Conference report

Complex technologies

At the recent ERA International Avionics Conference and Exhibition, a range of advanced topics was addressed. The event was subtitled "Are we coping with the complex technology?"and was divided into nine sessions. It attracted delegates from airlines,manufacturers, government bodies and academic institutions and embraced current and future organisational and technical issues that are shaping the avionics industry. Such was the diversity of subjects discussed that some attempt has been made to concentrate on those which it is felt will be of interest to the majority of readers.

The papers presented for the initial session on Project Updates concentrated on military aircraft and was opened by a presentation from the Defence Evaluation and Research Agency (DERA)at Farnborough which discussed avionic technologies for the future offensive air system (FOAS). The FOAS is a UK programme that aims to replace the RAF's Tornado GR4 aircraft within approximately 20 years. A variety of options are being considered which include derivatives of current aircraft (such as the Eurofighter Typhoon) and completely new ideas. The study has been conducted by six working groups, each being drawn from the UK and French bodies.

Significant enhancements in weapon system effectiveness will be required, including improved targeting,enhanced survivability, improved operational flexibility, reduced life cycle cost, and improved in-service support. Technologies that require specific funding by the FOAS were identified, covering Integrated Modular Avionics,Electro-optics technologies, RF technologies, Sensor fusing, Mission management,Navigation and Communication. Many areas were identified if they were to reach the required development status by an assumed in-service date of 2015. A set of 35 technologies were seen to need specific FOAS project funding to ensure that the risks of meeting the requirements of that project are reduced to an acceptable level.

A presentation from Raytheon Systems dealt with the F-22 Common Integrated Processor (CIP) and was described as a case study in obsolescence management. Diminishing manufacturing resources (DMS) refers to all the ways that a system is deprived over time of the components used for its manufacture, including obsolescence. Today, DMS management of digital avionics has become critical to weapons system viability. The F-22 CIP has faced this reality several years before deployment.

The difference between lifecycles of commercial products and those of the military are marked. The military digital systems engineer has to adapt to a typical commercial device produced for maybe a year to five years. This compares with a complex weapon system such as the F-22 which requires more than ten years to even develop.

Given by several authors from the UK, France and Germany, was a description of progress of the ASAAC Phase II programme sponsored by the three countries for work over five years in two stages to establish a complete set of standards for military core avionics. Work started in 1997 and the first stage is just about completed.

The Allied Standard Avionics Association Council (ASAAC) Phase II programme has the main objectives of the Integrated Modular Avionics concept with the idea of Open Architecture balancing the conflicting requirements of life-cycle costs, mission performance and operational performance. For this, alternative technologies will be evaluated and a set of well defined and stable interface standards for hardware,software and networks will establish the basis of a flexible avionics architecture applicable to a large number of platforms for post-year 2000.

The first results of Stage 1 include Architecture concept refinement and software concept refinement and work on common functional modules and network. System issues also play an important part. Demonstrations are planned for Stage 2 to achieve a complete and efficient validation of ASAAC requirements and standards. The construction of demonstrators will start in 1999.

Integrated modular avionics (IMA)

Two sessions were devoted to IMA which reflects the importance of this topic. The first of these contained contributions from both the civil and military fields and also the University of York. An interesting paper from British Aerospace Airbus addressed the safety and certification issues associated with IMA and reflected work undertaken by the company. It is apparent that current certification practices will become inadequate due to the increasing use of complex computer systems which are physically modular, yet functionally highly integrated. They differ from current avionics systems in that they are software intensive and they share common resources. Particular attention is given in the paper to the integration of critical systems and functions.

First, a proposed IMA architecture is described. This consists of cabinets that will replace Line Replaceable Units (LRUs). Each cabinet will contain standard removable components ­ Line Replaceable Modules (LRMs) that will perform a variety of avionics functions. They will share processing, memory and I/O function and power supplies. The LRMs are not necessarily dedicated to one particular function and features include the sensors and actuators being connected to the LRMs via databuses. The processing facilities are distributed rather than centralised.

Referring to Figure 1, the architecture uses a dual-dual ARINC 629 bus which links cabinets, Remote Data Concentrators (RDCs) and a Global Health Monitor. The dual-dual 629 bus contains four channels, which are configured into two pairs with each pair carrying the same information. The cabinets, which are duplicated, contain LRMs which communicate with the 629 bus via a dual 659 bus and four gateway modules. Each LRM implements one or more functions on a pair of dissimilar processors.

