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VALVE
Overheat Anti-Ice Shutoff Valve is normally closed
(must be energized open
" UGMRIOTUTNHDERTMEOMSPWEIRTACTHURE ® Electrical ground on this wire turns
WSHLD HT light on
(SmQaUkAesTcSoWnnIeTcCtHionRwEhLeAnY p Electrical ground on this wire turns
aircraft is on the ground) WSHLD OV HT light on
p If input is power oropen circuit,
the output is an electrical ground
Learjet 31A
Pilot's Manual
WSHLD HEAT SWITCH
The windshield anti-ice system is controlled through the WSHLD
HEAT switch in the ANTI-ICE group on the center switch panel. The
switch has three positions: On (WSHLD HEAT), HOLD, and OFF.
When the BATTERY switches are set ON, the windshield anti-ice overheat
shutoff valve is energized to the open position. When open, the
overheat shutoff valve allows engine bleed air to flow downstream to
the anti-ice modulating valve. When the WSHLD HEAT switch is
placed in the On position, a circuit is completed to the anti-ice modulating
valve and WSHLD HT indicator light. The anti-ice modulating
valve will move toward full open until the valve is fully open or the
WSHLD HEAT switch is set to HOLD. When the switch is in theHOLD
position, the anti-ice modulating valve will remain in its last attained
position, and allow bleed air to the anti-ice heat exchanger. When the
WSHLD HEAT switch is set to OFF, the anti-ice modulating valve will
move towards the closed position until the valve is fully closed or the
WSHLD HEAT switch is set to HOLD. The anti-ice modulating valve
will fully open or close in approximately 15 seconds. The anti-ice heat
exchanger cools the bleed air with ram air regulated by a ram air modulating
valve. This valve is controlled by the downstream anti-ice duct
temperature sensor and regulates the anti-ice bleed air temperature by
varying the amount of ramair allowed into the heat exchanger.
WSHLD HT LIGHT
The green WSHLD HT light, located on the glareshield annunciator
panel, provides the crew with a visual indication of windshield heat
operation. The light will illuminate when the anti-ice modulating valve
is in any position except fully closed and remain illuminated until the
applicable (ground or inflight) low-limit thermoswitch trips or the
WSHLD HEAT switch is set to OFF.
WSHLD OV HT LIGHT
Illumination of the red WSHLD OV HT warning light, on the
glareshield annunciator panel, indicates that the bleed air temperature
in the windshield outlet nozzles has reached the applicable (ground or
inflight) high-limit thermoswitch setting and the windshield anti-ice
system has been shut down. If the outlet nozzle bleed air temperature
reaches 250°F (121°C) during ground operation, the ground high-limit
overheat thermoswitch will close the anti-ice overheat shutoff valve
and illuminate the WSHLD OV HT warning light. If the outlet nozzle
bleed air temperature reaches 270°F (132°C) in flight, the inflight highlimit
overheat thermoswitch will perform the same function. When the
PM-121
6-g
Change 3
Learjet 31A
Pilot's Manual
nozzle bleed air temperature cools sufficiently, the overheat thermoswitches
will reset and allow the anti-ice overheat shutoff valve to
open, extinguishing the WSHLD OV HT warning light. To avoid a false
WSHLD OV HT indication upon landing, the ground overheat thermoswitch
circuitry is disabled for 10 seconds after touchdown, after
which normal functioning will resume .
WINDSHIELD DEFOG
Windshield internal defogging is accomplished using electrically heated
windshield panels . The system is designed to be activated before
takeoff and remain on until shutdown. The system consists of two
windshield panels with integral heaters, windshield heat control unit,
system switch, L and R WS DEFOG annunciators, and associated aircraft
wiring. The system utilizes a 163 VAC output from the inverter
system to power the integral heaters. The control circuit receives 28
VDC through the 5-amp L WSHLD DEFOG and RWSHLD DEFOG circuit
breakers on the pilot's and copilot's circuit breaker panels .
WSHLD DEFOG SWITCH
The windshield defog system is controlled through the WSHLD DEFOG
switch in the ANTI-ICE group on the center switch panel. The
switch has two positions: OFF and On (WSHLD DEFOG). When the
WSHLD DEFOG switch is set to the On position, the integral heaters
will be supplied 163 volts AC power from the inverter system via the
windshield heat control unit.
NOTE:
The inverter powering the left AC bus will power the left
windshield panel while the inverter powering the rightAC
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