Flight control modes

Last updated
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 Aeroflot Boeing 777 inflight.jpg
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 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 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. [1] [2] [3] [4]


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 Air Data Inertial Reference Unit (ADIRU). [5]

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 damping.[ citation needed ]

Two aircraft manufacturers produce commercial passenger aircraft with primary flight computers that can perform under different flight control modes. The most well-known is the system of normal, alternate, direct laws and mechanical alternate control laws of the Airbus A320-A380. [3] The other is Boeing's fly-by-wire system, used in the Boeing 777, Boeing 787 Dreamliner and Boeing 747-8. [4] [6]

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 ]

Flight control laws (Airbus)

A330-200 in flight Gulfair.a330-200.a40-kc.arp.jpg
A330-200 in flight

Airbus aircraft designs after the A300/A310 are almost completely controlled by fly-by-wire equipment. These newer aircraft, including the A320, A330, A340, A350 and A380 operate under Airbus flight control laws. [7] 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. [7]

Airbus A321 cockpit Airbus A321 cockpit - G-EUXG British Airways.jpg
Airbus A321 cockpit
Illustration of the air-data reference system on Airbus A330 Airspeed indication system - fly by wire.png
Illustration of the air-data reference system on Airbus A330

The aircraft is controlled by three primary control computers (captain's, first officer's, and standby) and two secondary control computers (captain's and first officer's). In addition there are two flight control data computers (FCDC) that read information from the sensors, such as air data (airspeed, altitude). This is fed along with GPS data, into three redundant processing units known as air data inertial reference units (ADIRUs) that act both as an air data reference and inertial reference. ADIRUs are part of the air data inertial reference system, which, on the Airbus is linked to eight air data modules: three are linked to pitot tubes and five are linked to static sources. Information from the ADIRU is fed into one of several flight control computers (primary and secondary flight control). The computers also receive information from the control surfaces of the aircraft and from the pilot's aircraft control devices and autopilot. Information from these computers is sent both to the pilot's primary flight display and also to the control surfaces.[ citation needed ]

There are four named flight control laws, however alternate law consists of two modes, alternate law 1 and alternate law 2. Each of these modes have different sub modes: ground mode, flight mode and flare, plus a back-up mechanical control. [7]

Normal law

Normal law differs depending on the stage of flight. These include:[ citation needed ]

During the transition from take-off to cruise there is a 5-second transition, from descent to flare there is a two-second transition, and from flare to ground there is another 2 second transition in normal law. [7]

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 but manual settings (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. [7]

Flight mode

The flight mode of normal law provides five types of protection: pitch attitude, load factor limitations, high speed, high-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. [8]

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 ]

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 ]

High speed protection will automatically recover from an overspeed. There are two speed limitations for high altitude aircraft, VMO (maximum operational velocity) and MMO (maximum operational Mach) the two speeds are the same at approximately 31,000 feet, below which overspeed is determined by VMO and above which by MMO.[ citation needed ]

Flare mode

A380 in take off Airbus A380.jpg
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 landing 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. [7]

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 alternate law mode. [8]

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. [7]

Alternate law 2 (ALT2) loses normal law lateral mode (replaced by roll direct mode and yaw alternate mode) along with pitch attitude protection, bank angle protection and low energy protection. Load factor protection is retained. High angle of attack and high speed protections are retained unless the reason for alternate law 2 mode is the failure of two air-data references or if the two remaining air data references disagree. [8]

ALT2 mode is entered when 2 engines flame out (on dual engine aircraft), faults in two inertial or air-data references, with the autopilot being lost, except with an ADR disagreement. This mode may also be entered with an all spoilers fault, certain ailerons fault, or pedal transducers fault. [7]

Direct law

Direct law (DIR) introduces a direct stick-to-control surfaces relationship: [7] control surface motion is directly related to the sidestick and rudder pedal motion. [3] 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. and not-too-sensitive control with an aft C.G. [9]

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. [7]

Mechanical control

In the mechanical control back-up mode, pitch is controlled by the mechanical trim system and lateral direction is controlled by the rudder pedals operating the rudder mechanically. [3]

Boeing 777 primary flight control system

The cockpit of the 777 is similar to 747-400, a fly-by-wire control simulating mechanical control Boeing 777-200ER cockpit.jpg
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. [10] Because the system is controlled electronically, the flight control system can provide flight envelope protection.

The electronic system is subdivided between two levels, the four actuator control electronics (ACE) and the three primary flight computers (PFC). The ACEs control actuators (from those on pilot controls to control surface controls and the PFC). The role of the PFC is to calculate the control laws and provide feedback forces, pilot information and warnings. [10]

Standard protections and augmentations

The flight control system on the 777 is designed to restrict control authority beyond certain range by increasing the back pressure once the desired limit is reached. This is done via electronically controlled backdrive actuators (controlled by ACE). The protections and augmentations are: bank angle protection, turn compensation, stall protection, over-speed protection, pitch control, stability augmentation and thrust asymmetry compensation. The design philosophy is: "to inform the pilot that the command being given would put the aircraft outside of its normal operating envelope, but the ability to do so is not precluded." [10]

Normal mode

In normal mode the PFCs transmit actuator commands to the ACEs, which convert them into analog servo commands. Full functionality is provided, including all enhanced performance, envelope protection and ride quality features.[ citation needed ]

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. [5] [4]

Related Research Articles

<span class="mw-page-title-main">Avionics</span> Electronic systems used on aircraft

Avionics are the electronic systems used on aircraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fitted to aircraft to perform individual functions. These can be as simple as a searchlight for a police helicopter or as complicated as the tactical system for an airborne early warning platform.