 Figure 1 Dual-dual ARINC 629 bus linking cabinets, remote data concentrators, and a global health monitor

The Safety Argument Manager (SAM) tool has been developed by the University of York and has been used to develop a goal structure, which encapsulates the high level augmentation and supporting evidence for a safety case. The primary safety assessment processes are Functional Hazard Assessment, Preliminary System Safety Assessment, and Common Cause Analysis. The last named identifies failures or development errors which bypass or invalidate redundancy or independence. They may generate derived requirements in order to preserve independence in subsequent stages of design.

The safety analysis problems of IMA are such that, with increased integration and complexity, the assumption that each system can be treated independently becomes less and less tenable. New safety analysis techniques are needed. Specific areas will have to address logical partitioning, common mode events, credit for fault tolerance,type certification of modules, and software certification. Work is continuing to provide solutions to all these issues.

From the military side came a discussion on Integration ­ Performance with Safety. A technology demonstrator has been produced by BAe in collaboration with UK MoD and industrial partners. It is an integrated navigation, flight and control system rig, which in many respects represents the core elements of presently perceived vehicle management systems (VMS) for future combat aircraft.

The programme was initially instigated in 1985 and the current phase started in 1996. The latter is developing a rig demonstration system comprising at its core a computing system called the Development Test Bed (DTB) which implements the navigation,flight and propulsion control functions. It is a rig standard equivalent of anticipated on-aircraft equipment. The key themes of VMS architecture embraced in the work are physical integration, functional integration, vehicle executive,common modules (including spare capacity), and distributed architecture (i.e. core, remote sensing, and remote effector control).

This demonstrator programme provides a facility where the effects of integration can be investigated. It is not itself safety critical, but the systems it implements will be safety critical on a future aircraft system. The single most important point is that safety, and the arguments that need to be constructed to demonstrate safety must be considered at the beginning. Even lower costs at ever increasing performance are essential requirements. With the current and envisaged future technologies, integration is an effective way of addressing both these targets.

The Integration of AI Techniques into IMA was presented by the University of York which dealt with the emergent use of artificial intelligence (AI) techniques in real-time embedded systems.The impact that these techniques have on an IMA architecture is outlined and the extent to which IMA can support AI computing components such as rule-based systems.

Two motivations are given for the introduction of AI into IMA. The first of these is to reduce the cognitive load on an operator and, thereby, aid situation assessment and planning. The more complex the command and control system becomes, the more information is available (via sensor readings). Knowledge-based techniques can be used to help the so-called "helmet fires". In these systems, response times of the order of seconds are required.

The second motivation is to improve the active control of the aircraft. The more demands that are placed on the aircraft itself, the greater the requirement for autonomous control functions. Knowledge-based fault diagnosis and error recovery, for example, may be required in order to obtain timely contact when severe disruption of base-level services occurs (perhaps to the computing infrastructure or the physical sensor/actuators). In these systems, response times of the order of 100 milliseconds may be required.

The key intention of IMA is that all on-board software should run on a single (distributed) hardware platform. Hence, any AI software should also be executed on this shared resource. To use AI techniques to obtain intelligent behaviour in real time,decision algorithms are required.

In summary, many aspects of IMA architecture are supportive of what is considered necessary for integrating AI and standard components. The following examples are quoted:memory protection between processes; policing of the use of channels;guaranteeing of real-time behaviour; CPU time monitoring; and support for mode changing and controlled reconfiguration.

Fibre optics for IMA systems were described by BAe Military Aircraft & Aerostructures. Third generation avionic systems have introduced fibre optic communications and reliability has been improved. Based on this, the technologies for IMA communications have to be considered. There is strong pressure to use commercial technologies wherever possible to minimise the cost of military equipment. For the fibre, the aim is to use a standard one with a 125 micron cladding diameter in order to allow the use of standard connector ferrules. Either single mode(9/125) fibre or graded-index multimode fibre could be used, the various issues being, as far as single mode fibres are concerned: bandwidth, test and measurement, connection issues and environmental issues.

Single-mode technology is an attractive choice for IMA applications as it has wide support in the telecommunications industry, is compatible with future technologies such as optical switches, and offers unlimited bandwidth. However, the technology has not been developed with aircraft applications in mind and there are several areas to be tackled before it could be confidently recommended. Multimode technology is more mature and understood. Both the single and multimode fibres have some advantages over the other, but it is felt that the latter,graded-index multimode fibres, will meet both the near- and long-term requirements for IMA applications.

