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F-15 Eagle

Service Life

In 2010 the F-15E fleet, with an average age of over 16 years, had and aircraft availability rate of 59 percent. The Air Force plans for the F-15E to be an integral part of the Nations force through at least 2035. As of 2010 the F-15 C/D air superiority fighter fleet had an average age of over 25 years. The Air Force projected the F-15C/D fleet to be viable until 2025-2030, and will consider the airframes service life extension requirements following full-scale fatigue testing. This testing is scheduled to begin this summer and conclude in FY14. The Air Force was managing the fleet through scheduled field and depot inspections under an individual aircraft tracking program. For FY10, the F-15C/Ds aircraft availability was 65 percent.

In FY10 the USAF reduced F-15C/D force structure by 132 permanently assigned aircraft and retired 112 aircraft to the Aerospace Maintenance and Regeneration Group at Davis-Monthan AFB in Arizona. This left 199 permanently assigned aircraft (250 total active inventory) for FY11 and beyond. The Air Force expects these efforts to successfully enable the 176 F-15C/D Long-Term Fleet to operate safely and effectively through at least 2025, as determined by the full-scale fatigue test.

The F-15 initial operational requirement was for a service life of 4,000 hours. Testing completed in 1973 demonstrated that the F-15 could sustain 16,000 hours of flight. Subsequently operational use was more severely stressful than the original design specification. With an average usage of 270 aircraft flight hours per year, by the early 1990s the F-15C fleet was approaching its service-design-life limit of 4,000 flight hours. Following successful airframe structural testing, the F-15C was extended to an 8,000-hour service life limit. An 8,000-hour service limit provides current levels of F-15Cs through 2010. The F-22 program was initially justified on the basis of an 8,000 flight hour life projection for the F-15. This was consistent with the projected lifespan of the most severely stressed F-15Cs, which by the turn of the century had averaged 85% of flight hours in stressful air-to-air missions, versus the 48% in the original design specification.

Full-scale fatigue testing between 1988 and 1994 ended with a demonstration of over 7,600 flight hours for the most severely used aircraft, and in excess of 12,000 hours on the remainder of the fleet. A 10,000-hour service limit would provide F-15Cs to 2020, while a 12,000-hour service life extends the F-15Cs to the year 2030. The APG-63 radar, F100-PW-100 engines, and structure upgrades would be mandatory. The USAF cannot expect to fly the F-15C to 2014, or beyond, without replacing these subsystems. The total cost of the three retrofits would be under $3 billion. The upgrades would dramatically reduce the 18 percent breakrate prevalent in the mid-1990s, and extend the F-15C service life well beyond 2014.

In 1998 an F-15D Eagle air superiority fighter became the first in the U.S. Air Force's inventory to reach 6,000 flight hours. The F-15D, assigned to the 33rd Fighter Wing at Eglin Air Force Base, Fla., reached the 6,000-hour mark on 16 December 1998. It first flew on Jan. 7, 1982, and was delivered to the U.S. Air Force on March 3 that same year. It was the 702nd F-15 manufactured in St. Louis and the 30th D model (two seat) off the production line. The 6,000-hour milestone was not unexpected. Full scale fatigue tests of various models of the F-15 airframe had demonstrated a structural life exceeding 16,000 flight hours for primary safety of flight structure. While the milestone was noteworthy, the number of flight hours represented less than half the useful economic life of the aircraft.

By 2005 the F-15 Sustaining Engineering and Supply Chain Management efforts were being seriously impacted by continuing platform service life extensions. This necessitated a proactive engineering response to Diminishing Manufacturing Sources and Material Shortages (DMSMS) management. DMSMS is a condition brought about when the last known manufacturer announces the intention to discontinue production of an item or group of items still required by DoD activities for systems support. In FY 2006 the Aging Aircraft program established enabling avionics capabilities that can be affordably inserted into the legacy force structure, facilitating a force multiplier combat capability across diverse platforms. It worked to develop an affordable F-15 Heads Up Display (HUD) cathode ray tube (CRT) replacement item that can be transparently inserted into fielded assets as part of the normal repair cycle. Planned CRT advancements will eliminate an inherent F-15 failure mode, increasing the incurred CRT mean time between failure rate from under 400 hours to over 3,000 hours, and will be transferable to alternate platforms experiencing marginal HUD CRT reliability performance.

