A B73-300 BACK TAXIES AT MDT DURING CONSTRUCTION AND GOES INTO CONSTRUCTION ZONE. THE ACFT DROPS ABOUT 3 INCHES INTO A ROUGH SURFACE; THEN TURNS BACK TO RISE OUT OF THE DEPRESSION OF THE CONSTRUCTION ZONE. PLT UNABLE TO DETERMINE WHERE CONSTRUCTION ZONE IS LOCATED.

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MDT RWY 31/13 IS UNDERGOING RESURFACING. TO DEPART ON RWY 31; ACFT MUST BE CLRED BY TWR CTLR TO BACK-TAXI FROM INTXN 'D.' ATIS INFO STATES THAT ACFT DEP ON RWY 31 MUST BACK-TAXI FROM INTXN 'D' TO THE NEW THRESHOLD MARKINGS. APPROX 1500 FT OF THE APCH END OF RWY 31 IS UNDERGOING THE RE-GROOVING AND PAVING WORK. I WAS GIVEN CLRNC TO BACK-TAXI AND TKOF. AS I BEGAN TO BACK-TAXI FROM INTXN 'D' I COULD SEE A BLOCKADE (BARRELS; ETC) WHERE THE NEW THRESHOLD APPEARED TO BE. APPROX 500 FT PRIOR TO REACHING THE BLOCKADE WE FELT A BUMP; WHICH TURNED OUT TO BE A DROP OF APPROX 3 INCHES OF THE RWY SURFACE. AT THAT TIME; TWR CTLR INFORMED US THAT WE HAD ENTERED THE CLOSED PORTION OF THE RWY. WE TURNED AND RE-ENTERED THE OPEN PORTION OF RWY AND WERE CLRED FOR TKOF. ARRIVING AT OUR DEST I CALLED THE MDT TWR SUPVR. HE STATED TO ME HIS CONCERN ON THIS MATTER; BECAUSE MDT HAD HAD VARIOUS SIMILAR INCURSIONS IN THE PAST. HE WAS CONCERNED WITH THE POTENTIAL FOR LIABILITY TO THE ARPT IF DAMAGE WOULD OCCUR. HE ALSO ASKED ME FOR INPUT AS TO WHAT COULD BE DONE TO PREVENT FURTHER PROBS. I FEEL THAT THE NEW MARKINGS SHOULD HAVE BEEN MORE APPARENT; AND THAT; THE BLOCKADE 500 FT FURTHER DOWN; PROVIDED A VISUAL TRAP AS TO WHERE THE BACK-TAXI COULD BE CONTINUED. ALSO; THE 3 INCH DROP IS NOT EASILY DETECTED FROM THE HT OF AN AIRLINER COCKPIT. MOVING THE BLOCKADE TO THAT POINT SEEMS LIKE THE MOST LOGICAL WAY TO MARK THE WORK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE WAS LOOKING FOR THE BLOCKADES ON THE RWY TO INDICATE HOW FAR DOWN THE RWY HE COULD TAXI BEFORE TURNING AROUND AND BEGIN HIS TKOF ROLL. HE WAS SURPRISED TO FEEL THE ACFT DROP THE 3 INCHES IT DID AS THAT INDICATED TO HIM THAT HE WAS IN THE CONSTRUCTION AREA; BUT THE BLOCKADES WERE STILL AHEAD OF HIM. HE TURNED THE ACFT AROUND FOR TKOF AND MOVED AHEAD TO GET TO THE LEVEL OF THE RWY. HE NOTICED THAT WHEN HE WAS ON THE FINISHED SURFACE OF THE RWY; HE WAS ABOUT 500 FT AWAY FROM THE BLOCKADE. UPON LNDG AT HIS DEST HE CALLED THE ARPT MGR AS REQUESTED ONLY TO FIND OUT THE ARPT MGR WAS VERY CONCERNED ABOUT ACFT THAT TAXI TOO FAR DOWN THE RWY. HE TOLD THE RPTR THAT THE CONTRACTOR MOVED THE BLOCKADE; COMPOSED OF BARRELS; 500 FT AWAY FROM THE CONSTRUCTION END AS THE BARRELS WERE ALWAYS GETTING BLOWN OVER. MANY OTHER ACFT HAD PROBS TRYING TO IDENT EXACTLY WHERE THE EXACT END OF THE RWY WAS LOCATED AS THERE WAS NO EASILY IDENTIFIABLE MARKINGS AT THE RWY END.