From Boeing came a paper on development of an Open System Architecture (OSA) for Integrated Electronics. The Integrated Sensor System (ISS) programme includes a task to define an OSA for Radio Frequency (RF) electronics, which now represent the largest portion of an advanced aircraft's flyaway cost. The ISS programme is being worked by Boeing and Lockheed Martin and what at first seemed dissimilar developments have achieved some common formats.

An integrated architecture such as described in the paper reduces costs in a number of ways, including time-sharing, centralization of resources, and reduction of the number of unique module types. The OSA approach extends these cost reductions by simplifying technology insertion, using well-understood standards, and increasing use of commercial hardware and software. Innovative approaches to OSA development have been applied and considerable effort has gone into the development of a process using metrics that assess the life cycle cost benefits of an OSA.

A longer-term view of civil IMA was presented by Smiths Industries Aerospace. The purpose of the paper is to increase awareness of issues connected with IMA and promote discussion on what may be achieved. IMA in many sectors of the air transport industry has largely become the assumed way forward for the implementation of future avionics. Progress has already been demonstrated with first generation civil IMA systems such as ELMS and AIMS on the Boeing 777.These are quite different implementations, having been optimised for their specific system domains, and therefore appear to go only some way towards meeting the ultimate goals anticipated by the industry. As technology has advanced, so there has been a continuing trend of avionics integration. Although modularity has been with us for some time, a primary goal of IMA is to establish the application of a"standard" set of hardware modules, directly line-replaceable, encompassing as much of the total avionics suite as possible. Concerning aircraft and systems architecture issues, the advances that have enabled rack-mounted LRUs to be reduced in size or be integrated together over the years have also, in combination with serial data bus interfaces and other advances, enabled electronics to be used increasingly in various locations around the aircraft. With advanced microcircuits it is already possible to provide considerable functionality with many system peripheral devices without significantly affecting the space they occupy or reducing their inherent reliability.

The concept of IMA has come about in response to these and other trends. Various implementation architectures are described in ARINC 651, the industry's overall design guidance document for IMA. They share the common theme of a number of "cabinets" which are connected together and with other peripheral equipment by means of a number of multiple-access serial data buses, as shown in Figure 2. Each cabinet contains a selected mix of line-replaceable-modules (LRM), e.g. core processor,input/output (I/O), power supply, and other special LRMs, all interconnected via an internal back-plane bus. The goal of standard, reusable and interchangeable modules is central to the concept of IMA (Figure 2).

 Figure 2 IMA overall architecture

Although many wish to see the idea of common modules applied as widely as possible, differing systems or functions may be more optimally implemented with different platform architecture and module suites. In the longer term as the aircraft systems become more and more digital "end-to-end" through the use of smart peripherals and remote data concentrators the need for I/O modules within IMA platforms will diminish. Also,the processing within these platforms need not be burdened with functions associated with peripheral devices, and can be simplified to that needed to perform upper-level system functions only. Therefore, the possibility exists for cabinets to become general-purpose avionics computers networked together and to the peripherals by serial digital data buses.

The more-electric aircraft

Practical Considerations related to this subject were outlined by Rolls-Royce. It is generally held that the more-electric system will offer significant benefits for the aircraft in terms of weight, reliability and operating cost. There are limitations to today's aircraft electrical power systems (engine and airframe): aircraft power levels are increasing, even without more-electric load additions; more 1.5 Vac power generation options need to be considered; there are practical power switching/feeder limitations; emergency power requirements are increasing; power generation heat rejection is an issue; and power conversion/semi-conductor limitations need to be addressed.

The most common generator size in use in contemporary medium to large civil aircraft today is 90kVA;typical in most Boeing and Airbus aircraft including the 747-400. The 777 and,more recently, the 767-400 electrical systems have increased to 120kVA. In addition, the 777 has a 20kVA/channel VSCF Backup system and some small PMGs to supply power to the Flight Control DC System to assure the supply of 28dc power to the FBW system. In all of these systems it is usual for the power generation drive shafts to be driven from an accessory gearbox run by the engine IP shaft and they are therefore single shaft solutions. The A3XX and other large aircraft developments in the medium term could utilize a simple dual shaft arrangement.

The secondary power required can cause problems for the engine in certain regimes and solutions are needed to overcome these emerging and difficult problems. A constant running APU is one solution. However, there would be a need for electrical power generation for the aircraft to be retained on the main engines using electrical starter/generator systems. A combination of the two may provide a solution.

