The Air Force is likely to order dozens of its F-15 fighter jets permanently grounded because of critical structural flaws, significantly reducing the number of planes available to protect the United States, officials said Tuesday. After one of the fighters broke apart during a simulated dogfight in November, Air Force officials grounded the entire F-15 fleet, nearly 700 planes, fearing such a defect. The newest versions of the fighter jets were allowed to resume flying shortly afterward, but 440 of the older model F-15s have remained out of service. The Air Force plans to allow about 260 of the remaining grounded planes to return to duty today. About 180 will remain idle because of suspected structural flaws. “Many of them may never fly again,” said a senior Air Force officer. The officer, like others interviewed, spoke on condition of anonymity because results of the investigation were not to be made public until today.