If the APU is running with ground power
(GPU) connected to the aircraft, the APU gen-
erator will remain off-line. The green “RUN”
indication will be displayed, however, the
GEN AMPS window will display “000.”
The APU starter/generator can provide up to
300 amps of power and can be operated in
conjunction with an aircraft engine generator.
APU amps above 300 will cause an “APU
AMPS HIGH” amber CAS message to be
posted. When the APU is providing electrical
power, the fuel flow increases to keep the op-
erating speed at 100%. The amount of power
extracted from the generator is displayed on
the GEN AMPS indicator on the APU control
panel and on the EICAS electrical system
schematic page. Any load higher than 500
amps will not be indicated, and the display will
remain at 500 amps until the load drops below
this value.
APU Normal Shutdown
The APU is shutdown by pressing the APU
START/STOP S/I. The ECU executes a com-
manded shutdown by testing the overspeed
circuits causing the fuel solenoid and fuel
torque motor to close. Overspeed protection
in the ECU includes two solenoids (for the
fuel solenoid and fuel torque motor). This
provides two independent shutdown paths in
case of overspeed.
The ECU will detect a failure of the overspeed
circuits during a commanded shutdown. This
failure would cause an “APU FAIL/FAULT”
indication on the Maintenance Panel.
A commanded shutdown using the APU MAS-
TER switch results in an APU shutdown and
an ECU power down. A commanded shutdown
using the START/STOP switch results in an
APU shutdown with the ECU remaining pow-
ered until another start is commanded or the
APU MASTER switch is turned off.
Automatic Shutdown and
Start Inhibit
The ECU provides automatic shutdown for
certain conditions. Some fault conditions are
detectable prior to a start attempt. These con-
ditions become start inhibits and will prevent
an APU start until the condition has been cor-
rected.
An auto shutdown, except loss of overspeed
protection, is cleared by re-initiating a start
command. A loss of overspeed protection fault
can be cleared by cycling the APU MASTER
switch or the 28 VDC power to the ECU.
Following a shutdown resulting from the de-
tection of a bleed air leak, the APU may be
restarted after cycling the APU MASTER
switch on the APU control panel.
The occurrence of a protective shutdown or
start inhibit is annunciated by the illumination
of an amber “APU FAIL” light on the APU con-
trol panel and an “APU FAIL” amber CAS.
When “FAIL” is illuminated the APU will not
start or run.
The ECU provides Protective Shutdown (PS)
and Start Inhibit (SI) as follows:
1. Fire (PS/SI)
2. Low Oil Pressure (PS)
3. High Oil Temperature (PS)
4. Overtemperature (PS)
5. Overspeed (PS)
6. ECU failure (PS/SI)
7. Loss of overspeed protection (PS/SI)
8. Loss of speed sensor signal (PS)
9. Loss of EGT thermocouple (PS)
10. Slow start (PS)
11. DC power loss (PS)
12. No flame (PS)
13. Loss of HOT sensor (PS)
14. Loss of inlet air T2 temperature (PS)
15. BAV position vs. command (SI)
6-10
FOR TRAINING PURPOSES ONLY
LEARJET 45 PILOT TRAINING MANUAL
FlightSafety
international
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