Fault Message:

APU UNDERSPEED

Fault Code:

4960137APU

Associated CAS:

APU SHUTDOWN
(Advisory)

The Full-Authority Digital Engine-Controller (FADEC) commanded an automatic Auxiliary Power Unit (APU) shutdown when both conditions that follow occur:

• The APU is not in essential mode
• The APU decreased to 20% below the set point for 10 seconds

• Combustor
• Ignition Exciter
• Fuel Control Torque Motor
• Fuel Solenoid Valve (V2)
• Inlet Pressure Sensor (MT1)
• Inlet Temperature Sensor (MT3)
• Exhaust Gas Temperature Sensor
• Impending Bypass Valve and Switch (S4)
• Inlet-Duct
• Igniter Plugs
• Igniter Plug Leads
• Fuel Control Unit (FCU) (A3)
• Fuel Flow-Divider Valve
• Auxiliary Power Unit (APU)
• Associated Wiring

Advisory Wire/Service Bulletin:

• Honeywell Service Bulletin - SIL-APU-106 - Implementation of a Combustor Wash Program
• Honeywell Service Bulletin - SIL-APU-80 - Uncommanded shutdowns of APU

Forum Articles/Infoservice/Newsletter: None

1. On the CAIMS PMAT, get access to the active faults:
   Select the CAIMS message:4960118APU - FADEC CONTROLLER4960149APU - OPEN CKT - EGT SENSOR/CKT 1 WIRING4960150APU - SHORT - EGT SENSOR/CIRCUIT 1 WIRING4960154APU - OPEN CKT - EGT SENSOR/CKT 2 WIRING4960155APU - SHORT - EGT SENSOR/CIRCUIT 2 WIRING4960164APU - SHORT CKT-FCU [SHUT OFF SOL]/WIRING4960165APU - OPEN CKT-FCU [SHUT OFF SOL]/WIRING4960174APU - OPEN CKT- IMPDG BYPASS VLV & SW/WRG4960175APU - APU FUEL FILTER IMPENDING BYPASS4960176APU - FCU [TRQ MOT SINGLE COIL OPEN CKT]4960177APU - OPEN CKT - FCU/FCU TORQUE MOTOR WRG4960179APU - SHORT - FCU/FCU TORQUE MOTOR WRG4960181APU - SHORT - INLET PRESS SENSOR/EXC WRG4960182APU - OPEN CKT-INLET PRESS SENSOR/EXC WRG4960189APU - INLET PRESSURE SENSOR [SIG OUT HI]4960190APU - INLET PRESSURE SENSOR [SIG OUT LO]4960191APU - OPEN CKT- INLET PRESSURE SENSOR/WRG4960192APU - SHORT- INLET PRESS SNSR/POS SIG WRG4960193APU - GROUND SHORT- INLET TEMP SENSOR/WRG4960194APU - INLET TEMP SENSOR [SIGNAL OUT HIGH]4960195APU - INLET TEMP SENSOR [SIGNAL OUT LOW]4960196APU - OPEN CKT - INLET TEMP SENSOR/WIRING4960197APU - SIG SHORT- INLET TEMP SENSOR/WIRING4960198APU - CH A Built-In Test (BIT) SHORT-IGNITION EXCITER/WRG4960199APU - CH B Built-In Test (BIT) SHORT-IGNITION EXCITER/WRG4960200APU - SHORT - IGNITION EXCITER/WIRING4960201APU - OPEN CKT - IGNITION EXCITER/WIRING4960204APU - CH A - IGN PLUG/LEAD/EXCITER/WIRING4960205APU - CH B - IGN PLUG/LEAD/EXCITER/WIRING4960206APU - OPN/SHT CKT CH A&B- IGN EXCITER/WRG4960222APU - SHORT - FUEL SOLENOID VALVE/WIRING4960223APU - OPEN CKT-FUEL SOLENOID VALVE/WIRING

   1. If any of the following active faults show on the CAIMS PMAT, troubleshoot them first.
   2. If any of the following active faults does not show on the CAIMS PMAT, continue with next step.

      1. With any of the above messages APU SHUTDOWN (Advisory) shows on the EICAS primary display, clean the combustor, go to step 2.
      2. With any of the above messages APU SHUTDOWN (Advisory) does not show on the EICAS primary display, examine the engine branched wiring-harness for bare wiring, kinks, worn areas and other damage (WM 49-12).

         1. If damaged or worn, replace engine branched wiring-harness and do close out.
         2. If APU SHUTDOWN (Advisory) still shows on the EICAS primary display, on the APU control panel, set the APU switch to RUN and continue with next step.
            NOTE: The FADEC will cause the inlet door to open to the fully-open position.

2. Put the insertion tube of the optic fiber borescope through the inlet door.
3. Move the borescope insertion tube, as necessary, to examine the inlet-duct forward section for blockage.

   1. If the inlet-duct forward section has blockage, remove the inlet-duct forward section and remove the blockage and do close out.
   2. If the inlet-duct forward section has no blockage, continue with next step.

4. APU SHUTDOWN (Advisory) still shows on the EICAS primary display, replace fuel control unit.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

5. Replace fuel solenoid valve.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

6. Replace flow-divider valve.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

7. Replace igniter plugs.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

8. Replace igniter plug leads.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

9. Replace APU.
10. Do close out.