Gulfstream 550 Intermittent Fuel Quantity

June 2025

Duncan Aviation airframe technicians recently addressed recurring intermittent fuel quantity issues in two Gulfstream 550 aircraft. The left wing and total fuel indications would intermittently dash with an FQMS (Fuel Quantity Measuring System) Maintenance Required CAS message. 

These discrepancies could not be duplicated on the ground. 

FQMS Troubleshooting

After defueling, sumping the aircraft, and performing wing ventilation, our airframe technicians started troubleshooting. Below is an overview of the steps taken, but it is not an all-inclusive list.

• GSCP (Ground Service Control Panel) Fault Check: Stored faults were noted on both aircraft.
• Capacitance and Insulation Resistance Check: No discrepancies were identified, and capacitance values were within the specified limits of AMM.
• Lo-Z Circuit Inspection: The resistance of the Lo-Z wiring between the Fuel Quantity Signal Conditioner and the probes in the affected fuel cell were measured. Also, the torque of the probe terminals was checked.
• Hi-Z Signal Verification: The Hi-Z signal integrity was checked between FQMC (Fuel Quantity Management Computer) and probe 19.
• Hi-Z Wiring Examination: Hi-Z wiring was inspected for any core wiring shorted to the shielding.

FQMS Troubleshooting Results

On the first aircraft, the GSCP indicated a CELL L4 Shorted fault. While troubleshooting did not reveal any definitive findings, a closer inspection of the strain relief clamps at the probes within the faulted Cell L4 revealed chafing on the Lo-Z wire under the clamp, causing the shorted indication fault. This was corrected. As a precaution, the customer elected to replace the probes in this cell. The aircraft was returned to service with no recurrence of the issue.

The second aircraft displayed CELL L3 Open on the GSCP. Although troubleshooting did not reveal any definitive findings, the customer elected to replace the fuel probes within the affected cell and the complete left wing fuel quantity wiring harness.  Replacing the harness is fairly extensive and is normally only accomplished after all other troubleshooting steps have failed to show a definitive fault. The aircraft was returned to service, and no subsequent issues were reported after multiple flights.