Interpolating POH performance charts
Aircraft performance charts often publish values at selected temperatures, altitudes, weights or wind conditions rather than every possible point. Interpolation estimates a value between published points when the chart or procedure allows it.
Read the entire chart first
Before doing any math, identify the chart title, units, notes, conditions and limitations. Performance charts may assume a specific weight, configuration, runway surface, technique or power setting.
Linear interpolation idea
If the desired condition lies halfway between two published inputs, a simple linear interpolation places the output halfway between their values. If the input is one quarter of the way across the interval, use one quarter of the output difference.
A midpoint between chart values
If a chart gives 1,000 feet at one condition and 1,200 feet at the next, a halfway condition would be estimated at 1,100 feet only when linear interpolation is appropriate for that chart.
Do not extrapolate casually
Interpolation estimates between known points. Extrapolation goes beyond published data and can be unreliable or prohibited. If the operating condition is outside the chart, the chart may not support the operation.
Multiple variables
Many performance problems require repeated interpolation for pressure altitude, temperature, weight or wind. Work one variable at a time and keep the units visible so you can audit the process.
Chart reading errors can dominate the math
A perfectly calculated interpolation is useless if the wrong line, axis or condition was selected. Cross-check the result against nearby chart values and the expected trend.
Related PilotDesk tools
Pressure altitude · Density altitude · Weight & balance
Between 3,000 and 5,000 feet
Suppose a hypothetical chart lists a 1,000 ft result at 3,000 ft pressure altitude and 1,600 ft at 5,000 ft, with every other input held constant. A 4,000 ft condition is halfway between the published altitudes, so linear interpolation estimates 1,300 ft: 1,000 + 0.5 × (1,600 − 1,000). These invented figures explain the method; they are not aircraft performance data.
If the condition is one third of the way from the lower to upper input, use one third of the output difference. Interpolate one variable at a time, only when the chart instructions permit it, and do not extrapolate beyond published values. For an actual takeoff or landing decision, use the exact approved chart and all of its notes, corrections and margins.