An aircraft is described comprising a transmission unit with a first module and a lubrication system; the first module comprises a casing and a movable member; the lubrication system comprises a header, a nozzle fed with the lubricating fluid and designed to feed the lubricating fluid inside the casing of the first module, a collection tank for the lubricating fluid injected by the nozzle, and recirculation means designed to cause the recirculation of the lubricating fluid from the collection tank to the feed header; the first module comprises a valve available in a first configuration, in which it enables the outflow of said lubricating fluid from said module to the recirculation means when the pressure of the lubricating fluid inside the header is greater than a threshold value; and in a second configuration, in which it fluidically isolates the module from the recirculation means when the pressure of the lubricating fluid inside the header is less than the threshold value.