located on the structure that supports the upper
battery tray in the aft equipment bay (Figure
6-7).
Both fans are thermostatically controlled and
operate automatically. The APU may be op-
erated at ambient temperatures up to 38° C
(100° F) with the amber “APU FAN FAIL”
indicator illuminated.
APU SYSTEMS
The APU systems include the following:
• Lubricating System
• Ignition System
• Electrical Circuit Protection System
• Fuel System
• APU Protection System
• APU Fire Detection and Extinguishing
System
Lubricating System
The APU lubricating system is a self-con-
tained wet-sump system with a capacity of
approximately 2 quarts. Oil level is remotely
sensed and indicated on the APU Maintenance
P a n e l l o c a t e d i n t h e t a i l c o n e ( S e e A P U
Maintenance Panel and Oil Servicing, this
chapter). Oil pressure will vary between 60 and
80 psi, depending upon oil temperature. A re-
lief valve is set at 200 psi to prevent over-
pressure.
The ECU continuously monitors both oil tem-
perature and oil pressure. If high oil temper-
ature or low oil pressure is sensed, the ECU
will either inhibit an APU start or automati-
cally command a protective shutdown.
The lubrication pump assembly also houses the
oil filter and a magnetic chip detector is in-
stalled into the oil drain plug. When the filter
becomes contaminated, a decrease in oil pres-
sure results in an automatic shutdown.
The lower portion of the gearbox is cast to form
the oil tank reservoir. The APU has no support
bearings in the turbine area. As a result, no oil
cooler is required and no external oil service
is necessary. The maintenance oil filler loca-
tion is located on the left side of the gearcase.
Ignition System
The APU ignition system consists of an APU-
mounted ignition exciter with a single ignitor
plug installed into the combustion chamber.
The ignition system is fully automatic and op-
erates in response to input from the ECU. The
ignition system is activated by the ECU dur-
ing start at 5% rpm. Four seconds after reach-
ing 95% the ignition unit is de-energized. An
automatic re-light feature operates whenever
the rpm drops below 94% after the ECU has
terminated ignition, turning the ignition back
on, unless the APU is in a protective or nor-
mal shutdown mode.
Electrical Circuit Protection
System
All APU systems and operation are protected
by four (4) circuit breakers located in the APU
group on the copilot’s circuit breaker panel.
Power for APU operation is provided from
the right main bus.
6-5
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
international
Figure 6-7. APU Cooling Fan Fail Indicator
and Reset Switch
