FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A pilot's airspeed indicator, corrected for instrument and position error to give calibrated airspeed (CAS), reads 150 knots while flying at 8,000 feet pressure altitude in standard atmospheric conditions. Using the general rule of thumb that true airspeed increases approximately 2% of CAS per 1,000 feet of altitude, what is the approximate true airspeed?
- A166 knots
- B174 knots
- C158 knots
- D150 knots
Show answer & explanationAnswer & explanation
Correct answer: B. 174 knots
At 8,000 feet, the correction is approximately 2% × 8 = 16% of CAS. 150 knots × 0.16 = 24 knots added to CAS, giving 150 + 24 = 174 knots TAS.
Why the other options are wrong
- A. This applies about 11% instead of the correct 16% correction.
- C. This undercalculates the correction; 8,000 ft requires 16%, not roughly 5%.
- D. This ignores the altitude correction entirely, which is only valid at sea level.
True Airspeed (TAS) Correction
TAS is CAS corrected for nonstandard density altitude; as altitude increases, air density decreases, so TAS exceeds CAS by roughly 2% per 1,000 feet.
- TAS > CAS at altitude due to lower air density
- Rule of thumb: add ~2% of CAS per 1,000 ft altitude
- Exact values require a flight computer or airspeed correction chart
Memory trick: Thinner air, faster TAS — climb and the gap grows.