INSTALLATION SECRETS Adding GPSS to Existing S-TEC Autopilots by Scott Howard, Manager, Customer Service With the great interest generated by the news of S-TEC's GPSS Converter, Product Support has received many calls from Dealers with questions regarding installation. Many callers want to know the wiring requirements for adding it to an existing S-TEC autopilot. The GPSS Converter converts the 429 data from the GPS receiver into an analog DC steering signal usable for the S-TEC flight control systems. (See Jim's Fly 'Em Better, page 3, for more information on how GPSS works.) When a GPS Receiver is installed in an aircraft, the crosstrack deviation (left-right) signal output can be switched to the autopilot's NAV inputs to achieve GPS tracking. Figure 1 shows a simplified wiring configuration for that switch. Figure 1. Typical Autopilot Wiring Using the new S-TEC GPSS Converter requires both an autopilot computer modification and a slight wiring change. That modification occurs in the roll computer, but this can be performed in the field with instructions that will be provided with the GPSS installation kit. S-TEC's autopilots will interface to a multitude of electronic Heading or Compass systems, including numerous DGs and HSIs. A typical wiring interface is configured for either AC or DC signal inputs to the roll computer, which is internally modified for the heading system as required to obtain usable HDG/CRS information. With the Converter, the GPSS signal to the autopilot will be DC, and the heading system's modification will take place in the GPSS Converter unit. Either AC to DC or DC to DC conversion will be performed, depending on heading system type. The converter unit will be modified at the factory as requested before shipment. Provisions will also be made for field modification if necessary, in the event of heading system change for instance. In the aircraft, existing wiring between heading system and autopilot must be intercepted and routed through the converter. The GPS Receiver must provide ARINC 429 to the Converter. Figure 2 shows simplified wiring using GPSS. (Note: Disregard "CRS" signal for systems Twenty, Thirty, 40, and 50.) Figure 2. Autopilot/GPSS Wiring NAV switching remains the same, and the Converter switch will now allow the pilot to select heading bug or GPS roll steering for autopilot operation in the "HDG" mode. As you can see, adding the GPSS function to an existing S-TEC autopilot is a relatively simple task.