AN ACR MD11 FREIGHTER FLC DSNDED BELOW THEIR ASSIGNED ALT WHEN THE CAPT MISINTERPRETED THE CLRNC AND THE FO DID NOT VERIFY THE ALT AS PER STANDARD PROCS. THERE WAS; APPARENTLY; LTSS.

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WHILE IN LEVEL CRUISE AT FL330; ZID ISSUED A DSCNT CLRNC AT 'PLT'S DISCRETION' TO 11000 FT. I DIALED 11000 FT IN THE PRE-SELECT ALT WINDOW AND THE FO ACKNOWLEDGED. SHORTLY THEREAFTER THE CTLR CLRED US TO FL290; PLT'S DISCRETION TO 11000 FT (OR SO I THOUGHT). THE FIRST CLUE I HAD SOMETHING WAS AMISS WAS WHEN I NOTICED ANOTHER ACFT 6-7 MI AWAY GOING DOWN OUR R SIDE. WHEN I MENTIONED TO THE FO; OR POINTED OUT THE OTHER ACFT TO HIM; HE TOLD ME WE WERE ONLY CLRED TO FL290. WE WERE APCHING FL270 SO I IMMEDIATELY BEGAN A CLB BACK TO FL290. ABOUT THAT TIME THE CTLR CAUGHT OUR MISTAKE AND BEGAN QUESTIONING WHAT WE WERE DOING. I HAD SIMPLY MISUNDERSTOOD OUR CLRNC AS FL290 BEING OUR CLRNC LIMIT. THE FO HAD BEEN BUSY TALKING TO OUR COMPANY ON THE #2 RADIO WHILE COMMUNICATING WITH ATC AND HAD ASSUMED I UNDERSTOOD THE CLRNC. WE HAD ANOTHER COMPANY PLT ONBOARD; NOT FAMILIAR WITH THE MD11; WHO WAS EQUALLY CONFUSED BUT ELECTED NOT TO QUESTION THE CLRNC BECAUSE HE SAID HE WASN'T SURE EITHER. WHEN I BEGAN THE DSCNT OUT OF FL330 I DID SO IN VERT SPD MODE AT 1000 FPM. WHEN I NOTICED OUR DSCNT PROMPT SHOWING BEHIND THE DSCNT SCHEDULE I RETURNED THE AUTOPLT ON FMS TO PROFILE MODE WHICH INCREASED OUR DSCNT RATE. IF I HAD LEFT IT IN VERT SPD MODE THE ALT INCURSION WOULD PROBABLY NEVER HAPPENED; WHICH IS MOSTLY IRRELEVANT. THE ONLY THING WHICH COULD HAVE PREVENTED THIS INCIDENT OTHER THAN PAYING CLOSER ATTN AND BETTER CREW COM WOULD HAVE BEEN NOT BEING DISTRACTED WITH NON PERTINENT DUTIES UNRELATED TO FLT SUCH AS COMMUNICATING WITH OUR COMPANY AT AN INOPPORTUNE TIME. SUPPLEMENTAL INFO FROM ACN 391016: SCHEDULED FLT FROM MEMPHIS TO INDIANAPOLIS; MD11. APPROX 4 MINS LATER; CTLR (ZID) GAVE A SUBSEQUENT CLRNC TO DSND TO FL290 AT PLT DISCRETION. ACKNOWLEDGED BY PNF AND DSCNT WAS COMMENCED FROM FL330. DSNDING THROUGH FL277 WE BECAME VISUAL WITH ANOTHER ACFT OFF OUR R WING; APPROX 4-5 NM LATERALLY. IT APPEARED THAT WE WERE CO-ALT (WITH US DSNDING AND THEM CLBING). AT THAT MOMENT FO STATED WE'RE BELOW FL290 AND DSCNT WAS STOPPED BY CAPT. DURING ABOVE STATED CONVERSATION WITH CTLR; A CONVERSATION WAS GOING ON WITHIN COCKPIT WITH DEADHEADING CREW MEMBER IN COCKPIT. (MAY HAVE CONTRIBUTED WITH FO AND CAPT NON- VERIFICATION OF ALT ASSIGNMENT WITH EACH OTHER.) WITH BRIEFING/RECEIVING ATIS/CALLING 'IN RANGE CALL;' CREW HAD SEVERAL TASKS IN PROGRESS WHILE NOT VERIFYING TO EACH OTHER. CLR INTERP OF CTLR'S REQUEST OR LACK THEREOF. PNF AND PF NOT VERBALLY REVERIFYING ALT ASSIGNMENT. THIS WILL CLR UP MISUNDERSTANDING BTWN CTLR AND AIR CREW.