Thanks for all the replies. I've done a bit of checking, and AiCarriers never shows up in the icons accessed from the system tray. After shutting down "properly", FSX disappears from Task Manager and only the little Steam icon shows in the system tray popup, but if I try to exit it I get the message that I must shut down FSX first, even though I have done so. However, if I go to Task Manager AiCarriers is there, and if I close it (or, indeed Steam) then FSX does shut down as far as Steam is concerned. It seems as if AiCarriers is to blame and preventing a normal shutdown of FSX, but this is a new problem after months of use. How do I fix it?