Date: 201507
Local time of day: 0001-0600
Location: SEA.Airport, WA
Light: Dawn
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B SEA
ATC / advisory: TRACON S46; Tower SEA
Second aircraft: B777 Undifferentiated or Other Model
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: ATC Issue All Types; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Environment - Non Weather Related; Procedure
Primary problem: Ambiguous

Narrative: A large B-777 preceded us on takeoff out of SEA.  Just as the B-777 began his takeoff role; SEATAC Tower assigned us to line up and wait; which we did; slowly; giving the B-777 time to get down the runway and its takeoff thrust time to dissipate.  After what seemed like a sufficient amount of time (I did not start my clock as I usually do when following a heavy); we were cleared for takeoff.  The winds were approximately 320 at 10/G15.  On takeoff roll; we felt a couple of minor buffets; which we attributed to the gusts.  In second stage climb at approximately 1500 FT AGL; in clean configuration; our aircraft experienced two moderate wake turbulence events in close succession; each lasting about five seconds.  During the events; it was obvious the flying pilot (the first officer) was controlling the aircraft but with moderate difficulty.  Climb gradient dropped to approximately 500 FPM; speed increased to approximately 260 knots (or 30 knots above target) and the pilot flying (PF) needed to constantly recover from wake induced angles of bank of up to approximately 25 degrees.  During the second event; I encouraged the PF to keep the nose up to keep the climb in and avoid airspeed increase.  I then informed Seattle Approach Control that we were experiencing wake turbulence off the preceding B-777 and requested a vector to the left to get out of its wake.  Approach immediately gave us an initial vector of (I believe if was) 300 degrees and promised us direct BANGR.  Upon turning to the new heading we immediately ceased experiencing the wake turbulence; and did not experience further wake turbulence events.In my safety and welcoming announcements to the passengers after FL180; I apologized to the passengers for the bumps after takeoff and attributed them to gusty winds and some wake turbulence off a preceding aircraft; but assured them that the rest of the flight should be much smoother.  It may have been the mandated separation time behind the heavy.  Because I did not time it (as I usually do); I don't know; but it obviously was not actually enough time for real wake dissipation to occur.   I have noticed that the time pressure caused by the lack of the center runway which is under construction may be encouraging all concerned (pilots and controllers) to shorten required separations to the limits; for both arriving and departing aircraft; in order to limit the delays that this construction and the increase in traffic is causing.




Synopsis: B737 Captain reported encountering wake turbulence in trail of a B777 after takeoff from SEA.