The traps identified are associated with "too little and too late" consideration of BIT requirements relative to initial system design. A significant amount of analysis and tradeoffs must be done before a cost-effective BIT design approach can be detailed. As presented in the Joint Services BIT Design Guide 1, the operational and maintenance requirements of performance monitoring (failure direction) and fault localization, using tradeoff relationships with cost, weight, and volume, to determine the type and depth of BIT needed. Included in this would be the consideration as to whether the requirement would be better filled via automatic test equipment, manual test techniques, or mixes of the three approaches. The operational and maintenance BIT requirements then must be coordinate with Integrated Logistics Support (ILS) and production test personnel.
The BIT design concept can be taken too far, ending up with an expensive, impractical, product if the requirement is to isolate to a single replaceable item for a large system. Tradeoff analysis should be used to justify or reject such an approach. The obvious advantages of isolating to a single replaceable item are (1) reduced diagnostics time and (2) reduced occurrences of "no trouble found" removals.
Built-in test function should be planned early in the program for maximum effectiveness in both field operation and production integration and test. The "real" field maintenance environment should be determined, not an imagined or "should be" scenario. Trade studies should be performed to determine the most effective BIT parameters for the system. Ultimate production test needs will have a significant impact on BIT philosophy and should be determined early in the project development phases.
A detailed BIT approach should be developed prior to design start. This should included not only prime contractor BIT design, but also subcontractor and major vendor considerations, since BIT concepts can have a major impact on design approach. Failure to take these positive steps early in the program will result in a BIT philosophy which evolves piecemeal, rather than a well understood design concept which drives the conceptual and detail designs.
It is critical to effective BIT design to involve test and production discipline in trade studies to determine optimum approaches. Often, ultimate production cost can be prohibitive if the needs of integration and test functions are not considered. Test engineering personnel can analyze production steps and assist in choosing optimum BIT for production testability. A thorough testability analysis should be performed at this time to guide the system design. This integrated BIT design approach will assist the design engineer in selecting techniques which are best for both field supportability and production test efficiency.
A single point responsibility for BIT design at the
system level is a key factor. Tracking implementation of the BIT plan as it
evolves will ensure that redesigns are not necessary and that BIT "works" for
successful evaluation testing of the total design. The Joint Services BIT
Design Guide1 presents the fundamentals of BIT, provides an overview of the different approaches available to the designer and acquisition manager, and discusses standardized methods for evaluation of these different approaches. This guide is an invaluable tool for the personnel responsible for test, maintainability, reliability, and logistics support for present and future systems.
One of the prime objectives of the BIT design guide is to provide acquisition managers with guidelines for selection of analytical techniques, specification of BIT requirements, and for determining why BIT should be specified at all. The guide also gives guidance to designers responsible for translating BIT requirements into integral features of equipment design. The guide aids the designer in evaluating alternative BIT concepts and configurations, in choosing the preferred alternative, and in verifying the adequacy of choice.
Designers should receive adequate training in the latest BIT technology. Use of both manufacturing and test personnel to critique the design will allow production transition without redesign, and prevent an integration/test scheme which is "tacked on." This approach will minimize schedule slips which are inevitable when BIT requirements are redefined late in the design cycle.
The early evaluation of BIT design is essential. The
detail design reviews of the system should include a thorough review of the
conformance of the design to the BIT plan by senior designers as well as test,
production, and field personnel. This should be followed by structured
verification in the initial system integration and early measurement in the
use environment by end item users. This should prevent marginal BIT design and
should ensure efficient production and field test capabilities.
1Joint Services Built-in Test (BIT) Design
NAVMAT P9405/ DARCOMP 34-1
AFLCP 800-39/ AFSCP 800-39/
2721, 19 March 1981