This study researches the heat and mass transport as a result of a fire source in the hidden area located in the overhead space of the aircraft test article cabin. The overhead area involves complex geometry, a highly curved ceiling, and densely cluttered obstructions. It presents a challenge to aircraft fire safety and, more specifically, to timely fire detection and suppression. Visual observation of major fire signatures, such as flame luminance and smoke, in the hidden area is usually delayed and less reliable. Therefore, fire in the hidden area must be detected at the earliest stage, which further requires better understanding of the heat and mass (including hot gases and smoke) transport within complex geometry. The current research focuses only on the hot gas transport by measuring the temperature rise at the ceiling level as a result of a fire source. Smoke is not introduced in the fire source because of the potential contamination to the test article. It is speculated that smoke carried by the buoyancy plume will follow the hot gas movement in the overhead area. Qualitative assessment of the smoke density is not within the scope of the current study. In addition to the full-scale fire test in the aircraft overhead area, Computational Fluid Dynamics (CFD) is used as a primary tool in this study. CFD carries the advantages of lower cost, faster assessment of design variations, more comprehensive information, and ability to explore conditions not possible in full-scale tests. The CFD simulations were performed using the high- performance computing system built at the FAA William J. Hughes Technical Center at the Atlantic City International Airport, NJ. This system has more than 200 computation cores and can significantly reduce computation time through parallel computation. The following tasks were accomplished during this research: Task 1: Performed full-scale fire tests at the selected region in the test article Boeing 747-SP cabin overhead area. The test measures the ceiling temperature at 50 thermocouple locations at the ceiling level and allows the vertical temperature gradient to be assessed using thermocouples at different heights. Task 2: Imported the computer-aided design model of the overhead area to the CFD model, and generated a mesh system for the imported geometry. Each mesh is assigned to one computation core. Task 3: Performed CFD simulation at the full-scale test condition. The simulated results are compared with the test results for validation. Task 4: Performed CFD simulation at reduced pressure. The simulated results are compared with the simulation at normal pressure for pressure effect. Task 5: Performed CFD simulation with different fire source locations. CFD is found to predict the temperature field very well. CFD also revealed that the lower ambient pressure at cruise altitude decreases air entrainment in the fire and, therefore, results in a higher ceiling temperature. Complex geometries at the ceiling height obstruct the gas flow and create hotoA comprehensive study (both experiments and simulations) of the transport of heat, smoke, and carbon monoxide/dioxide from burning solid fuels in the cluttered spaces (e.g., cargo compartment, cabin, overhead area) is anticipated in the future. Besides the pressure’s effect on fire size, air entrainment, and plume movement, and its effect on solid fuel pyrolysis rate, combustion products (especially smoke) are to be studied. The results from the current study combined with those studies will be used to guide the placement and certification of smoke detectors, temperature sensors, and CO detectors in transport and cargo aircraft.INTRODUCTION The FAA Advisory Circular (AC) 120-80A [1] defines hidden fires as those that are not readily accessible, may be difficult to locate, and that are more challenging to extinguish. Figure 1 shows the diagram of a typical wide-body aircraft cross-section with possible locations of hidden fires [1]. Causes of in-flight fires include wiring failures, electrical component failures, lightning strikes, and overheating of batteries. The indications of hidden fires include abnormal operation or disassociated component failures, circuit breakers, hot spots, fumes, and visual sighting of smoke. In particular, the plume induced by the fire source, coupled with the generated smoke, are the signatures of a hidden fire at the earliest stage. Figure 1. Airplane cross-section showing possible locations of hidden-area fires Hidden fires always involve solid material pyrolysis and combustion, and the heat output from burning is complex and unsteady.ELIZABETH HUNTER is a ten-time USA Today bestselling author of romance, contemporary fantasy, and paranormal mystery. Based in Central California and Addis Ababa, she travels extensively to write fantasy fiction exploring world mythologies, history, and the universal bonds of love, friendship, and family. She has published over forty wo