ATR42 Model : 400/500
PNEUMATIC SYSTEM
AIR
P 1
MAY 98
001
1.03
.
20
20.1 DESCRIPTION
(See schematic p 7/8)
Compressed air is bled from the engine compressors at the LP or HP stages.
The pneumatic system consists of all the systems designed to supply air to the
various aircraft systems, zones or engines, with associated control, monitoring and
indicating components. It supplies under pressure air for air conditioning,
pressurization and ice protection system.
A protection against overheat due to possible leakage around the hot air ducts is
provided.
AIR BLEED
- Air conditioning and pressurization
The system is designed to :
S
select the compressor stage from which air is bled, depending on the pressure
and/or temperature existing at these stages.
S
regulate air pressure in order to avoid excessive pressures
Air is generally bled from the low compressor stage (LP). At low engine speed when
pressure from LP stage is insufficient, air source is automatically switched to the
high compressor stage HP. (This may occur on ground and during descent at F.I.).
Transfer of air is achieved by means of a pneumatically operated and electrically
controlled butterfly valve, (HP valve) which remains closed in absence of electrical
supply :
S
when the HP valve is closed, air is directly bled from the LP stage through LP
bleed air check valves.
S
when the HP valve is open, the HP air pressure is admitted into the LP pneumatic
ducting and closes the check valves ; air is therefore bled from HP stage only,
without any recirculation into the engine.
- Wing and engine de-icing.
Air is bled from the HP compressor stage. Transfer of air is achieved through a
pressure regulating valve which is electrically controlled.
ISOLATION
Downstream of the junction of the LP and HP ducting, air is admitted into the duct by
a pneumatically operated, electrically controlled butterfly bleed valve which acts as a
shut off valve. It includes a single soleno
ï
d which locks the valve closed when
deenergized. The bleed valve automatically closes in the following cases :
- Bleed duct OVHT
- Bleed duct LEAK
- Actuation of associated ENG FIRE handle
- Engine failure at TO (UP TRIM signal)
- PROPELLER BRAKE selected ON (for left bleed valve only).
In the absence of air pressure, the valve is spring-loaded closed regardless of
electrical power supply.
Note : During a starting sequence, the bleed valves opening is inhibited.
AA
R
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01/07//2018 REV 1 These notes are not revised after issue For Training Purposes Only
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