FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceMedium
Using a landing performance table, a pilot finds a ground roll of 1,400 ft at a pressure altitude of 2,000 ft and 1,600 ft at a pressure altitude of 4,000 ft, with all other variables held constant. What is the interpolated ground roll for a pressure altitude of 3,000 ft?
- A1,500 ft
- B1,400 ft
- C1,600 ft
- D1,450 ft
Show answer & explanationAnswer & explanation
Correct answer: A. 1,500 ft
3,000 ft is exactly halfway between 2,000 ft and 4,000 ft. The difference in ground roll is 1,600 − 1,400 = 200 ft, so half of that (100 ft) is added to the lower value: 1,400 + 100 = 1,500 ft.
Why the other options are wrong
- B. This is the value at 2,000 ft, not the interpolated value.
- C. This is the value at 4,000 ft, not the interpolated value.
- D. Only a quarter of the difference was added instead of half.
Linear Interpolation in Performance Charts
A method to estimate a value between two known table entries by proportionally distributing the difference.
- Find the fraction of distance between the two known points
- Multiply that fraction by the difference in outcome values
- Add the result to the lower known value
Memory trick: Halfway in, halfway out on the chart