BOMBARDIER BD-100-1A10, Operating Manual

Page: 182 / 544

AUXILIARY POWER UNIT
Volume 2 Flight Crew Operating Manual Sep 13/2004
05-01-08
CSP 100-6
REV 1
PNEUMATIC SUPPLY
DESCRIPTION
APU compressor bleed air is used to pressurize the right side of the bleed air manifold to provide pneumatics for engine
starting and air conditioning. Reverse airflow protection is provided by a one way check valve.
COMPONENTS AND OPERATION
The APU pneumatic control system consists of a bleed valve and a surge control valve.
BLEED VALVE MODULATION/BLEED AIR LOADS
The pneumatic load on the APU is controlled by the ECU as a function of demands sent from the APU bleed valve. When
the bleed valve is opened, a portion of the compressor discharge air is diverted from the plenum chamber to the right side
of the bleed air manifold. The bleed valve opens when the flight deck APU bleed load switch is placed in the open position.
The APU engine reaction to bleed loads is different from shaft loading. Bleed air extraction from the APU diverts pneu-
matic energy from the turbine and some of the cooling air from the combustion system. As with the shaft load, the rpm
drops, and with the corresponding ECU command fuel flow increase, recovers to full governed speed.
Air diverted into the aircraft bleed manifold is no longer available to drive the turbine or cool the combustion system. The
loss of pneumatic energy is replaced with thermal energy (fuel) with significant temperature increase.
The following variables influence exhaust gas temperature (EGT) under bleed air load conditions:
- Aircraft demand for bleed air
- Ambient air temperature and density
Should EGT reach the limit, the ECU will reduce bleed air flow by modulating the bleed valve toward the closed position
to prevent over temperature.
BLEED VALVE
The APU bleed valve controls airflow to the aircraft bleed system and protects the APU from an overtemperature condition.
The valve is controlled electrically and the valve mechanism operates a butterfly valve pneumatically. When the bleed
VALVE is open, the ECU adjusts the butterfly plate position to supply the correct quantity of bleed air. These adjustments
are related to the required quantity of bleed air and APU EGT.
SURGE CONTROL VALVE
The surge control valve is automatically controlled by the ECU. The surge valve is commanded open when the APU is
supplying electrical loads at flight altitudes above 15 000 ft. This function reduces APU surge potential when operating in
an unloaded condition.
BLEED AIR AVAILABILITY
For the ECU to energize the bleed-air supply system, the APU must be more than 95% of its maximum speed.
BLEED AIR LIMITATIONS
Refer to the Approved Airplane Flight Manual for bleed air limitations.