LGT ACFT CLRED TO BACK TAXI ON PARTIALLY CLOSED RWY TO DISPLACED THRESHOLD; FLC TAXIED PAST THE POORLY MARKED THRESHOLD. TWR ADVISED THEM AND THEY RETURNED ACFT TO THRESHOLD FOR TKOF.

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DURING TAXI OUT FOR TKOF ON RWY 31 AT MDT; TWR CLRED US TO BACK TAXI ON RWY 31 WHICH IS PARTIALLY CLOSED FOR CONSTRUCTION. ARPT NOTAM XX STATES 'WHEN SE 2500 FT OF RWY 13/31 IS CLOSED; NW 7000 FT AVAILABLE ALL OPS EXCEPT 6000 FT AVAILABLE FOR RWY 13 LNDG TO THE DISPLACED THRESHOLD. BACK TAXI AND 180 DEG TURN MAY BE REQUIRED FOR RWY 31 TKOF.' CURRENT COMMERCIAL CHART ARPT DIAGRAM PAGE 21-1 DOES NOT DEPICT CONSTRUCTION AREA. WE BACK TAXIED ON RWY 31 PER TWR INSTRUCTIONS. DURING 180 DEG TURN AROUND TWR ADVISED THAT WE HAD TAXIED BEYOND RWY TEMPORARY THRESHOLD. THE PORTION OF RWY 31 THAT WE WERE ON AT THAT TIME WAS COMPLETELY PAVED AND WE OBSERVED NUMEROUS TIRE MARKS FROM PREVIOUS ACFT THAT HAD PERFORMED 180 DEG TURNS IN THIS AREA. WE PERFORMED 180 DEG TURN NEAR THE CONES WHICH WE INTERPRETED AS THE END OF AVAILABLE RWY WITH PWR AT IDLE. WE TAXIED BACK DOWN RWY 31 TO THE POINT OF ACTUAL TEMPORARY THRESHOLD WHICH IS POORLY MARKED. TKOF FROM THIS POINT WAS UNEVENTFUL. ARPT NOTAMS; DIAGRAMS AND ACTUAL RWY MARKINGS DO NOT CLRLY DEPICT USEABLE PORTION OF RWY 31/13.