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being vectored or deviating for weather, due to the
proximity to another waypoint in the route. This can
be prevented by placing the receiver in the
nonsequencing mode. When the receiver is in the
nonsequencing mode, bearing and distance are
provided to the selected waypoint and the receiver
will not sequence to the next waypoint in the route
until placed back in the auto sequence mode or the
pilot selects a different waypoint. On overlay
approaches, the pilot may have to compute the along
track distance to stepdown fixes and other points due
to the receiver showing along track distance to the
next waypoint rather than DME to the VOR or ILS
ground station.
m. Conventional Versus GPS Navigation Data
There may be slight differences between the heading
information portrayed on navigational charts and the
GPS navigation display when flying an overlay
2/19/04 AIM
Air Navigation Aids 1−1−37
approach or along an airway. All magnetic tracks
defined by a VOR radial are determined by the
application of magnetic variation at the VOR;
however, GPS operations may use an algorithm to
apply the magnetic variation at the current position,
which may produce small differences in the displayed
course. Both operations should produce the same
desired ground track. Due to the use of great circle
courses, and the variations in m agnetic variation, the
bearing to the next waypoint and the course from the
last waypoint (if available) may not be exactly 180°
apart when long distances are involved. Variations in
distances will occur since GPS distance−to−waypoint
values are along track (straight−line) distances (ATD)
computed to the next waypoint and the DME values
published on underlying procedures are slant range
distances measured to the station. This difference
increases with aircraft altitude and proximity to the
NAVAID.
n. Departures and Instrument Departure Procedures
(DPs)
The GPS receiver must be set to terminal (±1 NM)
CDI sensitivity and the navigation routes contained in
the database in order to fly published IFR charted
departures and DPs. Terminal RAIM should be
automatically provided by the receiver. (Terminal
RAIM for departure may not be available unless the
waypoints are part of the active flight plan rather than
proceeding direct to the first destination.) Certain
segments of a DP may require some manual
intervention by the pilot, especially when radar
vectored to a course or required to intercept a specific
course to a waypoint. The database may not contain
all of the transitions or departures from all runways
and some GPS receivers do not contain DPs in the
database. It is necessary that helicopter procedures be
flown at 70 knots or less since helicopter departure
procedures and missed approaches use a 20:1 obstacle
clearance surface (OCS), which is double the
fixed−wing OCS, and turning areas are based on this
speed as well.
o. Flying GPS Approaches
1. Determining which area of the TAA the
aircraft will enter when flying a “T” with a TAA must
be accomplished using the bearing and distance to the
IF(IAF). This is most critical when entering the TAA
in the vicinity of the extended runway centerline and
determining whether you will be entering the right or
left base area. Once inside the TAA, all sectors and
stepdowns are based on the bearing and distance to
the IAF for that area, which the aircraft should be
proceeding direct to at that time, unless on vectors.
(See FIG 5−4−3 and FIG 5−4−4.)
2. Pilots should fly the full approach from an
Initial Approach Waypoint (IAWP) or feeder fix
unless specifically cleared otherwise. Randomly
joining an approach at an intermediate fix does not
assure terrain clearance.
3. When an approach has been loaded in the
flight plan, GPS receivers will give an “arm”
annunciation 30 NM straight line distance from the
airport/heliport reference point. Pilots should arm the
approach mode at this time, if it has not already been
armed (some receivers arm automatically). Without
arming, the receiver will not change from en route
CDI and RAIM sensitivity of ±5 NM either side of
centerline to ±1 NM terminal sensitivity. Where the
IAWP is inside this 30 mile point, a CDI sensitivity
change will occur once the approach mode is armed
and the aircraft is inside 30 NM. Where the IAWP is
beyond 30 NM from the airport/heliport reference
point, CDI sensitivity will not change until the
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