Autopilot Nav Switching
by Scott Howard
Tech RepThere 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 Sys-tem 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. To select from Nav 1/Nav 2/GPS or Loran all that is required is two double pole toggle switches. (See your S-TEC Dealer)
The same functions can be accomplished with a single rotary switch which, when using a third pole, could also provide an indicator light to display GPS/Loran selected.
S-TEC System 55's, 60's and 65's which are equipped with radio couplers require additional switch contacts for the additional signals being input.
As a function of the coupler these systems incorporate a 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/Loran display. In this installation the Nav switching is not affected by the course input.
However, in an installation with an 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 an HSI installation is to display two Nav sources on the HSI using a relay switching device and an HSI select switch. Then the pilot can choose Nav 1 or GPS/Loran for the HSI display. The autopilot is then connected directly to the HSI and will track whichever source is selected.
When you also want to select Nav 2 it is a little more challenging. Unless the Nav 2 indicator contains a course synchro, the HSI course arrow will be the constant input to the autopilot in Nav mode.
You have two options for incorporating Nav 2 coupling. The first is to switch the heading bug output to the course input when Nav 2 is selected ( this can only be accomplished with a compass system HSI that has equal output levels for Heading and Course signals). That makes the HSI operation similar to that of a DG in Nav 2 position.
The second option is to remove the course input when Nav 2 is selected. This will reduce the functionality of the coupler to that of a tracker on Nav 2, meaning the pilot must place the aircraft on course before engaging Nav mode. One positive of this configuration is that it provides a backup in the event of a failure of the HSI.
If you have questions on heading systems or navigational system interfacing give us a call at S-TEC Technical Support any time.
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