On August 9, 2003, approximately 1300 mountain daylight time, a Piper PA-18-150, N115HP, registered to/operated by Hawkins & Powers Aviation Inc., and being flown by an airline transport rated pilot, sustained substantial damage during a loss of control on landing roll, approximately one nautical mile north of Rapids, Montana. The pilot was uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal, was operated under 14 CFR 91, and had originated from Greybull, Wyoming, approximately noon on the same day. The pilot reported that upon arriving in the vicinity of his destination, he over flew the intended landing site, a gravel road, to determine its condition and the direction of the prevailing winds. He then made a standard traffic pattern entry landing to the east. The pilot reported that the gravel road was 25 feet wide, down sloping to the east and 1,000 feet in length that was "slightly winding." During the landing roll (decelerating through about 35-40 miles per hour), the pilot determined that the aircraft was drifting right and began correcting for the drift. He then noticed that the right tire was deflated and rolling on the rim. The aircraft's right wheel entered a shallow ditch at the edge of the road and the aircraft then departed the landing surface and traveled over rough down sloping terrain before nosing over and coming to rest inverted. The pilot, who held a mechanic's license, reported that upon examining the right wheel following the accident he found the wheel's valve stem to be broken.