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referred to as stand-alone GPSs. They are considered
nonprecision approaches, offering only LNAV and
circling minimums. Precision minimums are not
authorized, although LNAV/VNAV minimums may
be published and used as long as the on-board system
is capable of providing approach approved VNAV.
The RNAV (GPS) Runway 18 approach for
Alexandria, Louisiana incorporates only LNAV and
circling minimums. [Figure 5-36]
Figure 5-35. Traditional GPS Overlay Approach.
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For a non-vertically guided straight-in RNAV (GPS)
approach, the final approach course must be aligned
within 15° of the extended runway centerline. The final
approach segment should not exceed 10 NM, and when
it exceeds 6 NM, a stepdown fix is typically incorporated. A minimum of 250 feet obstacle clearance is also
incorporated into the final approach segment for
straight-in approaches, and a maximum 400-foot per
NM descent gradient is permitted.
The approach design criteria are different for
approaches that use vertical guidance provided by a
Baro-VNAV system. Because the Baro-VNAV guidance is advisory and not primary, Baro-VNAV
approaches are not authorized in areas of hazardous ter-
rain, nor are they authorized when a remote altimeter
setting is required. Due to the inherent problems associated with barometric readings and cold temperatures,
these procedures are also temperature limited.
Additional approach design criteria for RNAV
Approach Construction Criteria can be found in the
appropriate Order 8260 series directives.
RNAV (GPS) APPROACH USING WAAS
WAAS was commissioned in July, 2003, with initial
operational capability (IOC). Although precision
approach capability is still in the future, initial WAAS
currently provides a new type of approach with vertical
guidance (APV) known as LPV. Approach minimums as
low as 200 feet HAT and 1/2 SM visibility are possible,
Figure 5-36. Alexandria International (KAEX), Alexandria, Louisiana, RNAV (GPS) RWY 18.
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even though LPV is semi-precision, and not considered
a precision approach. WAAS covers 95 percent of the
country 95 percent of the time.
NOTE: WAAS avionics receive an airworthiness
approval in accordance with Technical Standard Order
(TSO) C-145A, Airborne Navigation Sensors Using
the (GPS) Augmented by the Wide Area Augmentation
System (WAAS), or TSO-146A, Stand-Alone Airborne
Navigation Equipment Using the Global Positioning
System (GPS) Augmented by the Wide Area
Augmentation System (WAAS), and installed in
accordance with AC 20-130A, Airworthiness
Approval of Navigation or Flight Management
Systems Integrating Multiple Navigation Sensors, or
AC 20-138A, Airworthiness Approval of Global
Positioning System (GPS) Navigation Equipment for
Use as a VFR and IFR Navigation System.
Precision approach capability will become available
when LAAS becomes operational. LAAS further
increases the accuracy of GPS and improves signal
integrity warnings. Precision approach capability
requires obstruction planes and approach lighting systems to meet Part 77 standards for ILS approaches.
This will delay the implementation of RNAV (GPS)
precision approach capability due to the cost of certifying each runway.
ILS APPROACHES
Notwithstanding emerging RNAV technology, the ILS
is the most precise and accurate approach NAVAID
currently in use throughout the NAS. An ILS CAT I
precision approach allows approaches to be made to
200 feet above the TDZE and with visibilities as low
as 1,800 RVR; with CAT II and CAT III approaches
allowing descents and visibility minimums that are
even lower. Nonprecision approach alternatives cannot
begin to offer the precision or flexibility offered by an
ILS. In order to further increase the approach capacity
of busy airports and exploit the maximum potential of
ILS technology, many different applications are in use.
A single ILS system can accommodate 29 arrivals
per hour on a single runway. Two or three parallel
runways operating consecutively can double or triple
the capacity of the airport. For air commerce this
means greater flexibility in scheduling passenger
and cargo service. Capacity is increased through the
use of parallel (dependent) ILS, simultaneous parallel (independent) ILS, simultaneous close parallel
(independent) ILS, precision runway monitor
(PRM), and converging ILS approaches. A parallel
(dependent) approach differs from a simultaneous
(independent) approach in that the minimum distance
between parallel runway centerlines is reduced; there is
no requirement for radar monitoring or advisories; and a
　
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