Date: 201308
Local time of day: 1201-1800
Location: W55.Airport, WA
Flight conditions: VMC
Weather elements / visibility: 50
Light: Daylight
Aircraft: Beaver DHC-2
Operator: Air Taxi
Operating under: Part 135
Flight plan: None
Mission: Ferry / Re-Positioning
Flight phase: Final Approach
Airspace: Class G W55
ATC / advisory: CTAF W55
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Taxi
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Human factors: Communication Breakdown
Anomaly: Conflict NMAC
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Airport; Human Factors; Airspace Structure
Primary problem: Ambiguous

Narrative: I used standard uncontrolled airport procedures for the Lake Union traffic pattern including appropriate radio reporting point calls. I called; 'South of Sand Point; Webster Point; Husky Stadium; the I 5 bridge; Gasworks Park; left downwind; turning base; and final landing north.' Each call included my Aircraft ID; my location; and my intentions. While crossing the I-5 bridge I noticed an aircraft on wheels over mid Queen Ann hill moving north bound west of Lake Union. He looked to be about 1;100 FT about a mile west of Lake Union heading north. I heard a call from the pilot while he was over Queen Ann to say he was proceeding northbound.While on base descending; I heard the pilot announce he was over Lake Union and asked me if I had him in sight. I replied no. A few seconds later the aircraft flew under us from left to right approximately 80 FT below and 100 FT in front of us and exclaimed he had been making radio calls as he passed from underneath us to slightly above us heading from north west to south east then proceeded to turn south toward Boeing field. After he passed us and reported he was southbound to Boeing I asked if he would cross the final approach of an uncontrolled airport at 900 FT. He did not respond. (I realized he was actually lover than 900 as I as about 900 descending on base.)A second pilot on board my aircraft sitting in the back seat saw the aircraft cross from behind his left rear side window and called the traffic. I shallowed my descent. I landed and declined to fly for the remainder of day for the safety reasons.There were about four other aircraft landing at Lake Union during the time I made my first call and encountered the near miss. The pilot of aircraft involved seemed to be sight seeing in the traffic pattern of a busy uncontrolled airport environment unaware of the danger of his decisions and actions. He was aware of the common traffic advisory frequency as he was using it.While I was aware the wheeled aircraft was in the area and out of place; my attention was focused on the choosing a landing path through the busy boat traffic on a warm summer day on Lake Union. My attention should be divided between the two.




Synopsis: A float plane inbound to the Lake Union waterstrip north of SEA experienced an NMAC with another aircraft.  Both had been broadcasting on CTAF; but did not have visual contact with one another.