Longeron upgrade would keep F-15Cs flying
24 MAY, 2017 SOURCE: FLIGHTGLOBAL.COM BY: LEIGH GIANGRECO WASHINGTON DC
The release of president Donald Trump’s fiscal year 2018 budget, which continues millions of dollars in funding for ongoing upgrades to the Boeing F-15C – as well as a new service life-extension programme (SLEP) for the aircraft’s longerons – has allowed the US Air Force to officially quash rumours of a near-term retirement for the fighter.
In order for the F-15C/D fleet to meet its planned service life within acceptable risk margins, the proposed budget sets aside $7 million in FY2018 for a new longeron SLEP activity.
“The longeron SLEP entails replacing 14 primary tension members in the structure of the forward fuselage and is critical to the safety of flight of these aircraft,” the USAF says in budget documents. “There are other structural issues with the F-15Cs besides the longerons, and full-scale fatigue testing is ongoing to assess these matters.”
Boeing is conducting fatigue testing with a full-scale F-15C and F-15E article, Steve Parker, vice-president of F-15 programmes at Boeing, told reporters in St. Louis, Missouri last week. The aircraft have gone through 30,000h of testing, he says. By replacing the longerons on the fighter – an endeavor that would cost about $1 million per aircraft – Boeing could push the F-15C’s service life into the 2030s.
An earlier USAF analysis examined updating the F-15C’s fuselage, wings and landing gear, which could stretch the aircraft’s life into the 2040s – but would cost $30 million per airframe, Parker says.
“We believe that’s the most costly scenario,” he says. “We don’t believe the scenario is required – we don’t believe the air force is even looking at that scenario, because for $1 million they can replace the longerons.”
Rumours of the F-15C/D fleet’s possible retirement swirled around Washington DC earlier this year after a USAF official mentioned an analysis to replace the fighters with Lockheed Martin F-16s.
But Trump’s proposed budget would continue modifications to keep the F-15C flying into the mid-2020s, including the Eagle passive active warning and survivability system and an integrated defensive electronic countermeasures system. These would autonomously locate radio frequency threats and deny RF threat systems.
The proposed budget also requests $57 million for an infrared search and track system in FY2018, and $16.7 million for upgrades to the platform’s Raytheon APG-63(V)3 radar.
Original post flightglobal.com
The McDonnell Douglas F-15 was the winner of an Air Force competition for a new air superiority fighter aircraft in December 1968. The first flight of the aircraft was in July 1972 and IOC was in January 1976. Since the start of the program, five different models have been built for the Air Force (plus models for foreign military sales). These are the single-seat A and C models, the two-seat B and D models, and the dual-role two-seat F-15E Strike Eagle. The Air National Guard currently has 116 of the A and B models, and the Air Force has 621 of the other models. The structural configuration of the aging A through D models are much the same.
The F-15A/D models were designed under Air Force ASIP requirements in the late 1960s prior to the adoption of damage tolerance requirements; however, full-scale fatigue and static testing were also conducted. A complete DADTA was performed in the early 1980s to update the maintenance program based on the damage tolerance approach. In the initial design of the F-15, McDonnell Douglas incorporated a fatigue-resistant interference fastener system, but ignored its beneficial effects when establishing the operating stress levels for the structure. This turned out to be fortuitous in that it allowed some margin for increase in severity of the loads spectrum. In fact, the growth in weight of the aircraft and changes in load factor severity have significantly increased the spectrum severity, causing the time required to grow an initial flaw to critical size to be reduced to about one-fourth of its original value. This increased severity was noted through the IATP conducted by the Warner-Robins ALC, and because the change was so significant, it was decided to conduct an additional full-scale fatigue test to the increased severity spectrum. This test was conducted at the Wright Laboratories test facility at Wright-Patterson AFB. The results of this test indicate that the original operational service life goal of 8,000 hours should still be attainable. However, the increased use severity will increase the inspection burden, and some of the wing inspections could become particularly onerous because of the current lack of NDE capability to inspect for small cracks without removal of the fasteners. McDonnell Douglas is currently using the results of the tear-down inspection of the fatigue test aircraft and crack growth analyses to obtain a better estimate of the actual service life expectancy of the F-15.
When the F-15E was designed, the MIL-STD-1520 and MIL-A-83444 damage tolerance requirements had been implemented. This meant that some areas of the original F-15 structures design had to be changed to meet these requirements, and some additional testing was required to prove the structural integrity. To date, the E models seems to be flying close to their design use spectrum.
