About 60 percent of the U.S. Air Force’s Boeing F-15 Eagle fighter fleet has been found fit and ready to fly again.
About 260 F-15s were returned to full operational duty Jan. 8 after receiving nose-to-tail inspections following the Nov. 2 midair breakup of a Missouri Air National Guard single-seat F-15C as it flew south of St. Louis during a training mission, senior Air Force officers told reporters today at the Pentagon following today’s release of the accident investigation report. Much of the Air Force’s nearly 700 F-15s had been grounded since the incident.
The report cited the failure of a structural component called a longeron as causing the $42 million F-15C to break into two parts. Although injured, the pilot was able to successfully eject from the aircraft and parachute safely to earth.
Technical study of the F-15’s recovered wreckage determined that the component in question did not meet the manufacturer’s structural blueprint specifications and had developed fatigue cracks that caused it to fail, according to the report. These cracks expanded under life cycle stress, causing the longeron to fail, which initiated a catastrophic failure of the remaining support structures and led to the aircraft breaking apart in flight. The upper right longeron is one of four metal beams that help hold the cockpit to the main fuselage.
“When the upper right longeron broke, the remaining structure along the forward fuselage was unable to distribute the loads in that area, which caused the aircraft to break into two at a point just aft of the cockpit area.” explained USAF Colonel William Wignall, president of the accident investigation board.
“We’ve had great involvement from Boeing during the investigation,” Wignall said. “In fact, they’re the ones who determined the longeron was the problem. This was then confirmed by the Air Force Research Laboratory.” In his accident report, Wignall noted that no inspection requirements existed for detecting a crack in the longeron.
“This is not just a bad part, this is a bad part that’s been under huge stress” for nearly three decades, said USAF Gen. John D.W. Corley, commander of Air Combat Command, at Langley AFB, VA. The accumulated fatigue and stress on that part caused it to crack and eventually to break, he said.
F-15 aircraft returned to duty have undergone extensive inspections, said Air Force Maj. Gen. Thomas J. Owen, commander of Warner Robins Air Logistics Center, Air Force Material Command, Robins Air Force Base, Ga. Owen’s unit is responsible for logistics support for various Air Force aircraft, including F-15s.
“No (operational) restrictions will be imposed on the aircraft that have passed the inspections, although future recurring inspections will be required to the upper-right and left longerons,” Owen said. He said it will cost an estimated $250,000 to replace a faulty longeron.
About 182 F-15 A through D models manufactured between 1978 and 1984 remain out of service pending additional tests, officials said. Nine other F-15s have been found to have longeron-fatigue cracks and have been grounded. About 441 F-15s in the Air Force inventory are model A through D, while 224 others are of the newer E series.
Inspections performed on the F-15 fleet are more than 90 percent complete. They include thorough checks of hydraulic and electrical lines, fasteners, aircraft fuselages and skins, and all internal structural components, including the longerons, Owen explained.
“This isn’t just about one pilot in one aircraft with one bad part,” Corley said. “I have a fleet that is 100 percent fatigued, and 40 percent of that has bad parts. The long- term future of the F-15 is in question.”
“We don’t have a full and healthy fleet, so we’ve gotten behind on training missions, instructor certifications, classes and exercises,” he said. “And in the meantime, our pilots have to be ready to deploy.”
For some of the nine F-15s that have longeron cracks, it may be cost prohibited to repair them, Corley said. The Air Force is scheduled to retire some of these aircraft in the next fiscal year.
“We’re going over each and every aircraft to make a determination,” he said. “We will take some F-15s out of the inventory. It just doesn’t make sense to spend the time and money if it won’t be worth it for some aircraft.”