Article 49 Montreal Convention
Article 49 of the Montreal Convention (Convention For the Unification of Certain Rules for International Carriage by Air) is titled Mandatory Application. It is under Chapter VI (Other Provisions).
Any clause contained in the contract of carriage and all special agreements entered into before the damage occurred by which the parties purport to infringe the rules laid down by this Convention, whether by deciding the law to be applied, or by altering the rules as to jurisdiction, shall be null and void.
See also: Article 48 Montreal Convention