X-Plane 11.30 offers to equip planes with preconfigured autopilots, in addition to the many configurable options of previous X-Plane versions. Some of the available autopilots come with additional features.
On the Systems page of Plane Maker, you can select the autopilot from the drop-down list. Depending on your selection, additional options in the form of checkboxes and radio buttons will or will not be available for further customization. The options to select from are:
- X-Plane Custom: This is the backward-compatible option that all planes created prior to 11.30 default to. It gives you the full customization of the X-Plane autopilot, and allows you the finest grained control.
- Airliner: Like X-Plane Custom, but with the addition of new auto throttle modes suitable for airliners, flight director selection instead of source selection, auto land capability and a different behaviour of CWS mode, explained below.
- GFC-700: Default autopilot of the Garmin G1000 digital flight deck. This hides other configuration options and configures the autopilot internally to behave like the Garmin GFC-700, which is a high-end position-based digital autopilot.
- S-Tec 55: High-end general aviation dual-axis rate-based digital autopilot. Hides other configuration options and configures the autopilot to behave like an S-Tec 55. Most notably, this autopilot does not have buttons with toggle logic, so you cannot press the button of an active mode to go back to a default mode. You always have to select a new mode to cancel an old mode. To go back to attitude hold, use the Control Wheel Steering button. This autopilot supports GPSS, by pressing the NAV button twice.
- S-Tec 55 with alt preselector: Like the S-Tec 55, but with an altitude pre-selector that allows arming altitude capture. Note that the pre-selector takes its pressure altitude from the static system in X-Plane and needs to be baro-calibrated in addition to the pilot altimeter, or it will level off at surprising altitudes. The altimeter setting for the altitude preselector is on the dataref
Note that unlike the GFC700, vertical speed mode does not default to arm the altitude capture. In order to engage vertical speed with armed altitude capture, press the command
(on the real autopilot, this is accomplished by pressing the VS and ALT buttons both simultaneously).
- KAP-140 single-axis: Hides other configuration options and configures the autopilot to behave like this single-axis general aviation rate-based autopilot. This acts on the roll axis only, does not have an elevator or trim servo, and defaults to zero turn rate (wings level) for roll mode. This autopilot supports GPSS through the heading mode (see below)
- KAP-140 dual axis: dual-axis general aviation rate-based autopilot. Adds vertical speed hold and altitude hold to the functions of the KAP-140.
- KAP-140 dual axis with alt preselector: Like the dual-axis KAP-140, but with an altitude pre-selector that allows arming altitude capture. Note that the pre-selector takes its pressure altitude from the static system in X-Plane and needs to be baro-calibrated in addition to the pilot altimeter, or it will level off at surprising altitudes. The altimeter setting for the altitude preselector is on the dataref
Unlike the S-Tec with alt preselection, the KAP will always auto-arm altitude capture when you dial in a new target altitude.
- Piper Autocontrol: Hides other configuration options and configures the autopilot to behave like this generic low-tech non-microprocessor rate-based autopilot. Has the usual dual-axis modes, but does not have any logic for automatic mode reversions. Will not change modes on its own, does not have advanced logic like dual-mode intercepts or altitude capture.
At runtime, the type of installed auto-pilot can be read from
with the number corresponding to the above list. Note that this cannot be changed at runtime.
Rate-based versus Position-based
The GFC-700 autopilot is position-based, S-Tec, KAP and Piper autopilots are rate-based. The custom autopilots can be configured to be either. Position-based autopilots calculate their steering cues based on the attitude gyro, and hold a roll angle and a pitch angle. Rate-based autopilots calculate their steering cues based on the turn coordinator gyro and the absolute pressure transducer, they hold turn rates instead of bank angles and vertical speeds instead of pitch angles.
The desired turn rate is reflected in the
dataref, and the status of this roll mode is reflected in the
Auto throttle modes
If the autopilot is of type airliner, the auto throttle system gains additional modes that are reflected in the
- Arm: Auto throttle ready, but not engaged. The throttles can be moved freely.
- Speed Hold: The auto-throttle tries to achieve the selected indicated airspeed or mach number.
- Thrust Reference: The auto-throttle tries to achieve the set target N1 value per engine:
or the set target EPR value per engine:
The throttles are driven by a PID-controller off the N1 indication or EPR indication for each engine. The PID constants can be set on the Plane Maker “custom autopilot constants” preference pane, just like other autopilot constants.
The primary parameter can be selected N1 or EPR on the engine limits pane of Plane Maker.
- Retard: The throttles gently retard to flight idle
- Reserved: Reserved for future modes.
Note that if the autopilot is of type airliner, it will automatically use the throttle REF and RETARD modes for level change climbs and descends.
The autopilot interacts with pitch in two ways: First, it directly acts on the pitch axis, thus actuating the elevator. The amount of input on the elevator axis can be seen in
. Then, it actuates the elevator trim, to eliminate the force acted on the elevator. This is to achieve both quick reactions to small disturbances and long term stability without excessive control forces. When changing the trim, it is limited by the speed of the electric pitch trim,
. Thus with a fast trim, it is not unusual to see only minuscule control deflection on servo_pitch_ratio.
In X-Plane 11.30, it is possible to switch off the trim servo, but keep the elevator servo by using the dataref
. This allows simulating autopilots without trim servos, that need pilot interaction to trim excessive forces. In this case, the dataref sim/cockpit2/autopilot/pitch_mistrim will alert the pilot of trim needed (-1 for trim down, +1 for trim up, 0 for no trim).
Source selection and dual channel
The airliner type autopilot differs from the other types in the way it deals with on-side vs cross-side sources.
