AIM
12/2/21
ground
−
based NAVAIDs or associated aircraft
avionics to fly the approach. Monitoring the
underlying approach with ground
−
based NAVAIDs is
suggested when able. Existing overlay approaches
may be requested using the GPS title; for example,
the VOR or GPS RWY 24 may be requested as “GPS
RWY 24.” Some GPS procedures have a Terminal
Arrival Area (TAA) with an underlining RNAV
approach.
(c)
For flight planning purposes,
TSOC129() and TSOC196()
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equipped users
(GPS users) whose navigation systems have fault
detection and exclusion (FDE) capability, who
perform a preflight RAIM prediction for the
approach integrity at the airport where the RNAV
(GPS) approach will be flown, and have proper
knowledge and any required training and/or
approval to conduct a GPSbased IAP, may file
based on a GPS
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based IAP at either the destination
or the alternate airport, but not at both locations. At
the alternate airport, pilots may plan for:
(1)
Lateral navigation (LNAV) or circling
minimum descent altitude (MDA);
(2)
LNAV/vertical navigation (LNAV/
VNAV) DA, if equipped with and using approved
barometric vertical navigation (baroVNAV) equip
ment;
(3)
RNP 0.3 DA on an RNAV (RNP) IAP,
if they are specifically authorized users using
approved baroVNAV equipment and the pilot has
verified required navigation performance (RNP)
availability through an approved prediction program.
(d)
If the above conditions cannot be met, any
required alternate airport must have an approved
instrument approach procedure other than GPS
−
based that is anticipated to be operational and
available at the estimated time of arrival, and which
the aircraft is equipped to fly.
(e) Procedures for Accomplishing GPS
Approaches
(1)
An RNAV (GPS) procedure may be
associated with a Terminal Arrival Area (TAA). The
basic design of the RNAV procedure is the “T” design
or a modification of the “T” (See Paragraph 545d,
Terminal Arrival Area (TAA), for complete informa
tion).
(2)
Pilots cleared by ATC for an RNAV
(GPS) approach should fly the full approach from an
Initial Approach Waypoint (IAWP) or feeder fix.
Randomly joining an approach at an intermediate fix
does not assure terrain clearance.
(3)
When an approach has been loaded in
the navigation system, 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 not
already 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 and the approach is armed, the CDI
sensitivity will not change until the aircraft is within
30 miles of the airport/heliport reference point.
Feeder route obstacle clearance is predicated on the
receiver being in terminal (
±
1 NM) CDI sensitivity
and RAIM within 30 NM of the airport/heliport
reference point; therefore, the receiver should always
be armed (if required) not later than the 30 NM
annunciation.
(4)
The pilot must be aware of what bank
angle/turn rate the particular receiver uses to compute
turn anticipation, and whether wind and airspeed are
included in the receiver’s calculations. This informa
tion should be in the receiver operating manual. Over
or under banking the turn onto the final approach
course may significantly delay getting on course and
may result in high descent rates to achieve the next
segment altitude.
(5)
When within 2 NM of the Final
Approach Waypoint (FAWP) with the approach
mode armed, the approach mode will switch to
active, which results in RAIM and CDI changing to
approach sensitivity. Beginning 2 NM prior to the
FAWP, the full scale CDI sensitivity will smoothly
change from
±
1 NM to
±
0.3 NM at the FAWP. As
sensitivity changes from
±
1 NM to
±
0.3 NM
approaching the FAWP, with the CDI not centered,
the corresponding increase in CDI displacement
may give the impression that the aircraft is moving
further away from the intended course even though it
is on an acceptable intercept heading. Referencing the
digital track displacement information (cross track
1
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1
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Navigation Aids