A P3 MIL PIC DSNDS TOO SOON ON THE MULLAN PASS TRANSITION; THINKING HE HAD CLRNC FOR THE DSCNT.

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WHILE ON A VECTOR/DIRECT WHEN ABLE TO MLP; ZSE (126.1) CLRED US FOR THE 'MULLAN PASS TRANSITION TO THE MSO ILS RWY 11 APCH;' THEN PASSED US TO SPOKANE APCH CTL (124.9). WHAT I THOUGHT I HEARD/UNDERSTOOD WAS 'CLRED FOR DSCNT VIA MULLAN PASS TRANSITION TO MSO ILS RWY 11.' SO I BEGAN A DSCNT FROM 13000 FT MSL TO 9600 FT MSL AS PER TRANSITION. UPON CHKING IN WITH SPOKANE APCH CTL; I WAS PASSING 11500 FT MSL AND WE WERE TOLD 'WE HAD NOT BEEN CLRED FOR DSCNT; CLB TO AND MAINTAIN 12000 FT.' THE REST OF THE APCH AND FLT WERE UNEVENTFUL. POSSIBLE CONTRIBUTING FACTORS: FATIGUE -- THIS WAS A MIL RESERVE EVENT FLOWN LATE IN THE DAY; WHERE I HAD ALREADY BEEN UP FOR OVER 13 HRS HAVING STARTED MY DAY COMPLETING A 4 DAY REDUCED REST TRIP WITH MY COMMERCIAL AIRLINE WITH A 3-4 HR BREAK BTWN FINISHING MY CIVILIAN FLYING THEN GOING TO DO RESERVE FLYING. COMBINE THE ABOVE FATIGUE WITH MARGINAL ARR WX AT MSO; AND MY DESIRE TO GET ESTABLISHED ON MSO ILS RWY 11 PARAMETERS AND I PROBABLY HEARD A CLRNC THAT I WAS ANTICIPATING BUT NOT YET DIRECTED. I PROBABLY MISPERCEIVED CORRECT DSCNT CLRNC.