21B Pressurization
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TTP |
B1-L3 |
ATA 21B |
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Beech 90 Series |
B2-L3 |
Pressurization |
Pressurization,
with a maximum pressurization
differential of 5.0
psid, is regulated
through a pressurization
controller and a switch on
the pedestal. The CABIN ALT knob of the controller is used to select the desired
cabin pressure altitude on the controller
dial, while the controller RATE knob regulates the cabin rate of
climb and descent. During normal operations, the cabin controller positions the outflow valve on the rear
pressure bulkhead. A safety valve, inboard of the outflow valve, protects the pressure vessel from overpressurization
when airborne and prevents pressurization on the ground.
A cabin rate-of-climb gauge and a coaxial gauge, showing cabin pressure altitude and cabin psid, are on the console in front of the pedestal.
The
cabin pressure switch on
the pedestal is labeled CABIN PRESS DUMP–PRESS–TEST.
• When
this switch is in the DUMP (forward) position, the safety valve is held open so the cabin depressurizes and/or remains
unpressurized.
• When in the PRESS (center) position, the safety valve is closed in flight and the pressurization controller modulates the outflow valve to
pressurize the cabin.
• When the switch is held
in the spring- loaded TEST (aft position), the left landing gear safety switch is bypassed
and the safety valve closes to facilitate testing of
the pressurization system on the ground.
During
ground operations, with the cabin pres-
sure control switch in the PRESS (pressurize)
position, the dump solenoid, located
forward of the aft
pressure bulkhead
adjacent to the out-
flow valve, is powered to the open position and vacuum pressure
pulls the safety
valve open. This prevents the cabin from pressurizing. Addition- ally, the cabin preset solenoid is powered to the
closed position, which prevents
vacuum pressure from operating
the cabin controller, and, thus, the outflow valve closes.
Prior to takeoff, the cabin altitude selector knob is adjusted so the ACFT ALT scale altitude indicator reads 1,000 feet above the planned cruise altitude. At this setting, the CABIN ALT scale should indicate an altitude of at least 500 feet above the takeoff-field pressure altitude. If the CABIN ALT scale were to read below the takeoff- field pressure altitude, the system will, at takeoff, attempt to pressurize to a level below the take- off field altitude. The rate control selector knob is adjusted as desired. Setting the index mark between the 9- and 12-o’clock positions provides the most comfortable cabin rate of climb. Check the cabin pressure switch to ensure it is in the PRESS position.
At takeoff, power is removed from the dump and the preset solenoids. The dump solenoid closes, and without vacuum, the safety valve closes. The preset solenoid opens and vacuum pressure is applied to the cabin controller. In response to reference vacuum pressure from the controller, the outflow valve then automatically meters the outflow of cabin air.
As the aircraft climbs to (or descends from) altitude, the position set on the rate selector knob on the cabin controller determines how fast the cabin pressure gradient changes. The rate of change selected may vary from approximately 200 feet per minute with MIN selected to 2,000 feet per minute with MAX selected. Generally, a pilot operates the rate selector at the middle or 12 o’clock position.
At altitude or level off, the actual cabin altitude is determined by the setting on the cabin controller dual-scale CABIN ALT dial. Both scales rotate together on a common face. The outer scale shows the cabin altitude selected and the controller modulates the outflow valve to maintain this altitude. The inner scale shows an aircraft altitude that represents the maximum pressure differential (5.0 psid) at the selected cabin altitude (outer scale) value.
If
the pilot were to inadvertently select a cabin altitude (outer scale) or aircraft altitude
(inner scale) that would
pressurize greater than the
5.0 psid limit, the cabin controller
would, in fact, try to establish this excessive pressure differential. However, as
the cabin started to overpressurize,
either the safety valve or the outflow valve or
both would control the cabin at the maximum
5.0 pressure differential and would not allow the over-
pressure to occur.
PRESSURIZATION (LJ-1 AND AFTER, LW-1 AND
AFTER)
65-90
Pressurization
for the cabin comes from a hydraulic motor driven Roots type
supercharger located in the left nacelle. Power for the motor
is taken from a hydraulic pump mounted on the engine
accessory case. The hydraulic reservoir is preloaded by the engine bleed air to 70 psi.
To assure proper
hydraulic system preloading, start the right engine first.
Air
enters the supercharger through a screen covered opening
in the left engine oil cooler air scoop.
If the screen should ice over, a spring
loaded door beside the screen opens allowing continued air flow. Pressure air from the supercharger passes
through a flow control valve, controlled by a downstream
venturi, to regulate the air flow into the cabin at 12 pounds per minute.
After regulation, the pressure
air enters the air conditioning plenum chamber
for distribution in the cabin. The air-to-air heat exchanger
in the pressure air duct removes the
heat generated by the supercharger from the air before it enters the air conditioner plenum.
