National Airspace System [electronic resource] : FAA has implemented some free flight initiatives, but challenges remain : report to congressional requestors / United States General Accounting Office.
Mode of access via the GPO Access Web site. Address as of 4-13-01: http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=gao&docid=f:rc98246.pdf; current access is available via PURL.