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Thread: PFD CB pulled in flight

  1. #1
    Senior Member Andy's Avatar
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    Default PFD CB pulled in flight

    The FX-3 with Exec Glass panel powers the GDU 465 from Main Bus and IBBS. Either should be sufficient to keep the display fully operational.

    Today, in an inflight redundancy management test, I pulled the PFD circuit breaker. The GDU should have remained powered by IBBS and nothing should have changed. Instead the GDU shutdown and stayed shut down until the PFD circuit breaker was reset. This is not the expected system response. The AFCS kept AP engaged in LVL mode. The only other mode that was selectable was ALT.

    I plan to do more testing and will report what I find. My GDU powers fine on IBBS when on the ground so this does not appear to be a power wiring issue.
    Last edited by Andy; 10-07-2020 at 09:21 AM.

  2. #2
    Senior Member Andy's Avatar
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    Default Re: PFD CB pulled in flight

    The "anomaly" was easily reproduced on the ground with no need to run the engine. The sequence in which power is applied or interrupted doesn't seem to make any difference. Whether the PFD/GDU is operational or blanked is described by this table:


    PFD CB IBBS Master Display
    In On Off Yes
    In On On Yes
    In Off On Yes
    Out On Off Yes
    Out Off On No
    Out On On No


    According to the CubCrafters G3X schematic the PFD circuit breaker connects Main Bus to GDU 465 Aircraft Power 1 (pin 32) and has no other connection. I do not yet understand why the GDU reacts differently for loss of Aircraft Power 1 as a result of PFD CB pull than it does for loss of Aircraft Power 1 as a result of turning Master off.

  3. #3
    Senior Member Andy's Avatar
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    Default Re: PFD CB pulled in flight

    It took another trip to the hangar and a review of the IBBS documentation but I now understand what is happening.

    IBBS documentation states:

    "Pin 5 provides charging and bus voltage sensing for the IBBS unit and must be connected to an aircraft power bus. When the voltage on pin 5 falls below about 11 volts the system automatically transfers the load current on the output pins to the internal back-up battery source."

    This means the IBBS output power is not continuously available to GDU 465 pin 31. It only becomes available when the charging input to IBBS drops below 11 volts. This explains all the test observations.

    What does this mean:

    1. IBBS will provide backup power to the G3X system if main bus voltage drops below 11 volts.

    2. IBBS will not provide backup power to the G3X system if the PFD circuit breaker opens or if the wire between the circuit breaker and the GDU 465 breaks.

    For failure case 2 the display can be recovered by pulling the IBBS breaker.
    Last edited by Andy; 10-08-2020 at 10:36 AM.

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