WINDSHIELD ANTI-ICE/DEFOG
AND RAIN REMOVAL SYSTEM
Defog System—Model 23
Windshield defogging is accomplished by di-
recting engine bleed air over the outside of the
windshield or across the inside of the windshield,
or by allowing conditioned cabin air to flow be-
tween the dual panes of the windshield. The sys-
tem may be used for emergency pressurization
as well as external and internal defogging.
External Defog
In most cases, there are three defog push-pull
knobs located in the cockpit. One external and
one internal control knob is located next to the
pedestal on the left (Figure 10A-5), and one ex-
ternal control control knob is on the right side.
For outside airflow, pull both external control
knobs out, and push the internal knob in, then
toggle the DEFOG switch to the DEFOG po-
sition for three to four seconds. This should pro-
duce the desired airflow. To turn off the defog
system, toggle the DEFOG switch to OFF po-
sition for three to four seconds until the valve
is closed, then place the external control knobs
in and pull the internal control knob out.
On SNs 23-003 through -008, a thermal switch
in the defog duct automatically closes the
defog shutoff valve (via the defog relay) when
the temperature reaches 205ºF (Figure 10A-
6). The defog relay then deenergizes when
the temperature drops to 180ºF, at which time
the defog system can be turned back on.
On SNs 23-009 through -099, a thermal switch
in the defog duct illuminates the red W/S
O’HEAT light on the warning panel when tem-
perature reaches 205ºF. the light extinguishes
at 180ºF. The defog system should be turned
off or airflow decreased when this light illu-
minates in order to protect the windshield
from possible distortion.
The water accumulator, installed in the ex-
ternal defog ducting forward of frame 5, pre-
vents any accumulated water in the external
nozzles from entering the internal ducting.
T h e a c c u m u l a t o r s a r e ve n t e d ove r b o a r d
through the nose compartment. A check valve
is also installed between the internal and ex-
ternal outlets to prevent a loss of cabin pres-
surization through the external defog nozzles.
Internal Defog
For internal defog or emergency pressuriza-
tion, the two external control knobs must be
pushed in and the internal control knob pulled
out. Toggle the spring-loaded DEFOG switch
to the DEFOG position for three to four sec-
onds until maximum airflow is attained.
Conditioned cabin air is routed through a clear
plastic dehumidifier before passing between the
window panes. This prevents moisture, dirt, or
smoke from collecting between the panes.
Defog System—SNs 24-100
through -180, and 25-003
through -024
Windshield defogging is accomplished by di-
recting engine bleed air over the outside of the
windshield and conditioned cabin air over the
inside of the windshield. The defog system
(Figure 10A-7) can be used for either exter-
nal defogging or emergency pressurization.
10A-5
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 10A-5. Defog Push-Pull Knobs