Narrative:

Aircraft X, msp to spf checked on at 6000 ft. Aircraft Y checked on shortly after at 8000 ft. Aircraft X requested 8000 ft. I issued 8000 ft, immediately reissued 7000 ft, advised overtaking traffic at 8000 ft. I then issued VFR traffic aircraft X could climb through. Pilot acknowledged with roger, looking. Conflict alert activated as aircraft X climbed through 7300 ft. I told aircraft X to check altitude and altimeter, assigned altitude 7000 ft, rwf altimeter 2992. No response, so repeated my transmission. Pilot advised he had read back 8000 ft, but would descend to 7000 ft. Contributing factors: non standard phraseology -- I told pilot to maintain 7000 ft, but did not use 'amend altitude' which I almost always do. Also, after assigning and amending altitude I issued IFR and VFR traffic, may be information overload. Aircraft X thought he read back 8000 ft, but it was not on tape. Pilot may have keyed up, earlier during my transmission, but no evidence found on tape recording. Corrective action: I should have asked for an altitude readback. When pilot rogers, that puts onus on him or her to have correct clearance information. However, check and double-checks would have averted a pilot deviation. We need to be on same page, and I should have questioned whether we really were. Supplemental information from acn 629571: I'm an ATP pilot with 6700 hours flight time and an instrument flight instructor with approximately 2800 hours of dual given. I also have 20+ yrs in the FAA working as an air traffic controller and more recently as an area supervisor. We were on an IFR flight from lvn to spf. I was providing flight instruction to another pilot, with 2 passenger in the rear seats. The makeup of these occupants is significant information. In the rear seat there was an operations manager and an air traffic controller, both from ZMP. In the front seat, receiving dual instruction, was an area supervisor, and myself, also an area supervisor, both from ZMP. Of the 4 occupants, 3 were instrument-rated pilots. We were all wearing headsets and listening to ATC, making note of the service being provided. We were wbound at 6000 ft, when PF requested 8000 ft. The controller cleared us up to 8000 ft, PF responded with 'leaving 6000 ft for 8000 ft, aircraft X.' when he let up on the microphone button, the controller was giving us VFR traffic at approximately 2 O'clock position, 5 mi, 6500 ft, swbound. PF acknowledged with 'roger, looking for the traffic.' all 4 of us started looking for the VFR traffic. While climbing through 7500 ft, the controller told us to check altitude and altimeter setting, and that we were only cleared to 7000 ft. PF immediately responded by telling the controller that we were cleared up to 8000 ft, and that he read it back to him, and does he now want us to go back down to 7000 ft. The controller told us to descend and maintain 7000 ft. We descended and reported level within 15 seconds. Separation was lost between our aircraft and a BE1900D overtaking us at 8000 ft. From the FAA side and what shows up on the recorded tape is the following: the controller cleared us up to 8000 ft, paused, then reclred us to maintain 7000 ft, then issued the overtaking traffic at 8000 ft, then issued VFR traffic at approximately 2 O'clock position, 5 mi, 6500 ft, swbound, all in one transmission. The controller then took our last transmission of 'roger, looking for the traffic,' as an acknowledgement or readback to his entire transmission. What appears to have happened, is that the controller cleared us up to 8000 ft, then paused, at which time PF started transmitting with the readback. With both of us transmitting, neither one received the other's transmission. The recorded tape at ZMP only recorded the center controller's transmission, and nothing else. After doing some research, I found that when a controller is transmitting, the tape only records what the controller says, and nothing that a pilot or pilots may be transmitting at the time. Out in the sector, other airplanes will hear the garbled sound when a stepped-on transmission occurs, but none of that will show up on the recorded tape, if the controller has their microphone keyed. I believe this occurrence took place due to an equipment limitation, that doesn't allow either party, to be aware of or have knowledge that the xmissions were never received by the other party. I see this as a rare occurrence, though one that will probablyrepeat itself in the future.

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Original NASA ASRS Text

Title: A ZMP ARTCC RADAR CTLR IMMEDIATELY REALIZED A MISTAKE IN ISSUING A CLRNC TO A C182 UP TO 8000 FT, WHICH WAS OCCUPIED BY B1900, AND RECLRED THE ACFT TO 7000 FT. THE XMISSION WAS BLOCKED AND THE C182 CONTINUED CLB RESULTING IN A LOSS OF SEPARATION.

