Something special happens as a plane climbs through 28,000 feet. At around this altitude (give or take for atmospheric conditions), the airspeed reading transitions from indicated airspeed to Mach. Let’s talk about what Mach is and why aircraft speed is measured by it.
What is Mach?
Mach is named after Austrian physicist Ernst Mach, whose work in the late 18th and early 19th centuries was seminal for understanding shock waves and fluid dynamics. Mach is the ratio of the velocity of an object relative to the speed of sound. Regarding aircraft, Mach measures how fast the plane moves through the air mass at its current altitude relative to the speed of sound, which is 761 miles per hour at sea level on a standard day.
Mach is an effective measurement for the velocity of an aircraft because the speed that sound can travel varies significantly with temperature. Sound waves travel faster through warmer air, so the Mach ratio adjusts for an object’s true velocity within a relative medium. For planes, this means that ground speed increases on warmer atmospheric days given the same Mach number compared to a cooler atmospheric day at the same altitude. This follows the general principle of density altitude.
Aerodynamic considerations
Theoretically, pilots don’t need to change how speed is measured when they reach 28,000 feet. Indicated airspeed remains a viable measurement and still is a good general indicator for the aerodynamic performance of the plane. The importance of transitioning to the Mach scale is to have a more precise metric for understanding how fast the aircraft moves relative to the speed of sound.
Due to how planes are designed, some portions of the wing might near or exceed the sound barrier even when the aircraft’s velocity is well below it. There are enormous aerodynamic penalties for flying too near the speed of sound in a commercial airliner, and it’s generally unsafe to do so. Manufacturers thus assign a speed called Mmo, or the maximum Mach operating speed. If this speed is exceeded, portions of the wing could approach Mach 1 and experience structural issues.
Transitioning from knots to Mach
The speed transition from indicated airspeed to Mach usually occurs between 27,000 and 28,000 feet because this is where the two speeds intersect. A plane at a lower altitude is met with substantially more resistance from denser air. Thus, it cannot accelerate anywhere near the speed of sound without exceeding structural limits at that altitude. At around 28,000 feet, the air density is low enough that the Mach value provides the more accurate measure of speed relative to maintain structural integrity.
Pilots input two speeds into their flight computers for climbing and descending: The first is indicated airspeed, and the second is Mach. The autopilot (and autothrottles, if available) will target the indicated airspeed the pilots selected until reaching 28,000 feet before honoring the selected Mach value. With both speeds input in the flight computer, the aircraft is safe from overspeed and underspeed in knots and Mach so long as all relevant parameters are met.
There’s a good chance your plane’s speed is measured in Mach for most of your flight unless it’s a short trip or flown at an excessively low altitude. Most planes cruise between Mach .75 and .85, with the fastest private jets achieving velocities above Mach .90. Time will tell if passenger planes will exceed Mach 1.0 again as the Concorde did regularly; some companies and airlines have ambitions of doing so.
Source: simpleflying.com