How to align the FC board for level flight

I am in the process of fitting an Omnibus F4 Pro flight controller to a Bixler 3 and I need to make sure that the flight controller board is mounted “level” with the Bixler 3.

I assume that when the Bixler 3 is trimmed correctly and cruising at a steady altitude, that the stabiliser is “level” (perfectly horizontal) and thus the flight controller board needs to be perfectly parallel with the stabilser/elevator to maintain a constant cruising altitude?

Is that correct?

perfect ‘leveled’ mounting is almost impossible, not practical and not necessary.

It is easy to make it level autopilot by calibrating sensors inside and that’s what ArduPilot supports.

Mount flight controller and calibrate sensors as wiki or GCS guides, check it is level, if not press calibrate level button on GCS. then ArduPilot will find sensor offset for level.

Thanks for your prompt reply.

Are you saying to do this “calibrate level” while the plane is in level flight?

Or, is this an iterative process where the plane is flown, checked for level flight and then landed and calibrated again on the ground with the plane sitting at an attitude that is opposite to the flight attitude?

Nope, you should check it before flight, in ground while level your plane as much as you can.

Everything should be checked before flight
roll, pitch, yaw, servo movements (aileron, elevator, rudder …), etc.

Please refer following documents

I apologise if I am being overly pedantic, but it is the definition of “level” as it applies to a plane that has me confused. I have built several multi-copters and “level” is quite obvious for them - it is the horizontal plane that all the propellers lie in.

So, when you say:

how do you define level as it applies to a plane?

Is that level with the stabiliser/elevator, or level with the bottom of the wing, or level with the line that goes from the centre of the leading edge to the centre of the trailing edge of the wing, or what?

I should add that I am referring to “level” in the pitch direction. “Level” in the roll direction is quite obvious and easy to establish.

The concept of level shouldn’t be that hard.
It is just same as even, leveled ground.
If you have a stand for your plane or landing gear then you can align with it.

It is the same concept as any kind of aircrafts, and any axis applies the same.
you can imagine the virtual line that starts from center of mass(or gravity) towards
virtual axes of reference NED frame.
roll, pitch, yaw is view from the ground (i.e inertial frame).

I think that you’ll find that “level” doesn’t have to be as perfect as you think it does. Arduplane uses a lot more than just “level” to determine how it flies, you won’t notice any difference if it’s a degree or two off. Just set the airplane down in a way that it isn’t obviously tilted forward or back, and run the calibration.