Table of Contents
Unit definitions and an ideal-gas speed example for learning. This article is not a flight-planning tool or cockpit procedure. Equations use a decimal point.
1Length, speed and mass
The foot and nautical mile are length units; the knot is a speed unit. Here lb denotes avoirdupois mass, not pound-force. The conversion factors below do not describe altitude reference systems or aircraft performance. [1] 1 ft = 0.3048 m; 1 nmi = 1852 m; 1 kn = 1 nmi/h = 1.852 km/h; 1 lb = 0.45359237 kg.
2Temperature and sound speed
For an ideal gas, a = √(γRT), with absolute temperature T in kelvin. In this illustrative air model, γ and R are held constant. Starting from the rounded reference a1 = 661.5 kn at 15 °C (288.15 K), the ratio a2/a1 = √(T2/T1) gives the following estimate at −50 °C (223.15 K). It is a model calculation, not a measured atmospheric value. [2] a2 = 661.5 × √(223.15 / 288.15) ≈ 582.1 kn.
3A Mach-number example
Mach number compares speed relative to the gas with its local sound speed. Using the same temperature and model as above, multiply Mach by a2. A fixed Mach number therefore has no single temperature-independent value in kn or km/h. [2] TAS = Mach × a2 = 0.85 × 582.12866 ≈ 494.8 kn.
Results depend on the stated assumptions. Sources checked on 2026-10-03.
