Towards all-electric

Citation

Ford, T. (2002), "Towards all-electric", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 4. https://doi.org/10.1108/aeat.2002.12774daf.002

Towards all-electric

Over the years progress with aircraft systems of all kinds has been characterised by more and more reliance on electrics as the main source of power. More recently the speed of developments has enabled arrangements that cater for all possible modes of failure and achieve maximum operational economy. This has to be viewed against the background of continuous evolution and a system with even fewer and more reliable components.

Essential reliability and robustness is demonstrated, for example, by the Smiths industries Fuel Quantity Indicating System (FQIS) for the Airbus A319 which consists of six principal elements: tank units; cadensicon; preselector unit; indicator; fuel quantity indication computer (FQIC); and the harness (Figure 1).

Figure 1 Fuel Quantity Indicating System components for the A319

The tank units are capacitive probe sensors mounted vertically which are capable of signalling the degree of immersion in the fuel. Selected tank units also measure specific fuel properties of temperature and dielectric constant. In each of the base fuel tanks is a cadensicon which is a combination sensor that determines the primary fuel characteristics of density and permittivity that are needed for gauging calculations. A preselector unit mounted on the refuel panel enables automatic refuelling by allowing manual preselection of a total fuel mass. Also included is an indicator on the refuel panel which displays continuously the fuel mass in each tank.

The FQIC forms the heart of the system. It is a two-channel dual redundant, data processing unit that receives electrical signals from all in-tank sensors independently and initially converts each signal into digital form and value that is directly readable by the internal microprocessors. Interpretation of these signals allows continuous determination of the exact height of the top surface of the fuel and associated fuel properties.

Central Maintenance System (CMS)

Developed from the Airbus A320 Centralized Fault Display System (CFDS) which entered service on the aircraft in 1988, the A330/ A340 Central Maintenance System (CMS) includes two fully redundant Central Maintenance Computers (CMC). Four user interfaces are defined and the mode of operation of the CMCs has been split within three main functions; the reporting mode, interactive mode, and servicing mode.

The reporting mode is active in flight to record all events tied to maintenance actions to be performed on the ground. The units connected to the CMCs transmit automatically and permanently all failures affecting them, whether of internal or external origin. The failures are divided into three classes, depending on their impact on the current flight. Class 1 failures are indicated to the crew by means of warnings and flags. Class 2 failures are indicated to the crew on the ground only and have no impact on the current flight; Class 3 failures are not indicated to the crew at all and are available on request only.

The CMC builds a Post Flight Report by merging all Class 1 and 2 messages received from all computers as well as all Flight Deck Effects received from the Flight Warning Computer and the Data Management Computer. This report gives the maintenance personnel an overall view of what occurred during the last flight leg, and is an important complement to the flight log.

The interactive mode or menu mode allows the connection of the Built in Test Equipment (BITE) of any unit via the MCDUs, in order to display the data memorized and formatted by each member system. Activated on the ground only, of of its important purposes is ease activation of system tests. The tests bare divided into system, complementary and guided types, thus ensuring that at least one test can always be activated at the ramp by a mechanic.

An optional servicing report has been designed including hydraulic levels, that should help the maintenance personnel to easily check what has to be performed regarding the servicing of the aircraft.

Integrated information

Designed to provide information management within the aircraft and between the aircraft and the ground, Rockwell Collins' Integrated Information System (IS) streamlines collection and distribution of operational, maintenance, safety and administrative information through both airborne and ground-based communications networks.

The IS system links the aircraft to the airlines' corporate information networks using wireless communication technology. Data such as real-time graphical weather can be transferred via VHF, HF or satellite communication systems. Maintenance diagnostics, navigation databases, flight plan data and other information are exchanged through a low-power, microwave airport gatelink system.

Programs underway to implement the IS technology include a developing technical specification for the Aircraft Integrated Network System together with Airbus which is being certificated on the A340-500/600 in 2002. Based on Ethernet and wireless network technology, the links enable a wide range of applications to support flight and maintenance personnel.

Another effort is Rockwell Collins participation in the US NASA sponsored tests of a satellite-based graphical weather information service (SWIS), which will provide worldwide, real-time data to aircraft, allowing pilots to make earlier decisions during changing weather conditions. A third effort is the Condor Aircraft Integrated Network (CAIN) programme, in which Rockwell Collins is teamed with Condor Flugdienst, the charter affiliate of Lufthansa. The trials began in 1999 and the system is certificated on several aircraft.

Fly-by-Wire

The most significant of the developments has to be Fly-by-Wire (FBW), first by Airbus and then Boeing and other manufacturers. The entry into service of the Airbus A320 in 1988 and the emergence of the FBW concept was the start of an aircraft family with technical and operational commonality as initial design objectives. For the flight control system these meant similar aircraft handling characteristics for all the aircraft of the family, minimizing the transition training time, and allowing safe and efficient mixed fleet flying capability.

