Date: 201708
Local time of day: 1201-1800
Location: S56.TRACON, UT
Light: Daylight
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B SLC
ATC / advisory: TRACON S56
Second aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Departure; Instructor
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Time Pressure; Workload; Distraction; Human-Machine Interface; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Aircraft; Procedure
Primary problem: Aircraft

Narrative: I was performing OJTI on the Lake and Gunnison sectors combined. This is the Developmentals first positions at his first FAA facility. We had a light to moderate level of traffic; with some VFR aircraft and a few weather deviations adding to the complexity. Aircraft X departed on the NSIGN departure OCS transition assigned climb via 230 kts. Aircraft Y departure 3.5 in trail EKR transition assigned the same. Separation increased to about 3.78. We were working on climbing and coordinating speeds with the traffic as Aircraft Y started to overtake Aircraft X since Aircraft X did not continue to speed up to 230 kts as assigned. As soon as the Developmental was finished with the other traffic; he caught Aircraft X being overtaken and sped him up to 250 kts. This caused the already slow climbing aircraft to not be able to make the next crossing restrictions which we need to get him above terrain. Aircraft X clearing the terrain became the focus. Aircraft Y was turned to the east as soon as his climb rate indicated he would clear the terrain; however separation had decreased to 2.6 miles. At this point my main focus is Aircraft X. Tower can give us aircraft less than 3 if the routes diverge if they get visual separation on the ground; this is indicated on the strip. However; the Developmental was not using strips since we are not required to. We are used to seeing this kind of proximity on a daily basis; I did not notice the separation decrease since the main focus was on Aircraft X's climb. Aircraft X advised us he could not make the crossing restriction. As Aircraft X made the eastbound turn on the departure I realized he would not make the MVA and issued a turn to the northwest away from terrain. Aircraft X just barely entered the 11000 MVA climbing through 10600. Aircraft X speeds were deleted to expedite the climb; however he increased his speeds further by 20 knots and swung wider than anticipated. I definitely should have turned him off the departure earlier. However; we use judgment to start the east turn prior to clearing the MVA; we don't use crossing restrictions; at that point even a slow underperforming [aircraft type] will clear 11000 out of 9800 at that point with an expeditious climb. As far as turning Aircraft X earlier; next time if there is any doubt I'll take him off the departure. When these unexpected situations happen on this sector; and aircraft then continue to not operate as expected; there are very little outs. We need to get above 12000 to turn east; the downwind is descending to 11000 about 6 miles west of us. Getting visual separating is technically an 'out' but does not increase the safety situation. It is still up to us to fix something. Visual off the ground; though not used here; puts aircraft in very close proximity with little outs when something goes wrong and increased workload greatly. It also leads us to a point that we expect visual separation with less than three miles. The fact they put it on the strip doesn't help since we are not required to use strips and the expectation to when aircraft are departed 1.5 in trail is too difficult to keep up with. I suggest we get all departures 3 in trail. Then if we see separation collapsing we don't have to scramble to get or verify visual separation. This procedure has caused multiple safety situations throughout the years.




Synopsis: S56 TRACON Controller reported a loss of separation due to a slower speed aircraft in front of a faster departure.