THE AIRPLANE EXECUTED A STEEP LEFT TURN, STALLED, AND DESCENDED NOSE LOW INTO A TREE AND THE GROUND WHILE SEARCHING FOR A PIPE LEAK IN AN OIL FIELD. INSPECTION OF THE WRECKAGE REVEALED ADEQAUTE FUEL, CONTINUITY AND NO MECHANICAL MALFUNCTION. ENGINE DISASSEMBLY FAILED TO UNCOVER ANY DISCREPANCY. A WITNESS OBSERVED THE AIRPLANE FLYING LOW TO THE GROUND AND SLOWED DOWN TO WATCH. THE WITNESS SAW THE AIRPLANE CIRCLE AND GO DOWN.