Per MIL-HDBK-338B, the environmental conversion factors stated therein were “based on a grouping of approximately 18,000 parts from a number of equipment reliability predictions performed on various military contracts. Ratios were developed using this database and the MIL-HDBK-217F algorithms”. The relative percentages of each part type in the database (and upon which the MIL-HDBK-388B conversion factors are based) are as follows: transistors-2%, capacitors-28%, resistors-27%, integrated circuits-18%, inductors-17%,diodes-5%, miscellaneous-3%. The conversion factors are therefore most appropriate to use when a system’s part type distribution resembles these percentages. Since the failure rates listed in NPRD are mostly for mechanical equipment and are based upon field data as reported to RIAC, rather than MIL-HDBKL-217F failure rate models, it would be inappropriate to use the MIL-HDBK-338B conversion factors to adjust the NPRD data.

Richard Wisniewski,
Senior Reliability Engineer