A power transmission gearbox provided with at least one lubrication system including a lubrication pump. The lubrication pump is in fluid flow communication with a fluid flow circuit leading to at least one spray means. The gearbox includes an emergency tank suitable for containing a lubrication fluid, the emergency tank being in fluid flow communication with the fluid flow circuit, the emergency tank being connected by at least one pipe to the fluid flow circuit upstream from each spray means, the lubrication fluid being moved in the fluid flow circuit to each spray means by a gas that is set into motion by the lubrication pump in the event of a leak of the lubrication liquid.