Why do Passenger Planes Fly so High in the Sky?
Why do aircraft fly at high altitude?
Why Do Commerical Aircraft Fly So High?
The reason passenger aeroplanes have a high cruising altitude (typically between 30,000 ft – 42,000 ft), is due to the high levels of fuel efficiency they can achieve at that level. At high altitude, aircraft fuel burn is much less per mile travelled when compared to low level flight because of the reduced aerodynamic drag (due to thinner air) and improved jet engine efficiency. This means a jet aeroplane can fly much faster whilst using less fuel at a high altitude.
Flying above approximately 30,000 ft also has the benefit of allowing the aeroplane to fly above most weather systems making it more comfortable for the passengers.
This article will look at the main two reasons why passenger planes fly so high; improved engine efficiency and reduced air density.
Engine Efficiency, Airspeed & Density
Modern jet engines on commercial passenger aircraft (referred to as Turbo-Fans), are most efficient when they are operated at high altitude. This is because jet engines gain an efficiency benefit when they are run at close to their maximum RPM limit or maximum (exhaust) temperature limitations. At lower altitudes, the engines can only be run at maximum thrust during take-off or perhaps the climb or else the aircraft would quickly exceed its maximum speed limitation. If you tried to fly straight and level at 10,000ft with more than 70% thrust set, you would quickly overspeed most commercial jet aircraft. At 70% thrust, the engine isn’t running very efficiently.
As the aircraft climbs in altitude, the jet engines produce less thrust (as the air is thinner), but it maintains a high compression ratio and thermal efficiency. As the air is thinner, the plane is able to achieve a much higher True Air Speed (TAS) than lower down, meaning the aircraft travels much faster whilst the engines burn less fuel.
The higher in the air you go, the less dense the air is, or put in a different way, the thinner it is. Therefore, there is less resistance (or friction) there is to stop the aircraft moving through the air. This aerodynamic friction is called resistance “drag”. Less friction means that less thrust is needed from the engines to propel the aircraft through the air, or in other words, it the aircraft can fly faster for the same high (and therefore efficient) thrust setting.
Here’s an example to demonstrate how the air thins out as you get higher; imagine moving your hand through water and golden syrup. If you want to move your hand through both of these liquids at the same speed, you need much more effort to move your hand through the golden syrup than the water. This is the same principle with an aircraft flying at a higher altitude when compared to a lower altitude.
If you found this article of interest, check out our page on How fast do planes fly?