Experiments have been made for investigation of a flame accelerating during its propagation in gaseous mixtures of propane-butane-air in the large-scale semiopen horizontal tube (diameter 1.45 m, length 100 m) with combustion initiated near the closed end of the tube. We found that in most experiments the flame velocity, S, increases linearly with a distance L, from the end of the tube. At a distance of 15-20 m from the closed end of the tube, the change of the regime of flame propagation may take place with the appropriate change of the proportionality coefficient between S and L. The experiments showed that the combustion process duration, in a fixed tube cross-section that is detected by photodiodes and electric potential transducers, decreases exponentially with the distance from the closed end of the tube. The exponent index does not depend on a combustible mixture composition. The linear relationship between the flame velocity and the distance from the closed end of the tube was qualitatively explained based on the concept of a flame accelerating due to formation of an elongated flame front.