About the Influence of the Use of Geometric Altimetry on the Flight Efficiency in Climb and Descent

For decades, the sky has been navigated using a system that relies on the air itself to tell a pilot how high they are flying. Instead of measuring distance from the ground, aircraft use barometric altimeters, which sense atmospheric pressure. As an airplane climbs, the air gets thinner and the pressure drops; as it descends, the pressure rises. By calibrating this pressure against a standard model of the atmosphere, pilots can determine their altitude. This method is robust and simple, but it is not perfect. Because the atmosphere changes with weather and location, the pressure at a specific height can vary. To manage this, pilots constantly adjust their instruments to match the local pressure at sea level, a setting known as QNH. When flying high, they switch to a standard pressure setting to ensure all aircraft in the sky are using the same reference, creating "flight levels" that keep planes safely separated. However, this system requires a transition zone where pilots switch between local and standard settings, and it means that the actual height of an aircraft relative to the ground can drift slightly depending on the weather, even if the instrument reads the same number.

Imagine trying to walk a straight line while the ground beneath your feet is constantly shifting up and down; that is the challenge of flying with pressure-based altitude. A new approach, explored by researchers at the German Aerospace Center, suggests replacing this shifting reference with a fixed one: geometric altitude. This method uses satellite signals, specifically from global navigation satellite systems, to measure the exact distance from the aircraft to the center of the Earth, providing a precise, unchanging height regardless of the weather. The question driving recent research is whether this switch would make flying more efficient, saving fuel during the critical phases of climbing out of an airport and descending back to land. While the idea of a perfectly accurate height gauge sounds like an obvious improvement, the physics of flight are complex, and the answer is not as simple as "more precision equals less fuel."

André Koloschin, a researcher at the German Aerospace Center, conducted a detailed study to answer this question, focusing on the Airbus A320, a common commercial airliner. He did not fly real planes; instead, he used a highly accurate computer simulation model that had been verified against real flight test data. This virtual aircraft was put through a series of rigorous tests, simulating 294 different flight scenarios. These scenarios covered climbs and descents at three major airports in the London area: Heathrow, Stansted, and Luton. The goal was to compare how much fuel the virtual plane would burn using the traditional pressure-based system versus the new satellite-based geometric system. Crucially, the study looked at two different ways the new system could be used. First, it tested the direct effect of simply swapping the altitude measurement tool while keeping the flight paths exactly the same as they are today. Second, it explored the indirect benefits: what if the new, more precise system allowed air traffic controllers to design better, more efficient flight paths that are currently impossible because the old system is too imprecise to guarantee safety?

The results of the simulation revealed a nuanced picture. When the researchers simply swapped the altitude measurement tool without changing the flight paths, the difference in fuel consumption was incredibly small. In many cases, the direct benefit of using geometric altitude was so slight that it could be considered negligible. The study found that the amount of fuel saved or wasted depended heavily on the weather conditions at the time, specifically the atmospheric pressure. On days with certain pressure settings, the geometric system might save a tiny amount of fuel; on others, it might use a tiny bit more. Over a long period with changing weather, these small gains and losses would largely cancel each other out. The researchers concluded that the direct advantage of switching to geometric altitude, in isolation, is not a major driver for fuel savings.

However, the story changes when the researchers allowed the flight paths themselves to be optimized for the new system. Because geometric altitude is so precise, it removes the need for the wide safety margins that are currently required when using pressure-based measurements. In the simulation, the researchers designed "solution" flight paths that were tighter and more direct than the current "baseline" paths. These new paths removed unnecessary flat segments where the plane had to level off and climb again, and they allowed the aircraft to stay at higher, more efficient altitudes for longer during descent. When these optimized paths were flown, the fuel savings were significant. For the descent scenarios tested, the optimized path saved approximately 23 kilograms of fuel per flight, which represented about 6.6 percent of the total fuel used for that phase of the journey. This saving was not because the satellite system itself was more efficient, but because the system's precision enabled a smarter flight path that would be too risky to fly with the old, less precise system.

The climb scenarios told a slightly different story. Here, the fuel savings from optimization were much smaller, around 2 kilograms per flight, or roughly 0.25 percent of the fuel used. The researchers found that while removing level-off segments helped save fuel, the new system required the aircraft to follow a strictly fixed climbing angle between points. This rigid requirement actually worked against the plane's natural performance. An aircraft climbs most efficiently by adjusting its angle as it gets higher and the air gets thinner, but the fixed-angle rule forced it to fly less efficiently in some parts of the climb. This counteracted the benefits of the smoother path, leaving only a tiny net gain. In some simulated cases, the rigid rules even caused the plane to use more fuel than the current system. This suggests that while geometric altitude could improve climbs, the rules governing those climbs would need to be flexible to truly unlock the potential.

The study also highlighted a critical distinction between the climb and descent phases. During a descent, an aircraft's performance is relatively stable and predictable, making it well-suited to the fixed-angle paths enabled by geometric altitude. During a climb, however, performance varies greatly depending on the aircraft's weight and the air density, making a one-size-fits-all fixed path less efficient. The researchers noted that the biggest fuel savings in their study came from the descent optimization, where the ability to fly a continuous, smooth path without stopping to level off made a real difference. They also pointed out that the London airspace used in the study is exceptionally busy and complex, with many flight paths crossing each other. In less crowded skies, the need for such tight, optimized paths might be less urgent, and the fuel savings might be smaller.

Despite the modest direct fuel savings, the researchers argue that the case for geometric altitude remains strong for reasons beyond just fuel. The current system requires pilots to manually adjust their instruments based on local pressure reports, a process that introduces the risk of human error. There have been past incidents where a wrong setting led to an aircraft flying too low, a danger that would be eliminated if the altitude was measured directly by satellites. Furthermore, the current system requires a transition layer where pilots switch between local and standard pressure settings, a zone that limits how many planes can fly in a given area. Removing this layer could increase the capacity of the sky. The study also noted that while geometric altitude is precise, it relies on satellite signals, which can be vulnerable to interference or jamming, a risk that does not exist with the independent pressure sensors on board.

Ultimately, the research suggests that the future of flight efficiency may not lie in the altitude measurement tool itself, but in what that tool allows us to do. The direct switch from pressure to geometric altitude offers little immediate fuel benefit on its own. The real value lies in the ability to redesign flight procedures to be smoother, more direct, and safer. If the aviation industry can solve the challenges of satellite reliability and design flexible flight paths that take advantage of the new precision, the indirect benefits could be substantial. For now, the study recommends that while the climb procedures need careful rethinking to avoid fuel penalties, the descent procedures are ready for this optimization. The path forward is not just about changing a number on a gauge, but about reimagining how aircraft move through the sky to make the most of the air around them.