Summary: GAO discussed the Federal Aviation Administration's (FAA) plans to modernize its air traffic control (ATC) system through the Advanced Automation System (AAS). GAO noted that AAS, which will cost $3.2 billion, is expected to: (1) enable the consolidation and replacement of en route and terminal ATC facilities; (2) increase controller productivity and system availability; (3) reduce operating costs; (4) save fuel; and (5) automate many of the functions that controllers currently perform. AAS is scheduled to be implemented in steps beginning in 1991. GAO also noted that FAA faces a number of technical and operational risks in implementing AAS, including: (1) the complete replacement of ATC hardware; (2) the use of a new computer language; (3) the system's extremely high availability requirements; (4) problems inherent in interfacing with other ATC systems that are still under development; (5) unvalidated system performance models; and (6) uncertainty over the adequacy of the planned controller work station. In addition, GAO noted that FAA cannot be certain that AAS will be cost-effective because it used questionable assumptions in its AAS cost-benefit study, which may have inflated the FAA estimate of savings that will result from AAS implementation.