What is a letter of design approval?
An FAA design approval for an article manufactured outside the United States that meets a specific TSO.

How do I obtain a letter of TSO design approval?
The FAA will issue a letter of TSO design approval only to manufacturers located in a country with which the U.S. has concluded a bilateral agreement applicable to the article in question.  The exporting Civil Aviation Authority (CAA) is responsible for oversight of both the design and production of such articles.

An applicant for a FAA letter of TSO design approval should apply through the exporting CAA with a request that the application and required data be forwarded to the FAA.  The exporting CAA should contact the FAA for information concerning the latest FAA technical policy and procedures whenever the CAA receives an application for an FAA letter of TSO design approval to a TSO performance standard for which the CAA has not previously made compliance findings.

The FAA issues a letter of TSO design approval only for articles of a kind for which a minimum performance standard has been published in an FAA Technical Standard Order (TSO).