Fairing device for nacelle of an aircraft propulsion power unit

A nacelle, a part of which is the fairing device, surrounds a gas turbine engine (4) and a support pylon (7) on an aircraft fuselage (15). The pylon extends into a substantially horizontal plane of symmetry containing the longitudinal axis of the propulsion power unit (1), and comprises a central compartment (7b) that is open on the top and on the bottom and provided for housing devices. The pylon is attached to the fuselage (5) by a distal edge (7d) extending parallel to the longitudinal axis of the propulsion power unit (1). The fairing device consists of an upper half-cowl (2) and a lower half-cowl (3) substantially symmetrical relative to said plane of symmetry, and lockable therebetween, in a closed position. The upper half-cowl (2) and lower half-cowl (3) are individually pivotably mounted on the distal edge (7d) of the pylon (7) such that, in an open position, access is provided at the same time to the engine (4) and to the central compartment (7b) of the pylon (7).