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Home > Flying the Plane > Why fly at such heights when it makes more sense to fly lower?
Why fly at such heights when it makes more sense to fly lower?
Flying - Flying the Plane
Thursday, 14 June 2007 10:56

Good afternoon,

I have a rather unusual question that I am unable to find a definitive answer to. From what I have found, a large amount of fuel is consumed just to get the aircraft up to cruising altitudes from ground level.

My question is, why travel at such heights when it seems to make more sense to fly lower? I realize, over land, the noise and building height is an issue as well as migrating birds but over water, it seems like there is little issue.

Is there an aeronautical explanation or element of physics that I am not aware of? Obviously the higher you go, the further you need to travel to go from point A to point B; the other benefit of flying low would be safety.

It may not be very comforting at that speed to drop into the ocean, but 30,000 feet is not all that comforting either. Maybe, turbulence is worst the lower you are or people are able to see how fast they are going, could be a concern.

If you would not mind, I would appreciate any insight you may have on the topic.

Thank you for your time.

Russell

Hi Russell,


The main reason why jetliners fly high is because it is more efficient and economical to do so. You can also see my explanation in my earlier FAQ
which I am restate below:

"At higher altitude, the temperature decreases. The colder the air temperature, the better it is for the jet engine as it convert fuel more efficiently at low outside air temperatures. Basically, the efficiency is directly proportional to the temperature differential. The greater that temperature differential is, the more efficient the engine.

Colder air is also good for jet engine operation. Colder mass means more air. Remember the principle of a gas turbine engine - it requires air to be sucked at the front, compressed, combusted then expanded and blasted out through the jet nozzle (a more simplistic explanation - suck, squeeze, blow and go!)

Further, when a plane climb higher, the air outside gets thinner and less dense. This has the effect of reducing the forward resistance.

So, overall, a gas turbine engine is more fuel efficient when operated at higher altitudes than nearer to the surface."

Yes, most modern airlines employ the computer systems to optimize the overall efficiency of their operations by taking into account various factors such as the price of fuel or forecast winds to come out with an optimum cruising altitude. More often than not, the best altitudes are in the region of 35,000 feet, NOT 10,000 feet or 15,000 feet!

It is not necessary true that the higher you go, the further you have to fly (theoretically, yes) but for a long journey, it is insignificant. Further, although the fuel consumption on the climb is high, but overall, is worth the climb. In fact, it is safer to fly high as you would avoid most of the turbulent clouds, get better communication range, and not forgetting, in the very unlikely event of loss of all engines, you have a longer distance and time to glide home :-( !

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retired
With fs 2004,I've noticed that when I was flying F4 jet fighter at low altitudes,the speed was very high , but at the high altitudes the speed was dropping dramatically. I would think that the reason is that the high air is of less density then the volume of the air coming inside the engine is less than that of air at low altitudes.Does it make sense?,
walid halabi , 28 Sep, 2009

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