EMB ERJ 170/175 ER/LR
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Human Factors
I was conducting OJTI during the inbound cargo rush. Within about 10 minutes of the session beginning; the Developmental turned an aircraft on left base abeam the FAF for Runway XXL. Weather conditions dictated that we vector for ILS approach. I questioned the Developmental as to why he was basing the aircraft in such an impossible turn on position. I received a blank stare in return and then some senseless babble about he didn't think it was too tight. By this time the aircraft was already established in the turn; so I issued instructions to fly heading 170; attempting to get enough room to turn the aircraft to join the localizer.  Further discussion ensued; and the Developmental turned the aircraft to a base heading of 140. I said; 'That is not enough;' because the indicated airspeed was 280 knots. The Developmental did not take action to correct; and turned to me to say; 'I am way back on my heels here.'  I issued a turn to join and asked the pilot to make his best rate of turn to capture the localizer. The pilot responded with words to the effect that he was going through and turning at the maximum rate. The aircraft subsequently slid all the way to the XXR localizer course; approximately 4 miles behind the heavy jet on Runway XXR approach. Separation was lost by the time the pilot unkeyed; and I threw in a 'caution wake turbulence' or some such. The aircraft subsequently reported the airfield insight; and a visual approach was issued. I believe that this event happened because I allowed it to happen. I let the trainee go too far down the error chain before I tried to pull it back. I failed to correctly analyze the aircraft's airspeed and tail wind configuration and attempted to take corrective action too late.  In addition; I wasted mental effort trying to understand what was 'going on inside the trainee's head' instead of giving the traffic the attention it deserved. I was honestly flummoxed by the trainee's lack of judgment and lackadaisical attitude.
TRACON Controller described a loss of separation event during an OJT session when failing to intercede and correct the trainee's poor vectoring technique.