The Technician's Bench
Setting The Autopilot NAV Gain For GPS Coupling & Tracking
by Scott Howard, Technical Representative
Based on factory flight testing and reports from the field, it has been determined that during enroute tracking and coupling using a GPS signal, autopilot tracking performance will be enhanced by utilizing a higher NAV sensitivity level than normally used with a VOR signal.
This is easily accomplished with System 40's, 50's and 55's by engaging the Approach Mode ("APR"). This provides an intermediate gain level (CAP SOFT) as opposed to the VOR low gain (SOFT).
System 60's and 65's however, do not allow the manual engagement of the "APR" mode, which is activated only by the ILS energize signal (LOC Switch) from the NAV receiver.
The roll flight guidance computer in a System 60-1, 60-2 or 65 can be instructed to remain in approach logic mode gain status without enunciating "APR" or enabling Glideslope functions while coupled to the GPS.
NOTE: Contact switching should occur in conjunction with the NAV/GPS select switch, either on the mechanical switch or the remote relay switch.If annunciation of the "APR" light is desired during GPS Approaches, the "APR" logic signal from the GPS (if available) may be switched to the AP LOC input. Under these conditions, the GS flag inputs to the pitch computer must be opened to avoid possible GS enabling. Note: The LOC switch input must be logic low to activate.
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