Mode of access: Internet at the FAA web site. Address as of 3/25/09: http://www.airweb.faa.gov/Regulatory%5Fand%5FGuidance%5FLibrary/rgAdvisoryCircular.nsf/0/686e99b0c138785686256dc50049be9a/$FILE/AC150-5210-21.pdf; current access available via PURL.