ACR X ALTDEV. NON ADHERENCE TO ATC CLRNC. DSCNT TO WRONG ALT.

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ACR X AT FLT LEVEL WE WERE CLRED TO 10000 FT. THE COPLT WHO WAS FLYING AT THE TIME ROGERED AND SET 10000 FT IN THE ALT ALERTER WHEN WE VACATED OUR CRUISE ALT I TOLD CTR THAT WERE LEAVING FLT LEVEL FOR 10000 FT. I DO NOT REMEMBER IF HE ACKNOWLEDGED THAT OR NOT. WHEN THE CTLR NEXT SPOKE TO US HE ASKED US WHAT ALT WE WERE AT; AND I SAID 10000FT; AND THAT WAS WHAT I HAD TOLD HIM WE WERE GOING TO. HE SAID THAT WE WERE SUPPOSED TO BE AT 11000 FT; AND TO CONTACT TYS APCH AT THAT TIME. WE WERE NEVER AWARE OF ANY OTHER TFC OR ANY OTHER POSSIBLE CONFLICTS. WE CONTINUED ON TO TYS FOR AN UNEVENTFUL LNDG. I WAS VERY TIRED AT THE TIME; AS I HAD FLOWN ALL NIGHT. MY DUTY HAD STARTED ABOUT XX00 THE NIGHT BEFORE AND THIS WAS MY 3RD LEG OF THE NIGHT. I KNOW OUR TIREDNESS CONTRIBUTED TO THIS AND WE PROBABLY WEREN'T AS ALERT AND LISTENING AS WHEN WE ARE FRESH. HOWEVER 2 TIMES WE TOLD THE CTLR THAT WE WERE GOING TO 10000 FT AND HE DID NOT CORRECT US. I IMAGINE HE WAS PROBABLY AS TIRED AS WE WERE; BECAUSE WE HAD BEEN AT 10000 FOR SOME TIME BEFORE HE NOTICED US.