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THE APU MUST NOT BE LEFT UNATTENDED DURING
GROUND OPERATIONS.
APU LOADING
The FADEC controls the ready to load signals, one for electrical loading and one for
pneumatic loading. Electrical loading is activated 2 seconds after the APU speed
reaches 99%. If the APU drops below 95%, electrical loading is no longer available
(on ground and in flight).
On the ground, pneumatic loading is available 60 seconds after the APU reaches 99%
if the APU pre-start EGT was < 149°C. If the APU prestart EGT was ≥ 149°C,
pneumatic loading is available on the ground 2 seconds after the APU reaches 99%.
In flight, pneumatic loading is available 2 seconds after the APU reaches 99%,
regardless of the pre-start EGT. Bleed air extraction is limited to 30,000 feet.
The APU LCV is opened upon receiving a bleed air request from the airplane bleed
management control system or if the APU BLEED switch on the BLEED/AIR COND/
ANTI-ICE panel is selected. Open and closed positions of the control valve are
determined by the status of weight-on-wheels, main engine starting, and
environmental control system signals. The bleed and load management systems are
responsible to ensure that the bleed air request is activated only when another source
of bleed air is not commanded. Example: L engine bleed valve open and/or the R
engine bleed valve open and crossfeed valve open, the LCV will remain closed to
prevent damage to the APU.
The normal position for the APU Bleed switch is AUTO. The AUTO position
automatically integrates control of the APU LCV. The manual ON position will
command the LCV to open as selected, but only if allowed by the FADEC (to
guarantee APU self-preservation). The manual OFF position will command the LCV
to close as selected.
APU SHUTDOWN
When the “OFF” position on the APU control panel is selected, the FADEC controls
the shut down sequence. The ready-to-load signals are cancelled, commanding the
LCV closed and the generator is tripped off line. During cooldown, the APU speed is
gradually lowered to 70 percent rpm over 60 seconds below 20,000 feet or remains at
100 percent rpm for 60 seconds above 20,000 feet.
4-18 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
NOTE
If the APU RUN switch is selected to RUN prior to completion of
the 60 second cooldown period, the APU returns to 100 percent
and continues normal operation.
After cooldown is completed, the APU FADEC shuts down the APU. When the speed
drops below 25 percent the inlet door is closed, APU operating hours and fault
information are stored in the FADEC. APU indications are removed from EICAS
when the rpm drops below 5 percent.
If the BATT MASTER switch is selected OFF with no AC power on (i.e. APU GEN,
VFGs, Ext AC) or if either the APU shutoff switches (External Services Panel or APU
compartment) are selected the APU FADEC will cancel the cooldown period and
initiates an immediate shutdown.
APU RPM AND EGT INDICATIONS
For normal operations, two APU parameters (rpm and EGT) are displayed on the
status page when the RUN position is selected on the APU control panel. With the
APU switch selected to OFF the display remains active until APU speed drops below
5 percent, and the APU door is closed. The numerical readouts change to red when
their limit values are exceeded (AFM, Chapter 2, LIMITATIONS).
EMERGENCY SHUTDOWN
The following methods are considered non-normal shutdown procedures, but may be
used in an emergency situation. Actioning any or all of these procedures will result in
shutdown.
NOTE
Carrying out any of the following shutdown procedures will
cancel the “cooldown” mode and shutdown will occur
immediately.
• BATT MASTER switch selected to OFF with no AC power on
• APU DISCH handle pulled
• APU SHUTOFF button on the external services panel pushed in momentarily and
released, or
• APU compartment SHUT DOWN button, located inside the APU compartment
(on the forward bulkhead) pushed in momentarily and released
For Training Purposes Only
Sept 04
4-19
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
EXTERNAL SERVICES PANEL
RESTART AFTER EMERGENCY SHUTDOWN
In order to restart the APU following an emergency shutdown procedure, two
conditions have to be met:
• The BATT MASTER switch has to be in the ON position
• The switch on the APU control panel must be placed to the OFF position before
moving it to the momentary START position (to reset the FADEC start logic)
BATTERY
MASTER
GROUND
SERVICE
EXT AC EXT DC
　
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