Why are high altitudes best for for commerical airliners?

Question

Or, why is it beneficial for any airplane to fly above sea level? What is better about air at 35,000 ft.?

But why is less fuel required? And doesn't less air mean less lift, which would require more thrust to overcome?

Answer 1

As you rise above the earth the air get's thinner and thus less drag on the plane as it is 'shoved through the air'. Also the turbulance in the upper astmosphere is less due to the thinner air which allows planes to cruise relatively smoothly above thunderstorms and such. There are even cases where planes and rise up into a 'jet stream' of fast moving upper astmosphere that they can utilize to save on time and fuel.

Hope this helps!

Answer 2

There is one primary reason and one secondary reason why flying higher is more efficient.

The primary reason:

The temperature of the air in the atmosphere cools at the rate of approximately 2 degrees Celcius per thousand feet of altitude. As such, the higher the airplane goes the cooler the temperature. The efficiency of an engine is directly proportional to the temperature differential of the "hot sink and cold sink". In this case, the hot sink is the combustion chamber and the cold sink is the air that enters the engine. The greater that temperature differential, the more efficient the engine. This is true with any internal combustion engine.

The secondary reason:

Fuel and air combusts at a specific mixture ratio. Modern day engines have computer controlled fuel injectors that optimize the volumetric flow rate of fuel to be injected into the combustion chamber depending on the position of the throttle and the environment. The higher the airplane goes the less air molecules that exist. As such, a proportionally lower volumetric flow rate of fuel can be injected into the combustion chamber for any given throttle position and any constant set of environmental conditions. This is again, because a constant ratio of air and fuel have to exist in the combustion chamber for any specific throttle position and environmental conditions.

This secondary reason of reduced fuel flow at higher altitudes is something rarely considered in the pilot community. Most of us are more concerned about finding the lowest possible temperatures as well as the greatest tailwind component or lowest headwind component. All of this helps us save fuel.

One answer above mentions that jet engines burn less fuel at cruise altitude than they do when idling. Maybe some engines do this, but I haven't witnessed it. The lowest cruise fuel flow I have seen in the C-5 was 16,000 pounds of fuel per hour. This was at 43,000 feet. On the ground, we typically idle at about 1,000 pounds per hour.

Answer 3

There are a few different reasons. For starters, jet engines work better at high altitude. Secondly, in the thinner air, there is less air resistance. If you're flying from west to east, you also have the added benefit of the jet stream pushing the plane, so it requires less fuel to fly at the FAA-prescribed speed.

Answer 4

If you are flying at a higher altitude, there is less air up there, meaning less drag on the plane.
There are many smaller airplanes at altitudes lower than 15,000 feet, so it is much easier to fly at higher altitudes for longer flights.

Answer 5

Ohhhhh....just little things like the increased efficiency do to thinner air, taking advantage of the high altitude jet stream, increased speed realized in decent, smooth air well above most weather phenomenon ohhh yes, the view.

Answer 6

The higher the altitude, the lower the drag on the plane from air resistance. They actually burn less fuel at cruise at 35K feet than when they're idling on the runway.

Answer 7

At higher altitudes the air is thinner which makes it easier for the plane to move through it, thus the plane operates more efficiently

Answer 8

There is plenty of lift... the saving comes from decreased drag