REPORTEDLY, THE NON-RATED PLT HAD NO PREVIOUS FLT EXPERIENCE, BUT WAS BRIEFED ON THE ULTRALIGHT VEHICLE'S OPERATION, CONTROL RESPONSE & OTHER ASPECTS, AS WELL AS THE TRAFFIC PATTERN, DIRECTION, ETC. DURING TAKEOFF, THE ULTRALIGHT ENTEREDA DIVE AFTER CLIMBING 30 TO 40 FT, BUT THE PLT LEVELED OFF JUST ABOVE THE RWY & CONTINUED CLIMBING IN THE TRAFFIC PATTERN TO APRX 300 FT. A WITNESS STATED THAT ON THE DOWNWIND LEG, THE VEHICLE SUDDENLY ENTERED A STEEP NOSEDOWN DIVE & CRASHED. HE OBSERVED NO WING ROCKING, NO TURN & NO ROLLING OR YAWING. DURING AN EXAM OF THE WRECKAGE, NO PREIMPACT, MECHANICAL MALFUNCTION/FAILURE WAS FOUND. THE VEHICLE WAS NOT EQUIPPED WITH A SHOULDER HARNESS, THE SEAT BACK WAS NOT RIGID, THE ONLY FOOT/LEG SUPPORT WAS A TUBE EXTENDING EITHER SIDE OF THE NOSEWHEEL WITH NO HEEL CUP. THE SINGLE FLT CONTROL OPERATED THE RUDDER & ELEVATOR (NO AILERONS WERE INSTALLED). FORE & AFT MOVEMENT OF THE CONTROL WOULD PROVIDE NOSE DOWN & NOSE UP RESPONSE OF THE ELEVATOR. LEFT & RIGHT MOVEMENT OF THE CONTROL WOULD PROVIDE LEFT & RIGHT RUDDER.