PA-28R Cherokee Arrow All Series
Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Instrument
Flight Crew Last 90 Days 15; Flight Crew Total 1700; Flight Crew Type 700
Communication Breakdown; Situational Awareness
Conflict NMAC
General None Reported / Taken
Human Factors; Procedure
I was working Cape Approach while enroute to MVY from the southwest. Cape handed me off to the tower who advised me to report a right downwind entry. From my direction of flight a right base would have been a better line up. I requested a right base and was granted same.  There were multiple aircraft in the area to both land and takeoff. It became clear as I approached that a right base would put me in conflict with another aircraft on a right base further out to my left. I advised the tower that I thought a right downwind would in fact be better and he concurred and I altered course for the right downwind. While on approach and established parallel to the runway on the right downwind at or near pattern altitude the tower controller requested I extend my downwind for other aircraft. I acknowledged same. In a split second out of the corner of my eye I saw another aircraft to my left (Aircraft Y) on a right base. Aircraft Y passed underneath me by a very slim margin; less than 200 feet; perhaps less than 100 feet. It was very quick. If we had been at the same altitude we would have collided. I advised the tower controller of the event and the only response was 'that's why I told you to extend the downwind' The airport went from quiet to extremely busy in a very short period of time. I am not sure if the controller did not know of Aircraft Y's position or if I misinterpreted my instructions but if the latter I did repeat everything and I believe the controller acknowledged same. I was expecting Aircraft Y to be inside my position and not outside my position. My attention was focused towards the airport with only occasional glances to my left. I was asked once again to extend the downwind and continued the downwind to fall in behind a PC-12 on final; the rest of the flight was uneventful. I don't know how the pilot of Aircraft Y could have missed me; I was right in front of him. My only explanation is that he may have been looking to his right for me; thinking I was more abeam the tower area on the right downwind.
PA28 pilot on a right dowinwind reports a NMAC in the pattern at MVY with an aircraft crossing on a right base. The Tower did not advise either pilot of the impending conflict.