The F-15 was reportedly [Flight International, February 14, 2006] cleared for 18,000 flight hours [this must reference the F-15E]. As of 2006 the Air Force planned to upgrade and maintain around 170 F-15s through 2025. Planned improvements included the installation of a new active electronically scanned array (AESA) radar, which was expected to enter competition early in FY07, as well as integrating a new digital radar-warning receiver. The Air National Guard was upgrading 48 F-15C fighters between 2006 and 2012 with a new AESA radar, the Raytheon APG-63(V)3.

On 02 November 2007 an F-15C mishap resulted in the loss of the aircraft. The Accident Investigation Board found defects which indicated potential structural damage in the rest of the fleet. A failure of the upper right longeron, a critical support structure in the F-15C Eagle, caused the crash of a Missouri Air National Guard F-15C, four miles south-southeast of Boss, Missouri. The initial findings of the Accident Investigation Board, while at the mishap site Nov. 27, indicate the fleet of F-15s A-D might not be airworthy after a metallurgical analysis of the mishap aircraft. The findings focus specifically on the upper longerons, major structural components of the aircraft, which are located near the canopy of the aircraft and run along the side of the aircraft lengthwise.

The discovery of more structural damage in the F-15s prompted a 03 December 2007 stand-down order from Air Combat Command Commander Gen. John Corley. The stand-down does not impact the operational status of the F-15 E Strike Eagle. Maintainers performed methodical and time-intensive inspections on all F-15 Eagle A, B, C and D model aircraft, which revealed more cracks in the aircraft longerons. Maintainers at Langley initially found no cracks or evidence of fatigue in F-15 longerons; however, throughout the Air Force, maintainers had found cracks in the upper longerons of eight F-15s (as of 10 December 2007). Four of these aircraft were assigned to the Air National Guard's 173rd Fighter Wing, Kingsley Field, Ore.; two were assigned to the 18th Wing, Kadena Air Base, Japan; another was assigned to the 325th Fighter Wing, Tyndall AFB, Fla.; and one assigned to the ANG 131st Fighter Wing, St. Louis, Mo.

Every aircraft underwent all previously published time compliance technical order inspections. However, the cleared aircraft did not immediately return to flight. Technical experts at Warner Robins Air Logistics Center, Ga., developed new inspection techniques based on findings in parts of the mishap aircraft. These inspections were performed as soon as the new TCTO [time compliance technical order] was available for the affected F-15s. As part of the previous TCTO, maintenance crews around the Air Force stripped paint and performed non-destructive inspections in the F-15's upper longeron just aft of the canopies. Inspections are more than just a visual check. After the paint is stripped and bare metal is exposed, Airmen from the non-destructive inspection shop apply chemicals that reveal cracks under a black light. Other inspections in hard-to-see areas are done with a borescope - a tool that uses a tiny camera and fits in tight areas.

According to the Air Combat Command Accident Investigation Board report [released 10 January 2008], a technical analysis of the recovered F-15C wreckage determined that the longeron didn't meet blueprint specifications. This defect led to a series of fatigue cracks in the right upper longeron. 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 one longeron, already not up to design specifications, cracked apart under the stress of a 7G turn, the colonel said. This led to the other longerons failing as well, which then caused the cockpit to separate from the rest of the fuselage. The pilot was able to eject, but suffered a broken arm when the canopy snapped off.

Air Combat Command officials cleared a portion of its F-15 A through D models to begin flying on 09 January 2008. As of that date, the Air Force had approved 60 percent of F-15 A through D models to return to service with no flight restrictions.

The F-15E structure is rated at 16,000 flight hours, double the lifetime of earlier F-15s.



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