Narrative:

Cpr X was at FL310 going djb direct azo (approximately 310 heading). Cpr Y was eastbound direct djb climbing to FL290. The sector was extremely busy with over 17 aircraft. I told a non-involved air carrier flight, 'turn left heading 270', he said 'roger 270', next I said 'cpr X turn left fly heading 270 vector for traffic.' he said 'ok, leaving 310 for 270.' I thought he meant he was leaving a 310 heading for a 270 heading, just like the air carrier flight meant he was going to a 270 heading. So I went on to fix all the other traffic situations. But cpr X started descending even though his readback made no mention of 'descent, altitude, or flight level.' so I had to climb him from FL300 back to FL310 but he was right on top of cpr Y at FL290. I think to prevent this pilots should be required to give complete readbacks with proper ATC phraseology. Supplemental information from acn 168612. I believe the traffic complexity itself was a causal factor and cpr X that was inbound expecting a descent clearance; took a heading assignment as a descent clearance and his phraseology was misleading to the controller. I also think the fact that another aircraft in handoff status that was in and out of a boundary, causing track control difficulty was a distraction. I was moved from the radar associate to the tracker position. Another radar associate was called back and we were in a positional transition. There was numerous coordination calls and I was not aware of the clearance and the readback. Supplemental information from acn 168893. Distraction: the transition to a handoff position assignment of another radar associate. The activation of a false conflict alert. The problem was pointed out by a controller next to our pvd. Cle were assigned headings for traffic, an en route was assigned a heading for traffic (by using proper phraseology). The conflict alert flashed on the opp of our scope which drew attention. Cpr X took the heading for an altitude change west/O saying descending or stating he was leaving his assigned altitude. The problem could have been avoided if pilots were required to read back their clearance verbatim. Supplemental information from acn 169147. Cpr X en route at FL310, with moderate radio congestion, we received what I felt was a clearance to descend to FL270. My response was, 'cpr X is leaving three one zero for two seven zero.' the PF did not hear center's original call and therefore listening intently to my response, begins a descent while calling for a descent checklist. After approximately twenty seconds, passing through 30400', cleveland center asks us, 'cpr X, confirm level at 310.' my immediate response was 'cpr X is descending to FL270.' center then requests, 'cpr X maintain FL310,' followed by 'cpr X is climbing to FL310, we gave you a readback!' about 4 mins later, and after a frequency change, cleveland center states that they have a possible altitude excursion and to please call a certain telephone number, which they gave us. As you can suspect, both crew members stomachs turned once with immediate anxiety. The rest of the flight, approximately 20 mins, was focused on reliving the event through cockpit conversations and mental review. Twice, the cockpit conversation came up that both of us needed to focus our attention to the upcoming approach and landing, but ultimately we continued to discuss the cause of our tension. I called the telephone number only to find out that the supervisor knew nothing as of yet and that the tapes were being reviewed. We then had to fly back to our original departure point, again under a tremendous amount of stress.

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Original NASA ASRS Text

Title: CPR X DESCENDED FROM ASSIGNED ALT WITHOUT CLRNC AND HAD LESS THAN STANDARD SEPARATION WITH CPR Y. SYSTEM ERROR.

