14 CFR § 16.207 - Intervention and other participation.
(b) A person may submit a written motion for leave to intervene as a party. Except for good cause shown, a motion for leave to intervene shall be submitted not later than 10 days after the notice of hearing and hearing order.
(c) If the hearing officer finds that intervention will not unduly broaden the issues or delay the proceedings and, if the person has an interest that will benefit the proceedings, the hearing officer may grant a motion for leave to intervene. The hearing officer may determine the extent to which an intervenor may participate in the proceedings.
(d) Other persons may petition the hearing officer for leave to participate in the hearing. Participation is limited to the filing of a posthearing brief and reply to the hearing officer and the Associate Administrator. Such a brief shall be filed and served on all parties in the same manner as the parties' posthearing briefs are filed.
(e) Participation under this section is at the discretion of the hearing officer, and no decision permitting participation shall be deemed to constitute an expression that the participant has such a substantial interest in the proceeding as would entitle it to judicial review of such decision.