14 C.F.R. § 302.20 – Formal intervention.

(a) Who may intervene. Any person who has a statutory right to be made a party to an oral evidentiary hearing proceeding shall be permitted to intervene. Any person whose intervention will be conducive to the public interest and will not unduly delay the conduct of such proceeding may be permitted to intervene.
(b) Considerations relevant to determination of petition to intervene. In passing upon a petition to intervene, the following factors, among other things, will be considered and will be liberally interpreted to facilitate the effective participation by members of the public in Department proceedings:
1. (1) The nature of the petitioner's right under the statute to be made a party to the proceeding;
2. (2) The nature and extent of the property, financial or other interest of the petitioner;
3. (3) The effect of the order that may be entered in the proceeding on petitioner's interest;
4. (4) The availability of other means whereby the petitioner's interest may be protected;
5. (5) The extent to which petitioner's interest will be represented by existing parties;
6. (6) The extent to which petitioner's participation may reasonably be expected to assist in the development of a sound record; and
7. (7) The extent to which participation of the petitioner will broaden the issues or delay the proceeding.
(c) Petition to intervene—(1) Contents. Any person desiring to intervene in a proceeding shall file a petition in conformity with this part setting forth the facts and reasons why he or she thinks he or she should be permitted to intervene. The petition should make specific reference to the factors set forth in paragraph (b) of this section.
(2) Time for filing. Unless otherwise ordered by the Department:
1. (i) A petition to intervene shall be filed with the Department prior to the first prehearing conference, or, in the event that no such conference is to be held, not later than fifteen (15) days prior to the hearing.
2. (ii) A petition to intervene filed by a city, other public body, or a chamber of commerce shall be filed with the Department not later than the last day prior to the beginning of the hearing.
3. (iii) A petition to intervene that is not timely filed shall be dismissed unless the petitioner shall clearly show good cause for his or her failure to file such petition on time.
(3) Answer. Any party to a proceeding may file an answer to a petition to intervene, making specific reference to the factors set forth in paragraph (b) of this section, within seven (7) days after the petition is filed.
(4) Disposition. The decision granting, denying or otherwise ruling on any petition to intervene may be issued without receiving testimony or oral argument either from the petitioner or other parties to the proceeding.
(d) Effect of granting intervention. A person permitted to intervene in a proceeding thereby becomes a party to the proceeding. However, interventions provided for in this section are for administrative purposes only, and no decision granting leave to intervene shall be deemed to constitute an expression by the Department that the intervening party has such a substantial interest in the order that is to be entered in the proceeding as will entitle it to judicial review of such order.