  The minimization of the Direct Operating Cost (DOC) for hydrogen-powered
aircraft is formulated in this paper as an optimal control problem and is
solved based on Pontryagin's minimum principle. As a consequence, the optimum
cruise flight speed is determined assuming cruising at a constant altitude. The
optimization criterion corresponds to the minimization of a functional
representing the trade-off between the cost of hydrogen fuel and time-dependent
costs, which are related by a parameter denoted by cost index. The value of
this parameter is introduced by a pilot into the Flight Management System (FMS)
of the aircraft. The HY4 aircraft model is used to obtain numerical results for
the proposed methodology.