The non-airliner type autopilot has one flight director and one set of servos that work based off this flight director, and the
dataref selects whether the sole flight director works on the captain’s or copilot’s instruments (altimeter, radios, HDG bug, etc..).
The airliner type autopilot instead has two sets of flight directors and two sets of servos, each hardwired to their on-side instruments. That is, FD1 always works off the captain’s altimeter, nav radio, etc, while FD2 always works of the copilot’s altimeter, nav radio, etc. AP1 always works with FD1 and AP2 always works with FD2. You can check for the availability of the second flight director and autopilot by either checking the preconfigured_ap_type dataref, or the
Instead of a source selector, this autopilot has a master flight director selector:
. With the master flight director set to 0 (captain), the copilot’s flight director will follow the captain’s. With the master flight director set to 1 (copilot), the captain’s FD will follow the co-pilot’s. With the master flight director set to 2, each flight director will indicate independently, which is required for dual channel operation, and in many aircraft used for approach and go-around modes. Each FD’s AP can be selected to actuate the flight control servos. If the master FD is set to independent mode (2), both APs can be selected to actuate the servos simultaneously. If a discrepancy between the individual APs’ steering commands is detected, they will disengage. Thus, a dual-channel operation with redundant controls can be achieved that immediately alerts you if the sources disagree due to mis-tuning or a failure.
Note that the rules as to which flight director assumes the master role, and when dual-channel can be selected, differ between different types of airliners. X-Plane checks against non-sensical combinations (e.g. it does not allow feeding the autopilot from a flight director that is selected off) when using commands to change the autopilot or flight director modes. If you use the datarefs instead of the commands, X-Plane makes no attempt to second-guess what the master FD setting should be in any given situation of flight. It is up to the specific airplane’s plugin or scripting to decide on the master FD setting and set the dataref accordingly, if the individual flight directors are manipulated by
In case of the airliner type autopilot, a third set of servos is available that is not tied to an on-side flight director but either to the selected master flight director, or it gets its loc/gs information from the third indexed nav radio if the flight director selection is set to independent. Keep in mind X-Plane has a total of 10 nav receivers that can be accessed via the array datarefs. Indexed radios 0 and 1 are naturally accessed as NAV1 and NAV2 receiver. The third set of servos is tied to indexed radio 2, and can thus be utilized for a triple-channel ILS approach. Note that in a plane with triple-channel configuration, you will normally see the autopilots labelled “Left” tied to captain’s side, “Center” tied to the master FD and “Right” tied to the copilot’s side. In that configuration, the X-Plane autopilots 0, 2 and 1 correspond to Left, Center, and Right.
Note that since specific triple channel configurations vary subtly by aircraft type, X-Plane makes no guess as to when to activate the third autopilot or when to tune the third radio, and leaves that to plugin or script of the particular aircraft.
Note that for the third autopilot, no flight director is available on the user-facing side.
If dual- or triple-channel mode is selected on approach, all autopilot input and output parameters agree, the master flight director is set to independent mode, below 1500ft AGL the flare and rollout modes will arm.
will indicate 1 for arm and 2 when active. Note that there are no commands to arm these modes, they just need to match their preconditions instead. The beginning and end of the flare mode in terms of radio altitude can be defined for each airplane in Plane Maker. Flare mode will roundout the descend, retard the throttles and also de-crab in a crosswind condition. Rollout will de-rotate the plane, disarm the auto throttle and keep the plane on the localizer and thus runway centerline using rudder. For the directional channel, see the autopilot parameterization article for more information.
Control Wheel Steering modes
Control Wheel Steering means two different things for airliner and general aviation autopilots.
For anything but the airliner-type autopilot, CWS is a momentary action while the CWS button on the yoke is pressed and held down. While this button is pressed, the servos are declutched and the flight director will be synced to the current pitch and bank for a position-based autopilot, or the current rate of turn and vertical speed for a rate-based autopilot. Additional parameters will be synced depending on the specific type of the autopilot.
For the airliner type autopilot, CWS is a mode in pitch and/or bank rather than a momentary action. This mode can be accessed by placing either autopilot in CWS mode by using the commands sim/autopilot/CWSA and sim/autopilot/CWSB respectively. The autopilot will remain in this mode until a different mode is selected, rather than requiring a yoke button held pressed to stay in this mode. In CWS mode, the autopilot acts as a pitch and bank hold, while the pitch and bank can be modulated by exerting a force on the yoke – in X-Plane, this force corresponds to any deflection of the joystick input by more than 20% of its maximal input.
If the autopilot is engaged in a mode other than CWS on both axis, exerting a force on the roll axis reverts the roll mode to CWS roll and exerting a force on the pitch axis reverts the pitch mode to CWS pitch. In X-Plane, this force corresponds to a deflection of the joystick input by more than 20% of its maximum deflection, in the respective axis.
Servo or trim disconnect
X-Plane has a new set of commands for autopilot servo disconnect, that can optionally also disconnect trim and yaw damper servos, but leaves the flight director on. One of these commands is usually triggered by an AP DISC button located on the yoke or stick:
sim/autopilot/servos_off_any sim/autopilot/servos_yawd_off_any sim/autopilot/servos_yawd_trim_off_any
This will disconnect whatever servo is active, no matter if it is the left, right, center or all autopilots.
GPS steering is available in two different ways:
- A modern autopilot like the S-Tec 55 will receive the GPS steering input directly, not affecting the ability to follow the heading bug, and can engage in GPSS as opposed to NAV mode to follow the GPS steering cues. See the article on VORLOC vs GPS modes for more information.
- An older autopilot like the KAP-140 can use GPSS as a heading input, overriding the heading bug. To switch the input between HDG bug and GPSS, use the dataref
to switch to GPSS in which case the heading bug is ignored and the autopilot follows GPSS in HDG mode.