Transcript BLEED AIR PNEUMATIC AND VACUUM SYSTEMS

BLEED AIR PNEUMATIC AND VACUUM
SYSTEMS
Pneumatic air is utility air used by various systems in the aircraft.
It originates from the P3 section of the engine, is routed through
firewall shutoff valves and regulated to 18 PSI. Pneumatic air
supplies pneumatic pressure and vacuum for the following
systems:
Pressure
Vacuum
• Brake Deice
• Bleed Air Warning System
• Rudder Boost System
• Pressurization Controller
• Pressurization Safety
Valve
•De-Ice Boots
General Information
Bleed air comes off of each engine and is brought
back to the fire seal. Right after the fire seal and
before the flow pack, we tap off the little P3. Air
passes through a N.O valve (power required to close
it). The air then passes through a one way check
valve installed in the bleed air line which ensures
adequate air supply during single engine operation
by preventing air from escaping back into the
compressor of the inoperative engine. Next both
engines P3 air joins at a T fitting and goes to a
regulator that steps the pressure down from 120 to
18 PSI.
Brake Deice
• First system to use the pneumatic air
• System consists of:
• Plumbing
• Two solenoid operated valves operated by
Brake Deice switch
• Distributor manifold on each main gear
• Electrical circuit containing a time delay PCB
Brake Deice
• If the aircraft takes off with the brake deice
control switch on, a circuit is completed
between uplock switch and timing circuit
• Solenoid closed after 10 minutes
• Can’t activate till gear cycled down
• If operation does not auto terminate after
10 minutes; turn brake Deice switch-OFF
• Don’t operate brake deice > 15 degrees C
• >85% N1 required to operate brake deice
and surface deice at the same time
Bleed Air Warning System
• The engine uses 25% of air for combustion
and the other 75% to cool engine and drive
pressurization and pneumatic systems
• System provides for visual indication in the
cockpit of a ruptured bleed air line
• Bleed air lines are accompanied / paralleled
by bleed air warning lines of plastic
(polyflow) tubing from the engines to the
cabin
• Bleed air warning lines are plugged and
have an internal pressure of 2 psi.
• If a bleed air line fails, escaping hot air wil
melt the paralleling warning line.
• Resulting drop in pressure (1 psi) will
actuate a pressure switch and illuminate
the L or R BL AIR FAIL warning light
• Steady illumination of the warning light
indicates a possible ruptured bleed air line
aft of the firewall. Follow procedure in
checklist / operator’s manual.
Bleed Air System Emergency
Procedures / Knowledge Check
• Procedure: Brake Deice switch-off, ITT
and TQE monitor (note readings),Bleed Air
switch- OFF, Cabin Pressurization-check
• Break in big P3 inside vessel will not
result in change in torque or ITT.
• Break in little P3 will always result in drop
in torque and rise in ITT.
• Break in P3 forward of firewall = drop in
torque and rise in ITT but no warning light.
Is the paint blistering?
Rudder Boost
• Aids pilot in maintaining directional
control in the event of engine failure
• Differential pressure valve accepts high
pressure bleed air pressure from each
engine
• If pressure differential reaches preset
tolerance, rudder boosting servos activate
cables to compensate for asymmetrical
thrust
• System controlled by RUDDER BOOST
switch and tested during run-up
Rudder Boost Continued…..
• Moving either bleed air valve switch to
PNEU & ENVIR off will disengage the
system, however, note that both switches
must be in OPEN for system to operate
correctly (environmental switches
complete electrical circuit required for
activation)
• Discuss unscheduled Rudder Boost
Activation Emergency Procedure
Surface De Ice
• System removes ice accumulation from
leading edges of the wing and horizontal
stabilizer by alternately inflating and
deflating boots.
• The inflation and deflation phases are
controlled by a surface deice distributor
valve and automatic timer
• Actuated by three position switch;
MANUAL – OFF - SINGLE CYCLE AUTO
• Spring loaded to OFF
Surface De Ice Continued…
• Pressure regulated bleed air supplies
pressure to inflate the boots.
• A venturi ejector, also operated by bleed
air, creates vacuum to deflate boots and
hold them down while not in use.
• Single Cycle Auto – via distributor –
inflates wing boots for 6 seconds, timer
deflates wing boots and 4 second inflation
begins on horizontal stabilizer boots.
When deflated, cycle complete.
