Chapter 2
Introduction
The en route phase of flight is defined as that segment of
flight from the termination point of a departure procedure
to the origination point of an arrival procedure. The
procedures employed in the en route phase of flight are
governed by a set of specific flight standards established
by 14 CFR [Figure 2-1], FAA Order 8260.3, and related
publications. These standards establish courses to be flown,
obstacle clearance criteria, minimum altitudes, navigation
performance, and communications requirements.
En Route Operations
Figure 2-1. Code of Federal Regulations, Title 14 Aeronautics and Space.
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.
Airways
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
Figure 2-2. Airways depicted on an aeronautical chart.
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
Figure 2-3. Victor airways.
Figure 2-4. Jet routes.
ZID
ZMP
ZOB
ZBW
ZNY
ZNY
ZDC
ZAU
ZKC
ZME ZTL
ZJX
ZMA
ZHU
ZFWZAB
ZDV
ZLA
ZOA
ZLC
ZSE
ZHN
ZAN
Cleveland Center
Albuquerque Center
Seattle Center
Atlanta Center
Chicago Center
Boston Center
Washington
Center (DC)
Denver Center
Fort Worth Center
Houston Center
Indianapolis
Center
Jacksonville Center
Kansas City Center
Los Angeles Center
Salt Lake City Center
Miami Center
Memphis Center
Minneapolis Center
New York
CenterOakland Center
Honolulu
Center
Anchorage
Center
Figure 2-5. Air Route Traffic Control Centers.
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
• Vertically—
• 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.
Figure 2-6. Boston Air Route Traffic Control Center.
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 2-7. Flight progress strips.
Figure 2-8. Fort Worth Air Route Traffic Control Center.
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.
Sectors
The airspace controlled by a Center may be further
administratively subdivided into smaller, manageable
pieces of airspace called sectors. A few sectors extend from
the ground up, but most areas are stratified into various
levels to accommodate a wide variety of traffic. Each sector
is staffed by a set of controllers and has a unique radio
frequency that the controller uses to communicate with
the pilots. As aircraft transition from one sector to another,
they are instructed to change to the radio frequency used
by the next sector. Each sector also has secure landline
communications with adjacent sectors, approach controls,
areas, ARTCCs, flight service centers, and military aviation
control facilities.
Figure 2-9. Low altitude sectors.
Figure 2-10. Intermediate altitude sectors.
Figure 2-11. High altitude sectors.
Figure 2-12. Ultra high altitude sectors.
The ARTCC at Fort Worth, Texas is subdivided into sectors
that are categorized as follows:
• Eighteen low altitude sectors. [Figure 2-9]
• Seven intermediate altitude sectors. [Figure 2-10]
• Sixteen high altitude sectors. [Figure 2-11]
• One ultra high altitude sector. [Figure 2-12]
From one to three controllers may work a sector, depending
upon the amount of air traffic. Each controller is assigned
to work the positions within an area of specialization.
Controllers have direct communication with pilots, with
surrounding sectors and Centers, plus the towers and Flight
Service Stations (FSS) under their jurisdiction. Each control
position is equipped with computer input and readout
devices for aircraft flight plan data.
The Center controllers have many decision support
tools (computer software programs) that provide vital
information to assist the controllers in maintaining safe
separation distances for all aircraft flying through their
sector. For example, one tool available allows the controller
to display the extended route of any aircraft on the radar
screen called a vector line. This line projects where the
aircraft will be within a specified number of minutes,
assuming the aircraft does not change its course. This is
a helpful tool to determine if aircraft flying intersecting
routes pass safely within the separation standard, or if they
conflict with each other. In addition to vector lines, the
controller can also display a route line for any given aircraft
on his or her radar screen. This tells the controller where
a particular aircraft is in specified number of minutes, as
well as the path the aircraft will fly to get there. Decision
support tools such as these help each controller look ahead
and avoid conflicts.
In-flight Requirements and Instructions
The CFRs require the pilot in command under IFR in
controlled airspace to continuously monitor an appropriate
Center or control frequency. When climbing after takeoff,
an IFR flight is either in contact with a radar-equipped local
departure control or, in some areas, an ARTCC facility. As
a flight transitions to the en route phase, pilots typically
expect a handoff from departure control to a Center
frequency if not already in contact with the Center.
The FAA National Aeronautical Information Services
publishes en route charts depicting Centers and sector
frequencies. [Figure 2-13] During handoff from one Center
to another, the previous controller assigns a new frequency.
In cases where flights may be out of range, the Center
frequencies on the face of the chart are very helpful. In
Figure 2-13, notice the boundary between Memphis,
Tennessee and Atlanta, Georgia Centers, and the remote
sites with discrete very high frequency (VHF) and ultra high
frequency (UHF) for communicating with the appropriate
NOT FOR NAVIGATION
MEMPHIS
Huntsville
120.8 307.0
ATLANTA
Chattanooga
132.05 353.8
MEMPHISATLANTA
Figure 2-13. Air Route Traffic Control Centers and sector frequencies.
ARTCC. These Center frequency boxes can be used for
finding the nearest frequency within the aircraft range.
They also can be used for making initial contact with the
Center for clearances. The exact location for the Center
transmitter is not shown, although the frequency box is
placed as close as possible to the known location.
During the en route phase, as a flight transitions from one
Center facility to the next, a handoff or transfer of control
is required as previously described. The handoff procedure
is similar to the handoff between other radar facilities, such
as departure or approach control. During the handoff,
the controller whose airspace is being vacated issues
instructions that include the name of the facility to contact,
appropriate frequency, and other pertinent remarks.
