Narrative:

Air carrier X was en route to dtw with clearance from FL350 to FL280. Cpr Y, was en route to mbs at FL310. Cpr Y flight path was under air carrier X. Air carrier X was then given clearance to descend and maintain 'FL330 only.' air carrier X read back descent to FL330 and after listening to tapes, also added to cross next fix at FL280. I did not hear the descent to FL280. Traffic was issued for both aircraft and air carrier X responded with traffic in sight. Air carrier X then started a descent to FL280 with cpr Y within separation error standards. Air carrier X was issued climb back to FL330. Supplemental information from acn 246275: we were in cruise at FL350. We received a clearance to cross aylmer VOR (yqo) at FL280. Since we were still quite some distance from the VOR, we stayed at FL350. After a period of time (I believe some 5-8 mins) the controller came back on and said to descend to FL330 now. I read back 'leaving FL350 for FL330 to cross aylmer at FL280. Nothing was said about this readback and no corrections made. As the aircraft was passing FL313, the controller asked us to verify our altitude as FL330. I said we were still in a descent so as to cross aylmer at FL280. The controller asked us to climb back to and maintain FL330. If the second clearance was to be an amended clearance, then it should have been stated as such and that the clearance to cross aylmer had been cancelled.

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Original NASA ASRS Text

Title: ACR X UNAUTH DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM CPR Y. PLTDEV. SYS ERROR.

Narrative: ACR X WAS ENRTE TO DTW WITH CLRNC FROM FL350 TO FL280. CPR Y, WAS ENRTE TO MBS AT FL310. CPR Y FLT PATH WAS UNDER ACR X. ACR X WAS THEN GIVEN CLRNC TO DSND AND MAINTAIN 'FL330 ONLY.' ACR X READ BACK DSCNT TO FL330 AND AFTER LISTENING TO TAPES, ALSO ADDED TO CROSS NEXT FIX AT FL280. I DID NOT HEAR THE DSCNT TO FL280. TFC WAS ISSUED FOR BOTH ACFT AND ACR X RESPONDED WITH TFC IN SIGHT. ACR X THEN STARTED A DSCNT TO FL280 WITH CPR Y WITHIN SEPARATION ERROR STANDARDS. ACR X WAS ISSUED CLB BACK TO FL330. SUPPLEMENTAL INFO FROM ACN 246275: WE WERE IN CRUISE AT FL350. WE RECEIVED A CLRNC TO CROSS AYLMER VOR (YQO) AT FL280. SINCE WE WERE STILL QUITE SOME DISTANCE FROM THE VOR, WE STAYED AT FL350. AFTER A PERIOD OF TIME (I BELIEVE SOME 5-8 MINS) THE CTLR CAME BACK ON AND SAID TO DSND TO FL330 NOW. I READ BACK 'LEAVING FL350 FOR FL330 TO CROSS AYLMER AT FL280. NOTHING WAS SAID ABOUT THIS READBACK AND NO CORRECTIONS MADE. AS THE ACFT WAS PASSING FL313, THE CTLR ASKED US TO VERIFY OUR ALT AS FL330. I SAID WE WERE STILL IN A DSCNT SO AS TO CROSS AYLMER AT FL280. THE CTLR ASKED US TO CLB BACK TO AND MAINTAIN FL330. IF THE SECOND CLRNC WAS TO BE AN AMENDED CLRNC, THEN IT SHOULD HAVE BEEN STATED AS SUCH AND THAT THE CLRNC TO CROSS AYLMER HAD BEEN CANCELLED.

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.