Rules of thumb

Use your mach meter for direct reading of ASF above FL 150. Explanation: The speed of sound equals 600 KT at -35ºC. that is the temperature in standard atmosphere at FL 250. Therefore: Mach 1,0 = TAS 600 KT → ASF = 10 (Nm/min) or. Mach 0,7 = TAS 420 KT → ASF = 7 (Nm/min) or. Mach 0,6 = TAS 360 KT → ASF = 6 (Nm/min) etc. ................
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