Since everyone now days uses an EFB, many pilots can not, or do not work back from TAS to figure out their needed IAS to fly their flight plan. So lets explore:
CAS/IAS from TAS
Inputs: TAS (kt), Pressure Altitude (ft), OAT (°C). Output: CAS ≈ IAS (172).
Estimated CAS/IAS: — kt
Density Ratio (σ = ρ/ρ₀): —
Assumes troposphere, PA uses 29.92, ignores compressibility & minor CAS↔IAS differences.
Quick student note
- Thinner air (higher PA or hotter OAT) → lower CAS/IAS for same TAS.
- We compute air density from PA & OAT, then use CAS/IAS ≈ TAS × √σ.
- In a 172, CAS and IAS are within a few knots → presented together.
Why Does My EFB Show TAS, But I Fly IAS? And What the Heck is CAS?
A pilot-friendly guide to understanding TAS, IAS, and CAS—without the aerodynamic jargon.
✅ True Airspeed (TAS): The Planner’s Speed
Your EFB cares about TAS because:
- TAS is your actual speed through the air mass.
- It matters for navigation and timing—how fast you move over the ground after wind correction.
- Performance charts (climb, cruise, fuel burn) assume TAS under standard conditions.
Think of TAS as the speed your airplane would show if the air were sea-level dense everywhere. At altitude, the air is thinner, so for the same TAS, your pitot tube feels less pressure.
✅ Indicated Airspeed (IAS): The Flyer’s Speed
Your airspeed indicator shows IAS, which is basically:
- The dynamic pressure
- Why do we fly IAS? Because lift, stall speed, and control feel depend on dynamic pressure, not on how fast you’re slicing through thin air.
Example: At 8,000 ft, you might be cruising at 120 KTAS, but your IAS could be only 105 knots. The wing doesn’t care about TAS—it cares about IAS because that determines lift.
✅ Calibrated Airspeed (CAS): The Reality Check
CAS is just IAS corrected for:
- Instrument error (your gauge isn’t perfect).
- Position error (pitot/static placement quirks).
In a Cessna 172, CAS and IAS are usually within a couple of knots, so for training, we often treat them as the same. But in faster or more complex aircraft, CAS matters for accurate performance calculations.
🛫 Why It Matters
- EFB flight plans use TAS for timing and fuel burn.
- You fly IAS/CAS because that’s what keeps you safe—stall speeds, approach speeds, and maneuvering speeds are all IAS-based.
- At altitude, IAS drops while TAS stays high. That’s why your ground speed can be smoking fast, but your stall speed hasn’t changed.
✅ Quick Example
Cruising at 8,000 ft on a cool day:
- TAS: 120 knots (EFB shows this for planning).
- IAS: ~105 knots (your indicator shows this for flying).
- CAS: ~104 knots (after tiny corrections).
Same airplane, same wing, same stall speed—just thinner air.
💡 Bottom Line
- Plan with TAS (EFB, navigation, fuel).
- Fly with IAS/CAS (performance, safety).
- CAS is the “true” indicated speed after corrections—important for POH charts and precision flying.