Learjet 20 Series Pilot Training Manual page 253

Cabin Safety Valve
A cabin safety valve (Figure 12A-9) is in-
stalled in the aft pressure bulkhead to relieve
a cabin overpressure differential. In flight,
the valve normally remains fully closed unless
opened by the pilot valve or activated by the
cabin safety (dump) switch. It serves to limit
maximum cabin pressure in case the outflow
valve does not operate properly. On SNs 23-
003 through 24-139 and 24-170 through 24B-
184,-186,-187, and -189, and 25-011 through
-026 and 25-028, the safety valve is opened on
the ground by a squat switch.
Differential Pressure Relief
(Cabin Safety Valve)
A pilot valve in the cabin safety valve prevents
overpressurization by admitting ambient air
into the reference chamber and increasing cabin
altitude. It is set to maintain 8.7 psid on model
23 and 9.12 psid on models 24, 24B, and 25.
CABIN AIR FLOW and AIR
BLEED Switches
AIR BLEED switches on SNs 23-029 and ear-
lier allow engine bleed air to flow to the bleed-
air manifold and then to the flow control valve.
Airplanes with a single AIR BLEED switch
cause the flow control valve to move to the
closed position when turned off. On all air-
planes, the CABIN AIR FLOW switch deter-
m i n e s w h e t h e r a i r f l ow i s r e g u l a t e d t o
approximately 180 cfm or whether the flow
control valve moves to the fully open position.
12A-10
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
Figure 12A-10. Cabin Altitude and Differ-
ential Pressure Indicator
INDICATORS
Cabin Altitude and Differential
Pressure Indicator
Cabin altitude and differential pressure are in-
dicated on a single indicator (Figure 12A-10)
incorporating two scales and two pointers on
SNs 24-156 and 25-010 and subsequent, and on
airplanes with glareshield warning lights.
Figure 12A-11.
FlightSafety
international
Cabin Vertical Speed
Indicator