ACR Y UNAUTHORIZED DSCNT THROUGH OCCUPIED ALT DSCNT TO WRONG ALT HAD LTSS FROM SMT X. PLTDEV. RADAR CTLR FAILED TO HEAR ACR Y INCORRECT READBACK OF WRONG ALT. SYS ERROR.

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METERING; HOLDING WITH APPROX 12-13 MIN DELAYS FOR MSP. APPROX 13-16 ACFT ON THE FREQ. SMT X WAS AN OVERFLT THROUGH THE SECTOR AT 110. ACR Y WAS CLRED OUT OF HOLD AND ISSUED 120. PLT READ BACK 100 AND I MISSED IT. AT THE TIME OF HIS READBACK; I WAS SEARCHING AROUND THE SECTOR FOR THE LISTING OF AWOS FREQS AS I HAD 2 DIFFERENT ACFT GOING INTO 2 DIFFERENT ARPTS WITHIN THE SECTOR AND THE WX WASN'T VERY GOOD. ACR Y PROBABLY WASN'T LISTENING UP VERY WELL EITHER AS I HAD TO ISSUE A PREVIOUS CLRNC TO HIM 2 TIMES. BUT THAT'S NO EXCUSE. SUPPLEMENTAL INFO FROM ACN 207848: DURING THE INITIAL OUTBOUND TURN AS I ENTERED THE HOLDING PATTERN AT MEINZ INTXN; CTR SAID TO CONTINUE TURN TO 310 DEGS; INTERCEPT THE ARR; MAINTAIN 10000 FT AND INCREASE SPD TO 270 KTS (HAD PREVIOUSLY BEEN MAINTAINING 250 KTS PER ATC). I READ BACK THE CLRNC AND COMPLIED. (FO WAS ON THE PA ADVISING PAX WE WERE NOW CLRED TO CONTINUE TO MSP). PASSING 11000 FT; CTR ASKED IF WE WERE AT 12000. TOLD FO TO REPLY NEGATIVE DSNDING TO 10000 AND DID THEY WANT US TO RETURN TO 12000. CTR TOLD US TO DSND AND MAINTAIN 10000. A LITTLE LATER; HE ADVISED US THERE WAS A 'POSSIBLE ALTDEV' AND GAVE A TELEPHONE NUMBER TO CALL IF PROBLEMS AROSE BECAUSE OF 2 THINGS: 1) 4 ITEMS OF INSTRUCTION IN 1 STRING; 2) FLYING PLANE AND RESPONDING ON RADIO AT SAME TIME IN BUSY PHASE OF FLT. I COULD HAVE PREVENTED THE SITUATION BY TELLING FO TO HOLD UP ON PA UNTIL WE WERE WELL ESTABLISHED IN HOLDING; AND QUERYING CTR ON ALT ASSIGNED AS THE ARR SAYS TO EXPECT TO CROSS PEPPER INTXN AT 10000 FT TO MAINTAIN 9000 FT LNDG ON RWY 29. APRIL 92 -- AFTER WRITING THE ABOVE; I FOUND OUT TODAY THAT MSP CTR HAD PULLED THE TAPES ON THIS INCIDENT. APPARENTLY; CTLR HAD IN FACT CLRED ME TO 12000 FT. I READ BACK 10000 FT AND THEY DID NOT CATCH THE ERROR IN MY READBACK.