What happened
On 25 February 2019, a Boeing 737-800, registration 9M-MXH, operating a scheduled service from Jakarta to Kuala Lumpur, declared an emergency due to a suspected fuel leak. While cruising at FL 380, the flight crew observed a fuel imbalance between the main tanks. Despite selecting the fuel pumps for tank No. 1 to the OFF position, the fuel quantity in that tank continued to decrease at an accelerated rate compared to tank No. 2.
As the "IMBAL" and "LOW" fuel alerts were triggered, the crew initiated the required non-normal checklists. Although the crew did not complete every specific verification step in the checklist, the rapid decrease in total fuel quantity led them to conclude a leak was present. The aircraft diverted to Sultan Thaha Syaifuddin Airport (DJB) in Jambi, Indonesia. Because the airport was not a standard diversion site for the operator, the crew lacked specific approach charts and had to rely on radar vectors and information provided by another aircraft on the frequency.
The investigation
AAIB Malaysia examined the aircraft' and the flight crew's actions following the landing at DJB. Upon engine shutdown, the fuel indication for tank No. 1 unexpectedly rose from 630 kg to 2380 kg, suggesting the issue was not a physical leak. Maintenance engineers from GMF AeroAsia performed a physical inspection of the wings and engines, finding no evidence of leakage or water contamination.
Subsequent technical inspections in Kuala Lumpur focused on the fuel quantity system. Investigators discovered that the Fuel Quantity Processor Unit (FQPU) had a damaged J1 connector and a faulty clock signal on the circuit card assembly. Replacing the FQPU resolved the intermittent erroneous data.
Findings
- The primary cause of the incident was a technical fault within the Fuel Quantity Processor Unit (FQPU), specifically a damaged connector and a faulty clock signal, which produced unreliable fuel quantity readings.
- The flight crew's decision to divert was driven by the misleading fuel imbalance and low-level alerts.
- The crew did not perform all verification steps in the fuel leak checklist, which might have altered their initial assessment.
- The lack of approach charts for the diversion airport required the crew to rely on external information from other aircraft.
Safety action
Following the investigation, the following actions were noted:
- The flight crew involved received coaching regarding checklist sequencing and diversion decision-making.
- Recommendations were made to monitor the FQPU for reliability issues with the manufacturer.
- It was suggested that training simulators include more high-workload scenarios involving multiple checklist conditions.
- The operator was advised to establish a single point of contact for communicating with captains during diversions to prevent the crew from being overwhelmed by multiple departmental inquiries.