"A technical report is a document that describes the process, progress, or results of technical or scientific research or the state of a technical or scientific research problem. It might also include recommendations and conclusions of the research." https://en.wikipedia.org/wiki/Technical_report
Technical reports are produced by corporations, academic institutions, and government agencies at all levels of government, e.g. state, federal, and international. Technical reports are not included in formal publication and distribution channels and therefore fall into the category of grey literature.
Aerospace / Aviation