Cal. PUC § 21693

In order to facilitate the making of investigations by the department in the interest of public safety and promotion of aeronautics, the reports of investigations or hearings or any part of the reports, shall not be admitted in evidence or used for any purpose in any suit, action, or proceeding growing out of any matter referred to in the investigation, hearing, or report, except in case of any suit, action, or proceeding, civil or criminal, instituted by or in behalf of the department or in the name of the state under the provisions of this part or other laws of the state relating to aeronautics. Any member of the department, the director, or any officer or employee of the department shall not be required to testify to any facts ascertained in, or information gained by reason of, his official capacity, nor be required to testify as an expert witness in any suit, action, or proceeding involving any aircraft. Subject to these limitations, the department may make available to appropriate federal, state, and political subdivision agencies information and material developed in the course of its investigations and hearings.