Applying for an Aviation Reference Number (ARN) An aviation reference number (ARN) is similar to an account or customer number. You will need an ARN if you hold, or intend to hold, any licence, permission or authorisation with CASA (Civil Aviation Safety Authority). You can apply for your ARN through the CASA Portal. You will require 100 points of identification, if your documents cannot be immediately verified, you will need to upload a certified copy of your documents.