On May 30, 2004, at 1355 Pacific daylight time, a Cessna T210M single-engine airplane, N121JR, landed with the main landing gear retracted at the Charles M. Schulz - Sonoma County Airport (STS), Santa Rosa, California. The pilot owned and operated the airplane under the provisions of 14 CFR Part 91 as a personal flight. The private pilot and five passengers were not injured; the airplane sustained substantial damage. The flight departed Upper Lake, California, approximately 1300, and was terminating when the event occurred. Visual meteorological conditions prevailed, and a flight plan had not been filed. During a telephone interview with the pilot, he stated that he landed at Upper Lake earlier in the day. Prior to takeoff, the pilot conducted a walk around inspection of the airplane and noted no anomalies. Upon arriving at STS, the pilot noticed that when he went to extend the landing gear, the hydraulic power pack motor kept running and the landing gear down-and-locked light did not illuminate. The pilot looked at the reservoir and noticed that there was plenty of hydraulic fluid in the tank. He attempted to retract and lower the landing gear again, to no avail. He then performed the Pilot Operating Handbook's recommended procedures for landing gear malfunctions, also to no avail. The pilot contacted the STS control tower and notified them of the situation. Another aircraft flew by the accident airplane, and examined the landing gear. The pilot of the other aircraft notified the accident pilot that the nose landing gear was extended, but the main landing gear remained only partially extended. The pilot briefed his passengers of the ensuing landing scenario. He performed a soft-field landing on runway 19 with crash/fire rescue personnel standing by. Upon touchdown, the airplane tilted and veered to the left. The airplane continued to the left where it departed the runway and impacted a taxiway sign before coming to rest upright. According to the pilot, the hydraulic power pack motor was replaced with a repaired motor 3 months prior to the accident. In addition, approximately 1.5 weeks prior to the accident, the right main landing gear door actuator was repaired due to a leaking o-ring. According to a mechanic located at the STS airport, the left wing, horizontal stabilizer, and elevator sustained structural damage. The mechanic noted that the right main landing gear door actuator had come undone, and hydraulic fluid was along the right side of the airframe aft of the door. He also noticed that the retaining snap-ring that holds the actuator together had been installed improperly (the rounded, or pressed, side of the snap-ring was facing away from the actuator piston). A work order, dated May 11, 2004, indicated that at an aircraft total time of 4,492.3 hours, the "starboard main gear door actuator" was leaking. The same work order indicated that mechanics "removed the right gear door actuator. Found o-rings worn. R&R [Removed and Replaced] with new o-rings. Reinstalled actuator with new locknuts. Ops check good, leak check good." The airplane accumulated 10.7 hours since the right main landing gear door actuator work.