Narrative:

During en route phase to the ssi VOR, center rerted us to over the sav VOR to avoid military airspace. We normally do not get a change of course. En route to sav, we were given a descent clearance. Normally, en route to the ssi VOR, we expect a descent clearance down to FL240. A descent to FL240 is what I believe I heard and accepted and read back. As I was descending through FL295, the controller said that I should be at FL310 and I need to return immediately, which I did. I stated to the controller that I thought that I had read back FL240. He stated that I was only cleared to FL310. I didn't argue with him. I did not hear him say anything to any other aircraft so I don't believe there was a conflict. I had been on duty for 10 hours and the sun was just peeking above the hangar, so I was a little tired when this occurred. The sic was doing paperwork when this occurred. The sic is also a qualified PIC. It is common practice for the PF to fly and answer the radios en route while the PNF catches up on paperwork, so he didn't hear the situation. If I read the clearance back right (FL310) then I had a mental lapse in assuming it was FL240. I will correct this by making sure the PNF is also in the loop. If I read back FL240 then both myself and the controller are at fault -- me for reading it back wrong, and him for not catching and correcting it.

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Original NASA ASRS Text

Title: A CARGO FLC OVERSHOOTS THEIR ASSIGNED ALT IN DSCNT WHEN THE CAPT MISUNDERSTANDS THE CLRNC NNW OF SAV, GA.

Narrative: DURING ENRTE PHASE TO THE SSI VOR, CTR RERTED US TO OVER THE SAV VOR TO AVOID MIL AIRSPACE. WE NORMALLY DO NOT GET A CHANGE OF COURSE. ENRTE TO SAV, WE WERE GIVEN A DSCNT CLRNC. NORMALLY, ENRTE TO THE SSI VOR, WE EXPECT A DSCNT CLRNC DOWN TO FL240. A DSCNT TO FL240 IS WHAT I BELIEVE I HEARD AND ACCEPTED AND READ BACK. AS I WAS DSNDING THROUGH FL295, THE CTLR SAID THAT I SHOULD BE AT FL310 AND I NEED TO RETURN IMMEDIATELY, WHICH I DID. I STATED TO THE CTLR THAT I THOUGHT THAT I HAD READ BACK FL240. HE STATED THAT I WAS ONLY CLRED TO FL310. I DIDN'T ARGUE WITH HIM. I DID NOT HEAR HIM SAY ANYTHING TO ANY OTHER ACFT SO I DON'T BELIEVE THERE WAS A CONFLICT. I HAD BEEN ON DUTY FOR 10 HRS AND THE SUN WAS JUST PEEKING ABOVE THE HANGAR, SO I WAS A LITTLE TIRED WHEN THIS OCCURRED. THE SIC WAS DOING PAPERWORK WHEN THIS OCCURRED. THE SIC IS ALSO A QUALIFIED PIC. IT IS COMMON PRACTICE FOR THE PF TO FLY AND ANSWER THE RADIOS ENRTE WHILE THE PNF CATCHES UP ON PAPERWORK, SO HE DIDN'T HEAR THE SIT. IF I READ THE CLRNC BACK RIGHT (FL310) THEN I HAD A MENTAL LAPSE IN ASSUMING IT WAS FL240. I WILL CORRECT THIS BY MAKING SURE THE PNF IS ALSO IN THE LOOP. IF I READ BACK FL240 THEN BOTH MYSELF AND THE CTLR ARE AT FAULT -- ME FOR READING IT BACK WRONG, AND HIM FOR NOT CATCHING AND CORRECTING IT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.