What is mission time in functional safety and reliability engineering?

In reliability engineering, mission time refers to the period during which a system, component, or device is expected to operate without failure under specified conditions it is also called as an operational time. It is the total duration or time interval over which the performance of the system is evaluated for reliability.

Mission time is crucial in determining failure rates and reliability metrics because it sets the operational time window for which a system must meet reliability requirements. During this time, the system is assumed to function as needed, and any failures within this period are recorded to assess its reliability.

It is generally a measurement applicable to situations where on-line repair is not possible, such as an unmanned space flight or an airborne aircraft.

For example, in a reliability analysis of a spacecraft, the mission time might be defined as the total operational duration from launch to the end of the space mission, and reliability assessments would focus on ensuring the spacecraft remains functional throughout this defined period.