Mean Time Between Failures(MTBF)

The average time from the start of operation until the first failure of a non-repairable item, or the average time between replacement and next failure for items that are replaced rather than repaired. MTTF is technically correct for non-repairable components (bearings, seals, filters, lamps) while MTBF applies to repairable systems. In practice, the terms are often used interchangeably.

The average time required to repair a failed item and restore it to operational status, measured from the point of failure detection to the point of verified restoration. MTTR includes fault diagnosis, logistics (obtaining parts and tools), physical repair, testing, and recommissioning. It does not include administrative delays or waiting time unless specifically defined as Mean Time To Restore.

The average distance traveled or cycles completed between consecutive failures, used for equipment where usage is measured in distance or cycles rather than operating hours. MDBF is the distance/cycle-based equivalent of MTBF, appropriate for mobile equipment, rotating machinery measured in revolutions, or reciprocating equipment measured in strokes.

The average number of failures per unit of time for a population of items, typically expressed as failures per hour, per year, or per million hours. Failure rate (lambda) is the reciprocal of MTBF for constant-failure-rate systems. It is a statistical average measure, distinct from the instantaneous hazard rate, and is most accurate when the failure rate is approximately constant (beta near 1).

The probability that an item will perform its required function under stated conditions for a specified period of time. Reliability is a quantitative measure expressed as a probability (0 to 1) or percentage. It is a function of the failure rate and time: for constant failure rate systems, R(t) = e^(-lambda*t). Higher reliability means lower probability of failure over the same period.

The proportion of time that an item is in a state to perform its required function under given conditions, assuming that the required external resources are provided. Availability combines reliability (how often it fails) and maintainability (how quickly it can be restored). Inherent availability considers only corrective maintenance; operational availability includes all downtime including preventive maintenance and logistics.