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APU Control System: Which Box Do I Send in for Repair?

April 2016

The Duncan Aviation /Garrett APU (Auxiliary Power Unit) system, an option in Dassault Falcon 20, Gulfstream Astra SPX / G100, SAAB 340, and Cessna 650 aircraft, interfaces with the aircraft’s electrical system and includes controls for operating, monitoring, and regulating the APU. Duncan Aviation’s Avionics/Instruments department has repair capabilities on the 3 major components for this system;

EM2095-(X) Control Head

EM2099-(X) Regulator Controller

DA650-184-1 Interface Computer

As seldom as these APU systems fail, when they do, it can be confusing to determine which box is causing the problem.

For example:

  • The fault codes displayed in the cockpit are generated in the interface computer, processed by the regulator controller and displayed on the control head, and are a clue to what may be causing the issue.
  • The signal for the Caution and Max annunciators on the control head is generated in the regulator controller, however are based on voltage values on the generator shunt ground.
  • The Speed and Temp displays on the control head are processed in the interface computer.
  • Flashing lights not flashing or flashing when they shouldn't in the control head may indicate an issue with the regulator controller, not the control head.
  • A control head with a “flickering” Caution or Max annunciator can indicate a bad solder joint in the regulator controller; whereas a steady “flashing” APU ON annunciator is a valid error signal indicating a fuel valve that has failed in transit.

As always, a good detailed squawk can be invaluable in narrowing down the cause of the problem. However, a good cell phone video of the squawk can be worth its weight in gold.

Below are several common and valid squawks for each of the three units listed above;


  • Displays blank, locked up, too dim, too bright, missing segments
  • No start or fails to come online unless buttons cycled multiple times
  • Dimming INOP
  • Annunciators INOP
  • No power from control head
  • Button sticks or will not latch
  • Attempts start on A/C power up


  • Over/under voltage
  • Out of parallel with main generators and cannot be adjusted via voltage adjust
  • Generator drops off and APU continues to run
  • Intermittent fail lamp
  • Fail light on (despite a valid ESU Ready signal from the generator)
  • Caution or Max annunciators flickering (erratic)
  • Fire test or Fire Detect INOP
  • Shutdown after xx minutes, when cold, when hot etc.
  • Will not display error codes on control head
  • Will not display bite codes on regulator controller
  • Generator won’t come online or trips off.
  • A flashing annunciator that is not flashing at a steady and constant rate


  • Intermittent shutdown with error code
  • Shutdown and failed to restart
  • Will not crank, no starter rotation
  • RPM or TEMP readings out of tolerance
  • Starter won’t disengage
  • Generator won’t come online
  • Intermittent to start, shuts down on its own
  • Shutdown after XX minutes, when cold, when hot etc.
  • Intermittent or random fault codes, after shutdown
  • Will not display error codes on Control Head
  • A code that doesn’t reflect a physical problem such as:
    • Generator overheat
    • Fire detect
    • Fire wall shutoff
    • Overspeed test
    • Emergency kill
    • Start contactor
    • Generator contactor
    • Bleed lead detect
    • Bleed mod valve

It is good to remember the squawks listed above are the ones most commonly associated with each unit. Occasionally you will run into the exception to the rule. If in doubt or under time constraints due to flight schedules, it is perfectly acceptable to send in two or all three of the boxes in order to isolate the squawk, make necessary repairs and function test and return any boxes that are found to be problem free.