valve closes, and on SNs 24-350 and subse-
quent, and 25-227 and subsequent, the safety
valve simultaneously opens.
EMERGENCY
PRESSURIZATION
An emergency source of pressurization bleed
air is provided to increase the flow of air into
the cabin in the event of a pressure leak.
SNs 24-230 through -349, and
25-067 through -226
Emergency pressurization is provided by use
o f t h e w i n d s h i e l d a n t i - i c e / d e f o g s y s t e m
(Chapter 10). This is accomplished by pushing
the IN NORMAL/OUT DE-FOG knob (Figure
12-8) in, then positioning the WSHLD HEAT
AUTO–MAN switch to AUTO. This causes the
defog valve to fully open and also illuminates
the WSHLD HEAT light. These actions intro-
duce air directly into the cabin area through the
pilot’s footwarmer and bypass possible leaks
in the conditioned bleed-air distribution system.
To isolate bleed-air leaks downstream of the
flow control valve, it is necessary to place the
AIR BLEED switch to OFF.
SNs 24-350 and Subsequent,
and 25-227 and Subsequent
Emergency pressurization is accomplished by
routing air directly into the cabin from either
or both engines through the emergency pres-
surization valves. This air completely by-
passes the entire manifold and conditioned
bleed-air distribution system (see Chapter 9).
The valves are spring-loaded to the emergency
position and require both servo bleed-air pres-
sure and DC power to position them to normal.
Cockpit control of the valves is provided by
the three-position (OFF–ON–EMER) AIR
BLEED switches (on some airplanes, the
s w i t c h e s a r e l a b e l e d “ B L E E D - A I R ” ) .
Automatic functioning occurs as a result of ex-
cessive cabin altitudes or DC power failure.
With the AIR BLEED switches in ON, a solenoid
on each emergency valve is energized, causing
servo bleed-air pressure to move the valve to the
normal position.
Positioning either AIR BLEED switch to EMER
deenergizes the respective solenoid, blocking
the servo bleed-air pressure, and the valve repo-
sitions to emergency by spring pressure.
The emergency pressurization valves are also
controlled by two cabin aneroid switches (one
for each valve). The aneroids are set to oper-
ate at 9,500 feet ±250 feet cabin altitude.
Should cabin depressurization occur for any
reason, the aneroid switches deenergize their
respective valve solenoids, causing the valves
to position to emergency. The aneroids reset
when cabin altitude decreases to approxi-
mately 8,300 feet.
Finally, the same sequence of events occurs as a
result of DC power failure. On unmodified air-
planes, the L and R ECS VAL circuit breakers, lo-
cated on the left and right main buses, provide DC
power to hold the respective emergency valves in
the normal position.
12-10
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 12-8.
IN NORMAL/OUT
DE-FOG Knob
