The portable equipment
usually consists of a container, regulator, mask outlet,
and pressure gauge. Aircraft oxygen is usually stored in
high pressure system containers of 1,800–2,200 psi. When
the ambient temperature surrounding an oxygen cylinder
decreases, pressure within that cylinder decreases because
pressure varies directly with temperature if the volume of a
gas remains constant. A drop in the indicated pressure of a
supplemental oxygen cylinder may be due to the container
being stored in an unheated area of the aircraft rather than
an actual depletion of the oxygen supply. High pressure
oxygen containers should be marked with the psi tolerance
(i.e., 1,800 psi) before filling the container to that pressure.
The containers should be supplied with oxygen that meets
or exceeds SAE AS8010 (as revised), Aviator’s Breathing
Oxygen Purity Standard. To assure safety, periodic inspection
and servicing of the oxygen system should be performed.
An oxygen system consists of a mask or cannula and a
regulator that supplies a flow of oxygen dependent upon
cabin altitude. Most regulators approved for use up to 40,000
feet are designed to provide zero percent cylinder oxygen
and 100 percent cabin air at cabin altitudes of 8,000 feet or
less, with the ratio changing to 100 percent oxygen and zero
percent cabin air at approximately 34,000 feet cabin altitude.
[Figure 7-43] Most regulators approved up to 45,000 feet
are designed to provide 40 percent cylinder oxygen and 60
percent cabin air at lower altitudes, with the ratio changing
to 100 percent at the higher altitude.
Pilots should be aware of the danger of fire when using
oxygen. Materials that are nearly fireproof in ordinary air may
be susceptible to combustion in oxygen. Oils and greases may
ignite if exposed to oxygen and cannot be used for sealing
the valves and fittings of oxygen equipment. Smoking during
any kind of oxygen equipment use is prohibited. Before
each flight, the pilot should thoroughly inspect and test all
oxygen equipment. The inspection should include a thorough
examination of the aircraft oxygen equipment, including
available supply, an operational check of the system, and
assurance that the supplemental oxygen is readily accessible.
The inspection should be accomplished with clean hands and
should include a visual inspection of the mask and tubing
for tears, cracks, or deterioration; the regulator for valve
and lever condition and positions; oxygen quantity; and the
location and functioning of oxygen pressure gauges, flow
indicators, and connections. The mask should be donned and
the system should be tested. After any oxygen use, verify that
all components and valves are shut off.
Figure 7-44. Cannula with green flow detector.
Oxygen Masks
There are numerous types and designs of oxygen masks in
use. The most important factor in oxygen mask use is to
ensure that the masks and oxygen system are compatible.
Crew masks are fitted to the user’s face with a minimum of
leakage and usually contain a microphone. Most masks are
the oronasal type that covers only the mouth and nose.
A passenger mask may be a simple, cup-shaped rubber
molding sufficiently flexible to obviate individual fitting. It
may have a simple elastic head strap or the passenger may
hold it to his or her face.
All oxygen masks should be kept clean to reduce the danger
of infection and prolong the life of the mask. To clean the
mask, wash it with a mild soap and water solution and rinse
it with clear water. If a microphone is installed, use a clean
swab, instead of running water, to wipe off the soapy solution.
The mask should also be disinfected. A gauze pad that has
been soaked in a water solution of Merthiolate can be used
to swab out the mask. This solution used should contain
one-fifth teaspoon of Merthiolate per quart of water. Wipe
the mask with a clean cloth and air dry.
Cannula
A cannula is an ergonomic piece of plastic tubing that runs
under the nose to administer oxygen to the user. [Figure 7-44]
Cannulas are typically more comfortable than masks, but
may not provide an adequate flow of oxygen as reliably as
masks when operating at higher altitudes. Airplanes certified
to older regulations had cannulas installed with an on-board
oxygen system. However, current regulations require aircraft
with oxygen systems installed and certified for operations
above 18,000 feet to be equipped with oxygen masks instead
of cannulas. Many cannulas have a flow meter in the oxygen
supply line. If equipped, a periodic check of the green flow
detector should be a part of the pilot’s regular scan.
Diluter-Demand Oxygen Systems
Diluter-demand oxygen systems supply oxygen only when
the user inhales through the mask. An automix lever allows
the regulators to automatically mix cabin air and oxygen or
supply 100 percent oxygen, depending on the altitude. The
demand mask provides a tight seal over the face to prevent
dilution with outside air and can be used safely up to 40,000
feet. A pilot who has a beard or mustache should be sure it is
trimmed in a manner that will not interfere with the sealing
of the oxygen mask. The fit of the mask around the beard or
mustache should be checked on the ground for proper sealing.
Pressure-Demand Oxygen Systems
Pressure-demand oxygen systems are similar to diluter
demand oxygen equipment, except that oxygen is supplied to
the mask under pressure at cabin altitudes above 34,000 feet.
