NORMAL OPERATION
Before Takeoff
During ground operation, the CAB AIR or
AIR BLEED switch is normally not turned to
ON until just prior to takeoff unless engine
bleed air is desired for cabin heating.
When accomplishing the Before Starting
Engines checklist from the approved AFM,
the crew normally sets the AUTO–MAN switch
to AUTO, the expected cruise altitude on the
ACFT (inner) scale of the CABIN CON-
TROLLER dial, and the RATE knob to ap-
proximately the nine o’clock position.
When the CAB AIR or AIR BLEED switch is
positioned on prior to takeoff, the flow con-
trol valve is opened, allowing engine bleed air
to enter the cabin. On SNs 24-350 and subse-
quent, and 25-227 and subsequent, there is a
delay of approximately 10 seconds before the
safety valve closes.
FLIGHT OPERATION
Automatic
At liftoff, the squat switch relay box deener-
gizes all pneumatic solenoids and pressur-
ization begins. The cabin altitude begins to
climb at a rate based on the RATE knob set-
ting. It should be adjusted, as necessary, to
maintain a comfortable cabin altitude climb
rate of approximately 600 feet per minute. As
the aircraft climbs to cruise altitude, the
CABIN CONTROLLER automatically adjusts
the outflow valve to desired cabin climb rate
until the altitude set on the controller dial is
reached. As the airplane continues to climb,
the differential pressure increases while the
cabin altitude remains constant until the air-
craft arrives at the selected aircraft altitude.
If it is observed that the DIFF PRESS indica-
tor is riding on the yellow/red line, a slightly
higher cabin altitude should be selected. Adjust
the CABIN CONTROLLER as necessary when
changing cruise altitude.
Monitor cabin altitude and differential pres-
sure throughout the flight.
Manual
If the CABIN CONTROLLER is not function-
ing properly, follow the Manual Mode Operation
procedures in Section 2 of the approved AFM.
Manual mode operation is established when the
AUTO–MAN switch is placed to MAN. This
closes the manual mode solenoid valve, block-
ing the automatic pneumatic circuit.
The UP–DN lever (cherry picker) then controls
the outflow valve directly by using the static
air source or existing cabin pressure to cause
the cabin to climb or descend.
The manual control valve can be very sensi-
tive, and even small, momentary displace-
ments of the lever generates significant cabin
climb or descent rates.
In manual mode, the cabin altitude must be
monitored much more closely than in auto-
matic mode. The outflow valve position must
be adjusted frequently during climbs and de-
scents and when making power adjustments.
Descent
During descent for landing, destination field
elevation should be set on the CABIN scale of
the CABIN CONTROLLER dial. The airplane
rate of descent should be controlled so the
cabin descent rate is comfortable (approxi-
mately 600 feet per minute).
Landing
As the aircraft descends and reaches the pre-
selected cabin altitude, the outflow valve mod-
ulates toward the open position. The cabin
should be unpressurized at landing. At touch-
down, the squat switch relay box actuates
three pneumatic solenoid valves in the con-
troller, causing the outflow valve to open com-
pletely and ensure cabin depressurization. In
addition, when the CAB AIR or AIR BLEED
SWITCH is placed to OFF, the flow control
12-9
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
