Article 6
1. The requesting Member State shall undertake to readmit the third-country national forthwith if: (a) the transit by air authorisation was refused or revoked under Article 3(3) or (5); (b) the third-country national entered the requested Member State without authorisation during the transit; (c) removal of the third-country national to another transit country or to the country of destination, or boarding of the connecting flight, was unsuccessful; or (d) transit by air is not possible for another reason. 2. The requested Member State shall assist with the readmission of the third-country national to the requesting Member State in the cases referred to in paragraph 1. The requesting Member State shall bear the costs incurred in returning the third-country national.