Date: 201605
Local time of day: 1201-1800
Location: SEA.Airport, WA
Light: Daylight
Aircraft: Medium Transport; High Wing; 2 Turboprop Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class B SEA
ATC / advisory: Tower SEA
Second aircraft: Medium Transport; High Wing; 2 Turboprop Eng
Reporter organization: Government
Reporter function: Instructor; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Distraction; Situational Awareness; Training / Qualification
Second reporter function: Departure
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic
Contributing factors: Procedure; Human Factors
Primary problem: Procedure

Narrative: Aircraft X departed straight out on the Seattle 4 departure and Aircraft Y was put into position by LCE (Local Control East). As Aircraft Y was going into position S46 called and stopped our automatic turns to 020 heading and I told LCE expecting them to cross aircraft in front of the departure with both aircraft now being straight out instead of turning the second aircraft. LCE departed Aircraft Y and issued visual separation instructions to the pilot which he accepted.  After Aircraft Y became airborne I realized that they were going faster and out climbing Aircraft X; I told the LCE trainee that they needed to do something with Aircraft Y; LCE then stopped Aircraft Y at 2000 ft. I then called departure Sector A and told them that Aircraft Y was stopped at 2000 ft and was told 'I would expect so' by the departure controller. As I was on with the departure controller I incorrectly assumed LCE had climbed Aircraft Y to 2100 ft because the altitude readout on the STARS showed the aircraft at 2100 ft and they were approaching a 2100 ft MVA area North of Boeing Field.


Second narrative: Aircraft X departed SEA climbing straight out climbing to 120. Aircraft Y departed SEA climbing straight out climbing to 030; with +VV in the secondary scratchpad. Almost immediately Aircraft Y began speed overtake of Aircraft X. LA (Local Airport) called me on Dep A to coordinate and advise Aircraft Y was stopped at 020 due to the overtake with Aircraft X ahead with the same departure procedure. At this point Aircraft Y was nearly directly beneath Aircraft X. I immediately issued a turn for Aircraft Y to establish divergence so I could climb the aircraft.It appears the tower controllers at SEA are unfamiliar with aircraft characteristics. If this continually reoccurring problem cannot be trained and fixed I would recommend discontinuing to allow SEA to use visual separation between departures; and instead make them use standard IFR separation. This problem could be remedied by just taking aircraft characteristics into account prior to launching IFR aircraft.


Synopsis: Seattle Tower and Seattle TRACON controllers reported of an operation where the second departure overtook the first departure. Controllers used altitude to separate traffic.