GPS Overlay Approaches

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