THE PLT REPORTED THAT AFTER TAKING OFF, HE STAYED LOW OVER FARM FIELDS. HE REPORTED THAT HE TRAVELED ABOUT 1 1/4 MI FROM THE ARPT & DECIDED TO RETURN & LAND. HOWEVER, DURING A LEFT TURN, THE ULTRALIGHT VEHICLE CRASHED. THE PLT WAS SERIOUSLY INJURED & DID NOT REMEMBER DETAILS OF THE ACCIDENT AFTER REGAINING CONSCIOUSNESS. A GROUND WITNESS REPORTED THE ULTRALIGHT WAS FLYING SLOW WHEN THE NOSE PITCHED DOWN & IT ENTERED A SPIRAL & HIT THE GROUND. THE PLT STATED THAT HE BEGAN THE TURN WHILE AT APRX 75 TO 100 FT AGL & WAS INCREASING POWER WHEN THE RIGHT WING CAME UP & THE NOSE DROPPED. AN EXAM OF THE FLT CONTROLS SHOWED CONTINUITY THRU-OUT THE SYS. AN INSPECTION OF THE ENG REVEALED THE URETHANE HOSE WAS PINCHED & MELTED AT A LOCATION BETWEEN THE ENG CASE & THE PULSATING FUEL PUMP. A FURTHER INVESTIGATION REVEALED THAT ON A 50 DEG DAY, THE TEMP IN THAT AREA WOULD BE APRX 175 TO 200 DEG & THAT THE HOSE WOULD COLLAPSE & BECOME SEATED TOGETHER AT 200 DEG. THE TEMPERATURE ON THE DAY OF THE ACCIDENT WAS 98 DEG.