Why Are Jet Engines More Efficient at Higher Altitudes?

When flying a jet engine aircraft, they are best operated at higher altitudes upwards of 30,000 to 40,000 ft. At this elevation, pilots can take advantage of optimal cruising speeds where fuel economy is best and true airspeed (TAS) can be obtained. As fuel consumption often decreases among jet engines when flown at high altitudes, the constant decrease in air density and air temperature allows them to function at their maximum RPM.

Whether piloting civilian, business, commercial, or military jet-powered aircraft, one should always strive for peak jet engine efficiency. In this blog, we will briefly go into detail the reasons why airliners, and other jets, fly so high in the atmosphere.  

Upon takeoff, pilots will utilize an altimeter to gauge altitude. This is because altitude is relative to air pressure, and as you climb in elevation, air density falls, resulting in “thin air” and reduced drag. This thinning of air is caused by earth’s gravity pulling on denser particles and trapping bigger molecules like oxygen (O), nitrogen (N), and carbon dioxide (CO₂) closer to the earth’s surface; therefore, the more elevation you gain, the less molecules (O, N, CO₂) there will be to interact with each other, leading to less dense air.

Determined by the pilot based on altimeter value, once cruising altitude has reached roughly above 30,000 ft, engine thrust can be reduced, and the plane can be leveled to fly parallel along the horizon. Additionally, due to the lack of particulates within the stratosphere which are known to cause friction and turbulence in the troposphere, many passenger airliners choose to fly at these heights for increased safety and passenger comfort. This is because, at this elevation, jet engines are able to fly with reduced air friction, allowing them to traverse the stratosphere faster with the aid of thin air. In other words, because wind resistance and other atmospheric phenomena are greatly reduced at cruising altitudes, jet engines are able to use less fuel to go faster which bolsters power output, all the while reducing engine stress.

However, while thin air is needed for jet engines to function optimally, they still require oxygen to perform key functions. This means that aircraft constructed with jet engines also have maximum predetermined altitude levels which should not be ignored. As oxygen decreases the higher you go in the atmosphere, if exceeding an elevation outside of an engine's capabilities, there will not be enough oxygen to effectively promote the combustion of air/fuel mixtures.

While the engines also need oxygen to properly combust fuel, supplemental oxygen must always be supplied to passengers and crew members when flying anywhere above 10,000 ft. Although a rare occurrence, if a heavier-than-air jet powered aircraft were to experience a sudden case of cabin depressurization, additional oxygen can be supplied to those within the vehicle through the ejection of compartmentalized masks.

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1. johnson scottish
2. Posted on July 18, 2022
aviation

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