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T-34C INSTRUMENTS
CHAPTER SEVEN
2.
There are non-precision approaches which are not suited to
the GPS operational characteristics and which are not included in
the database. Verify the database contains desired approaches for
desired airports prior to flight.
7.
GENERAL PROCEDURE FOR GPS NON- PRECISION APPROACHES
In order to conduct an approach one must be loaded into the active flight plan. Approaches can
only be entered in FPL 0, the active flight plan, and must be loaded prior to the final approach fix.
As the aircraft flies towards the airfield, the "approach arm" mode is activated automatically at
30 NM. At that time, the CDI scale factor smoothly changes from the enroute mode of + 5.0 (NM
not degrees) to the terminal mode of + 1.0 NM over a 30 second transition period. If the GPS
APR switch is pressed when the aircraft is more than 30 NM from the airport, the approach arm
mode is activated but the CDI scale remains + 5 NM until the aircraft is within 30 NM of the
airport. When the approach arm mode is activated the GPS will cue the pilot to press ALT to
update the altimeter setting.
WARNING:
Only the pilot with avionics command can adjust the course into the
CDI and the information will be repeated in the non-avionics
command cockpit CDI. Both cockpits can enter information into
the GPS regardless of avionics command.
The most significant point to understand when using the GPS for non-precision approaches is the
use of the OBS mode. When the GPS is in OBS mode, rotating the OBS knob will rotate the
course select line on the GPS, but will not change course select line when in LEG mode. When
given radar vectors to the FAF, select OBS mode on the GPS, select the FAF as the active
waypoint, and set the final approach course in the CDI. This will display the final approach
course through the FAF so that you can intercept it when being vectored to final. Once
established on the inbound course and prior to the FAF, reselect LEG mode on the GPS. This
will provide the automatic waypoint sequencing required for the approach mode to activate.
The "approach active" mode (indicated by the green ACTV light on the annunciator control
panel) can only be activated by the GPS itself and occurs automatically when they aircraft is
approximately 2 NM from the FAF with an approach loaded into the active flight plan.
Additionally, the pilot must select LEG mode, adequate integrity monitoring must be available to
complete the approach, RAIM must be available at the FAF and MAP, the aircraft must be
heading towards the FAF and the FAF or co-located IAF/FAF must be the active waypoint. If
any of the these conditions are not met, and conditions do not change prior to the FAF, the GPS
will not transition to approach active and a missed approach must be performed. Do not delay the
switch from OBS to LEG mode. A late switch could inhibit the "approach active" mode or
compress the scale factor change on the CDI/RMI. An abrupt scale factor change could be
mistaken for actual deviations on final causing the pilot to make unnecessary corrections. Once
in the approach active mode the CDI scale will change to +0.3 NM. If the GPS APR switch is
INSTRUMENT NAVIGATION 7-35


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