Flight control modes: Difference between revisions

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{{Short description|Aircraft control computer software}}

{{refimprove|date=July 2014}}

[[File:Aeroflot Boeing 777 inflight.jpg|thumb|upright=1.14|Modern aircraft designs like the [[Boeing 777]] rely on sophisticated flight computers to aid and protect the aircraft in flight. These are governed by computational laws which assign flight control modes during flight.]]

{{cleanup rewrite|section=yes|date=January 2015}}

A '''flight control mode''' or '''flight control law''' is a computer software algorithm that transforms the movement of the [[Yoke (aeronautics)|yoke]] or [[joystick]], made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the [[Aircraft flight control system| flight control system]] is [[fly-by-wire]], the movements the pilot makes to the yoke or joystick in the [[cockpit]], to control the flight, are converted to electronic signals, which are transmitted to the [[Flight control computer|flight control computers]] that determine how to move each control surface to provide the aircraft movement the pilot ordered.<ref>{{Cite web|url=https://www.skybrary.aero/index.php/Flight_Control_Laws|title=Flight Control Laws |website=SKYbrary Aviation Safety|access-date=2019-07-03}}</ref><ref>{{Cite web|url=https://leehamnews.com/2016/03/25/bjorns-corner-flight-control-part-3/|title=Flight control part 3|website=Bjorn's corner|date=25 March 2016 }}</ref><ref name="urlCrossing the Skies » Fly-by-wire and Airbus Laws">{{cite web |url=http://www.crossingtheskies.com/fly-by-wire-airbus-laws/ |title=Crossing the Skies » Fly-by-wire and Airbus Laws |work= crossingtheskies.com|archive-url = https://web.archive.org/web/20090308133740/http://www.crossingtheskies.com/fly-by-wire-airbus-laws/|archive-date =8 March 2009}}</ref><ref name="Boeing 777">{{cite web |url=http://euler.ecs.umass.edu/ece655/Boeing777.ppt |title=The Boeing 777 |format=powerpoint |work= by Saurabh Chheda }}</ref>

A reduction of electronic flight control can be caused by the failure of a computational device, such as the flight control computer or an information providing device, such as the [[ADIRUAir Data Inertial Reference Unit]] (ADIRU).<ref name="Skybrary: Flight Control Laws">{{cite web |url=http://www.skybrary.aero/index.php/Flight_Control_Laws |title=Skybrary: Flight Control Laws}}</ref>

[[File:Aeroflot Boeing 777 inflight.jpg|thumb|upright=1.14|Modern aircraft designs like the [[Boeing 777]] rely on sophisticated flight computers to aid and protect the aircraft in flight. These are governed by computational laws which assign flight control modes during flight]]

A '''flight control mode''' or '''flight control law''' is a computer software algorithm that transforms the movement of the [[Yoke (aeronautics)|yoke]] or [[joystick]], made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the [[Aircraft flight control system| flight control system]] is [[fly-by-wire]], the movements the pilot makes to the yoke or joystick in the [[cockpit]], to control the flight, are converted to electronic signals, which are transmitted to the flight control computers that determine how to move each control surface to provide the aircraft movement the pilot ordered.<ref>{{Cite web|url=https://www.skybrary.aero/index.php/Flight_Control_Laws|title=Flight Control Laws |website=SKYbrary Aviation Safety|access-date=2019-07-03}}</ref><ref>{{Cite web|url=https://leehamnews.com/2016/03/25/bjorns-corner-flight-control-part-3/|title=Flight control part 3|website=Bjorn's corner}}</ref><ref name="urlCrossing the Skies » Fly-by-wire and Airbus Laws">{{cite web |url=http://www.crossingtheskies.com/fly-by-wire-airbus-laws/ |title=Crossing the Skies » Fly-by-wire and Airbus Laws |work= crossingtheskies.com|archive-url = https://web.archive.org/web/20090308133740/http://www.crossingtheskies.com/fly-by-wire-airbus-laws/|archive-date =8 March 2009}}</ref><ref name="Boeing 777">{{cite web |url=http://euler.ecs.umass.edu/ece655/Boeing777.ppt |title=The Boeing 777 |format=powerpoint |work= by Saurabh Chheda }}</ref>

A reduction of electronic flight control can be caused by the failure of a computational device, such as the flight control computer or an information providing device, such as the [[ADIRU]].<ref name="Skybrary: Flight Control Laws">{{cite web |url=http://www.skybrary.aero/index.php/Flight_Control_Laws |title=Skybrary: Flight Control Laws}}</ref>

[[Aircraft flight control systems|Electronic flight control systems]] (EFCS) also provide augmentation in normal flight, such as increased protection of the aircraft from overstress or providing a more comfortable flight for passengers by recognizing and correcting for [[turbulence]] and providing [[yaw damper|yaw damping]].{{Citation needed|date=October 2013}}

