X-Plane provides two ways to trim the elevator or stabilizer of a plane: Mechanical or with an electric servo. The presence of an electric servo has consequences to the type of commands you can use the move the trim, and the commands you want to set up for hardware. This article explains the interaction of the commands and X-Plane systems code.
Trim commands for pilots – how to configure your flight control hardware
X-Plane provides separate commands for electric and mechanical trim, and a catch-all command for trim that would work with either system. Most of the time, you’d want your flight control yoke or joystick to trigger the generic command, so you can fly with any type of aircraft:
will dispatch the command to an electric trim servo, if the plane has one, and to moving the trim wheel, if that’s the only way of trimming the plane has.
If you have a more advanced setup involving more hardware, you can easily configure your yoke trim rocker to actuate the electric commands, while a switch on your throttle console acts on the mechanical commands. This way, you can realistically deal with a servo failure in a plane that has one: As long as the servo is on, you trim with your thumb on your yoke, and if the servo is off, you can still “grab the wheel” on your console to move the trim. This obviously also works if you have an actual hardware wheel which you assign to an axis instead of a button. The respective commands are:
an even more advanced yoke might also have split trim switches where both channels need to be active in order to trim. Commands for this are:
Another important difference is that electric trim by pilot action will necessarily trim-interrupt and disconnect the autopilot, while displacing the mechanical trim wheel (by command or by axis) will not!
Trim commands for airplane creators
A legacy aircraft will detect the type of trim system based on the presence of an autopilot: A plane with a single-axis autopilot is assumed to have no electric trim servo. A plane equipped with a dual-axis or better autopilot is assumed to have an electric trim servo and use it as primary means of trim.
New aircraft can specify the trim servo with the option “electric trim servo equipped” in Plane Maker on the trim controls page.
When you have an electric trim servo it can be switched on or off with the dataref
switching it off will not only prevent the autopilot from using it, but also cause electric trim commands to no longer work. The same thing happens when the “Elevator trim actuator” failure is triggered – the servo will fail and you can only trim mechanically.
If your plane does not have an electric servo (by means of not having selected one, or by being a legacy plane with a single-axis auto-pilot), assigning any bus or amperage to the trim actuator does nothing. The failure of the trim actuator then refers to a jamming of the mechanism, rather than a servo failure.
Make sure to assign the “Elevator trim actuator” a bus and an amperage in Plane Maker, so it only runs when the correct electrical system is powered.
When it comes to the 3D cockpit, assign the manipulator of the physical wheel to either the _mech commands, or to the trim dataref. Assign the _elec commands to rocker switches. Since you know which system your specific aircraft has, you should never need to assign the catch-all commands to anything in your 3D cockpit.