Maintaining power generation for these systems may pose problems; one solution may be to move the main electrical power generation to the LP shaft on the main engines.To meet ETOPS needs, power installations for twin-engine aircraft require multiple power generation capability. Provisioning an HP shaft driven motor/generator, for main engine start, together with an LP shaft driven main generator integrates well with this need. Clearly, the introduction of more electric technology to aircraft will bring major benefits, but solutions will only be found by considering the whole systems and their overall integration.

Another paper in this session was from Lucas Aerospace, and dealt with reliability enhancement of electronic equipment. Supporting this reliability enhancement and also reliability assessment are the partners in the REMM project, British Aerospace­ Military Aircraft & Aerostructures, Lucas Aerospace, GEC Aerospace,and Smiths Industries Aerospace, as well as the Universities of Loughborough and Strathclyde. The RAF Reliability Group are also active participants. The research work to be undertaken aims to develop a holistic model of all reliability activities and functions used in a product life cycle, and so enhance design and manufacturing processes of complex avionic systems.

There are problems with the current approach to reliability and the REMM project will: investigate and provide recommendations for improvements to the application of current reliability technology both individually and collectively, in order to support the construction of a reliability case; also, it will develop a statistical model which takes input from each technique, including expert knowledge, and outputs reliability performance measures that will be used for assessment.

The project started in April 1998. Two work packages have made significant progress; WPI overall specification of the tool set, and WPB effective use of physics of failure methods. The whole project will be completed in May 2001.

CNS/ATM

A series of papers referred to various aspects of Communications Navigation Surveillance/Air Traffic Management. The first, from London University, concerned new generation satellite communications. Increasing reliance is being placed on digital datalinks and, to avoid limitations being put on certain services, a new range of satellite systems is being prepared.

For example, the Japanese Civil Aviation Bureau will implement a full CNS concept based on a satellite system. It will offer air traffic services (ATS) fully compliant with ICAO, that is Automatic Dependent surveillance (ADS) and Controller Pilot Data Link Communication (CPDLC). The system will consist of two geostationary satellites at an altitude of about 42,000km and an earth segment. The system will offer voice and data exchange between controller and pilot, surveillance via ADS and navigation using GPS augmentation concepts similar to GNSS1. It is scheduled to be operational in 1999.

A Satellite Data Link System (SDLS) is projected in Europe for continental airspace, in contrast to the ADS-Europe trials which are exclusively considering trans-oceanic use. The main operational requirements are provision of ADS, CPDLC, and flight information services. In addition, provision of airline operations communications will be considered with air traffic services having priority. The space segment will be based on existing satellite services (Figure 3).

A number of new non-geostationary satellite constellations are also planned, in Low Earth Orbit(LEO) and Medium Earth Orbit (MEO). Typical lifetimes are around five years for LEO and ten to 12 years for MEO. The most advanced of these systems is Iridium which is now in service. The next system to come on line, in 1999, is Globalstar, and two others are projected, ICO and Teledesic. In these and other satellite systems, there is a necessity to introduce as much commonality as possible to reduce the need for expensive retro-fitting of avionics.

 Figure 3 Satellite orbit schematic

From Rockwell Collins came a paper on the Future Air Navigation and Traffic Avoidance Solution Through Integrated CNS (FANTASTIC). This programme's goal is to couple commercial technology with modular electronics to implement new ATC functional compliance capability into USAF fighter platforms. The programme also targets general aviation aircraft where space, weight, power and cooling are similarly constrained. The three-year FANTASTIC programme was started in 1997. Called the Future Air Navigation System (FANS) by the civil sector and Global Air Traffic Management (GATM) by the military, the core requirements are; communications systems, navigation/flight management systems, and communication management systems. These can be broken down into their commercially developed solutions that are idealized for FANS/GATM solutions. The communication management is the only capability that needs to be developed.

For a modular solution, a multi-function radio is required to perform an existing function in addition to the GATM required functionality, and the FANTASTIC programme is utilizing a Direct Conversion Receiver Exciter module that is capable of operating from 30MHz to2GHz. This module can accomplish the VHF data link functionality required and will culminate in a demonstration in the future.

