Why do Planes Fly so High?

Why do commercial planes fly so high?

The reason passenger aircraft have a high cruising altitude (between 30,000 feet and 42,000 feet) is due to the high levels of fuel efficiency they achieve at this level. At high altitude, aircraft fuel consumption is much less per mile flown compared to low altitude flight due to reduced aerodynamic drag and improved jet engine efficiency. This means that a jet aircraft can fly much faster while consuming less fuel at high altitudes.

Flying above 30,000 feet also has the advantage of allowing the aircraft to fly above most weather systems, making it more comfortable for passengers.

This article will look at the two main reasons why passenger planes fly so high; Improved engine efficiency and reduced air density.

Motor efficiency, speed and density

Modern jet engines in commercial passenger aircraft (called turbofans) are most efficient when operated at high altitudes. This is because jet engines gain efficiency when operated near their maximum rev limit or maximum (exhaust) temperature limits. At lower altitudes, the engines can only be operated at maximum thrust during takeoff or possibly climb, otherwise the aircraft would quickly exceed its maximum speed limitation. If you tried to fly straight and level at 10,000 feet with more than 70% thrust, you would quickly outlast most commercial jet aircraft. At 70% thrust, the motor does not spin very efficiently.

As the aircraft increases in altitude, the jet engines produce less thrust (because the air is thinner), but they maintain a high compression ratio and thermal efficiency. As the air is thinner, the aircraft is able to reach a much higher true air speed (TAS) than at bottom, which means the aircraft moves much faster while the engines consume less fuel. fuel.

Thinner air

The higher you go in the air, the less dense the air, or in other words, the thinner it is. Therefore, there is less resistance (or friction) to keep the plane from moving through the air. This is called resistancedragged“. This means that it takes less thrust from the engines to propel the aircraft through the air, or in other words, the aircraft can fly faster for the same high (and therefore efficient) thrust setting.

Here is an example to show how the air gets thinner as you go up; imagine running your hand through water and golden syrup. If you want to move your hand through these two liquids at the same speed, you need a lot more effort to move your hand through golden syrup than through water. It is the same principle with an airplane flying at a higher altitude compared to a lower altitude.


If you found this article of interest, check out our page on How Fast Do Airplanes Fly?