Threads can quickly move off down the list if no one posts in them, also after so many days, (a few hundred I think), threads seem to get 'archived'. Best way to find anything is to use the Advanced Search, (in the Search drop down). It allows you to search in thread titles, (not just every post like Search does), you can even set which forums to search in, plus more options, and it finds threads in the 'archive'.
I just tried an advanced search for 'Tritle' with it set to just search the thread titles and just in the Aircraft-Electric-Airplanes forums. it came back with 66 threads, so they are still around.