A turboprop engine includes a power jet mounted coaxially to the engine axis (X-X'), and has a fan case and a gas ejecting system provided with two afterbody half-cowlings (10b) each of which is pivotable around an axis (A.sub.2) between open and closed positions, wherein the half-cowlings are connected to the downstream of the fan case through a `V-blade/V-groove` system. In a first embodiment, the pivot axes (A.sub.2) make an angle .alpha.>3.degree. to the engine axis (X, X') and a joint plane (Z.sub.2-Z.sub.2) between the afterbody half-cowlings and the downstream of the fan case makes to the plane (Y, Y') perpendicular to the engine axis (X, X') an angle .beta. in such a way that the difference between the angle .alpha. and the angle .beta. is less than or equal to 3.degree.. In the other embodiment, the axes are converged to each other in a downstream direction.