meaningful for items with replaceable parts only when failure modes have adequate coverage. Readiness Readiness is meaningful when the item does not require down time for periodic maintenance. This is a useful measurement for items that incorporate automatic recovery or condition based maintenance. Readiness is the probability that an item will operate as expected when used at any random time while the item is in the correct mode of operation. Mean Time To Recover form manual actions is generally measured or estimated. The following is an example of the kind of values that could be used for estimating the mechanical portion of the recovery time associated with replacing a failed circuit card. Static wrist strap 120 seconds Bolts and screws with captive nut remove 15 seconds; replace 30 seconds Bolts and screws with loose nut remove 30 seconds; replace 60 seconds Small cables disconnect 15 seconds; reconnect 60 seconds Circuit card remove 30 seconds; insert 120 seconds Readiness calculations are meaningful for items with replaceable parts only when failure modes have adequate coverage. Coverage Maintenance coverage evaluates the proportion of faults detected by CBM and PMS. A rough estimate of coverage can be made by observing the ratio between operational failures and maintenance actions. Availability calculations, readiness calculations, and related claims are only valid if coverage exceeds availability. Military Versus Commercial Military systems and large commercial systems share reliability constraints. The ability to for a military system to continue operating after battle damage is survivability. Military Maintenance Policy (MMP) is required for defense systems. Designs typically include redundancy with automatic fault detection, automatic fault isolation, and automatic fault bypass. These reconfigure systems without human intervention after combat damage and normal failure. Most Commercial Off The Shelf (COTS) items are deployed in a benign environment, but electronic devices fail much like constant random battle damage. This effect grows worse as size grows. Excessive down-time is a type of design defect that impacts all large systems. As an example, if a system is built from 1,000 individual computers each with a 3-year Mean Time Between Failure (MTBF), then the whole system will have an MTBF of 1 day. If Mean Time To Repair (MTTR) is 3 days, then the system will never work. If the same system includes 1,010 computers, then failure will be rare if the system includes automatic fault detection, automatic fault isolation, and automatic fault bypass. This shows why