ALTDEV ALT EXCURSION FROM ASSIGNED ALT BY AN OVERBURDENED FLT CREW ON FREQ WITH ZMP; MN.

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AS AN FO IN A PART 121 OP; THE FLT WAS AT A CRUISING ALT OF 24000 FT. FLT PLAN WAS TO OMA. ALTDEV OCCURRED 80-90 MI OUTSIDE OF OMA. TSTMS WERE THROUGHOUT THE AREA. IT SEEMED AS IF EVERY ACFT THAT ZMP WAS CTLING; WERE REQUESTING/ASSIGNED VECTORS AND ALT CHANGES WITH EVERY COM. OUR CTLR WAS WORKING MULTIPLE FREQS TO INCLUDE SOME MIL OPS WHICH SIMPLY ADDED TO CONGESTION ON THE FREQ. OUR FLT WAS NO LONGER ON OUR ORIGINAL FLT PLAN RTE DUE TO VECTORS AS PREVIOUSLY MENTIONED. I BELIEVE WE WERE ON OUR 3RD ALT AND 5TH VECTOR BY THIS TIME. WITH 15-20 MINS LEFT IN THIS FLT; I BEGAN CALCULATING LNDG WT AND PERFORMANCE DATA FOR OUR ARR INTO OMA (TO INCLUDE CURRENT WX). IN THE MIDDLE OF THESE CALCULATIONS ZMP CALLS US: 'ACR X; TURN TO A HDG OF TWO ONE ZERO' (THIS WAS FROM OUR CURRENT HDG OF I BELIEVE 160 DEGS). I; THE PNF RESPOND: 'HDG OF TWO ONE ZERO FOR ACR X.' I TURNED TO THE CAPT AND REPEATED 'HDG TWO ONE ZERO.' THE CAPT REPLIED; 'OK; TWO ONE ZERO -- HERE WE GO.' I CONTINUED ON WITH MY PERFORMANCE CALCULATIONS. DURING MY CALCULATIONS; I HEARD CTR CALL A COMPANY FLT (MULTIPLE COMPANY CALL SIGNS WERE ON OUR AND OTHER FREQS BEING OPERATED BY OUR CTLR; PRETTY TYPICAL). AFTER THE CTLR'S 3RD ATTEMPT AT REACHING MY CAPT (PF) QUERIED THE CTLR; 'SIR; DO YOU MEAN; COMPANY Y?' CTR: 'YES; ACR X; YOU'RE AT FL240; AREN'T YOU?' AT THIS TIME I GOT THIS AWFUL FEELING IN MY STOMACH AND AS I LOOKED UP FROM ACARS AND PAUSED PREPARATION PER LNDG; I SEEN THAT OUR ALT PRESELECT (WHICH ONLY THE PNF IS SUPPOSED TO CHANGE; ACCORDING TO COMPANY PROFILE) HAD BEEN DIALED DOWN FROM 24000 FT TO 21000 FT; AND AT THIS TIME WE WERE DSNDING THROUGH 22300 FT FOR 21000 FT. CAPT: 'NO SIR; WE ARE DSNDING THROUGH 22300 FT FOR 21000 FT.' CTR: 'I TOLD YOU TO TURN TO A HDG OF TWO ONE ZERO NOT DSND TO TWO ONE ZERO.' CAPT: 'DO YOU WANT US TO CLB BACK TO TWO FOUR ZERO?' CTR: 'ACR X; TURN TO A HDG OF TWO ONE ZERO; DSND TO FLT LEVEL TWO ONE ZERO AND CONTACT CTR ON XXX.XX.' CAPT REPEATED BACK DIRECTION AND CONTINUED AS DIRECTED. I SIGNED ON WITH THE NEXT CTLR AND HE INSTRUCTED US TO DSND TO 11000 FT. AFTER THE INCIDENT; WE NEVER HEARD ANYTHING MORE FROM CTR REGARDING THE INCIDENT. I FEEL RESPONSIBLE FOR THIS FOR I DID NOT BACK THE CAPT UP TO INSURE THAT HE DIALED IN 210 ON THE HDG BUG VERSUS HIS ALT PRESELECT. I SIMPLY RESUMED GETTING WX AND LNDG DATA INTO OMA. CONTRIBUTING FACTORS: TSTMS/WX; FREQ CONGESTION; MULTIPLE VECTORS/ALT CHANGES; WORKLOAD AT THE SPECIFIC POINT OF THE FLT; POSSIBLE PLT FATIGUE -- ONLY HAD 8 HRS AND 1 MIN OFF BTWN DUTY DAYS - FLT TIME LOGGED ON PREVIOUS DAY WAS 7 HRS AND 5 MINS -- DUTY DAY WAS 14 HRS AND 45 MINS. 'BUT IT'S LEGAL.'