Narrative:

I was the PF on a routine FAA part 121 flight from pit to mdw, on the first leg of a planned 2 leg day. During climb out, the captain mentioned he was going to xfeed in order to balance fuel between the wing tanks. As we were in clouds (1200 ft overcast in light rain at pit), my attention was focused primarily on flying, navigation, and watching for icing conditions. Shortly after leveloff, the captain indicated the xfeeding operation seemed to be proceeding slower than normal. At this time, the fuel balance was within limits (and there are alternate means of balancing the fuel supply). Approximately 50 mins into the flight, with the autoplt on, the captain concluded that he was performing the xfeeding procedure correctly, but the imbalance was increasing. He used the QRH checklist, which called for using differential thrust. This procedure did not help the situation, so we requested vectors direct to mdw (we were just beginning the STAR at this time), thereby declaring an emergency. The captain took over as PF, and I assumed PNF duties. An uneventful landing was made at mdw. WX was a minor factor. Although my attention was directed to flying duties due to the climb out conditions, it was predominately VFR on top. At that point when I was able to devote more attention to the problem, the captain had essentially already performed all the appropriate actions. There is a lack of guidance in the B737-200 manual regarding fuel imbalance limits. The manual lists several limits, but not what the consequences or appropriate actions are if those limits are exceeded. It does not address whether exceeding those limits constitutes a need for an immediate emergency landing, or just a minor maintenance adjustment. Also, the manual fails to mention that the underlying cause of the fuel imbalance may also cause the fuel in a particular tank to be unusable, effectively cutting the range, endurance, and fuel supply in half. This would compound the situation at a time when the main concern might be aerodynamic control due to a center of gravity imbalance.

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Original NASA ASRS Text

Title: AN ACR B737 FLC DISCOVERS THAT THEY ARE UNABLE TO CORRECT A FUEL IMBALANCE PROB. THE ARTCC RADAR CTLR CLRED THEM DIRECTLY TO THEIR DEST WHERE THEY LANDED SAFELY.

Narrative: I WAS THE PF ON A ROUTINE FAA PART 121 FLT FROM PIT TO MDW, ON THE FIRST LEG OF A PLANNED 2 LEG DAY. DURING CLBOUT, THE CAPT MENTIONED HE WAS GOING TO XFEED IN ORDER TO BAL FUEL BTWN THE WING TANKS. AS WE WERE IN CLOUDS (1200 FT OVCST IN LIGHT RAIN AT PIT), MY ATTN WAS FOCUSED PRIMARILY ON FLYING, NAV, AND WATCHING FOR ICING CONDITIONS. SHORTLY AFTER LEVELOFF, THE CAPT INDICATED THE XFEEDING OP SEEMED TO BE PROCEEDING SLOWER THAN NORMAL. AT THIS TIME, THE FUEL BAL WAS WITHIN LIMITS (AND THERE ARE ALTERNATE MEANS OF BALANCING THE FUEL SUPPLY). APPROX 50 MINS INTO THE FLT, WITH THE AUTOPLT ON, THE CAPT CONCLUDED THAT HE WAS PERFORMING THE XFEEDING PROC CORRECTLY, BUT THE IMBALANCE WAS INCREASING. HE USED THE QRH CHKLIST, WHICH CALLED FOR USING DIFFERENTIAL THRUST. THIS PROC DID NOT HELP THE SIT, SO WE REQUESTED VECTORS DIRECT TO MDW (WE WERE JUST BEGINNING THE STAR AT THIS TIME), THEREBY DECLARING AN EMER. THE CAPT TOOK OVER AS PF, AND I ASSUMED PNF DUTIES. AN UNEVENTFUL LNDG WAS MADE AT MDW. WX WAS A MINOR FACTOR. ALTHOUGH MY ATTN WAS DIRECTED TO FLYING DUTIES DUE TO THE CLBOUT CONDITIONS, IT WAS PREDOMINATELY VFR ON TOP. AT THAT POINT WHEN I WAS ABLE TO DEVOTE MORE ATTN TO THE PROB, THE CAPT HAD ESSENTIALLY ALREADY PERFORMED ALL THE APPROPRIATE ACTIONS. THERE IS A LACK OF GUIDANCE IN THE B737-200 MANUAL REGARDING FUEL IMBALANCE LIMITS. THE MANUAL LISTS SEVERAL LIMITS, BUT NOT WHAT THE CONSEQUENCES OR APPROPRIATE ACTIONS ARE IF THOSE LIMITS ARE EXCEEDED. IT DOES NOT ADDRESS WHETHER EXCEEDING THOSE LIMITS CONSTITUTES A NEED FOR AN IMMEDIATE EMER LNDG, OR JUST A MINOR MAINT ADJUSTMENT. ALSO, THE MANUAL FAILS TO MENTION THAT THE UNDERLYING CAUSE OF THE FUEL IMBALANCE MAY ALSO CAUSE THE FUEL IN A PARTICULAR TANK TO BE UNUSABLE, EFFECTIVELY CUTTING THE RANGE, ENDURANCE, AND FUEL SUPPLY IN HALF. THIS WOULD COMPOUND THE SIT AT A TIME WHEN THE MAIN CONCERN MIGHT BE AERODYNAMIC CTL DUE TO A CTR OF GRAVITY IMBALANCE.

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.