For a jet aircraft and propeller aircraft, two methods estimate their maximum flight range and endurance, respectively; one assumes that the aircraft weight over air density is constant, and the other assumes that the thrust over air density is constant. This paper investigates the difference in the maximum flight range of a jet aircraft and endurance of a propeller aircraft calculated by these two methods. For the former, there is no significant difference between two methods. For the latter, the results show that the difference in the flight endurance changes depending on the ratio of the lift coefficient assumed for two methods. It has been shown that the difference in maximum endurance calculated by two methods is less than 10% as long as the lift coefficient is within an ordinary range.