Surface De Ice Continued…
• Manual position; all boots inflate
simultaneously and remain inflated till
switch released, then go into vacuum
hold-down condition until actuated again.
• Use of system below –40 degrees C can
cause permanent damage to the boots
• Allow minimum ½ inch of ice to form
before attempting ice removal.
• (Discuss SOP requirements)
Vacuum System
• Vacuum is created by pneumatic bleed air
flowing through the venturi ejector
mounted beneath the cabin floor. The
venturi effect creates a vacuum as bleed
air passes through the venturi cone. The
vacuum regulator valve maintains a
constant vacuum level in the system by
bleeding in ambient air through the
vacuum regulator filter.
Vacuum System Cont…
• The surface deice system uses vacuum as
previously discussed.
• The pressurization system uses vacuum
directly to operate the safety valve to
depressurize the cabin. The
pressurization controller uses vacuum to
control the cabin pressure outflow valve.
Each will be covered in discussing the
Pressurization System.
Pressurization
BACKGROUND
Turbine aircraft are more efficient in terms of True
airspeed and fuel flow at higher altitudes. The
crew needs either supplemental oxygen or a
pressurized cabin to operate at these altitudes
because atmospheric pressure decreases with an
increase in altitude. Pressurization is desirable in
an airplane because it allows the altitude of the
cabin to be lower than the altitude of the airplane.
DEFINITION’S CONT…
• Pressure vessel means the portion of the
aircraft which is designed to withstand the
pressure differential. The pressure vessel
extends from a forward pressure bulkhead
between the cockpit and nose section to a
rear or aft pressure bulkhead just behind
the baggage compartment, with exterior
skins making up the outer seal. Side
windows are of a round design for
maximum strength. All cables, wire
bundles, and plumbing passing through
the pressure vessel boundaries are sealed
to reduce leaks.
PRESSURIZATION REQT’S
To have a pressurization system we
must have an air source to increase
the pressure inside the pressure
vessel, a means to regulate the
pressure, and an emergency relief
system. The pressurization system
in the C-12 pumps engine bleed air
in and we control the rate it leaks
out.
AIR DELIVERY SYSTEM
Engine bleed air is ducted from the last
compressor stage (station 3) of each
engine. This P3 air line is routed to the
electronic flow control unit (sometimes
called the flow pack) mounted at the
firewall. The flow control unit regulates
the mixture of engine bleed air with
ambient air from the cowling intake.
Air Delivery Cont…..
• Pilot controls inflow with ENVIRO & PNEU
BLEED AIR switches. ON = Valve in Flow
Pack open; air allowed into duct system
• Heated air may be retained for heating, or
cooled through air-to-air heat exchanger
on the way to cabin
• Check valve installed in environmental
(Big P3) lines to prevent pressure loss in
the event of engine failure
• Air flows through mixing plenum to ducts
for windshield defrost, flight compartment
air, and floor outlets for cabin air.
ELECTRONIC FLOW
CONTROL UNIT
• Controls flow of ambient and bleed air as
a function of atmospheric temperature, for
cabin heating and pressurization.
• Regulates the inflow of air into the
pressure vessel
• Flow control unit attempts to always
provide a relatively constant inflow.
Advantage of electronic flow pack is that it
can maintain scheduled pressurization
even with power levers at idle
FLOW CONTROL UNIT CONT…
• After T/O, the main landing gear safety
switch prevents pressure bumps by
allowing left ambient air valve to open
first. Through a 4-6 second time delay, the
right ambient air valve will open.
• As aircraft climbs and temperature
decreases, ambient flow valve closes
incrementally to maintain sufficient
heating until 0 degrees Celcius when it
becomes completely closed.
PRESSURIZATION CONTROL
SYSTEM AND COMPONENTS
5 Primary Components:
1. Controller to select cabin altitude and rate of
climb/descent
2. System switch to select pressurization, depressurization, or test of the system
3. Indicators to inform the flight crew of system
performance
4. An outflow valve to control cabin altitude
5. A safety valve to protect against overpressurization and to provide emergency depressurization
Pressurization Control Cont…
• Pressurization Controller maintains
selected cabin altitude dialed into it by
adjusting flow of air out of the pressure
vessel through the outflow valve.
• Rate control knob regulates the rate at
which the cabin pressure ascends or
descends to the selected altitude.
