AFTER ASSEMBLING HIS ULTRALIGHT, THE PLT RECEIVED DUAL INSTRUCTION IN A 2 PLACE MODEL. AFTER HIS 2ND DUAL FLT, HE TOLD HIS INSTRUCTOR (CFI) HE HAD SOLOED HIS VEHICLE. THE CFI CAUTIONED HIM AGAINST FURTHER SOLO FLTS UNTIL HE WAS QUALIFIED &EMPHASIZED THE NEED TO WEAR A HELMET. 3 DAYS LATER, WITNESSES SAW THE VEHICLE CRUISING AT APRX 300 TO 500 FT AGL WHEN ITSUDDENLY ROLLED RIGHT & ENTERED A STEEP NOSE DOWN ATTITUDE & CRASHED. NONE OF THE WITNESSES COULD VERIFY THE ENG STATUS BEFORE IMPACT. NEITHER OF THE PROP BLADES HAD ROTATIONAL DAMAGE, BUT 1 BLADE BROKE WHEN IT HIT THE GROUND. THE ENG WAS STARTED WITH FUEL REMAINING IN THE FUEL TANK & OPERATED NORMALLY. THE IGNITION SWITCH WAS MOUNTED ON THE CONTROL STICK NEXT TO THE TWIST GRIP THROTTLE & WAS FOUND IN THE OFF POSITION. THE ENG WAS EQUIPPED WITH A CENTRIFUGAL CLUTCH & COULD NOT BE STARTED BY A WINDMILLING PROP. THE PLT WAS NOT WEARING A HELMET & HAD SUFFERED EXTENSIVE HEAD INJURIES. HIS CFI HAD WARNED HIM THAT HIS SPOILER LINES WERE TOO TIGHT & COULD RESULT IN UNWANTED SPOILER ACTION WTIH AFT STICK MOVEMENT.