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RNAV systems were originally used within the en
route environment as a tool for flying conventional
ground−based routes including departure procedures
(DPs) and standard terminal arrivals (STARs).
RNAV terminal procedures are being designed with
RNAV systems in mind. There are several potential
advantages to this including:
1. Routings which save both time and fuel,
2. Reduced dependence on radar vectoring,
altitude, and speed assignments allowing a reduction
in required ATC radio transmissions, and
3. More efficient use of airspace.
b. RNAV Operations. RNAV procedures, such
as DPs and STARs, demand strict pilot awareness and
maintenance of the RNAV procedure course
centerline. Pilots should possess a working knowledge
of their aircraft navigation system to ensure
RNAV procedures are flown in an appropriate
manner. Additionally, pilots should have an understanding
of the various waypoint and leg types used
in RNAV procedures, these are discussed in more
detail below.
1. Waypoints. A waypoint is a predetermined
geographical position that is defined in terms of
latitude/longitude coordinates. Waypoints may be a
simple named point in space or associated with
existing navaids, intersections, or fixes. A waypoint
is most often used to indicate a change in direction,
speed, or altitude along the desired path. RNAV
procedures make use of both fly−over and fly−by
waypoints.
(a) Fly−by waypoints. Fly−by waypoints
are used when an aircraft should begin a turn to the
next course prior to reaching the waypoint separating
the two route segments. This is known as turn
anticipation.
(b) Fly−over waypoints. Fly−over waypoints
are used when the aircraft must fly over the
point prior to starting a turn.
NOTE−
FIG 1−2−1 illustrates several differences between a fly−by
and a fly−over waypoint.
FIG 1−2−1
Fly−by and Fly−over Waypoints
2. RNAV Leg Types. A leg type describes the
desired path proceeding, following, or between
waypoints on an RNAV procedure. Leg types are
identified by a two−letter code that describes the path
(e.g., heading, course, track, etc.) and the termination
point (e.g., the path terminates at an altitude, distance,
fix, etc.). Leg types used for procedure design are
included in the aircraft navigation database, but not
normally provided on the procedure chart. The
narrative depiction of the RNAV chart describes how
a procedure is flown. The “path and terminator
concept” defines that every leg of a procedure has a
termination point and some kind of path into that
termination point. Some of the available leg types are
described below.
2/19/04 AIM
1−2−2 Area Navigation (RNAV) and Required Navigation Performance (RNP)
(a) Track to Fix. A Track to Fix (TF) leg is
intercepted and acquired as the flight track to the
following waypoint. Track to a Fix legs are
sometimes called point−to−point legs for this reason.
Narrative: “via 087 track to CHEZZ WP.” See
FIG 1−2−2.
(b) Direct to Fix. A Direct to Fix (DF) leg is
a path described by an aircraft’s track from an initial
area direct to the next waypoint. Narrative: “left
turn direct BARGN WP.” See FIG 1−2−3.
FIG 1−2−2
Track to Fix Leg Type
FIG 1−2−3
Direct to Fix Leg Type
AIM 2/19/04
Area Navigation (RNAV) and Required Navigation Performance (RNP) 1−2−3
(c) Course to Fix. A Course to Fix (CF) leg
is a path that terminates at a fix with a specified course
at that fix. Narrative: “via 078 course to PRIMY
WP.” See FIG 1−2−4.
FIG 1−2−4
Course to Fix Leg Type
(d) Radius to Fix. A Radius to Fix (RF) leg
is defined as a constant radius circular path around a
defined turn center that terminates at a fix. See
FIG 1−2−5.
FIG 1−2−5
Radius to Fix Leg Type
(e) Heading. A Heading leg may be defined
as, but not limited to, a Heading to Altitude (VA),
Heading to DME range (VD), and Heading to Manual
Termination, i.e., Vector (VM). Narrative: “climb
runway heading to 1500”, “heading 265, at 9 DME
west of PXR VORTAC, right turn heading 360”, “fly
heading 090, expect radar vectors to DRYHT INT.”
3. Navigation Issues. Pilots should be aware
of their navigation system inputs, alerts, and
annunciations in order to make better−informed
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AIM航空信息手册2004上(54)
