In the United States, the first jet-powered airplanes that most non-military pilots transition to, are those of the regional airlines or to a light- to medium-sized business jet. The regional airlines are typically flying either the Bombardier Canadair Regional Jet (CRJ) or Embraer E-Jet family, and there are numerous options for pilots to transition into a business jet airplane including single-engine jets such as the Cirrus Vision SF50.
As a relatively new first officer with a regional airline, I was perplexed with the concept of Mach transition during climb and descent. As a general-aviation (GA), fixed-wing, piston-engine pilot/instructor and a turboshaft-powered U.S. Army helicopter aviator, the concept of Mach transition made little sense to me. I wondered why cost-index (CI) -based flight planning and air traffic control (ATC) services at higher cruise altitude referred to a Mach number rather than indicated airspeed in knots (KIAS). For instance, typical Mach transition speeds in the ERJ-175 for climb and decent are as follows: 250-270 KIAS during climb until Mach 0.70-0.75. The assigned Mach number is maintained for the remainder of the climb after transitioning to Mach. During decent Mach 0.70-0.75 is maintained until reaching 250-300 KIAS. Assigned indicated airspeed is maintained after transitioning from Mach to KIAS. For reference, the airspeed limitation for the ERJ-175 is Mach 0.82 or 320 KIAS above 10,000 ft and 300 KIAS below 8,000 ft.
Let us consider the following climb scenario: You takeoff from an airport at sea level and climb to FL410. If a constant indicated airspeed climb of 250 KIAS is maintained from sea level to FL410, the corresponding Mach number is 0.84. This will exceed the ERJ-175’s Mach Maximum Operating (MMO) of Mach 0.82. On a similar note, a decent from cruise altitude, even at a relatively constant low Mach number (e.g., Mach 0.6 from FL270 to sea level) will yield 397 KIAS at Sea Level and thus exceed the Velocity Maximum Operating (VMO) of 300 KIAS below 8,000 ft.
The table shows Calibrated Airspeed (KCAS) for different Mach numbers and altitudes. For the ERJ-175, it is reasonable to assume KIAS≈KCAS. The table shows that if a pilot selects 270KIAS and Mach 0.70 as the transition speed, transition altitude from KIAS to Mach is approximately FL290. Similarly, a selection of 270KIAS and Mach 0.75 as the transition speed yields a Mach transition altitude of FL320. Intriguingly, the relationship between KIAS, KCAS, Equivalent Airspeed (KEAS) and Mach number does not depend on air temperature since air density changes at a given flight level (pressure level) and has been accounted for in the calculation and conversion. However, deviations from standard temperature impacts true airspeed (KTAS). The higher the density altitude, the faster the KTAS. For example, Mach 0.75 at FL300 yields 446 KTAS at -40C, and 456 KTAS at -30C.
During climb, the transition from KIAS to Mach implies a reduction in indicated airspeed with altitude and results in a steeper climb gradient since altitude is gained at the expense of airspeed (i.e., trading altitude for airspeed or kinetic energy for potential energy). During decent, the transition from Mach to indicated airspeed decreases the true airspeed (KTAS) and thus promotes a steeper decent gradient. However, this idealization is often not observed since the decent gradient is typically held constant (e.g., 3.0°) if vertical navigation (VNAV) path mode is selected.
References
Airspeed Conversions (CAS/EAS/TAS/Mach) by AeroToolbox.com
