Cabin Safety Valve
A cabin safety valve is installed in the aft
pressure bulkhead. Its purpose is to relieve a
cabin overpressure or a negative pressure dif-
ferential caused by a malfunction in the nor-
mal control system. In flight, it normally
remains fully closed unless the secondary dif-
ferential pressure-relief valve opens it to re-
lieve an overpressure condition. In the case of
a negative differential pressure condition, am-
bient pressure opens the safety valve, allow-
ing an inward flow to equalize the cabin and
ambient pressures.
Operation of the safety valve is automatic in
flight; there is no crew control. On SNs 24-350
and subsequent, and 25-227 and subsequent,
a fourth solenoid valve is installed in the pneu-
matic control circuit to allow control of the
safety valve on the ground with the engine
running and the AIR BLEED switches in ON.
The solenoid valve is powered open when the
CAB AIR switch is turned to OFF and opens
the safety valve. The valve closes 10 seconds
after the CAB AIR switch is turned ON and
closes the safety valve. The solenoid is deen-
ergized in flight regardless of CAB AIR switch
position.
On airplanes prior to SNs 24-350 and 25-227,
the safety valve does not open on the ground.
Differential Pressure-Relief
Valve (Secondary)
The secondary pressure-relief valve functions
in association with the safety valve. Should the
primary pressure-relief valve not operate prop-
erly, the secondary pressure-relief valve opens
the safety valve to limit cabin pressure. The
safety valve limits differential pressure to 9.2
psi on SNs 24-230 through -349, and 25-067
through -226. On subsequent SNs, the pressure
is limited to 9.7 psi.
Cabin Altitude Limiter
(Secondary)
The cabin altitude limiter for the cabin safety
valve serves the same purpose as the cabin al-
titude limiter for the outflow valve. If the sec-
o n d a r y d i ff e r e n t i a l p r e s s u r e - r e l i e f va l ve
malfunctions, causing the safety valve to open,
the cabin altitude climbs. Should cabin altitude
climb to 11,000 ±1,000 feet on SNs 24-349 and
25-226 and prior, or 11,500 ±1,500 feet on
later airplanes, the cabin altitude limiter in-
troduces cabin air pressure into the safety
valve. This causes the valve to modulate and
maintain cabin altitude at the limiter value.
CAB AIR Switch (SNs 24-350
and Subsequent, and 25-227
and Subsequent)
The CAB AIR switch (Figure 12-6) controls
the flow control valve. Switch positions are la-
beled “ON” and “OFF.” With the switch in
ON, bleed air flows through the flow control
valve and into the air distribution system. In
the OFF position, the flow control valve closes,
blocking airflow into the cabin. The switch op-
erates in conjunction with the three-position
(ON–OFF–EMER) AIR BLEED switches. The
AIR BLEED switches control bleed-air flow
from the engines to the bleed-air manifold.
AIR BLEED Switch (SNs 24-349
and Earlier, and 25-226 and
Earlier)
The three-position AIR BLEED switch (Figure
12-6) is labeled “OFF–NORM–MAX.” The
OFF position closes the flow control valve. The
MAX position opens the valve completely.
The NORM position permits the flow control
valve to open and modulate to maintain a con-
stant airflow regardless of altitude or engine
power setting.
12-7
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
