Editing Fly-by-wire (section)

== Digital systems ==
[[File:Fly by wire.jpg|thumb|The NASA F-8 Crusader with its fly-by-wire system in green and Apollo guidance computer]]

A digital fly-by-wire flight control system can be extended from its analog counterpart. Digital signal processing can receive and interpret input from multiple sensors simultaneously (such as the [[altimeter]]s and the [[pitot tube]]s) and adjust the controls in real time. The computers sense position and force inputs from pilot controls and aircraft sensors. They then solve [[differential equation]]s related to the aircraft's [[equations of motion]] to determine the appropriate command signals for the flight controls to execute the intentions of the pilot.<ref name=avihand >{{cite web|url=http://davi.ws/avionics/TheAvionicsHandbook_Cap_12.pdf |archive-url=https://web.archive.org/web/20110812213704/http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_12.pdf |archive-date=2011-08-12 |url-status=live|title=The Avionics Handbook|website=davi.ws|access-date=24 April 2018}}</ref>

The programming of the digital computers enable [[flight envelope protection]]. These protections are tailored to an aircraft's handling characteristics to stay within aerodynamic and structural limitations of the aircraft. For example, the computer in flight envelope protection mode can try to prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits on the aircraft's flight-control envelope, such as those that prevent stalls and spins, and which limit airspeeds and [[g force]]s on the airplane. Software can also be included that stabilize the flight-control inputs to avoid [[pilot-induced oscillation]]s.<ref name=airbus >{{cite web |url = http://personales.upv.es/juaruiga/teaching/TFC/Material/Trabajos/AIRBUS.PDF |title = Airbus A320/A330/A340 Electrical Flight Controls: A Family of Fault-Tolerant Systems |archive-url=https://web.archive.org/web/20090327095042/http://personales.upv.es/juaruiga/teaching/TFC/Material/Trabajos/AIRBUS.PDF |archive-date=27 March 2009 |url-status=dead}}</ref>

Since the flight-control computers continuously feedback the environment, pilot's workloads can be reduced.<ref name=airbus/> This also enables [[military aircraft]] with [[relaxed stability]]. The primary benefit for such aircraft is more maneuverability during combat and training flights, and the so-called "carefree handling" because stalling, spinning and other undesirable performances are prevented automatically by the computers. Digital flight control systems (DFCS) enable inherently unstable combat aircraft, such as the [[Lockheed F-117 Nighthawk]] and the [[Northrop Grumman B-2 Spirit]] [[flying wing]] to fly in usable and safe manners.<ref name=avihand/>

=== Legislation ===
The United States [[Federal Aviation Administration]] (FAA) has adopted the [[Radio Technical Commission for Aeronautics|RTCA]]/[[DO-178C]], titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software. Any [[safety-critical]] component in a digital fly-by-wire system including applications of the laws of [[aeronautics]] and computer [[operating system]]s will need to be certified to DO-178C Level A or B, depending on the class of aircraft, which is applicable for preventing potential catastrophic failures.<ref>{{Cite web|url=http://www.aviationexplorer.com/Fly_By_Wire_Aircraft.html|title=Fly-By-Wire Aircraft Facts History Pictures and Information|last=Explorer|first=Aviation|website=aviationexplorer.com|access-date=2016-10-13}}</ref>

Nevertheless, the top concern for computerized, digital, fly-by-wire systems is reliability, even more so than for analog electronic control systems. This is because the digital computers that are running software are often the only control path between the pilot and aircraft's [[flight control surfaces]]. If the computer software crashes for any reason, the pilot may be unable to control an aircraft. Hence virtually all fly-by-wire flight control systems are either triply or quadruply [[redundancy (engineering)|redundant in their computers and electronics]]. These have three or four flight-control computers operating in parallel and three or four separate [[Bus (computing)|data buses]] connecting them with each control surface.{{Citation needed|date=May 2010}}

=== Redundancy ===
The multiple redundant flight control computers continuously monitor each other's output. If one computer begins to give aberrant results for any reason, potentially including software or hardware failures or flawed input data, then the combined system is designed to exclude the results from that computer in deciding the appropriate actions for the flight controls. Depending on specific system details there may be the potential to reboot an aberrant flight control computer, or to reincorporate its inputs if they return to agreement. Complex logic exists to deal with multiple failures, which may prompt the system to revert to simpler back-up modes.<ref name=avihand/><ref name=airbus/>

