How does Mach number interact with the airspeed indicator at high altitudes?

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Multiple Choice

How does Mach number interact with the airspeed indicator at high altitudes?

Explanation:
At high altitude the speed limit shifts from the indicated airspeed to Mach number. The airspeed indicator shows indicated airspeed based on dynamic pressure and is calibrated for sea‑level standard air, so as the air becomes thinner and compressibility effects grow, the IAS reading becomes less representative of the airplane’s actual speed in the thin air. The aircraft’s maximum safe speed is set by the Mach limit (Mmo), which stays essentially fixed with altitude. Because the Mach limit governs and the IAS value that corresponds to that Mach tends to approach the instrument’s redline in the thinner air, you’ll reach the IAS redline before you actually exceed Mmo as altitude increases. So, the Mach limits become the controlling factor, and the IAS will tend toward its redline first.

At high altitude the speed limit shifts from the indicated airspeed to Mach number. The airspeed indicator shows indicated airspeed based on dynamic pressure and is calibrated for sea‑level standard air, so as the air becomes thinner and compressibility effects grow, the IAS reading becomes less representative of the airplane’s actual speed in the thin air. The aircraft’s maximum safe speed is set by the Mach limit (Mmo), which stays essentially fixed with altitude. Because the Mach limit governs and the IAS value that corresponds to that Mach tends to approach the instrument’s redline in the thinner air, you’ll reach the IAS redline before you actually exceed Mmo as altitude increases. So, the Mach limits become the controlling factor, and the IAS will tend toward its redline first.

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