TAS Calculation

Calculating True Airspeed (TAS) is a core part of General Navigation.

🧭 1️⃣ Understanding TAS

True Airspeed (TAS) is the actual speed of the aircraft through the air mass.

It’s different from:

• IAS (Indicated Airspeed): What you read on the airspeed indicator.
• CAS (Calibrated Airspeed): IAS corrected for instrument and position error.
• EAS (Equivalent Airspeed): CAS corrected for compressibility (important above ~200 kt or 10,000 ft).
• TAS: EAS corrected for air density (altitude & temperature).

So:

TAS = EAS × √(ρ₀ / ρ) where ρ₀ = air density at sea level, ρ = actual air density

✈️ 2️⃣ Practical Rules of Thumb

Rule of Thumb 1 — per 1,000 ft

Add roughly 2% of IAS per 1,000 ft altitude

Example:

• IAS = 150 kt
• Altitude = 10,000 ft
• TAS = 150 + (150 × 0.02 × 10) = 180 kt

✅ This gives a quick and fairly accurate result for subsonic flight.

🧮 3️⃣ More Precise Calculation

Step-by-step method:

1. Start with IAS → Correct to CAS (using aircraft charts)

2. Correct for compressibility → CAS → EAS

3. Use altitude and temperature → EAS → TAS

Formula:

$$ TAS = EAS × \sqrt{\frac{T}{T_0}} × \frac{1}{\sqrt{\delta}} $$

where:

• (T) = total temperature (Kelvin)
• (T_0) = standard temperature at sea level (288.15 K)
• (\delta) = pressure ratio = (P/P_0)

🧩 4️⃣ Using Flight Computer (E6B or CRP-5)

On a mechanical flight computer:

1. Set pressure altitude opposite outside air temperature.
2. Read TAS opposite your IAS. ✅ Easy and used in exams or flight planning.

🧠 5️⃣ Example Problem

Given:

• IAS = 140 kt
• Pressure altitude = 8,000 ft
• OAT = –5°C
• Assume no instrument error

Step 1: Approximate TAS using rule of thumb TAS ≈ 140 + (140 × 0.02 × 8) = 140 + 22.4 = 162 kt

Step 2 (optional precise method): You’d get a very similar number using E6B.

✅ Answer: TAS ≈ 162 kt

🧭 Summary Table