Narrative: ACFT X, MSP TO SPF CHKED ON AT 6000 FT. ACFT Y CHKED ON SHORTLY AFTER AT 8000 FT. ACFT X REQUESTED 8000 FT. I ISSUED 8000 FT, IMMEDIATELY REISSUED 7000 FT, ADVISED OVERTAKING TFC AT 8000 FT. I THEN ISSUED VFR TFC ACFT X COULD CLB THROUGH. PLT ACKNOWLEDGED WITH ROGER, LOOKING. CONFLICT ALERT ACTIVATED AS ACFT X CLBED THROUGH 7300 FT. I TOLD ACFT X TO CHK ALT AND ALTIMETER, ASSIGNED ALT 7000 FT, RWF ALTIMETER 2992. NO RESPONSE, SO REPEATED MY XMISSION. PLT ADVISED HE HAD READ BACK 8000 FT, BUT WOULD DSND TO 7000 FT. CONTRIBUTING FACTORS: NON STANDARD PHRASEOLOGY -- I TOLD PLT TO MAINTAIN 7000 FT, BUT DID NOT USE 'AMEND ALT' WHICH I ALMOST ALWAYS DO. ALSO, AFTER ASSIGNING AND AMENDING ALT I ISSUED IFR AND VFR TFC, MAY BE INFO OVERLOAD. ACFT X THOUGHT HE READ BACK 8000 FT, BUT IT WAS NOT ON TAPE. PLT MAY HAVE KEYED UP, EARLIER DURING MY XMISSION, BUT NO EVIDENCE FOUND ON TAPE RECORDING. CORRECTIVE ACTION: I SHOULD HAVE ASKED FOR AN ALT READBACK. WHEN PLT ROGERS, THAT PUTS ONUS ON HIM OR HER TO HAVE CORRECT CLRNC INFO. HOWEVER, CHK AND DOUBLE-CHKS WOULD HAVE AVERTED A PLTDEV. WE NEED TO BE ON SAME PAGE, AND I SHOULD HAVE QUESTIONED WHETHER WE REALLY WERE. SUPPLEMENTAL INFO FROM ACN 629571: I'M AN ATP PLT WITH 6700 HRS FLT TIME AND AN INST FLT INSTRUCTOR WITH APPROX 2800 HRS OF DUAL GIVEN. I ALSO HAVE 20+ YRS IN THE FAA WORKING AS AN AIR TFC CTLR AND MORE RECENTLY AS AN AREA SUPVR. WE WERE ON AN IFR FLT FROM LVN TO SPF. I WAS PROVIDING FLT INSTRUCTION TO ANOTHER PLT, WITH 2 PAX IN THE REAR SEATS. THE MAKEUP OF THESE OCCUPANTS IS SIGNIFICANT INFO. IN THE REAR SEAT THERE WAS AN OPS MGR AND AN AIR TFC CTLR, BOTH FROM ZMP. IN THE FRONT SEAT, RECEIVING DUAL INSTRUCTION, WAS AN AREA SUPVR, AND MYSELF, ALSO AN AREA SUPVR, BOTH FROM ZMP. OF THE 4 OCCUPANTS, 3 WERE INST-RATED PLTS. WE WERE ALL WEARING HEADSETS AND LISTENING TO ATC, MAKING NOTE OF THE SVC BEING PROVIDED. WE WERE WBOUND AT 6000 FT, WHEN PF REQUESTED 8000 FT. THE CTLR CLRED US UP TO 8000 FT, PF RESPONDED WITH 'LEAVING 6000 FT FOR 8000 FT, ACFT X.' WHEN HE LET UP ON THE MIKE BUTTON, THE CTLR WAS GIVING US VFR TFC AT APPROX 2 O'CLOCK POS, 5 MI, 6500 FT, SWBOUND. PF ACKNOWLEDGED WITH 'ROGER, LOOKING FOR THE TFC.' ALL 4 OF US STARTED LOOKING FOR THE VFR TFC. WHILE CLBING THROUGH 7500 FT, THE CTLR TOLD US TO CHK ALT AND ALTIMETER SETTING, AND THAT WE WERE ONLY CLRED TO 7000 FT. PF IMMEDIATELY RESPONDED BY TELLING THE CTLR THAT WE WERE CLRED UP TO 8000 FT, AND THAT HE READ IT BACK TO HIM, AND DOES HE NOW WANT US TO GO BACK DOWN TO 7000 FT. THE CTLR TOLD US TO DSND AND MAINTAIN 7000 FT. WE DSNDED AND RPTED LEVEL WITHIN 15 SECONDS. SEPARATION WAS LOST BTWN OUR ACFT AND A BE1900D OVERTAKING US AT 8000 FT. FROM THE FAA SIDE AND WHAT SHOWS UP ON THE RECORDED TAPE IS THE FOLLOWING: THE CTLR CLRED US UP TO 8000 FT, PAUSED, THEN RECLRED US TO MAINTAIN 7000 FT, THEN ISSUED THE OVERTAKING TFC AT 8000 FT, THEN ISSUED VFR TFC AT APPROX 2 O'CLOCK POS, 5 MI, 6500 FT, SWBOUND, ALL IN ONE XMISSION. THE CTLR THEN TOOK OUR LAST XMISSION OF 'ROGER, LOOKING FOR THE TFC,' AS AN ACKNOWLEDGEMENT OR READBACK TO HIS ENTIRE XMISSION. WHAT APPEARS TO HAVE HAPPENED, IS THAT THE CTLR CLRED US UP TO 8000 FT, THEN PAUSED, AT WHICH TIME PF STARTED XMITTING WITH THE READBACK. WITH BOTH OF US XMITTING, NEITHER ONE RECEIVED THE OTHER'S XMISSION. THE RECORDED TAPE AT ZMP ONLY RECORDED THE CTR CTLR'S XMISSION, AND NOTHING ELSE. AFTER DOING SOME RESEARCH, I FOUND THAT WHEN A CTLR IS XMITTING, THE TAPE ONLY RECORDS WHAT THE CTLR SAYS, AND NOTHING THAT A PLT OR PLTS MAY BE XMITTING AT THE TIME. OUT IN THE SECTOR, OTHER AIRPLANES WILL HEAR THE GARBLED SOUND WHEN A STEPPED-ON XMISSION OCCURS, BUT NONE OF THAT WILL SHOW UP ON THE RECORDED TAPE, IF THE CTLR HAS THEIR MIKE KEYED. I BELIEVE THIS OCCURRENCE TOOK PLACE DUE TO AN EQUIP LIMITATION, THAT DOESN'T ALLOW EITHER PARTY, TO BE AWARE OF OR HAVE KNOWLEDGE THAT THE XMISSIONS WERE NEVER RECEIVED BY THE OTHER PARTY. I SEE THIS AS A RARE OCCURRENCE, THOUGH ONE THAT WILL PROBABLYREPEAT ITSELF IN THE FUTURE.

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.