True Airspeed (TAS) Study Guide & Exam Prep
Demystify IAS, CAS, EAS, and TAS conversions. Master the 2% per 1,000 feet rule of thumb and compressibility math.
Editorially reviewed by the AviCalculator team
1. Airspeed Hierarchy & Definitions
Understanding the exact aerodynamic relationships between cockpit instrument readings and true velocity is heavily tested on FAA private and commercial pilot exams.
- Indicated Airspeed (IAS): Direct uncorrected reading from cockpit airspeed indicator measuring dynamic ram pitot pressure.
- Calibrated Airspeed (CAS): IAS corrected for instrument manufacturing tolerance and static port position errors across varying angles of attack (crucial during slow flight/flaps down).
- Equivalent Airspeed (EAS): CAS corrected for adiabatic ram air compressibility shockwaves inside the pitot tube. Only mathematically significant above Mach 0.3 (~200 knots).
- True Airspeed (TAS): Actual physical velocity of the aircraft moving through the undisturbed air mass. CAS corrected for non-standard air density (ρ).
2. Governing Mathematical Formulas
The 2% Rule as an Oral-Exam Demonstration
Designated pilot examiners routinely ask you to estimate TAS in your head without a calculator. The accepted cockpit shortcut is to add roughly 2% of indicated airspeed for every 1,000 feet of altitude. Practice stating it aloud as you would on a checkride: “Two percent per thousand, so at ten thousand feet I add about twenty percent to my indicated.” The calculator page proves the exact number; here you are demonstrating you can reason about it live.
Exact Aerodynamic Density Scaling Formula
3. Practical Exam Calculation Walkthrough
Exam Scenario: Cruising at 10,500 ft MSL. IAS reads 125 knots. OAT is standard. Estimate TAS.
- Apply 2% Rule: 10.5 (thousands) × 2% = 21% speed increase over indicated.
- Calculate Increase: 125 × 0.21 = 26.25 knots.
- Sum for TAS: 125 + 26.25 = 151.25 KTAS.
4. Checkride Trap Mistakes to Avoid
Students frequently confuse which airspeed flies aerodynamic stall speeds vs which navigates maps. Remember: You Fly the airplane referencing IAS (because stall angle of attack is governed by dynamic ram pressure), but You Navigate referencing TAS (because map checkpoint calculations require true air velocity combined with wind vectors).
Test Your Knowledge
You've reviewed the theory. Now apply it to real FAA/EASA checkride questions to verify your understanding.