Differential Pressure Relief
(Outflow Valve)
A pilot valve in the outflow valve (Figure
12A-7) prevents overpressurization by ad-
mitting ambient air into the reference cham-
ber to open the outflow valve. This valve is set
to maintain 8.3 psid on SNs 23-003 through
23-099 and 8.77 psid on SNs 24-100 through
-299 and 25-003 through 25-063.
Cabin Altitude Limiter
A cabin altitude limiter may be installed on the
outflow and cabin safety valve. This device ad-
mits cabin air to the reference chamber to
close the valve and keep the cabin altitude
from exceeding 11,000 ±1,000 feet.
Cabin Pressure Relief Valve
S N s 2 3 - 0 0 3 t h r o u g h 2 4 - 1 8 9 a n d 2 5 - 0 0 3
through 25-029 have a cabin pressure relief
valve installed. The cabin pressure relief valve
is a mechanical spring-loaded valve located be-
tween the pilot’s rudder pedals on frame 5. The
valve starts to open to relieve excessive cabin
pressure differential at 9.3 psid and is fully
open at 10 psid.
Cabin Safety (Dump) Switch
On SNs 23-003 through 24-184, -186, -187,
and -189, and 25-003 through -026 and -028,
a guarded safety or dump switch (Figure 12A-
8) installed on the pressurization module per-
mits depressurizing the cabin by opening the
cabin safety valve. On some airplanes, this
switch is deactivated by an aneroid switch
above 10,000 feet and does not reset until the
airplane descends below 8,000 feet.
Cabin Under Pressure Switch
Above 10,000 feet cabin altitude, an aneroid
switch directs all signals from the controller
amplifier to the outflow valve down solenoid.
Cabin Altitude Warning Horn
The horn is activated by an aneroid switch
forward of the instrument panel above 10,000
feet cabin altitude. A silence switch, located
on the pedestal forward of the thrust levers,
may be used to silence the horn for periods of
30 to 50 seconds. The horn may be tested using
the cabin altitude test switch located next to
the silence switch.
12A-9
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 12A-9.
Cabin Safety Valve (Typical)
NORMAL STATIC SOURCE
CABIN ALTITUDE
PRESSURE LIMITER
DUMP INTO TAILCONE
CABIN SAFETY
VALVE
CABIN PRESSURE
STATIC PRESSURE
LEGEND
Figure 12A-8.
Cabin Safety Dump Switch