Enroute controller described a TCAS RA event when he/she failed to note an incorrect read back on an...

Narrative:

An A319 was originally climbed to FL230 by me. A citation was level at FL250 needing to start his descent. The controller working the citation was sitting next to me and pointed out the citation descending to FL220. I amended the A319's altitude to FL210 and missed his read back. Listening to the audio; another aircraft checked on immediately after I issued FL210 to the A319. Then the A319 said 'center; flight XXX?' I only heard this in the audio; I was off line approving a point out when he said this. I then issued traffic to the A319; 'traffic 1 - 2 o' clock; 10 miles eastbound; lifeguard citation descending to FL220; higher when clear. After a minute passed I noticed the A319's altitude readout was 211. I should have said something immediately but figured he went slightly above leveling off. The next hit was 216 and I immediately keyed up to stop him; but he started talking a split second before I did and said he was responding to an RA. I immediately told the controller next to me to stop the citation. The citation leveled off at 230; and the A319 never went above 218. I gave the A319 a safety alert for the same traffic I had told him about; 'safety alert; 3 o' clock 2-3 miles eastbound citation descending to FL220'. He said they were finishing the maneuver. I told him to maintain his present altitude at that time which was 217. It is easy to see why this happened; the whole time the A319 thought he was cleared to FL230. I missed a crucial read back because I got distracted and didn't realize the A319 never got the clearance. When I issued traffic descending to FL220 'higher when clear'; the pilots of the aircraft should have questioned their clearance to 230. It's possible they questioned it and didn't say anything; or because there were a few miles between them and the citation they didn't think twice about it. Recommendation; I need to be more diligent about crucial read backs. Possibly pilots should question traffic calls like that more often; or the altitude should be reinstated to maintain after a traffic call.

Original NASA ASRS Text

Title: Enroute Controller described a TCAS RA event when he/she failed to note an incorrect read back on an altitude assignment; indicating required coordination distraction/s contributed to this event.

Narrative: An A319 was originally climbed to FL230 by me. A Citation was level at FL250 needing to start his descent. The Controller working the Citation was sitting next to me and pointed out the Citation descending to FL220. I amended the A319's altitude to FL210 and missed his read back. Listening to the audio; another aircraft checked on immediately after I issued FL210 to the A319. Then the A319 said 'Center; Flight XXX?' I only heard this in the audio; I was off line approving a point out when he said this. I then issued traffic to the A319; 'Traffic 1 - 2 o' clock; 10 miles eastbound; Lifeguard Citation descending to FL220; higher when clear. After a minute passed I noticed the A319's altitude readout was 211. I should have said something immediately but figured he went slightly above leveling off. The next hit was 216 and I immediately keyed up to stop him; but he started talking a split second before I did and said he was responding to an RA. I immediately told the Controller next to me to stop the Citation. The Citation leveled off at 230; and the A319 never went above 218. I gave the A319 a safety alert for the same traffic I had told him about; 'Safety Alert; 3 o' clock 2-3 miles eastbound Citation descending to FL220'. He said they were finishing the maneuver. I told him to maintain his present altitude at that time which was 217. It is easy to see why this happened; the whole time the A319 thought he was cleared to FL230. I missed a crucial read back because I got distracted and didn't realize the A319 never got the clearance. When I issued traffic descending to FL220 'higher when clear'; the pilots of the aircraft should have questioned their clearance to 230. It's possible they questioned it and didn't say anything; or because there were a few miles between them and the Citation they didn't think twice about it. Recommendation; I need to be more diligent about crucial read backs. Possibly pilots should question traffic calls like that more often; or the altitude should be reinstated to maintain after a traffic call.

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	946490
Time
Date	201105
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	A319
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	Citation V/Ultra/Encore (C560)
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Published Material / Policy

Enroute controller described a TCAS RA event when he/she failed to note an incorrect read back on an altitude assignment; indicating required coordination distraction/s contributed to this event.