Ever heard of grey literature? It sounds like the name of an anatomy textbook. Grey literature is essentially non-commercially published materials like technical manuals, conference proceeding, government documents and so on. It is the kind of material that was traditionally put into vertical files in the library. I must confess that I have never kept a vertical file and all the vertical file information that was kept in my library I have thrown out. The only materials that I keep that would even approximate this kind of information are the educational catalogues that arrive in alarming volume at our school every month.
So it take a new set of skills and thinking to access this literature which is mostly part of the invisible web. The invisible web is that part of the Internet that search engines can't access. One needs to search for databases sites that warehouse this kind of information. A search engine cannot search a database for you. However you can use search engines to help locate off ramps to the invisible web when you include the term database in your search. Searching for statistic database or aircraft accident database is one way to find access points to the invisible web and to a wealth of grey literature.