AIRLINE CREW IS DISTRACTED BY AIRPORT CONSTRUCTION; CHANGING WIND CONDITIONS AND PERFORMANCE ISSUES CAUSED BY A SHORTENED RWY. IT LEADS TO A TAXI ONTO THE ACTIVE RWY WITHOUT CLRNC.

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WHILE TAXIING FOR TAKEOFF AT EFD AT XA00LCL ON TAXI D; WE WERE GIVEN MANY DIFFERENT OPTIONS FOR TAKEOFF RWY BASED ON WINDS. I ELECTED TO STILL USE RWY 04 SINCE WINDS WERE R X-WINDS 11 KTS. DURING THE RADIO CALLS I GOT THE IMPRESSION WE WERE OK ONTO THE RWY. HOWEVER; NO POS AND HOLD CALL WAS GIVEN TO US. I UNDERSTAND THE PHRASEOLOGY OF 'TAXI TO RWY 04' DOESN'T MEAN CLRED ONTO THE RWY. HOWEVER; THE TONE OF OUR CONVERSATION LED US TO BELIEVE THE RWY WAS OURS. OBVIOUSLY OUR MISUNDERSTANDING. WE VISUALLY CLRED THE APCH AND DEP ENDS AND PROCEEDED ONTO THE DISPLACED THRESHOLD FOR BACK TAXI. WHEN I ASKED IF WE WERE CLEARED FOR 'BACK TAXI' IS WHEN THE MISCOMMUNICATION BECAME EVIDENT. I SAID WE'D CLR THE RWY AND THE CTLR TOLD US TO POS AND HOLD; BUT THAT WE WERE ON A CLOSED PORTION OF THE RWY. WE WERE; HOWEVER; POSITIONING ON THE TEMPORARILY DISPLACED THRESHOLD; WELL CLR OF THE CONSTRUCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RWY IN QUESTION WAS INVOLVED IN CONSTRUCTION THAT MADE THE FIRST 1000 FT UNUSABLE AND REQUIRE A TAXI BACK TO GET USE OF THE MAXIMUM LENGTH. THERE WAS A LOT OF BACK AND FORTH CONVERSATION BETWEEN THE TWR AND THE ACFT RELATIVE TO THE RWY TO BE USED. THERE WAS NO OTHER TFC AT THE ARPT. THE RPTR NOTICES THAT GENERALLY MILITARY TWRS DO NOT GIVE AS MUCH SPECIFIC DIRECTION AS FAA CTLRS. HE SAID THIS MAY HAVE CONTRIBUTED TO TAXIING ONTO THE RWY WITHOUT CLRNC; ALONG WITH THE FACT THERE WAS NO OTHER TFC.