B737-700 DISPLAYS FLT CHARACTERISTICS CONSISTENT WITH AN AFT CG AND EXCESSIVE GROSS WEIGHT. FLT CREW ALSO SUSPECTS FAULTY FLT GUIDANCE SYS.

Narrative

CLBING OUT OF FL370; ACFT FLASHED BUFFET ALERT WHILE IN VNAV MODE; AUTOPLT THEN DISENGAGED; MACH .63; AS 220. MANUALLY ACCELERATED ACFT TO NORMAL CLB. ACFT WOULD NOT ACCELERATE IN LEVEL FLT AT FL370 WITH N1 PWR. INITIATED A DSCNT TO RECOVER AIRSPD. CLBED NORMALLY TO FL400. UPON REACHING FL400; VNAV REENGAGED. SPD COMMAND WAS FOR .788; ACFT WOULD NOT SLOW BELOW .804. REPROGRAMMED TO .76; ACFT SLOWED; REPROGRAMMED TO 'ECON' AND ACFT RESPONDED NORMALLY. NORMAL DSCNT UNTIL BEGINNING TO SLOW FOR THE APCH. AS WE PUT OUT THE FLAPS AT 210K; ACFT BEGAN A SLIGHT PORPOISE MOTION AND CONTINUED AS WE SLOWED TO 170 AND FLAPS 15 DEGS. I DISENGAGED THE AUTOPLT AND HAND FLEW THE APCH. THE ACFT EXHIBITED AFT CTR OF GRAVITY TENDENCIES WITH SLOW CTL RESPONSE AND A LAG IN RESPONSE TO VERT CTL INPUTS. LANDED FLAPS 30 DEGS UNEVENTFUL. THE FO COMMENTED AFTER LNDG AND DISCUSSION ABOUT THIS THAT HE THOUGHT THE ROTATION WAS RAPID AND THINKING BACK THE ACFT ACTUALLY ROTATED ITSELF; BUT I KIND OF WROTE IT OFF TO 'ONE OF THOSE THINGS.' CTR OF GRAVITY WAS GIVEN TO US AS 24.7% AND TKOF WT WAS 128000 LBS. THINKING BACK; CTR OF GRAVITY COULD HAVE BEEN A BIG FACTOR IN THE ACFT NOT ACCELERATING AT 370. BETTER ATTENTIVENESS TO ACFT CONDITION IN THE AUTOFLT MODE. BE MORE ALERT TO LOADING CONDITIONS AND BE MORE SUSPECT OF THINGS THAT SOMETIMES ARE LOGGED UP TO 'ONE OF THOSE THINGS.'