A four-jet aircraft must be equipped with an in-flight fuel jettisoning system in order to reduce the aircraft weight in an emergency:
until the central tank is empty in order to cope with the wing and landing gear constraints at landing touchdown.
unless it is capable of meeting the climb requirements : 2.7% in approach configuration with 1 engine inoperative and 3.2% in landing configuration with all engines operative.
in order to reach the maximum structural landing weight in less than 15 minutes after activation of the jettisoning system.
in order to reduce the landing distance to 60% of the effective runway length.