Windshear

A Challenging night last night. One of our three IRUs (Inertial Reference Unit) was bad, so we were restricted from CAT 3 approaches, which are the most demanding of the navigation system. That was not a huge deal since the visibility was good going in to Ontario. The other failure this produces, if it's the left IRU (it was) is that the windshear warning system does not work. We were therefore restricted from going into areas of actual or forecast windshear. Now LLWS (Low Level Windshear) warnings are produced all the time, especially near mountainous areas. Usually they're just that: a warning to be vigilant, but we were legally prohibited from landing anywhere with windshear warnings. Ontario showed a forecast with windshear after we got airborne from Fort Worth. Fortunately we were late enough that the forecast expired and we landed as scheduled. Otherwise, we would have had a dicey little divert to LAX or Vegas with very little fuel to play with since no alternate was planned for in our release. Ho ho ho.