14 CFR §406.137

1. (a)A person may file with the Federal Docket Management System and serve on each other party a motion for leave to intervene as party in an adjudication. Except for good cause shown, a motion for leave to intervene must be filed not later than 10 days before the hearing.
2. (b)The administrative law judge may grant a motion for leave to intervene if the administrative law judge finds that—
   1. (1)Intervention will not unduly broaden the issues or delay the proceedings, and
   2. (2)The intervener will be bound by any order or decision entered in the action or the intervener has a property, financial, or other legitimate interest that may not be addressed adequately by the parties.
3. (c)The administrative law judge may determine the extent to which an intervener may participate in the proceedings.