Singapore B772 near Singapore on Apr 16th 2014, fuel discrepancy
Last Update: June 18, 2018 / 11:35:10 GMT/Zulu time
Singapore's AAIB (SIAAIB) released their final report concluding the probable causes of the incident were:
- The B777-200ER aircraft was overfuelled by about 41 tonnes before its departure from Singapore. The increasing discrepancy between calculated fuel quantity remaining and the measured fuel quantity remaining on the aircraft prompted the flight crew to return to Singapore.
- The cause of the overfuel situation was that the B777-200ER aircraft was erroneously recognised as a B777-200 version by the FQIS owing to a fault in the Program Switch Module (PSM). This had resulted in the eight sensors within the mid-section of the centre fuel tank of the B777-200ER not being computed by the aircraft’s FQIS and caused an under-reading of the fuel quantity. The nature of the fault in the PSM could not be established.
- The discrepancy between actual fuel quantity uplifted to the aircraft by the refuelling dispenser and the fuel quantity indication on the aircraft prompted the maintenance crew to perform manual fuel quantity check using the magnasticks. The magnastick check performed by the maintenance crew did not discover the overfuel situation. It was likely that the maintenance crew did not perform the magnastick check correctly.
- The Certiying Technican (CT) and Lead Technician (LT) underwent training on refuelling operation which included theory on magnastick check. However, there were no practical session on magnastick check during their training on refuelling operation. In addition, magnastick check was not a commonly performed task in the course of their work.
- There was no regulatory requirement on duplicate inspection, performed by qualified personnel, for magnastick check. Neither the Aircraft Maintenance Service Provider (AMSP) nor the airline operator concerned had such a requirement.
- Apart from the abnormally high fuel uplift receipt from the Refuelling Dispenser Operator (RDO) which prompted the magnastick check, there was no indicator to provide an alert when incorrect aircraft model was referenced. It would be desirable to have an aircraft safety system that could detect or alert the flight crew or maintenance crew when there was a mismatch between the aircraft model referenced by the FQIS and the actual aircraft model.
The SIAAIB reported:
The aircraft was installed with a program switch module (PSM) consisting of pin switches that could be preset. The PSM pin switch pattern for the Boeing 777-200ER was different from that for the Boeing 777-200 and allowed the FQPU to accept the inputs from the eight additional sensors on the Boeing 777-200ER centre fuel tank. In other words, the PSM made the FQPU “know” which aircraft model it was dealing with so that the FQPU would get inputs from the correct sensors and compute the fuel quantity accordingly.
A review of the FQPU data by the aircraft manufacturer after the incident showed that the FQPU was operating in the Boeing 777-200 mode, instead of the Boeing 777-200ER mode. Thus, the inputs from the eight extra centre fuel tank sensors on the Boeing 777-200ER model were not taken into account and the FQIS was under-reading the fuel quantity, registering only 86 tonnes when the actual fuel quantity was some 127 tonnes.
However, when the PSM was inspected after the incident, it was found that the PSM had the correct pin switch pattern for a Boeing B777-200ER. The FQIS had also been operating normally prior to this incident flight and there was no recent maintenance work done to the FQIS or PSM.
The SIAAIB reported that despite observed more wear than usual the PSM did not show any electrical or mechanical fault when tested. No problem was also identified in the connection between PSM and FPQU.
The SIAAIB analysed:
The aircraft manufacturer had found that, while the PSM switches were correctly set for B777-200ER, they showed signs of wear from multiple movement, along with ink marks which it believed to be consistent with the use of a pen to move the switches. The degree of wear was also higher than on other PSMs that were returned to the aircraft manufacturer from service. In the opinion of the aircraft manufacturer, such wear had the potential to cause a fault in the FQPU and might cause it to default to the B777-200 mode. There had been no recent maintenance work done to the FQIS or PSM prior to the occurrence, but the investigation team understands that aircraft maintenance personnel may at times need to reset the PSM switch pattern in the course of troubleshooting the fuel quantity indication system. Such troubleshooting practice could probably explain the ink marks and wear. However, it cannot be established whether the wear had affected the FQPU and caused it to default to the B777-200 mode.
The CT and LT had limited practical experience in performing the magnastick check. The magnastick readings taken for the incident flight were likely not correct. As such, the CT, the LT or the flight crew were not alerted to the overfuel condition. If the overfuel condition had been detected through correct magnastick check, the overfuel condition, and perhaps later the erroneous FQIS fuel quantity reading, would have been detected.
This article is published under license from Avherald.com. © of text by Avherald.com.
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