The Air Force F-22 fighter, also known as the Raptor, is "the most capable fighter aircraft ever built, period." Procurement of F-22s began in FY1999, and a total of 195 (177 production aircraft, 16 test aircraft, and 2 development aircraft) were procured through FY2009. Ongoing issues for Congress regarding the F-22 program include questions regarding the F-22's supply of oxygen to its pilots, the possible resumption of production, the reliability and maintainability of in-service Raptors, the F-22 modernization program, and the potential sale of F-22s to other countries. Congress's decisions on all these issues could affect Department of Defense (DOD) capabilities and funding requirements, the U.S. tactical aircraft industrial base, and U.S. relations with other countries. Following a November 2010 fatal crash of an F-22 in Alaska, the Air Force began an effort to understand why pilots were "reporting hypoxia-like symptoms in the air. Hypoxia is a condition that can bring on nausea, headaches, fatigue or blackouts when the body is deprived of oxygen." At least 25 "physiological incidents" were recorded among F-22 pilots; 14 prior to the Raptor being grounded from May to September 2011, for investigation of the issue, and at least 11 subsequently. In an August 2012 review, the Air Force announced that the oxygen deprivation issue was due to "a 'mosaic' of interrelated cockpit equipment issues that led to a chain reaction of glitches resulting in symptoms similar to hypoxia." The equipment problems included issues with valves in vests designed to help pilots breathe during high-g maneuvers; improper "scheduling," or programming, of those vests; and effects of the charcoal canisters added by the Air Force during the investigation in order to reduce possible toxins in the oxygen supply. While the valve has performed satisfactorily for pilots of older F-15 and F-16 fighters, the F-22 is designed to fly at higher altitudes, which requires continuously pumping more oxygen to the pilots, Lyon said. The valve wasn't strong enough to prevent that higher pressure oxygen from inflating the pilots' vests at lower altitudes. ... the Air Force did "rudimentary testing" on the valve some years ago and found that it caused the high-pressure vest to inflate early, hampering pilot mobility. However, the vest was found to be unnecessary on the F-15 and F-16. When the vest was re-introduced on the F-22 with the same valve, the problem wasn't detected. Also in response to the 2010 crash, the Air Force is "replacing handles that engage the F-22 Raptor fighter jet's emergency oxygen system." Difficulty in operating the backup oxygen system was cited as contributing to the crash. Beginning April 4, 2013, Air Combat Command returned F-22s that had had emergency backup oxygen systems installed to unrestricted flight status. The full fleet was expected to be in this status by July 2014. On May 3, 2012, Lockheed Martin delivered the 195 th F-22 to the Air Force, completing the fleet. That final aircraft had come off the Lockheed Martin production line in Marietta, GA, on December 13, 2011. Following its assembly, Lockheed Martin had begun to store "a total of more than 30,000 jigs, fixtures and other 'tooling' used to build the plane" in order to "leave open an option to restart the premier plane's production relatively cheaply." Lockheed Martin had also documented the assembly process, including videotaping each step of assembly, in order to more quickly train workers to assemble F-22s in the event of a production line restart. About 5,600 Lockheed employees worked on the F-22 program at its peak in 2005, including 944 in Marietta. The current number is 1,650 companywide, 930 in Marietta.... However, 600 Marietta employees will handle technical support and modernizations for the existing F-22 fleet. Conscious of DOD's having preserved the F-22 production tooling and Lockheed Martin's actions to ease a potential restart, a number of commentators have posited that the F-22 could be returned to production, whether to enable foreign sales, increase fleet numbers, or act as a hedge against difficulties in the F-35 program. The potential cost of a restart, and its concomitant effect on F-22 unit costs, is unclear. By one account: Bringing back the F-22 line would take less than $200 million, "a fraction of the costs seen in previous line restarts of other weapons systems," Alison Orne, a Lockheed spokeswoman, said by email, citing preliminary analysis. In a 2009 study conducted for the Air Force prior to the termination