Narrative:

Air carrier X en route from cdc to slc at FL270. ATC instructed us to cross 40 mi south of slc at 17000', pilot's discretion. Thereafter ATC came back and instructed us to descend to FL240 west/O cancelling our pilot discretion to 17000' or crossing restriction. During our descent to 17000' (at 23600'), ATC said, 'air carrier X, say altitude.' responded, '23600 descending 17000', pilot's discretion!' they responded by telling us to climb immediately to 24000' and instructed air carrier Y to descend to 22000' immediately. We later questioned the clearance and there was obviously a miscom between ATC and ourselves. We thought and still believe that we were required to descend to 17000' at 40 mi south, that we were needed out of 27000' then (for ATC convenience), and to start our descent (which we did!). Controller apparently wanted us to descend and hold 24000' for he had apparently vectored or cleared an aircraft into the airspace previously assigned to us! The 5 mi bubble was broken and the 1000' sep was minimized. Suggestions on how to avoid this occurrence: 1) ATC should have communicated an amended clearance cancelling pilot's discretion to 17000', or he shouldn't have placed air carrier Y in the airspace we had been given. Or, he could have told us air carrier Y would be at 23000' and we had an amended clearance. Or, we could have understood the clearance to mean what ATC had intended! We (ATC and ourselves) both made errors! Better and precise communications needed. Supplemental information from acn 171683: ATC gave us clearance to descend to cross 17000' at 40 mi southwest suc VOR. He then came back and told us to descend to FL240 (we were at FL270). We understood the restriction to cross 40 mi south at 17000' to still be in effect. At the appropriate distance we started our descent for the restriction, and at 23500' were asked our altitude and told to immediately climb to FL240. Traffic was turned out below us. ATC then gave us a frequency change and thanked us for our help, twice. ATC then listened to the tapes and called us on the ground to tell us it was a pilot deviation.

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Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. UNAUTH DESCENT THROUGH OCCUPIED ALT. PLT DEVIATION.

Narrative: ACR X ENRTE FROM CDC TO SLC AT FL270. ATC INSTRUCTED US TO CROSS 40 MI S OF SLC AT 17000', PLT'S DISCRETION. THEREAFTER ATC CAME BACK AND INSTRUCTED US TO DSND TO FL240 W/O CANCELLING OUR PLT DISCRETION TO 17000' OR XING RESTRICTION. DURING OUR DSNT TO 17000' (AT 23600'), ATC SAID, 'ACR X, SAY ALT.' RESPONDED, '23600 DSNDING 17000', PLT'S DISCRETION!' THEY RESPONDED BY TELLING US TO CLB IMMEDIATELY TO 24000' AND INSTRUCTED ACR Y TO DSND TO 22000' IMMEDIATELY. WE LATER QUESTIONED THE CLRNC AND THERE WAS OBVIOUSLY A MISCOM BTWN ATC AND OURSELVES. WE THOUGHT AND STILL BELIEVE THAT WE WERE REQUIRED TO DSND TO 17000' AT 40 MI S, THAT WE WERE NEEDED OUT OF 27000' THEN (FOR ATC CONVENIENCE), AND TO START OUR DSNT (WHICH WE DID!). CTLR APPARENTLY WANTED US TO DSND AND HOLD 24000' FOR HE HAD APPARENTLY VECTORED OR CLRED AN ACFT INTO THE AIRSPACE PREVIOUSLY ASSIGNED TO US! THE 5 MI BUBBLE WAS BROKEN AND THE 1000' SEP WAS MINIMIZED. SUGGESTIONS ON HOW TO AVOID THIS OCCURRENCE: 1) ATC SHOULD HAVE COMMUNICATED AN AMENDED CLRNC CANCELLING PLT'S DISCRETION TO 17000', OR HE SHOULDN'T HAVE PLACED ACR Y IN THE AIRSPACE WE HAD BEEN GIVEN. OR, HE COULD HAVE TOLD US ACR Y WOULD BE AT 23000' AND WE HAD AN AMENDED CLRNC. OR, WE COULD HAVE UNDERSTOOD THE CLRNC TO MEAN WHAT ATC HAD INTENDED! WE (ATC AND OURSELVES) BOTH MADE ERRORS! BETTER AND PRECISE COMS NEEDED. SUPPLEMENTAL INFO FROM ACN 171683: ATC GAVE US CLRNC TO DSND TO CROSS 17000' AT 40 MI SW SUC VOR. HE THEN CAME BACK AND TOLD US TO DSND TO FL240 (WE WERE AT FL270). WE UNDERSTOOD THE RESTRICTION TO CROSS 40 MI S AT 17000' TO STILL BE IN EFFECT. AT THE APPROPRIATE DISTANCE WE STARTED OUR DSNT FOR THE RESTRICTION, AND AT 23500' WERE ASKED OUR ALT AND TOLD TO IMMEDIATELY CLB TO FL240. TFC WAS TURNED OUT BELOW US. ATC THEN GAVE US A FREQ CHANGE AND THANKED US FOR OUR HELP, TWICE. ATC THEN LISTENED TO THE TAPES AND CALLED US ON THE GND TO TELL US IT WAS A PLT DEVIATION.

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.