FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceMedium
A pilot is flying at an indicated airspeed of 120 knots at a density altitude of 8,000 ft. Using the rule of thumb that true airspeed increases about 2% per 1,000 ft of density altitude above indicated airspeed, what is the approximate true airspeed?
- A140 knots
- B120 knots
- C130 knots
- D150 knots
Show answer & explanationAnswer & explanation
Correct answer: A. 140 knots
At 8,000 ft density altitude, TAS increases about 16% over IAS (2% × 8): 120 × 1.16 = 139.2 knots, approximately 140 knots.
Why the other options are wrong
- B. This ignores the density altitude correction entirely.
- C. This applies only about an 8% increase, half of the correct amount.
- D. This overstates the correction, applying about 25% instead of 16%.
TAS vs IAS with Density Altitude
As density altitude increases, true airspeed exceeds indicated airspeed by approximately 2% for every 1,000 feet of density altitude.
- TAS ≈ IAS × (1 + 0.02 × DA in thousands)
- Thinner air requires higher TAS for same dynamic pressure
- Used for quick in-flight TAS estimates
Memory trick: Two Percent per Thousand — thin air needs more speed