Date: 201302
Local time of day: 1801-2400
Location: BFI.Airport, WA
Flight conditions: IMC
Weather elements / visibility: 10
Light: Night
Aircraft: Falcon 50
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class D BFI
ATC / advisory: TRACON S46
Second aircraft: B757 Undifferentiated or Other Model
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach; Flight Crew Returned To Clearance
Contributing factors: Airspace Structure; Environment - Non Weather Related
Primary problem: Environment - Non Weather Related

Narrative: Flying at 2;200 FT; IMC; autopilot engaged; Seattle Approach cleared us for ILS 13R. Approach advised that we were following a Boeing 757 and issued a 'caution wake turbulence' advisory. We were instructed to 'fly no faster than 170 to TOGAE'. The published missed approach instructions require the aircraft to cross OCEZE (3.9 miles southeast of the airport) at or below 1;500 then climb to 6;400 FT while completing the procedure. I briefed the co-pilot to set 1;500 FT in the altitude selector once the glide slope was captured. Crossing ISOGE; the aircraft began to pitch and roll erratically. After a few moments I disconnected the autopilot to have better control in case the oscillations intensified and I needed to recover from a wake induced roll. The oscillations continued as we descended and I decided to go around rather than risk a low altitude/IMC wake encounter. The go around was initiated prior to TOGAE (FAF); which has a 1;600 FT crossing altitude. My copilot announced the missed approach as I activated the go around button; applied power; and pitched up to climb out of the turbulence. ATC called back immediately and instructed us to fly the missed approach as published. I believe we reached an altitude of 2;400 FT MSL when my copilot stated that we were above 1;500 MSL.I immediately began a descent but did not aggressively pitch down. I was hand flying in IMC; did not want to over control; and I wanted to stay above the 757's flight path. We captured 1;500 FT MSL at the MAP and ATC began vectoring us for our second approach; which was normal. I called Approach after landing and learned that their separation standards had been compromised. No TCAS alerts were triggered on our aircraft. There are no mandatory altitudes published other than OCEZE. However; SEA arrivals overfly the BFI arrivals. Throughout training pilots are rigorously taught how to conduct missed approaches and avoid wake turbulence. However; in Seattle's unique airspace; years of repetitive training ran counter to carefully constructed local flight patterns. We abandoned the approach prior to the 1;500 MSL missed approach altitude restriction. Rather than pitching up I could have leveled off a few moments to put the aircraft above the 757's flight path and then descended to 1;500 MSL. Pilots should practice missed approaches prior to the MAP. There are some approaches within the national airspace that may require a descent (due to abandoning an approach early; or respecting mandatory altitudes).  This situation is counter-intuitive and should be trained. Two other factors that night were fatigue and ATC communications. We conducted the first approach at night after a 5-hour flight. I felt tired and abandoned the approach as trained; but should have considered SEA traffic above. I felt that ATC complicated the situation by over communicating throughout the maneuver. My copilot was so engaged with Seattle Approach; that I had to make AFCS mode selections while hand flying the missed approach procedure. This also led to our configuration changes being made later than I would have liked.


Second narrative: We perceived that we were getting too close to the B757 and out of an abundance of caution executed a missed approach when the aircraft moved in an abrupt manner that resulted in a brief loss of separation.


Synopsis: DA50EX flight crew on approach to BFI reported wake vortex encounter in trail behind a B757 on approach to SEA that resulted in porpoising and roll. Captain decided to go around and there was some confusion when they realized they were above missed approach level off altitude.