FUEL IMBALANCE
(Caution)

• Fuel Management and Quantity Gauging Computer (FMQGC) (A47)
• Associated Wiring

Advisory Wire/Service Bulletin:

• AW700-28-0496 - Fuel Quantity Indication Fluctuation

Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): None

NOTES:

• The most common cause for fuel qty indication issues is wiring and very rarely caused by the fuel computer. Do not replace the fuel computer (FMQGC) without having performed the complete troubleshooting plan. 
• Keep in mind that a single defective fuel probe can make the adjacent fuel probes seem out of range in CAIMS.
• As viewed in CAIMS, the Probe Status "GOOD PROBE" indicates the probe is communicating with FMQGC and the probe is reporting a capacitance within a nominal range (between full dry and full wet). The OMS may report the Probe Status as "GOOD PROBE", yet a dry capacitance check per the AMM may find a probe out of AMM limits. The only way to verify if a probe is reporting accurately is to do the Dry Capacitance check IAW AMM task 28−41−00−760−801 Capacitance Test of the Fuel Quantity Probes.
  

1. Sump the fuel tanks of any standing water.
2. Move the A/C on the ground to "slosh" the fuel in the tanks to dislodge possible water droplets contaminating fuel probes. Is the fault resolved?

   1. If YES, wait 24 hours and sump the fuel tanks of any standing water again. If fault remains cleared, do close out.
   2. If NO, continue with next step.

3. Perform a dry capacitance check of the fuel probes per AMM 28-41-00-760-801 and record values on the following data entry form.

   1. If capacitance values are good, go to step 8.
   2. If any probe capacitance value is out of range, continue with next step.

4. Remove the FMQGC. Inspect the connectors and pins on the unit and on the tray. Make sure none of the contacts are loose/unlocked, damaged or contaminated.
   NOTE: Applying small movement by hand to the harness behind the FMQGC while the unit is installed and aircraft is powered may help to identify a connection issue behind the FMQGC if the fuel qty values fluctuate while doing so.

   1. If any of the contacts are loose/unlocked, damaged or contaminated, repair as required and continue with next step.
   2. If inspection results are good, continue with next step.

5. Re-rack the FMQGC making sure it is fully and properly inserted into place and perform a system test. Is the fault still present?

   1. If NO, do close out.
   2. If YES, continue with next step.

6. Using System Schematic Manual (SSM 28-41-00), examine if the affected probes are all connected to the same excitation from the FMQGC.

   1. If the affected probes are connected to the same excitation line (i.e. EXCIT A, EXCIT B, EXCIT C...), continue with next step.
   2. If the affected probes are connected to different excitation lines or a single probe is out of range, go to step 8.

7. Perform a wiring check of the affected excitation wiring. Make sure to inspect every connector along the line from the FMQGC to the wing and/or belly fairing disconnects. Check that the coaxial connectors of the excitation are properly secured into place without any looseness.

   1. If defects are found, rectify as required and do close out.
   2. If no defects are found, continue with next step.
      NOTE: Excitation voltage coming from the FMQGC can be measured at various points as an indication of the health of the signal. Voltage should be between 5.3 VAC to 6 VAC. Any measurement below these values is an indication of a possible connection issue.

8. Familiarize yourself with the content of Fuel qty fluctuation AW700-28-0496. Check all connections to suspected fuel probes. Inspect the terminal lugs on the probes for damage, corrosion or contamination. Inspect the terminal lug screws for loose or under-torqued condition.

   1. If the terminal lug screws are loose or under-torqued, re-torque as required and do close out.
   2. If there is damage, corrosion or contamination, clean/repair/replace as required and do close out.
      NOTE: Refer to the Cleaning of the Fuel Management and Quantity Gauging System (FMQGS) Components (AMM TASK 28-41-00-110-801).
   3. If there is no damage, corrosion or contamination and the terminal lugs are torqued properly, continue with next step.

9. Remove the first fuel probe on the affected excitation line. Using Wiring Manual diagram Feed Tank, temporarily splice together the terminal lugs that are normally connected together on the same fuel probe terminal to ensure the excitation voltage carries on to the next fuel probe as they are "daisy chained". Verify the CAIMS capacitance values to see if probes that were previously showing out of range are still out of range.
   NOTE: If a low excitation voltage was previously measured (below 5.3 VAC), you may also measure the excitation voltage on the last fuel probe of the affected excitation line to see if removing a suspected probe has affected the result positively.

   1. If the capacitance value (or measured excitation voltage) of the remaining fuel probes downstream of the removed fuel probe are now showing within range, replace the removed faulty fuel probe and do close out.
   2. If there is no change to either the capacitance value or excitation voltage downstream of the removed fuel probe, continue with next step.

10. Remove the next fuel probe on the daisy chain and repeat the previous step until a defective fuel probe is identified.

    1. If a defective fuel probe is identified, replace the defective fuel probe and do close out.
    2. If no defective fuel probe is identified, reinstall all fuel probes per AMM and continue with next step.

11. Replace the FMQGC and do close out.
12. Do close out.