Climatology of Estimated Altimeter Error Due to Nonstandard Temperatures

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  • 1 Department of Applied Aviation Sciences, Embry-Riddle Aeronautical University, Daytona Beach, Florida
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Abstract

General-aviation (GA) controlled flight into terrain accidents often occur when a pilot is unaware their aircraft’s true altitude is lower than the altitude indicated by the pressure altimeter due to colder than standard temperatures. However, little guidance is available that quantifies the magnitude of these altimeter errors and their variation with season. In this study, the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5) data set is combined with the pressure-altitude equation to construct a 30-year monthly climatology covering much of the U.S. and Canada of D-value (i.e., true altitude minus pressure altitude) corrected for the standard atmosphere height separation between the altimeter setting and standard mean sea-level pressure. This “corrected” D-value therefore provides a useful estimate of the error between true and altimeter-indicated altitude. During winter, the mean corrected D-values reach values as low as −350 m (~ −1,200 feet) in northern, low-terrain regions for flights near a pressure altitude of 3,600 m, meaning the aircraft would be nearly 350 m lower than the altimeter indicates. Furthermore, the minimum (maximum negative) corrected D-values are nearly double their mean values for the same time period. In addition, the reanalysis-based corrected D-values are compared to estimated values calculated using a simple rule-of-thumb based solely on the air temperature at altitude and the surface elevation. The rule-of-thumb tends to under-predict the magnitude of the estimated error, in some cases by 70 m (~200 feet), and therefore gives a lower margin of safety.

Corresponding author: Thomas A. Guinn, guinnt@erau.edu

Abstract

General-aviation (GA) controlled flight into terrain accidents often occur when a pilot is unaware their aircraft’s true altitude is lower than the altitude indicated by the pressure altimeter due to colder than standard temperatures. However, little guidance is available that quantifies the magnitude of these altimeter errors and their variation with season. In this study, the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5) data set is combined with the pressure-altitude equation to construct a 30-year monthly climatology covering much of the U.S. and Canada of D-value (i.e., true altitude minus pressure altitude) corrected for the standard atmosphere height separation between the altimeter setting and standard mean sea-level pressure. This “corrected” D-value therefore provides a useful estimate of the error between true and altimeter-indicated altitude. During winter, the mean corrected D-values reach values as low as −350 m (~ −1,200 feet) in northern, low-terrain regions for flights near a pressure altitude of 3,600 m, meaning the aircraft would be nearly 350 m lower than the altimeter indicates. Furthermore, the minimum (maximum negative) corrected D-values are nearly double their mean values for the same time period. In addition, the reanalysis-based corrected D-values are compared to estimated values calculated using a simple rule-of-thumb based solely on the air temperature at altitude and the surface elevation. The rule-of-thumb tends to under-predict the magnitude of the estimated error, in some cases by 70 m (~200 feet), and therefore gives a lower margin of safety.

Corresponding author: Thomas A. Guinn, guinnt@erau.edu
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