A LOW TIME C172 PVT PLT LOSES HIS SITUATIONAL AWARENESS ON A PLANNED VFR NIGHT OP WHEN HE RUNS INTO WX CONDITIONS THAT ARE TOTALLY UNEXPECTED; EXPERIENCES AN UNAUTH ENTRY INTO SEATAC CLASS B AIRSPACE; FORCING A RETURN AND LAND TO BFI; WA.

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A VFR FLT WAS PLANNED IN DEPTH WITH 4 WAYPOINTS FROM BFI TO PDX. THERE WERE 3 ON BOARD (2 PAX; AND PLT; WHO FILED THIS RPT). THE DETAIL OF THE FLT PLAN INCLUDED ACCURATE COMPASS VECTORS; WAYPOINT ETA'S WITH VOR RADIALS AS BACKUP. A WX BRIEFING AND A FLT PLAN WAS FILED APPROX 10 MINS BEFORE THE FLT. THE ACFT WAS EQUIPPED FOR IFR FLT AND HAD A MOVING MAP GPS UNIT. TKOF WAS FROM RWY 13L. JUST AFTER TKOF THE PLT OPENED THE FLT PLAN AND THEN EXPERIENCED HIGH LOAD CONDITIONS DUE TO NIGHT LIGHTING AND MODERATE TURB AND WINDSHEAR. A CLASS B INCURSION OCCURRED; DUE TO LOAD AND FIXATION FROM SEVERAL DISTRACTIVE FACTORS: 1) TURB AND WINDSHEAR: TRYING TO KEEP THE ACFT STRAIGHT AND LEVEL DUE TO TURBULENT CONDITIONS CAUSED LOSS OF AWARENESS OF THE FLT PATH. 2) A REAR SEAT PAX WAS DISTRACTING THE PLT WITH A HEADSET MALFUNCTION. 3) THE PLT WAS INEXPERIENCED -- 115 HRS TOTAL TIME AND LIMITED HRS AT NIGHT ALTHOUGH WAS NIGHT CURRENT. 4) DEV FROM THE FLT PLAN. IT WAS INTENDED TO ORIGINALLY TAKE THE VASHON WBOUND DEP BUT DECIDED ON TKOF TO TAKE THE SBOUND DEP THAT WAS UNFAMILIAR. 2 MINS INTO THE FLT THE PLT LOST SPATIAL AWARENESS AND BECAME DISORIENTED. CONCERNED THAT THE ACFT WAS GETTING CLOSE TO THE SEATAC AIRSPACE OR RENTON ARPT AIRSPACE THE PLT TOOK EVASIVE ACTION AND DUMPED HT TO GET BELOW 1100 FT; INSTINCTIVELY FEELING THIS WAS A SAFE LEVEL IN THE AREA. THE INTENTION AT THIS POINT WAS TO RETURN TO A PATH THAT WAS FAMILIAR (THE VASHON DEP). IT WAS AT THIS POINT THAT THE PLT REALIZED THAT THE ACFT IN FACT WAS MUCH FURTHER S THAN PREVIOUSLY THOUGHT. A VISUAL SCAN EXTERNALLY CONFIRMED THIS AS AN ACFT ON FINAL FOR SEATAC WAS OFF THE STARBOARD SIDE AT 3 O'CLOCK POS; APPROX 1 NM. FURTHER EVASIVE ACTION WAS TAKEN BY STEERING AT HIGH BANK NBOUND TO AVOID XING THE SEATAC CTRLINE IN ORDER TO AVOID THE ILS CORRIDOR. SHORTLY AFTERWARD THE PLT CHANGED TO A WESTERLY COURSE TO ATTEMPT TO INTERCEPT THE VASHON WBOUND DEP THAT WAS FAMILIAR TO HIM IN AN EFFORT TO REGAIN SPATIAL AWARENESS. AFTER CHANGING TO A WESTERLY PATH BFI TWR MADE CONTACT AND REQUESTED THAT IN THE FUTURE TO NOTIFY THEM OF ANY DEP DEV. THE ACFT CONTINUED OVER THE SHORELINE INTO THE PUGET SOUND WHERE THE TURB STOPPED AND SMOOTH FLT CONDITIONS RETURNED. AT THIS POINT A DECISION WAS MADE TO TERMINATE THE FLT AND RETURN TO BFI. A LNDG WAS MADE ON THE LONG RWY 13R. AFTER LNDG THE TWR NOTIFIED THE PLT THAT A TELEPHONE CALL WAS NEEDED TO SEATAC. CHAIN OF EVENTS: 1) IN PLANNING THE FLT IN THE AFTERNOON IT WAS KNOWN THAT THE DAYTIME WX WAS IFR. THE FORECAST FOR TKOF WAS VFR BUT CLOSE TO MINIMUMS AND TURBULENT. 2) A DEV FROM THE INTENDED WBOUND DEP WAS MADE. 3) AN UNFAMILIAR DEP WAS FLOWN. 4) DISTR FROM THE REAR PAX DUE TO THEIR FAULTY HEADSET. 5) EVASIVE ACTIONS WERE TAKEN. 6) RECOGNIZING THAT: A) INCURSION OCCURRED AND B) THE CONDITIONS WERE ABOVE SKILL LEVEL THE PLT ELECTED TO ABORT THE FLT AND RETURN TO BFI. HUMAN PERFORMANCE CONSIDERATIONS: 1) ALTHOUGH LICENSED TO FLY IN THE WX CONDITIONS THE PLT WAS NOT EXPERIENCED TO DO SO EFFECTIVELY. 2) LIMITED NIGHT EXPERIENCE. 3) BAD COCKPIT MGMNT OF RESOURCES. A GPS UNIT WITH MOVING MAP DISPLAYING THE CLASS B BOUNDARIES WAS AVAILABLE BUT THE ZOOM FUNCTION WAS SET TO A SCOPE OF LIMITED USE. A MISINTERP OF THE CLASS B BOUNDARIES WAS MADE. PREVENTIVE SUGGESTIONS: 1) MAINTAIN ORIGINAL PLANNING. 2) BE FAMILIAR WITH ALL PATHS OF FLT AVAILABLE. 3) MORE EXPERIENCE REQUIRED IN NIGHT AND MARGINAL CONDITIONS. 4) UNDERSTAND PERSONAL LIMITATIONS.