What is TAS?
TAS, or True Airspeed, is the actual speed of an aircraft through the surrounding air mass. It’s what you get when you take the Indicated Airspeed (IAS) shown on the airspeed indicator and correct it for altitude and temperature — two factors that directly affect how dense the air is, and therefore how fast the aircraft is really moving through it.
How TAS Works
The airspeed indicator in any aircraft measures pressure difference — specifically the gap between the air ramming into the pitot tube and the static pressure around the aircraft. At sea level in standard conditions, that reading is reasonably close to TAS. But as you climb, the air thins. The same pressure difference now represents a higher actual speed through the air, so TAS diverges from IAS with every thousand feet you gain.
As a rough rule of thumb, TAS is approximately 2% higher than IAS for every 1,000 feet of altitude. So at FL350, a typical cruise IAS of around 280 knots might correspond to a TAS of 460 knots or more. Temperature deviation from standard also matters — warmer-than-standard air is less dense, which pushes TAS even higher for the same IAS.
This is why TAS is the speed used in flight planning and navigation. SimBrief uses TAS when calculating estimated time en route and fuel burn — not IAS, which would produce meaningless results above a few thousand feet. The FMC in a modern airliner works in TAS for its wind calculations and route predictions, too.
What TAS doesn’t account for is wind. If you’re doing 460 knots TAS into a 60-knot headwind, your groundspeed is 400 knots — and groundspeed is what actually determines how long the flight takes. TAS, groundspeed and IAS all measure different things and appear in different places in the cockpit for good reason.
At high altitudes, pilots and aircraft systems often switch from TAS to Mach number. Mach is TAS expressed as a fraction of the local speed of sound, which itself varies with temperature. Above roughly FL280 in most airliners, you’ll transition from an IAS limit to a Mach limit for the cruise phase — something the FMC manages automatically.
Key Considerations
- TAS is what SimBrief and your FMC use for flight planning calculations. If your planned flight time doesn’t match what you’d expect from your IAS readout, TAS is the reason.
- Confusing TAS with groundspeed is one of the most common stumbling blocks for newer sim pilots. TAS is your speed through the air; groundspeed is what the ground actually sees.
- TAS increases significantly with altitude for the same IAS — it’s perfectly normal for your airspeed indicator to read 280 knots while your FMC shows a TAS of 460+ at cruise level.
- In MSFS and X-Plane, TAS is displayed in the FMC or EFB. Comparing it against your SimBrief planned TAS during cruise is a quick and useful sanity check.
