CAPT MISUNDERSTANDS LENGTH OF RWY IN BKK. FULL LENGTH OF RWY IS NEVER USED FOR HIS B747-200 ACFT.

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I JUST FINISHED A 1 MONTH ASSIGNMENT FLYING OUT OF BANGKOK; THAILAND TO EITHER THE MIDDLE EAST OR JAPAN. MOST ARRS AND DEPS FROM BANGKOK WERE AT NIGHT. ALL USED RWY 21R (PRIMARY) OR RWY 21L (SECONDARY). ALL PERFORMANCE DATA USED RWY 21L WHICH IS SHOWN AS 11483 FT LONG; BUT RWY 21R; WHICH IS SHOWN AS 12139 FT LONG; WAS USED APPROX 75 PERCENT OF THE TIME FOR THE ACTUAL TKOF. REDUCED THRUST BASED ON ACTUAL TKOF GROSS WT WAS CALCULATED FOR ALL (AND USED FOR MOST) TKOFS. ON LEAVING BKK AS A PAX ON DEC/XX/96; I NOTICED FOR THE FIRST TIME WHILE LOOKING OUT THE WINDOW ON THE L SIDE OF THE ACFT THAT THERE IS A 10000 FT RWY MARKER AT THE BEGINNING OF THE NORMAL TKOF FROM RWY 21L. THUS; THERE IS AN APPARENT DISCREPANCY BTWN THE 11483 FT USED FOR PERFORMANCE IN THE EVENT OF A LAST MIN CHANGE IN CLRNC TO THE SECONDARY RWY AND THE ACTUAL USABLE LENGTH OF THIS RWY (21L). RECOMMENDATION: I RECOMMEND THAT COMMERCIAL CHARTS AND ALL OTHER ARPT AND PERFORMANCE DATA SYS FOR ALL ACFT OPERATING OUT OF BANGKOK; THAILAND BE CHKED AND VERIFIED FOR CONSISTENCY AND INTEGRITY OF DATA REGARDING THE ACTUAL USABLE LENGTH OF RWY 21L. A SHORTAGE OF APPROX 1200 FT IS AN APPARENT SAFETY HAZARD WHICH COULD HAVE SERIOUS CONSEQUENCES FOR HVY ACFT ACCEPTING A LAST MIN RWY CHANGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPT WAS SUBMITTED BY FO WHO WAS FRUSTRATED THAT CAPT WOULD NEVER USE FULL LENGTH OF RWY FOR TKOF. ALL PERFORMANCE DATA WAS BASED ON 11500 FT OF RWY BUT CAPT ONLY USED 10000 FT OF RWY. FO WAS TRYING TO GET CAPT TO TAXI TO END OF RWY BY USING TXWY 'X.' THE CAPT WOULD LOOK AT THE RWY AND TXWY MAP TO FIND THE TXWY MARKED 'X;' AND THEN STATE THAT THE 'X' ON THE TXWY MEANT THE TXWY WAS CLOSED. AS THEY WOULD APCH THE 'X' TXWY AND SEE A SIGN MARKED 'X;' THE CAPT WOULD STATE THAT THE 'X' ON THE SIGN MEANT THE TXWY WAS UNUSABLE. FO HAS SPOKEN TO HIS COMPANY OPS OFFICIALS AND FEELS THE SIT IS BETTER UNDERSTOOD SO OTHER FLCS WON'T MAKE THE SAME ERROR. THE FO HAS MADE IT CLR TO COMPANY OFFICIALS HE WILL NOT FLY WITH THAT CAPT AGAIN.