19 CFR § 122.86 - Substitution of aircraft.
(a) Application. The residue cargo procedure applies when an airline must substitute aircraft to reach a destination due to weather conditions or operational factors which prevent an aircraft on arrival of the flight at the first port from continuing inbound to interior ports scheduled for that flight.
(d) Transporting cargo -
(1) Forwarding. The carrier may forward all cargo which arrived on one aircraft by transferring it to another aircraft of the same airline to complete the inbound flight. The transfer shall be done under Customs supervision.
(2) Conditions. All of the residue cargo from more than one inbound flight of an airline may be laden on one substitute aircraft of the airline. The substitute aircraft shall finish the inbound transport of the residue cargo.