FAA Sport Pilot Airplane (SPA)Airspace and NavigationHard
A pilot's airspeed indicator reads a calibrated airspeed (CAS) of 100 knots while flying at a pressure altitude of 8,000 feet under standard temperature conditions. Using the rule of thumb that true airspeed increases approximately 2% above CAS for every 1,000 feet of altitude, what is the approximate true airspeed (TAS)?
- A124 knots
- B108 knots
- C100 knots
- D116 knots
Show answer & explanationAnswer & explanation
Correct answer: D. 116 knots
Using the rule of thumb, TAS increases about 2% per 1,000 feet: 8,000 ft ÷ 1,000 = 8, so 8 × 2% = 16% increase. 100 knots × 1.16 = 116 knots TAS.
Why the other options are wrong
- A. This overstates the correction, corresponding to roughly a 24% increase (12,000 ft).
- B. This corresponds to only an 8% increase (4,000 ft), not the full 8,000 ft altitude given.
- C. This ignores the altitude correction entirely, which would only be valid at sea level.
True Airspeed (TAS) Correction
As altitude increases, air density decreases, causing true airspeed to exceed calibrated/indicated airspeed for a given dynamic pressure.
- Rule of thumb: TAS ≈ CAS + 2% of CAS per 1,000 ft of altitude
- At sea level with standard temperature, CAS and TAS are nearly equal
- TAS is used for actual navigation and groundspeed calculations
- An E6B flight computer gives a more precise TAS conversion
Memory trick: 'IAS, CAS, TAS, GS' — like climbing stairs, each speed grows with altitude and wind