How does an airplane flying around the world remain at the same altitude?

Question

Maybe this is a stupid question, but I started thinking about this and I know there probably is a simple logical answer. The world is round, so how does an airplane that flys from New York to Japan at 30,000 feet remain at the same altitude if the world is round? Gravity? I am not asking how the airplane flys or stays straight & level

Answer 1

It would be much easier if you start picturing atmospheric flight as a summation of discrete legs of straight lines. The flight around the globe at 30,000 feet will be flown along small tangents off a circle of radius 20,908,126* + 30,000 feet. It is a continuous feed back loop, altitude will be gained or lost in each short leg of straight line flight, but attempted to be corrected in the next. Or in other words, the flight around the world is not in a circle, but along a polygon.

*mean radius of earth (6,372.797 km or 20,908,126 feet)

Answer 2

The density of the air decreases as the altitude goes up. This is a roughly uniform pattern everywhere on the planet.
An airplane flying is at an equilibrium. The thrust form the engine balances with the drag of going at the speed it is traveling at, and the lift of the wing balances the weight of the airplane.
Now, what would happen if the plane was to gain altitude, just a bit? The airplane would slow down a tad (because some energy from the engines would be used to get the airplane a bit higher) but also the density of the air would decrease also, and the wind depend on that to keep the lift equally to the weight. Less density means less lift and the airplane comes down, picking up speed.
In fact, airplane are going though this slow speed and altitude trade off all the time, in a motion that is called phugoid (look this up in Wikipedia: http://en.wikipedia.org/wiki/Phugoid )

An airplane in motion will tend to return to some condition if it deviates from them. This stability is actually part of the design and certification requirement.

Answer 3

The solution to this is that aircraft altimeters are set to show altitude above sea level, which is constant everywhere in the world. Thus, a pilot's altitude above ground level will vary even though the above-sea-level altitude will not. This is how, for example, a pilot could think he is flying at an altitude of 2500 feet and still run into a mountain.

I just re-read your question and it looks like you might be asking something else. Are you asking, basically, why an airplane flying in a straight line doesn't fly straight into space? The answer to that is the measured altitude above sea level is based on atmospheric pressure - as a result, a pilot will calibrate his altimeter (depending on weather conditions) so that it can be inferred from the day's sea level pressure. If a pilot flew on a tangent out of the atmosphere, this would require flying through higher levels of the atmosphere which would register as lower pressure and thus higher altitude.

Answer 4

There are different altitudes for different applications. Airliners fly so high because it is economical. Other planes like military have their own air space to play in although they can fly anywhere with a flight plan. Same with general aviation. There is also a method of assigning altitudes such as those flying North fly and an odd height and those going South fly at an even altitude (just an example).

Answer 5

Below 18000 feet, pilots calibrate their altimeters to local pressure (since an altimeter is really a calibrated pressure gauge). Above 18000 feet, they set to standard barometric pressre (29.92 millibars) and call the altitude a 'flight level.' So 30000 feet is flight level 300.

They of course are subject to gravity, so they fly at constant altitude; however, they don't on really long flights, maintain the same altitude; rather, they gradually climb to thinner air (for reduced drag) as they burn off fuel.

Answer 6

The altimeter is set to sea level. As the plane climbs, the altimeter shows altitude above sea level. As the aircraft travels around the world, the pilot adjusts his altitude as necessary to stay at a particular height above sea level i.e. 30,000'.

Answer 7

It remains at a particular altitude above sea level not above ground level which varies above or below sea level.

The plane remains a specific distance above sea level. In doing so, it automatically follows the contour of the spherical earth. It follows a straight line relative to the earth's surface which is actually an arc.

Answer 8

The plane doesn't fly a straight line - it flies an arc always being 30,000 feet over land or water.

Answer 9

gravity caused by the rotation of the earth... thats why the apollo missions used to get over the earth, pick up speed and then shoot off on a tangent becaue there was no gravity, that cant be done inside earth's atmosphere...

Answer 10

it's all magic!