〈measure〉 probability for an item not to be in a state to perform as required at a given instant
Note 1 to entry: The unavailability is the complementary of the availability: U(t)=1− A(t )
Note 2 to entry: The unavailability is called “Probability of Failure on Demand” (PFD) by the standards related to
the functional safety of safety related/instrumented systems (e.g. IEC 61508).
Note 3 to entry: Note 3 to entry: When dealing with safety U(t) is generally small compared to 1 and this property
is used to develop approximated formulae (see Clause 7).
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards