diff --git a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
index 7c19f38..24602a8 100644
--- a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
+++ b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
@@ -471,6 +471,15 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         let mut outcome = if let Some(iqn) = iqn.as_mut() {
             iqn.set_tolerance(tol_step).unwrap();
             iqn.solve(&d_predicted, pass)
+        } else if cfg.coupler == "fixed" {
+            // P6-c (`docs/overset_metal_campaign.md` §5.19): the constant
+            // relaxation `OMEGA0` — at 1.0 the plain staggered loop, the
+            // only one whose candidate is the FEA's own output, so the
+            // Robin wall's datum (the previous pass's tractions) IS the
+            // structure's traction at the candidate (Robin–Neumann).
+            Subiterated::relaxed(cfg.initial_relaxation, cfg.max_subiterations, tol_step)
+                .unwrap()
+                .solve(&d_predicted, pass)
         } else {
             Subiterated::aitken(cfg.max_subiterations, tol_step)
                 .unwrap()