As of X-plane 10.30+, there are many options to configure the autopilot of a plane, and especially where this autopilot gets its input data from.
Some of these options have been available since much earlier versions, but they are not widely used correctly by designers, so they deserve a more thorough explanation.
Attitude source
This is the most important setting. It allows you to select AHRS, electric gyro or vacuum gyro. It determines under which circumstances the autopilot will stay functional in abnormal situations.
- AHRS – choose this option when you are making a modern plane with G1000 or Avidyne glass panels or when making a modern airliner. The autopilot will use the electronic attitude and heading reference system to determine steering commands. AHRS only requires electric power, so it depends on a power source, alternator or battery, to function. AHRS is pretty robust as it works entirely without moving parts, using solid-state accelerometers and rotation rate sensors. The latter are sometimes called gyroscopes, but are really solid-state electronics and have no rotating parts.
- vacuum gyro – choose this option when modeling a general aviation aircraft with vacuum gyros equipped with an attitude-based two- or three-axis autopilot. Modern autopilots like the S-TEC Fifty Five are examples of this kind of autopilot. This system uses air pressure (or, lack thereof) to drive mechanical gyros which provide attitude information to the pilot’s attitude indicator instrument and the autopilot. Vacuum systems are usually less reliable and vacuum pumps tend to malfunction at the worst possible times.
- electric gyro – choose this option when making a general aviation plane equipped with a rate-based two-axis autopilot like the popular Bendix-King KAP-140. This option is very popular in real-world installations, as the autopilot will still work with a vacuum failure! It will of course stop working in case of a complete electrical failure, that is with a broken generator and a depleted battery. The electric gyro in the turn coordinator is very reliable and provides bank rate and turn rate information to the autopilot, thus giving an indirect indication of bank angle. Because of that it can only provide two, but not three-axis guidance.
Heading source
This determines what provides heading information to the autopilot and the kind of performance to expect from that:
- AHARS – electronic AHARS systems determine yaw rate by solid-state gyroscopes and use a fluxgate sensor to detect the earth’s magnetic field. They provide extremely accurate and reliable information on the magnetic heading of the airplane. Choose this option when making a modern glass-cockpit GA aircraft or an airliner.
- vacuum gyro – the directional gyro found in most non-glass general aviation planes is driven by the vacuum system, just like the attitude indicator. These directional gyros are not slaved to fluxgate sensors, but must be manually adjusted by the pilot, by comparing their indication to the magnetic compass in unaccelerated straight-and-level flight. This must be done to compensate for gyro precession errors that inevitably occur. With failure of the vacuum pump, this kind of heading indicator will stop working completely. Choose this option for non-glass general aviation planes with vacuum systems.
- electric gyro: This kind of gyro works like the vacuum one in that it is not fluxgate-slaved, but rather than using the vacuum system to spin the gyro, it uses an electric motor for that, which is often more reliable. Choose this option when making a general-aviation plane without a vacuum system, like early Cirrus SR-20s and -22s before they were equipped with glass-cockpits.
This is a new option as of X-Plane 10.30, with an additional setting added in X-Plane 11. To intercept and track a nav source, like a VOR radial, a localizer, or a GPS track, the autopilot must know the radial or DTK (desired track). This angle information can come from different sources with different types of autopilots, so you can choose the one that most closely reflects the setup of the airplane you are modelling:
- GPS/LOC: the autopilot takes course to intercept in nav/loc mode from either GPS or localizer and from OBS selector when tracking a VOR radial. The autopilot will know what course to track and intercept when flying a localizer or GPS track, even if the HSI is misplaced. This is what is usually found in modern airplanes, with autopilots like G1000 or S-Tec Fivty Five or in airliners. This type of autopilot is capable of tracking and coupling, and can do dual-mode intercepts. This is the default setting that X-Plane 9 always used. When combining this type of autopilot with the new X-Plane 430 GPS navigator, it supports GPSS-mode (GPS steering) with turn anticipation and can also fly DME arcs or fixed-radius RNAV legs.
- OBS: the autopilot takes the course to intercept in nav/loc/gps mode from the OBS setting of the selected nav source and the associated HSI or CDI. Here it is very important that the pilot selects the correct front course on the HSI/CDI in order for the autopilot to track localizers or a GPS track correctly. General aviation autopilots like most Centuries and the Bendix/King KAP-140 work like this when the plane is equipped with an HSI. Tracking/coupling and dual-mode intercepts are supported. In combination with the new 430 GPS navigator it has no GPSS support and no automatic turn anticipation. Automatic flying of DME arcs is not possible unless the pilot adjusts the HSI manually.
- HDG: the autopilot takes the course to intercept in nav/loc mode from the heading bug, even when tracking a VOR radial. It is important that the pilot always sets the heading bug to the desired front course, otherwise the autopilot cannot intercept and track a radial, localizer, or GPS track correctly. Tracking and coupling works when the correct front course is selected using the heading bug. Dual-mode intercepts are not possible with that type of autopilot, nor is GPSS. Simpler and older general aviation autopilots work like this. The popular Bendix-King KAP-140 works like this when no HSI is installed in the plane.
- The HDG with GPSS mode has been replaced by two new types of using GPSS, which are available with the autopilots of X-Plane 11.30.
HDG with GPSS: This setting is new as of X-Plane 11. The autopilot takes the course to intercept in VOR nav/LOC app mode from the heading bug when tracking a VOR radial or localizer. It is important that the pilot always sets the heading bug to the desired front course, otherwise the autopilot cannot intercept and track a radial or localizer. Tracking and coupling works when the correct front course is selected using the heading bug. Dual-mode intercepts to VOR radials and localizers are not possible with that type of autopilot. However, when the navigation source is the GPS, the autopilot gets GPS steering (GPSS) information and thus works with turn anticipation and can also fly DME arcs or fixed-radius RNAV legs. The S-Tec Fivty-Five autopilot works like this when installed in an airplane which does not have an HSI. - None: The autopilot gets no information at all on front course or desired track in any mode. This is a very old and basic type of autopilot that can only track a VOR radial once captured, but not intercept it. It only reacts to raw CDI deflection. Classic Sperry autopilots found in vintage planes worked like this. Obviously, only course tracking is possible, but not coupling or any kind of automatic intercept or even GPSS.