fuselage skin beside the copilot seat, and a duct
temperature sensor located in the tailcone.
These sensors determine what the H-valve po-
sition should be to produce a cabin tempera-
t u r e e q u a l t o t h a t s e l e c t i o n o n t h e c a b i n
temperature control knob. If automatic cabin
heat is not functioning properly, manual cabin
heat should be used.
Manual Mode Operation
With 28 VDC available through the MAN
CABIN HEAT circuit breaker, turn the cabin
temperature control knob full counterclock-
wise into the detent, then toggle the spring-
loaded HOT–COLD switch for the desired
cabin temperature.
NOTE
H-valve movement may be observed
by reference to the H VALVE posi-
tion indicator (optional on some air-
craft). See Figure 11-9.
Figure 11-9.
H VALVE Indicator
Duct Temperature Sensor
A duct temperature sensor is installed in the
cabin bleed-air duct between the H-valve and
the cabin. In automatic temperature control
mode, it monitors the temperature of the bleed
air entering the cabin and provides H-valve
repositioning signals. The signals compen-
sate for changes in bleed-air temperature due
to changes in engine power setting. The duct
temperature sensor does not function in the
manual temperature control mode.
Duct Temperature Limiter
A duct temperature limiter, adjacent to the
duct temperature sensor, functions in both au-
tomatic and manual temperature control modes
and limits the maximum temperature of the
cabin bleed-air duct. Excessive duct temper-
atures provide a signal voltage that causes the
H-valve to drive toward the cold position.
Cabin Temperature Sensor
A cabin temperature sensor, located behind the
copilot’s seat, monitors cabin temperature and
provides signals to reposition the H-valve.
The cabin temperature sensor is functional
only in the automatic mode of cabin temper-
ature control.
Skin Temperature Sensor
On SNs 24-255 through -349 and 25-090
through -226, a skin temperature sensor is
m o u n t e d o n t h e a i r p l a n e s k i n b e l ow t h e
copilot’s position. It provides signals to repo-
sition the H-valve in response to atmospheric
temperature changes. The skin temperature
sensor is functional only in the automatic
mode of cabin temperature control.
AIR-CONDITIONING SYSTEM
(SNs 23-003 THROUGH 24-229)
Bleed-Air Shutoff Valves
(SNs 23-003 through 23-029 not
Modified by SK-140)
Figure 11-10 illustrates the air-conditioning
system.
The bleed-air shutoff valves control the flow of
engine bleed air through the flow control valve
and through the heat exchanger. These valves
are controlled by the two AIR BLEED switches.
Bleed-Air Check Valves
Two bleed-air check valves are installed in
the engine bleed-air outlet lines to prevent
loss of compressor bleed air during single-
engine operation.
11-10
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
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