Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2.5
Situational Awareness; Workload; Training / Qualification; Distraction; Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Human Factors; Procedure
Training on both departure sectors combined; trainee is a Certified Professional Controller in Training CPC-IT supervisor with years of air traffic experience. South flow; MVFR; multiple layers with bases around 2;000' and tops around 10;000'.The trainee released multiple aircraft off BFI; which combined with SEA departures to create a complex situation which was beyond their capability. I had to take over the position with numerous situations developing; the first of which was Aircraft X and Aircraft Y.  Aircraft X was a BFI departure to BFI; but needed to climb to 15;000' to check a system before returning. Aircraft Y's SID off SEA includes an automatic turn to the East; which brought him in conflict with Aircraft X who had been left runway heading off BFI. After taking over; I determined Aircraft X's intentions and initiated a turn away from Aircraft Y and a climb; but it was too late to maintain separation. This led into the next portion of the incident; because I did not climb Aircraft Y. I was attempting to recover from the loss with Aircraft X. A few moments later; I became aware that Aircraft Y was at 5;000' and about to enter a 5;700' MVA.  I instructed Aircraft Y to turn and climb; but it was too late to avoid the higher MVA. The final portion of the incident was in all of this I had turned a subsequent BFI departure; Aircraft Z; to the East as well. I did not climb Aircraft Z and by the time I was aware of it the aircraft was at 3;000' in a 5;000' MVA.  I issued instructions for Aircraft Z to make a left turn to the Northwest; which I knew would parallel terrain in the area; and climb immediately.To my knowledge; all aircraft involved subsequently proceeded without further incident. Management was immediately aware of the incidents.The root cause of the incident(s) is my failure as an On the Job Training Instructor (OJTI) to keep the situation under positive control. I should not have allowed the trainee to release so many BFI departures. My intention was to allow the releases to add some complexity and increase the workload slightly; which would provide an opportunity for me to discuss with the trainee later how it can be smarter to allow fewer releases and have the traffic be more manageable instead. Each release adds complexity to the situation and I was not as aware as I should have been of the specifics of each aircraft the trainee released. Specifically; I would not have released Aircraft X without asking the BFI controller if they were aware of the aircraft's intentions. Knowing ahead of time that Aircraft X wanted to maneuver in the airspace at 15;000' would have allowed better planning. I also should have taken over the position much sooner. I was overconfident in both my ability to recover the sector; as well as the trainee's ability to work the traffic. I was lulled into a false sense of security by the trainee's previous experience and overall performance on the previous training session. I was not aware that the trainee had only a few hours on either departure sector. Had I been aware of that fact; I should have requested the departure sectors be split instead of training on both sectors combined.
Seattle TRACON Controller reported that while instructing; the sector became overloaded and aircraft where in Minimum Vectoring Areas below the required altitudes.