W END OF RWY 10-28 NOTAMED 700 FT CLOSED. WHEN QUERIED TWR CTLR DECLARED ENTIRE 9200 FT AVAILABLE.

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ACR FLT MSY TO ORD. FLT WAS 'WT CRITICAL'. CAPT ASKED ENGINEER FOR CONFIGN FOR TKOF. DISCUSSION ENSUED AS TO ACTUAL RWY LENGTH AVAILABLE. W 700 FT OF 10-28 (9200) WAS NOTAMED UNAVAILABLE DUE TO CONSTRUCTION; BUT CONSTRUCTION APPEARED TO BE LOCATED BEYOND END OF RWY. CREW THEN ASKED TWR IF 'FULL LENGTH OF RWY' WAS AVAILABLE. TWR ANSWERED 'AFFIRMATIVE'. CREW; NOT SATISFIED; THEN ASKED IF 9200 FT WAS AVAILABLE. TWR AGAIN ANSWERED 'AFFIRMATIVE'. BY THIS TIME WE WERE HOLDING SHORT OF RWY 10. CAPT COUNTED MARKERS FROM TAXIWAY TO RWY END AND GUESSED THAT ONLY 8500 FT OPEN. ENGINEER NOTED THAT EITHER LENGTH WAS SUFFICIENT; AND HAD ACTUALLY PLANNED FOR 8500 FT. WHEN WE TAXIED TO RWY END; MARKERS CONFIRMED LENGTH OF 8500 FT. CREW DISCUSSED WHY TWR WOULD GIVE INACCURATE INFO; AND DECIDED THAT CONSTRUCTION HAD BEEN IN PLACE SO LONG THAT CTLRS CONSIDERED 8500 FT TO BE 'NORMAL' FULL LENGTH. AS OBSERVER; I MUST QUESTION THE CREW'S FAILURE TO CALL THE CTLR'S ATTN TO HIS ERROR; AS WELL AS THE ERROR ITSELF. MISTAKES THAT ARE ALLOWED TO PASS; ARE LIKELY TO BE REPEATED.