Climb · IAS, Mach and crossover
Follow a speed schedule through changing density and temperature.
Why does the climb change from IAS to Mach?
In a constant-IAS climb, TAS generally increases as density falls. Mach also rises; if that IAS were held indefinitely, MMO could become limiting. At the crossover altitude, the scheduled IAS corresponds to the scheduled Mach, so the aircraft transitions to a constant-Mach climb. Crossover depends on the selected IAS/Mach pair and the atmosphere; it is not one universal flight level.
The display is not the energy reference
In a constant-IAS climb, TAS generally increases as density falls. Mach also rises; if that IAS were held indefinitely, MMO could become limiting. At the crossover altitude, the scheduled IAS corresponds to the scheduled Mach, so the aircraft transitions to a constant-Mach climb. Crossover depends on the selected IAS/Mach pair and the atmosphere; it is not one universal flight level.
In the troposphere, temperature and speed of sound normally decrease with height. At constant Mach, TAS therefore decreases. In an isothermal lower stratosphere, constant Mach gives approximately constant TAS. IAS decreases with height in the constant-Mach part. Compressibility matters, so the simple low-Mach IAS≈EAS density formula is not the correct high-altitude conversion.
Follow the envelope and energy change
A normal transport climb uses the approved low-altitude speed restrictions, an IAS schedule, then Mach. When TAS increases during constant-IAS climb, some excess power accelerates the aircraft instead of increasing potential energy. A constant-Mach schedule in cooling air can give a different ROC because TAS is no longer increasing.
Operational limits can be set by VMO, MMO, buffet margins, turbulence speeds, configuration or the available climb thrust. Changing temperature, mass or thrust changes the achieved ROC and may move the applicable limit. Above the tropopause, use the actual temperature profile rather than automatically assuming that temperature keeps falling.
Can you make the performance decision?
Four original questions. Work from the stated conditions and review the reasoning.