On April 17, 1998, about 2005 hours Pacific daylight time, a Grumman American AA5B, N28565, operated by the pilot, was substantially damaged following a wire strike on approach to runway 28, Lake Riverside Estates Airport, Aguanga, California. The pilot was not injured. Visual meteorological conditions prevailed for the 14 CFR Part 91 personal flight and no flight plan was filed. The flight originated at Lancaster, California, at 1915 hours. The maintenance technician/pilot was returning the airplane to its owner located on the private airstrip after an annual inspection. He stated that he entered the traffic pattern by overflying the airport and entering on the 45 for the downwind and runway 28. As he lined up with the runway, he said that he looked down at the airspeed indicator and heard a "kunk," at which point he noticed the propeller had stopped. He then noticed some power lines in his path and at the same time he noticed the runway lights flicker and go out. A postaccident examination of the airplane revealed a 3- to 4-inch gash in the spinner and some marks on one propeller blade, which the pilot said "looked like it had scraped on something." The pilot reported "apparently there are two rows of power lines and somehow I hit a wire on the first row." A maintenance worker looked at the power lines on the first row furthest from the runway and reported that none of the power lines were broken, but one had been pulled out of it's support. The pilot confirmed that the sun had set and it was dusk. He said he had no knowledge of wires on the approach. Neither the Los Angeles sectional chart nor his publications mentioned or illustrated wires in the vicinity of the runway. He also stated that there were no displaced threshold markings of any type to indicate obstructions in the 20:1 or 3-degree approach slope to runway 28. Information about the private Lake Riverside Estates Airport was not found in the current Flight Guide or the AOPA Airport Directory. Information was obtained from a member of the airport committee, and according to him, there are two sets of wires. The farthest set of wires from the runway threshold is 1,300 feet, and the nearest set is 700 feet from the threshold. Runway 28 has a 1,000-foot displaced threshold. The way the displaced threshold is marked on the dirt runway was not reported. A Federal Aviation Administration (FAA) Airport Master Record, form 5010-1, was obtained from the Airports Division. The airport data form provided limited information and had never actually been inspected by an FAA inspector because of the private status. According to the data for runways 10/28, runway 28 has ball marked power lines within 300 feet of the threshold. It also indicates that the runway has nonstandard lighting with six red/green lights at each threshold. There is no mention of a displaced threshold for a clear 20:1 approach on either runway 28 or 10. According to a Safety Board Sun/Moon program, official sunset had occurred at 1924.