Fire development is influenced by many factors, such as oxygen concentration, external radiation, and the inclination angle of the combustible material. This article examines the effect of these factors on flame spread over a charring solid using a series of experimental studies. The results show that the flame spread can be divided into three regimes based on oxygen concentration: an unsustained flame spread regime (Y-o <= 0.8), a stable combustion regime (0.8 < Y-o < 1.3), and an accelerating flame spread regime (Y-o = 1.3), where Y-o is the oxygen concentration ratio in mole fraction. The critical angle for the transition from the stable combustion regime to the accelerating combustion regime is between 10 degrees and 20 degrees, measured from the horizontal. When the fuel is exposed to external radiation, the flame propagation velocity generally increases. At reduced oxygen concentrations, the increase in flame spread rate is greater with external radiation, whereas at a higher oxygen concentration level, the oxygen concentration, rather than the external radiation, dominates the increased flame spread rate.