Learjet 20 Series Pilot Training Manual page 183

When the knob is pushed in to NORMAL,
bleed air is diverted to the pilot's and copilot's
footwarmers, increasing the amount of flow
into the cabin, which effectively becomes the
emergency source for cabin pressurization.
The knob is normally kept pushed in.
When the knob is pulled out to the DE-FOG po-
sition, the bleed air is directed to the external
windshield duct outlets for heating and anti-icing.
Two WSHLD HEAT switches (Figure 10-8) are
located on the anti-icing panel. One is labeled
"ON" and "OFF" and is spring-loaded to the
neutral position; the other switch is labeled
"AUTO" and "MAN."
Figure 10-8. WSHLD HEAT Switches
Bleed air from the manifold is routed through
two valves—the shutoff valve and the pressure-
regulator valve. Figure 10-9 illustrates the
windshield heat system.
The shutoff valve is motor driven by either of
the two switches on the anti-ice control panel
and takes four to five seconds to cycle fully.
Selecting AUTO opens the shutoff valve. The
green WSHLD HEAT light illuminates when-
ever the shutoff valve is not fully closed. If
MAN is selected, the shutoff valve may be
opened with the ON–OFF switch. Since this
switch is spring-loaded to neutral, it must be
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
held in the ON position while the valve drives
toward the fully open position. The switch
can be released at any intermediate position,
and the valve will stop moving. Since the
WSHLD HEAT light illuminates when the
valve is in any position other than fully closed,
there is no cockpit indication of intermediate
shutoff valve position. Once the valve has
been opened, it can be closed only by holding
the ON–OFF switch to OFF (with MAN se-
lected) for at least four seconds.
The pressure-regulator valve is solenoid op-
erated and is deenergized closed. It is energized
open when airplane power is on. Its function
is to regulate the engine bleed air from the man-
ifold to 16 psi.
Automatic Operation
The flow of bleed air to the windshields is con-
trolled in the automatic mode by the high (250ºF)
and low (215ºF) temperature thermoswitches
installed in each windshield outlet nozzle.
On the ground, if the low-limit thermoswitch
senses 215ºF, it closes the shutoff valve, ex-
tinguishes the green WSHLD HEAT light, and
illuminates the red WSHLD OV'HT light. If
the low-limit switch fails or the shutoff valve
fails to close fully, the temperature may rise
sufficiently to trigger the high-limit temper-
ature switch, which causes the pressure-
regulator valve to close. The red WSHLD
OV ' H T l i g h t i l l u m i n a t e s , a n d t h e g r e e n
WSHLD HEAT light remains illuminated be-
cause the shutoff valve is not fully closed.
In flight, the windshield heat cycles off the
low-limit thermoswitches. When the temper-
ature reaches 215ºF, the low-limit switches
complete a circuit that drives the shutoff valve
to the closed position, extinguishing the green
WSHLD HEAT light. When the switch cools,
the shutoff valve opens, the WSHLD HEAT
light illuminates, and the cycle repeats. If the
high-temperature limit thermoswitches reach
250ºF due to failure of the low-limit switches,
t h e p r e s s u r e - r eg u l a t o r va l ve c l o s e s , t h e
WSHLD OV'HT lights illuminate, and the
WSHLD HEAT light remains illuminated.
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10-7