Correct answer is... 811 km/h. It is equal to 438 kn or 504 mph.
A cruise speed is the speed of a plane in relation to the air masses surrounding it. If we wanted to figure out how fast the plane is moving in relation to earth we would have to take into account the factor of wind. In windless weather the cruise speed equals the speed in relation to earth. However, the higher the plane flies the stronger the wind.
To give an example, the cruise speed of McDonnell Douglas C-9 flying at 300 meters against the wind of 40 km/h will equal 771 km/h. With the wind, it will equal 771 km/h. If there is a side wind, the speed will vary between 771 and 851 km/h, depending on whether it’s more against or with the wind.
Disclaimer: Our articles and data connected to cruise speed of McDonnell Douglas C-9 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about cruise speed from manufacturer of McDonnell Douglas C-9.