HILO, Hawai'i — The National Transportation Safety Board is blaming mechanical problems for an accident in which a helicopter was forced to ditch in the ocean last year about 50 yards offshore near Honokohau National Historical Park north of Kailua, Kona.

Other reports released by the NTSB blamed pilot error for two other helicopter accidents this year, including one where a helicopter rolled on its side at the Kailua, Kona, airport, and another where a pilot landing in a wooded area on Maui allowed the helicopter's main rotor blades to strike a tree.

The agency blamed a fourth accident, a hard landing near McGregor Point on Maui in 2005, on contaminated fuel.

In the April 23, 2005, accident in Kona, a single-engine Robinson Helicopter Co. R22A had just departed the Kailua airport with a flight instructor and a student aboard when the pilot was forced to ditch in the ocean.

The helicopter, owned by Mauna Loa Helicopters, was climbing above the 1,300-foot level when the engine "just quit," the pilot told the NTSB. Neither the instructor nor the student was injured, and the helicopter ended up in about eight feet of water.

The NTSB listed the probable cause of the accident as failure of one of the engine's exhaust valves, adding that "failure of maintenance personnel to adequately check" portions of the exhaust valve assembly contributed to the accident.

In a second mishap, the NTSB said the rotor blades of a Hughes 369D were ruined on July 7 in Kihei when a Windward Aviation Inc. pilot allowed the blades to strike a tree while attempting to land to retrieve an animal.

The flight was a reconnaissance and eradication mission by the Tri-Isle Resource Conservation and Development Council, which is under the U.S. Department of Agriculture, according to the NTSB report.

The three people aboard were uninjured, and the helicopter landed a short distance away after the mishap near Kihei. The helicopter was later flown back to Kahului for repairs, according to the NTSB.

Pilot error was also blamed in another accident at the Kona airport on March 23, when a Robinson R-22B helicopter rolled onto its left side while hovering above the tarmac during an instructional flight.

The aircraft was operated by Mauna Loa Helicopters, and the pilot and student were uninjured. The NTSB ruled the accident was caused by the instructor's "delayed remedial action" when an inexperienced student encountered problems.

Contaminated fuel was blamed for a hard landing by a Windward Aviation Hughes 369D near McGregor Point on Maui after the aircraft lost power on May 16, 2005. The pilot and four passengers were not injured.

According to the NTSB, a contaminated fuel sample was discovered two days before the accident, and the maintenance crew examined the helicopter but did not examine and clean the entire fuel system.

That accident was attributed to maintenance personnel who failed to check out the entire fuel system and to make daily quality assurance checks on a fuel storage tank. The tank had a broken cover that allowed water and contaminants to mix with the fuel, according to the NTSB.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.