CABIN CONTROLLER
In the automatic mode of operation, the CABIN
CONTROLLER regulates cabin pressure in
relation to the altitude that is set on the alti-
tude selector knob. Rotating the knob on the
face of the CABIN CONTROLLER either
turns a dial or aligns a window to indicate two
scales with a fixed index between them. The
outer scale represents cabin altitude, and the
inner scale represents aircraft altitude.
Rate Control
A RATE knob is located to the lower left of the
CABIN CONTROLLER to control the rate at
which the cabin climbs and descends. The
RATE control knob allows variable control
w i t h i n t h e a p p r o x i m a t e l i m i t s o f 1 7 5
feet/minute and 2,500 feet/minute. In auto-
m a t i c m o d e , t h e C A B I N C O N T RO L L E R
maintains the desired rate of climb or descent
until the selected altitude is attained.
Manual Cabin Altitude
Control Valve
A lever labeled UP–DN (Figure 12-4) can be
used to pneumatically control the outflow valve.
Because of the red knob on the end of the lever,
it is frequently referred to as the “cherry picker.”
The lever is spring-loaded to the center posi-
tion and is guarded on later airplanes to pre-
vent inadvertent activation.
The lever can be used to increase or decrease
cabin altitude in either the automatic or man-
ual mode. However, if it is used in the automatic
mode, the CABIN CONTROLLER also attempts
to control the outflow valve, and, as soon as the
lever is released, the CABIN CONTROLLER
returns cabin pressure to the original value.
Differential Pressure-Relief
Valve (Primary)
The primary differential pressure-relief valve
functions in association with the CABIN CON-
TROLLER. Its purpose is to prevent exceed-
ing normal differential limits.
On SNs 24-230 through -349, and 25-061 and
-070 through -226, the relief valve is set for 8.9
psi. For later SNs, the valve is set for 9.4 psi.
The primary differential pressure-relief valve
is not functional in the manual control mode.
During a rapid aircraft climb, with a low set-
ting on the RATE knob, it is possible to reach
the differential pressure-relief setting prior
to attaining the selected aircraft altitude.
Cabin Altitude Limiter
(Outflow Valve)
A cabin altitude limiter is installed on SNs 24-
218, -350 and subsequent, 25-227 and subse-
quent, and earlier SNs incorporating AMK
78-5. The limiter functions to restrict the loss
of cabin pressure due to a malfunctioning con-
troller or inadvertent operation of the primary
differential pressure-relief valve.
If cabin altitude reaches 11,000 ±1,000 feet on
SNs prior to 24-350 and 25-227, or 11,500
±1,500 feet on later SNs, the altitude limiter
modulates the outflow valve to restrict the
cabin altitude to the listed level.
12-5
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 12-4. UP–DN Lever
