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UNDERSTANDING GPS APPROACHES
The GPS 155 is the first GPS certified to meet the requirements of TSO C-129
Category A1. When using the GPS 155 for non-precision approaches, you’ll
encounter two types of approaches: overlay approaches (based on existing proce-
dures) and GPS approaches (new approaches designed specifically for GPS). The FAA
has approved a large number of overlay approaches, with plans to add as many GPS
approaches as possible to take advantage of the safety and convenience of GPS.
Flying a GPS approach is not difficult, and varies from conventional approaches
only in the operation of the equipment being used. Although you’ll often be following
the same flight path used in conventional approaches, the equipment operation pro-
cedures will be different from typical NDB or VOR approaches.
The following definitions, guidelines and examples will help you understand the
basic rules of GPS approaches, and offer four examples to guide you through some
typical approaches. Think through the approach examples— this will assist you in
getting the greatest benefit from the GPS 155.
Basic Rules of GPS Approaches
There are a few basic rules that apply to all GPS approaches. Remembering these
rules will assist you in understanding the approach procedures and ensure the great-
est margin of safety for your flight.
• The approach to be flown must be in the aviation database, and the database
must be current.
• You may select the desired approach and arm the approach mode at any
time after the destination airport is selected. If the GPS APPR switch is not set
to ARM within 2 nm inbound to the final approach fix, the approach will not
become active, which precludes descent at the FAF.
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APPROACH NAVIGATION
Understanding GPS Approaches
TSO C-129 requires that satellite coverage and
navigational accuracy provided by the GPS sys-
tem meets minimum standards.
A ‘No RAIM from FAF to MAP’ message will
appear if RAIM is predicted to be unavailable for
an approach. Do not plan on conducting a GPS
approach. Refer to Appendix C for a complete list
of RAIM messages and page 111 for more infor-
mation on RAIM during approach navigation.
GPS approaches must be in the current aviation
database to be approved.
155man (GOOD) 7/24/98 1:57 PM Page 71