Two aircraft manufacturers produce commercial passenger aircraft with primary flight computers that can perform under different flight control modes (or laws). The most well-known areis the system of ''normal'', ''alternate'', ''direct laws'' and ''mechanical alternate control'' laws of the [[Airbus]] [[Airbus A320 family|A320]]-[[Airbus A380|A380]].<ref name="urlCrossing the Skies » Fly-by-wire and Airbus Laws"/> The other is [[Boeing]]'s fly-by-wire system, used in the [[Boeing 777]], [[Boeing 787 Dreamliner]] and [[Boeing 747-8]].<ref name="Boeing 777"/><ref name="urlAvionics Magazine :: Boeing 787: Integration’s Next Step">{{cite web |url=http://www.aviationtoday.com/av/categories/commercial/932.html |title=Avionics Magazine :: Boeing 787: Integration's Next Step |access-date=2009-07-19 |archive-date=2015-09-23 |archive-url=https://web.archive.org/web/20150923180905/http://www.aviationtoday.com/av/categories/commercial/932.html |url-status=dead }}</ref>

[[Boeing]]'s fly-by-wire system is used in the [[Boeing 777]], [[Boeing 787 Dreamliner]] and [[Boeing 747-8]].<ref name="Boeing 777"/><ref name="urlAvionics Magazine :: Boeing 787: Integration’s Next Step">{{cite web |url=http://www.aviationtoday.com/av/categories/commercial/932.html |title=Avionics Magazine :: Boeing 787: Integration's Next Step }}</ref>

These newer aircraft use electronic control systems to increase safety and performance while saving aircraft weight. These electronic systems are lighter than the old mechanical systems and can also protect the aircraft from overstress situations, allowing designers to reduce over-engineered components, which further reduces the aircraft's weight.{{Citation needed|date=October 2013}}

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[[File:Gulfair.a330-200.a40-kc.arp.jpg|thumb|right|A330-200 in flight]]

Airbus aircraft designs after the [[Airbus A300|A300]]/[[Airbus A310|A310]] are almost completely controlled by fly-by-wire equipment. These newer aircraft, including the [[Airbus A320 family|A320]], [[Airbus A330|A330]], [[Airbus A340|A340]], [[Airbus A350|A350]] and [[Airbus A380|A380]] operate under Airbus flight control laws.<ref name="SmartCockpit - A330 Flight Controls">{{cite web|url=http://www.smartcockpit.com/pdf/plane/airbus/A330/systems/0010/ |title=Airbus 330 – Systems – Flight Controls |work=SmartCockpit – Airline training guides, Aviation, Operations, Safety |access-date=July 12, 2009 |url-status=dead |archive-url=https://archive.istoday/20090612084314/http://www.smartcockpit.com/pdf/plane/airbus/A330/systems/0010/ |archive-date=June 12, 2009 }}</ref> The flight controls on the Airbus A330, for example, are all electronically controlled and hydraulically activated. Some surfaces, such as the rudder, can also be mechanically controlled. In normal flight, the computers act to prevent excessive forces in pitch and roll.<ref name="SmartCockpit - A330 Flight Controls"/>[[File:Fly by wireAirbus A321 cockpit - G-EUXG British Airways.jpg|thumb|leftright|Airbus A321 cockpit]]

[[File:Airspeed indication system - fly by wire.png|thumb|Illustration of the air-data reference system on Airbus A330]]

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====Ground mode====

The aircraft behaves as in direct mode: the autotrim feature is turned off and there is a direct response of the elevators to the sidestick inputs. The horizontal stabilizer is set to 4° up <!-- Stab nose up? Clarify--> but manual settings <!-- Manual settings, keyed in to the FMS? Clarify -->(e.g., for center of gravity) override this setting. After the wheels leave the ground, a 5-second transition occurs where ''normal law – flight mode'' takes over from ''ground mode''.<ref name="SmartCockpit - A330 Flight Controls"/>

====Flight mode====

The flight mode of ''normal law'' provides five types of protection: pitch attitude, load factor limitations, high speed, high-[[angle of attack|AOA]] and [[bank angle]]. ''Flight mode'' is operational from take-off, until shortly before the aircraft lands, around 100 feet above ground level. It can be lost prematurely as a result of pilot commands or system failures. Loss of ''normal law'' as a result of a system failure results in ''alternate law 1'' or ''2''.<ref name="urlAirbus Flight Control Laws">{{cite web |url=http://www.airbusdriver.net/airbus_fltlaws.htm |title=Airbus Flight Control Laws }}</ref>

