About the influence of the use of geometric altimetry on cruise flight efficiency under nominal operations

Abstract The introduction of global navigation satellite systems (GNSS) generally enables the use of the geometric altitude for altimetry instead of the currently used barometric altitudes, which is indeed a simple and robust method for altimetry but is also subject to various measurement errors. By using geometric instead of barometric altimetry, a more efficient airspace design is be expected. However, the use of geometric altimetry also influences the aircraft performance, which generally follows barometric relations. A fast-time simulation study was performed to investigate the influence of the use of geometric altimetry instead of barometric altimetry on cruise flight efficiency. A series of more than 600 real scheduled flights was analysed and re-simulated applying geometric altimetry in order to determine the fuel consumption in the theoretical event that the respective flight had been conducted with geometric instead of the actual barometric altimetry. Furthermore, factors such as the risk to exceed the maximum barometric altitude and the influence on flight time were analysed. The simulations revealed that, on average, the flights would have consumed more fuel in cruise flight with geometric altimetry than with barometric altimetry. Also, a considerable risk to exceed the maximum barometric altitude was found. This would require additional step descents in order to prevent the exceeding of the maximum altitude. Although some assumptions had to be taken (e.g. on the aircraft take-off mass), the study is designed as a comparison study, so that systematic mistakes through wrong assumptions can be neglected as they cancel each other out throught the comparison of the same flight with barometric and geometric altimetry. Based on the results of this study, it can be concluded that the introduction of geometric altimetry in cruise flight would affect flight efficiency negatively and would probably lead to more level changes.