Pressure-demand regulators create airtight and oxygen-tight
seals, but they also provide a positive pressure application of
oxygen to the mask face piece that allows the user’s lungs
to be pressurized with oxygen. This feature makes pressure
demand regulators safe at altitudes above 40,000 feet. Some
systems may have a pressure demand mask with the regulator
attached directly to the mask, rather than mounted on the
instrument panel or other area within the flight deck. The
mask-mounted regulator eliminates the problem of a long
hose that must be purged of air before 100 percent oxygen
begins flowing into the mask.
Continuous-Flow Oxygen System
Continuous-flow oxygen systems are usually provided for
passengers. The passenger mask typically has a reservoir
bag that collects oxygen from the continuous-flow oxygen
system during the time when the mask user is exhaling.
The oxygen collected in the reservoir bag allows a higher
aspiratory flow rate during the inhalation cycle, which
reduces the amount of air dilution. Ambient air is added to
the supplied oxygen during inhalation after the reservoir bag
oxygen supply is depleted. The exhaled air is released to the
cabin. [Figure 7-45]
Electrical Pulse-Demand Oxygen System
Portable electrical pulse-demand oxygen systems deliver
oxygen by detecting an individual’s inhalation effort and
provide oxygen flow during the initial portion of inhalation.
Pulse demand systems do not waste oxygen during the
Figure 7-45. Continuous flow mask and rebreather bag.
Figure 7-46. EDS-011 portable pulse-demand oxygen system.
Figure 7-47. Onyx pulse oximeter.
breathing cycle because oxygen is only delivered during
inhalation. Compared to continuous-flow systems, the pulse-
demand method of oxygen delivery can reduce the amount
of oxygen needed by 50–85 percent. Most pulse-demand
oxygen systems also incorporate an internal barometer
that automatically compensates for changes in altitude by
increasing the amount of oxygen delivered for each pulse as
altitude is increased. [Figure 7-46]
Pulse Oximeters
A pulse oximeter is a device that measures the amount of
oxygen in an individual’s blood, in addition to heart rate.
This non-invasive device measures the color changes that
red blood cells undergo when they become saturated with
oxygen. By transmitting a special light beam through a
fingertip to evaluate the color of the red cells, a pulse
oximeter can calculate the degree of oxygen saturation
within one percent of directly measured blood oxygen.
Because of their portability and speed, pulse oximeters are
very useful for pilots operating in nonpressurized aircraft
above 12,500 feet where supplemental oxygen is required.
A pulse oximeter permits crewmembers and passengers of
an aircraft to evaluate their actual need for supplemental
oxygen. [Figure 7-47]
Servicing of Oxygen Systems
Before servicing any aircraft with oxygen, consult the
specific aircraft service manual to determine the type of
equipment required and procedures to be used. Certain
precautions should be observed whenever aircraft oxygen
systems are to be serviced. Oxygen system servicing should
be accomplished only when the aircraft is located outside
of the hangars. Personal cleanliness and good housekeeping
are imperative when working with oxygen. Oxygen under
pressure creates spontaneous results when brought in contact
with petroleum products. Service people should be certain to
wash dirt, oil, and grease (including lip salves and hair oil)
from their hands before working around oxygen equipment. It
is also essential that clothing and tools are free of oil, grease,
Figure 6-48. Deicing boots on the leading edge of the wing.
Tubes deflated
Tubes inflated
and dirt. Aircraft with permanently installed oxygen tanks
usually require two persons to accomplish servicing of the
system. One should be stationed at the service equipment
control valves, and the other stationed where he or she
can observe the aircraft system pressure gauges. Oxygen
system servicing is not recommended during aircraft fueling
operations or while other work is performed that could
provide a source of ignition. Oxygen system servicing while
passengers are on board the aircraft is not recommended.
Anti-Ice and Deice Systems
Anti-icing equipment is designed to prevent the formation
of ice, while deicing equipment is designed to remove ice
once it has formed. These systems protect the leading edge
of wing and tail surfaces, pitot and static port openings, fuel
tank vents, stall warning devices, windshields, and propeller
blades. Ice detection lighting may also be installed on some
aircraft to determine the extent of structural icing during
night flights.
Most light aircraft have only a heated pitot tube and are not
certified for flight in icing. These light aircraft have limited
cross-country capability in the cooler climates during late
fall, winter, and early spring. Noncertificated aircraft must
exit icing conditions immediately. Refer to the AFM/POH
for details.
Airfoil Anti-Ice and Deice
Inflatable deicing boots consist of a rubber sheet bonded to
the leading edge of the airfoil. When ice builds up on the
leading edge, an engine-driven pneumatic pump inflates the
rubber boots. Many turboprop aircraft divert engine bleed
air to the wing to inflate the rubber boots. Upon inflation,
the ice is cracked and should fall off the leading edge of the
wing. Deicing boots are controlled from the flight deck by
a switch and can be operated in a single cycle or allowed to
cycle at automatic, timed intervals. [Figure 7-48]
In the past, it was believed that if the boots were cycled
too soon after encountering ice, the ice layer would expand
instead of breaking off, resulting in a condition referred to as
ice “bridging.” Consequently, subsequent deice boot cycles
would be ineffective at removing the ice buildup. Although
some residual ice may remain after a boot cycle, “bridging”
does not occur with any modern boots. Pilots can cycle the
boots as soon as an ice accumulation is observed. Consult
the AFM/POH for information on the operation of deice
boots on an aircraft.