Unlike conventional controls, in ''normal law'' vertical side stick movement corresponds to a load factor proportional to stick deflection independent of aircraft speed. When the stick is neutral and the load factor is 1g, the aircraft remains in level flight without the pilot changing the elevator trim. Horizontal side stick movement commands a roll rate, and the aircraft maintains a proper pitch angle once a turn has been established, up to 33° bank. The system prevents further trim up when the [[angle of attack]] is excessive, the load factor exceeds 1.3g, or when the bank angle exceeds 33°.{{Citation needed|date=October 2013}}

Alpha protection (α-Prot) prevents stalling and guards against the effects of windshear. The protection engages when the angle of attack is between α-Prot and α-Max and limits the angle of attack commanded by the pilot's sidestick or, if autopilot is engaged, it disengages the autopilot.{{Citation needed|date=October 2013}}

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====Flare mode====

[[File:Airbus A380.jpg|thumb|right|A380 in take off]]

This mode is automatically engaged when the [[radar altimeter]] indicates 100 feet above ground. At 50 feet the aircraft trims the nose slightly down. During the [[flarelanding (aviation)|flare]], ''normal law'' provides high-[[angle of attack]] protection and bank angle protection. The load factor is permitted to be from 2.5g to −1g, or 2.0g to 0g when slats are extended. Pitch attitude is limited from −15° to +30°, and upper limit is further reduced to +25° as the aircraft slows.<ref name="SmartCockpit - A330 Flight Controls"/>

===Alternate law===

There are four reconfiguration modes for the Airbus fly-by-wire aircraft: ''alternate law 1'', ''alternate law 2'', ''direct law'' and ''mechanical law''. The ground mode and flare modes for ''alternate law'' are identical to those modes for ''normal law''.

'''Alternate law 1''' (ALT1) mode combines a ''normal law'' lateral mode with the load factor, bank angle protections retained. High angle of attack protection may be lost and low energy (level flight stall) protection is lost. High speed and high angle of attack protections enter alternativealternate law mode.<ref name="urlAirbus Flight Control Laws"/>

ALT1 may be entered if there are faults in the horizontal stabilizer, an elevator, yaw-damper actuation, slat or flap sensor, or a single air data reference fault.<ref name="SmartCockpit - A330 Flight Controls"/>

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===Direct law===

''Direct law'' (DIR) introduces a direct stick-to-control surfaces relationship:<ref name="SmartCockpit - A330 Flight Controls"/> control surface motion is directly related to the sidestick and rudder pedal motion.<ref name="urlCrossing the Skies » Fly-by-wire and Airbus Laws"/> The trimmable horizontal stabilizer can only be controlled by the manual trim wheel. All protections are lost, and the maximum deflection of the elevators is limited for each configuration as a function of the current aircraft centre of gravity. This aims to create a compromise between adequate pitch control with a forward C.G.center of gravity and not-too-sensitive control with an aft C.Gcenter of gravity.<ref name="A320AFM">Airbus A320 AFM (requires page number, publisher, etc)</ref>

DIR is entered if there is failure of three inertial reference units or the primary flight computers, faults in two elevators, or flame-out in two engines (on a two-engine aircraft) when the captain's primary flight computer is also inoperable.<ref name="SmartCockpit - A330 Flight Controls"/>

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==Boeing 777 primary flight control system==

[[File:Boeing 777-200ER cockpit.jpg|thumb|right|The cockpit of the 777 is similar to 747-400, a fly-by-wire control simulating mechanical control.]]

The fly-by-wire electronic flight control system of the Boeing 777 differs from the Airbus EFCS. The design principle is to provide a system that responds similarly to a mechanically controlled system.<ref name="Boeing B-777: Fly-By-Wire Flight Controls">{{cite web |url=http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf |title=11 Boeing B-777: Fly-By-Wire Flight Controls | author=Gregg F. Bartley – Boeing | date=May 4, 2008 | access-date=October 8, 2016}}</ref> Because the system is controlled electronically, the flight control system can provide [[flight envelope]] protection.

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===Secondary mode===

Boeing ''secondary mode'' is comparable to the Airbus ''alternate law'', with the PFCs supplying commands to the ACEs. However, EFCS functionality is reduced, including loss of flight envelope protection. Like the Airbus system, this state is entered when a number of failures occur in the EFCS or interfacing systems (e.g., ADIRU or [[SAARU]]). Moreover, in case of a complete failure of all PFCs and ACEs, the ailerons and selected roll spoilers are connected to the pilot controls by control cable, permitting mechanical control on a temporary basis.<ref name="Skybrary: Flight Control Laws" /><ref name="Boeing 777"/>

==See also==

*[[Index of aviation articles]]

*[[Dual control (aviation)]]

==References==

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{{Aircraft components}}

[[Category:Technology systems]]

[[Category:Aircraft instruments]]

[[Category:Aerospace engineering]]

[[Category:Aircraft instruments]]

[[Category:Flight control systems]]

[[Category:Technology systems]]