AA.I.D.K3FAA-S-ACS-11A · Nov 2023Knowledge
Relationship between Mach number, indicated airspeed, true airspeed, and change over al...
Relationship between Mach number, indicated airspeed, true airspeed, and change over altitudes.
Bottom line
Mach number is the ratio of the aircraft's true airspeed to the speed of sound in the same atmospheric conditions. As altitude increases, air density decreases, so a faster true airspeed is required to produce the same pitot pressure for the same KCAS. At low/warm altitudes, the speed of sound is so high that an aircraft is limited by indicated airspeed; at high/cold altitudes, the speed of sound is lower so the aircraft is limited by Mach.
Key numbers
| Number | What it means |
|---|---|
| ≈661 KCAS at sea level; 574 KCAS at FL 380 | Speed of sound |
| 0.82 MMO = 261 KCAS at FL 380 | MMO to KCAS |
| Mach 0.7 to Mach 0.90 | Jet cruise |
FAA references
- PHAK ch. 5 (p. 5-45) — FAA-H-8083-25C (2023)
- PHAK ch. 5 (pp. 5-45–5-46) — FAA-H-8083-25C (2023)
- AFH ch. 16 (p. 16-4) — FAA-H-8083-3C
- AC 61-107, para. 1-4 — AC No: 61-107B
- PHAK — FAA-H-8083-25C (2023)
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