THE ACFT WAS ON A FLT TO A PRIVATE RANCH LANDING STRIP AT NIGHT. THE STRIP HAD AN UNLIGHTED WINDSOCK AT MIDFIELD & PORTABLE RWY EDGE LIGHTS WERE IN USE. DURING ARRIVAL, THE AIRCREW ESTIMATED THE VISIBILITY WAS 3 TO 4 MI & REPORTED THEY WOULD BE ABLE TO LAND. PERSONNEL AT THE RANCH HEARD THE ACFT FLY OVER. ONE OF THEM REPORTED THAT EARLIER, THE WIND HAD BEEN BLOWING HARD & THE AIR WAS FULL OF DUST, BUT AT THE TIME THE ACFT ARRIVED, DUST HAD SETTLED. A DEPUTY SHERIFF NOTED THAT AT TIMES, THE VISIBILITY WAS GOOD, BUT AT OTHER TIMES, IT WAS POOR. THE ACFT OWNER, WHO WAS AT THE RANCH, SAID HE COULD SEE THE ACFT LIGHTS AT TIMES (IN THE TRAFFIC PATTERN), BUT AT TIMES, HE COULD NOT SEE THEM. AFTER MAKING 2 PASSES, THE ACFT CRASHED IN THE AREA WHERE IT SHOULD HAVE BEEN ON A BASE LEG. AN EXAM OF THE WRECKAGE REVEALED IT WAS IN A LEFT TURN DURING IMPACT. AFTER INITIAL IMPACT, IT BOUNCED/SLID 663 FT, THEN HIT A TREE COVERED CANAL LEVEE & BURNED. NO EVIDENCE OF A PREIMPACT/MECHANICAL MALFUNCTION OR FAILURE WAS FOUND.