Speed at Altitude — Visualized
At high altitude, the airspeed indicator (IAS) and the Mach number tell different stories. Pick which one to hold steady, then move the altitude slider to see what the others do.
Hold Constant
Inputs
KT
°C
35000 ft · FL350
SLFL100FL200FL300FL400
IAS / CAS
250KT
Mach
0.741
TAS
427KT
OAT
-54.3°C
Speed of sound
576KT
Density Alt
34995ft
What you're seeing
- Holding IAS constant: as you climb, air gets thinner, so for the same dynamic pressure you have to fly faster through the air → TAS rises, and so does Mach.
- Holding Mach constant: as you climb, the speed of sound drops (colder air), so TAS climbs slowly while IAS — which only feels dynamic pressure — drops sharply.
- ISA deviationchanges only temperature: it shifts TAS (warmer air → faster sound) but leaves the IAS↔Mach relationship untouched, because that's a pressure-only conversion.