| abstract | - During the period in question, maneuverability was enhanced by relaxed static stability, made possible by introduction of the fly-by-wire (FBW) flight control system (FLCS), which in turn was possible due to advances in digital computers and system integration techniques. Analog avionics, required to enable FBW operations, became a fundamental requirement and began to be replaced by digital flight control systems in the latter half of the 1980s. The further advance of microcomputers in the 1980s and 1990s permitted rapid upgrades to the avionics over the lifetimes of these fighters, incorporating system upgrades such as Active Electronically Scanned Array (AESA), digital avionics buses and Infra-red search and track (IRST). Due to the dramatic enhancement of capabilities in these upgraded fighters and in new designs of the 1990s that reflected these new capabilities, the US government has taken to using the designation 4.5th generation to refer to these later designs. This is intended to reflect a class of fighters that are evolutionary upgrades of the 4th generation incorporating integrated avionics suites, advanced weapons efforts to make the (mostly) conventionally designed aircraft nonetheless less easily detectable, and trackable as a response to advancing missile and radar technology (see stealth technology). Inherent airframe design features exist, and include masking of turbine-blades and application of advanced sometimes radar-absorbent materials, but not the distinctive low-observable configurations of the latest aircraft, referred to as fifth-generation fighters or aircraft such as the F-22 Raptor and the F-35 Lightning II. The United States Government defines 4.5 generation fighter aircraft as fourth generation jet fighters that have been upgraded with AESA radar, high capacity data-link, enhanced avionics, and "the ability to deploy current and reasonably foreseeable advanced armaments."
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