Mastering the Honeywell Two-Button Digital Engine APU Controller

Mastering the Honeywell Two-Button Digital Engine APU Controller A critical APU troubleshooting tool By John Casey April 2000 On a number of aircraft, a Digital Electronic Control Unit controls the Auxiliary Power Unit (APU). The...


On the Citation X, for example, the FAULTS processor can display a PRESENT fault record and up to five history records. A fault shutdown causes the present record to become HISTORY-1. If there was a previous HISTORY-1 fault stored in memory, it is moved to HISTORY-2, etc. Once all histories are full, an additional fault shutdown will be stored as PRESENT, and all prior histories are moved down, ending with the previous HISTORY-5 being removed from memory.

There are also preventative maintenance faults, or LRU faults stored within the FAULTS mode. For example, if the APU bleed load control valve (BLCV) electrically failed with an open circuit, or a broken wire in the harness, the APU would start and operate. However, it would not be able to provide a bleed load to the aircraft. Interrogation of the ECU on the Citation X would go something like this:

Image• Pressing the FAULT button will scroll down the modes until FAULTS is displayed.
• Press SELECT to enter the faults matrix and PRESENT will be displayed (this is the current, or present fault), pressing MAINT will enter the present faults processor.
• The next display will be SHUTDOWN. Press FAULT to scroll down to the shutdown mnemonic (or reason for shutdown), because there was not an automatic shutdown, NO SDN is displayed. • Pressing fault at this time will scroll down through to the LRUs display and pressing FAULT one more time will display the faulted component, BLCV T/M. This indicates an electrical fault within the BLCV torque motor, or its wiring. Before running off to change the bleed load control valve, check the error code and maintenance action(s).
• Press abort (one time), to return to the SHUTDOWN display, when SHUTDOWN is displayed, press MAINT and hold for 5 seconds, allowing entrance to the error code matrix.
• Press FAULT to scroll down through the error code(s); the ECU checks the LRUs for both open and shorted circuits. In this scenario, the faulted component is an open circuit in the bleed load control valve harness. We could see ERROR137 displayed.
• After referring to the applicable APU maintenance manual, we see that the Probable Cause is "BLCV T/M open circuit." The maintenance tasks and corrective action tell the technician to check the wiring for open circuit first, then checks the valve. There are currently 240 error codes listed, for the 36-150[CX], in the maintenance manual, referring to them will help isolate faults and reduce maintenance costs.

The ECU for the Falcon 2000 is a little different; upon entering the "SHUTDOWN" display, all of the faults and error codes will scroll automatically. All you have to do is watch and record the information (there may be a lot of information, writing it down will help).

During the operation of the APU, real time parameters are easily accessed through the ECU, giving the technician additional data. Information such as APU oil temperature, compressor discharge pressure, inlet temperature, exhaust gas temperature, APU speed in percent RPM, and even data like altitude and ambient temperature is available to the technician. From the top-level mode display (this is the display that exhibits the APU status), RUN, STANDBY, FAILURE, START. The technician can enter this mode and actually watch the temperatures and pressures change. Exit this mode by pressing both buttons at the same time (abort) and return the display to the top level.

The ECU also stores its own part number and software version, as well as the APU serial number, operating hours and start cycles. During APU or ECU replacement, the technician can reset the information on the APU. The ECU part number and software version obviously can not be changed. Following installations and maintenance tasks, the fault histories are often erased, Honeywell does not recommend clearing the fault data every time one accesses the ECU. This data can provide an important history of faults and occurrences; once cleared, the history is lost.

ImageMany technicians, not being familiar with the operation of the ECU, get "lost" in the matrixes and some may even become intimidated by the overwhelming amount of information available. To make matters worse, the technician may work on several different aircraft and ECU data acquisition can be a little different. Reference to the maintenance manual exhibits a matrix chart and directions that help technician navigate the bewildering maze of displays. The technician needs to become familiar with the operation of the ECU to interrogate it while troubleshooting, setting new data if the APU or ECU is changed and clearing fault data that is no longer needed. Unless the technician enters and erases data when asked ERASE?, or enters the SET DATA menu (the Fokker 70 and 100 ECUs will erase the APU data upon entering), they can interrogate the ECU without changing anything.

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