AIM 

4/20/23

 

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28

Navigation Aids

(a)

GPS overlay approaches are designated non

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precision instrument approach procedures that pilots are

authorized to fly using GPS avionics. Localizer (LOC), localizer type directional aid (LDA), and simplified
directional facility (SDF) procedures are not authorized. Overlay procedures are identified by the “name of
the procedure” and “or GPS” (e.g., VOR/DME or GPS RWY 15) in the title. Authorized procedures must be
retrievable from a current onboard navigation database. The navigation database may also enhance position
orientation by displaying a map containing information on conventional NAVAID approaches. This approach
information should not be confused with a GPS overlay approach (see the receiver operating manual, AFM,
or AFM Supplement for details on how to identify these approaches in the navigation database).

NOTE

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Overlay approaches do not adhere to the design criteria described in paragraph 5

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4

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5m, Area Navigation (RNAV)

Instrument Approach Charts, for stand

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alone GPS approaches. Overlay approach criteria is based on the design criteria

used for ground

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based NAVAID approaches.

(b)

Stand

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alone approach procedures specifically designed for GPS systems have replaced many of the

original overlay approaches. All approaches that contain “GPS” in the title (e.g., “VOR or GPS RWY 24,” “GPS
RWY 24,” or “RNAV (GPS) RWY 24”) can be flown using GPS. GPS

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equipped aircraft do not need underlying

ground

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based NAVAIDs or associated aircraft avionics to fly the approach. Monitoring the underlying approach

with ground

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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, TSO-C129() and TSO-C196()

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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 GPS-based 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 (baro-VNAV) equipment;

(3)

RNP 0.3 DA on an RNAV (RNP) IAP, if they are specifically authorized users using approved

baro-VNAV 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

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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 5-4-5d, Terminal
Arrival Area (TAA), for complete information).

(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 

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5 NM either side of centerline to 

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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