In addition, most of the early digital fly-by-wire aircraft also had an analog electrical, mechanical, or hydraulic back-up flight control system. The [[Space Shuttle]] had, in addition to its redundant set of four [[digital computer]]s running its primary flight-control software, a fifth backup computer running a separately developed, reduced-function, software flight-control system – one that could be commanded to take over in the event that a fault ever affected all of the other four computers. This backup system served to reduce the risk of total flight control system failure ever happening because of a general-purpose flight software fault that had escaped notice in the other four computers.<ref name=suth/><ref name=avihand/>

=== Efficiency of flight ===
For airliners, flight-control redundancy improves their safety, but fly-by-wire control systems, which are physically lighter and have lower maintenance demands than conventional controls also improve economy, both in terms of cost of ownership and for in-flight economy. In certain designs with limited relaxed stability in the pitch axis, for example the Boeing 777, the flight control system may allow the aircraft to fly at a more aerodynamically efficient angle of attack than a conventionally stable design. Modern airliners also commonly feature computerized Full-Authority Digital Engine Control systems ([[FADEC]]s) that control their engines, air inlets, fuel storage and distribution system, in a similar fashion to the way that FBW controls the flight control surfaces. This allows the engine output to be continually varied for the most efficient usage possible.<ref>{{Cite web|date=29 June 2001|title=Full Authority Digital Engine Control|url=https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_33.28-1.pdf|access-date=3 January 2022|work=Compliance Criteria For 14 CFR §33.28, Aircraft Engines, Electrical And Electronic Engine Control Systems|author=Federal Aviation Administration|author-link=Federal Aviation Administration|archive-url= https://web.archive.org/web/20200624041757/https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_33.28-1.pdf|archive-date= 24 June 2020|url-status= live}}</ref>

The [[Embraer E-Jet E2 family|second generation Embraer E-Jet family]] gained a 1.5% efficiency improvement over the first generation from the fly-by-wire system, which enabled a reduction from 280&nbsp;ft.² to 250&nbsp;ft.² for the [[horizontal stabilizer]] on the E190/195 variants.<ref name="AWSTEmbraerE2CertificationTestsSetToAccelerate">{{cite news|last1=Norris|first1=Guy|title=Embraer E2 Certification Tests Set to Accelerate|url=http://aviationweek.com/commercial-aviation/embraer-e2-certification-tests-set-accelerate|access-date=6 September 2016|work=[[Aviation Week & Space Technology]]|publisher=[[Aviation Week]]|date=5 September 2016|url-access=registration }}</ref>

=== Airbus/Boeing ===
{{Main|Flight control modes}}

Airbus and Boeing differ in their approaches to implementing fly-by-wire systems in commercial aircraft. Since the [[Airbus A320 family|Airbus A320]], Airbus flight-envelope control systems always retain ultimate flight control when flying under normal law and will not permit pilots to violate aircraft performance limits unless they choose to fly under alternate law.<ref>{{cite magazine|url=https://www.popularmechanics.com/flight/a3115/what-really-happened-aboard-air-france-447-6611877/ |title=Air France 447 Flight-Data Recorder Transcript – What Really Happened Aboard Air France 447 |magazine=Popular Mechanics |date=6 December 2011 |access-date=7 July 2012}}</ref> This strategy has been continued on subsequent Airbus airliners.<ref>Briere D. and Traverse, P. (1993) "[http://personales.upv.es/juaruiga/teaching/TFC/Material/Trabajos/AIRBUS.PDF Airbus A320/A330/A340 Electrical Flight Controls: A Family of Fault-Tolerant Systems] {{webarchive|url=https://web.archive.org/web/20090327095042/http://personales.upv.es/juaruiga/teaching/TFC/Material/Trabajos/AIRBUS.PDF |date=27 March 2009 }}" Proc. FTCS, pp. 616–623.</ref><ref>North, David. (2000) "Finding Common Ground in Envelope Protection Systems". ''Aviation Week & Space Technology'', 28 Aug, pp. 66–68.</ref> However, in the event of multiple failures of redundant computers, the A320 does have a mechanical back-up system for its pitch trim and its rudder, the [[Airbus A340]] has a purely electrical (not electronic) back-up rudder control system and beginning with the A380, all flight-control systems have back-up systems that are purely electrical through the use of a "three-axis Backup Control Module" (BCM).<ref>Le Tron, X. (2007) [http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2007_09_27_A380_Flight_Controls.pdf A380 Flight Control Overview] Presentation at Hamburg University of Applied Sciences, 27 September 2007</ref>

