THE NATIONAL AIRSPACE UNDER NEXTGEN SYSTEMS WILL BE HIGHLY AUTOMATED. FOUR-DIMENSIONAL TRAJECTORY-BASED OPERATIONS AND CLEARANCES WILL REPLACE ATC SECTOR CONTROL, ELIMINATING THE NEED FOR MANY COMMUNICATIONS BETWEEN FLIGHT CREWS AND ATE. MOREOVER, WHEN COMMUNICATION DOES OCCUR BETWEEN AIRCRAFT OR BETWEEN GROUND AND AIRCRAFT, IT IS LIKELY TO BE VIA DATA-LINK SYSTEMS (PERHAPS DIRECTLY BETWEEN GROUND AND FLIGHT COMPUTERS) RATHER THAN VOICE CHANNELS. AIRCRAFT ONBOARD DATA ACCESS (E.G., VIA ADS-B, GPS, DATA-LINK, CDTI) WILL ALLOW SELF-SEPARATION CAPABILITY, AND VIRTUALLY ALL-WEATHER OPERATIONS. WHEN AIRCRAFT ARE SO EQUIPPED, AIR TRAFFIC CONTROLLERS WILL ASSUME THE ROLE OF 'MONITORS' OR 'SERVICE PROVIDERS' RATHER THAN ACTIVE CONTROLLERS. "INTELLIGENT DECISION SUPPORT" SYSTEMS WILL BE USED TO MANAGE INFORMATION OVERLOAD. THESE CHANGES WILL ENABLE AIR TRAFFIC FLOW THAT IS TWO OR THREE TIMES THE CURRENT LEVEL - A SIGNIFICANT INCREASE IN TRAFFIC DENSITY IN BOTH ENROUTE AND TERMINAL OPERATIONS. ALONG WITH THE POSSIBILITY OF ENHANCED TRAFFIC FLOW AND EFFICIENCY, HOWEVER, COME ISSUES ASSOCIATED WITH ACCOMPANYING CHANGES IN ROLES AND RESPONSIBILITIES FOR FLIGHT CREWS, AS WELL AS WITH THE CAPABILITIES AND LIMITATIONS OF HUMAN DECISION MAKING.