THE FE ON A B727 FORGOT TO TURN OFF THE XFEED VALVES AFTER XFERRING FUEL DURING CRUISE CAUSING A FUEL IMBALANCE. THE MISTAKE WAS DISCOVERED DURING POSTFLT WHEN THE CAPT NOTICED THE FUEL IMBALANCE AFTER CHKING THE QUANTITY IN EACH TANK. THE CAPT DID NOT NOTICE ANY ACFT TRIM DIFFICULTY WHEN MANUALLY FLYING THE ACFT DURING APCH AND LNDG.

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I WAS THE CAPT ON A B727 WHICH DEPARTED LAX FOR DEN WITH 46000 LBS OF FUEL. WE LANDED WITH 24000 LBS; AS PLANNED; BUT WITH ONE MAJOR PROB -- THE INDIVIDUAL FUEL TANK QUANTITIES READ #1 -- 11600 LBS; #2 -- 800 LBS; #3 -- 11600 LBS. THE XFEED VALVES HAD NEVER BEEN CLOSED. IN THE VICINITY OF ZION NAT'L PARK; THE SO ANNOUNCED 'XFEEDING COMPLETE' AND TURNED ON THE BOOST PUMPS IN TANKS #1 AND #3; WHICH I OBSERVED. AT THE TIME; I HAD BEEN POINTING OUT THE GEOGRAPHICAL SIGHTS TO THE PAX OVER THE PA; AS WELL AS PASSING ON TO MY CREW NUMEROUS DETAILS OF THE AREA. AS THE SO LATER RECALLED HE HAD DELAYED CLOSING THE XFEED VALVES BECAUSE #2 WAS JUST SLIGHTLY HIGH YET; AND HE WAS GOING TO 'PERFECTLY' BAL ALL 3 TANKS. HE WAS THEN DISTRACTED BY MY COMMENTARY; AND FORGOT TO COMPLETE THE PROCESS. THIS CONDITION WAS NOT NOTICED DURING SUBSEQUENT FUEL CHKS BECAUSE THE SO HAD A UNIQUE 'SHORTCUT' FOR TOTALING THE FUEL GAUGE READINGS (WHICH HE HAS SINCE ABANDONED) AND I WAS ONLY LOOKING BACK AT THE TOTALIZER FOR A QUICK CHK OF THE FUEL. THIS IMBALANCE WAS NOT NOTICED UNTIL AFTER BLOCK IN. THE FLT LANDED WITHOUT INCIDENT; AND INCREDIBLY TO ME; THE OUT OF LIMIT CTR OF GRAVITY PROB THAT WE ARE LED TO EXPECT WAS NOT NOTICEABLE TO ME AS I HAND FLEW THE ACFT THROUGH DSCNT AND LNDG. (THE CABIN WAS FULL.) THE SO HAS ENUMERATED HIS REMEDIES SEPARATELY; BUT FROM THE PERSPECTIVE OF THE L SEAT (CAPT) I OFFER 3 POINTS TO HEED: 1) TREAT THE COMPLETION OF FUEL XFEEDING WITH THE SAME SERIOUSNESS WE GIVE TO ALT LEVELOFFS. CUT OUT ANY DISTRACTIONS AT THAT TIME. 2) AS THE CAPT; NOTICE MORE THAN THE BOOST PUMPS ON; WITH THE LIGHTS OUT. LOOK AT THE XFEED VALVE POS. 3) DON'T LET DOWN YOUR GUARD BECAUSE THE PLT WITH WHOM YOU'RE FLYING IS ABOVE AVERAGE EXPERIENCE/TALENT OR YOU'VE FLOWN TOGETHER MANY TIMES.