14 CFR 16.223 - Evidence.
(a) General. A party may submit direct and rebuttal evidence in accordance with this section.
(b) Requirement for written testimony and evidence. Except in the case of evidence obtained by subpoena, or in the case of a special ruling by the hearing officer to admit oral testimony, a party's direct and rebuttal evidence shall be submitted in written form in advance of the oral hearing pursuant to the schedule established in the hearing officer's prehearing conference report. Written direct and rebuttal fact testimony shall be certified by the witness as true and correct. Subject to the same exception (for evidence obtained by subpoena or subject to a special ruling by the hearing officer), oral examination of a party's own witness is limited to certification of the accuracy of written evidence, including correction and updating, if necessary, and reexamination following cross-examination by other parties.
(c) Subpoenaed testimony. Testimony of witnesses appearing under subpoena may be obtained orally.
(d) Cross-examination. A party may conduct cross-examination that may be required for disclosure of the facts, subject to control by the hearing officer for fairness, expedition and exclusion of extraneous matters.
(e) Hearsay evidence. Hearsay evidence is admissible in proceedings governed by this part. The fact that evidence is hearsay goes to the weight of evidence and does not affect its admissibility.
(f) Admission of evidence. The hearing officer admits evidence introduced by a party in support of its case in accordance with this section, but may exclude irrelevant, immaterial, or unduly repetitious evidence.
(g) Expert or opinion witnesses. An employee of the FAA or DOT may not be called as an expert or opinion witness for any party other than the agency except as provided in Department of Transportation regulations at 49 CFR part 9.