The structural problems that have been encountered in service on the F-15 fall into the five following general categories:
damage to honeycomb structure
corrosion in nonhoneycomb structure
low-cycle fatigue cracking
The F-15C and E models have experienced honeycomb water intrusion, corrosion, disbonds, cracks, and in-flight loss of various secondary structures such as wing tips, ailerons, flaps, fin leading edges, and horizontal tail components. These problems have been caused by leak paths, inadequate bond durability, and unexpected dynamic loading. The current solution has been to perform a patch repair or to replace the components with improved honeycomb components.
The areas of the F-15 structure that have encountered buffet-induced cracking are illustrated in Figure A-4. Twintailed aircraft, such as the F-15 and the Navy’s F-18, use vortices generated from the fuselage at high angles of attack to provide additional rudder power for control. Unfortunately, these same vortices provide a very turbulent flow field at intermediate angles of attack and subject the tails to a high-frequency, asymmetric loading that causes early high-cycle fatigue cracking and partial failure of the tail structure. The first sign of cracking due to these loads in the F-15 was in the pod attachments at the top of the tails. Local repairs did nothing but move the failure points and reduce the life. It took a careful analysis of the entire tail response and fuselage attachment stiffness by McDonnell Douglas to simulate the tail vibration modes and deflections that led to these failures and provide the insight to arrive at a solution. This involved increasing the overall stiffness of the tail by adding graphite composite plies to attenuate the vibration. In the case of wing cracking due to buffet, as indicated in Figure A-4, the cause was flow detachment over the outer wing at even modest angles of attack that resulted in high-frequency out-of-plane loading. These loads vibrated the skins and integral stiffeners and caused cracking of the rib mouseholes through which the spanwise stiffeners ran. Again, local repairs did not solve the problem. Eventually, damping systems were applied to the stiffener/rib connections to reduce the problem, and an alternate method of connecting the stiffener cap to the rib was developed.
The primary acoustically induced high-cycle fatigue cracking on the F-15 was encountered on the E model after stores (externally mounted weapons and systems) were qualified for use on the aircraft. The E model is configured for both air-to-air and air-to-ground attack missions, and in the air-to-ground mission radar evasion often requires low-altitude, high-speed cruise and dash to the target. With multiple stores attached to pylons beneath the wings, shocks are formed, which cause high acoustic vibrations to occur on certain skin panels of the fuselage. These vibratory loads have been high enough to cause high-cycle fatigue cracking of some skin panels. To permanently fix such damage, it is necessary to design the repaired structure such that its natural frequency is out of the range of the shock impingement frequency. This is a complex problem that requires knowledge of both the excitation sources and the structural responses. The current approach to fixing these problems on the F-15 has been to replace the damaged structure with parts with greater thickness to increase strength and to apply damping material. Additional research in understanding and developing repairs or modifications for these types of problems (e.g., composite repairs and better damping materials) appears worthwhile.
The corrosion problems in nonhoneycomb structure on the F-15 have been minimal. There have been some problems in the fuselage fuel tank, the outboard leading-edge structure of the wings, and the flap hinge beam. The current solution has been to improve drainage, repair, and replace.
The primary low-cycle fatigue cracking that has occurred in service to date has been in the upper surface of the wing in compression-designed structure that was not sized for fatigue during the initial design. Also, there has been one fuselage cracking problem. The specific low-cycle fatigue cracking locations were as follows:
upper wing surface stringer runouts
upper wing spar cap seal grooves
front wing spar conduit hole
upper in-board longeron splice plate holes
FIGURE A-4 F-15 buffet-induced problems.
None of these problems is life limiting, and in all cases preventive repairs have been designed and installed on the aircraft. Source Synopses of Air Force Aging Aircraft Structural Histories
Possible Longeron that is being the issue on the F-15’s highlighted in above image – Source abovetopsecret.com
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, Nov 2.
On 2 November 2007, a Boeing Co. F-15C Eagle fighter jet flown by the Missouri Air National Guard crashed during a training exercise. The following day, the U.S. Air Force grounded the country’s global fleet of 676 F-15s out of “airworthiness concerns” while they conducted inspections to investigate a possible structural failure in the aircraft. An animated video simulation of the Missouri F-15’s mid-air breakup was prepared during the course of the investigation, and the images displayed above are frames taken from that simulation:
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