The Mode S aircraft identification code is listed on AC Form 8050-3, Certificate of Aircraft Registration, as the International Civil Aviation Organization (ICAO) Aircraft Address Code. If the aircraft registration does not contain this information, aircraft owners with a Mode S transponder can obtain an aircraft identification code from the FAA Aircraft Registry in Oklahoma City, Oklahoma. This may be accomplished by writing to the following FAA address:

Federal Aviation Administration
Aircraft Registration Branch AFS-750
P.O. Box 25504
Oklahoma City, OK 73125-0504

You may also call:
866-762-9434
405-954-3116