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 Digital Electronic Control Unit is called several names; an Electronic Control Unit (ECU), an Electronic Control Box (ECB), or a Full Authority Digital Engine Controller (FADEC).

So what is the correct nomenclature? Different aircraft manufacturers refer to the Digital Electronic Controller by different names. Because the aircraft and APUs are different, the controlling parameters are also different. However, the digital controllers discussed here all use the same basic processor that provides safe, trouble-free control for the engine. For purposes of this article, the Digital Electronic Controller will be referred to as the ECU.

Please note, this article contains hints and suggestions. Always refer to the applicable maintenance manual before performing any maintenance tasks.

There are a number of APUs that are controlled by this style of ECU, to name a few: Citation X (36-150CX), Falcon 2000 (36-150F2M), the IAI Galaxy (36-150IAI), Fokker 70 (36-150R), Fokker 100 (36-150RR), Bombardier CL601RJ (36-150RJ), MD80 (36-280D) and MD90 (131-9D).


The ECU is generally mounted near the rear on the aircraft in an electrical bay. For example, the ECU for the Citation X is located in the forward section of the aft electrical bay. On the Falcon 2000, the ECU is located just forward of the tail-cone, adjacent to the flight recorders.

The ECU controls all normal aspects of the APU operation during acceleration, on-speed, and shutdown and provides emergency shutdown of the APU in the event that critical parameters are exceeded. In many installations, the ECU communicates the APU status to the aircraft where APU parameters, as well as fault data, can be viewed from the flight deck.

There are two buttons on the ECU that are typically labeled "FAULT" and "MAINT," or "SELECT" and "ENTER" (The nomenclature depends on the specific installation) and an eight character digital display. APU data, shutdown and fault histories are stored in a non-volatile memory comprised of five to seven multi-level matrixes.

Once energized, the ECU will perform a "Pre-Start" BITE (Built-in Test Equipment) check and then waits for further commands. If the operator is starting the APU, "START" is selected and the ECU will take over and control all aspects of the APU's operation. The ECU not only performs the pre-start BITE test that checks its own circuitry and the APU electrical components, but continues to monitor APU parameters and ECU circuits. If a fault is found, the ECU will log it into memory and if the fault is critical, the ECU will initiate a protective shutdown.

Faults and protective shutdowns are stored in five to seven histories: Present plus History-1, History-2, etc. This allows the technician to interrogate fault histories and shutdown logic.

One area of particular importance to the technician is that the ECU also references the maintenance manual error codes. This allows the technician to troubleshoot the faulted condition through an extensive array of coded maintenance actions, eliminating the "shotgun" approach to troubleshooting and reducing the replacement of operational hardware. It also saves time in troubleshooting — Why change the fuel control unit when the ECU tells you the APU fuel solenoid shutoff valve is faulted?

On the Analog ECUs (used with the 36-100 and some 36-150 APUs), a low oil pressure switch that fails to close on shutdown will allow the APU to accelerate to 95 percent, plus 10 seconds, before a protective shutdown is initiated. The digital ECU, on the other hand, will not allow the starter to be engaged; interrogation will exhibit LOP SW, the applicable error code and maintenance task.

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.

On the Citation X, one can change the APU Data and erase faults while in DISPLAY mode (only the battery switch "ON") and the ECU will not save the changes. The ECU must be powered on by the APU Master switch before updating can take place. This should be remembered if trying to change data and the ECU is in DISPLAY mode.

Remember, isolate the error code(s) and refer to the applicable manual for fault error codes and maintenance tasks. Honeywell Aerospace Academy will soon complete an extensive electronic simulation for the ECU installed in the Cessna Citation X. The training simulation, although designed to be instructor led, will include a comprehensive instruction text box to help lead the student through the course.

Remember, take the time to learn and understand the ECU — being able to use the unit correctly will help to isolate faults and reduce operating costs.