On September 27, 2011, at 1030 Pacific daylight time, a Bell 47D1, N9158R, collided with trees while descending near Ahwahnee, California. The helicopter was being operated under the provisions of Title 14 Code of Federal Regulations Part 91. The helicopter was substantially damaged. The private pilot sustained serious injuries, and the passenger sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed. The pilot departed about 1020 for the local area flight. The Federal Aviation Administration (FAA) accident coordinator responded to the accident site. The helicopter had impacted terrain in a nose down attitude and the tail and tail rotor systems were resting upright against a tree. The pilot submitted a written statement on May 14, 2012. He reported that he was conducting a run on landing and the rotor revolutions per minute (RPM) decreased. He turned away from the trees hoping to recover the RPM. The helicopter then settled into a tree. He said that there were no mechanical malfunctions or failures. In an interview with the NTSB investigator, the passenger said that he had been maintaining the helicopter for about 5 years. The owner had reported what he surmised to be a problem with the number one bearing being loose on the tail rotor drive shaft. Additionally, they had previously installed an oil filter and they were going to change the oil lines. The pilot and mechanic completed the pre-flight inspection and review and noted no problems with the tail rotor drive shaft. They decided to take the helicopter on a short flight to warm the oil. The mechanic had flown with the pilot previously and had no concerns about his operation of the helicopter. They departed from the private landing pad which was located at the pilot's residence. They flew for about 15 minutes with no problems noted. They returned to the area of the landing pad which was surrounded by trees and terrain. The helicopter gradually descended and the mechanic told the pilot that he did not think they were going to maintain clearance from a large tree to the west of their position. About 10 yards from the tree, they were below treetop level and the power [rotor RPM] continued to decrease while the helicopter descended. The pilot attempted to pull out of the landing to avoid the tree and turned the helicopter to the right while increasing collective. The helicopter then collided with the tree. The mechanic had flown in and out of the area with the pilot multiple times. He felt on this occasion, due to the confined area there were no options to recover the helicopter prior to impact with the tree. The mechanic indicated that no mechanical malfunctions occurred. A witness was working as a fire lookout in Miami Mountain Fire Lookout Tower, which was located about 1 mile from the accident site. She heard the helicopter and saw a cloud of dust appear. The helicopter ascended out of the dust cloud and flew approximately 500 feet above ground level down a valley. The helicopter then turned and appeared to be returning to the landing pad. The landing pad was surrounded by trees and houses. As the helicopter approached the landing pad, it made a gradual descent while almost coming to a stop. She then saw the tail move from left to right, and the helicopter quickly descended and impacted a tree. The wind was calm at the time. There was no postimpact fire. She immediately called emergency response personnel.