MIAMI – At low altitudes, pilots are interested in airspeed rather than ground speed. We need to be aware of stall speeds, to calculate safe climb speeds, approach speeds, and optimum cruise speeds.
At higher altitudes and speeds, in less dense air, we’re more concerned with high-speed buffet, low-speed buffet, minimum drag speed, best range speed, etc… and these are best measured as a function of Mach number.
There are other variables (weight and temperature) but using the ratio of TAS (True Air Speed) to the Speed of Sound simplifies things. If we used IAS (Indicated Air Speed) values would vary all over the place, as weight, altitude, and temperature changed.
Mach 1 is the Speed of Sound and airliner cruise speeds are represented as a percentage of this speed, e.g. Mach 0.82 representing 82% of the Speed of Sound.
Mach number changes with altitude, temperature, and density. Mach 1 at sea level is approximately 661 knots (761 mph/ 1224 kph) and 573 knots (660 mph/ 1062 kph) at 40000 feet.
Apart from the Concorde, which flew at Mach 2.0 (2 x the Speed of Sound) all of today’s modern airliners fly at sub-sonic speeds.
Flying the Airbus A330, A350
I fly both the Airbus A330 and A350, which are designed to fly at different speeds. The A330 is most efficient at speeds between M0.81 to M0.82, this is due to wing and airframe design and available thrust. The A330 can fly faster, up to M0.85 for short periods of time, but the fuel burn is too high to make a prolonged cruise at this speed practical.
The newer A350 has an optimum cruise speed of M0.85 and does so with a similar fuel burn as the A330. The A350 can (temporarily) fly at M0.87 or M0.88 although again the fuel burn becomes un-economical. Interestingly, if you fly below the optimum speed of M0.85 the fuel burn will also be higher.
This short video was taken from an A350 at M0.85 “under-taking” an A330 at M081, the difference being M0.04 which equates roughly at 6 knots per M0.01 or 24 knots (28 mph / 44 kph) which is noticeable when flying just 1000 feet below.
So how much quicker does an A350 get from London to New York, than the A330?
Featured image: Aeroflot Airbus A330-300 VQ-BMX. Photo: Davide Calabresi/Airways