<span class="mw-page-title-main">Fly-by-wire</span> Electronic flight control system

Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control computers determine how to move the actuators at each control surface to provide the ordered response. It can use mechanical flight control backup systems or use fully fly-by-wire controls.

<span class="mw-page-title-main">Attitude indicator</span> Flight instrument which displays the aircrafts orientation relative to Earths horizon

The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions.

<span class="mw-page-title-main">Stick shaker</span> Mechanical device in an aircraft cockpit to warn the pilot of an imminent stall

A stick shaker is a mechanical device designed to rapidly and noisily vibrate the control yoke of an aircraft, warning the flight crew that an imminent aerodynamic stall has been detected. It is typically present on the majority of large civil jet aircraft, as well as most large military planes.

<span class="mw-page-title-main">Autopilot</span> System to maintain vehicle trajectory in lieu of direct operator command

An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations.

<span class="mw-page-title-main">Glass cockpit</span> Aircraft instrumentation system consisting primarily of multi-function electronic displays

A glass cockpit is an aircraft cockpit that features electronic (digital) flight instrument displays, typically large LCD screens, rather than the traditional style of analog dials and gauges. While a traditional cockpit relies on numerous mechanical gauges to display information, a glass cockpit uses several multi-function displays driven by flight management systems, that can be adjusted to display flight information as needed. This simplifies aircraft operation and navigation and allows pilots to focus only on the most pertinent information. They are also popular with airline companies as they usually eliminate the need for a flight engineer, saving costs. In recent years the technology has also become widely available in small aircraft.

<span class="mw-page-title-main">FADEC</span> Computer used for engine control in aerospace engineering

A full authority digital engine (or electronics) control (FADEC) is a system consisting of a digital computer, called an "electronic engine controller" (EEC) or "engine control unit" (ECU), and its related accessories that control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines.

<span class="mw-page-title-main">Aircraft flight control system</span> How aircraft are controlled

A conventional fixed-wing aircraft flight control system (AFCS) consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed.

<span class="mw-page-title-main">Electronic flight instrument system</span> Display system in an aircrafts cockpit which displays flight information electronically

In aviation, an electronic flight instrument system (EFIS) is a flight instrument display system in an aircraft cockpit that displays flight data electronically rather than electromechanically. An EFIS normally consists of a primary flight display (PFD), multi-function display (MFD), and an engine indicating and crew alerting system (EICAS) display. Early EFIS models used cathode ray tube (CRT) displays, but liquid crystal displays (LCD) are now more common. The complex electromechanical attitude director indicator (ADI) and horizontal situation indicator (HSI) were the first candidates for replacement by EFIS. Now, however, few flight deck instruments cannot be replaced by an electronic display.

<span class="mw-page-title-main">Autoland</span>

In aviation, autoland describes a system that fully automates the landing procedure of an aircraft's flight, with the flight crew supervising the process. Such systems enable airliners to land in weather conditions that would otherwise be dangerous or impossible to operate in.

<span class="mw-page-title-main">Air data computer</span> Avionics component

An air data computer (ADC) or central air data computer (CADC) computes altitude, vertical speed, air speed, and Mach number from pressure and temperature inputs. It is an essential avionics component found in modern aircraft. This computer, rather than individual instruments, can determine the calibrated airspeed, Mach number, altitude, and altitude trend data from an aircraft's pitot-static system. In some very high speed aircraft such as the Space Shuttle, equivalent airspeed is calculated instead of calibrated airspeed.

<span class="mw-page-title-main">Air France Flight 296Q</span> Aviation accident at Habsheim air show

Air France Flight 296Q was a chartered flight of a new Airbus A320-111 operated by Air France for Air Charter International. On 26 June 1988, the plane crashed while making a low pass over Mulhouse–Habsheim Airfield as part of the Habsheim Air Show. Most of the crash sequence, which occurred in front of several thousand spectators, was caught on video.

An Air Data Inertial Reference Unit (ADIRU) is a key component of the integrated Air Data Inertial Reference System (ADIRS), which supplies air data and inertial reference information to the pilots' electronic flight instrument system displays as well as other systems on the aircraft such as the engines, autopilot, aircraft flight control system and landing gear systems. An ADIRU acts as a single, fault tolerant source of navigational data for both pilots of an aircraft. It may be complemented by a secondary attitude air data reference unit (SAARU), as in the Boeing 777 design.

