Airspeed and Mach concepts
Overview
Airspeed and Mach concepts tests your ability to move between the different speed definitions and to relate TAS to the local speed of sound. Questions typically involve conversions, altitude effects and Machmeter logic.
Core rules
- IAS is what the instrument shows. CAS corrects IAS for position (and instrument) error. EAS corrects CAS for compressibility; TAS corrects EAS for air density. GS adds wind.
- At low speed EAS ≈ CAS. At altitude TAS = EAS/√σ where σ = ρ/ρ0.
- Mach number M = TAS/a. The speed of sound depends only on temperature: a = 38.94√T(K) knots, about 661 kt at ISA sea level.
- In the ISA troposphere temperature falls about 1.98 °C per 1,000 ft, so a falls with altitude. M_MO is a constant Mach limit; V_MO is an IAS limit.
- A Machmeter uses pitot and static pressures; the pressure ratio determines M, so no outside air temperature input is needed.
Worked example
FL350, ISA T = 218.8 K. a = 38.94√218.8 = 576 kt. TAS 460 kt gives M = 460/576 = 0.80. If the air is 10 °C warmer, a rises to about 589 kt and M falls to 0.78 for the same TAS.
Traps
- Using IAS as TAS in Mach calculations.
- Thinking the speed of sound depends on pressure or density.
- Assuming M_MO can always be reached at low altitude, where the IAS limit V_MO applies first.