British Airways discussed a smooth transition from active to passive air traffic surveillance and suggested ways to achieve this. The existing active surveillance techniques are,commonly, a combination of primary and secondary surveillance radar in Terminal Manoeuvring Areas (TMA). Locations out of radar range usually rely on HF radio. Other solutions such as Automatic Dependent Surveillance ­ Contract, are being developed but are not discussed further since radar is the primary focus of this paper.

Existing surveillance techniques have limitations. Primary Surveillance Radar plays a significant role but does not provide identification or altitude information. Secondary Surveillance Radar is very effective but has limited range. Other considerations are cost and capacity. As far as air to air traffic surveillance is concerned,similar disadvantages to the ground side of SSR could be applied to the airborne implementation of SSR interrogators.

The alternative to active surveillance is passive surveillance where the requirement for a bi-directional flow of data is not required between the aircraft and the ground. The concept of broadcasting surveillance information automatically is known as Automatic Dependent Surveillance Broadcast (ADS-B). Although both ground and air applications have been developed, those most vigorously pursued have been mainly concerned with air to air functions. Which transmission medium to use is an important feature and the main three are mentioned here. The Mode S SSR system already includes an unsolicited broadcast called a squitter, transmitting twice per second on the 1090MHz frequency.

Another medium is VHF­ Synchronised Time Division Multiple Access (STDMA) which is at present used as a medium for voice communication and some limited data link applications. The broadcast data formats could be defined in the same way as the extended squitter for Mode S and therefore be used for an ADS-B system. The third medium is Universal Access Transponder (UAT), which system is basically the same as the Mode S 1090MHz solution but put on a different frequency.

The advantages of passive surveillance include reduced complexity and hence cost, and range and capacity,particularly in the case of Mode S. STDMA gives great range but at the expense of capacity without the use of multiple channels. The use of multiple channels greatly reduces the "see all" benefit of ADS-B and may also require interaction from the ground.

It is considered that with the wide equipage of SSR transponders, the case for transfer to another technology is very difficult to make. It is clear that for the implementation of passive surveillance in the guise of ADS-B to be successful, it is imperative that the transition path be as smooth as possible in all aspects. With the benefits unquantified and the route to implementation unclear, cost will be a significant factor. Therefore, 1090MHz implementation of ADS-B is the best way forward.

From Rockwell Collins came a paper on VHF Communications, the 8.33kHz Program. The implementation of 8.33kHz channel spacing in Europe has resulted in full-scale development and production programmes at all of the main avionics vendors. The deadline for implementation has been extended to October 1999, and there are still challenges to be faced before the programme requirements are satisfied.

Rockwell Collins used the following existing radio designs that were included in the 8.33kHz development programme: the 618M-3/3A/4/4A is an ARINC 556A compliant product that has been in production for a number of years; the VHF-700A which is an ARINC 716 compliant product also developed some time ago; and, the VHF-900 which was designed before ICAO decided on 8.33kHz channel spacing, which has been modified.

Program development for the 8.33kHz began in 1995 and was completed in the autumn of 1997. Field implementation of 8.33kHz capable VHF radios includes providing modification kits and Service Bulletins for existing VHF radios already in service and supplying new 8.33kHz capable VHF. Issues that may affect the distribution of the modification kits and new radios are: certification of 8.33kHz VHF transceivers in the field; aircraft electromagnetic RF interference (EMI) and its effect on aircraft certification; and aircraft wiring. In summary, aircraft manufacturers are working very hard to certify 8.33kHz capability using Type Certificates. Additionally, the avionics manufacturers, along with the airlines,are certifying this capability on the aircraft using Supplemental Type Certificates. The delay of the implementation date means that by late next year 8.33kHz operation will have been provided for aircraft in Europe.

Design for maintenance and support

The first paper in this session came from Smiths Industries Aerospace and concerned an Integrated Data Acquisition and Recording System with HUMS Growth (IDARS with HUMS Growth). The first IDARS unit was delivered to Raytheon in 1997 for use on the Joint Primary Aircraft Training System (JPATS).

Application of this IDARS will lower acquisition costs, provide improved system availability, lower operating costs, provide reduced weight, extend useful life of the aircraft, and provide increased safety. The basic generic IDARS provides integrated data acquisition, crash survivable cockpit voice recording, and crash survivable flight data recording in a single line replaceable unit. The modular and expandable design of the IDARS architecture allows easy integration of additional functions to provide Health and Usage Monitoring System (HUMS)capability. Making use of the variety of configurations possible, the Smiths Industries IDARS/ HUMS has been selected for a variety of fixed and rotating wing aircraft.

