ZKC controller and PC12 pilot both described a loss of separation event when traffic was issued and ...

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.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	969582
Time
Date	201109
Local Time Of Day	1801-2400
Place
Locale Reference	ZKC.ARTCC
State Reference	KS
Aircraft 1
Make Model Name	PC-12
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Person 2
Function	Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Altitude Overshoot Deviation - Procedural Clearance

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.