CAPT OF AN ACR DECLARED AN EMER DURING APCH TO JAN WHEN THE AUTO SLAT FAIL WARNING LIGHT CAME ON WHEN THE FLAPS WERE EXTENDED. SUCCESSFUL LNDG WAS MADE WITH FLAPS AT 15 DEGS.

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ON DSCNT INTO JAN; ATIS INDICATED WX DETERIORATING. CURRENTLY; WX WAS 2400 FT RVR WITH FOG. NO ALTERNATE NOTED ON FLT PLAN. GREEN PAGES FOR JAN INDICATED MEI; BTR; AND MLU. NEITHER ONE OF US HAD EVER BEEN TO MEI. BTR HAD 1/4 MI VISIBILITY AND FOG. MLU WAS CLR. WE CHOSE MLU AS OUR ALTERNATE. ON FINAL APCH; RVR WAS STILL 2400 FT. WE STARTED TO CONFIGURE THE ACFT. WHEN FLAPS EXTENDED TO FLAPS 15 DEGS; AUTO SLAT FAIL MESSAGE ILLUMINATED ON OAP. WE BROKE OFF THE APCH AND MADE A PA THAT WE WERE SETTING UP FOR ANOTHER APCH. POM IN FLT CTL INDICATED NORMAL OPS; JUST MAINTAIN LESS THAN 240 KIAS WITH SLATS EXTENDED. I MOVED THE FLAP HANDLE BACK TO FLAPS 11 DEGS HOPING THE MESSAGE WOULD CLR; WHICH IT DID. MOVED FLAP HANDLE BACK TO FLAPS 15 DEGS. AUTO SLAT FAIL MESSAGE REAPPEARED. WATCHED SLAT POS INDICATOR FOR MOVEMENT. BLUE TKOF INDICATOR REMAINED ILLUMINATED. WE DISCUSSED THE FACT THAT THE BLUE TKOF INDICATOR SHOULD NOT HAVE REMAINED ON WHILE THE FLAPS MOVED FROM FLAPS 11 DEGS TO FLAPS 15 DEGS. THE POM DID NOT SEEM TO HAVE A CHKLIST FOR THE SIT WE WERE IN. I CHOSE TO MOVE TO FLAPS 23 DEGS. THE BLUE TKOF LIGHT REMAINED ILLUMINATED. WE WERE STILL NOT SURE WE KNEW WHERE THE SLATS WERE -- MID RANGE OR FULLY EXTENDED. I DECIDED TO MOVE THE FLAPS BACK TO FLAPS 15 DEGS; WITH THE INTENTION OF RETRACTING THE FLAPS TO START ALL OVER AGAIN. AFTER I MOVED THE FLAP HANDLE; WE HEARD A THUMP COMING FROM THE BACK OF THE ACFT. AT THIS POINT SINCE WE STILL HD A LOW RVR AND FOG AT THE ARPT; WE DECIDED TO LEAVE THE FLAPS 15 DEGS AND TAKE OFF. WE HAD SPDS FROM THE POM (129 KIAS). WE CHKED THE ODM AND FOUND OUR LNDG DISTANCE AT 100000 LBS WAS 3100-3300 FT. RWY AVAILABLE WAS 8500 FT. WE INFORMED THE FLT ATTENDANTS OF THE SIT; AND COMMENCED THE APCH. APCH CTL ASKED IF WE WANTED TO DECLARE AN EMER AND I SAID YES. WE MADE AN UNEVENTFUL APCH AND LNDG AT JAN.