Maintainability and Availability
Maintainability and Availability
This domain tests repair-time distributions, maintainability functions, MTTR, and the different availability measures used in operations and contracts.
Core rules
- Maintainability M(t)=P(repair≤t). Exponential repair: M(t)=1-e^(-μt) and MTTR=1/μ.
- Inherent availability A_i=MTBF/(MTBF+MTTR).
- Achieved availability excludes logistics delay; operational availability A_o=MTBM/(MTBM+MDT), where MDT includes repair, administrative, and supply delay.
- Preventive maintenance affects availability through scheduled downtime and restoration quality.
Worked example
MTBF=500 h and MTTR=5 h give A_i=500/505=0.9901. If mean logistics delay is 10 h, MDT=15 h and A_o=500/515=0.9709. Improving repair time from 5 h to 2 h raises A_i to 0.9960, but the 10 h logistics delay still caps A_o at 500/512=0.9766 unless spares and support are improved.
Traps
- Using MTTR in operational availability while omitting supply delay.
- Confusing maintainability, a probability, with MTTR, a mean.
- Counting standby or scheduled downtime as MTBF operating time.
- Assuming repair is exponential when data are lognormal.