FAA Federal Aviation Regulations (FARS, 14 CFR)

(a) The Presiding Officer may designate additional persons as parties to the investigation, if in the discretion of the Presiding Officer, it will aid in the conduct of the investigation.

(b) The Presiding Officer may designate any person as a party to the investigation if that person --

(1) Petitions the Presiding Officer to participate as a party; and

(2) Is so situated that the disposition of the investigation may as a practical matter impair the ability to protect that person's interest unless allowed to participate as a party, and

(3) Is not adequately represented by existing parties.