True Airspeed Calculator
Calculate true airspeed (TAS) from indicated airspeed, altitude, and temperature
Calculator
Flight Parameters
Results
Where ρ₀ = sea level density, ρ = density at altitude
True Airspeed Reference Table
TAS values for various altitudes at standard temperature (IAS = 100 knots baseline)
| Altitude | Std Temp | TAS (100 kt IAS) | TAS Increase | Density Ratio |
|---|---|---|---|---|
| Sea Level | 15°C / 59°F | 100 kts | 0% | 1.000 |
| 5,000 ft | 5°C / 41°F | 108 kts | +8% | 0.862 |
| 10,000 ft | -5°C / 23°F | 117 kts | +17% | 0.738 |
| 15,000 ft | -15°C / 5°F | 128 kts | +28% | 0.629 |
| 20,000 ft | -25°C / -13°F | 141 kts | +41% | 0.533 |
| FL250 | -35°C / -31°F | 155 kts | +55% | 0.448 |
| FL350 | -55°C / -67°F | 194 kts | +94% | 0.310 |
| FL410 | -57°C / -70°F | 224 kts | +124% | 0.217 |
How True Airspeed Is Calculated
The Formula
True Airspeed:
TAS = CAS × √(ρ₀ / ρ)
ρ₀ = Sea level air density (1.225 kg/m³)
ρ = Air density at altitude
Density Ratio:
ρ/ρ₀ = (P/P₀) × (T₀/T)
P = Pressure at altitude
T = Temperature at altitude (Kelvin)
Quick Rules of Thumb
True Airspeed Regulatory Requirements
FAA Requirements (United States)
14 CFR 91.117 - Aircraft Speed
Speed limits are based on indicated airspeed. Pilots must understand TAS to ensure compliance while maintaining safe navigation.
14 CFR 91.159 - VFR Cruising Altitude
VFR altitude rules apply based on magnetic course. TAS calculations are essential for fuel planning at various altitudes.
AC 00-45H - Aviation Weather Services
Provides guidance on using winds aloft data, which requires TAS for ground speed calculations.
EASA Requirements (Europe)
SERA.5005(f) - Pre-flight Action
Pilots must determine estimated times en route, requiring accurate TAS calculations for flight planning.
SERA.6001 - VMC Requirements
Visibility requirements reference speeds that must be understood in terms of both IAS and TAS.
Professional Best Practices
Calculation Accuracy
- Use current altimeter setting
- Verify OAT readings
- Cross-check with GPS ground speed
- Account for non-standard conditions
Flight Planning
- Calculate TAS for each altitude
- Consider temperature deviations
- Update TAS en route as needed
- Factor wind into ground speed
Navigation
- Use TAS for wind triangle solutions
- Calculate accurate ETAs
- Monitor fuel consumption rates
- Verify with actual ground speed
Frequently Asked Questions
What is the difference between indicated airspeed and true airspeed?
Indicated Airspeed (IAS) is the speed shown on the aircraft's airspeed indicator, while True Airspeed (TAS) is the actual speed of the aircraft through the air mass. At sea level in standard conditions, they are nearly equal. As altitude increases, air density decreases, causing TAS to become progressively higher than IAS for the same indicated speed.
Why does true airspeed increase with altitude?
True airspeed increases with altitude because air density decreases at higher altitudes. The airspeed indicator measures dynamic pressure, which depends on air density. In thinner air, the aircraft must move faster through the air mass to create the same dynamic pressure on the pitot tube. At FL350, TAS is typically about 1.7 times higher than IAS.
How do I calculate true airspeed from indicated airspeed?
True airspeed is calculated by correcting indicated airspeed for altitude and temperature using the formula: TAS = CAS × √(ρ₀/ρ), where ρ₀ is sea level density and ρ is air density at altitude. A quick rule of thumb: TAS increases approximately 2% per 1,000 feet of altitude under standard conditions.
What is calibrated airspeed (CAS)?
Calibrated Airspeed (CAS) is indicated airspeed corrected for instrument and position errors. Most modern aircraft have minimal errors, making CAS very close to IAS in normal flight regimes. The correction is typically documented in the aircraft's POH/AFM and varies by aircraft type and configuration.
Why is true airspeed important for flight planning?
True airspeed is essential for accurate flight planning because it represents the actual speed through the air. When combined with wind data, TAS allows calculation of ground speed, fuel consumption estimates, and accurate time en route. GPS ground speed equals TAS only when there is zero wind.
How does temperature affect true airspeed?
Higher temperatures reduce air density, which increases true airspeed for a given indicated airspeed. For every 10°C above standard temperature, TAS increases by approximately 2%. Conversely, colder temperatures increase air density and reduce TAS. This is why the OAT (Outside Air Temperature) is needed for accurate TAS calculations.
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