Hawker 800 XP, Pilot Training Manual

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during the airplane’s descent or at any other
time that the LP bleed-air pressure drops below
27 psi. The supplemental HP bleed air keeps
the pressure above 27 psi for operation of the
dependent systems.
From each main air valve, the air is ducted via
a nonreturn valve, and then via a common duct
to a pressure regulator and shut-off valve unit
(PRSOV), and a pressure reducing and flow re-
stricting venturi to an air cycle machine. (Refer
to Chapter 11, “Air Conditioning.”). Bleed air
from the No. 2 engine may be used for auxil-
iary heating or emergency pressurization.
CONTROLS AND INDICATORS
The pneumatic system’s controls consist of two
MAIN AIR VLV 1 and 2 switches. Each switch
has three positions labeled “OPEN,” “LP ON,”
and ”CLOSE” and is located on the roof panel.
(See Figure 9-1.)
Engine Air Bleeds
Bleed air from each engine enters the rear bay
via two ducts in each pylon. Low pressure
(LP) air is ducted from a flow limiting venturi
to a mixing valve.
High pressure (HP) air, when both main air
valves (MAVs) are selected open, is ducted
from a flow limiting venturi to a solenoid-op-
erated shutoff valve then to the mixing valve.
Mixing Valve
The mixing valve regulates HP air (when it is
demanded by LP air). It mixes the HP and LP
air supplies, maintaining a minimum pressure
of 18 psi at the valve outlet and limiting the
temperature to approximately 200°C.
The amount of HP air used is dependent on the
LP air pressure available. When the LP air
pressure falls to 27 psi, the HP valve is opened
but the mixing valve does not mix any HP air
into the system until the LP air pressure falls
to 18 to 22 psi and below.
Main Air Valve (MAV)
Air from each mixing valve is fed through an
electrically actuated main air valve (MAV)
and a nonreturn valve (NRV) into a common
supply duct. The MAV and HP valve selections
are made by means of two switches, MAIN
AIR VLV 1 and MAIN AIR VLV 2, on the
flight compartment roof panel (Figure 9-1).
The switch selections of CLOSE, LP ON, and
OPEN affect the system as follows:
• CLOSE: MAV and HP valve are both se-
lected closed.
• LP ON: MAV selected open and HP
valve selected closed.
• OPEN: MAV selected open. HP valve
‘armed’ but the valve position is con-
trolled by the LP duct pressure switch.
Time delay units are incorporated in the “open”
circuit of the MAVs to prevent an initial surge
of air to the cabin. The No. 1 MAV has a stag-
gered open/delay logic control, and takes ap-
proximately 20 seconds to reach the fully open
position. The No. 2 MAV is fast acting but is
delayed by 30 seconds before running fully
open in approximately one second.
Revision 1 9-2
FOR TRAINING PURPOSES ONLY
HAWKER 800 XP PILOT TRAINING MANUAL
FlightSafety
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Figure 9-1. Pneumatic System
Control Switches