Why do airliners fly so high in the sky?
Why do aircraft fly at high altitude? Is it to save fuel?
The average commercial passenger jet aircraft cruises at an altitude between 30,000 and 42,000 feet (ft) (9,000 – 13,000 meters). This equates to approximate 5 – 7 miles up in the air. The reason for this is due to fuel efficiency. A jet engine operates more efficiently at higher altitude where the air is much thinner, allowing an aircraft to travel faster whilst burning less fuel.
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 produces 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 the altitude the less dense the air, or put in a different way, the thinner the air is. Therefore there is less resistance (or friction) to stop the aircraft moving through the air. We call this resistance “drag”.
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.