created which distends the detector diaphragm,
actuating the diaphragm switch. This action
moves the system to the sensing phase by en-
ergizing the interpreter arming relay and di-
recting current to the blade of the diaphragm
switch. When the probe perforations become
blocked with ice, the resulting attenuated pres-
sure differential allows the diaphragm to relax
and return to its normal position. Electrical
power illuminates the red ICE DETECT warn-
ing light (see “Annunciator Panel” section) and
heats the probe. The pressure differential is
then restored, and the system reverts to the
sensing phase. It cycles between the sensing and
deicing phases until icing conditions abate.
The system is completely automatic and can-
not be adjusted.
For those airplanes without tip tank taxi lights
or the above system, windshield ice detection
lights (Figure 10A-3) may be installed.
The ice detection lights consist of two probes
mounted on top of the glareshield. Inside each
probe is a red light. Each probe receives 28 VDC
from a separate ICE DETECT circuit breaker.
During night flight, the windshield ice de-
tection lights (if installed) cause red areas,
approximately 1
1
/
2
inches in diameter, to appear
on the windshield if particles of ice or moisture
form. The light on the pilot’s side is located in
the defog airflow stream, and the light on the copi-
lot’s side is outside the defog airflow stream. If
the windshield defog system is operating, the
copilot must monitor the light on his side for in-
dication of ice or moisture formation. The wind-
shield ice detection lights indicate moisture
encounters when OAT is above freezing. At below
freezing OAT, the lights indicate ice encounters.
ANTI-ICE SYSTEMS
ENGINE ANTI-ICE SYSTEM
(NACELLE HEAT)
Nacelle heat is controlled by a LH or RH NAC
HEAT switch (Figure 10A-4) on the main
switch panel or pilot’s subpanel. Nacelle lip
heating uses 28 VDC, and front frame heat is
provided by engine bleed air from the re-
spective engine. The bleed air is used to heat
the hollow inlet struts, variable inlet guide
vane, and the engine nose cone. After passing
through the front frame, the bleed air is dis-
charged into the engine inlet.
10A-3
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
WINDSHIELD
(REF)
ICE DETECTOR
ASSEMBLY
FORWARD
GLARESHIELD
UP
FWD
Figure 10A-3. Ice Detection Lights