FAA Commercial Pilot Airplane (CAX)Navigation and Flight PlanningHard
An airplane's calibrated airspeed is 140 knots while cruising at 8,000 feet MSL under standard conditions. Using the rule of thumb that true airspeed increases approximately 2% for every 1,000 feet of altitude above sea level, what is the approximate true airspeed?
- A162 knots
- B168 knots
- C155 knots
- D148 knots
Show answer & explanationAnswer & explanation
Correct answer: A. 162 knots
At 8,000 feet, the increase is approximately 2% x 8 = 16%. TAS ≈ 140 x 1.16 = 162.4 knots, which rounds to approximately 162 knots.
Why the other options are wrong
- B. This overstates the increase, closer to a 20% correction.
- C. This reflects roughly a 10-11% increase, still below the expected 16%.
- D. This reflects only about an 6% increase, far too small for 8,000 feet.
TAS Rule of Thumb
A quick estimation method stating that true airspeed increases approximately 2% above calibrated/indicated airspeed for every 1,000 feet of altitude gained under standard conditions.
- 2% increase per 1,000 feet is an approximation, not exact
- Actual TAS also depends on non-standard temperature deviations
- More precise TAS is found using an E6B flight computer with pressure altitude and temperature
Memory trick: 'Two percent per grand' — TAS climbs 2% for every 1,000 feet up.