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GPS Overlay Approaches - P-8710169

 

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T-6A INSTRUMENT NAVIGATION
CHAPTER EIGHT
Enroute Mode. Enroute phase, prior to the execution of the instrument approach, the display
sensitivity full-scale deflection is 5 NM either side of centerline.
Terminal Approach Mode. Upon activation of the approach mode, the display sensitivity should
smoothly transition from a full-scale deflection of 5 NM either side of centerline to 1 NM either
side of centerline within 30 NM of the destination airport. The approach mode must be active to
proceed past the FAF on a non-precision approach.
Final Approach Mode. At a distance of 2 NM inbound to the FAF waypoint, the display
sensitivity begins to transition to a full-scale deflection of 0.3 NM either side of centerline.
Some GPS avionics may provide an angular display between the FAF and MAP that
approximates the course sensitivity of the LOC portion of an ILS.
Missed Approach Mode. When navigation to the missed approach holding point is activated, the
CDI display sensitivity transitions back to terminal area sensitivity (+1 NM).
GPS OVERLAY APPROACHES
Overlay approaches consist of GPS waypoints overlaid on conventional (VOR, VOR/DME, etc.)
non-precision approaches. Though overlays have the same minimums as the underlying
approach, they offer the opportunity to fly an IAP using GPS. Because GPS approaches are
designed as a TO-TO system (meaning you navigate to one fix, cross it and proceed to the next
fix), overlays are not a perfect fit with the underlying procedure. Some conventional approaches,
for example, have no FAFs, so GPS would not be able to navigate to a fix, nor would it know
when to switch the CDI sensitivity. This problem is solved by inserting synthetic, GPS-specific
"sensor FAFs" on approach overlays when required. There are various GPS overlay approach fix
naming standards involving characters and numbers. It is not critical that the student memorize
the naming methods, however, it is important to recognize the GPS database uses this protocol.
Listed below are examples of GPS overlay fixes the student may see as they scroll through the
fix list:
D006J - A DME arc fix, with the "D" meaning DME, 006 the radial, and "J" the distance, 10
miles (J is the 10th letter of the alphabet).
Along track fix at DME 19
ORW19
FF142
FAF on 142 course
Missed approach fix on 142 course
MA142
RW14
MAP, runway 14
CF151
Capture or course fix on the 151 radial
If any of the above listed fixes are an actual 5-letter named fix, they will have a dash and a small
letter suffix at the end of the waypoint. These suffixes are intended to assist the pilot in
GLOBAL POSITIONING SYSTEM
8-3


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