Watch the speed tape in an airliner's climb and you will notice something odd. The aircraft holds a steady indicated airspeed for thousands of feet, then the target changes to a Mach number, and the indicated airspeed slowly falls as the aircraft keeps climbing. The point where the change happens is the crossover altitude.
IAS, TAS and Mach: a quick refresher
- Indicated airspeed (IAS) reflects the dynamic pressure of the air on the aircraft. It governs lift, handling and structural loads, so it is what pilots fly at lower levels.
- True airspeed (TAS) is the real speed through the air. For the same IAS, TAS increases as the air gets thinner with altitude.
- Mach number is TAS divided by the local speed of sound. The speed of sound depends on temperature, and it falls as the air gets colder with altitude.
What happens in a constant-IAS climb
Climbing at, say, 290 knots IAS, the aircraft's TAS keeps rising while the local speed of sound keeps falling. Both effects push the Mach number up. Eventually the Mach number reaches the value planned for the climb, for example Mach 0.78. That altitude, where the selected IAS and the selected Mach number are equal, is the crossover altitude. Above it the aircraft holds the Mach number, and the IAS gradually decreases.
Watch: crossover altitude in 60 seconds
Why switch to Mach at high altitude?
Every jet has two speed limits: VMO (maximum operating speed, as IAS) and MMO (maximum operating Mach number). Lower down, VMO is the limit. Higher up, the aircraft would reach MMO long before VMO, so Mach becomes the limit that matters. Flying a Mach number at altitude keeps the aircraft away from compressibility effects that appear as airflow over the wing approaches the speed of sound.
A related danger: Mach tuck
If a jet goes beyond its critical Mach number, shock waves form on the upper surface of the wing. The centre of pressure moves aft and the nose tends to pitch down, which increases speed further. This is Mach tuck. Modern jets have a Mach trim system that automatically counters it, and MMO is set to keep the aircraft clear of it.
In the descent, it works in reverse
Descending at a constant Mach number, the IAS increases as the air gets denser. At the crossover altitude the target changes back to an IAS, and the aircraft continues down at that IAS.
Exam summary
- Constant IAS climb → TAS and Mach increase.
- Crossover altitude: where selected IAS and selected Mach coincide.
- Above it, fly Mach; the IAS decreases as you climb.
- VMO limits at lower levels, MMO at higher levels.
- Mach tuck: nose-down pitch beyond the critical Mach, countered by the Mach trim.
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