Many deicing boot systems use the instrument system suction
gauge and a pneumatic pressure gauge to indicate proper boot
operation. These gauges have range markings that indicate
the operating limits for boot operation. Some systems may
also incorporate an annunciator light to indicate proper boot
operation.
Proper maintenance and care of deicing boots are important
for continued operation of this system. They need to be
carefully inspected during preflight.
Another type of leading edge protection is the thermal anti-ice
system. Heat provides one of the most effective methods for
preventing ice accumulation on an airfoil. High performance
turbine aircraft often direct hot air from the compressor
section of the engine to the leading edge surfaces. The hot
air heats the leading edge surfaces sufficiently to prevent the
formation of ice. A newer type of thermal anti-ice system
referred to as ThermaWing uses electrically heated graphite
foil laminate applied to the leading edge of the wing and
horizontal stabilizer. ThermaWing systems typically have
two zones of heat application. One zone on the leading edge
receives continuous heat; the second zone further aft receives
heat in cycles to dislodge the ice allowing aerodynamic forces
to remove it. Thermal anti-ice systems should be activated
prior to entering icing conditions.
An alternate type of leading edge protection that is not as
common as thermal anti-ice and deicing boots is known
Figure 7-50. Prop ammeter and anti-ice boots.
The boot is divided into two sections: inboard and outboard.
When the anti-ice is operating, the inboard section heats on
each blade, and then cycles to the outboard section. If a boot
fails to heat properly on one blade, unequal ice loading may
result causing severe vibration.
Prop anti-ice boot
200
10
PROP DEICER
AMPS
When the system is operating,
the prop ammeter indicates
normal operating range. As each
boot section cycles, the ammeter
fluctuates.
Prop anti-ice ammeter
Outboard section
Inboard section
Figure 7-49. TKS weeping wing anti-ice/deicing system.
as a weeping wing. The weeping-wing design uses small
holes located in the leading edge of the wing to prevent
the formation and build-up of ice. An antifreeze solution
is pumped to the leading edge and weeps out through the
holes. Additionally, the weeping wing is capable of deicing
an aircraft. When ice has accumulated on the leading edges,
application of the antifreeze solution chemically breaks down
the bond between the ice and airframe, allowing aerodynamic
forces to remove the ice. [Figure 7-49]
Windscreen Anti-Ice
There are two main types of windscreen anti-ice systems.
The first system directs a flow of alcohol to the windscreen.
If used early enough, the alcohol prevents ice from building
up on the windscreen. The rate of alcohol flow can be
controlled by a dial in the flight deck according to procedures
recommended by the aircraft manufacturer.
Another effective method of anti-icing equipment is the
electric heating method. Small wires or other conductive
material is imbedded in the windscreen. The heater can be
turned on by a switch in the flight deck, causing an electrical
current to be passed across the shield through the wires to
provide sufficient heat to prevent the formation of ice on
the windscreen. The heated windscreen should only be used
during flight. Do not leave it on during ground operations, as
it can overheat and cause damage to the windscreen. Warning:
the electrical current can cause compass deviation errors by
as much as 40°.
Propeller Anti-Ice
Propellers are protected from icing by the use of alcohol or
electrically heated elements. Some propellers are equipped
with a discharge nozzle that is pointed toward the root of the
blade. Alcohol is discharged from the nozzles, and centrifugal
force drives the alcohol down the leading edge of the blade.
The boots are also grooved to help direct the flow of alcohol.
This prevents ice from forming on the leading edge of the
propeller. Propellers can also be fitted with propeller anti-ice
boots. The propeller boot is divided into two sections—the
inboard and the outboard sections. The boots are imbedded
with electrical wires that carry current for heating the
propeller. The prop anti-ice system can be monitored for
proper operation by monitoring the prop anti-ice ammeter.
During the preflight inspection, check the propeller boots for
proper operation. If a boot fails to heat one blade, an unequal
blade loading can result and may cause severe propeller
vibration. [Figure 7-50]
Other Anti-Ice and Deice Systems
Pitot and static ports, fuel vents, stall-warning sensors,
and other optional equipment may be heated by electrical
elements. Operational checks of the electrically heated
systems are to be checked in accordance with the AFM /POH.
Operation of aircraft anti-icing and deicing systems should be
checked prior to encountering icing conditions. Encounters
with structural ice require immediate action. Anti-icing and
deicing equipment are not intended to sustain long-term flight
in icing conditions.
Chapter Summary
All aircraft have a requirement for essential systems such
as the engine, propeller, induction, ignition systems as well
as the fuel, lubrication, cooling, electrical, landing gear, and
environmental control systems to support flight. Understanding
the aircraft systems of the aircraft being flown is critical to
its safe operation and proper maintenance. Consult the AFM/
POH for specific information pertaining to the aircraft being
flown. Various manufacturer and owners group websites can
also be a valuable source of additional information.