Airliners cruise near the speed of sound, but the speed of sound itself is not consistent: It decreases with temperature from 1,230 km/h at 20ºC to 1,060 km/h at the ‑57ºC you find at cruising altitude. That’s why physicist Ernst Mach formulated a ratio between the speed of an object (airplane) and the speed of sound. This method of measuring velocity becomes more and more pertinent as pilots take airplanes to higher altitudes (above 25,000 feet) and increase speed. At cruising altitude, Mach .78 is 828 km/h whereas Mach .86 is 913 km/h. For a flight from Toronto to San Francisco that means a difference of about 25 minutes.
Pilots are required to maintain their speed to within .01 accuracy of their Mach number.
Air Canada’s narrow‑body fleet cruises at Mach .74 to .80. The wide‑body fleet zips through the air at Mach .80 to Mach .88.
The Boeing 787, Air Canada’s fastest plane, is capable of Mach .90 (90% the speed of sound) but we fly it from Mach .84 to Mach .87.
The only airliner to fly faster than Mach 1 was the Concorde, at a whopping Mach 2 (twice the speed of sound).
Turboprop aircraft such as the Dash 8 Q400 fly at 25,000 feet or lower, so Mach speed is not pertinent.