Citation V/Ultra/Encore (C560)
Air Traffic Control Developmental
Confusion; Communication Breakdown
ATC Issue All Types; Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
Procedure; Human Factors
A Citation V was instructed to descend via the arrival except to maintain 9;000 for traffic. Aircraft Y was given traffic of the Citation V and eventually called the jet in-sight. The Citation V was given the prop traffic and never reported the prop in-sight. When the Citation V read back the descent instructions they were incorrect and I did not catch the incorrect readback. The last scan that I had of the 2 aircraft is that they were separated by 1;000 FT. The Citation V descended to 8;300; was only about 1 mile north of Aircraft Y and the Conflict Alert (CA) went off. I immediately asked the Citation V if they were level at 9;000 and they just read back their call sign. Then the Citation V keyed up again to advise me that they were indeed climbing from 8;300 to 9;000. A few miles later when I gave the Aircraft Y a frequency change to ZDC he advised me that the Citation V did not pass overhead at 1;000 FT; but at 300 FT. I advised Aircraft Y that we were looking into it. Upon review of the recordings is when I realized that I had missed the incorrect readback. Change the phraseology for the difference in altitude when an aircraft is on a 'descend via' clearance. It would be helpful if it were something more distinct and direct so that it would be less questionable. The current phraseology is questioned by many pilots of all calibers. Having something more blatant and less wordy might be better. Perhaps pilots could provide more direct feedback.
PCT Controller described an NMAC event acknowledging a failure to correct an incorrect readback. Controller listed the 'descend via' phraseology as problematic.