FAA Advisory Circular 43-16A No. 135 states, "AD Numbering and
Identification - AD numbers are composed of three basic elements (example:
89-01-02). The first two digits represent the year in which the AD was
issued. The second two digits indicate the biweekly period within the year
(normally, a year contains 26 biweekly periods). The last two digits are
sequential numbers assigned to each AD issued within a biweekly period
(sequential numbers assigned to telegraphic AD's begin with number 51
instead of 01). Thus, AD 89-01-02 identifies the second AD issued during the
first biweekly period of 1989."

The sequential numbers do not always indicate the order of the effective
dates. For example, AD 2005-07-17 is effective May 12th and AD 2005-07-18 is
effective May 11th. The effective dates of sequential ADs are sometimes much
further apart -- not that it makes any difference.

"O. Sami Saydjari" wrote in message
...
Do Airworthiness Directive Numbers have any meaning and structure? For
example, AD 82-27-3. Does one part mean year issued? Another refer to
type equipment effected? Another, perhaps a serial number of the AD
itself? Just Curious.

-Sami