(5) Protective Shutdown Logic The protective shutdown logic automatically shuts down the APU whenever continued operation of the APU would result in damage to the A/C, the APU or the ECB. The protective shutdown sequence is rapid:
-
The cool-down cycle is suppressed,
-
The ECB removes the power from the solenoids and torque motors
-
The APU fault light is energized,
-
The FAULT indication in the APU MASTER SW is energized,
-
The APU rolls down.
The shutdown reason is stored in the ECB memory.
If the ECB operates in TSO mode (discrete input by A/C) most of the
protective shutdowns are inhibited during flight, except the
-Overspeed shutdown
-ECB shutdown,
-Emergency shutdown.
If the ECB is operating in the JAR mode all protective shutdowns are
active in flight.
(a) Overspeed Shutdown The ECB has two independent possibility that protect the APU against overspeed.
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1_ Software Overspeed Shutdown
The ECB software compares the APU speed with the programmed
overspeed limit of 106 % rpm. If APU speed is more than 106 %
rpm for 40 ms, the ECB commands an APU shutdown.
2_ Hardware Overspeed Shutdown
The ECB has two separate hardware overspeed shutdown circuits.
Each of these circuits uses a different speed sensor input to
shutdown the APU. The overspeed limit is also 106 % rpm.
The hardware circuits are tested at every normal (commanded)
shutdown of the APU. If both hardware circuits are declared as
faulty, the ECB will disable itself (see ECB shutdown).
(b)
ECB Shutdown The ECB BITE includes continuous self-monitoring of its circuitry and behavior. A fail-safe logic in the ECB will cause the ECB to command an APU shutdown if the:
-Watchdog timer logic of the microprocessor fails
-Overspeed detection circuits are faulty,
-ECB fails to detect or control as it should.
(c)
Emergency Shutdown The ECB receives three emergency signals on the EMER SIGNAL input via the:
-
APU FIRE PUSH P/BSW on the ENG/APU FIRE cockpit panel,
-
APU SHUT OFF P/B
-
APU fire and overheat detection system (only on ground). The ECB will command the APU to shutdown if any of these emergency signal is active.
(d)
Underspeed Shutdown The ECB will initiate an APU shutdown if
-
the APU is in on-speed governed mode and
-
the actual speed is < 85 % rpm and
-
the acceleration is < 0.5 %/s for 10 seconds.
(e)
Overtemperature Shutdown The ECB has two overtemperature shutdown logics:
-
an overtemperature during the APU start sequence,
-
an overtemperature with the APU on-speed.
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1_ Overtemperature Shutdown during APU start
The ECB calculates a limit for the EGT (T5) based on the speed
and the APU inlet temperature (T2). If the measured T5 is
higher than the calculated EGT limit for more than 2 seconds,
the ECB commands an APU shutdown.
2_ Overtemperature Shutdown with APU on-speed
The on-speed overtemperature shutdown is controlled by the
turbine inlet temperature (T4) limit. The on-speed T4 is
calculation is based on the EGT (T5), the APU inlet
temperature (T2) and the APU inlet pressure (P2). If the
calculated T4 is higher than the T4 limit of 2200 deg.F
(1204.44 deg.C), the ECB will initiate an APU shutdown after a
1 second delay.
(f)
Loss of Speed Shutdown If both speed measurement channels fail while the APU operates, the ECB commands an APU shutdown. A failure in the measurement channel could be caused by either the speed sensor or its associated ECB conditioning circuit. A failure of one channel does not cause a shutdown.
(g)
Sensor Fail Shutdown
1 Loss of EGT
_ The ECB shuts down the APU during APU starting phase when both thermocouples or both theromcouple measurement channels are declared as faulty. During the APU is on-speed condition the shutdown is inhibited when:
-the main engine are on,
-in flight.
2 Low Oil Level and Low Oil Pressure (LOP) switch failure
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