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A fire shutdown occurs if you pull one of the APU fire handles or
if the fire detection system detects a fire.
An inlet door shutdown occurs if the ECU commands inlet door
open and the ECU does not get an open signal from the inlet
door position switch in 30 seconds. An inlet door shutdown also
occurs if the door open signal is lost for 1 second after APU
speed is more than 7 percent.
An inlet overheat shutdown occurs if the compressor inlet
temperature is more than 350F (180C) for 3 seconds.
A loss of EGT shutdown occurs if both EGT thermocouples fail.
Loss of one EGT thermocouple does not shutdown the APU.
A loss of speed shutdown occurs if both speed sensing circuits
fail. Loss of one speed circuit does not shutdown the APU. A
speed circuit includes the speed sensor, aircraft wiring and the
ECU.
APU CONTROLS - ELECTRONIC CONTROL UNIT - PROTECTION SYSTEM
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A no acceleration shutdown occurs if, after ignition and before
95 percent, acceleration is less than 0.2 percent per second for
12.5 seconds.
A no APU rotation shutdown occurs if speed is less than 7
percent 20 seconds after the ECU gives the start command to
the SCU.
A no flame shutdown occurs if EGT is less than 300F (149C) 20
seconds after the ECU commands the fuel solenoid valve open.
An oil filter shutdown occurs if these conditions occur:
* Oil temperature is more than 100F (38C)
* Both main engines are off for more than 90 seconds
* Airplane is on ground
* Starter-Generator Oil filter is clogged for more than 5
seconds.
A oil temperature shutdown occurs if the oil temperature is
more than 290F/143C for 10 seconds and the sensor is good.
An overtemperature shutdown occurs if the turbine inlet
temperature or exhaust gas temperature goes too high. When
APU speed is above 95 percent, the maximum turbine inlet
temperature is 2200F (1204C). The ECU uses exhaust gas
temperature to calculate turbine inlet temperature. When APU
speed is less than 95 percent, the EEC uses these values to find
the maximum exhaust gas temperature:
* APU speed
* Inlet temperature (T2)
* Inlet pressure (P2).
A reverse flow shutdown occurs if load compressor air flow
decreases to approximately zero for 6 seconds. The ECU
commands the BAV closed when the air flow decreases to
approximately zero for one second. With the BAV closed, bleed
air can not reverse flow from the airplane to the APU load
compressor. This prevents most reverse flow shutdowns.
A sensor failure shutdown occurs if the inlet temperature (T2) or
the oil temperature sensor fails and the airplane is on the
ground.
An underspeed shutdown occurs if these conditions occur for 10
seconds:
* APU acceleration is less than 0.5 percent per second
* APU speed is less than 85 percent
* APU is not starting.
An overspeed protection shutdown occurs if the APU fails the
overspeed circuit test. The ECU does the overspeed circuit test
during APU shutdown.
An overspeed protection shutdown also occurs if the fuel
solenoid valve fails in the open position. The fuel solenoid valve
fails in the open position if these conditions occur:
APU CONTROLS - ELECTRONIC CONTROL UNIT - PROTECTION SYSTEM
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-60-00-006
* Power is removed from fuel solenoid valve for 10 seconds
* APU speed is more than 90 percent.
An overspeed shutdown occurs if the APU speed is more than
106 percent.
An oil pressure shutdown occurs if the APU is on speed and oil
pressure is low for 20 seconds.
MAINT Light
The ECU uses the MAINT light to tell you when the startergenerator
has a shorted rotating diode. Repair the startergenerator
as soon as possible.
The ECU also uses the MAINT light to tell you the APU oil level
is at low oil quantity (LOQ). When the oil level is at LOQ, there is
enough oil in the APU for 30 - 50 hours of operation at the
maximum oil consumption rate before the APU will shutdown
because of low oil pressure. However, if there is an oil leak, the
APU will not operate the 30 - 50 hours before a low oil pressure
shutdown occurs. Add oil to the APU as soon as possible.
You can also use the ECU to deactivate the MAINT light
because of incorrect low oil quantity (LOQ) indication. It is
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