Narrative:

During a return to service taxi of aircraft X; it was noticed that the #4 fuel boost pumps were not operating properly. The pumps would indicate normally if both were on; but would not indicate normally if operated independently. Extensive troubleshooting was performed at the gate and it was determined that the problem was not electrical. The problem was suspected to be in the fuel tank at the inlet to the #4 fwd and aft pressure switches. Aircraft returned to the maintenance gate where the fuel crew found fuel lines from the fwd and aft boost pumps to their respective press switches reversed. The lines were installed correctly and operations checked per MM.callback conversation with reporter revealed the following information: reporter stated the problem was noted while they were taxiing the B747-400. Mechanic was individually verifying the #4 fwd and aft fuel boost pump low pressure light would go out at the overhead panel after each pump was activated. While troubleshooting; maintenance initially thought the pump canon plugs were reversed. But that was not the case. Reporter stated the control portion in each of the fwd and aft boost pump overhead switches functioned properly; but the indication portion for the overhead pressure switch was coming from a different circuit. The fuel pressure line from the #4 fwd boost pump that should have been connected to the fwd pressure switch was actually connected to the #4 boost pump aft pressure switch. Reporter also stated this aircraft had been flying for some time in this configuration and the problem was traced to an international maintenance repair station that had replaced one of the #4 pump fuel pressure lines for being damaged. But a fuel pressure light check was not accomplished. With the fuel lines reversed and if an emergency occurred; the flight crew would not have a clear indication of what was actually going on with the #4 fuel boost system. Reporter stated if mechanic or pilot activates all the boost pumps; they may not notice a discrepancy of when each low pressure light goes out. The only way is to individually select a fuel pump and watch for the low pressure light to go out. He does not know what the flight crew procedures are for the fuel pump pressure check at departure.

Related reports:

Google
 

Original NASA ASRS Text

Title: DURING A RETURN TO SERVICE TAXI OF A B747-400; MECHANIC NOTICES THE #4 FUEL BOOST PUMPS WERE NOT OPERATING PROPERLY. FUEL CREW FOUND FUEL LINES FROM THE FWD AND AFT BOOST PUMPS TO PRESSURE SWITCHES REVERSED.

Narrative: DURING A RETURN TO SERVICE TAXI OF ACFT X; IT WAS NOTICED THAT THE #4 FUEL BOOST PUMPS WERE NOT OPERATING PROPERLY. THE PUMPS WOULD INDICATE NORMALLY IF BOTH WERE ON; BUT WOULD NOT INDICATE NORMALLY IF OPERATED INDEPENDENTLY. EXTENSIVE TROUBLESHOOTING WAS PERFORMED AT THE GATE AND IT WAS DETERMINED THAT THE PROBLEM WAS NOT ELECTRICAL. THE PROBLEM WAS SUSPECTED TO BE IN THE FUEL TANK AT THE INLET TO THE #4 FWD AND AFT PRESSURE SWITCHES. ACFT RETURNED TO THE MAINT GATE WHERE THE FUEL CREW FOUND FUEL LINES FROM THE FWD AND AFT BOOST PUMPS TO THEIR RESPECTIVE PRESS SWITCHES REVERSED. THE LINES WERE INSTALLED CORRECTLY AND OPS CHECKED PER MM.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE PROBLEM WAS NOTED WHILE THEY WERE TAXIING THE B747-400. MECHANIC WAS INDIVIDUALLY VERIFYING THE #4 FWD AND AFT FUEL BOOST PUMP LOW PRESSURE LIGHT WOULD GO OUT AT THE OVERHEAD PANEL AFTER EACH PUMP WAS ACTIVATED. WHILE TROUBLESHOOTING; MAINT INITIALLY THOUGHT THE PUMP CANON PLUGS WERE REVERSED. BUT THAT WAS NOT THE CASE. RPTR STATED THE CTL PORTION IN EACH OF THE FWD AND AFT BOOST PUMP OVERHEAD SWITCHES FUNCTIONED PROPERLY; BUT THE INDICATION PORTION FOR THE OVERHEAD PRESSURE SWITCH WAS COMING FROM A DIFFERENT CIRCUIT. THE FUEL PRESSURE LINE FROM THE #4 FWD BOOST PUMP THAT SHOULD HAVE BEEN CONNECTED TO THE FWD PRESSURE SWITCH WAS ACTUALLY CONNECTED TO THE #4 BOOST PUMP AFT PRESSURE SWITCH. RPTR ALSO STATED THIS ACFT HAD BEEN FLYING FOR SOME TIME IN THIS CONFIGURATION AND THE PROBLEM WAS TRACED TO AN INTERNATIONAL MAINT REPAIR STATION THAT HAD REPLACED ONE OF THE #4 PUMP FUEL PRESSURE LINES FOR BEING DAMAGED. BUT A FUEL PRESSURE LIGHT CHECK WAS NOT ACCOMPLISHED. WITH THE FUEL LINES REVERSED AND IF AN EMER OCCURRED; THE FLT CREW WOULD NOT HAVE A CLEAR INDICATION OF WHAT WAS ACTUALLY GOING ON WITH THE #4 FUEL BOOST SYSTEM. RPTR STATED IF MECHANIC OR PLT ACTIVATES ALL THE BOOST PUMPS; THEY MAY NOT NOTICE A DISCREPANCY OF WHEN EACH LOW PRESSURE LIGHT GOES OUT. THE ONLY WAY IS TO INDIVIDUALLY SELECT A FUEL PUMP AND WATCH FOR THE LOW PRESSURE LIGHT TO GO OUT. HE DOES NOT KNOW WHAT THE FLT CREW PROCS ARE FOR THE FUEL PUMP PRESSURE CHECK AT DEP.

Data retrieved from NASA's ASRS site as of May 2009 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.