das hier findet sich dazu im PMDG-Forum bei AVSIM:
I had this exact issue. It's going to be a bit of a pain, but..... Delete your dll.xml and fsconfig files. Then un-install the 747 and MD11. Start FSX and let it rebuild.
Then re-install the 747 and MD11. They now both work a treat for me. I must have got a lot of mess in the dll.xml and Fsconfig files because now all the strobes beacons and landing lights display correctly. I haven't had these display correctly for ages, but just put up with it.
So doesn't hurt to have a clean out every now and then.
Klingt nach Arbeit ...