One of the objectives of the CleanSky-2 project is to develop an Environmentally Friendly Fire Protection (EFFP) system to substitute halon for the aircraft cargo hold. For this, an aircraft demonstrator including the cargo hold was equipped with a nitrogen-based fire suppression system. The demonstrator is located in the Flight Test Facility (FTF) low-pressure vessel and can thus be subjected to realistic cruise pressure conditions and take-off and descent pressure profiles. As a design tool, a zonally refined simulation model to predict the local oxygen and nitrogen concentration distribution in the cargo hold has been developed using the Indoor Environment Simulation Suite (IESS). The model allows for fast transient simulations of the suppression system operation. This paper presents a model validation case of knockdown during cruising, followed by a holding phase and descent.