-Overspeed shutdown
-ECB shutdown
-Emergency shutdown.
(a)
Overspeed Shutdown The ECB has two independent possibilities which protect the APU against overspeed.
(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 < 88 % rpm and
-
the acceleration is < 0.2 %/s for 1 second (5 seconds for speed < 50 % rpm).
(e)
Overtemperature Shutdown The ECB has two overtemperature shutdown logics:
-
an overtemperature during the APU start sequence,
-
an overtemperature with the APU on-speed.
(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_ Software Overspeed Shutdown
The ECB software compares the APU speed with the programmed
overspeed limit of 107 % rpm. If APU speed is more than 107 %
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 107 % 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).
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 1 second,
the ECB commands an APU shutdown.
2_ Overtemperature Shutdown with APU on-speed
The on-speed overtemperature shutdown is controlled by an
on-speed T5 limit. The on-speed T5 is calculation is based on
the APU inlet temperature (T2) and the APU inlet pressure
(P2). If the actual T5 is higher than the calculated T5 limit,
the ECB will initiate an APU shutdown after a 1 second delay,
or during flight or MES mode after 1 to 4 seconds delay.
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
_ In case of a low-oil level condition and a failed LOP switch the ECB commands an APU shutdown.
(h)
Low Oil Pressure Shutdown An LOP input causes a shutdown, after a time delay, when the speed is greater than 95%, or when cooldown is active and the speed is greater than 60%. The time delay is 15.5 seconds if the APU oil temperature is less than 25F or if the aircraft is on the air. The time delay shall be 7 second if the aircraft is on the ground and the APU oil temperature is greater than 25F.
(i)
No Flame Shutdown The ECB shuts down the APU if APU light-off (adequate rise in EGT) does not occur within 40 seconds after the fuel shutoff valve is opened.
(j)
Flap Not Open Shutdown The APU is shutdown if the ECB does not receive the INLET DOOR FULLY OPEN signal within 30 seconds after the APU MASTER SW is set to ON. Additionally the ECB shuts down the APU whenever the APU speed is more than 7 % rpm and the INLET DOOR FULLY OPEN signal is lost for more than 0.5 seconds.
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