The risk of collision is one of the crucial factor for the applications of Ekranoplans in civil transportation. In fact, the extremely low flight altitude of these aircraft increases dramatically the chances of interference between their flight path and the multitude of obstacles populating the surrounding area. Examples of those potential obstacles are: ships, small boats, and stumbling blocks. In this work we consider the optimal collision avoidance problem between a cruising Ekranoplan and a

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... steady obstacle located on the ground. The following assumptions are employed: (i) initially the Ekranoplan is moving in a quasisteady levelled cruise trajectory; (ii) after the avoidance manoeuvre a recovery manoeuvre is executed by the aircraft to return to the initial cruise path; (iii) both the avoidance manoeuvre and the recovery manoeuvre lie on the same vertical plane identified by the initial cruise trajectory. The investigation of the collision avoidance performances on the longitudinal flight is encouraged by the relatively large quasi-flat turn radius of the Ekranoplans.