A B737-300 first officer reported flying the aircraft on an ETOPS flight with the fuel crossfeed val...

Narrative:

After 4:30 hours of flight; we had a 2;200 pound imbalance of fuel between the wing tanks. The center tank was empty for the flight and read zero always. We never suspected a fuel leak because the fuel flow was exactly even on each engine and the totalizer and calculated fuel levels were within 100 pounds of each other. Notwithstanding the possibility of a fuel leak; the aft engine fuel cross feed valve was deferred. How can this be a legal deferral for an ETOPS flight? It seems that this was an improper deferral and we should never have accepted it. The deferral of the crossfeed valve; I believe; is possibly the culprit in our unexplained fuel imbalance. We the flight crew are not given enough information concerning mels and the maintenance compliance with them to make safe; sound; and informed decisions before accepting the aircraft. Instead; I feel we were duped into flying a fully loaded aircraft all the way to hawaii.

Original NASA ASRS Text

Title: A B737-300 First Officer reported flying the aircraft on an ETOPS flight with the fuel crossfeed valve MEL'ed and subsequently developing an unexplained 2;200 LB fuel imbalance which they could not equalize.

Narrative: After 4:30 hours of flight; we had a 2;200 LB imbalance of fuel between the wing tanks. The center tank was empty for the flight and read zero always. We never suspected a fuel leak because the fuel flow was exactly even on each engine and the totalizer and calculated fuel levels were within 100 LBS of each other. Notwithstanding the possibility of a fuel leak; the aft engine fuel cross feed valve was deferred. How can this be a legal deferral for an ETOPS flight? It seems that this was an improper deferral and we should never have accepted it. The deferral of the crossfeed valve; I believe; is possibly the culprit in our unexplained fuel imbalance. We the flight crew are not given enough information concerning MELs and the maintenance compliance with them to make safe; sound; and informed decisions before accepting the aircraft. Instead; I feel we were duped into flying a fully loaded aircraft all the way to Hawaii.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	990091
Time
Date	201201
Local Time Of Day	0601-1200
Place
Locale Reference	ZHN.ARTCC
State Reference	HI
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Fuel System
Person 1
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 80 Flight Crew Total 15000 Flight Crew Type 2700
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural MEL

A B737-300 first officer reported flying the aircraft on an ETOPS flight with the fuel crossfeed valve MEL'ed and subsequently developing an unexplained 2;200 pound fuel imbalance which they could not equalize.