Date: 201308
Local time of day: 0601-1200
Location: SEA.Airport, WA
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
ATC / advisory: TRACON S46
Second aircraft: B737-800
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Flight Engineer; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Ambiguous

Narrative: We departed SEA Runway 34C in clear weather. [A] 737-800 had preceded us on departure. First Officer; hand-flying; with LNAV and VNAV selected on the MCP; MOUNTAIN6 Departure. SID calls for over flying NEZUG and then a turn right to a heading of 070; about 2 miles prior to NEZUG; at 5;000 FT; we [experienced] a wake turbulence upset; which caused nearly 30 degrees of right bank. First Officer appeared to react properly; as the bank angle initially came back to about 20 degrees of right bank; but the turbulence continued. The bank angle then varied between 20 to 25 degrees of right bank. I was focused on the aircraft attitude; and was concerned that the First Officer was having trouble leveling the wings; so I asked him; 'Are you able to level the wings?' I was about to take control of the aircraft. I didn't hear him answer; as ATC directed us to turn left to a heading of 340. The First Officer turned to that heading. Shortly thereafter; we were given an easterly heading; and the Controller asked if we had programmed the departure correctly. I told him that we had plugged it in and hooked it up; but that we had experienced a wake turbulence upset that caused us to turn right; and that we were trying to level the aircraft and turn back to the left. He responded 'OK.' The First Officer and I then had a discussion about what had happened and he told me that; at some point after the onset of the upset; he was leading the turn a NEZUG. He thought it was a flyby waypoint; like those that he had experienced at international airport departures in the past. I told him that NEZUG is not a flyby waypoint; and this fact is reemphasized in our company [briefing] page for Seattle. I was unaware of his thought process; and because this started as a wake turbulence upset; I thought he was still having difficulty leveling the aircraft. In reality; somewhere in the process; he had decided to lead the turn; and he was doing that. I don't know if there was a loss of separation with another aircraft in this event; but the departure is noise sensitive; and the Controller seemed to be watching us closely. This event would probably have prevented if we had held a more detailed discussion in the preflight briefing; so that I knew the First Officer understood that NEZUG is not a flyby waypoint. The company [briefing] page for SEA emphasizes this. When I fly these types of SIDs; I normally always engage the autopilot soon after takeoff; and a properly programmed departure is always precisely flown. I don't like to hand-fly these things; and I don't like watching first officers chase the flight director; sometimes imprecisely; on any of these departures. In the future; I will spend more time discussing these SIDs; so that we both understand what is expected; and I will be more forceful in my recommendation to always use the autopilot to fly the departure.




Synopsis: B757 Captain reported wake vortex encounter in trail of a B737-800 on departure from SEA that resulted in a course deviation.