The GPS knows exactly what heading changes need to be made and when to make them in order to guide the aircraft around a procedure turn and intercept the final approach course. This is exactly what is happening except that the very skilled pilot happens to be the GPS receiver. Just imagine that a very skilled pilot is operating the HGD bug on the DG and the pilot knows exactly when and how much heading change to make in order to turn smoothly on the next route segment or intercept that route segment if off course. The autopilot has no idea that he is being driven buy the GPS while in GPSS mode. As far as the autopilot is concerned he is just getting heading commands from a HDG bug on the directional gyro.

In order to implement this the GPSS module has a switch to change from HDG mode to GPSS mode. When in NAV mode the autopilot responds to the CDI for lateral guidance (and the CDI glide slope if the autopilot has that capability like my 60-2 does.) When in HDG mode the autopilot would normally respond to the Heading Bug on the Directional Gyro but if GPSS is installed the autopilot actually has two separate functions while in HDG mode: 1) Follow the HDG bug on the DG 2) Follow the GPS Roll Steering commands that are provided by the GPS via ARINC 429 output that is routed to the autopilot GPSS module and converted into Heading Commands as if the HDG bug were providing them. First we need to understand that after installing GPSS the autopilot has two completely different interfaces. As I did this installation myself I am quite familiar with the wiring involved which helps in understanding how the autopilot responds so differently to GPSS (GPS Roll Steering) than it does to the CDI (Course Deviation Indicator). As I recently installed a Garmin GTN 650 and add S-TEC GPSS to my 35 year old S-TEC 60-2 autopilot I thought I might respond also. Give any one of the Duncan Aviation Avionics Tech Reps a call, we are happy to lead you through the traps to determine the correct action.Pugz provided some very good information back on September 29th. When troubleshooting Auto Pilot squawks, donРІР‚в„ўt be discouraged if the answer isnРІР‚в„ўt obvious. We havenРІР‚в„ўt addressed cable tensions or servo motors. A pitch failure could be caused by any number of other parameters that will appear more often during altitude hold, due to the tight reference.Р”Р„s you can see, there is no obvious answer to an Autopilot Altitude Hold INOP squawk and these questions are not all-inclusive. Even though it may appear to be only an altitude hold failure, we need to remember in altitude hold, the A/P is trying to hold a tight reference of altitude against pitch attitude input. If the A/P hunts for the altitude, this could indicate either an altitude hold or pitch channel problem. If the Auto Trim does not come in and take the effort away from the elevator servo after initial correction, the servo may run out of authority to hold the correction and allow the aircraft to drift off. Another thing would be to check the Auto Trim. However, if the flight command bars respond with corrective action when the A/P drifts off but the A/P doesnРІР‚в„ўt respond, I would took into the A/P amp or its connection to the FD computer. If the error can be confirmed as having reached the FD computer, then the computer is faulty. This could be a faulty Air Data Sensor or FD computer.

For instance, if the A/P drifts off the altitude and the flight command bars do not recognize it and give the command to correct, the altitude error from the Air Data Sensor is not getting to the flight computer. If the A/P is drifting off altitude, first determine of the flight command bars are responding appropriately. If both signals are being transmited correctly to the FD computer, you can assume the FD computer is faulty. There are two main things required by the Flight Director (FD) computer in order for the Altitude Hold Mode to engage: 1) Mode Select voltage, and 2) Valid logic from the Air Data Sensor. If the altitude mode does not show engaged, the next step is to determine if the fault lies with the flight computer or air data. Below are some of the basic questions we ask that lead us in the right direction towards a correct solution. Altitude Hold INOP is a common A/P squawk that we encounter nearly every day. It involves a rather complicated decision tree that includes several steps and scenarios. Common Troubleshooting Questions for Autopilot Squawk РІР‚вЂњ Altitude Hold INOP March 2012Р”Р‡iguring out the cause of an autopilot (A/P) squawk is more involved than just looking up the symptoms and determining a course of action.

Stec 50 autopilot altitude hold