Article 42 Montreal Convention
Article 42 of the Montreal Convention (Convention For the Unification of Certain Rules for International Carriage by Air) is titled Addressee of Complaints and Instructions. It is under Chapter V (Carriage by Air Performed by a Person other than the Contracting Carrier).
Any complaint to be made or instruction to be given under this Convention to the carrier shall have the same effect whether addressed to the contracting carrier or to the actual carrier. Nevertheless, instructions referred to in Article 12 shall only be effective if addressed to the contracting carrier.
See also: Article 41 Montreal Convention