The IDARS acquires and processes all aircraft sensor data, stores relevant data on the Crash Protected Memory (CPM) and/or the external data transfer system, and displays relevant exceedences, alerts and data on the Cockpit Control Unit (CCU). The IDARS consists of a chassis assembly and a set of plug-in Circuit Card Assemblies(CCAs). Up to five CCAs are accommodated with all data communications and power distribution handled though the rigid multiple layer motherboard. The CCAs are partitioned into the Flight Data Acquisition Unit (FDAU) and the Cockpit Voice and Flight Data Recorder CVFDR).

The FDAU provides monitoring and acquisition of flight data and sensors including analog, strain gauge, frequencies, low level AC/DC voltages, MIL-STD 1553, ARINC 429, RS-422 discrete signals, etc. It also provides outputs for various status and Built-In Test (BIT) data and outputs for use by other system components for crew alerts and data displays. The CVFDR provides data collection and incident/mishap recording of audio data, aircraft flight and system parameters to support post-incident analysis.

Growth beyond data acquisition and recording is envisaged and the integration of airborne monitoring and diagnostic systems with multifunction ground data analysis and support systems provides an evolving capability for very accurately tracking aircraft usage, system/sub-system life, and supporting fleet management and maintenance. These systems improve airworthiness, improve reliability, and reduce aircraft cost of ownership by detecting and diagnosing potential and actual failures; monitoring usage, automating test procedures, and providing advance warning of potential equipment failure and collecting valuable data for routine maintenance. The Smiths Industries GenHUMS extends IDARS into a proactive maintenance and diagnostic system. It monitors nearly 200 parameters vital to aircraft operation ­ continuously acquiring, processing and storing data. An operational GenHUMS system has two classes of components: the airborne equipment, consisting of the data acquisition and processing unit,cockpit control unit, cockpit interface panel, data transfer panel, data transfer system, and optical blade tracker; and the HUMS ground system. IDARS is claimed to be leading the market with light weight, small size, reduced power,and integrated single "box" solution.

A paper in this session from Boeing dealt with the use of industrial grade commercial parts for fielded fighter aircraft. During 1996-1997, Boeing conducted an Open System Demonstration Project (OSDP) co-sponsored by the Open Systems Joint Task Force and the F-15 Systems Program Office. The OSDP supported both a commercial operational and support savings initiative to develop a new core processor for the F-i5E aircraft and investigations into the insertion of commercial technology into high performance fighter aircraft. Specifically, the project included investigations into programme management, software processes,supportability and aircraft environmental specifications.

Vendor meetings, aircraft flight testing, analyses, and the review of research in related efforts were performed to accomplish the various objectives. The results suggest that:

• industrial grade, commercial components can be utilized with specific changes that will have minimal impact on F-15 operations;

• current design specifications are conservative; and

• commercial technology insertion into fielded fighter aircraft must be accompanied with a disciplined qualification process and supportability concept.

In a general sense, the results of the project are applicable to all aircraft and to all aircraft avionic systems.

From ERA and two UK companies came a discussion on an IMA packaging design for improved reliability and reduced instances of "no fault found" (NFF). The paper addresses an important new development of a robust mounting system appropriate for avionics installation in remote areas of large civil transport aircraft. Airlines would like to achieve a "fit and forget" policy for the avionics and this design arose to satisfy a requirement for the installation of IMA components.

To achieve the goal of open systems, defined in terms of system architecture by ARINC 651, and increased modularity, the packaging of certain IMA components needs to be reviewed. To be viable the package design, in addition to surviving harsh environments, would also have to take into account the considerations of limited access; ease of maintenance and re-usability (across different airframe platforms and materials); and architecture dependent functionality.

The design implementation addresses the following issues. The outer enclosure or case is equivalent to an aircraft LRU in so far as it houses a set of functional circuits and has to protect the circuitry from the aircraft environment. Also the internal circuit elements or modules are designed to accommodate the widest rage of commercially available circuit boards possible. Power supplies may be packaged and treated as a special instance of modules or sub-assemblies. In addition, the back panel may provide both the intermodule connections and power and possibly, I/O. Connectors need to be further developed, as well as an insertion-extraction device (IED)which is a mechanism to ensure a controlled force and sequence for the unit and its connector. A mounting tray, in addition, provides an option to ensure a controlled mechanical interface to the aircraft structure.

Terry Ford