Downtime (Td) is the Mission Duration times the sum of all of the different kinds of time required to transition from being down to the time to be fully operational divided by the Mean Time Between Failure.
Mean Time To Repair (MTTR) is time required to restore operations the level defined in the system specification
Mean Logistics Delay Time (MLDT) is time required to obtain parts from the part depot or from the manufacturer including transportation to the site