49 CFR §1503.621

1. (a)Filing and service. A party must file the amendment with the Enforcement Docket Clerk and must serve a copy of the amendment on the ALJ and all parties to the proceeding.
2. (b)Time. A party must file an amendment to a complaint or an answer within the following:
   1. (1)Not later than 15 days before the scheduled date of a hearing, a party may amend a complaint or an answer without the consent of the ALJ.
   2. (2)Less than 15 days before the scheduled date of a hearing, the ALJ may allow amendment of a complaint or an answer only for good cause shown in a motion to amend.
3. (c)Responses. The ALJ must allow a reasonable time, but not more than 20 days from the date of filing, for other parties to respond if an amendment to a complaint, answer, or other pleading has been filed with the ALJ.