FAA Federal Aviation Regulations (FARS, 14 CFR)

(a) The party on whose behalf a witness appears is responsible for paying any witness fees and mileage expenses.

(b) Except for employees of the United States summoned to testify as to matters related to their public employment, witnesses summoned by subpoena shall be paid the same fees and mileage expenses as are paid to a witness in a court of the United States in comparable circumstances.