Cabin Outflow Valve
The outflow valve is located on the forward
pressure bulkhead and maintains the desired
cabin pressure by regulating cabin air exhaust
flow. The outflow valve position is controlled
by two solenoid-operated valves which re-
ceive signals from the cabin altitude controller.
These solenoid-operated valves allow either
positive or negative air pressure to enter a ref-
erence chamber to open or close the outflow
valve. The cabin altitude manual control valve,
commonly referred to as the “cherry picker,”
provides manual control of air routed to the ref-
erence chamber. On SNs 24-140 through 24-
169, 24B-185, -188 and -190 through -229, and
25-003 through 25-010, and 25-027 and 25-029
through -063, except 25-061, the outflow valve
is opened on the ground by a squat switch or
a squat switch relay box.
Pressurization Jet Pump
On airplanes SNs 24-185, -188, -190, and
subsequent, and 25-027, -029, and subse-
quent, a pressurization jet pump is mounted
on the aft side of frame 25 and provides reg-
ulated vacuum and pressure to the outflow
valve. Starting with SNs 24-210 and 25-046,
the pressure regulator and the vacuum regu-
lator are combined into a single unit called
the “pressurization manifold.”
Pressurization Control
Module Typical
General
The pressurization control module (Figure
12A-6) is mounted in the copilot’s instru-
ment panel. The control contains the mode
switch (used to select manual or automatic
operation), an AIR BLEED switch(es), a
CABIN AIR FLOW switch (which selects
maximum airflow), a safety switch (on some
airplanes), a cabin altimeter and differential
pressure indicator, a cabin vertical speed
indicator, and a cabin altitude manual con-
trol valve. See Figure 12A-6 for a typical
pressurization module.
Mode Selector Switch
This switch, labeled “AUTO–MAN,” selects
either the automatic or manual mode of oper-
ation. In AUTO position, a combined signal
from the cabin altitude selector and the rate
sensor are amplified and sent to either the out-
flow valve open solenoid (higher cabin alti-
t u d e ) o r t h e c l o s e s o l e n o i d ( l ow e r c a b i n
altitude). In the MAN position, the signal from
the amplifier is interrupted, and control of the
outflow valve is provided by the cabin altitude
manual control valve.
Cabin Altitude Controller
The cabin altitude controller provides means
for automatically controlling the aircraft cabin
pressure during climb, descent, and level flight.
The automatic cabin altitude controller mode
is activated when AUTO is selected on the
mode selector switch. In AUTO, the cabin al-
titude controller automatically maintains the
cabin altitude as the airplane changes altitude.
The manual mode deenergizes the cabin alti-
tude controller, and the cabin altitude is con-
trolled by the cabin altitude manual control
valve. The system allows comfortable climb
and descent rates inside the cabin independent
of the climb and descent rates of the airplane.
Cabin pressure is automatically relieved by
12A-7
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 12A-6.
Pressurization Control
Module
