CAPT OF MD83 DISCOVERS CALCULATED WT OF ACFT IS LESS THAN ACTUAL AS A RESULT OF UNDERSPD WARNINGS GENERATED BY THE ACFT'S FMS PERFORMANCE COMPUTER.

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THIS IS YET ANOTHER IN A SERIES OF OVERWT EVENTS. THE USE OF THE ALLOWABLE YET FICTITIOUS PAX AND BAGGAGE WTS THAT THE FAA ALLOWS THE ACR TO USE ARE ONLY HALF THE WT THAT THE FAA'S OWN EMPIRICAL DATA HAS SHOWN TO BE TRUE. THIS OUTBOUND FLT WAS OVERWT BY APPROX 6000 LBS. THIS WAS DERIVED FROM THE 'SPD LOW' AT V2 +20 INDICATION. THEN AT INITIAL LEVELOFF 'ALPHA SPD' AT THE CLEAN 'MIN-MAN' BUG AS DETERMINED FROM THE TPS. TO EXTINGUISH THE ALPHA WARNING THE SPD BUG HAD TO BE MOVED TO 255 KTS. THUS BY INTERPOLATION; USING THE SPD CARDS GAVE A WT OF 154000 LBS INSTEAD OF 147800 LBS AT THAT POINT IN THE FLT. ON LNDG; ALPHA WAS SHOWN TO BE 233 KTS GIVING A WT OF 128000 LBS VERSUS OUR 'PAPER WT; OF 120000 LBS.' ON OUR RETURN FLT THE NEXT DAY; DIFFERENT ACFT; WE HAD SIMILAR TKOF INDICATIONS. BUT FOR LNDG; OUR ALPHA SPD SHOWED THAT WE ACTUALLY WEIGHED AND LANDED OVERWT AT APPROX 136000 LBS WHEN OUR PAPER WT HAD US AT 128000 LBS; AN ERROR OF NEARLY 8000 LBS. OBSERVED LNDG DISTANCE PUT US AT M6 ON RWY 9R INSTEAD OF THE MORE TYPICAL M5 TURNOFF. I WROTE UP IN THE LOGBOOK A REQUEST FOR AN OVERWT INSPECTION. I RESPECTFULLY REQUEST THAT THE FLT DEPT PUT OUT A FORMAL REQUEST OF ALL PLTS OF OUR ACR THAT ALL 'SPD LOW' AND 'ALPHA SPD' INDICATIONS BE DOCUMENTED AND THAT AN APPROVED USE OF THE SPD BUG TO DETERMINE ACTUAL ACFT WT; BASED ON THE OBSERVATION OF THESE 2 REAL TIME EMPIRICAL NUMBERS; BE CREATED. IF THE USE OF THE FAA'S N8300.112 RPT ARE VALID; THEN THERE SHOULD BE NO CASES OF OVERWT ACFT. MY OWN RANDOM SAMPLING HAS SHOWN THAT THE OPPOSITE WILL BE TRUE. DISORGANIZED ACCOUNTING METHODS AND THE DENIAL THAT THE FLYING PUBLIC IS LARGER AND CARRIES MORE ON TO OUR PLANES PUTS ALL OF US AND OUR ACR AT RISK. DO WE NEED AN ENG FAILURE ON ONE OF THESE FLTS TO DO A 'COST BENEFIT ANALYSIS' OR CAN WE; AS PROACTIVE PLTS AND SAFETY MGRS; DO SOMETHING IN ADVANCE? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CLARIFIED THAT THE MEANS BY WHICH HE DETERMINED THE ACTUAL WT OF THE ACFT -- AS COMPARED TO THE WTS PROVIDED BY THE COMPANY IN COMPLIANCE WITH FAA APPROVED PROCS -- WAS THE ACFT FLT DATA COMPUTER SYS WHICH ADVISES WHEN SELECTED AIRSPDS DO NOT COMPLY WITH ANGLE OF ATTACK REQUIREMENTS FOR THE CURRENT FLAP SETTING. BY DETERMINING THE SPD AT WHICH THE ANGLE OF ATTACK REQUIREMENTS WERE SATISFIED; THE ACTUAL WT OF THE ACFT COULD BE DETERMINED AND COMPARED TO THE SAME COMPUTATION BASED ON THE FINAL LOAD PLANNING WTS ISSUED BY THE COMPANY. RPTR STRESSED STRONGLY THAT HE FELT THE COMPANY IS USING FAA APPROVED METHODS OF DETERMINING ACFT WT THROUGH CANNED ASSUMPTIONS REGARDING THE WT OF PAX AND BAGGAGE; WHILE FULLY AWARE THE ACTUAL WTS ARE SIGNIFICANTLY HIGHER. THE RPTR ALLEGES; WHEN MULTIPLIED BY A COUPLE OF HUNDRED PAX; THE RESULTS CAN BE A SIGNIFICANT DISCREPANCY BTWN PLANNED AND ACTUAL WT; ENOUGH TO ALTER THE REQUIRED AIRSPD TO MEET PERFORMANCE SPECS FOR ALL PHASES OF FLT.