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by kuyakev on March 29th, 2007

kuyakev

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Why don't commercial airplanes fly at altitudes above 40,000ft? Isn't fuel consumption much efficient at higher altitudes?

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Answers. 6 helpful answers below.

  • by Av8trxx on April 7th, 2007

    Av8trxx

    Why don't airliners fly above FL400?

    Simply because most are not physically capable of doing so. They are designed to fly within a certain performance envelope and altitudes above FL400 are beyond it. The wing design needs to be able to perform up there as well as down low while maneuvering for landing. Getting a wing that can do both and expand that performance envelope to over 40,000ft is something only the latest generation of aircraft (like the Boeing 737 NG models at FL410) have and it's not by much. The 787 Dreamliner & A380 are to have a service ceilings of 43,000 ft which will be the highest of all airliners in service.

    True, you do use less fuel as you go higher because the air thins but something else to consider is there less air available at those altitudes to create lift and give the engines power. It's a matter of cost VS efficiency. Flying around under FL400 is fine as no airline is going to go out and buy a whole new fleet of aircraft so they can fly high so as to increase fuel savings. With the use of more composites, blended winglets, improved engine and more efficient wings airlines are already seeing increased efficiency so there isn't a drive to go higher.

    Plus, in the flight levels above 40,000 engine power output drops dramatically and the wing needs an increasing angle of attack, to create more lift to hold the airplane up as the air is so thin. At some point there won't be enough power available to accelerate the airplane and the angle of attack on the wing will continue to increase just to hold altitude. In that situation the airplane isn't able to speed up and an aerodynamic stall of the wing becomes inevitable.

    The other critical issue with high flight is the effect of Mach. Even though the airplane is flying well under the speed of sound, air accelerating over the wing can come close to, or exceed Mach 1. In that situation a shock wave develops and laminar airflow over the wing is disrupted. This can cause "Mach buffet" which can lead to a nose-down tuck and a loss of control of the aircraft.
    http://www.grc.nasa.gov/WWW/K-12/airplane/mach.html

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  • by mrdeux on February 10th, 2009

    mrdeux

    Service ceiling of the 767 is 43,100 ft, and the 747-400 is 45,100 ft. The aircraft weight generally doesn't get low enough in flight to make going above 40,000 worthwhile, though occasionally a combination of light weight, and winds can mean that it does happen.

    At FL430 in the 767, it is very quiet. I've taken a 747-400 to FL450, but it was on an empty ferry flight (weight would have been around 205 tonnes). Any payload and it would be too heavy to go there. And for those that understand...the yellow lines are very close.

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  • by Fly Forever on October 18th, 2007

    Fly Forever

    As others have put very well, the reasons for not going much above FL400 is to avoid the problem of thin air and engine power, as well as dealing with the effects of coming close to Mach 1 around the airflow over the wing. Most planes reach their maximum fuel effiency at or around 40,000 feet. If you go too much higher you risk what I just stated, if you go too much lower, you would also waste fuel.

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  • by Kumbaya on March 30th, 2007

    Kumbaya

    You also need to fly faster to have enough lift, and it's also more difficult to reach high speeds.

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  • by audiogal on March 29th, 2007

    audiogal

    The airplanes need to be certified for a rapid descent in case of depressurization, and most commercial planes are not rated for that high of an altitude (it is based on the rate of descent)

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  • by ymax77 on August 5th, 2011

    ymax77

    There is a need to have aircraft fly as high as they safely can that was not mentioned here. That is to reduce noise. No, not the noise inside the plane. Not just to reduce noise disturbances to passengers, but to minimize noise disturbances to those on the ground trying to live in peace. If you are unfortunate enough to live under a heavily traveled airway you know what I mean, especially if you live in a otherwise quiet rural area. Aviation noise pollution has been growing along with the growth of air travel. It gets worse with every passing year. It has become difficult to find a truly quiet spot not polluted by constant overhead flights. If they must fly over us, get them up there as high as possible. They are just too noisy to be allowed to fly as low as they do now, especially those unnecessary, short haul, low altitude, and annoyingly noisy commuter flights.

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