En Route Navigation
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 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).
The en route airspace structure of the National Airspace System (NAS) consists of three strata. The first stratum low altitude airways in the United States can be navigated using NAVAIDs, have names that start with the letter V, and are called Victor Airways. [Figure 2-3] They cover altitudes from approximately 1,200 feet above ground level (AGL) up to, but not including 18,000 feet above mean sea level (MSL). The second stratum high altitude airways in the United States all have names that start with the letter J, and are called Jet Routes. [Figure 2-4] These routes run from 18,000 feet to 45,000 feet. The third stratum allows random operations above flight level (FL) 450. The altitude separating the low and high airway structure varies from county to country. For example, in Switzerland it is 19,500 feet and 25,000 feet in Egypt.
Air Route Traffic Control Centers
The FAA defines an Air Route Traffic Control Center (ARTCC) as a facility established to provide air traffic control (ATC) service to aircraft operating on IFR flight plans within controlled airspace, principally during the en route phase of flight. When equipment capabilities and controller workload permit, certain advisory/assistance services may be provided to VFR aircraft.
ARTCCs, usually referred to as Centers, are established primarily to provide air traffic service to aircraft operating on IFR flight plans within the controlled airspace, and principally during the en route phase of flight. There are 21 ARTCC’s in the United States. [Figure 2-5] Any aircraft operating under IFR within the confines of an ARTCC’s airspace is controlled by air traffic controllers at the Center. This includes all sorts of different types of aircraft: privately owned single engine aircraft, commuter airlines, military jets, and commercial airlines.
The largest component of the NAS is the ARTCC. Each ARTCC covers thousands of square miles encompassing all or part of several states. ARTCCs are built to ensure safe and expeditious air travel. All Centers operate 7-days a week, 24-hours a day, and employ a combination of several hundred ATC specialists, electronic technicians, computer system specialists, environmental support specialists, and administrative staff. Figure 2-6 is an example of the Boston ARTCC. The green lines mark the boundaries of the Boston Center area, and the red lines mark the boundaries of Military Operations Areas (MOAs), Prohibited, Restricted, Alert, and Warning Areas.
Safe Separation Standards
The primary means of controlling aircraft is accomplished by using highly sophisticated computerized radar systems. In addition, the controller maintains two-way radio communication with aircraft in his or her sector. In this way, the specialist ensures that the aircraft are separated by the following criteria:
- Laterally—5 miles
- 1,000 feet (if the aircraft is below FL 290, or between FL 290 and FL 410 for RVSM compliant aircraft)
- 2,000 feet (if the aircraft is at FL 290 or above)
The controllers can accomplish this separation by issuing instructions to the pilots of the aircraft involved. Altitude assignments, speed adjustments, and radar vectors are examples of instructions that might be issued to aircraft.
En route control is handled by pinpointing aircraft positions through the use of flight progress strips. These strips are pieces of printed paper containing pertinent information extracted from the pilot’s flight plan. These strips are printed 20 minutes prior to an aircraft reaching each Center’s sector. A flight progress strip tells the controller everything needed to direct that aircraft. If the flight progress strips of each aircraft approaching a sector are arranged properly, it is possible to determine potential conflicts long before the aircraft are even visible on the Center controller’s display. In areas where radar coverage is not available, this is the sole means of separating aircraft.
The strips, one for each en route point from which the pilot reports his or her position, are posted on a slotted board in front of the air traffic controller. [Figure 2-7] At a glance, he or she is able to see certain vital data: the type of aircraft and who is flying it (airline, business, private, or military pilot), aircraft registration number or flight number, route, speed, altitude, airway designation, and the estimated time of arrival (ETA) at destination. As the pilot calls in the aircraft’s position and time at a predetermined location, the strips are removed from their slots and filed. Any change from the original flight plan is noted on the strips as the flight continues. Thus, from a quick study of the flight progress board, a controller can assess the overall traffic situation and can avoid possible conflicts.
Figure 2-8 shows the Fort Worth, Texas Air Route Traffic Control Center (ZFW) and the geographical area that it covers. The Center has approximately 350 controllers. Most are certified and some are in on-the-job training.