Technician's Bench Autopilot GPS Gain Select for 40's & 50's in the field by Scott Howard, Tech Rep In the January 1998 issue of the APPROACH, the "INSTALLATION SECRETS" article addressed the subject of Autopilot/GPS Gain. It was noted that all S-TEC Systems Twenty & Thirty and Systems 40 & 50 in current production include provisions for radio gain enhancement when a GPS navigation source is selected for Autopilot track. This generated great response from S-TEC dealers to our technical support group. Many wanted to know if this feature could be incorporated into the thousands of System 40's and 50's already in the field. The answer is yes. Typically, when using "APR" mode on the autopilot, systems 40/50 will adequately track most GPS receiver signals. In some cases however, due to a multitude of variables, the autopilot may have a tendency to let the aircraft drift slightly to one side of the course or the other before correcting. This occurs because a GPS signal is much "cleaner" than a VOR signal, which has a tendency to be "noisy" and therefore generates more rate. The new gain enhancement feature provides quicker response to slight course offsets. Upgrading an older 40/50 unit requires replacement of the Roll Flight Guidance Computer PCB (Ref: SL 98-001 - list price $204.00) and a minor modification to the controller motherboard. The NAV/GPS switch must then be rewired in aircraft as shown below. The exact part number of the 03118-( ) PCB can be determined by the first dash # of the unit's model #. Example: System 40, 0129-1 requires 03118-1 PCB. System 50, 0131-2-1 requires 03118-2 PCB. A P/N with no dash or -0 requires a plain 03118 PCB. Be sure to specify which HDG System type is in the aircraft when ordering. And don't hesitate to call S-TEC Product Support for modification instructions.

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