Installation Secrets Autopilot/GPS Interface by Scott Howard, Tech Rep With the growing popularity and installation of GPS navigation receivers in all types of aircraft, S-TEC's Tech Support Reps. have received an increase in questions from dealers regarding compatibility, interface, and operation. Now is a good time to mention a few facts regarding the autopilot's relationship to the GPS and to reiterate information on the subject from previous newsletters (Technician's Bench, Sept '96, Setting The Autopilot Gain For GPS Tracking and Installation Secrets, Jan '97, Autopilot NAV Switching). Most panel mounted GPS receivers on the market provide a standard analog DC output (±150 MV = full-scale deflection) for use with an external indicator and/or autopilot. Most hand held units provide only serial data outputs. Although digital to analog converters exist for interconnect, the FAA approves none, to our knowledge, for use. Prior to installation, several aspects of the Autopilot/GPS interface should be considered. Based on factory flight testing and reports from the field, we have 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 annunciating 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. There are numerous ways to wire an autopilot to track or couple signals from multiple navigation sources. The complexity of the job is dependent on both the navigation equipment and the autopilot. The S-TEC System 40/50 can be the easiest to wire for multiple Nav sources since the system has a radio tracker which is not dependent on course input, only on left-right deviation information. Switching from Nav 1/Nav 2/GPS, only requires two double pole toggle switches. S-TEC system 55's, 60's, and 65's that are equipped with radio couplers require additional switch contacts for the additional signals being input (NAV FLAG, LOC SW). As a function of the coupler, these systems incorporate course input in the Nav mode used for calculating intercepts and crosswind correction angles. In an installation with a Directional Gyro, this input is supplied by the heading bug, which should be set by the pilot on the same heading as the OBS course or to the bearing shown in the GPS display. In this type of installation, the Nav switching is not affected by the course input. However, in an installation with a HSI, autopilot-switching between multiple Nav sources is more difficult to configure. The reason is that with an HSI, the course pointer must be placed on the desired heading regardless of where the selected left-right information is displayed. The most common method for coupling the autopilot in a HSI installation is to display two Nav sources on the HSI using a relay switching device and a HSI select switch. Then, the pilot can choose Nav 1 or GPS for the HSI display. The autopilot is then connected directly to the HSI and will track whichever source is selected. If it is not desirable to have the GPS deviation displayed on the HSI, the navigation signals may be switched to the autopilot only, and the HDG bug used as the course input in the GPS position. This may only be accomplished with an HSI that has equal output levels for both HDG and CRS signals. Such as an ST-180 or most AC excited heading systems. An example of this arrangement is show in this figure: After installation be sure to adjust Roll Centering in the Nav Soft track mode with all CRS & Radio inputs centered.

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