Custom Autopilot Constants
X-Plane's default autopilot setup does
a pretty decent job with "average" aircraft. However, if the control
response of your plane is unusual, or just to get the autopilot really
right, you'll have to set up custom autopilot constants.
What do the autopilot constants
Basically, the autopilot applies
control inputs when the plane is not flying the way is should, given
the current autopilot settings. For example, let's say you're set the
autopilot for a 500 fpm climb. If the plane is currently flying level,
the autopilot will apply up elevator to pitch the plane up and make it
climb. The autopilot constants account for two critical aspects of the
- How much response you get for a certain amount of control input.
- How long it takes for the plane to respond to control input.
If the settings are too weak, you have a "lazy" aircraft that doesn't
fly to the autopilot settings. On the other hand, if the settings are
too strong, the plane will overshoot its corrections and might even
How do I fine-tune a plane's
There are two basic approaches: You can
adjust the autopilot constrants in PlaneMaker, then save the plane,
open it in X-Plane, and test fly it. Or you can adjust them in real
time while you fly the plane in X-Plane. Guess which is more
convenient. However, settings you change in X-Plane are not recorded in
the aircraft file! You still need to go back to PlaneMaker when you're
done to make your changes permanent.
Control - these values are used only for the auto-throttle
To adjust them in X-Plane, from the menu bar select Special->Set
To adjust them in PlaneMaker, open the plane in PlaneMaker and from
the menu bar select Expert->Artificial Stability; then select the
Autopilot tab. If the menu is blank, check the "Use custom autopilot
Either way you'll see the same menu with settings. (In X-Plane, you can
drag the menu to the bottom of the screen to get it out of the way
while you test-fly your current settings.) You'll see three major
sections, for throttle
control, roll, and pitch.
- This section controls the ailerons for all directional control modes
- wing leveler, heading select, and all LNAV modes.
Speed error for full throttle:
This is your sensitivity control. The autopilot will apply full
throttle if the plane is this much slower than the set speed. More
generally, decrease this value to increase the sensitivity, i.e., to
make a larger throttle correction in response to an error in speed.
Throttle control per second:
This value controls how fast the autopilot changes the throttle
setting. Keeping this value small damps out the throttle jiggling in
response to turbulence, but slows down the throttle response.
Speed prediction: This value
tells the autopilot how much delay there is in the aircraft's response
to a change in throttle. Increase this value if you find the aircraft
overshooting speed corrections; i.e., it's too slow, the autopilot
opens the throttle, the plane gets too fast, the autopilot backs off on
the throttle, the plane gets too slow, etc. Decrease this value if you
see the throttle open, and then back off before the plane is up to
- the third section contols the elevators for the autopilot pitch modes
- vertical speed, altitude hold, speed with pitch, and flight level
Roll error for full aileron:
Just like with the throttle, this is your sensitivity control. The
autopilot computes what your roll angle should be, based on your
current heading and what it ought to be, and then applies the necessary
aileron input to get the aircraft to the desired roll angle. As with
the throttle, decrease this value to get a stronger aileron input.
Roll prediction: This value
tells the autopilot how fast the plane rolls. As with the speed
prediction setting, increase this value to keep roll angle changes from
overshooting and decrease it if the autopilot's roll behavior feels
Roll tune time: The autopilot
won't get the roll behavior exactly right all the time, since control
response depends on the plane's speed and other factors. After this
time period has passed, the autopilot fine-tunes the roll angle by
applying small aileron corrections.
Localizer CDI gain: This value
and the next control the autopilot's response to a runway localizer.
The CDI gain value tells the autopilot how fast to turn the plane in
response to being off the localizer centerline. Localizers are the one
LNAV mode for which the autopilot does not know the correct course
heading, so it needs to "feel for the localizer" by watching changes in
the CDI while it turns toward the deviation indicator. For heading
select, VORs, and GPS routes, the autopilot knows the on-course
heading. VORs and GPS legs are intercepted at a 30 degree angle.
Localizer CDI prediction: This
value tells the autopilot how fast the plane can turn. It is used for
all LNAV modes to turn the plane smoothly onto the on-course heading as
it approaches the course centerline. As with the other prediction
values, too small a value will cause the plane to turn too late and
overshoot the course centerline; too large a value causes the plane to
turn to the course heading before it is on course, and so never get
quite on course.
Pitch error for full elevator:
As with speed error and roll error, this value controls the sensitivity
of the autopilot's elevator control. Decrease it to get a stronger
elevator response for pitch control.
Pitch prediction: This value
tells the autopilot how long the aircraft takes to change its pitch
attitude. As with the other prediction values, increase it to tune out
overshoot and oscillation; decrease it to make the aircraft hold pitch
Pitch tune time: This is the
same idea as roll tune time. After this time period, the autopilot
makes small elevator corrections to get the pitch angle exactly right.
Glideslope CDI gain: This is
the same idea as the localizer CDI gain. Increase this value to get a
stronger elevator response to the plane being off glideslope.
Glideslope CDI prediction: This
value predicts how long the plane takes to change its descent rate.
Increase it to eliminate overshooting and oscillating on the
glideslope; decrease it to hold glideslope more closely.
Pitch degrees per knot: This
value controls the rate of change of pitch for the speed with pitch and
flight level change modes. In these modes, the autopilot pitches the
aircraft upwards if it is flying too fast, and downwards if it is
flying too slowly.
Wow... How do I get all
these settings right?
The settings are complicated and they
interact. For example, if you increase control sensitivity (i.e.,
reduce the roll or pitch error values) you will probably have to
compensate by slightly increasing the prediction time to account for
the aircraft's more aggressive response.
The best way to get everything set is to start with the defaults. Then
inspect the settings for "sanity" - i.e., set the prediction times
according to how fast the aircraft responds to normal control inputs.
Set the sensitivity values according to how you would control the
aircraft - e.g., "I would apply full up elevator if the aircraft is
pitched too far down by X degrees".
Then set up some test scenarios for each of the modes, test-fly the
plane, and make adjustments until it responds correctly.
The pitch and roll control values need to be set up in stages. First
set the sensitivity (error value) so that you get reasonable response -
for example, pitch response to vertical speed changes in VS mode or
roll response to heading changes in HDG mode. ("Reasonable" depends on
the aircraft. A 747 rolls a lot slower than a Piper Cub.) If the
aircraft overshoots or undershoots pitch and roll attitude, increase or
decrease the prediction time, respectively. If you can't get rid of
oscillation, reduce the sensitivity by increasing the error value.
Set the pitch and roll tune time to be 3x or more larger than the
Now you can go on to the secondary settings (CDI gain and prediction).
Here you can select the prediction value first by using known targets.
For pitch, use ALT HOLD and set the prediction so the aircraft settles
in neatly to the set altitude. Similarly, for roll, use HDG select and
adjust the prediction time so that the aircraft rolls out properly to
the selected heading. Finally you can adjust the CDI gain by
intercepting a localizer and glideslope. Localizer intercept is
one of the hardest to get right. Set up a realistic worst case
situation (e.g., for a typical airliner, a 30 degree localizer
intercept angle, 10 miles out, at 250 kt) and make it work. You will
probably have to go back and tweak settings you made previously. Saving
and reloading a situation makes it convenient to rerun the same test
with different settings.
Recording your work
If you've made your autopilot
adjustments "on the fly" in X-Plane, they have not been permanently
recorded! Even reloading the plane (or any other plane) will erase
them. When you're done, write them down, open the plane in PlaneMaker
and use the autopilot settings screen to make your changes permanent.
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