B747-400
Confusion; Communication Breakdown; Situational Awareness; Troubleshooting
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Aircraft; Procedure; MEL; Human Factors
Prior to takeoff we found an improperly deferred maintenance item. The issue was resolved when the fuel pressure switch was replaced on the # 4 Forward Fuel Pump before takeoff. This same situation came up recently on a flight out of another location where I was part of the crew. The reason I am writing this is that there seems to be confusion regarding applying the MEL to this particular problem; leading to aircraft being flown with an improperly deferred item. The original deferral was done in September 2012 and the aircraft arrived with the deferred item. We were preparing to depart. The actual defect: when the Number-4 Fuel Boost Pump Switch is turned 'Off'; there was no indication of loss of fuel pressure; neither the Fuel Pressure light on the Overhead Panel switch; nor the Fuel Pump-4 Fwd EICAS message appear (the pressure light in the switch did illuminate properly during a light test). With this defect there is no way to confirm that the pump is actually producing proper output pressure; the system shows fuel pressure; even when the pump is 'Off'. The only deferral option available for this defect is to defer the pump 'inoperative'. That requires a significant unusable fuel penalty. But in this situation (and in the prior incident I mentioned earlier) the deferral used to allow the aircraft to continue was a deferral of the Fuel Pump Switch pressure light (MEL 28-42; Main Tank-4 Fuel Pump Pressure Light Inoperative). The only requirement for deferral of the light is that the remaining Pressure light in the associated tank must operate normally. In both situations the actual problem was a defective Pump Low Pressure Switch; replacing the switch fixed the problem. In fact in this instance; the action section of the Deferral on the Deferred List even had the comment: 'Suspect # 4 Low Pressure Switch'. I believe this is a serious issue. The improper deferral of the Low Pressure Switch could allow an aircraft with an inoperative [fuel] pump to be dispatched without the required unusable fuel supply. A pump failure en route that caused a drop in output pressure would not be noticed by the crew. The first step in the irregular procedure for a Fuel Pump Fwd/Aft EICAS caution message is to turn the pump switch 'Off' to remove power from the associated pump. Without the Low Pressure warning; a defective pump would remain powered. From my conversations with the Maintenance technicians and Maintenance Control during both of these instances; I believe there is a huge potential for confusion about the proper deferral. The Station AMTs insisted that this was a proper deferral; since that was the only deferral available related to low fuel pressure. When this same situation occurred at a second location; even the Maintenance supervisors were adamant that the deferral of the Switch Light was proper. In both cases they also were sure that the pump was operating properly because when the pump switch was turned 'On' the [fuel] pump icon on the EICAS synoptic display turned green. We called Maintenance Control on SatCom from overseas to discuss the issue since we couldn't resolve it with the station personnel. A couple of Maintenance controllers came on-line and after some discussion; they concurred that the deferral of the switch light wasn't correct; although they also felt that the information they had didn't directly cover this situation and could be subject to interpretation. My recommendation to prevent the chance of an improper deferral for this same defect would be to modify the MEL card for MEL 28-42 to include the requirement that the associated EICAS Fuel Pump () Fwd / Aft message appears when the pump is turned 'Off' and clears when the pump is turned 'On'. Although this isn't directly related to a broken pump switch pressure light; it would prevent this deferral from being used in error when there is a more serious underlying problem preventing the light from illuminating. Additionally; when I was talkingwith someone from Maintenance in Control about this; after we arrived at our destination; he looked up the history on the aircraft and showed me some messages from the B747-400 aircraft's Central Maintenance Computer (CMC) System from prior flights related to the # 4 Fuel Pump. There were messages from both preflight and shutdown phases that showed that the # 4 Fuel Pump was 'On' when commanded 'Off'. After each message was a code that he told me referenced a procedure for maintenance. He didn't look these up; but I wonder why the presence of these messages didn't result in a proper repair or deferral; perhaps the procedures used when these messages appear should be reviewed.
A First Officer describes why he recommends modifying MEL 28-42 to prevent an improper and unsafe deferral of Fuel Pump Switch pressure lights on B747-400 aircraft. When the #4 Fuel Boost Pump Switch was turned off; there was no indication of loss of fuel pressure; neither the Fuel Pressure light on the Overhead panel switch nor the Fuel Pump 4-Fwd EICAS message appeared. Fuel pump continued to run. Pressure switch was replaced.