Definition
The altitude at which a specified indicated airspeed (in knots) and a specified Mach number represent the same true airspeed. Below this altitude, the airspeed limit governs; above it, the Mach limit governs.
Plain English
The height at which a pilot stops controlling speed by knots and starts controlling it by Mach number. Below it, you watch the airspeed indicator. Above it, you watch the Mach indicator.
Context Anchor
Seen in high-altitude climb and descent planning, especially in turbine and jet aircraft speed schedules.
Derivation
From 'cross over,' meaning to switch from one thing to another. At this altitude, the pilot crosses over from using indicated airspeed as the limiting reference to using Mach number.
Why Pilots Care
Maintaining the correct speed schedule across this altitude protects performance, fuel efficiency, and handling margins.
Intuition Check
Crossover altitude is not an altitude where flight paths cross. It is the altitude where two speed references match and the speed schedule changes from one reference to the other.
Example Sentence 1
Climbing through the crossover altitude, the pilot transitioned from holding 280 knots to holding Mach 0.74.
Example Sentence 2
On descent the pilot held Mach 0.78 until passing through crossover altitude, then shifted to a constant 280-knot indicated airspeed.