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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) accidents involving controlled flight into terrain often occur when pilots are unaware that their aircraft’s true altitude is lower than the altitude indicated by the pressure altimeter as a result of 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 atmospheric reanalysis of the global climate (ERA5) dataset is combined with the pressure–altitude equation to construct a 30-yr monthly climatology, covering much of the United States 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 (~−1200 ft) in northern, low-terrain regions for flights near a pressure altitude of 3600 m, meaning the aircraft would be nearly 350 m lower than the altimeter indicates. Furthermore, the minimum (i.e., 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 with estimated values calculated using a simple rule of thumb that is based solely on the air temperature at altitude and the surface elevation. The rule of thumb tends to underpredict the magnitude of the estimated error, in some cases by 70 m (~200 ft), and therefore gives a lower margin of safety.

SIGNIFICANCE STATEMENT

When temperatures are colder than the defined standard atmospheric temperature for the same altitude, pressure altimeters (such as those used in general aviation) report altitudes that are higher than the aircraft is actually flying. This can put the pilots and passengers at risk of flying into terrain if not properly recognized. Although this issue is well known, a detailed climatic summary showing these potential altimeter errors has never been created to aid aviation weather education. This study uses high-resolution historical weather model data to create 1981–2010 30-yr monthly and seasonal average climatic values of estimated altimeter errors for flight levels that are frequented by general aviation pilots. For comparison, a second method for computing the altimeter error, based on a simpler rule of thumb, is also used to create a similar climatic description. Results from each method show that the mean errors can be as great as 350 m (~1200 ft) during the winter months, with the rule of thumb providing a slightly lower margin of safety for terrain avoidance.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

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

Abstract

General aviation (GA) accidents involving controlled flight into terrain often occur when pilots are unaware that their aircraft’s true altitude is lower than the altitude indicated by the pressure altimeter as a result of 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 atmospheric reanalysis of the global climate (ERA5) dataset is combined with the pressure–altitude equation to construct a 30-yr monthly climatology, covering much of the United States 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 (~−1200 ft) in northern, low-terrain regions for flights near a pressure altitude of 3600 m, meaning the aircraft would be nearly 350 m lower than the altimeter indicates. Furthermore, the minimum (i.e., 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 with estimated values calculated using a simple rule of thumb that is based solely on the air temperature at altitude and the surface elevation. The rule of thumb tends to underpredict the magnitude of the estimated error, in some cases by 70 m (~200 ft), and therefore gives a lower margin of safety.

SIGNIFICANCE STATEMENT

When temperatures are colder than the defined standard atmospheric temperature for the same altitude, pressure altimeters (such as those used in general aviation) report altitudes that are higher than the aircraft is actually flying. This can put the pilots and passengers at risk of flying into terrain if not properly recognized. Although this issue is well known, a detailed climatic summary showing these potential altimeter errors has never been created to aid aviation weather education. This study uses high-resolution historical weather model data to create 1981–2010 30-yr monthly and seasonal average climatic values of estimated altimeter errors for flight levels that are frequented by general aviation pilots. For comparison, a second method for computing the altimeter error, based on a simpler rule of thumb, is also used to create a similar climatic description. Results from each method show that the mean errors can be as great as 350 m (~1200 ft) during the winter months, with the rule of thumb providing a slightly lower margin of safety for terrain avoidance.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

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