There seems to be a lot of misinformation and guesswork in this thread. This is a confusing topic, since there are a couple of issues that can manifest as trim weirdness. Some of the issues are briefly mentioned in the posts above, but I thought it might be worth it to add some details to them. This will be a long post, but I’m hoping to bring something useful to the discussion.
There are several ways you can adjust the trim, and each of them have some quirks which may or may not be dependent on what kind of controller peripherals you have, how you have mapped the controls, and how you fly the plane. Some of them can be obvious for a long time sim fanatic, but they often forget that the less tech savvy simmers just want things to work without paying too much attention to how to make the controllers work with each other.
The elevator trim generally moves in one of four ways:
- Using a controller axis mapped as the elevator trim axis
- Using a controller button (or the keyboard) mapped to elevator trim up or down
- Interacting the virtual cockpit trim wheel with the mouse by clicking on it, or using the mouse wheel
- Using the autopilot
Not all of them work together, and some of them work in unexpected ways.
- Issue nr 1: Analogue trim axis (method 1) and autopilot (method 4) do not go together
Generally, mapping an analogue controller axis to the elevator trim is a bad idea. MSFS allows you to do it, but please don’t. This is not a bug in MSFS, but the way trims and analogue controllers work.
Some of the posts above complain about the trim behaving erratically when disconnecting the autopilot. This is most likely due to the trim being mapped to one of the controllers’ analogue axis (often named X, Y, or Z axes). In a typical real aircraft, the trim is in fact an analogue device, where it might make sense to map to an axis instead of buttons, but they work very badly in conjunction with the autopilot.
The autopilot adjusts the elevator trim when it tries to raise and lower the aircraft nose. In the real aircraft the autopilot trim adjustments are translated into the elevator trim wheel itself, so that the wheel rotates when the autopilot turns it. Your computer’s analogue controller axis, however, does not move automatically. If you have set the trim axis to 50% when you engage the autopilot, the autopilot will adjust it along the way depending on multiple factors, and it might be set at 30% when you disengage the AP. At that moment the sim will reset the trim from 30% back to 50%, resulting your nose shooting up violently.
Notice that many trim wheel peripherals, like the Honeycomb Bravo trim wheel might look like analogue axes but in fact simulate button presses instead (via so-called rotary encoders) so they are not affected by this issue.
- Issue nr. 2: Controllers with on/off buttons and the infamous Controller acceleration bug
The linked thread explains the issue far better I ever could, but it is one likely culprit for the oversensitive trim some of the people are experiencing.
In short: if you have a controller that sends continuous button presses to the sim (like an on/off rocker switch), it will trigger a bug where the elevator trim and many other controls are boosted so that pressing the key once results in 10 keypresses. This control acceleration happens whether you use the keyboard, the mouse wheel, joystick buttons or the Honeycomb Bravo or Logitech trim wheel peripherals.
I believe this issue will get fixed in the Sim update 3 scheduled some time in March, but there are multiple workarounds available that you can try if you dare.
- Issue nr. 3: Hidden controller mappings
The OP in this thread touched this subject already, but I want to include this for the sake of completeness.
Some controllers are automatically mapped by MSFS in strange ways, if there are no proper templates built-in in MSFS for the exact controller model. One example is the old CH Throttle Quadrant, which have button detents at couple of positions of the lever axes. In some circumstances some of those buttons are inadvertently mapped to trim up and trim down functions, which will result in the elevator trim shooting up or down when you move the throttle or prop levers.
Make sure to go through all mapped buttons one-by-one and unmap the extras.