- How do you calculate true airspeed from indicated airspeed?
- Divide IAS by the square root of the air density ratio (sigma) at your altitude: TAS = IAS / sqrt(sigma). As a rule of thumb, TAS increases about 2% per 1,000 ft - so 100 kts IAS at 10,000 ft is roughly 120 kts TAS. This calculator applies the exact ICAO standard atmosphere maths.
- What is the difference between IAS and TAS?
- IAS is what your airspeed indicator reads - it reflects dynamic pressure on the pitot tube. TAS is your actual speed through the air mass, corrected for air density. At high altitudes where air is thinner, TAS is significantly higher than IAS.
- Why does Mach matter in cruise?
- At high altitudes, airframe buffet onset and critical Mach limits are expressed in Mach rather than knots. Most jet aircraft have a Mmo (maximum operating Mach) around M0.82-0.92. Monitoring Mach prevents overspeed in the thin air of the flight levels.
- When should I enter OAT instead of leaving it blank?
- Use OAT when the day is significantly warmer or colder than ISA. A large ISA deviation (ISA+20 is common on hot days) meaningfully changes TAS and Mach, and the default ISA assumption would give inaccurate results.