general, the pneumatics system provides compressed air at a constant flow
rate to each of the air conditioning units, where its temperature is controlled
and it is ducted into the cabin.Boeing refers to the air conditioning units as packs.The pressurization control system restricts the escape of this
air from the cabin to maintain proper pressurization in the cabin.The pneumatic system also provides compressed air for engine starting.
The engine compressors. auxiliary power unit, or a ground unit can be
used to supply the pneumatic system.The pneumatic manifold is normally supplied from engines one and
three, with backup from engine two.Engines 1 and 3 are usually referred to as the "pod"
engines.The APU or an external air source may also be used.AC powered valves in the pneumatic manifold control the delivery
of engine bleed air into the manifold.The engine 2 bleed valves, when closed, isolate the two sides of
the pneumatic manifold so that each air conditioning pack is supplied
by a separate air source.
The bleed air switches flight engineers panel normally control the engine
bleed valves.The engine
2 bleed switches also control the APU's bleed air valve.The fire switches will close the bleed valves when those switches
are pulled.The number 2
fire switch closes both engine 2 bleed valves.In order for the engine bleed valves to respond to the positions
of the bleed switches and fire switches, the AC buses must be powered.
There are two bleed valves an each pod engine.The 8th stage bleed normally provides most of the air to the manifold
except at low engine power, at which time the 13th stage valve opens automatically
to augment the flow.In normal
operation, pneumatic flow is arranged so that engine one provides air
for the left air conditioning pack, and engine three provides the right
pack.As the compressed air
passes from the pneumatic manifold to an air conditioning pack, it is
fed through a flow sensing venturi.If the flow rate is sensed to be too low, the venturi signals the
13th stage bleed valve to open and increase flow.
At high engine powers there is more than sufficient air-flow from an 8th
stage bleed for the associated air conditioning pack.As the compressed air leaves the pneumatic manifold it passes
through a modulating and shut off valve on its way to the flow sensing
venturi.The modulating and
shut off valve is signaled from the flow sensing venturi to control the
flow at high power settings.
As the 13th stage valve on a pod engine opens or closes, the temperature
of the air from that engine varies.The temperature of this bleed air must be controlled for air conditioning
pack operation.This is accomplished
automatically on engines one and three by a pre-cooler on the bleed lines
from each engine.The pre-cooler
uses fan stage air to cool the bleed air.A temperature sensitive valve controls the rate of flow of fan
stage air through the cooler.There
is no flight deck control for the pre-cooler.
To protect against excessively high temperature in the pneumatic duct
from a pod engine, an automatic trip off feature is installed.When the temperature of the bleed air is too high, the bleed
air valve closes and a trip off light next to the affected bleed switch
After a trip has occurred and the temperature of the bleed air has dropped
sufficiently, pressing the reset button on the air conditioning panel
will return the bleed valve to normal operation.If the condition that caused the trip to occur still exists, however,
the bleed valve trip will reoccur.
Engine No, 2 supplies air only from the 8th stage.Since supplemental air is not supplied from the 13th stage on this
engine, a pre-cooler is not fitted.To warn of excessively high temperature in the engine No 2 bleed
system, a high temperature light is provided.No automatic trip off is associated with illumination of this light.
Opening the engine No. 2 left bleed switch will open the left engine 2
bleed valve to supply air to the left air conditioning pack.The engine 1 bleed switch should be closed in this case so that
only one engine is supplying bleed air to that pack.
Pressure in the bleed air distribution system can be read on the duct
pressure gauge at the flight engineers panel.There are duct pressure transmitters installed on both sides of
the ducting.If both engine
2/APU bleed switches were open, the left and right pressures would be
equal indicating common pressure.
There is provision for using an external air cart for pneumatic supply.The external air cart is connected to the bleed air distribution
system between the right No. 2 and No. 3 bleed valves.On the exterior of the airplane this connection is on the aft right
side of the fuselage.
The APU can be used on the ground to deliver compressed air to the air
conditioning packs or the pneumatic manifold.The APU bleed air valve, which bleeds compressed air from the APU
will open when either (or both) engine 2/APU bleed switch is in the open
On the 200 series aircraft there is a flow multiplier.The purpose of the flow multiplier is to augment the bleed
air output of the APU so that there will be sufficient quantity of compressed
air to operate both air conditioning packs from the APU for ground operation.
With both air conditioning packs operating the augmented APU bleed air
is ducted directly to both packs.If the air conditioning packs are not operating, APU bleed air
travels back through the ducting, which contains the flow sensing venturi
and modulating and shutoff valves to the pneumatic manifold.This allows the APU bleed air to be used for engine starting.
With the APU operating, when one air conditioning pack is turned on, both
modulating and shutoff valves close to isolate the air conditioning packs
from the pneumatic manifold.Therefore,
with at least one engine 2/APU bleed switch open and one air conditioning
pack on, there is no flow of air through either modulating and shutoff
valve.The duct pressure
gauges are installed in the pneumatic manifold. If the APU is the only
source of bleed air to the pneumatic system, and at least one air conditioning
pack is turned on, no compressed air will reach the gauges, and they will
read zero pressure.
If the air conditioning packs are not operating, the APU provides compressed
air to the pneumatic duct through the modulating and shutoff valves.This is the normal configuration for engine starting.
Heat from a broken pneumatic or anti-ice bleed air duct could cause damage
to the airplane structure.Three
detection systems are installed in the areas of these ducts to give warning
of duct failures.
A detection system is installed in each pod engine strut area inboard
of its engine firewall.A
third system combines several sensors to detect overheat in what is referred
to as lower aft body.These
lower aft body sensors are located on either side of the aft airstairs,
above the ceiling of the aft cargo compartment, and in the fuselage keel
beam.An overheat sensed
by any of the three detection systems is reflected in the flight deck
by illumination of the appropriate amber warning light on the flight engineers
panel.The adjacent test
button is used to test simultaneously the light bulbs and the continuity
of the overheat detection sensors for the struts and lower aft body.