An onboard system and a method for draining an aeroengine. The onboard system comprises a buffer tank having an admission passage for admitting under gravity fluid coming from the aeroengine, a primary tank situated above the buffer tank and having an emptying passage for emptying the fluid coming from the aeroengine, and a lift circuit connecting the buffer tank to the primary tank in order to transfer the fluid coming from the aeroengine from the buffer tank to the primary tank.