Narrative:

The before start normal procedures call for the captain, if the center tank is fueled, to turn on the center fuel pumps. There is a note that states, 'if the center tank quantity is low, consider making the takeoff with center pumps off to avoid a master caution light coming on during takeoff.' the climb normal procedure state to burn center tank fuel first. Regardless of when center tank fuel will be burned, the fuel xfeed remains closed. One would assume that the aircraft fuel system, once the fuel in the center tank is exhausted (assuming that the fuel xfeed valve is closed), is designed so that tank 1 and tank 2 supply fuel directly to the respective engine. I have found several, and I suspect that many, of our aircraft will, when the center tank fuel is exhausted, feel both engines out of either out of either tank 1 or tank 2. The usual scenario is when the center tank fuel is exhausted one of the center tank fuel pumps low pressure light fails to illuminate. In my experience, it is usually the left center tank fuel pump low pressure light that remains off with 0 center tank fuel quantity. With both center tank fuel pump switches on, the fuel annunciator and master caution lights are only armed for operation when both center tank low pressure lights are simultaneously illuminated. Therefore, if one of the center tank low pressure lights fails to illuminate when the fuel in the center tank is exhausted, there is no timely indication to alert the pilots. As I was new to the aircraft and somewhat unsure of myself, I was not alarmed by the first few instances of exhaustion of center tank fuel followed by only 1 wing tank feeding both engines. I would just open the fuel xfeed and balance the fuel. It has finally dawned on me that the potential for disaster is attendant to the problem of unwanted fuel xfer. Carried to the ultimate a crew could remain totally unaware that both engines are feeding out of 1 wing tank. Not only would the wing tank fuel maximum unbalance limits be vastly exceeded, but flameout of at least 1 engine would occur upon exhaustion of the fuel in the 1 wing tank that was feeding both engines. I have written up the unwanted fuel xfer as log book discrepancy items. Both center tank fuel pumps low pressure lights never simultaneously illuminated prior to exceeding the wing tank fuel maximum unbalanced limits. Both center tank fuel pump low pressure lights illuminated when approximately 800# out of balance. In each instance maintenance was unable to find anything that was obviously wrong. The discrepancies were cleared and the aircraft released for flight. I do not think that it is normal, with the fuel xfeed valve closed, for both engines to feed out of 1 wing tank when the center tank fuel is exhausted, but the center tank fuel boost pumps remain on. Somewhere there must be a malfunctioning valve in the fuel system. Callback conversation with reporter revealed the following: reporter agreed with analyst that with the xfeed closed it should not be possible for both engines to feed from 1 tank, but insists that it did occur on 2 aircraft that he flew. His company has been made aware of the possible anomaly and is following up on his report. He also gave his information to his safety rep.

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Original NASA ASRS Text

Title: REPORTER STATES THAT IN 2 ACFT HE FLEW ON SUCCESSIVE DAYS BOTH ENGINES FED FROM 1 TANK WHEN CENTER TANK WAS EXHAUSTED WITH CROSSFEED CLOSED.

