Autopilot elevator trim behavior

Dear Asobo, I wonder if I am configuring something wrong, or this is a general
behavior of the simulator. For several projects I have been working with, the
autopilot always seems to reset the elevator trim current position as soon as
it is enabled. This causes the aircraft to pich up / down very quickly while
the AP then moves once again the elevator trim to stabilize the aircraft. In
the real world this is not how AutoPilot behaves, in the real world the
AP would engage and starts operating the elevator trim from the “current
position” set by the captain / Pilot instead of resetting it to start again.
This behavior is particularly visible the first time you engage the AP, is
there anything that could be done to prevent the behavior? like passing a
variable or something to the AP either via WASM, .XML or Simconnect when the
AP is engaged or prior to be engaged? this way perhaps we could avoid the
behavior and simulate what happens in the real world much better. Currently in
order to avoid these aircraft projects to be so affected by this behavior, I
have been forced to tweak the AP altitude PIDs values so the AP moves the
elevator trim more aggressive, correcting / moving the elevator pitch trim
faster after being resetted, as otherwise, these airplanes would depart
controlled flight causing a lot of trouble. Unfortunately, this makes the AP
altitude a behavior a bit aggressive… is a trade off… but you still notice
the aircraft elevator trim being reset to NULL when you enable the AP and then
it moves once again to a setting where the airplane stays in controlled flight
in accordance with the sim for very short seconds… Thanks in advance, Simbol

You can try the pitch_use_trim = 0 parameter in the systems.cfg. The AP should
then use the elevator directly to control pitch and trim subsequently.

Unfortunately this is not correct either because Autopilots in real world do
use the trim… in addition when the AP uses the elevator instead of the trim,
it is more difficult for the PIDs to hold the altitude, specially at higher
sim rates… or speeds, or when speed changes. In addition when you disconnect
the autopilot… the aircraft will suffer either high pitch or low pitch
behaviors because now the airplane is not trimmed… also totally outside of
how real world operations are performed for many auto pilots. With all my
tests, setting to use the elevator directly caused worse behaviors with the
AP. Best, Simbol

They use the trim to trim, not for primary pitch control.

I don’t think you understand… if you configure the AP to work with the
elevator only, instead of trim elevator… and your aircraft is let say 145
kts… which requires a lot of elevator pressure “down” to keep the aircraft
level, as soon as you disconnect the AP, that elevator pressure induced by the
AP will returns to null… as a result your aircraft with pitch UP very heavily
unless the user put back the pressure on the yokes. This is not the desired
behavior and how airplanes work with autopilot in the real world… instead
what should happen is the AP moves the elevator trim… and when you remove the
AP the aircraft will remains at the TRIM that it was set by the AP… leaving
the aircraft in controlled flight so the pilot can take over and avoid losing
control over the aircraft. Similarly, if a pilot has set the aircraft elevator
trim in a desired position to keep the aircraft at any desired attitude
pitch… or climbing speed, etc. and hits the AP button… the autopilot should
engage and use the current elevator **trim **position as the starting
point… if it requires more or less trim, it will adjust it… but under no
circumstances it in the real world would reset the elevator trim back to NULL
position, in order to then decide what is the required position to control the
aircraft. The situation I am reporting, is the fact that currently ,
MSFS AP implementation is resetting the trim to NULL as soon as it is
engaged… and then move it back to the previous position to control the
aircraft. This creates a yoyo effect in the airplane where it goes pitch up or
pitch down… then again up… while the AP takes control… which obviously
causes anomalies and lots of complains with current customers, who indicate
this is not how the AP should work… and they are right… It seems to me, the
attitude / altitude PID code is resetting the signal to zero to get feedback
at that point in order to calculate the require trim… rather than using the
current signal at the current elevator trim pitch as the starting point.
Regards, Simbol

I understand you, and this is not how autopilots work. They use the elevator
for changes in pitch, and then trim the aircraft with the trim after said
pitch changes, so that the aircraft is in trim when disengaging. They do use
the trim, except not for initiating the changes in pitch. As for MSFS
resetting the trim upon engagement, that would indeed be a problem.