-
. On-speed governor loop,
-IGV control logic,
-SCV control logic,
-Normal shutdown logic
-Protective shutdown logic,
-BITE.
(1)
Fuel Control Logic
The Fuel Control Logic is divided into two sub-logics:
-Timed acceleration logic
and
-On-speed governor loop.
(a)
Timed Acceleration Logic The timed acceleration logic accelerates the APU from zero speed (0% rpm) to governed speed (95% rpm) within controlled EGT limits. The timed acceleration logic provides 'required-to-run' fuel to the APU, in relation to a given schedule (ramp), by computing the FCU Torque Motor current. The ECB does the computation and makes the correction every 20 mS.
(b)
On-speed Governor Loop During governed APU speed the on-speed governor loop controls the fuel flow to the APU. This keeps a constant APU speed (99.1% rpm or 100% rpm). The fuel flow is limited by programmed min. and max. fuel schedules.
(2)
IGV Control Logic The ECB measures the IGV position set-point in relation to the APU operation mode. This set-point is dependent on three factors:
-
APU operating mode (i.e. MES, ECS etc.)
-
APU inlet temperature (T2),
-Inlet pressure (P2) The ECB gets feedback from the IGVs LVDT and computes and corrects the commanded position every 20 mS. When the EGT reaches the OVERTEMP limit the IGV setpoint is reduced in order to decrease the mechanical load on the APU.
R EFF : 001-049, 101-105, 107-107, 151-199, 49-61-00Page 24 201-210, Config-2 Aug 01/05
CES
(a)
ECS Mode The ECS mode operates when the APU BLEED pushbutton switch is pushed and the air conditioning packs are in operation. Then the Zone Controller 8HK sends the ECS demand through the ARINC 429 bus to the ECB (Ref. 21-63-00). This sets the Inlet Guide Vanes (IGV) position in proportion to the ECS demand signal. The ECS min position corresponding to zero % demand depends on T1 and T2.
(b)
MES Mode The MES mode operates when the APU BLEED pushbutton switch is pushed and a main engine start is initiated (Ref. 73-25-00). Then the FADEC sends a signal to the ECB. This sets the IGV position in a fixed position of 92 degrees. When the engine starter valve closes, the FADEC removes the MES signal to the ECB and the ECB switches back to the normal operation mode.
(3)
SCV Control Logic The SCV control logics prevents load compressor surge for all modes of APU operation, ambient conditions (T2/P2) and A/C airflow requirements. The ECB gets feedback from the SCV LVDT,computes the commanded position and corrects the current position every 5 mS.
(4)
Normal Shutdown Logic
(a)
Commanded Shutdown A normal APU shutdown is initiated when the operator pushes the APU MASTER SW from the ON to the OFF position. The ECB closes the IGVs to the min. position (15 degrees). The LCV closes and the SCV opens and the cool-down cycle of the APU starts.
(b)
Cool-down cycle The function of the cool-down timer is to delay the APU shutdown and to allow the APU to run for 60 seconds at a low stabilized temperature at 100% speed (N). The cool-down cycle starts immediately when the ECB receives the CLOSED signal of the LCV. The AVAIL signal remains active, the generator remains loaded if selected. If the LCV was not in the OPEN position during the entire APU operation the cool-down timer is set to zero (0) seconds. The APU enters the shutdown state immediately when commanded to shutdown. If APU Bleed is set to OFF more than 60 seconds before the APU MATER SW is set to OFF the APU is shutdown immediately. If the APU MASTER SW is pushed before completion of the cool-down cycle the APU will shutdown only after the cooldown cycle is complete.
R EFF : 001-049, 101-105, 107-107, 151-199, 49-61-00Page 25 201-210, Config-2 Aug 01/05
CES
(c)
Overspeed Test After the end of the cool-down cycle, the ECB sends an overspeed test signal to the overspeed shutdown circuit. This causes the overspeed shutdown logic to start an APU shutdown by closing the fuel solenoid in the FCU. In this mode no protective shutdown is recorded by the ECB. After overspeed protection circuit is checked, the ECB attempts to modulate the fuel flow to keep the speed at 100 % rpm. If the ECB would be able to keep speed at 100 % rpm, it indicates that the fuel solenoid sticks in the open position.
(d)
APU Rolldown During APU rolldown the IGVs are set to the position corresponding to Bleed OFF (15 / 22 / 30 degrees) until APU speed has dropped down to 80 % rpm. Then the IGVs are set to 15 deg. An APU re-start is possible at any time if the speed is more than 7 % rpm. When the speed has decreased to 7 % rpm the ECB commands the inlet door to close. Seconds after the speed is below 7 % rpm the ECB removes power from itself. The APU can be restarted at any time during the rolldown when the APU MASTER SW is set to ON and the START P/B is pushed. But the APU accelerates only when the APU speed has fallen below 7% speed.
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