ZBW controller described a loss of separation event during a parallel vectoring descent attempt; the...

Narrative:

I was working the radar at alb sector. There was air carrier Y going to ewr at 160 and aircraft X going to cdw at FL240. I wanted to get aircraft X below air carrier Y so I descended him to 140. I cleared him direct agnez and saw that was enough so I turned 10 degrees; then another 10 degrees to parallel the air carrier Y flight. I asked him to maintain 1;500 ft for greater on his descent and he said he was doing 2;000 ft. I then proceeded to tell him leaving 150 thousand cleared direct agnez. He read back 150 direct agnez. I thought he read back the right clearance and in reviewing the tapes he thought he was cleared to 150 and turned towards agnez causing lost of separation. When I asked him to be below 150 in 1 minute he said he was leveling at 150 and I inquired if he was descending to 140 then he proceeded to tell me he was descending to 140. His phraseology was not clear in the situation. I should have verified with the aircraft about the read back and trusted my instinct when I thought he had turned. I should have also had him report out of 150.

Original NASA ASRS Text

Title: ZBW Controller described a loss of separation event during a parallel vectoring descent attempt; the reporter identified a clearance readback error that was not immediately recognized causing the separation loss.

Narrative: I was working the RADAR at ALB Sector. There was Air Carrier Y going to EWR at 160 and Aircraft X going to CDW at FL240. I wanted to get Aircraft X below Air Carrier Y so I descended him to 140. I cleared him direct AGNEZ and saw that was enough so I turned 10 degrees; then another 10 degrees to parallel the Air Carrier Y flight. I asked him to maintain 1;500 FT for greater on his descent and he said he was doing 2;000 FT. I then proceeded to tell him leaving 150 thousand cleared direct AGNEZ. He read back 150 direct AGNEZ. I thought he read back the right clearance and in reviewing the tapes he thought he was cleared to 150 and turned towards AGNEZ causing lost of separation. When I asked him to be below 150 in 1 minute he said he was leveling at 150 and I inquired if he was descending to 140 then he proceeded to tell me he was descending to 140. His phraseology was not clear in the situation. I should have verified with the aircraft about the read back and trusted my instinct when I thought he had turned. I should have also had him report out of 150.

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	896085
Time
Date	201006
Local Time Of Day	1201-1800
Place
Locale Reference	ZBW.ARTCC
State Reference	NH
Aircraft 1
Make Model Name	Citation II S2/Bravo (C550)
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Altitude Undershoot Deviation - Procedural Clearance

ZBW controller described a loss of separation event during a parallel vectoring descent attempt; the reporter identified a clearance readback error that was not immediately recognized causing the separation loss.