On March 3, 2009, about 1020 Pacific standard time, a Maule M-5, N9166E, sustained substantial damage following an off airport landing in an open field three miles west of Calexico, California. The commercial pilot, the registered owner/operator of the airplane and passenger were not injured. The airplane was being operated as a visual flight rules (VFR) aerial observation flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight departed Calexico approximately 20 minutes prior to the accident. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported that about 20 minutes after takeoff, while maneuvering, the airplane developed a rough running engine and he initiated a precautionary landing to a nearby field. During the landing roll, the airplane encountered soft, uneven terrain and nosed over. Examination of the airplane by the pilot revealed that the vertical stabilizer and rudder were structurally damaged. Post accident examination of the engine and airframe, by the pilot, revealed no indication as to the basis of the rough running engine. Subsequent to the examination, the pilot began repairs on the engine and airframe. The engine and airframe were not examined by the National Transportation Safety Board.