“…Numerical simulations show that the initialization step for the direct method procedure is quite robust (a constant initial guess is enough). The results are reported on Figures 12,13 and 14. We observe that, when t ∈ [23,43], the control oscillates much with a modulus less than 1: this indicates that there is a singular arc in the "true" optimal trajectory, and therefore chattering Note that, along the singular arc, the variables ω x , ω y , p ωx , p ωy , p θ , p ψ and p φ are almost equal to 0, and we check that this arc indeed lives on the singular surface S defined by (26). Therefore, it turns out that there is a singular arc in the optimal trajectory, causing chattering at the junction with regular arcs.…”