ate all load requirements. The APU is said to
be at idle when running at 100% speed with
no power being extracted. When loads are ex-
erted on the APU, fuel flow increases while
maintaining a constant 100% speed.
AUXILIARY POWER
UNIT (APU)
GENERAL
The APU (Figure 6-1) has self-contained oil
and ignition systems and is equipped with a
fire detection and extinguishing system (See
Chapter 8, Fire Protection). The only external
requirements are an electrical power source for
starting, either from aircraft batteries or from
an external ground power unit (GPU), and a
fuel supply, from the aircraft main fuel sys-
tem.
APU operation is self-governing with auto-
matic start sequence, self-monitoring and both
normal commanded and automatic protective
shutdown features. An Electronic Control Unit
(ECU) provides monitoring which automati-
cally maintains the required rpm and operat-
ing exhaust temperature (EGT) throughout
start, acceleration, idle and full load operation.
If monitored limits are exceeded, the APU
shuts down automatically.
MAJOR SECTIONS
The APU has six major sections as follows
(Figure 6-2):
• Air Intake
• Compressor
• Combustion Section
• Turbine
6-2
FOR TRAINING PURPOSES ONLY
L E A R J E T 4 5
P I L O T T R A I N I N G M A N U A L
FlightSafety
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Figure 6-1. Auxiliary Power Unit (APU)