fly along the centerline when on a Federal airway or,  on routes other than Federal airways, along the direct  course between NAVAIDs or fixes defining the route. The  regulation allows maneuvering to pass well clear of other  air traffic or, if in visual meteorogical conditions (VMC),  to clear the flightpath both before and during climb or  descent.  Airway routing occurs along pre-defined pathways called  airways. [Figure 2-2] Airways can be thought of as three-  dimensional highways for aircraft. In most land areas of  the world, aircraft are required to fly airways between the  departure and destination airports. The rules governing  airway routing, Standard Instrument Departures (SID)  and Standard Terminal Arrival (STAR), are published flight  procedures that cover altitude, airspeed, and requirements  for entering and leaving the airway. Most airways are  eight nautical miles (14 kilometers) wide, and the airway  flight levels keep aircraft separated by at least 500 vertical  feet from aircraft on the flight level above and below  when operating under VFR. When operating under IFR,  between the surface and an altitude of Flight Level (FL)  290, no aircraft should come closer vertically than 1,000  feet. Above FL 290, no aircraft should come closer than  2,000 feet except in airspace where Reduced Vertical  Separation Minima (RVSM) can be applied in which case  the vertical separation is reduced to 1,000 feet. Airways  usually intersect at NAVAIDs that designate the allowed  points for changing from one airway to another. Airways  have names consisting of one or more letters followed by  one or more digits (e.g., V484 or UA419).  En route instrument flight rules (IFR) navigation is evolving  from the ground-based navigational aid (NAVAID) airway  system to a sophisticated satellite and computer-based  system that can generate courses to suit the operational  requirements of almost any flight. The FAA Global  Navigation Satellite System (GNSS) provides satellite-based  positioning, navigation, and timing services in the United  States to enable performance-based operations for all  phases of flight, to include en route navigation.  14 CFR Part 91, § 91.181, is the basis for the course to be  flown. Unless authorized by ATC, to operate an aircraft  within controlled airspace under IFR, pilots must either  The en route airspace structure of the National Airspace  System (NAS) consists of three strata. The first stratum low  Figure 2-1. Code of Federal Regulations, Title 14 Aeronautics and Space.  Figure 2-2. Airways depicted on an aeronautical chart.  Airways  En Route Navigation  2-2 