• System indicator:
Long needle = cabin altitude
Short needle = pressure differential
• Maximum differential is 6.5 +/- .1 PSI
OUTFLOW VALVE
FUNCTIONS:
• Modulates. The outflow valve modulates
to give us the desired pressure inside the
pressure vessel; we are controlling the
pressure going out. The valve is opened
or closed by the pressure controller using
vacuum.
• Negative Differential Relief. (Greater
pressure outside the airplane than inside)
If it were to happen, the outside pressure
will push on the negative relief diaphragm
and relieve the negative pressure.
SAFETY VALVE
• Safety valve prevents pressurization on the
ground, does nothing during normal pressurized
flight.
Primary Functions:
• Dump. Pilot opens the safety valve by placing
system control switch to DUMP.
• Maximum Pressure Relief. Safety valve senses
cabin differential. Relief valve set to trigger at 6.5
PSI will guard against over-pressurization. If
exceeded, valve opens and vents excessive
pressure outside the pressure vessel. Once
within limits, valve closes.
• Negative Differential Relief. Safety valve will
relieve negative pressure the same way as the
outflow valve.
Before
Engine
Start
(Engines
Off)
When DC electrical power is applied to the aircraft , the pressurization
system receives power. Power is routed through the PRESS CONT CB,
the pressurization system switch, the left main landing gear safety switch
to the N.C. Dump Solenoid and N.O Preset Solenoid. When these
solenoids receive power the Dump Solenoid opens and the Preset
Solenoid closes. Since the engines are not running there is no
environmental air at this point.
Engines
running
prior to
T/O
Bleed Air
Valves
Open
After the engines are started there is P3 air for both Pressurization and vacuum.
The pressurization or environmental air enters the Pressure vessel through the
duct system as depicted as a red arrow in the upper left hand corner. Vacuum is
blocked from the pressure controller by the closed preset solenoid. This keeps
the outflow valve closed. Since the dump solenoid is open, Vacuum reaches the
safety valve and opens it.. The environmental air is ported outside of the pressure
vessel through the safety valve preventing the aircraft from pressurizing on the
ground.
Pressurization
Test
To test pressurization, set the cabin altitude 500 feet below field pressure
altitude on the pressure controller. Then place the cabin pressure control
switch to TEST. Electricity is removed from both solenoids causing the
preset solenoid to open and the dump solenoid to close. Since we’re asking
for a lower cabin altitude it is necessary to increase the cabin pressure.
Since environmental air is continuing to enter the pressure vessel and the
outflow valve and the safety valve remains closed, the cabin pressure
increases as the system descends the cabin to the lower altitude. This is
shown as a descent on the cabin controller rate indicator. This verifies that
the pressurization system is working correctly.
After Takeoff
(Normal
Pressurization)
When the aircraft takes off, the left main landing gear squat switch deenergizes the preset solenoid open and the dump solenoid closed,
positioning them to their normal flight modes. As the airplane climbs, the
controller modulates the outflow valve based on commands from the rate
control chamber. By regulating how much vacuum is applied to the rate
chamber in the controller, the outflow valve is positioned to yield the desired
rate of climb. If the pilot increases the rate of climb, the leak in the lower
chamber is increased from the upper chamber causing an increase in
pressure in the upper chamber which moves the needle valve down allowing
more vacuum to the outflow valve. More vacuum causes the outflow valve
to open slightly more causing an increase in the rate of climb.
Ram Air Door
Pressurization
Dump
(Airborne)
> 12,500 Will illuminate
If the flight crew deems it necessary to de-pressurize the cabin because of some
emergency, the DUMP position on the system select switch will energize the dump
solenoid and de-energize the preset solenoid. When the dump solenoid is
energized vacuum reaches the safety valve and pulls it open, rapidly depressurizing the cabin. A small solenoid operated door is located in the right side
forward fuselage area ahead of the copilots feet. Commonly called the ram air
door, it is held tightly closed by a solenoid that receives 28 VDC through the
PRESS detent of the system control switch. When the switch is moved to the
DUMP position , the solenoid is de-energized and the door is opened by ram air
pressure.
DESCENT / ARRIVAL
On descent for landing, schedule the controller to
depressurize the cabin 500 feet above landing
airport field elevation; this ensures cabin will not
dump when the left main landing gear safety switch
activates.
When the weight of the airplane activates the safety
switch, the pressurization again reverts to the
ground mode. The preset solenoid is energized
closed and regulated vacuum for the controller is
off. The dump solenoid is energized open and the
safety valve is held open with vacuum. The ram air
door solenoid remains energized and closed.