Cruise Performance with Atmospheric Winds

An article appeared in the media implying that a B777 almost reached the speed of sound, as it was riding above a very strong jet stream. The journalist reports that the speed of sound is 761 mph on the ground (about 340 m/s), see inset.

Boeing B777-200 jet stream blasts news

Boeing B777-200 jet stream blasts news

These statements are incorrect. In the first place, the journalist/newspaper confuses air speed with ground speed. Ground speed governs the travel of the airplane, as measured by a ground station; air speed governs the aerodynamics. Aerodynamic drag and Mach numbers (the air speed with respect to to the local speed of sound) are governed by the air speed alone. In the absence of any wind speed, air speed and ground speed are equivalent.

With a head-wind, the air speed is equal to the ground speed plus the wind speed; with a tail-wind (like in the case at hand), the air speed is equal to the ground speed minus the wind speed. This means that an airplane encountering a strong tailwind will travel faster, as if riding on top of the wind. However, the true Mach number is nearly the same as in the absence of wind; therefore, the airplane could not possibly have reached supersonic speed.

At the normal cruise altitudes in the North Atlantic, it is not unusual that the jet stream is on the flight path – something that has been exploited for decades. Yet, that aricle makes it look like we did not know it.

Read full case study here