Lots of un- or ill-informed commentary here. Please note that the problem is resticted to specific pumps in particular installations. For those unfamiliar with the Boeing "quiet, dark cockpit" philosophy I offer the following explanation.
Prior to starting the engines, all the fuel pumps are turned "On" in all of the tanks, but the centre tank pumps are designed to develop higher pressure than main tank pumps. They therefore overpower or "override" the main tank pumps, giving them their name - "Jettison/Override Pumps". The centre tank thus empties into the engine fuel feed system first, providing wing bending load relief, while the main tanks remain full. Once the centre tank empties, the main tank pumps take over the fuel feed without crew intervention. The centre tank "Low Pressure" lights illuminate telling the crew that the centre tank is exhausted, whereupon they switch the centre tank pumps "Off". This action also extinguishes the 'Low Pressure" lights and any EICAS message.
The part of the pump that has sometimes been found overheated, is the mechanical part which actually moves the fuel, rather than the electrical motor that drives it. The pump is both lubricated and cooled by fuel and is designed so that some residual fuel is retained in the priming section where the bearing is located. The pump is intended to be capable of running for up to 11 minutes without fuel being available at the priming inlet, and can in theory be left rotating for this long after the fuel is exhausted. Beyond that time, bearing failure might ensue and the pump is liable to overheat. The thermal fuses intended to protect against overheating should isolate the pump before excessive heat develops, but it seems that in some cases this didn't happen until after remote parts of the pump heated up to >1000 degrees Celsius - hence the Airworthiness Directive. Meanwhile the hot impeller housing contains fuel vapour and, in the event that the vapour happens to be mixed with air in the right proportions, this becomes a possible source of ignition. In the overheat cases identified so far, the fuel vapour seems fortunately to been too rich for this to happen, however this still may be the underlying cause of accidents such as TWA 800.
A possible reason for a pump to continue running after being switched off is a jammed pump relay. In this situation the crew would be unaware that anything untoward was happening, as the "Low Pressure" lights are wired through the cockpit switch, so that switch operation maintains the "quiet, dark cockpit" philosophy. A pump could therefore be running for considerably longer than 11 minutes without anyone being aware of it. I have encountered this condition on two occasions on the ground, once on a B737-200 and once on a B747-100 and I reported these findings to the FAA during the investigation into TWA800.
The AD requires that a minimum quantity of fuel remains in the centre tank at all times, to ensure that the Jettison/Override pumps remain immersed in fuel regardless of aircraft attitude in roll or pitch. Thus they remain cool even if they continue running after being switched off. The ultimate corrective action is yet to be determined, but will almost certainly require a change to the design of the fuel pumps and their control/indication circuits.
I hope that this heavily simplified explanation helps those of you unfamiliar with the affected fuel feed systems to better understand what the subject is all about.
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Through difficulties to the cinema
Last edited by Blacksheep; 10th December 2002 at 05:49.