c. Insertion of improper computer programs
    d. Support-personnel fatigue
A third limitation of the parts-analysis approach discussed in Ref. [4] deals with the
subjectivity and invalid assumptions often used to· estimate piece/component reliabilities.
Here Booz•Allen quotes from a reporf1 by the Office of Technology Assessment, and we
do likewise:

    "The design reliability of proposed vehicles is generally estimated using:

          Data from laboratory tests of vehicle systems (e.g., engines and avionics) and
          components that have already been built;

          Engineer's judgments about the reliability- achievable in systems and
          components that have not been built;

          Analyses of whether a failure in one system or component would cause other
          systems and components, or the vehicle to fail; and

          Assumptions (often tacit) that:

              the laboratory conditions under which systems were tested precisely
              duplicate the conditions under which the systems will operate,
              the conditions under which the system will operate are those under which
              theywere designed to operate,

              the engineer's judgments about reliability are correct, and
              the failure analyses considered all circumstances and details that influence
              reliability:

    Such engineering estimates of design·reliability are incomplete and subjective...".

Effects influencing reliability that the analyst may fail to consider include:

    a. Lightning strikes
    b. Aging effects, particularly for solid propellants
    c. Corrosion
    d. Insufficient heat or cold insulation for critical components
    e.Idng
    f. Erroneous antennae patterns or instrumentation

Booz• Allen concludes as follows:·

    ''Finally, due to its nature, the engineering approach can not account for
    undetected design flaws. (If these flaws were detected, and could be modeled,


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