Crossover Altitude Calculator (CAS/Mach)
Where does 280 knots become Mach 0.74? Compute the exact crossover altitude for any climb speed schedule with the real pitot equations.
Below crossover the airspeed needle would hit the structural limit first; above it the Mach needle hits MMO first. The FMS switches automatically โ this is the altitude where it happens for your schedule.
Formula
โ ๏ธ For flight planning and education only โ verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
Where does 280 knots become Mach 0.74? Compute the exact crossover altitude for any climb speed schedule with the real pitot equations.
About Crossover Altitude Calculator (CAS/Mach)
Every jet climb is filed as a pair โ 280/.74, 300/.78 โ CAS below, Mach above, switching at the crossover altitude where both describe the identical true condition. This calculator finds that altitude exactly, using the real compressible pitot equations rather than chart approximations: the pressure ratio where your CAS's impact pressure equals your Mach's, inverted through the ISA. Type-rating students and simmer pilots get the same number the FMS computes.
How to use Crossover Altitude Calculator (CAS/Mach)
- 1Enter โ sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula ฮด_cross = [(1+0.2(CAS/aโ)ยฒ)^3.5 โ1] / [(1+0.2Mยฒ)^3.5 โ1]; invert ISA ฮด for altitude substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Crossover Altitude Calculator (CAS/Mach)?
- โInstant, free and private โ every calculation runs in your browser, nothing is uploaded
- โBuilt on the published formula ฮด_cross = [(1+0.2(CAS/aโ)ยฒ)^3.5 โ1] / [(1+0.2Mยฒ)^3.5 โ1]; invert ISA ฮด for altitude with sources cited on the page
- โBelow crossover the airspeed needle would hit the structural limit first; above it the Mach needle hits MMO first. The FMS switches automatically โ this is the altitude where it happens for your schedule.
- โSwitch units, tweak any input and watch every result update live
Frequently asked questions
What physically happens at the crossover altitude?+
Nothing โ and that's the point. Climbing at constant CAS, your Mach number rises (sound slows in colder air while TAS grows); at crossover, the rising Mach reaches your schedule's Mach target, and the crew or autothrottle simply re-references the hold from the CAS window to the Mach window. The airplane never feels the switch.
Why climb on a CAS/Mach schedule at all?+
Two different limits bound the envelope: dynamic pressure (structure, VMO) low down, and compressibility (shock onset, MMO) up high. Constant CAS respects the first while letting Mach drift up; capping at constant Mach respects the second while CAS drifts down. The two-segment schedule rides each limit's safe side in its own domain.
Does temperature change the crossover altitude?+
No โ elegantly, the crossover is a pure pressure-altitude phenomenon. Both CAS and Mach are defined through pressure ratios, so the ฮด where they agree is temperature-free; ISA deviation changes the TAS at crossover but not the altitude. (Our explain line quotes ISA TAS, but the altitude holds on any day.)
What are typical airline climb schedules?+
Single-aisles commonly climb 250 KIAS below 10,000 ft (regulatory), then 280โ300/.74โ.78 โ a 280/.74 schedule crosses near 29,500 ft; 300/.78 near 28,300 ft. Cost-index logic in the FMS nudges these for fuel-vs-time economics; heavy widebodies run similar numbers with higher Mach caps.
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