On 2 November 2007, an F-15C Eagle air superiority fighter of the Missouri Air National Guard disintegrated in mid-air, the pilot managing to eject safely from the stricken aircraft. As recounted here on 6 November, the cause of the accident was not immediately apparent, and the entire fleet of F-15s was grounded for precautionary inspections, pending completion of the accident investigation. An astute reader with hands-on experience believed that de-bonding honeycomb structures on the tail and ailerons could be at fault, with potentially grave implications for the airworthiness of the entire F-15 fleet. As it turned out, there were systemic problems, but from an entirely different cause. The precautionary inspections having failed to reveal any systemic faults in the aircraft, on 21 November the F-15s were cleared to fly once again. According to a message sent by General John D.W. Corley, Commander, Air Combat Command (ACC) to all F-15 pilots and their families:
However, on 27 November, analysis of the accident investigation revealed what USAF Air Combat Command called “possible fleet-wide airworthiness problems.” As detailed in an official ACC press release,
To understand the seriousness of this finding, one must understand something of how the F-15 was designed. For ease of manufacturing and maintenance, the aircraft is constructed in seven major subassemblies: the cockpit and forward fuselage; the fuselage center section; the engine compartment and aft fuselage; the wings; and the tail assembly. These are held together with a series of high-strength titanium bolts. This method of construction allows for the rapid maintenance and repair of the aircraft (e.g., a damaged wing can be unbolted and replaced with the wing of another plane), as well as allowing for distributed manufacturing of the subassemblies.
The longerons in question are a series of aluminum alloy beams running fore-and-aft down the length of the aircraft, and include the attachment points that connect the cockpit and forward fuselage to the fuselage midsection. If the longerons crack from metal fatigue or corrosion, then the cockpit could separate from the rear of the aircraft. On most aircraft, that particular area is not subject to severe stresses, but in the highly maneuverable F-15, which routinely pulls 5-6g turns (and is capable of 9g turns), the forward longerons are subjected to repeated stretching and bending forces, which over time can cause fatigue cracks. The accident investigation suspects that the F-15C which broke up over Missouri suffered precisely that kind of failure–and as an additional precautionary measure, all F-15A, B, C and D models have been grounded pending a more detailed inspection (probably including radiographic imaging of the longerons and bolts) of the critical area. If a widespread problem is detected, the affected models could be grounded for a long time, putting a major dent in USAF operational capabilities, particularly for air superiority and air defense operations. During the first grounding, NORAD had to rely less capable aircraft such as the F-16 Fighting Falcon and Canadian Forces CF-18 Hornets to fly air patrols and interceptions of suspicious aircraft. While capable fighters in their own right, they lack the F-15’s range, powerful radar and large air-to-air missile payload. The situation could be worse. The A-D models of the F-15 are optimized air superiority fighters, with little or no ground attack capability. Therefore, there is relatively little demand for their services at present, and, as during the first grounding incident, they can be supplemented by other aircraft such as the F-16A Air Defense Fighters assigned to the Air National Guard (as well as by allied aircraft). If the inspections reveal a significant number of F-15s in need of major repairs, pulling these aircraft off line will not create the same crisis as would have occurred had the F-15E Strike Eagle also been grounded. A “dual-role” fighter, the Strike Eagle retains the original Eagle’s superb dogfighting and beyond-visual-range (BVR) air combat capability, but is normally employed as a long-range strike-interdiction aircraft, capable of penetrating deep into enemy territory at treetop level in all weather and delivering a wide range of ordnance with great precision. Since the beginning of the wars in Iraq and Afghanistan, they have been used extensively as high-altitude precision guided munitions platforms, and are greatly valued because of their long range and endurance, as well as their awesome payload. Without the F-15E, the USAF would be severely handicapped in its offensive air missions. However, as noted in the ACC press release above, the F-15E is not affected by the current grounding, because its airframe was extensively reinforced to accommodate the very heavy weapons loads at which it operates (its maximum takeoff weight is a staggering 81,000 pounds). Repairing the F-15A-D models could prove expensive and time consuming. The fixes include everything from attaching “stiffening plates” to the longerons, to replacing the structural members entirely. The latter may prove prohibitively expensive for some of the older airframes, and the Air Force may choose instead to reduce the size of the fleet rather than pour scarce O&M dollars into a solution that may have only limited viability. Above all, the Air Force does not want to do anything that would threaten procurement of the new F-22 Raptor and F-35 Lighting II Joint Strike Fighter. The problems of the F-15 are symptomatic of a recurrent problem in U.S. defense procurement known as “block obsolescense”. Because of the military buildup in World War II and the beginning of the Cold War, the U.S. acquired a large number of aircraft, ships, tanks and other major equipment in a short period of time. After building up, it was difficult to get Congress to procure adequate numbers of replacement items, particularly at times when external threats appeared to be receding. The Vietnam war represented a major time-out in U.S. defense procurement, as war funding ate into procurement dollars, while the wartime tempo of operations ate into the service life of a rapidly aging force (a similar phenomenon may be occurring today because of Iraq and Afghanistan). By the late 1970s, an entire generation of U.S. weapons systems not only became technically outdated, but had reached the end of their material lives. All of them had to be replaced, albeit in smaller numbers, during the late 1970s and early 1980s–the years of the Reagan Buildup. But that was almost thirty years ago, and the systems we bought during that period are now reaching the end of their lives all at once, and will need to be replaced. But the cost of replacement weapons has risen exponentially in the interim, so that 1-for-1 replacement is simply unaffordable. Moreover, the nature the threats faced by the U.S., and the types of war we are likely to fight, are evolving very rapidly, making correct procurement decisions very difficult. Getting the right answer, and then funding and implementing it, will determine the effectiveness of all U.S. military services in the next generation.
