The controversial 130-turbine, 50-square-mile project slated for construction on Horseshoe Shoal hit a major roadblock late last week when a federal appeals court rejected a key finding from the Federal Aviation Administration that the wind farm would pose no hazard to pilots.

Senior circuit Judge Stephen F. Williams reached an opposite conclusion.

“A group of air traffic controllers summed it up by saying that adding the turbines to the area would be a ‘disaster waiting to happen,’ ” Judge Williams wrote, citing a submission from the National Air Traffic Controllers Association.

“The record contains numerous contentions indicating that the wind farm might pose just such a safety risk,” the decision reads. It continues:

“Petitioners cite evidence that the many pilots who regularly operate under visual flight rules near the proposed wind farm would have a difficult time staying beneath the foggy and otherwise inclement weather that often plagues Nantucket Sound, while at the same time maintaining a safe distance from the wind turbines. During such times there would be a ‘clear risk of collision with the wind turbine generators.’ ”

The court also cited the testimony from the chairman of the Barnstable Airport that the “finely balanced air space over Nantucket Sound is already one of the most congested, foggy and dangerous airspaces on the eastern seaboard.”

The appeal was brought against the FAA by the town of Barnstable and the Alliance to Protect Nantucket Sound, a vocal, privately-funded advocacy group formed some years ago to fight Cape Wind.

In separate press statements, the Alliance and Cape Wind reacted to the ruling.

“This represents a major setback for an already struggling project,” said Alliance president Audra Parker in a statement, also criticizing Gov. Deval Patrick for supporting a contract between the utility Nstar and the private developer Cape Wind.

Cape Wind communications director Mark Rodgers downplayed the decision.

“The FAA has reviewed Cape Wind for eight years and repeatedly determined that Cape Wind did not pose a hazard to air navigation,” he said in a statement. “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 . . . today’s court decision doesn’t change things very much because our existing determination of no hazard (the third we have received since we started with this project) was set to expire in just 90 days and we were going to have to re-apply at that time anyway.”

Earlier this year the Wampanoag Tribe of Gay Head (Aquinnah) filed a lawsuit against the wind developer claiming that the project would disturb submerged archeological and cultural resources and interfere with their tradition of welcoming the sun. That lawsuit is still pending.