DESCRIPTION AND
OPERATION
SNs 23-003 THROUGH 24-349
AND 25-003 THROUGH 25-226
Bleed-Air Check Valves
A check valve is installed in the bleed-air ducting
from each engine. Each check valve allows
airflow in one direction and blocks airflow
applied in the opposite direction. The check
valves prevent loss of bleed air during single-
engine operation (Figure 9-1).
NOTE
On SNs 23-003 through 23-029,
unless modified by SK140, an
additional bleed-air shutoff valve is
installed in the engine bleed-air
ducting between the engine and the
bleed-air check valves discussed
above. The bleed-air shutoff valves are
opened and closed by their respective
AIR BLEED switch located on the
copilot instrument panel (see note on
Figure 9-1).
Bleed-Air Manifold
The bleed-air manifold serves as a collection point
for engine bleed air from either or both engines.
From the manifold, bleed air is distributed to the
flow control valve for cabin pressurization and
heating, pressurization jet pump (except early 23,
24, and 25 models), windshield heat, wing heat,
and hydraulic reservoir pressurization on SNs 24-
297 and subsequent and 25-135, 25-181 and
subsequent (Figure 9-1).
Flow Control Valve
The flow control valve regulates the flow of bleed
air to the cabin for normal pressurization and
heating. On SNs 23-003 through 24-229 and 25-
003 through 25-064, excluding 25-061, the flow
control valve is controlled by an ON–OFF AIR
BLEED switch and a NORM–MAX CABIN
AIRFLOW switch. The AIR BLEED switch
opens or closes the valve while the CABIN
AIRFLOW switch selects either the normal or
maximum airflow rate. On SNs 24-230 through
24-349, 25-061 and 25-070, through 25-226, it is
controlled by a three-position AIR BLEED switch
labeled “OFF,” “NORM,” and “MAX,” on the
pressurization module. The OFF position closes
the valve completely. MAX position opens the
valve to full flow. NORM position allows sense
line pressure from the venturi (located
downstream) to modulate the flow control valve
to maintain a constant airflow to the cabin.
SNs 24-350 AND SUBSEQUENT
AND 25-227 AND SUBSEQUENT
LH and RH AIR BLEED Switches
The left and right bleed-air switches on the
copilot’s lower instrument panel control their
respective bleed-air shutoff valves (see Figure 9-2).
There are three-position ON–OFF–EMER
switches that draw DC power from the left or right
ECS valve circuit breaker on the respective left or
right main bus. In the ON position, the bleed-air
shutoff valve is open. In the OFF position, the
valve is closed. In the EMER position, the bleed-
air shutoff valve remains open, and electrical
power is removed from the respective emergency
pressurization valve, causing it to route bleed air
directly to the cabin.
LH and RH Emergency
Pressurization Valves
The emergency pressurization valves, one for
each engine (see Figure 9-2), are installed in the
engine bleed-air line between the bleed-air
shutoff valves and the bleed-air manifold in the
tailcone. The valve has two positions—normal
and emergency. In the normal position, bleed air
is directed to the bleed-air manifold; in the
emergency position, the bleed air is routed
directly to the cabin. The valve is spring-loaded
to the emergency position and requires electrical
power and servo air pressure to move it to the
9-2
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
