The under-card wear of control rods due to flow-induced vibration was discovered in 1986. To cope with this phenomenon, Framatome at once erected the full-scale Magaly test facility, in operation since 1988. Then in 1991 Framatome initiated exploratory numerical simulations using the Nat03 structural code specifically developed for this purpose. After introducing Magaly and Nat03, this paper illustrates how the following ill-posed inverse-problem has been attacked: from rod vibration measured with Magaly, how to calibrate via Nat03 computation a rustic model of the rod excitation created by the main flow coming from the fuel assembly. A single force applied on the control rod just beyond the continuous guidance section is sufficient to “support” after adjustment all the available experiment-computation comparisons: rod displacement (trajectory, standard deviation, PSD), contact force (average value, contact duration), and wear work rate, for three control rods differently located in the cards and nine card elevations with different flow rates.