page_title: Fourth-generation jet fighter
section_title: Fly-by-wire
hierarchical_section_title: Fourth-generation jet fighter / Design considerations / Fly-by-wire
caption_attribution_description: An F-14B TOMCAT, from the Fighter Squadron VF-103) "Jolly Rogers" and an F/A-18C(N) HORNET, from the Strike Fighter Squadron VFA-34 "Blue Blasters," demonstrate a high-speed, inverted pass.
context_page_description: Fourth-generation jet fighter is a general classification of jet fighters in service from approximately 1980 to the present and represent design concepts of the 1970s. Fourth-generation designs are heavily influenced by lessons learned from the previous generation of combat aircraft. Long-range air-to-air missiles, originally thought to make dogfighting obsolete, proved less influential than expected, precipitating a renewed emphasis on maneuverability. Meanwhile, the growing costs of military aircraft in general and the demonstrated success of aircraft such as the F-4 Phantom II gave rise to the popularity of multirole combat aircraft in parallel with the advances marking the so-called fourth generation.
  During the period in question, maneuverability was enhanced by relaxed static stability, made possible by introduction of the fly-by-wire flight control system, which in turn was possible due to advances in digital computers and system-integration techniques. Replacement of analog avionics, required to enable FBW operations, became a fundamental requirement as legacy analog computer systems began to be replaced by digital flight control systems in the latter half of the 1980s.
context_section_description: One of the new innovations on fourth generation jet fighters is fly-by-wire, while generation 4.5 introduced Active electronically scanned array radar.
  The YF-16, eventually developed into the F-16 Fighting Falcon, was the world's first aircraft intentionally designed to be slightly aerodynamically unstable. This technique, called "relaxed static stability" (RSS), was incorporated to further enhance the aircraft's performance. Most aircraft are designed with positive static stability, which induces an aircraft to return to its original attitude following a disturbance. However, positive static stability, the tendency to remain in its current attitude, opposes the pilot's efforts to maneuver. On the other hand, an aircraft with negative static stability will, in the absence of control input, readily deviate from level and controlled flight.
  An aircraft with negative static stability can therefore be made more maneuverable. At supersonic airspeed, a negatively stable aircraft can exhibit positive static stability due to aerodynamic center migration. To counter this tendency to depart from controlled flight—and avoid the need for constant minute trimming inputs by the pilot—the 4th gen aircraft has a quadruplex (four-channel) fly-by-wire (FBW) flight control system (FLCS). The flight control computer (FLCC), which is the key component of the FLCS, accepts the pilot's input from the stick and rudder controls, and manipulates the control surfaces in such a way as to produce the desired result without inducing a loss of control. The FLCC also takes thousands of measurements per second of the aircraft's attitude, and automatically makes corrections to counter deviations from the flight path that were not input by the pilot. Coordinated turn is also achieved in the same way, processing thousands of instructions per second to synchronize yawing and rolling to minimize sideslip drag in turns.
  Early fourth generation American fighters like the F-15 Eagle and F-14 Tomcat retained electro-mechanical flight hydraulics, while their newer and cheaper alternatives, the F-16 Fighting Falcon and F/A-18 Hornet, incorporated fly-by-wire. The newest derivative of the F-15, the F-15SA Strike Eagle for Saudi Arabia, has fly-by-wire instead of the previous Eagles' hybrid electronic/mechanical system.
caption: The F/A-18 inverted above an F-14 shown here is an example of fly-by-wire control.
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