<P> Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed (V). Combustion engines have an optimum efficiency level for fuel consumption and power output . Generally, piston engines are most efficient between idle speed and 25% short of full throttle . </P> <P> With aircraft, other factors affecting optimum cruise altitude include payload, center of gravity, air temperature, humidity, and speed . This altitude is usually where the higher ground speeds, the increase in aerodynamic drag power, and the decrease in engine thrust and efficiency at higher altitudes are balanced . </P> <P> The typical cruising airspeed for a long - distance commercial passenger aircraft is approximately 475--500 knots (878--926 km / h; 546--575 mph). </P>

What is the average speed of passenger airplane