FAA can issue two types of approval for a TSO part application: TSO Authorization and letter of TSO design approval. What's the difference?

Does it relate to the approval being design-only or production only?

Also, do other Airworthiness Authorities have a similar difference?

According to 14 CFR 21 Subpart O §21.601:

(2) A TSO authorization is an FAA design and production approval issued to the manufacturer of an article that has been found to meet a specific TSO;

(3) A letter of TSO design approval is an FAA design approval for an article that has been found to meet a specific TSO in accordance with the procedures of § 21.621;

(4) An article manufactured under a TSO authorization, an FAA letter of acceptance as described in § 21.613(b), or an article manufactured under a letter of TSO design approval described in § 21.621 is an approved article for the purpose of meeting the regulations of this chapter that require the article to be approved;


1 Answer 1


I don't know much about this, but it looks like a TSO Authorization is for domestic (US) manufacturers, and a letter of TSO design approval is for foreign manufacturers who want to import their products into the US. See 21.621:

§21.621 Issue of letters of TSO design approval: Import articles.

(a) The FAA may issue a letter of TSO design approval for an article—

(1) Designed and manufactured in a foreign country or jurisdiction subject to the export provisions of an agreement with the United States for the acceptance of these articles for import; and

(2) For import into the United States if— [...]

And the FAA's page on it says:

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

I have no idea about other aviation authorities, but I assume there are similar regulations in other countries because the same general issue exists everywhere: how do you ensure that both manufactured and imported aviation components meet all required standards?

  • $\begingroup$ As for your last question, dealing with imported parts is quite a tough problem as it depends on specific agreements between countries. See easa.europa.eu/document-library/… $\endgroup$
    – PPC
    Commented Nov 17, 2017 at 14:09

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