FLT CREW OF MD83 DISCOVER WEIGHT AND BALANCE FIGURES PROVIDED BY LOAD PLANNING ARE CONSIDERABLY LESS THAN ACTUAL. ACFT GROSS WEIGHT WAS UNDERSTATED BY 6-7000 LBS AND CTR OF GRAVITY APPARENTLY FORWARD OF FORWARD LIMIT.

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AT TKOF WITH A NORMAL PULL ON THE YOKE; THE NOSE DID NOT RISE NORMALLY. AT 5000 FT WE LEVELED OFF AND ALPHA SPD WARNING WAS ON AT FLAP MINIMUM MANEUVER SPD. ALPHA WENT OUT AT 251 KTS SPD SELECTED. OUR FUEL WAS 27.7 AND INDICATED WEIGHT WAS 141.3. THE CORRESPONDING SPD ON THE REF CARDS SHOWED A WEIGHT AT ABOUT 148;000 LBS. A 6 TO 7000 LB DIFFERENCE. ON LNDG WE WERE TO LAND WITH 9000 LBS OF FUEL AND 122;000 LBS INDICATED. ALPHA WARNING WAS ON AT 0 FLAP MINIMUM MANEUVER SPD GIVING US WEIGHT OF ABOUT 128;000 LBS AND A SPEED OF 232 VS 227. AT FLARE WITH A NORMAL PULL; I HAD NO NORMAL AMOUNT OF ELEVATOR REMAINING. I LOOKED AT THE TRIM FOR OUR STABLE APCH AND FOUND 12 UNITS. LADIES AND GENTLEMAN; IT WAS CRUISE DAY IN MIAMI. THE FAA WEIGHTS WE ALREADY KNOW BY THEIR OWN ADMISSION ARE NOT THE REAL WEIGHTS OF LUGGAGE OR PEOPLE. THIS ACFT WAS OVER WEIGHT AND OUT OF TRIM. PEOPLE ARE FAT AND THEIR LUGGAGE IS HEAVIER. REALITY INSTEAD OF COST BENEFIT ANALYSIS MUST PREVAIL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CLARIFIED THAT THE MEANS BY WHICH HE DETERMINED THE ACTUAL WEIGHT OF THE ACFT - AS COMPARED TO THE WEIGHTS PROVIDED BY THE COMPANY IN COMPLIANCE WITH FAA APPROVED PROCS - WAS THE ACFT FLT DATA COMPUTER SYSTEM WHICH ADVISES WHEN SELECTED AIRSPEEDS 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 WEIGHT OF THE ACFT COULD BE DETERMINED AND COMPARED TO THE SAME COMPUTATION BASED ON THE FINAL LOAD PLANNING WEIGHTS ISSUED BY THE COMPANY. RPTR STRESSED STRONGLY THAT HE FELT THE COMPANY IS USING FAA APPROVED METHODS OF DETERMINING ACFT WEIGHT THROUGH CANNED ASSUMPTIONS REGARDING THE WEIGHT OF PAX AND BAGGAGE WHILE FULLY AWARE THE ACTUAL WEIGHTS ARE SIGNIFICANTLY HIGHER. WHEN MULTIPLIED BY A COUPLE OF HUNDRED PAX THE RESULTS CAN BE A SIGNIFICANT DISCREPANCY BETWEEN PLANNED AND ACTUAL WEIGHT. ENOUGH TO ALTER THE REQUIRED AIRSPEED TO MEET PERFORMANCE SPECIFICATIONS FOR ALL PHASES OF FLT AND; UNDER THE MOST ADVERSE CONDITIONS - SUCH AS AN ENG FAILURE AT CRITICAL TKOF SPEED ON A BALANCED LENGTH TKOF -- TO RESULT IN POTENTIAL DISASTER. HE FELT A DEFINITE HAZARD ON THIS LNDG IN SO FAR AS THE CTR OF GRAVITY WAS SO FAR FORWARD AS TO RESULT IN INADEQUATE ELEVATOR AUTHORITY FOR THE FLARE MANEUVER; EVEN WHEN THEY HAD TAKEN PAINS TO ADJUST THE APCH SPD TO REFLECT THE GREATER WEIGHT.