Sorry my bad. In the refinement settings, you can define refinement levels on the fuselage, the wings, and landing gears, etc. You can also define refinement levels on a box that covers the plane, so that the cells within the box will have at least that refinement level. For example, with a background mesh of 0.5m generated from blockMesh, you can define level 8 refinement on the plane, and level 6 refinement on a box that covers the plane. Now, if you define a level 9 refinement on the box, then the entire plane will also have level 9 refinement, finer than what you need creating a lot of cells.
When doing mesh independency study, you can increase the level of refinement of that box to 7, but you don’t need to increase the level on the plane. If there are areas on the plane that are apparently not described by the surface mesh properly, then you refine it.