(a) No person may operate a multiengine aircraft unless the current empty weight and center of gravity are calculated from values established by actual weighing of the aircraft within the preceding 36 calendar months.
(b) Paragraph (a) of this section does not apply to—
(1) Aircraft issued an original airworthiness certificate within the preceding 36 calendar months; and
(2) Aircraft operated under a weight and balance system approved in the operations specifications of the certificate holder.
ACE rules page: Part 135 in plain English