CITABRIA PLT EXECUTES A GAR AT PAE AND EXPERIENCES AN NMAC WITH A PRIOR C172 DEP.

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I WAS ARRIVING PAE FROM THE W. I CALLED PAINE TWR (132.95) OVER LANGLEY AND WAS INSTRUCTED TO PROCEED TO AND RPT ARR AT THE MUKILTEO FERRY AND THEN ENTER DOWNWIND. I SUBSEQUENTLY DID SO AND WAS INSTRUCTED TO FOLLOW A CESSNA ON DOWNWIND. I DID SO. JUST PRIOR TO MY TURNING BASE; PAINE TWR INSTRUCTED ME TO EXTEND MY DOWNWIND TO MAKE SPACE FOR ANOTHER ACFT ARRIVING FROM THE S. AS THAT ACFT PASSED; I WAS INSTRUCTED BY TWR TO TURN BASE AND WAS CLRED TO LAND RWY 34L. ON VERY SHORT FINAL; I OBSERVED A C172 (OR POSSIBLY C182) ENTERING THE RWY AT THE APCH END OF THE RWY. I HAD NOT HEARD THE TWR COMMUNICATING WITH THE PLT OF THIS ACFT; SO I ASSUMED GND CTL HAD INSTRUCTED IT TO CROSS THE RWY. HOWEVER; JUST PRIOR TO MY XING THE FENCE AT APPROX 100 FT ALT; THE CESSNA TURNED ONTO THE RWY AND BEGAN A TKOF RUN. I IMMEDIATELY CALLED TWR AND CALLED; 'I HAVE A CONFLICT ON THE RWY.' RECEIVING NO IMMEDIATE RESPONSE; I AGAIN REPEATED; 'I HAVE A CONFLICT ON THE RWY.' I WAS ALREADY SLOWED TO FINAL APCH SPD SO I COULD NOT SAFELY SLOW FURTHER. HOWEVER; THE CESSNA AHEAD LIFTED OFF AND I DETERMINED THAT THE SAFE COURSE OF ACTION WOULD BE FOR ME TO LAND BEHIND THE DEPARTING ACFT; RATHER THAN GOING AROUND AND AGAIN PLACING OUR ACFT IN CONFLICT. JUST PRIOR TO LNDG; THE TWR CTLR THEN CALLED; 'ACFT X; GO AROUND.' EVEN THOUGH I JUDGED THAT IT WOULD HAVE BEEN SAFER TO LAND AT THAT POINT; I ALSO BELIEVED I COULD SAFELY GO AROUND. I REPLIED; 'ACFT X; GOING AROUND;' AND COMPLIED WITH THE INSTRUCTION. AS I GAINED ALT; I SIDESTEPPED TO THE R IN ORDER TO AVOID COLLIDING WITH THE CESSNA; WHICH WAS APPROX 50 FT AHEAD OF ME. AT THIS POINT; TWR CALLED; 'ACFT X; DO NOT TURN XWIND. SIDESTEP TO THE R TO AVOID THE CESSNA AHEAD.' I REPLIED; 'I HAVE ALREADY SIDESTEPPED.' 'ACFT X CHANGE FREQ TO 120.2 FOR INSTRUCTIONS TO ENTER THE PATTERN FOR RWY 34R.' THIS SEEMED A STRANGE INSTRUCTION; SINCE REMAINING IN THE PATTERN FOR RWY 34L WOULD HAVE BEEN THE SIMPLEST PROC. (AT NO TIME; DID I HEAR TWR COMMUNICATE WITH THE CESSNA AHEAD.) I CHANGED FREQS. 'PAINE TWR; CITABRIA.' 'ACFT X; TURN XWIND NOW.' I DID SO AND; WITHOUT FURTHER INSTRUCTION; I ALSO TURNED DOWNWIND FOR RWY 34R. AS I DID SO; I CALLED; 'ACFT X IS TURNING DOWNWIND.' 'ACFT X; FOLLOW THE CIRRUS AHEAD. CLRED TO LAND.'