FAA Instrument Flight Instructor (FII)Instruments and SystemsHard
An aircraft is flying at 8,000 feet MSL under standard atmospheric conditions with a calibrated airspeed of 150 knots. Using the general rule of thumb that true airspeed increases approximately 2% above calibrated airspeed for every 1,000 feet of altitude, the approximate true airspeed is
- A150 knots
- B162 knots
- C174 knots
- D186 knots
Show answer & explanationAnswer & explanation
Correct answer: C. 174 knots
Using the rule of thumb, the correction is 2% per 1,000 feet × 8 (thousands of feet) = 16%. Applying this to CAS: 150 kt × 0.16 = 24 kt correction. TAS ≈ 150 + 24 = 174 knots.
Why the other options are wrong
- A. This ignores the altitude correction entirely, treating TAS as equal to CAS.
- B. This corresponds to only an 8% correction, half of the correct 16% for 8,000 feet.
- D. This overstates the correction, corresponding to roughly a 24% increase instead of 16%.
True Airspeed (TAS) Rule of Thumb
TAS is approximately CAS increased by 2% for every 1,000 feet of altitude under standard atmospheric conditions, reflecting decreasing air density with altitude.
- Rule of thumb: +2% TAS per 1,000 ft above CAS
- Air density decreases with altitude, requiring higher TAS for same dynamic pressure
- IAS≈CAS≈EAS at low speed/altitude with corrections
- TAS is used for flight planning and navigation calculations
Memory trick: Two percent a grand, TAS climbs as planned.