Gulfstream V / G500 / G550
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Conflict NMAC
General None Reported / Taken
Human Factors; Procedure
I had Aircraft Y (I think) regional jet holding at XXXXX; just south of XXX. The aircraft was holding at 10;000 FT (showing 10;100 on the scope). I had a jet coming in to XXX also given holding; or was going to be given holding; can't remember. The aircraft was descending down to 11;000 FT or 12;000 FT. The aircraft inquired about cancelling and going in VFR. He then cancelled IFR and I told him to maintain VFR and I think I told; or at least suggested; he stay VFR at 10;500 FT or above. I thought I told him about the traffic holding at 10;000 FT. I thought he acknowledged the VFR 10;500 FT or above. As I went to call traffic to Aircraft Y; I noticed what seemed to be a very fast descent on Aircraft X. I quickly called to him and confirmed he was going to stay at 'ten-five' or above. I know it's not correct phraseology but I didn't feel I had time; that this was very serious. I had expected him to level so I wasn't expecting a serious situation that requires a traffic alert. Aircraft Y inquired as to what that aircraft was doing. I believe I told him he was VFR inbound to XXX; but can't remember if he said he was in sight or just using TCAS. Either way; it was very scary and close. Aircraft Y told me something about the aircraft not being VFR and I honestly didn't care; saying something to the effect of 'that's not my call' because I didn't care; I had RADAR and wanted to make sure they didn't hit; VFR or IFR. I think I needed to be more assured that the VFR aircraft was going to stay at 10;500 FT or above; even though that still doesn't guarantee he will. I have no idea how quickly it actually happened; but it felt very fast and I felt like I was very aware of the potential conflict; but just didn't expect a safety situation.
Enroute Controller described an unsafe event when IFR holding traffic and a VFR aircraft that cancelled IFR were in conflict; the Controller acknowledging increased altitude assurance from the VFR aircraft would have likely prevented the event.