Boeing airliners, such as the [[Boeing 777]], allow the pilots to completely override the computerized flight control system, permitting the aircraft to be flown outside of its usual flight control envelope.

=== Applications ===

[[File:F-BUAD A300B2-C1 Airbus Industry(3rd prototype) Farnborough SEP86 (12609347665).jpg|thumb|[[Airbus]] trialed fly-by-wire on an [[Airbus A300|A300]] registration F-BUAD as shown in 1986, then produced the [[Airbus A320|A320]].]]

* [[Concorde]] was the first production fly-by-wire aircraft with analog control.
* The [[General Dynamics F-16 Fighting Falcon|General Dynamics F-16]] was the first production aircraft to use digital fly-by-wire controls.<ref>{{Cite web|date=2000-06-01|title=Computers Take Flight|url=https://www.nasa.gov/wp-content/uploads/2021/04/182985main_DFBW_rev1.pdf|access-date=2024-06-10|website=NASA|language=en-US}}</ref>
* The [[Space Shuttle orbiter]] had an all-digital fly-by-wire control system. This system was first exercised (as the only flight control system) during the [[glider aircraft|glider unpowered-flight]] "Approach and Landing Tests" that began with the Space Shuttle ''[[Space Shuttle Enterprise|Enterprise]]'' during 1977.<ref>{{Cite journal|date=1975-08-01|title=Space Shuttle Flight Control System|url=https://www.sciencedirect.com/science/article/pii/S1474667017674822|journal=IFAC Proceedings Volumes|language=en|volume=8|issue=1|pages=302–310|doi=10.1016/S1474-6670(17)67482-2|issn=1474-6670|last1=Klinar|first1=Walter J.|last2=Saldana|first2=Rudolph L.|last3=Kubiak|first3=Edward T.|last4=Smith|first4=Emery E.|last5=Peters|first5=William H.|last6=Stegall|first6=Hansel W.}}</ref>
* Launched into production during 1984, the Airbus Industries [[Airbus A320 family|Airbus A320]] became the first airliner to fly with an all-digital fly-by-wire control system.<ref name="civilavionics">{{cite book |author1=Ian Moir |author2=Allan G. Seabridge |author3=Malcolm Jukes |title=Civil Avionics Systems |publisher=Professional Engineering Publishing Ltd. |location=London ([[iMechE]]) |year=2003|isbn=1-86058-342-3}}</ref>
* With its launch in 1993 the [[Boeing C-17 Globemaster III]] became the first fly-by-wire military transport aircraft.<ref>{{Cite web |title=C-17 Globemaster III Archives |url=https://www.airandspaceforces.com/weapons-platforms/c-17/ |access-date=2023-01-29 |website=Air & Space Forces Magazine |language=en-US}}</ref>
* In 2005, the [[Dassault Falcon 7X]] became the first [[business jet]] with fly-by-wire controls.<ref>{{cite news |url= http://aviationweek.com/business-aviation/pilot-report-falcon-7x-fly-wire-control-system |title= Pilot Report on Falcon 7X Fly-By-Wire Control System |date= 3 May 2010 |work= Aviation Week & Space Technology}}</ref>
* A fully digital fly-by-wire without a closed feedback loop was integrated in 2002 in the [[Embraer E-Jet family|first generation Embraer E-Jet family]]. By closing the loop (feedback), the [[Embraer E-Jet E2 family|second generation Embraer E-Jet family]] gained a 1.5% efficiency improvement in 2016.<ref name="AWSTEmbraerE2CertificationTestsSetToAccelerate"/>