Enroute controller described a near midair collision situation when he failed to note an incorrect d...

Narrative:

A phenom was descending on the arrival. The aircraft entered the sector at FL190; when the aircraft entered my airspace; I gave the pilot a decent to 140. The pilot hesitated in his response and read back descending to one zero. The transmission was weak and I missed the bad altitude. The phenom had southbound traffic a B350 at 130 with a restriction to cross the intersection at 040. When I saw the data block of the phenom drop below 140 I queried the pilot and asked to verify level at 140. I think I was stepped on; or the pilot didn't respond; and I asked again to verify level at 140. This time the pilot said no; [they] were descending to ten. I told the pilot of the phenom to stop his descent but by this time separation had been lost. I'm not sure what I can say; the only thing I can recommend is to be ultra vigilant in making sure that the proper read back is gotten for every transmission.

Original NASA ASRS Text

Title: Enroute Controller described a NMAC situation when he failed to note an incorrect descent clearance resulting in the conflict.

Narrative: A Phenom was descending on the arrival. The aircraft entered the sector at FL190; when the aircraft entered my airspace; I gave the pilot a decent to 140. The pilot hesitated in his response and read back descending to one zero. The transmission was weak and I missed the bad altitude. The Phenom had southbound traffic a B350 at 130 with a restriction to cross the intersection at 040. When I saw the Data Block of the Phenom drop below 140 I queried the pilot and asked to verify level at 140. I think I was stepped on; or the pilot didn't respond; and I asked again to verify level at 140. This time the pilot said no; [they] were descending to ten. I told the pilot of the Phenom to stop his descent but by this time separation had been lost. I'm not sure what I can say; the only thing I can recommend is to be ultra vigilant in making sure that the proper read back is gotten for every transmission.

Data retrieved from NASA's ASRS site as of July 2013 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	1059597
Time
Date	201301
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	Super King Air 350
Operating Under FAR Part	Part 91
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Embraer Phenom 300
Operating Under FAR Part	Part 91
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict NMAC Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

Enroute controller described a near midair collision situation when he failed to note an incorrect descent clearance resulting in the conflict.