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.