Narrative: CPR X WAS AT FL310 GOING DJB DIRECT AZO (APPROX 310 HDG). CPR Y WAS EBND DIRECT DJB CLBING TO FL290. THE SECTOR WAS EXTREMELY BUSY WITH OVER 17 ACFT. I TOLD A NON-INVOLVED ACR FLT, 'TURN L HDG 270', HE SAID 'ROGER 270', NEXT I SAID 'CPR X TURN L FLY HDG 270 VECTOR FOR TFC.' HE SAID 'OK, LEAVING 310 FOR 270.' I THOUGHT HE MEANT HE WAS LEAVING A 310 HDG FOR A 270 HDG, JUST LIKE THE ACR FLT MEANT HE WAS GOING TO A 270 HDG. SO I WENT ON TO FIX ALL THE OTHER TFC SITUATIONS. BUT CPR X STARTED DSNDING EVEN THOUGH HIS READBACK MADE NO MENTION OF 'DSNT, ALT, OR FLT LEVEL.' SO I HAD TO CLB HIM FROM FL300 BACK TO FL310 BUT HE WAS RIGHT ON TOP OF CPR Y AT FL290. I THINK TO PREVENT THIS PLTS SHOULD BE REQUIRED TO GIVE COMPLETE READBACKS WITH PROPER ATC PHRASEOLOGY. SUPPLEMENTAL INFO FROM ACN 168612. I BELIEVE THE TFC COMPLEXITY ITSELF WAS A CAUSAL FACTOR AND CPR X THAT WAS INBND EXPECTING A DSNT CLRNC; TOOK A HDG ASSIGNMENT AS A DSNT CLRNC AND HIS PHRASEOLOGY WAS MISLEADING TO THE CTLR. I ALSO THINK THE FACT THAT ANOTHER ACFT IN HDOF STATUS THAT WAS IN AND OUT OF A BOUNDARY, CAUSING TRACK CTL DIFFICULTY WAS A DISTR. I WAS MOVED FROM THE RADAR ASSOCIATE TO THE TRACKER POS. ANOTHER RADAR ASSOCIATE WAS CALLED BACK AND WE WERE IN A POSITIONAL TRANSITION. THERE WAS NUMEROUS COORD CALLS AND I WAS NOT AWARE OF THE CLRNC AND THE READBACK. SUPPLEMENTAL INFO FROM ACN 168893. DISTR: THE TRANSITION TO A HDOF POS ASSIGNMENT OF ANOTHER RADAR ASSOCIATE. THE ACTIVATION OF A FALSE CONFLICT ALERT. THE PROB WAS POINTED OUT BY A CTLR NEXT TO OUR PVD. CLE WERE ASSIGNED HDGS FOR TFC, AN ENRTE WAS ASSIGNED A HDG FOR TFC (BY USING PROPER PHRASEOLOGY). THE CONFLICT ALERT FLASHED ON THE OPP OF OUR SCOPE WHICH DREW ATTN. CPR X TOOK THE HDG FOR AN ALT CHANGE W/O SAYING DSNDING OR STATING HE WAS LEAVING HIS ASSIGNED ALT. THE PROB COULD HAVE BEEN AVOIDED IF PLTS WERE REQUIRED TO READ BACK THEIR CLRNC VERBATIM. SUPPLEMENTAL INFO FROM ACN 169147. CPR X ENRTE AT FL310, WITH MODERATE RADIO CONGESTION, WE RECEIVED WHAT I FELT WAS A CLRNC TO DSND TO FL270. MY RESPONSE WAS, 'CPR X IS LEAVING THREE ONE ZERO FOR TWO SEVEN ZERO.' THE PF DID NOT HEAR CTR'S ORIGINAL CALL AND THEREFORE LISTENING INTENTLY TO MY RESPONSE, BEGINS A DSNT WHILE CALLING FOR A DSNT CHKLIST. AFTER APPROX TWENTY SECS, PASSING THROUGH 30400', CLEVELAND CTR ASKS US, 'CPR X, CONFIRM LEVEL AT 310.' MY IMMEDIATE RESPONSE WAS 'CPR X IS DSNDING TO FL270.' CTR THEN REQUESTS, 'CPR X MAINTAIN FL310,' FOLLOWED BY 'CPR X IS CLBING TO FL310, WE GAVE YOU A READBACK!' ABOUT 4 MINS LATER, AND AFTER A FREQ CHANGE, CLEVELAND CTR STATES THAT THEY HAVE A POSSIBLE ALT EXCURSION AND TO PLEASE CALL A CERTAIN TELEPHONE NUMBER, WHICH THEY GAVE US. AS YOU CAN SUSPECT, BOTH CREW MEMBERS STOMACHS TURNED ONCE WITH IMMEDIATE ANXIETY. THE REST OF THE FLT, APPROX 20 MINS, WAS FOCUSED ON RELIVING THE EVENT THROUGH COCKPIT CONVERSATIONS AND MENTAL REVIEW. TWICE, THE COCKPIT CONVERSATION CAME UP THAT BOTH OF US NEEDED TO FOCUS OUR ATTN TO THE UPCOMING APCH AND LNDG, BUT ULTIMATELY WE CONTINUED TO DISCUSS THE CAUSE OF OUR TENSION. I CALLED THE TELEPHONE NUMBER ONLY TO FIND OUT THAT THE SUPVR KNEW NOTHING AS OF YET AND THAT THE TAPES WERE BEING REVIEWED. WE THEN HAD TO FLY BACK TO OUR ORIGINAL DEP POINT, AGAIN UNDER A TREMENDOUS AMOUNT OF STRESS.

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.