To provide information regarding a fuel system imbalance anomaly.
Boeing has received several operator reports of an imbalance anomaly pertaining to the 767 Fuel System. All operator reports describe simultaneous fuel consumption from the center fuel tank and either the left or right main fuel tank, or simultaneous fuel consumption between the center fuel tank and both left and right main fuel tanks.
Boeing analysis has determined this simultaneous fuel tank consumption anomaly is a result of the center fuel tank pumps not producing adequate pressure to override the main tank fuel pumps. With exception of an empty center fuel tank, the occurrence of the anomaly cannot be accurately predicted for any given flight or any given flight condition.
Boeing is continuing investigation of this anomaly. Several modifications have been made to the center fuel tank override pumps as directed by Federal Aviation Administration (FAA) Airworthiness Directive (AD) 2001-15-08, Amendment 39-12342, effective September 4, 2001, which may be responsible for the imbalance characteristic. FAA AD 2001-15-08 supercedes FAA AD 97-19-15, which is no longer in effect. Therefore, flight crews should be advised of this simultaneous fuel consumption anomaly and provided with recommended operating instructions as temporary corrective action.
The recommended operating instructions contained in this Operations Manual Bulletin have demonstrated in service to both terminate the simultaneous fuel consumption characteristic as well as prevent re-occurrence of the characteristic for the remainder of flight.
There are two means for the flight crew to determine if fuel is simultaneously being consumed from the center fuel tank and either or both main fuel tanks.
These are:
If the flight crew observes FQIS quantity indicators simultaneously decreasing from the center tank and either the left or right main fuel tank without the FUEL CONFIG advisory-level EICAS alert displayed and FUEL CONFIG light illuminated, accomplish the FUEL BALANCING supplementary procedure.
Note: Accomplishing the FUEL BALANCING supplementary procedure allows the center tank fuel pump on the low main fuel tank side to resume override pressure. The left and right main fuel tanks will remain unbalanced at the amount at which the low main fuel tank pumps are selected OFF. When center tank fuel is depleted, the main tanks will begin balancing.
If the EICAS advisory-level FUEL CONFIG alert displays accompanied by the FUEL CONFIG light, and the flight crew observes FQIS quantity indicators simultaneously decreasing from the center tank and either the left or right main fuel tank, accomplish the FUEL CONFIGURATION non-normal checklist
.Note: Accomplishing the FUEL CONFIGURATION non-normal checklist allows the center tank fuel pump on the low main fuel tank side to resume override pressure. The left and right main fuel tanks will remain unbalanced at the amount at which the low main fuel tank pumps are selected OFF. When center tank fuel is depleted, the main tanks will begin balancing.
If the flight crew observes FQIS quantity indicators simultaneously decreasing from the center tank and both the left and right main fuel tanks without the LOW FUEL caution-level EICAS alert displayed, continue normal operation.
Note: Operator in-service reports have indicated a higher rate of fuel consumption from the center fuel tank being observed in comparison to the rate of fuel consumption from the left and right main fuel tanks. In addition, Boeing analysis has determined that the center fuel tank will empty before the main fuel tank quantities are reduced by an appreciable amount.
If the EICAS caution-level LOW FUEL alert displays accompanied by the FUEL CONFIG light, accomplish the LOW FUEL non-normal checklist.
Note: In-service reports have indicated a higher rate of fuel consumption from the center fuel tank in comparison to the rate of fuel consumption from the left and right main fuel tanks. In addition, subsequent Boeing analysis has determined that the center fuel tank will empty before the main fuel tank quantities are reduced by an appreciable amount. Therefore, the LOW FUEL alert should be treated with all due vigilance, and the LOW FUEL non-normal checklist accomplished.
If the flight crew suspects a fuel leak may exist, accomplish the ENGINE FUEL LEAK non-normal checklist.
Note: Accomplishing the ENGINE FUEL LEAK non-normal checklist is appropriate if the flight crew suspects or confirms a fuel leak exists. One or more of the following may be evidence of a fuel leak:
If fuel jettison operation is required, accomplish the FUEL JETTISON non-normal checklist.
Note: Accomplishing the FUEL JETTISON non-normal checklist will exclusively isolate the center tank fuel for fuel jettison operation.
As a reminder, Auxiliary Power Unit (APU) operation on the ground or in flight may result in depletion of fuel from the left main fuel tank. Therefore, flight crews should be reminded that a left and right main fuel tank unbalance condition might be a result of APU operation.
Master Minimum Equipment List (MMEL) Item 28-41-1 allows airplane dispatch with an FQIS quantity indicator inoperative if the associated provisos are complied with. Boeing recommends not dispatching an airplane under MMEL Item 28-41-1, if the airplane has been modified by Service Bulletins 767-28-0052 or 767-28-0062, or production equivalents, and center tank fuel is loaded. With a main fuel tank FQIS quantity indicator inoperative, flight crew confusion can result in determining whether the fuel system anomaly described above is occurring or if a suspected engine fuel leak condition exists.
Boeing further recommends if a main fuel tank FQIS quantity indicator failure occurs after dispatch on an airplane modified by Service Bulletin 767-28-0052 or 767-28-0062 with center tank fuel loaded, the flight be terminated by taxiing back to parking or landing at the nearest suitable airport.
This condition is under investigation. This bulletin will be revised to include Service Bulletin information when available.