103 Fly-by-wire.docx

Fly-by-wire (FBW) is a flight control system that uses relatively light electrical wires and computers to replace the traditional direct mechanical linkage between a pilot’s cockpit controls and the moving control surfaces. The movements of flight controls are converted to electronic signals transmitted by wires (hence the fly-by-wire term), and flight control computers determine how to move the actuators at each control surface to provide the ordered response. The fly-by-wire computers continually act to stabilise the aircraft and adjust its flying characteristics without the pilot's input and to prevent the pilot operating outside of the aircraft's safe performance envelope (capabilities of its design). These systems have been used in guided missiles and subsequently in military aircrafts. They were implemented in commercial aircrafts much later due to the time required to develop appropriate failure survival technologies that would provide an adequate level of safety, reliability and availability. In "FBW" aircraft, the control inputs are instead sent to calculators which deliver actual orders to actuators. Each FWB system, in addition to introducing calculators into the pilot-control surface chains, also gets information from sensors to measure the aircraft response to orders (feedback). This design has two advantages:  

There is an actuator so the pilot has to exert on the control inputs only a fraction of the actual force that needs to be exerted by the actuators on the control surfaces. The calculator can supervise the pilot commands, so that the aircraft is never put in unwanted or dangerous configuration, e.g. stalled.

The leading commercial aircraft manufacturers, such as Airbus and Boeing, exploit FBW controls in their civil airlines. The flight controls on Airbus fly-by-wire aircraft are all electronically controlled and hydraulically activated. Some surfaces, such as the rudder and the horizontal stabilizer, can also be mechanically controlled. While in normal flight the computers act to prevent excessive forces in the pitch and roll axes. The following discussion is based on the A330 but much of the information also applies to other Airbus types. Some correspondence links the actuator deflection to the pilot input. These correspondences are usually named "flight control laws". Airbus uses 3 laws:   

Normal: When everything needed by the calculators is available. Maximum protection is available. It provides three axis control and flight envelope protection. Alternate: When some calculators required data are not available. Some protections may be lost. Direct: When no protection can be provided by the calculators, due to calculator failure, missing input, actuator or control surface failure. Basically, the aircraft is a non-FBW aircraft. Control surface motion is directly related to the sidestick motion.

Since the Airbus A320, Airbus flight-envelope control systems always retain ultimate flight control when flying under normal law, and will not permit the pilots to violate aircraft performance limits unless they choose to fly under alternate law. This strategy has been continued on subsequent Airbus airliners. 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 flightcontrol systems have back-up systems that are purely electrical through the use of a "three-axis Backup Control Module" (BCM)

There are two elevator and aileron computers and three spoiler and elevator computers. Each computer is partitioned into two different and independent channels. A failure is detected by comparing control/monitoring channel commands to predefined thresholds, and the channel is subsequently disconnected. The computers can operate without ventilation and are protected against electromagnetic impulses and indirect effects of lightning. Five flight control computers are active simultaneously in charge of control law computation and individual actuator control. The system incorporates redundancy to provide nominal performance and safety levels, making it possible to fly aircraft safely with only one active computer. Computer architecture is designed for failure detection. In an A330/A340, three hydraulic circuits can be pressurized by three sources: engine driven pump, an electric pump, and a ram air turbine. In the case of a double hydraulic failure, the high-level control law is still available. Redundancy in computer to actuator path is assured with the use of four computers. Two or four engine-driven generators (depending on aircraft type) provide redundancy and two batteries provide backup power Airbus aircrafts utilize a manoeuvre demand approach, i.e. pilots command the maneuver they want the aircraft to perform. The design of the Airbus flight control systems takes advantage of the potential of FBW to incorporate control laws. The major factor in human-computer interaction in FBW systems is the implementation of “flight envelope protection”. Airbus implements this protection according to the philosophy of “hard limits”. This means that even if pilots want to exceed these limits, such as the maximum bank angle, the system will not allow them to do so. Known as “alpha protection”, it is one of the crown jewels of the Airbus flight control system

Boeing airliners, such as the Boeing 777, allow the pilots to completely override the computerised flight-control system, permitting the aircraft to be flown outside of its usual flight-control envelope if they decide that it is necessary. Unlike modern Airbus aircraft, in the 777 both the rudder pedals and the yokes are mechanically interconnected so that either pilot feels and sees what the other pilot is doing when he operates the controls. In addition, all flight controls are "backdriven" by the Autopilot and/or Autothrottles when they are engaged, so as to keep the pilots in the loop and give them visual clues of what the AutoFlight System is doing; the exception being the rudder pedals which will not be backdriven by the Autopilot except during an Autoland approach below 1500 feet. No matter what the level of automation is, any pilot can grab any control and manually override the Autopilot or Autothrottles. The FBW system consists of the following basic components:    

Primary Flight Computers (PFC). They constitute the "heart" of the FBW system. Actuator Control Electronics (ACE). They are basically analog to digital and digital to analog converters. Power Control Units (PCU), They are electrically signaled but hydraulically powered. ARINC 629 Data Buses. The Bus transmits information among the various components of the system; essentially it's an information "highway".

The pilot generates an input signal by manipulation of the primary flight controls. This analog signal is received by the ACEs which digitalizes it and redirects it to the PFCs via the ARINC 629 Data Bus. The PFCs constantly gather and monitor information from a number of aircraft sources and will then generate actual control laws by "enhancing" the received signal. The PFC-generated control

command will then be sent back to the ACEs which will in turn convert it to an analog signal that the PCUs will use to move the control surfaces. When the Autopilot is engaged it communicates directly with the PFCs. The FBW system uses three modes of operation. Normal mode provides augmentations such as stall and bank angle protection. In secondary mode augmentation is lost. The direct mode is the most degraded mode of operation and would only be activated in the most improbable event of serious malfunction. In addition, the primary flight control system also supports maintenance functions, which interface with the onboard maintenance system. The Boeing 777 utilizes a lateral control system, where the pilot commands operation via the yoke and the aircraft adjusts the flight control surfaces independently. One of the design goals of the 777 was that operation and response of the airplane should be familiar to the pilots, based on their past experience and training.