Controller Solenoid Valves
Three solenoid-operated valves are installed in
the controller and are used to control the rout-
ing of pneumatic control pressure to the out-
flow valve. All three valves are energized on the
ground, causing the outflow valve to open,
thereby depressurizing the cabin.
One of the valves is used in flight to effect man-
ual control of the outflow valve and is referred
to as the “manual mode solenoid valve.”
For normal automatic in-flight operation, all
three solenoid valves are deenergized.
Aneroid Switches
Two aneroid switches are installed in the pres-
surization system—one for operation of the
manual solenoid and another for operation of
the horn.
Manual Aneroid Switch
The pressurization aneroid is located inside the
pressurization module.
On SNs 24-230 through -349 and 25-061
through -226, if cabin altitude increases to
10,000 feet or above, the aneroid switch com-
pletes a power circuit to the normally open
manual control solenoid valve. The solenoid
valve is energized closed, isolating all auto-
matic pneumatic circuits from the outflow
valve. The outflow valve, when isolated, main-
tains the last attained position. When cabin al-
titude decreases to 8,000 feet or below, the
aneroid resets and deenergizes the solenoid
valve open provided the AUTO–MAN switch
is in AUTO.
On SNs 24-350 and subsequent, and 25-227
and subsequent, the operation is the same as
early SNs except that the aneroid switch ac-
tuates at 8,750 ±250 feet and resets at 7,200
feet. Also, when actuated, it causes the amber
CAB ALT annunciator (see “Annunciator
Panel” section) to illuminate. When the aneroid
resets, the annunciator extinguishes.
Should the above cabin altitudes be reached
or exceeded, the “cherry picker” is the only
way to control the outflow valve.
Cabin Altitude Warning
Horn Aneroid Switch
The cabin altitude warning horn aneroid switch
activates the cabin altitude warning horn when-
ever the cabin altitude exceeds approximately
10,000 feet. A spring-loaded HORN SILENCE
switch (Figure 12-5) on the test panel forward
of the thrust levers may be used to silence the
horn for periods of 30 to 50 seconds. The horn
continues to reactivate after each use of the
HORN SILENCE switch until the aneroid re-
sets at approximately 8,000 feet cabin altitude.
The CAB ALT TEST switch on the pedestal
test panel is used to test the warning horn. If
the CAB ALT warning light is installed, it
does not illuminate during this test.
12-6
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 12-5. HORN SILENCE Switch