The pressurization duct may be isolated
from the engine, when necessary during emergency condition. This is accomplished by pulling the FIRE WALL SHUT-OFF control fully out to close
the valve.
65-A90/B90
Pressurization for the cabin comes from a mechanically driven Roots type supercharger connected to the left engine accessory case. Air enters the supercharger through a screen covered opening inside the left engine cowling. Pressure air from the supercharger passes through a firewall shut- off valve, heat exchanger and flow control valve. The air-to-air heat exchanger in the pressure duct, removes the heat generated by the supercharger from the air before it enters the cabin. The flow control valve, controlled by a downstream venturi, regulates flow into the cabin at 12 pounds per minute. The cabin pressurization duct may be isolated from the engine compartment, when necessary during emergency conditions. This is accomplished by pulling the FIREWALL SHUT- OFF control fully out to close the valve.
AUXILIARY PRESSURIZATION
LJ-327, LJ-377, LJ-389 THROUGH LJ-501
The
auxiliary pressurization system utilizes
engine bleed air from
the right engine to provide additional or
alternate pressurization should the
basic left engine supercharger system fail. The auxiliary pres- surization control switch is a three position ON, OFF and AUTO switch, located forward of the Pressure-Dump
switch on the pedestal. With the control
switch in the ON position the auxiliary pressurization is completely independent of the basic pressurization
system. In this mode of operation the right engine bleed air may be used for additional pressurization, or by closing the butterfly
valve in the heat exchanger
inlet duct, bleed air may be used to supplement the heating system. Two AUX-HEAT push pull controls
on the pilot subpanel control the left and right heat exchanger inlet valves. The controls are pulled
to close the valves.
When a shutdown of the left engine
is anticipated the auxiliary pressurization
switch may be placed in the ON position to put
the system into operation and avert the “bump”
that is experienced by
the lag of an auto pressurization changeover. With the control
in the AUTO position, the system is controlled by a pressure
sensing switch in the left-hand
center section connected to the left-hand
supercharger pressurization system.
Whenever the supply of air from
the left-hand system becomes insufficient to
maintain pressurization for the cabin, the
pressure switch completes
a circuit through a
10 second time delay relay to
energize the bleed air shutoff valve for the right engine.
When operating in AUTO mode, this time delay relay
prevents the auxiliary pressurization
to have time to become stabilized in its operation. The bleed air shutoff
valve for the right
engine is controlled by a pres-
sure switch in the right engine bleed air line, and
requires a pressure of 20 to 30 psi
to be available (engine operating above 65%) before the pressurization bleed air shutoff valve will open.
The bleed air is routed through ducts from the shutoff valve through the air-to-air heat exchanger near the right leading edge and into the cabin. The duct includes a check valve located adjacent to the connection to the air distribution duct. The AUX. PRESS ON light will be illuminated whenever the bleed air shutoff valve is open. An auxiliary pressure test relay in the circuit allows the auxiliary pressurization to be checked on the ground when the SCHGR-VENT switch is placed in the TEST position, bypassing the landing gear safety switch.
The auxiliary pressurization system operation may be checked along with the basic pressurization system ground checkout. With the left engine operating, set the PRESS Dump switch to PRESS and the cabin altitude pressurization control to read approximately 1,000 feet below field elevation. Place the SCHGR-VENT switch to the TEST position, the cabin climb indicator should show descent. With the right engine operating between 75% to 80%, move the auxiliary pressurization switch to AUTO position and pull out on the pressurization shutoff control. This simulates the loss of the left engine. Move the SCHGR-VENT switch to the test position, the cabin climb indicator should show descent and the AUX. PRESS ON annunciator should be illuminated.
Unpressurized Ventilation
When the battery switch is turned on, an electromagnetic coil adjacent to the ram-air door is energized to hold the ram-air door shut against any ram-air pressure from a scoop on the left side nose. In flight, the pressurization load on the door also holds the ram-air door closed. If the cabin pressurization switch is placed in the DUMP position, power is applied to the dump solenoid and vacuum pressure opens the safety valve. Existing cabin air pressure exits through the safety valve. Addition- ally, power is removed from the electromagnet and outside air pressure from the NACA-style ram-air scoop forces the ram-air door open and enters the evaporator plenum. Existing cabin air, supplied from the blower, then mixes with the outside ram air in the evaporator plenum. The ram-air door is always closed on the ground or in flight unless the cabin pressure is dumped.
CENTER PEDESTAL
Cabin Pressurization Switch
PRESS—Allows for normal pressurization when airborne. The cabin is prevented from pressurizing during ground operations.
CABIN
ALT KNOB—Used to set the desired pressurization on the dual scale indicator
dial. The difference
between the outer and inner scale represents the differential pressure
of 5.0 psi, the maximum differential capability of this system.
|
PTP Beech 90 Series |
B1 |
LOC |
FOT |
|
MEL |
TS |
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|
B2 |
LOC |
FOT |
|
MEL |
TS |
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