201 alternatively in the case of engines of the same power only one of both engines operates the other being kept in an over idling speed advantageously the mode m 2 also manages the conventional restart of the engine in an over idling speed when the phases b e or c are close to come to the end if this conventional restart fails the mode switches to the mode m 4 the mode m 3 manages the failure cases of the engine used by re activating the other engine through its emergency assistance device for example when the oversized turbo engine 1 used in operation alone during the phases of cruising flight b or e fails the u201c small u201d engine 2 is quickly re activated via its emergency assistance device u 2 step 300 on the same way if the u201c small u201d engine 2 alone in operation during the search phase c fails the u201c big u201d engine 1 is rapidly re activated via its emergency assistance device u 1 step 301 such mode m 3 also manages for a long time such cruising or searching phases when the engine initially provided in operation has failed and has been substituted by the other engine being reactivated in the case of the cruising phases b and e the emergency assistance device u 2 is disconnected the oei ratings of the u201c small u201d engine 2 being armed in accordance with the safety certifications step 310.