<span class="mw-page-title-main">Indian Airlines Flight 605</span> 1990 passenger aircraft landing crash in Bangalore, India

Indian Airlines Flight 605 was a scheduled domestic passenger flight from Bombay to Bangalore. On 14 February 1990, an Airbus A320-231 registered as VT-EPN, crashed onto a golf course while attempting to land at Bangalore, killing 92 of 146 people on board.

<span class="mw-page-title-main">Qantas Flight 72</span> 2008 aircraft incident

Qantas Flight 72 (QF72) was a scheduled flight from Singapore Changi Airport to Perth Airport by an Airbus A330. On 7 October 2008, the flight made an emergency landing at Learmonth Airport near the town of Exmouth, Western Australia, following an inflight accident that included a pair of sudden, uncommanded pitch-down manoeuvres that caused severe injuries—including fractures, lacerations and spinal injuries—to several of the passengers and crew. At Learmonth, the plane was met by the Royal Flying Doctor Service of Australia and CareFlight. Fourteen people were airlifted to Perth for hospitalisation, with 39 others also attending hospital. In all, one crew member and 11 passengers suffered serious injuries, while eight crew and 99 passengers suffered minor injuries. The Australian Transport Safety Bureau (ATSB) investigation found a fault with one of the aircraft's three air data inertial reference units (ADIRUs) and a previously unknown software design limitation of the Airbus A330's fly-by-wire flight control primary computer (FCPC).

<span class="mw-page-title-main">XL Airways Germany Flight 888T</span> 2008 aviation accident in the Mediterranean Sea

XL Airways Germany Flight 888T (GXL888T) was an acceptance flight for an Airbus A320 on 27 November 2008. The aircraft crashed into the Mediterranean Sea, 7 km off Canet-en-Roussillon on the French coast, close to the Spanish border, killing all seven people on board.

<span class="mw-page-title-main">Flight envelope protection</span>

Flight envelope protection is a human machine interface extension of an aircraft's control system that prevents the pilot of an aircraft from making control commands that would force the aircraft to exceed its structural and aerodynamic operating limits. It is used in some form in all modern commercial fly-by-wire aircraft. The professed advantage of flight envelope protection systems is that they restrict a pilot's excessive control inputs, whether in surprise reaction to emergencies or otherwise, from translating into excessive flight control surface movements. Notionally, this allows pilots to react quickly to an emergency while blunting the effect of an excessive control input resulting from "startle," by electronically limiting excessive control surface movements that could over-stress the airframe and endanger the safety of the aircraft.

<span class="mw-page-title-main">Atlas Air Flight 3591</span> Cargo flight involved in 2019 crash

Atlas Air Flight 3591 was a scheduled domestic cargo flight under the Amazon Air banner between Miami International Airport and George Bush Intercontinental Airport in Houston. On February 23, 2019, the Boeing 767-375ER(BCF) used for this flight crashed into Trinity Bay during approach into Houston, killing the two crew members and single passenger on board. The accident occurred near Anahuac, Texas, east of Houston, shortly before 12:45 CST (18:45 UTC). This was the first fatal crash of a Boeing 767 freighter.

<span class="mw-page-title-main">Maneuvering Characteristics Augmentation System</span> Boeing flight control system responsible for 346 deaths

The Maneuvering Characteristics Augmentation System (MCAS) is a flight stabilizing feature developed by Boeing that became notorious for its role in two fatal accidents of the 737 MAX, which killed all 346 passengers and crew among both flights. Systems similar to the Boeing 737 MCAS were previously included on the Boeing 707 and Boeing KC-46, a 767 variant.

<span class="mw-page-title-main">Loganair Flight 6780</span> Aviation incident in 2014

Loganair Flight 6780 was a scheduled domestic flight from Aberdeen Airport to Sumburgh Airport in the Shetland Islands, Scotland. On 15 December 2014, the Saab 2000 operating the flight was struck by lightning during the approach, and then plunged faster than the aircraft's maximum operating speed. The aircraft came within 1,100 feet (340 m) of the North Sea before the pilots recovered and returned to Aberdeen. All 33 passengers and crew were unharmed.


  1. "Flight Control Laws". SKYbrary Aviation Safety. Retrieved 2019-07-03.
  2. "Flight control part 3". Bjorn's corner. 25 March 2016.
  3. 1 2 3 4 "Crossing the Skies » Fly-by-wire and Airbus Laws". crossingtheskies.com. Archived from the original on 8 March 2009.
  4. 1 2 3 "The Boeing 777" (powerpoint). by Saurabh Chheda.
  5. 1 2 "Skybrary: Flight Control Laws".
  6. "Avionics Magazine :: Boeing 787: Integration's Next Step".
  7. 1 2 3 4 5 6 7 8 9 10 "Airbus 330 – Systems – Flight Controls". SmartCockpit – Airline training guides, Aviation, Operations, Safety. Archived from the original on June 12, 2009. Retrieved July 12, 2009.
  8. 1 2 3 "Airbus Flight Control Laws".
  9. Airbus A320 AFM (requires page number, publisher, etc)
  10. 1 2 3 Gregg F. Bartley – Boeing (May 4, 2008). "11 Boeing B-777: Fly-By-Wire Flight Controls" (PDF). Retrieved October 8, 2016.