ZKC Controller and PC12 pilot both described a loss of separation event when traffic was issued and the altitude of same was mistakenly interpreted as a new altitude assignment.
Synopsis
ZKC Controller and PC12 pilot both described a loss of separation event when traffic was issued and the altitude of same was mistakenly interpreted as a new altitude assignment.
Narrative
Aircraft X checked in descending to FL240 while over Sector 62. Sector 62 had traffic (Aircraft Y) climbing to FL230. Upon Aircraft X checking in; I issued the traffic at 10 o'clock out of FL205 climbing to FL230 and told the aircraft to expect lower when clear. The pilot's response was FL230. I did not believe the aircraft was descending to FL230; but rather was reading back the altitude that the aircraft was climbing to; as I told them to expect lower when clear of the traffic and that it was climbing to FL230. The aircraft did not state that they were descending or I would have caught the read back error and stopped the descent. When the aircraft were approximately nine miles apart and converging I saw that Aircraft X had descended to FL235 so I verified that they were level at FL240 thinking it might have been an bad RADAR hit. The aircraft said they were descending to FL230. I told the aircraft to climb and maintain FL240 and expedite the climb and reissued the traffic. Sector 27 above me tried to turn their aircraft to keep the two apart; but got no response; so they climbed them to provide some separation. They didn't know I had tried to climb Aircraft Z back up to level. At that time I descended Aircraft Z to provide some separation. The pilot did not read back a descent; so I thought the aircraft was reiterating the altitude that the traffic I had just issued was climbing to. I should have verified when I heard the aircraft read back an altitude; insuring they were not descending; but rather was confirming the altitude the traffic was climbing to. By the pilot not adhering to proper phraseology I did not realize they were descending. In the future I will be verifying anytime an aircraft reads back an altitude that doesn't involve an altitude I am assigning them. I would recommend the pilot utilize proper phraseology in the future as well.
Second reporter narrative
During our initial descent we were; to our understanding; cleared to descend from our cruise altitude of FL270 initially to FL240 then to FL230. Upon leveling at FL230 we were asked by ATC if we were at FL240. When we responded that we were at FL230; we were told to maintain FL240; since another aircraft was in the area. We immediately climbed to FL240. Upon reaching FL240; we were very quickly cleared to 10;000 FT. As a flight crew; we follow very strict procedures during altitude changes which includes a statement to ATC as to what altitude we are going to. As a result; we were surprised by this apparent deviation. Later in our descent; we were told to contact the center by phone since we 'may have been involved in a deviation'. By the time we landed and contacted the Center; they had listened to the tapes and determined that it was their error. The non-flying pilot did state to ATC we were on our way to FL230; the altitude was confirmed by me (flying pilot) and no corrective response was made by ATC. The controllers were cordial and suggested some potential improvements to communication that could help mitigate future miscommunication. While this issue was one of altitude; we will strive as a flight crew to be very specific in our communications whatever the instruction.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.