Essential FBW design considerations are that all flight control surfaces are electrically signalled and hydraulically actuated, and the trimmable horizontal stabilizer and rudder are also mechanically controlled. The electrical signalling of the flight control surfaces is achieved by two different types of computers, e.g. on the A330/A340, there are the Flight Control Primary Computers (FCPC) and the Flight Control Secondary Computers (FCSC), and in addition, two specific computers dedicated to manage the data from the flight control computers for indications, warnings, maintenance and recording purposes.

Design safety precautions ensure that redundancy is at all levels, there is dissimilarity in hardware and software and computer architecture is designed for failure detection. Redundancy in hydraulic generation is achieved by three hydraulic circuits; Green, Blue and Yellow. On the A330/A340 each Blue/Yellow circuit is pressurized by an engine driven pump (EDP) and an electric pump. The Green circuit is pressurized by two EDPs, an electric pump and a ram air turbine (RAT), in case of major failures. In case of a double hydraulic failure, the high level control law is still available with remaining flight controls.

Redundancy in electrical generation and distribution is provided by two or four engine- driven generators (depending on aircraft type), one APU generator, one constant speed motor/generator driven by hydraulic circuit (Green or Blue), driven by its dedicated pump or RAT, and two batteries. Additionally, extensive segregation rules have been applied to minimize common point risks.

The computer architecture is such that each computer is divided into two different and independent channels; the control channel and the monitoring channel which ensures that the control channel operates correctly. Each channel carries one or two processors, its own memories, I/O circuits, power supply and specific software. A flight control mechanical back-up is also provided, for pitch control and yaw/roll control (Figure 2).

Figure 2

The relation between the pilot input on the stick and the aircraft response is called control law. The control law determines the handling characteristics of the aircraft. When the pilot acts on the stick he directly orders an aircraft response (e.g. roll rate). The aircraft is thus linked to the pilot order as follows: the pilot order (electrical signal) is sent to the computer. The aircraft response to the pilot order is fed back to the computer. The computer gets the difference between the pilot order and the aircraft response and it processes the control surface deflection required to get the aircraft response as ordered by the pilot, on short term basis. The computer carries a high speed integrator which achieves a long term stabilization function by processing the deflection of the control surface required to maintain the aircraft flight path – stick free.

Protections are provided for high angle of attack, exceedance of certain pitch attitude values, load factor, high speed, and bank angle.

Airbus employ sidestick operation instead of the conventional yoke, the former being considered the most ergonomic solution for FBW, the best indication of who is flying, pilot or autopilot, the best solution in emergency situations, and an improvement in pilot comfort in flight, thus reducing fatigue.

The Boeing 777 FBW flight control system employs the conventional yoke and uses wires to carry electrical signals from the pilot control wheel, column and pedals to a primary flight computer (PFC). The PFC combines these pilot inputs with inertial data and air data from the air data inertial reference system to produce flight control surface commands. The PFC then sends the commands, also in the form of electrical signals, to the actuator control electronics, which in turn control the hydraulic actuators that move the control surfaces.

The Full Authority Digital Engine Control (FADEC) has for some time been a valuable addition and TRW Systems have a low cost system suited for the business and regional markets. This controls engine fuel flow by interfacing with the fuel metering unit and controls engine variable geometry via actuators, as well as being capable of controlling other standard engine functions. It has full BITE and maintenance functions.

TRW Systems have also produced a cargo system that incorporates advanced diagnostic capability, with greater speed in fault detection and isolation and greater efficiency in parts replacement. The diagnostic capability constantly monitors all activity during the loading process and automatically records all faults caused by damage or malfunction, including intermittent faults, to non-volatile memory, which allows the system to retain fault data even after the aircraft power is turned off.

More electric aircraft

The ability to produce a variety of complex system for actuation and control has allowed TRW Systems to play a leading part in the development of the More Electric Aircraft. This capability extends across the product lines and is characterised by the production of a number of flight control power-by-wire alternatives including electrohydrostatic actuation (EHA), electromechanical actuation (EMA) and the Integrated Actuation Package (IAP), with the aim of replacing the hydraulic power supply to primary and secondary flight controls with electric. Flight control actuation by FBW is controlled electrically but powered hydraulically. These systems rely on high pressure hydraulics throughout the aircraft to supply the power to move each control surface. The goal of power-by-wire is to eliminate the hydraulic connections, and its associated risks, by providing electrical power straight to the actuators. Enhanced survivability and maintainability will be realized as well as a reduction in weight and aircraft ground service time. The company is to supply the EHA concept for the A380.

To supply the higher electrical needs for power-by-wire, TRW has developed a variable frequency system for large aircraft by applying the experience gained from the supply of the first system of this kind to the Bombardier Global Express business jet. The system removes complexity by eliminating the hydro-mechanical constant speed drive required in conventional constant frequency systems, replacing constant frequency generators for improved reliability and maintainability, and reducing weight and cost. This is also directed at the A380.

Terry Ford