A flight path made dangerous by bad weather caused one of the worst helicopter crashes in Hawaii history, says a new report that blames the pilot, one of five fatalities.

A tour helicopter operated by Blue Hawaiian Helicopters crashed in mountainous terrain on Molo-kai on Nov. 10, 2011, killing four passengers, including a newlywed couple.

In a final report Wednesday, the National Transportation Safety Board blamed “the pilot’s failure to maintain clearance from mountainous terrain while operating in marginal weather conditions.”

As the helicopter was descending, part of the tail hit “vegetation and/or terrain,” causing the tail rotor assembly to fall off. That caused the Eurocopter EC-130 to lose control and plow nose-first into the ground, the report said.

Witnesses around Pukoo, near Kilo-hana Elementary School, reported seeing a fireball on impact.

Killed were pilot Nathan Cline, 30; Stuart Robinson of Toronto; Robinson’s companion, Eva Wannersjo of Toronto; and newlyweds Nicole Bevilacqua and Mike Abel of Pennsylvania.

In a statement Thursday from Washington, D.C., Blue Hawaiian Helicopters President Patricia Chevalier said: “First and foremost, we want to share our heartfelt condolences with all those who lost family members and friends as a result of the Molo-kai Island tour accident on November 10, 2011. We would also like to thank the NTSB investigators and NTSB leadership for their thoroughness, professionalism and transparency.

“We recently received the NTSB factual report, and this was classified as a Controlled Flight into Terrain accident. At this time, we are studying the report closely and taking the findings to heart to further improve our processes and procedures.”

The helicopter took off from Kahu-lui Airport at 11:44 a.m. for a tour of East Molo-kai.

The prearranged flight route took the aircraft along the north shore of West Maui and then across the Pai-lolo Channel to the Halawa Valley Waterfall, the NTSB said. The flight then continued along the cliffs to Papa-laua Falls.

In good weather the flight was to continue into Wai-lau Valley and then up and over the valley wall to the southern side of the island. In bad weather the flight was to fly around the eastern side of Molo-kai to reach the south coast.

Other pilots airborne at the time said the weather would not have allowed Cline to fly through Wai-lau Valley.

The last pilot to see the Eurocopter said it was flying west along the southern side of Molo-kai, just below the cloud ceiling at about 2,200 feet. That same pilot reported the weather was “continually deteriorating,” with a strong northeasterly wind and fast-moving squalls.

On the ground, several witnesses said they looked up when they heard a “woop woop woop” sound.

Witnesses also saw fragments of wreckage falling from the helicopter just before impact.

Rather than a traditional tail rotor, the Eurocopter has what is known as a “fenestron,” a rotor inside a circular shroud that provides a much quieter ride. Just forward of the fenestron are two horizontal stabilizers, which look like miniature wings.

The NTSB concluded that the right horizontal stabilizer and lower part of the fenestron hit vegetation or terrain. That triggered a fast-moving series of structural failures that led to the fenestron falling off.

“After the fenestron separated from the tail boom, the helicopter lost yaw (sideways rotation) control, and its center of gravity shifted forward, which caused it to become uncontrollable and, subsequently, descend to the ground.”

That was at 12:14 p.m.