DECAYED ACFT PERFORMANCE ON TKOF AND CLB; CRUISE ON A B737-700 WHEN THE FO INSERTS THE ZERO FUEL WT IN THE FMC VERSUS THE ACTUAL ACFT GROSS WT AT ZZZ.

Narrative

THE CAPT AND I WERE RECENTLY TRAINED TO FLY THE B737NG. AFTER RECEIVING THE LOAD MESSAGE DURING TAXI; I INADVERTENTLY ENTERED THE ZERO FUEL WT ON THE GROSS WT LINE IN THE FMC. SINCE THE FUEL LOAD WAS 38.5; THE DIFFERENCE WAS EXTREME; RESULTING IN V1-114; VR-115; AND V2-125 INSTEAD OF 138; 139; AND 145. FLAP RETRACT SOFT SPDS WERE ALSO INDICATING LOWER THAN THEY SHOULD HAVE BEEN ON DEP. I KNOW THAT THE B737-700 IS A REMARKABLE PERFORMER AND OUR PAX LOAD WAS LIGHT; SO THE V-SPDS DIDN'T REGISTER AS OUT OF LINE; ALTHOUGH IN RETROSPECT THEY WERE. I WAS IN THE HABIT OF VERIFYING THAT THE GROSS WT ON THE LOAD IS BELOW THE RELEASE WT; BUT WAS NOT IN THE HABIT OF VERIFYING THAT THE FMC GROSS WT MATCHES THE LOAD PAPERWORK GROSS WT; ALTHOUGH I AM NOW WHEN I ENTER THE DATA IN THE FMC; ALONG WITH VERIFYING THE GROSS WT. ROTATION FELT MUSHY; A LITTLE LIKE BEING TOO NOSE HVY; BUT NOT AS RESPONSIVE IN PITCH. THIS LED ME TO FLY AT HIGHER SPDS THAN BUGGED UNTIL CLEANED UP. I DISCUSSED WITH THE CAPT HOW I FELT THAT WE FELT A LITTLE MISS-TRIMMED ON TKOF. WE LEVELED OFF AT FL370 AND HAD CONTINUOUS LIGHT TURB; SO I LOOKED AT THE CRUISE PAGE AND IT SAID THAT FL410 WAS OPTIMUM WITH A HUGE FUEL SAVINGS; SO WE CLBED UP AND IT SMOOTHED OUT. CLB TO FL410 WAS NORMAL; BUT AFTER LEVELING; THE SPD BEGAN A SLOW DECAY TOWARD THE (ERRONEOUS) LOW SPD BUFFET YELLOW LINE. WE REQUESTED LOWER AND STARTED DSNDING JUST AS A SLIGHT BUFFET AT 7 KTS ABOVE THE (ERRONEOUS) YELLOW LINE. I SOON DISCOVERED MY ERROR AND THEN ENTERED THE CORRECT ZERO FUEL WT AND THE FLT CONTINUED NORMALLY. I NOW BRIEF NEARLY EVERYTHING ON THE TKOF PAGE; MENTIONING THE GROSS WT AND THAT IT IS BELOW RELEASE WT. THERE IS A GREAT POTENTIAL TO COMPLETELY CUT THE CAPT OUT OF THE LOOP AS FAR AS DATA ENTRY WHEN IT IS AN FO LEG AND THE CAPT IS BUSY TAXIING. I THINK ADDING THE GROSS WT TO THE BRIEFING HAS HELPED REMEDY THIS FOR ME; AS WELL AS JARRING MY MIND INTO THINKING ABOUT PERFORMANCE; SPDS AND RELEASE WT.