Date: 201301
Local time of day: 1801-2400
Location: ZZZ.TRACON, US
Aircraft: EMB ERJ 145 ER/LR
Flight plan: IFR
Flight phase: Descent
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: My role: OJT instructor of a Controller/Trainee within a month of his certification. Aircraft positions at start of scenario: Archer; 12 Northwest of airport and aligned with final for runway; in level flight (not landing) and Southeast bound; the Embraer 145 was 12 north northeast of the Archer on a 200 heading; aimed right at the Archer. The Cherokee Archer was at 4;000 FT in level flight. The Embraer 145 was on a vector toward final for a visual approach; and the projected flight path of the Embraer 145 toward runway would conflict with the Archer's southeast bound course; since the Embraer 145 was still above 8;000 FT and descending. It was clear to me how to handle this: aim the Embraer 145 slightly behind the Archer on a new vector. This new vector would create a 'crossing course' and set up the Embraer 145 for a long left turnaround for the visual approach. If we lost three miles lateral separation as the Embraer 145 descended behind the Archer; we would force the Embraer 145 to continue on that crossing course till they were at 3;000 FT below and behind the Archer; and thus able to turn back in. In trying to teach the trainee about vectoring safely yet 'shaving it close' to minimize the extra miles flown; and also to keep the Embraer 145's flight path within a reasonable zone so that the now-required long left turnaround back to the airport would be successful; I instructed the trainee - as the Embraer 145 was crossing two miles directly on the Archer's six o'clock and on that perpendicular-to-the-Archer's-path 220 heading at 4;000 FT at 220 KTS - to clear the Embraer 145 for the visual approach. My thoughts: this would create a sweeping wide left turn; allowing ample room for a normal turn and descent onto a five mile final for the runway; thus 'ducking under' the Archer. Background info: the trainee and I have worked a few dozen 'fast aircraft following slow aircraft' scenarios in the ETG simulated environment. As the trainee showed nearly complete memory loss of this instruction; frustration increased on my part. Result: my frustration resulted in my directing him to do what I would have done; and perhaps did it a bit to 'show off' that this was going to work all day long if done correctly. I'm still frustrated at his inability to see spacing on final. The Embraer 145 turned in more quickly than I'd expected and slowed their descent rate. This resulted in an overtake of the Archer; with us achieving 1;000 FT vertical separation below and behind the Archer as the Embraer 145 was 'about' three miles behind the Archer. What I feel most accountable for: it's been common knowledge in Approach Control that once an aircraft flies past or abeam its destination they will naturally slow down to stay 'closer to home;' and one can anticipate they will turn more steeply too. I forgot this. My surprise at how close it had become was then tempered by my realization I'd forgotten that a pilot will do that. Plan was flawed by forgetting the still unwritten rule; perhaps caused by frustration/impatience. This operation falls into what I've been coached on in the past: running 'em tight will occasionally end up with a loss of separation. Most recently I was coached that this is expected; 'don't worry about it.'




Synopsis: Tower Controller providing OJT tried to demonstrate expeditious vectoring techniques that resulted in a loss of separation event.