Accepting radar vectors from controllers does not relieve
pilots of their responsibility for safety of flight. Pilots must
maintain a safe altitude and keep track of their position,
and it is their obligation to question controllers, request
an amended clearance, or, in an emergency, deviate from
their instructions if they believe that the safety of flight
is in doubt. Keeping track of altitude and position when
climbing, and during all other phases of flight, are basic
elements of situational awareness (SA). Aircraft equipped
with an enhanced ground proximity warning system
(EGPWS), terrain awareness and warning system (TAWS),
or traffic alert and collision avoidance system (TCAS) help
pilots detect and/or correct for potential unsafe proximities
to other aircraft and increases pilot(s) situational awareness.
Regardless of equipment, pilots must always maintain SA
regarding their location and the location of traffic in their
vicinity.
High Altitude Area Navigation Routing
Special high altitude routes allow pilots routing options for
flight within the initial high altitude routing (HAR) Phase
I expansion airspace. Pilots are able to fly user-preferred
routes, referred to as non-restrictive routing (NRR), between
specific fixes described by pitch (entry into) and catch (exit
out of ) fixes in the HAR airspace. Pitch points indicate an
end of departure procedures, preferred IFR routings, or
other established routing programs where a flight can
begin a segment of NRR. The catch point indicates where
a flight ends a segment of NRR and joins published arrival
procedures, preferred IFR routing, or other established
routing programs.
The HAR Phase I expansion airspace is defined as that
airspace at and above FL 350 in fourteen of the western
and southern ARTCCs. The airspace includes Minneapolis
(ZMP), Chicago (ZAU), Kansas City (ZKC), Denver (ZDV),
Salt Lake City (ZLC), Oakland (ZOA), Seattle Centers (ZSE),
Los Angeles (ZLA), Albuquerque (ZAB), Fort Worth (ZFW),
Memphis (ZME), and Houston (ZHU). Jacksonville (ZJX)
and Miami (ZMA) are included for east-west routes only.
To develop a flight plan, select pitch and catch points
which can be found in the Chart Supplement (CS) based
upon your desired route across the Phase I airspace. Filing
requirements to pitch points, and from catch points, remain
unchanged from current procedures. For the portion of the
route between the pitch and catch points, NRR is permitted.
Where pitch points for a specific airport are not identified,
aircraft should file an appropriate departure procedure (DP),
or any other user preferred routing prior to the NRR portion
of their routing. Where catch points for a specific airport
are not identified aircraft should file, after the NRR portion
of their routing, an appropriate arrival procedure or other
user preferred routing to their destination.
Additionally, information concerning the location and
schedule of special use airspace (SUA) and Air Traffic
Control Assigned Airspace (ATCAA) can be found at http://
sua.faa.gov. ATCAA refers to airspace in the high altitude
structure supporting military and other special operations.
Pilots are encouraged to file around these areas when they
are scheduled to be active, thereby avoiding unplanned
reroutes around them.
In conjunction with the HAR program, area navigation
(RNAV) routes have been established to provide for a
systematic flow of air traffic in specific portions of the en
route flight environment. The designator for these RNAV
routes begin with the letter Q, for example, Q-501. Where
those routes aid in the efficient orderly management of air
traffic, they are published as preferred IFR routes.
Preferred IFR Routes
Preferred IFR routes are established between busier airports
to increase system efficiency and capacity. They normally
extend through one or more ARTCC areas and are designed
to achieve balanced traffic flows among high density
terminals. IFR clearances are issued on the basis of these
routes except when severe weather avoidance procedures
or other factors dictate otherwise. Preferred IFR routes are
listed in the CS and can also be found on www.fly.faa.gov,
which requires entering the following data: departure
airport designator, destination, route type, area, aircraft
types, altitude, route string, direction, departure ARTCC,
and arrival ARTCC. [Figure 2-14] If a flight is planned to or
from an area having such routes but the departure or arrival
point is not listed in the CS, pilots may use that part of a
preferred IFR route that is appropriate for the departure or
arrival point listed. Preferred IFR routes are correlated with
departure procedures (DPs) and STARs and may be defined
by airways, jet routes, direct routes between NAVAIDs,
Figure 2-14. Chart Supplement (includes Airport/Facility Directory
section).
waypoints, NAVAID radials/ distance measuring equipment
(DME), or any combinations thereof.
Preferred IFR routes are published in the CS for the low and
high altitude stratum. If they begin or end with an airway
number, it indicates that the airway essentially overlies
the airport and flights normally are cleared directly on the
airway. Preferred IFR routes beginning or ending with a fix
indicate that pilots may be routed to or from these fixes
via a SID route, radar vectors, or a STAR. Routes for major
terminals are listed alphabetically under the name of the
departure airport. Where several airports are in proximity,
they are listed under the principal airport and categorized as
a metropolitan area (e.g., New York Metro Area). One way
preferred IFR routes are listed is numerically, showing the
segment fixes and the direction and times effective. Where
more than one route is listed, the routes have equal priority
for use. Official location identifiers are used in the route
description for very high frequency omnidirectional ranges
(VORs) and very high frequency omnidirectional ranges/
tactical air navigation (VORTACs), and intersection names
are spelled out. The route is direct where two NAVAIDs, an
intersection and a NAVAID, a NAVAID and a NAVAID radial
and distance point, or any navigable combination of these
route descriptions follow in succession.
A system of preferred IFR routes helps pilots, flight crews,
and dispatchers plan a route of flight to minimize route
changes, and to aid in the efficient, orderly management
of air traffic using Federal airways. Preferred IFR routes
are designed to serve the needs of airspace users and
to provide for a systematic flow of air traffic in the major
terminal and en route flight environments. Cooperation by
Figure 2-15 Preferred IFR routes.