Date: 201709
Local time of day: 1201-1800
Location: S46.TRACON, WA
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B S46
ATC / advisory: TRACON S46
Second aircraft: Medium Large Transport
Reporter organization: Government
Reporter function: Departure; Instructor
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Distraction; Situational Awareness; Training / Qualification; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Staffing; Procedure; Human Factors; Chart Or Publication; Airspace Structure; Company Policy
Primary problem: Staffing

Narrative: I was conducting training on two departure sectors combined. It was supposed to be not too busy. I was told we could split off a sector if it started picking up. B737 off BFI departed on an RNAV departure which is a heading to a fix; expect vectors. The fix is about 5 miles off departure end. He [reached] the fix and turned on course which was about a 240 heading. He was issued 6000 feet. His English was not great. Meanwhile a conflicting departure heading 140 [off a different airport] climbing to 3000 feet came off. We saw B737 starting to turn and issued him a left turn to a 090 heading. He questioned it. I keyed up and said turn left heading 070 immediately. He read it back. I called Aircraft Y traffic for the B737. He had it in sight; I said maintain visual. He read it back. Separation was reestablished but then the departures starting coming out and it got busy fast.I keyed up the local line and said; 'Stop 140s' to help mitigate the traffic. I stayed on position and tried to get things under control. But because so many planes were out of position by the time we got the B737 situation under control; it was very hard to get control of the position. I had two frequencies and everyone was stepping on everyone. I had aircraft stuck under other aircraft; it was a nightmare. I have never been so busy on departures. I was already shaking from the B737 scare and I was feeling super panicked. I tried to establish separation with everyone. They tried to split off a Sector but I couldn't do that at the time so I asked that controller to work handoff instead to coordinate these aircraft that were out of position. I ended up with Aircraft Z below the MVA (minimum vectoring altitude). He was at 6000 feet in a 7000 feet MVA. I turned and climbed him but I called him the wrong call sign (I was flustered). That took multiple transmissions to rectify. With everyone stepping on each other and me having to repeat myself; and trying to coordinate and everything; it was bad. During all of this; tower kept calling for releases. I was trying to do a brasher statement to the B737 (never got a chance); center was calling; a [third aircraft] heading 140 departed after I stopped them. I really wanted to hit the stop departure lights but they were so far away and I couldn't reach them.First; the RNAV departure. I have been wanting to report this thing for a while. Ever since it changed; it now says a heading to a fix; expect vectors. I cannot tell you how many aircraft have turned on course after that fix instead of continuing on the 135 heading or asking for a Vector. From now on; I am going to issue a heading to every aircraft departing BFI. This is increasing workload and seems dumb but I don't care. Never again.Second; I should have de-combined the sectors. It wasn't supposed to get busy I thought my trainee could handle it. I wanted him to work a little volume. That was a mistake. I want him to work on picking up the pace so I thought if he had a few more airplanes but not too complex that it would be good training.Third; the check departure lights are positioned so that I would have to get up; walk away from my scope; climb up on the console (I am too short to reach them); and who knows what would have happened while I was walking away. I could have keyed the line I guess and told them to stop departures but I felt so busy that I couldn't. I probably felt busier than I was. My head was really spinning.Fourth; the low altitude alert never went off. Never. I was four miles into that MVA and it never went off. It all snowballed after that B737 and it got too busy too fast. I was trying my best to keep everyone separated. I was focused more in the southern portion of my airspace because that's where all the conflicts were happening. I forgot about Aircraft Z out there at 6000 feet. My scan got bad as I was so concentrated on the section with all the scary stuff. I was in full on recovery mode and it wasn't pretty.Hopefullychanging that RNAV departure will help a little. It's such a fine line of trying to challenge your trainee and it getting too busy. But that was busier than I could even handle. My supervisor was trying to split it off to help; but honestly it was all so fast and unexpected that it was too late. I wish someone would have seen that Aircraft Z out there and said something. I needed all the help I could get.

Callback: The reporter stated the departure procedure has been redesigned and the pilot confusion has been resolved.



Synopsis: TRACON Departure Controller reported becoming overloaded when a departure deviated from course and allowed another aircraft to fly below the Minimum Vectoring Altitude.