Height of aircraft from ground

Please can anyone help ? 
I have downloaded some aircraft mainly for AI use but I am finding that some appear to be either sinking into or floating above the ground.
Am I stuck with this or is it a matter of adjusting a file ?

I have downloaded some aircraft mainly for AI use but I am finding that some appear to be either sinking into or floating above the ground.
Am I stuck with this or is it a matter of adjusting a file ?