Cabin and Cockpit
Air Distribution
Conditioned airflow distribution to the cabin
and cockpit areas is essentially the same for
all airplanes (see Figure 11-1). The condi-
tioned air is routed from the tailcone into the
cabin area through two ducts, one on each
side of the cabin. The left duct ends at the
entry door, and the right duct continues for-
ward to the cockpit. A footwarmer diffuser, lo-
cated below the instrument panel just forward
of the center pedestal, directs continuous con-
ditioned air along the center floor. Two pic-
colo tubes, installed vertically on each side of
the windshield center support structure, direct
a continuous flow of conditioned air across the
forward section of each pilot’s windshield for
interior windshield defogging.
Temperature Control
Cabin temperature control is accomplished by
conditioning the engine bleed air, which is
also used for cabin pressurization. The cabin
heating system can be controlled both manu-
ally and automatically. In both modes, the
cabin temperature is regulated by the bypass
(H) valve which controls the amount of engine
bleed air that is allowed to pass through the
air-to-air heat exchanger. The bypass valve is
regulated either manually or automatically
through the cabin temperature knob located
on the climate control. The engine bleed air
follows a route from the engine(s) to the bleed-
air manifold, flow control valve, venturi, to the
bypass valve, and then either through the heat
exchanger to the cabin air distribution ducts or
directly to the cabin air distribution ducts
(bypassing the heat exchanger) depending on
the position of the H-valve. Rapid cabin tem-
perature fluctuations are prevented by a duct
temperature sensor in the supply duct and a
high-temperature limit switch prevents the
duct temperature from exceeding safe limits.
There are usually two 28 VDC circuit break-
ers—one for automatic and one for manual
cabin temperature control. A cabin tempera-
ture control knob (Figure 11-8) is used to
select MAN (in detent full CCW); the cabin
temperature is controlled by using the spring-
loaded HOT–COLD switch to operate the
bypass valve.
Figure 11-8.
Temperature Control Panel
Auto-Manual Selector
This rotary switch allows selection of either
automatic or manual control of cabin tempera-
ture. In the AUTO range, the rotary control
functions as a thermostat and allows the pilot
to set cabin temperature as desired. In the
automatic mode, the H-valve changes position
in response to signals from the cabin tempera-
ture sensor, the duct temperature sensor, and an
outside skin temperature sensor to maintain a
constant cabin temperature. With the selector
switch in MAN (full CCW), the H-valve may be
manually repositioned using the toggle switch
at the upper left of the climate control panel.
Auto Mode Operation
With 28 VDC available through the AUTO
CABIN HEAT circuit breaker, turn the cabin
temperature control knob to approximately
the 10 o’clock position (a comfortable set-
ting for cruise). Automatic cabin heat is now
controlled automatically using the cabin
temperature sensor located behind the copilot’s
seat, the skin temperature sensor located on the
11-9
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
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