A B737-300 FLT CREW EXPERIENCED ELEC STAB TRIM FAILURE AFTER TAKEOFF. THEY DECLARED AN EMERGENCY AND RETURNED TO THEIR DEPT ARPT.

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DURING BEFORE PUSH CHKLIST; DISCOVERED ELECTRIC TRIM WAS INOP. AS A RESULT; FLT DEPARTED LATE. MAINT ADVISED. MAINT TROUBLESHOT ISSUE AND SIGNED OFF THE ELECTRIC TRIM. PERFORMED NORMAL TKOF AND AROUND FLAP RETRACTION ALT; PF ADVISED THAT STABILIZER TRIM INOP. WE CONTINUED WITH NORMAL AFTER TKOF DUTIES. CREW DISCUSSED THAT AN INCREASE IN AIRSPD ABOVE 210 KIAS WAS A 'BAD IDEA' MAINTAINED 210 KIAS THROUGHOUT EVENT UNTIL LNDG PHASE. AS PLT MONITORING; I PERFORMED QRH AND HELPED WITH RADIO. PF PERFORMED ALL NOTIFICATIONS. MY ERROR OCCURRED DURING THE QRH AND NOT PROPERLY ATTAINING LNDG WT. I GOT DISTR WITH THE RADIO WHEN I OVERLOOKED THAT THE FLT DATA HAD DEFAULTED TO A LNDG WT NEARLY 5000 LBS LIGHTER THAN ACTUAL LNDG WT. I ONLY REALIZED THIS AFTER LNDG. ALTHOUGH THE OUTCOME OF THE LNDG WAS UNEVENTFUL; I NEVERTHELESS REALIZE THE SERIOUS NATURE OF MY MISTAKE BUT CAN ONLY SAY THAT I MESSED UP IN THE HEAT OF THE MOMENT. I ADVISED THE CAPT AFTER LNDG WHEN I REALIZED MISTAKE. I CAN ONLY SAY THAT ZZZ TWR AND DEP AND THE NUMEROUS CALLS TO US FROM ATC DISTR ME TO THE POINT THAT I MISSED CALCULATING OUR LNDG WT PROPERLY. I TAKE FULL RESPONSIBILITY. SUPPLEMENTAL INFO FROM ACN 804653: ON TKOF OUT OF ZZZ RWY X AT FLAPS UP; THE ELECTRIC STABILIZER TRIM FAILED. MANUAL TRIM WORKED. WE FOLLOWED THE QRH PROCS FOR STABILIZER TRIM FAILURE WHICH RESULTED IN AN UNEVENTFUL FLAPS 15 DEG LNDG. AN EMER WAS DECLARED WITH DEP; DISPATCH NOTIFIED THROUGH OPS; FLT ATTENDANTS; AND PAX WERE BRIEFED AND TOLD TO EXPECT A NORMAL LNDG. ON SHORT FINAL WITH FLAPS 15 DEGS; THE ELECTRIC TRIM STARTED TO WORK AGAIN. WHEN THE FLAPS WERE RETRACTED AFTER LNDG; THE ELECTRIC TRIM FAILED ONCE AGAIN.