Narrative: THE BEFORE START NORMAL PROCS CALL FOR THE CAPT, IF THE CENTER TANK IS FUELED, TO TURN ON THE CENTER FUEL PUMPS. THERE IS A NOTE THAT STATES, 'IF THE CENTER TANK QUANTITY IS LOW, CONSIDER MAKING THE TKOF WITH CENTER PUMPS OFF TO AVOID A MASTER CAUTION LIGHT COMING ON DURING TKOF.' THE CLB NORMAL PROC STATE TO BURN CENTER TANK FUEL FIRST. REGARDLESS OF WHEN CENTER TANK FUEL WILL BE BURNED, THE FUEL XFEED REMAINS CLOSED. ONE WOULD ASSUME THAT THE ACFT FUEL SYS, ONCE THE FUEL IN THE CENTER TANK IS EXHAUSTED (ASSUMING THAT THE FUEL XFEED VALVE IS CLOSED), IS DESIGNED SO THAT TANK 1 AND TANK 2 SUPPLY FUEL DIRECTLY TO THE RESPECTIVE ENG. I HAVE FOUND SEVERAL, AND I SUSPECT THAT MANY, OF OUR ACFT WILL, WHEN THE CENTER TANK FUEL IS EXHAUSTED, FEEL BOTH ENGS OUT OF EITHER OUT OF EITHER TANK 1 OR TANK 2. THE USUAL SCENARIO IS WHEN THE CENTER TANK FUEL IS EXHAUSTED ONE OF THE CENTER TANK FUEL PUMPS LOW PRESSURE LIGHT FAILS TO ILLUMINATE. IN MY EXPERIENCE, IT IS USUALLY THE LEFT CENTER TANK FUEL PUMP LOW PRESSURE LIGHT THAT REMAINS OFF WITH 0 CENTER TANK FUEL QUANTITY. WITH BOTH CENTER TANK FUEL PUMP SWITCHES ON, THE FUEL ANNUNCIATOR AND MASTER CAUTION LIGHTS ARE ONLY ARMED FOR OPERATION WHEN BOTH CENTER TANK LOW PRESSURE LIGHTS ARE SIMULTANEOUSLY ILLUMINATED. THEREFORE, IF ONE OF THE CENTER TANK LOW PRESSURE LIGHTS FAILS TO ILLUMINATE WHEN THE FUEL IN THE CENTER TANK IS EXHAUSTED, THERE IS NO TIMELY INDICATION TO ALERT THE PLTS. AS I WAS NEW TO THE ACFT AND SOMEWHAT UNSURE OF MYSELF, I WAS NOT ALARMED BY THE FIRST FEW INSTANCES OF EXHAUSTION OF CENTER TANK FUEL FOLLOWED BY ONLY 1 WING TANK FEEDING BOTH ENGS. I WOULD JUST OPEN THE FUEL XFEED AND BALANCE THE FUEL. IT HAS FINALLY DAWNED ON ME THAT THE POTENTIAL FOR DISASTER IS ATTENDANT TO THE PROB OF UNWANTED FUEL XFER. CARRIED TO THE ULTIMATE A CREW COULD REMAIN TOTALLY UNAWARE THAT BOTH ENGS ARE FEEDING OUT OF 1 WING TANK. NOT ONLY WOULD THE WING TANK FUEL MAX UNBALANCE LIMITS BE VASTLY EXCEEDED, BUT FLAMEOUT OF AT LEAST 1 ENG WOULD OCCUR UPON EXHAUSTION OF THE FUEL IN THE 1 WING TANK THAT WAS FEEDING BOTH ENGS. I HAVE WRITTEN UP THE UNWANTED FUEL XFER AS LOG BOOK DISCREPANCY ITEMS. BOTH CENTER TANK FUEL PUMPS LOW PRESSURE LIGHTS NEVER SIMULTANEOUSLY ILLUMINATED PRIOR TO EXCEEDING THE WING TANK FUEL MAX UNBALANCED LIMITS. BOTH CENTER TANK FUEL PUMP LOW PRESSURE LIGHTS ILLUMINATED WHEN APPROX 800# OUT OF BAL. IN EACH INSTANCE MAINT WAS UNABLE TO FIND ANYTHING THAT WAS OBVIOUSLY WRONG. THE DISCREPANCIES WERE CLRED AND THE ACFT RELEASED FOR FLT. I DO NOT THINK THAT IT IS NORMAL, WITH THE FUEL XFEED VALVE CLOSED, FOR BOTH ENGS TO FEED OUT OF 1 WING TANK WHEN THE CENTER TANK FUEL IS EXHAUSTED, BUT THE CENTER TANK FUEL BOOST PUMPS REMAIN ON. SOMEWHERE THERE MUST BE A MALFUNCTIONING VALVE IN THE FUEL SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR AGREED WITH ANALYST THAT WITH THE XFEED CLOSED IT SHOULD NOT BE POSSIBLE FOR BOTH ENGS TO FEED FROM 1 TANK, BUT INSISTS THAT IT DID OCCUR ON 2 ACFT THAT HE FLEW. HIS COMPANY HAS BEEN MADE AWARE OF THE POSSIBLE ANOMALY AND IS FOLLOWING UP ON HIS RPT. HE ALSO GAVE HIS INFO TO HIS SAFETY REP.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.