Mastering the Honeywell Two-Button Digital Engine APU Controller
A critical APU troubleshooting tool
By John Casey
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.
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