From a small building in Byron to a lab in Israel and a Boeing facility in St. Louis, a massive effort is under way that will play an important role in the future of the F-15 Eagle.
The two most extensive full-scale fatigue tests ever done on the venerated aircraft, along with complete teardowns of two others, will help determine what maintainers at Robins Air Force Base need to do to keep the aging fighter jet flying safely for years to come. Robins does depot maintenance, program management and life-cycle sustainment for all F-15s in the Air Force.
At Boeing, an F-15E Strike Eagle, which can take on air and ground targets, will undergo a test that will simulate at least 65,000 flying hours. If there are no major issues, that would certify it for a 32,500-hour service life. That’s more than three times the 8,000 hours currently certified for the plane.
For the F-15C, strictly an air-to-air fighter, the test will go to at least 36,000 hours, certifying it for 18,000, twice the current limit.
Randy Jansen, chief F-15 engineer at Robins, said the tests involve a complex system of hydraulics and tabs that apply pressure to aircraft structure as it hangs suspended, simulating air pressure from flying at varying altitudes.
It takes a year just to set it all up. The test has just begun on an F-15C Robins shipped to Boeing last summer, and the F-15E they shipped this summer is being prepped. Aside from pauses to do evaluations, the test will run around the clock until 2014. The F-15E test will go until 2015.
Jansen said the test gives an accurate simulation of the same stresses the aircraft would have during real flying.
“In order to continue to fly the aircraft, we need to know what happens in the future,” Jansen said. “This gives us insight into the future and allows us to do preventative maintenance to reduce stresses.”
Teardowns give look at current condition
While the fatigue tests give engineers a look into the future, the teardowns provide a look at how the aircraft is currently holding up.
In a building in Byron, a small team of contractors over the past two years have completely disassembled an F-15D and an F-15C. The purpose is to get a look at interior structural parts not seen even in programmed depot maintenance done at Robins.
Every square centimeter of the parts is carefully examined and analyzed for signs of problems. The evaluation of the F-15D teardown is finished and nothing of significance was found, Jansen said.
The evaluation of the F-15C teardown, which was only recently completed, is still under way.
Some of those parts have been shipped to Israel for metallurgical analysis. Israel is a major user of the F-15 and is involved because it has the expertise and a vested interest.
While Israel is the only foreign user with hands-on involvement, other nations using the F-15 have been keenly following both the teardowns and fatigue tests.
Tests may go even further
While 65,000 hours for the F-15E and 36,000 hours for the F-15C are the goals set for the test, Jansen said the true intent is to keep going until the planes completely break. He is confident the tests will exceed the goal hours.
Even if inspections show structural issues during the tests, it will not mean the tests will have to stop and the certified flying hours set lower than expected. Jansen said any issues discovered will help determine what maintainers need to look for during depot maintenance and what can be done to prevent those problems.
Current plans call for retirement of all F-15Cs by 2025 and all F-15Es by 2035. While theoretically the successful completion of fatigue tests might indicate those dates could be extended, that wouldn’t necessarily be the case.
Jansen said numerous factors other than structural issues could lead to retirement of the aircraft regardless of any projected date, such as more advanced fighters by potential enemies rendering the F-15 obsolete.
Missouri crash brought issues to the forefront
In 2007 a Missouri Air National Guard F-15C broke apart in midair while on a training mission. The pilot ejected safely, but the accident called into question the future of the F-15 and led to a grounding of the fleet. The investigation determined the accident was caused by the failure of the upper right longeron, a main part of the support structure.
The failure was blamed on the longeron being built thinner than blueprint specifications, but fatigue stress was a contributing factor. The aircraft had seen extensive use, said retired Col. Fritz Heck, who was commander of the Robins F-15 test flight fleet at the time of the accident.
“It had been flown hard,” he said, and that was ultimately what exposed the defect.
Heck said the teardowns and fatigue tests are important to preventing similar accidents in the future. The effort can help identify any such faults that can be corrected before a catastrophic failure occurs.
The accident led to inspections of all longerons, with replacements on five aircraft, and the grounding was lifted.
Aircraft remains important to national defense
A few years ago, the plan was for all F-15s to be retired except for the newer F-15E, with the F-15C’s air-to-air capability to be replaced by the F-22. However, the F-22 proved too expensive, and production was cut far short of what originally had been projected. The F-35 program also has seen extensive delays and problems.
It all means the F-15 is still vital to national defense and likely will be for many more years, Heck said. The F-15E has been used extensively in Iraq and Afghanistan, and the F-15C is the primary fighter for protecting America’s airspace. The F-15D is primarily used as a trainer.
The Air Force currently has 221 F-15Es and 250 C and Ds. Those numbers have dropped significantly in recent months as many of the older aircraft have been retired. The average flying hours have also dropped with the retirements, with the F-15Cs averaging 7,310 hours and the F-15Es at 5,611. Previously, many of the older aircraft had been flown well beyond the certified hours.
Despite the age of the aircraft, Heck said he still has high confidence in the capability and safety of the plane. He has 2,000 hours as an F-15E pilot, including stints in Afghanistan and Iraq. Heck’s son is in the Air Force Academy and wants to be a fighter pilot.
“I would have no problem whatsoever with him flying F-15s for years to come,” he said.
To contact writer Wayne Crenshaw, call 256-9725.