A federal appeals court today ruled there were major flaws in a key government study that led the US Interior Department last year to approve the nation’s first offshore wind farm in the waters off Cape Cod, further delaying a controversial project that has struggled to find financing and has been beset by legal challenges for the past decade.

The US Court of Appeals for the District of Columbia rejected the Federal Aviation Administration’s ruling that the Cape Wind project’s turbines present ‘‘no hazard’’ to aviation, finding the FAA failed to adequately determine whether the planned 130 turbines, each 440 feet tall in an area about the size of Manhattan, pose a danger to pilots flying by visual flight rules over Nantucket Sound.


“We find it ‘likely as opposed to merely speculative’ that the Interior Department would rethink the project if faced with an FAA determination the project posed an unmitigable hazard,” a panel of three judges wrote in the court’s ruling.

They vacated the FAA’s decision and ordered the agency to review its findings, a process that could take years and put the project’s lease at risk. “The FAA failed to supply any apparent analysis of the record evidence concerning the wind farm’s potentially adverse effects on [visual flight rules] operations,” the judges wrote.

Officials at Cape Wind Associates said they won’t appeal the decision but instead will seek a new no-hazard ruling from the FAA, which they said they would have had to do within 90 days anyway. A no-hazard determination lasts only two years, they noted, and the company has already received three such findings.

“The FAA has reviewed Cape Wind for eight years and repeatedly determined that Cape Wind did not pose a hazard to air navigation,” said Cape Wind spokesman Mark Rodgers. “The essence of today’s court ruling is that the FAA needs to better explain its Determination of No Hazard. We are confident that after the FAA does this, that their decision will stand and we do not foresee any impact on the project’s schedule in moving forward.”


But opponents of the wind farm called the court’s ruling a “resounding victory” in their long fight to block the turbines from being installed over a 25-square mile area in Nantucket Sound.

“This represents a major setback for an already struggling project,” said Audra Parker, president of the Alliance to Protect Nantucket Sound, the main opposition group. “It is time for Cape Wind and the Department of Interior to relocate this project to another site that will not only protect Nantucket Sound, but allow properly sited offshore wind development in a timely way. After 10 years, Cape Wind continues to face legal and financial challenges, while better and cheaper forms of green energy are widely available.”

David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.