decision, the RAND Corporation analyzed the costs of four industrial scenarios: shutting down the F-22 line permanently; restarting the line after a two-year shutdown; maintaining low-rate production; and maintaining full-rate production, in each case to produce an additional 75 F-22s. This study found that shutdown, hiatus, and restart would cost $513 million, or $434 million more than termination. According to another report, in 2010, "Lockheed officials ... told Japanese leaders it would cost $900 million to re-open the production line." Given the range of possible restart costs and the uncertainty of how many aircraft would be purchased, it is not possible to estimate the unit cost of any F-22s built following a potential production restart. The F-22A Raptor is the world's most capable air-to-air combat aircraft. It also has an air-to-ground (i.e., attack) capability. The F-22 incorporates a high degree of stealth, as well as supercruise, thrust-vectoring for high maneuverability, and integrated avionics that fuse information from on-board and off-board sensors. The F-22 and the multi-service F-35 Joint Strike Fighter (JSF) are considered the world's first (and to date only) fifth-generation tactical aircraft. Fifth-generation aircraft incorporate the most modern technology and are considered to be generally more capable than earlier-generation (e.g., fourth-generation and below) aircraft. The F-22 is intended to replace the Air Force's aging F-15 air superiority fighters, while the F-35A (the Air Force version of the F-35) is intended to replace the service's aging F-16 fighters and A-10 attack aircraft. The F-22 is more stealthy than the F-35, and more capable than the F-35 in air-to-air combat. The F-35A is intended to be a more affordable complement to the F-22, and is a strike fighter—a dual-role aircraft with significant capability in both air-to-ground (strike) and air-to-air (fighter) operations. If the F-15/F-16 combination represented the Air Force's earlier-generation "high-low" mix of air superiority fighters and more-affordable dual-role aircraft, then the F-22/F-35A combination might be viewed as the Air Force's intended future high-low mix of air superiority fighters and more-affordable dual-role aircraft. The Air Force states that: Fifth generation fighters like the F-22A and the F-35 are key elements of our Nation's defense and ability for deterrence. As long as hostile nations recognize that U.S. airpower can strike their vital centers with impunity, all other U.S. Government efforts are enhanced, which reduces the need for military confrontation.... Both the F-22A and the F-35 represent our latest generation of fighter aircraft. We need both aircraft to maintain the margin of superiority we have come to depend upon, the margin that has granted our forces in the air and on the ground freedom to maneuver and to attack. The F-22A and F-35 each possess unique, complementary, and essential capabilities that together provide the synergistic effects required to maintain that margin of superiority across the spectrum of conflict.... The F-22A Raptor is the Air Force's primary air superiority fighter, providing unmatched capabilities for air supremacy, homeland defense and cruise missile defense for the Joint team. The multi-role F-22A's combination of speed, stealth, maneuverability and integrated avionics gives this remarkable aircraft the ability to gain access to, and survive in, high threat environments. Its ability to find, fix, track, and target enemy air- and surface-based threats ensures air dominance and freedom of maneuver for all Joint forces. The F-22 program was initiated in the early 1980s with the aim of developing a highly capable successor to the F-15 that would be capable of defeating all known and projected enemy fighters, including those being developed at the time by the Soviet Union. The F-22 program was given Milestone I approval in October 1986. The first flight of an F-22 industry prototype occurred in August 1990, and the first flight of a development version of the aircraft occurred in September 1997. The program was granted approval for Low Rate Initial Production (LRIP) in August 2001, and the first LRIP F-22 was delivered in June 2003. The F-22 achieved Initial Operational Capability (IOC) in December 2005. Operational F-22s are currently assigned to Joint Base Langley-Eustis, Virginia; Joint Base Elmendorf-Richardson, Alaska; Holloman Air Force Base (AFB), New Mexico; and Joint Base Pearl Harbor-Hickam, Hawaii. A training unit is at Tyndall AFB, Florida.