In an aircraft propulsion system, such as a jet engine, the fan cowls (22) are articulated to the support strut or pylon (12) by which the centre or core engine (14) is connected to a wing element (10). The cowls (22) are kept closed by hooked locks (32a, 32b), which connect their lower edges. To eliminate any risk of accidental opening, guide devices (36, 38) are placed between the lower edges of the cowls (22) and structures (20; 24, 26) connected to the centre engine (14), at least at the front of the cowls. The front structure (20) opposes any axial and radial displacement of the cowls.