A DA20 FLC FLYING FREIGHT FROM GRR TO OKC DISCOVERED THAT THE LOAD MANIFEST SUPPLIED BY THE SHIPPER WAS IN ERROR RESULTING IN THEIR ACFT BEING FLOWN IN A HIGHLY OVERWT CONDITION.

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WE ARRIVED AT GRR AND THE FREIGHT WE WERE SCHEDULED TO DELIVER TO OKC ARRIVED AT XA00. UPON RECEIVING THE PAPERWORK; I NOTICED THAT THE ORIGINAL TOTAL WT OF 8828 LBS WAS CROSSED OUT AND REPLACED WITH 4400 LBS. AT THIS TIME I QUESTIONED THE TRUCK DRIVER ABOUT THE TOTAL WT. HE STATED THAT HE VERIFIED THE WT WITH THE LOADMASTER AND THE LOADMASTER STATED THAT THE 4400 LBS WAS THE CORRECT TOTAL WT. I THEN CALLED MY COMPANY DISPATCHER WHO IS A LICENSED PART 121 DISPATCHER AND STATED MY CONCERNS ABOUT THE WT. HE CALLED OUR CUSTOMER TO CONFIRM THE WT. THEY (CUSTOMER) CALLED THE SHIPPER AND AGAIN THEY CONFIRMED THE TOTAL WT AS 4400 LBS. ALSO TOLD ME THAT THEY (CUSTOMER) HAD BEEN RUNNING THE TRIP ALL WK WITH THE SAME LOAD OF PARTS AND THE SAME TOTAL WT OF 4400 LBS. I THEN TALKED TO THE LOADER (LOCATED IN GRAND RAPIDS) WHO STATED THAT THEY HAD BEEN LOADING THE SAME FREIGHT ALL WK LONG AND THE WTS WERE ALL THE SAME THAT BEING 5 BINS WITH A TOTAL WT OF 4400 LBS. WITH THIS IN MIND; I AGAIN TALKED TO DISPATCHER AND WAS RELEASED TO GO. WE COMPUTED A WT AND BAL AND ALL NECESSARY PERFORMANCE CHARTS AND ARRIVED AT A GROSS TKOF WT OF 27665 LBS WHICH WAS WELL WITHIN ALL OF THE PARAMETERS OF THE FLT. WHEN WE TAXIED OUT FOR TKOF THE ACFT FELT NORMAL. THE INITIAL TKOF ROLL FELT A BIT SLUGGISH; HOWEVER; OUR TIME TO 100 KTS OF 22 SECONDS WAS MET. AT V1; ALL SYS WERE NORMAL. AT VR; I ATTEMPTED TO ROTATE THE ACFT; HOWEVER; THE NOSE STAYED NO THE GND. I APPLIED FULL PWR; FULL BACK PRESSURE; AND STARTED RUNNING NOSE UP TRIM. THE ACFT LIFTED SLOWLY FROM THE RWY AND AS SOON AS THE WHEELS BROKE GND; I CALLED FOR GEAR UP. I CLBED TO 200 FT AGL; LEVELED OFF; AND ACCELERATED TO 170 KTS. I CALLED FOR FLAPS UP AND CONTINUED TO CLB TOWARDS FL280. UPON REACHING FL240; THE ACFT WAS CLBING AT LESS THAN 500 FPM. I ASKED THE CTLR TO LEVEL AT FL250; I SET CRUISE PWR AND CRUISED FOR APPROX 15 MINS WHEN I REALIZED THAT THERE WAS AN UNCERTIFIED FLOOR SCALE ABOARD THE ACFT. I RECOVERED A PART FROM ONE OF THE BINS AND WEIGHED IT IN AT 6.5 LBS. BY MULTIPLYING THE WT BY THE PIECE COUNT OF 1086 PLUS A TARE WT OF 1825 LBS; I HAD CALCULATED A TOTAL WT OF 8884 LBS. I RAN A NEW WT AND BAL WITH THIS NEW FIGURE AND CAME UP WITH A TKOF GROSS WT OF 32109 LBS WHICH WAS 3449 LBS OVER OUR MAX GROSS TKOF WT OF 28660 LBS. DUE TO THE LOWER CRUISING ALT; HEAVIER WT; AND UNFORESEEN WX AVOIDANCE; WE WERE UNABLE TO MAKE OUR DEST OF OKC. WE OPTED TO LAND IN TUL. BEING UNSURE OF THE ACTUAL WT; WE SUBTRACTED OUR FUEL BURN FROM 32109 LBS AND PLANNED FOR A MAX LNDG WT OF 27320 LBS. ALTHOUGH THE ACFT WAS EXTREMELY NOSE HVY AND REQUIRED MORE PWR THAN NORMAL WE HAD A VERY SMOOTH AND SUCCESSFUL LNDG. I INSISTED THAT THE FREIGHT BE REMOVED FROM THE ACFT AND REWEIGHED BY A CERTIFIED FACILITY. I STOOD BY THE SCALE AND WATCHED AS EACH BIN WAS WEIGHED IN. (BIN #1: 1699 LBS; BIN #2: 1711 LBS; BIN #3: 1700 LBS; BIN #4: 1725 LBS; AND BIN #5: 2015 LBS WITH A TOTAL WT OF 8850 LBS.) NOW KNOWING THE ACTUAL TOTAL WT; I REFUSED TO TAKE THE ACFT ANY FURTHER WITH FREIGHT ABOARD. AFTER ALL THE FREIGHT WAS UNLOADED; I WAS ASKED BY OUR DISPATCH AND CLRED BY OUR MAINT TO FLY THE ACFT TO YIP. THE ACFT WAS FLOWN TO YIP WITH NO FREIGHT ABOARD AND WITHOUT INCIDENT. I BELIEVE THAT THIS SIT OCCURS MORE OFTEN THAN WE WOULD LIKE TO BELIEVE; BUT PROBABLY NOT TO THIS EXTREME. IT IS NEARLY IMPOSSIBLE FOR CREW MEMBERS TO ACCURATELY CALCULATE THE WT OF FREIGHT BECAUSE OF THE LARGE QUANTITY OF PARTS THAT MAY BE SHIPPED AND THE INFO PROVIDED TO THEM IS NOT ALWAYS ACCURATE. FOR THE SAFETY OF THE CREW ABOARD; THE GENERAL PUBLIC; AND THE ACFT; I BELIEVE THAT THE FAA SHOULD REQUIRE ALL PART 135 FREIGHT OPERATORS TO ACCEPT ONLY FREIGHT THAT HAS BEEN WEIGHED AND CERTIFIED BY A THIRD PARTY OTHER THAN THE SHIPPER.