You can't install a previous version, because it would break compatibility with the Addon Manager that has been updated as well. Well, in theory you could, but it would require putting a mixmatch of files, that might create issues for the next updates.
We know there's a very minor issue that doesn't appear in SP2 so we thought it's not a big deal, first because it only happens if you put the airplane on grass, and then because the moment SP2 downloadable version will be out, nobody will probably want to use FSX SP1 anymore.
Apart for this, Zurich 1.2 is so vastly improved comparing to 1.1, that I don't see why you'll want to revert to 1.1 just to be able to see the airplane when it's on grass, expecially considering the problem will fix itself with FSX SP2.