On Monday 11 December Iberia will operate an Airbus A340-600 between Madrid and Brussels. The flight between Madrid and Brussels Airport is operated as a cargo-only flight, the way back to Madrid is available for passengers. Last week the A340-600 was also at Brussels but for cargo-operations only.
From Brussels Airport to Madrid: IB3209 17:15-19:40, price start from 109€
Note: on the website Iberia.com it is announced as an Airbus A340-500 but forum member CTBke explained to Aviation24.be that the aircraft will actually be an Airbus A340-600: https://www.aviation24.be/forums/viewtopic.php?f=4&t=62591