THE PLT & HIS PARTNER HAD COMPLETED BUILDING THE ULTRALIGHT VEHICLE SEVERAL DAYS PRIOR TO THE ACCIDENT. SINCE THEN, THE PLT HAD FLOWN IT APRX 8 HRS. A WITNESS OBSERVED THE VEHICLE CIRCLE THE ARPT IN WHAT HE INDICATED WAS A NORMAL LEFT TRAFFIC PATTERN FOR RWY 28. HOWEVER, INSTEAD OF TURNING TO A FINAL APCH, THE VEHICLE CONTINUED PAST THE RWY CENTERLINE, BUT KEPT TURNING TO PARALLEL THE RWY. AT ABOUT MIDFIELD, THE ULTRALIGHT CROSSED OVER THE RWY, THEN REVERSE TO A RIGHT TURN. DURING THIS TIME, THE VEHICLE STEADILY DESCENDED FROM AN ALT OF APRX 600 FT AGL (TRAFFIC ENTRY) TO APRX 400 FT ON BASE, THEN TO 100 FT WHEN IT REVERSED TO A RIGHT TURN. IT CONTINUED TURNING RIGHT (APRX 270 DEG), THEN IT ENTERED A STEEP NOSE DOWN ATTITUDE & CRASHED. AN EXAM OF THE FLT CONTROLS REVEALED THAT WHEN LEFT RUDDER WAS APPLIED, THE ELEVATORBELLCRANK WOULD JAM AGAINST THE RUDDER HORN & ONLY ABOUT NEUTRAL NOSE-UP ELEVATOR WAS AVAILABLE. AN EXPERIENCED BUILDER REPORTED THIS PROBLEM HAD OCCURRED BEFORE. THE KIT DID NOT PROVIDE A MIN CLEARANCE OR CAUTION CONCERNING THIS PROBLEM.