High Temperature Gas Reactors (HTGR) are a type of power plants that can provide industrial process heat and high efficiency, carbon-free electricity. The High Temperature Test Facility (HTTF) has been designed to provide data that can be used for the validation of system codes and verification of passive safety features in HTGRs. The HTTF is an integral effects test facility at Oregon State University (OSU). It is an electrical heated, ¼ height-scaled representation of the Modular High Temperature Gas Reactor (MHTGR) which uses helium as a system coolant. Air ingress from a double-ended guillotine break is one of the accident scenarios of HTGRs that needs further exploration. In particular, investigation is needed to understand when global natural circulation is initiated in the reactor core. Using several thermocouples place within the HTTF, the ingress of nitrogen, acting as air, can be tracked from initiation of the accident to onset of natural circulation and beyond. This work will first evaluate the theoretical time scales of air ingress and compare the fluid mechanism by which it enters the system. Secondly, this work will use experimental data to determine the point of onset of natural circulation and evaluate its relationship with initial core temperature.