The STAR I had actually been looking at was the TRUDO1 arrival for KMKE. Looking at it again, I notice that the instructions say "From over TRUDO to VEENA INT thence via the ATC assigned runway transition," and there are several paths on the plate that are labeled with transitions. So maybe ATC in this case says "descend via the TRUDO1 arrival with 19R transition," or something like that? Or perhaps it is automatically uploaded to the FMS? (My sim doesn't handle uploads and downloads, so I know almost nothing about that aspect of FMS operation.) Or maybe ATC gives you a runway and you select the appropriate transition on the FMC.
From what people are saying here, I understand that individual operators can actually cook up their own FMS databases? What would be the advantage? You'd still need the same fix and navaid data, and you couldn't really home-bake your own STARs, since they have to conform to the published ones (right?). At least this implies that you can modify the databases yourself (as an operator), which surprises me as I'd expect FMS databases to be locked by the vendor to make more money.
I also didn't know that FMS databases could be on a PAYG basis, but that part doesn't surprise me (money is everything). I guess the more you pay, the more you get in the database. If you are going somewhere that is unknown to the FMS, do you just enter all the routing data yourself by hand, or do you skip the FMS for the portion that it doesn't contain and just fly with the MCP, or by hand?
I know the Mt. Erebus problem was a navigation error, but not much more than that, I'll have to look it up again. I didn't realize that a FMS database error was involved.