Navs, Lorans & GPSs

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Let's try to simplify all these new navigational wonders that are available to aircraft owners today, and how to interface them to the autopilot. First, let's understand how an autopilot will use the information provided by the nav receiver. The receiver will output a voltage of +/- 150mvdc to indicate left or right deviation. This voltage is applied to the radio inputs of the autopilot. At this point all the autopilot knows is if it's supposed to bank left or right, or if the signal is 0 Vdc it will level the wings. This type of autopilot is known as a "tracker". Our Century I autopilot is this type (if it has the tracker board installed). Tracking is good, but that has it's limitations. Why? If the autopilot only knows it's supposed to bank left or right, it's possible for the autopilot to put the aircraft into a circular pattern, orbiting somewhere to the left or right of the radial you wanted it to track. Because the autopilot doesn't know at what heading it should level out at in order to start tracking that signal it must keep banking the aircraft in a certain direction until that radio signal becomes 0 Vdc. Therefore it has no intercept or crosswind capabilities.

So how does a system have intercept capabilities? What is required is for the autopilot to know what direction it needs to fly in order to get to that radial you wanted to track. You tell it by giving it a heading that corresponds to the radio you wanted to track. Most autopilots have an intercept angle of 45°. The polarity of the radio signal and heading signal inside the autopilot is such that they cancel out each other. Therefore you have the intercept capability.

Crosswind is the ability of the autopilot to keep the left-right needle centered and ignoring the airplane's true heading. Using an HSI is even simpler than a DG due to the fact you only have to turn the course knob in order to select the radial and course direction. So far so good using the VOR, but what about LORAN and GPS? They output left-right information just like the VOR does, except they also give you a bearing to fly. This means you must turn your heading bug on your DG, or course knob on your HSI to that bearing in order for the autopilot to track to that waypoint. One other point to keep in mind is that some LORANs and GPSs have programmable course widths. This is measured in nautical miles of full scale deviation. The range can be anywhere between 5 miles down to 1/4 a mile or lower full scale. Having a course width too wide would result in the autopilot being sluggish tracking the radio. The opposite would result if the course width was too narrow.

Interfacing these systems into the autopilot is not very difficult if you use a little common sense as to what will be required. Some autopilots only require left-right signal while others also need a nav flag signal. If the autopilot has a DG, it's just a matter of using a good quality switch or an electronic switch box and annunciator. If the system is using an HSI it's a good idea to present all navigational information on the HSI for the particular nav unit you want to use. As an example, if you are trying to switch NAV1 and NAV2 you will have 8 resolver stator wires, 4 resolver rotor wires, 4 nav flag wires and 4 to-from wires to properly display the information. Anything else could be confusing to the pilot.