ESDU 91020 presents data on the variability of civil aircraft and engine performance quantities drawn from a report of the ICAO Standing Committee on Performance, and also more recent information on the variability of turbo-fan engine performance quantities. These latter data are given as ranges of uncertainty and are converted to the ICAO form of a standard deviation using information from ESDU 91017. The variability data are then used to calculate the variability of take-off climb gradient; the methods of ESDU 91017 are again used, this time to calculate the overall variability arising from the cumulative effects of the individual variabilities in drag, thrust and weight.