4-1-1. Air Route Traffic Control Centers

AIM 4-1-1 introduces Air Route Traffic Control Centers (ARTCCs, or simply "Centers") — the FAA facilities that provide air traffic services to aircraft on IFR flight plans within controlled airspace, primarily during the en route phase of flight.

ARTCC scope and structure: The U.S. has 21 domestic ARTCCs covering the contiguous 48 states, Alaska, Hawaii, Puerto Rico, and Guam. Each ARTCC controls a defined geographic block of airspace (called a Center sector) up to the floors of Class A. Centers handle the bulk of IFR en route traffic — the airliners cruising at FL350 are talking to a Center, not Approach.

The handoff chain: A typical IFR flight transitions through multiple controlling facilities: Clearance Delivery → Ground → Tower → Departure (TRACON) → Center → another Center (if crossing boundary) → Approach (TRACON) → Tower → Ground. Center is the long en route segment between the TRACONs.

What Centers actually do: Provide separation (vertical, lateral, longitudinal) between IFR aircraft. Issue ATC clearances, route amendments, altitude changes. Provide weather information, traffic advisories, and (when workload permits) flight following to VFR aircraft.

VFR flight following on Center: Center will provide flight following / traffic advisory service to VFR aircraft when workload permits — this is sometimes called "VFR with flight following." The VFR pilot can request "radar advisories" from Center. Note: flight following is a workload-permitting service; Center can drop you if traffic load increases.

Cross-references in the FARs: § 91.123 (compliance with ATC clearances), § 91.181 (course to be flown), § 91.183 (IFR radio communications and reports).

Reference: FAA Order 7110.65 (Air Traffic Control) is the controller-side procedures manual; pilot-facing summary in AIM Chapter 4 Section 1.