49 C.F.R. § 801.23 – Extension.

(a) In unusual circumstances as specified in this section, the time limits prescribed in either § 801.21 or § 801.22 may be extended by no more than 10 days (excluding Saturdays, Sunday, and legal public holidays) by providing written notice to the requester setting forth the reasons for the extension and the date on which a determination is expected to be dispatched.
(b) If the request cannot be processed within the extended time limit specified in paragraph (a) of this section, the requester will be:
1. (1) Notified in writing;
2. (2) Given an opportunity to limit the scope of the request so that it may be processed within that time limit, or an opportunity to arrange with the agency an alternative time frame for processing the request or a modified request; and
3. (3) Advised of the requester's right to seek assistance from the NTSB's FOIA Public Liaison and seek dispute resolution services from OGIS.
(c) As used in this paragraph (c), “unusual circumstances,” as they relate to any delay that is reasonably necessary to the proper processing of the particular request, means:
1. (1) The need to search for and collect the requested records from field facilities or other establishments that are separate from the office processing the request;
2. (2) The need to search for, collect, and appropriately examine and process a voluminous amount of separate and distinct records which are the subject of a single request; or
3. (3) The need to consult with another agency that has a substantial interest in the disposition of the request or with two or more components of the agency having substantial subject-matter interest therein.