Date: 201710
Local time of day: 1201-1800
Location: MDT.TRACON, PA
Flight conditions: VMC
Light: Daylight
Aircraft: PA-60 600 Aerostar
Flight plan: IFR
Flight phase: Descent
Airspace: Class E MDT
ATC / advisory: TRACON MDT
Second aircraft: SR20
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Aircraft X was inbound on an IFR flight plan from the southwest initially at 5000 feet. Aircraft X was issued a descent to 3000 feet; at which time I noticed a VFR tag; which had recently departed climbing through 2500 feet southbound. I issued a preliminary traffic call to Aircraft X. When they were roughly 5 miles apart and Aircraft X was descending through 3800 feet; the VFR aircraft was climbing through 3200 feet; the VFR aircraft called me for advisories. I had them ident to be sure; then issued an immediate right turn westbound and issued a right turn eastbound to Aircraft X. The other aircraft; Aircraft Y; was very slow to take the turn; but continued his climb as Aircraft X continued his descent. The targets merged at 3600 feet; with the pilot of Aircraft X exclaiming 'that was close' on the frequency. Aircraft Y informed me that he had stopped his turn because he believed it would put him in more conflict with Aircraft X. My initial judgment based on their tracks and climb/descent rate led me to believe the best course of action was to use vectors to separate; but the delay and lack of commitment to the turn by Aircraft Y exacerbated the conflict and I did not issue any further altitude restrictions to try to mitigate the conflict; as I assumed they wouldn't be able to take effect until they were already through the other's altitude. I did not stress the immediate nature of the necessity to turn; or insist on a sharper turn rate for either aircraft; and the slow turns from each made the conflict more significant than if they had not turned at all.Although I recognized the potential for conflict early; I waited too long to issue positive control instructions to my IFR aircraft to steer well clear of the other aircraft. By the time I positively identified the VFR aircraft and gleaned his intentions; the need for resolution was imminent but I did not use words like 'immediate' to stress this to the VFR aircraft. Ultimately; if I had decided to use altitude instead of vectors at the time of attempted resolution; the conflict would've been far less severe even if it required climbing my IFR back up before descending him again for the approach. I misjudged the delay in the turn; prompted additional information from the VFR prior to issuing a turn to the IFR; and failed to consider aircraft speed; winds aloft; etc.; and their effects on both aircraft's ability to turn in a timely manner. When significant closure rates are happening and conflicting aircraft are inside of a certain distance apart; altitude separation is almost always the superior method to ensure some level of separation; even if not 'standard.'




Synopsis: A TRACON Controller reported his vectors for traffic were too late and insufficient to avoid a NMAC between their aircraft and an unidentified VFR track.