With an ECS VAL circuit breaker open, the
emergency pressurization valve will position
to emergency and the bleed-air shutoff valve
will fail open. In this case, positioning the
BLEED AIR switch to OFF will not stop air-
flow into the cabin since DC electrical power
is required to close the bleed-air shutoff valve.
On airplanes modified by AMK 90-3, the
emergency pressurization valves are powered
by the L and R EMER PRESS circuit break-
ers on the left and right main DC buses. On
these airplanes, the bleed-air shutoff valves are
powered by separate circuit breakers labeled
L and R BLEED AIR, also located on the left
and right main DC buses. With an EMER
PRESS circuit breaker open, the emergency
pressurization valve will position to emer-
gency and the bleed-air shutoff valve will re-
main open. In this case, positioning the BLEED
AIR switch to OFF will stop airflow into the
cabin since DC elecrical power, from the
BLEED AIR circuit breaker, will be available
to close the bleed-air shutoff valve.
If the emergency valves are positioned to emer-
gency for any reason, they may be reset by cy-
cling the AIR BLEED switches to OFF, then
back to ON if power is available, and the cabin
altitude is at or below 7,200 feet.
During the first engine start, the valves auto-
matically shift from emergency to normal as
servo air pressure from the engine becomes
available. A slight rush of air into the cabin is
normal during start.
The pressurization system automatic protection
and warning features relative to airplane serial
numbers are shown in Tables 12-1 and 12-2.
12-11
FOR TRAINING PURPOSES ONLY
• Pressurization aneroid automatically switches the system to manual control.
• Cabin altitude warning horn sounds—initiate emergency descent.
• Cabin altitude limiters actuate.
• Passenger oxygen masks are deployed. (See Chapter 17.)
*24-255 and subsequent, 25-090,-094 and subsequent and earlier airplanes modified by AAK 73-6.
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