The 2% true-airspeed rule: a quick estimate
A common training shortcut estimates true airspeed (TAS) by adding about 2% of calibrated airspeed (CAS) for each 1,000 ft of altitude. It is handy for a rough mental check at ordinary light-airplane speeds, but temperature, pressure altitude, airspeed calibration, and compressibility affect the actual result.
Rule of thumb
Approximate TAS = CAS × (1 + 0.02 × pressure altitude in thousands of feet)
For 110 kt CAS at 8,000 ft pressure altitude, the factor is 1 + (0.02 × 8) = 1.16. The shortcut gives 110 × 1.16 = 127.6 kt TAS, or about 128 kt. This is a shortcut calculation; the result is not an aircraft-system measurement or a performance-chart value.
Why true airspeed differs
An airspeed indicator responds to pressure rather than directly measuring speed through the air. TAS is the speed of the airplane through the surrounding air mass after accounting for air density. As density decreases, the airplane moves faster through the air for a similar indicated/calibrated value. The simple 2% rule assumes a rough density change with altitude and does not fully model nonstandard temperature.
On a warmer-than-standard day, air density is lower than the standard-atmosphere estimate for the same pressure altitude, so TAS for a given CAS tends to be higher than the shortcut suggests. Colder-than-standard air tends in the other direction. At higher speeds, compressibility also matters. For more precise work, use the aircraft's approved data, an appropriate E6B/flight computer, or a calculation using the correct CAS, pressure altitude, and outside-air temperature.
Use IAS, CAS, TAS, and groundspeed carefully
IAS is the instrument indication. CAS corrects IAS for instrument and position error. TAS corrects calibrated speed for air density. Groundspeed is speed over the ground after wind is considered. If the question provides IAS and the aircraft's calibration correction is relevant, apply that correction before treating it as CAS. The 2% rule is commonly taught from an indicated reading as a quick approximation; do not present it as a precise conversion between IAS and TAS.
These speeds answer different questions. Aircraft operating and performance speeds are commonly given as indicated values; flight planning uses TAS and then accounts for wind to estimate groundspeed. Do not substitute one just because the values share the unit “knots.”
When this shortcut is not enough
Use a more complete method if the answer affects a flight-plan estimate, a performance calculation, a high-altitude/high-speed operation, or a comparison close to a limit. Confirm which airspeed type the chart, instrument, or source expects and keep pressure altitude, temperature, and units consistent.
The FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 8, describes true airspeed as calibrated airspeed corrected for altitude and nonstandard temperature, and presents the 2% shortcut. For full wind and time planning, use the wind-triangle guide and the TAS calculator.