Flying level above a curved earth

[Latrobe]

Just a really weird (and possibly dumb) question that popped into my mind this morning. If an aircraft is flying along at 15,000ft level flight continuesly, will that aircraft eventually start to climb higher in altitude due to the Earth being curved and NOT level? Or, are aircraft attitude indicators designed to keep you flying level according to the curvature of the Earth?


My mind comes up with some wacky questions 

[dunnrite]

Not sure about this, but I would think that an altimeter uses gravitational force to calculate altitudes.  I assume auto pilot uses readings from an altimeter?

[Chalenge]

Altitude indicators go by barometric pressure and require periodic adjustments. Attitude is a gyroscopic measurement.

[dunnrite]

Altitude indicators go by barometric pressure and require periodic adjustments. Attitude is a gyroscopic measurement.

Wondered if that may be it as well, but I thought "attitude" was a alcoholic measurement.

[Fishu]

Just a really weird (and possibly dumb) question that popped into my mind this morning. If an aircraft is flying along at 15,000ft level flight continuesly, will that aircraft eventually start to climb higher in altitude due to the Earth being curved and NOT level? Or, are aircraft attitude indicators designed to keep you flying level according to the curvature of the Earth?

In aviation feets and flight levels are used to measure altitude. There is a transition altitude at which point the altitude is changed from feets to flight levels and vice-versa. In the USA and Canada the transition altitude is at 18,000ft. Altitude is always dependant on the atmospheric pressure. Below transition altitude feets are used to measure altitude above mean sea level and this requires the pilot to set the local atmospheric pressure. Stantardised pressure is used above transition altitude, which is 1013.25 millibars or 29.92 inches of mercury and altitude is referred to as flight level. Flight levels are not at a fixed altitude above MSL, but change according to the pressure. Because of the stantardized pressure everyone above the transition altitude are flying at similar altitudes. Like wise everyone below transition altitude are flying at similar altitudes as long as everyone is using the local pressure - if someone isn't using the local pressure, he'll be flying either above or below the indicated altitude above MSL. Which can be fatal without ground proximity warning radar (or GPWS) when below indicated altitude.

The short answer to your question is that air pressure along with stantardisation and flight regulations defines the altitude. Above transition altitude aircrafts are not flying at a fixed altitude above the sea and the actual altitude above the sea level depends on the pressure, however due to the stantardisation this is irrelevant. Below transition altitude aircraft are required to use the local pressure for indicated altitude, if they fail to do that then they will not be flying at the indicated altitude above sea level.

Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude, and from the pressure calculates and displays the corresponding altitude. To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes. This is a critical safety issue.

Flight levels solve this problem by defining altitudes based on a standard pressure. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure. Flight levels are described by a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100. Therefore an apparent altitude of, for example, 32,000 feet is referred to as "flight level 320". To avoid collisions between two aircraft due to their being at the same altitude, their 'real' altitudes (compared to ground level, for example) are not important; it is the difference in altitudes that determines whether they might collide. This difference can be determined from the air pressure at each craft, and does not require knowledge of the local air pressure on the ground.

Flight levels are usually designated in writing as FLxxx, where xxx is a one- to three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as "flight level two niner zero", in most jurisdictions. The phrase "flight level" makes it clear that this refers to the standardized pressure altitude

[Anaxogoras]

Just a really weird (and possibly dumb) question that popped into my mind this morning. If an aircraft is flying along at 15,000ft level flight continuesly, will that aircraft eventually start to climb higher in altitude due to the Earth being curved and NOT level? Or, are aircraft attitude indicators designed to keep you flying level according to the curvature of the Earth?

   No, an aircraft trimmed for level flight will not begin to climb because of the curvature of the earth.

Did you notice that your premise contains a contradiction?  That's why you were able to reach an absurd conclusion.

[wooly15]

Latrobe...I see what you are getting at....it seems if the plane made a continuously perpendicular to the earth flight, his alt would continually increase and he would eventually be in orbit.  That is if he isn't using a standard altimeter and just flying "straight".

Okay, so what about this....if two planes were traveling, one directly over the other....the first at 500 feet and the second at 50000 feet.  If both of the planes had to travel from point A to point B staying at the exact orientation throughout.  Both starting over point A and finishing over B at the same time....would the higher one have to travel faster to reach point B at the same time?  Because of the curvature of the earth, wouldn't he have to cover more distance in the same amount of time??  The same thing with a merry-go-round.  The outside seats would go faster because of the same principle....right? 

[Serenity]

Latrobe...I see what you are getting at....it seems if the plane made a continuously perpendicular to the earth flight, his alt would continually increase and he would eventually be in orbit.  That is if he isn't using a standard altimeter and just flying "straight".

Okay, so what about this....if two planes were traveling, one directly over the other....the first at 500 feet and the second at 50000 feet.  If both of the planes had to travel from point A to point B staying at the exact orientation throughout.  Both starting over point A and finishing over B at the same time....would the higher one have to travel faster to reach point B at the same time?  Because of the curvature of the earth, wouldn't he have to cover more distance in the same amount of time??  The same thing with a merry-go-round.  The outside seats would go faster because of the same principle....right? 

Yeah... since with the increased radius it's a longer distance. But fuel economy is better up there, so... is it worth the extra speed?

[MrRiplEy[H]]

If the plane wouldn't have to fight earths gravity in combination of thinning atmosphere, a direct flight path would indeed take it to outer space.

Physics dictate that a moving object moves in a straight line unless no force deviates it. In this case both gravity and thinning of atmosphere both force the aircraft to stay inside certain thresholds despite the attempts of the pilot to maintain an absolute level flight in 3D space (relative to earth's orbit and roll of course, nothing we do on this planet really goes directly from point a to point b in true 3D or 4D space).

[Cougar68]

It depends on if the treadmill is built to include the curvature of the earth.  If the treadmill is truly and definitely flat, than the airplane will be forced to continue completely flat and launch into space.  Assuming it could take off, of course.

[Kermit de frog]

It depends on if the treadmill is built to include the curvature of the earth.  If the treadmill is truly and definitely flat, than the airplane will be forced to continue completely flat and launch into space.  Assuming it could take off, of course.

[Anaxogoras]

This is the contradiction:

If an aircraft is flying along at 15,000ft level flight continuously, will that aircraft eventually start to climb...?

The answer is no, by definition.

[rabbidrabbit]

Crap,  You beat me to it...  Once the treadmill gets tossed in there all known physics goes out the window.

[FiLtH]

  I see a MythBuster episode in the making.


   "A plane takes off from San Fran and flies level at 100 ft at after takeoff....."

[Tec]

This is the contradiction:

He may not have worded it correctly, but it's pretty apparent what he's asking.