Accident Details
On September 27, 2011, at 1030 Pacific daylight time, a Bell 47D1 helicopter (registration N9158R) collided with trees while descending near Ahwahnee, California. The helicopter was being operated under Title 14 Code of Federal Regulations Part 91. The pilot, a private pilot, sustained serious injuries, and the passenger sustained minor injuries. The helicopter sustained substantial damage. Visual meteorological conditions prevailed, and no flight plan was filed. The pilot had departed approximately 1020 for a local area flight.
Investigation Findings
The Federal Aviation Administration accident coordinator responded to the site. The helicopter had impacted terrain in a nose-down attitude, with the tail and tail rotor systems resting upright against a tree.
Pilot and Passenger Statements
The pilot submitted a written statement on May 14, 2012. He reported that he was conducting a run-on landing when the rotor revolutions per minute (RPM) decreased. He turned away from trees hoping to recover RPM, but the helicopter settled into a tree. He stated there were no mechanical malfunctions or failures.
In an interview with the NTSB investigator, the passenger, who had been maintaining the helicopter for about five years, said the owner had reported a suspected problem with the number one bearing being loose on the tail rotor drive shaft. Additionally, they had previously installed an oil filter and were planning to change oil lines. The pilot and mechanic completed the pre-flight inspection and noted no problems with the tail rotor drive shaft. They decided to take a short flight to warm the oil. The mechanic had flown with the pilot previously and had no concerns about the pilot's operation.
Flight and Collision Sequence
They departed from a private landing pad at the pilot's residence and flew for about 15 minutes with no problems. Upon returning, the landing pad was surrounded by trees and terrain. The helicopter gradually descended, and the mechanic told the pilot he did not think they would maintain clearance from a large tree to the west. About 10 yards from the tree, they were below treetop level, and power (rotor RPM) continued to decrease while descending. The pilot attempted to pull out of the landing to avoid the tree, turning right while increasing collective. The helicopter then collided with the tree. The mechanic indicated no mechanical malfunctions occurred.
Witness Account
A witness working as a fire lookout at Miami Mountain Fire Lookout Tower about one mile away heard the helicopter and saw a cloud of dust. The helicopter ascended out of the dust cloud and flew approximately 500 feet above ground level down a valley, then turned and appeared to return to the landing pad. As it approached, it made a gradual descent while almost stopping. The witness saw the tail move left to right, then the helicopter quickly descended and impacted a tree. Wind was calm, and there was no post-impact fire. She called emergency responders.