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12 Answers

Is level flight parallel to earth surface?

Asked by: 2980 views Aerodynamics


Would an aircraft flying level flight fly parallel to earth surface with constant altitude or straight ahead, with slowly increasing altitude?

Would the pilot be required to make corrections that the level flight is maintained?


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12 Answers

  1. Koehn on Nov 20, 2012

    Level flight is, in theory, a constant distance from the center of an imaginary sphere roughly equivalent to sea level. An aircraft in such a state could, in theory, fly forever and never touch the surface. It would not slowly climb away from the imaginary sphere; that would be considered a climb, not level flight.

    In this way “level flight” is similar to orbit, although obviously the mechanics are completely different.

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  2. Brian on Nov 20, 2012


    Yes, level flight is defined as parallel to the earths surface.

    No altitude changes occur, assuming flat terrain beneath the aircraft.

    No corrections would need to be made, assuming the aircraft is perfectly trimmed in perfectly calm air.

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  3. Koehn on Nov 20, 2012

    Technically level flight is tangent to the earth’s surface. It cannot be parallel since the earth isn’t flat (apologies to all you flat-earthers).

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  4. Steven Porter on Nov 20, 2012

    From the forces on the aircraft point of view, level flight is perpendicular to the direction of the force of gravity. While this is usually ‘straight down’ it does vary slightly near mountainous areas and dense rock. The earth is not even a sphere, more of a squished ball (ellipsoid) with uneven lumps of mashed potatoes on the surface that bulges at the equator.

    To correct Koehn, it is not tangent, as it were one would be gaining altitude in level flight as the earth drops away from you.

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  5. Jim F. on Nov 20, 2012

    Yeah, not tangent, otherwise you’d end up in space. Not parallel either, since we’re dealing with 3-D spherical planes, not 2-D linear. The word you’re looking for in concentric, with the center point of your arced flight path being the center of the Earth.

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  6. Jim Post on Nov 20, 2012

    It sems that Brian answered the question. ” parallel to earth surface with constant altitude ”
    These other responses do no more than confuse a new aviator in his quest for knowlege, although it seems that more and more unprofessional responses are becoming predominate on this site. Bottom line-If you do not know the subject well enough to give an answer that is the accepted norm in the flight training community or if your teaching skills are limited, perhaps you should be listening rather than respond.

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  7. Brian on Nov 21, 2012


    The term parallel is similarly used in reference to geodesics, or curved paths who’s tangential vectors are parallel.

    A friend of mine once said, “whenever someone says something is technically ‘…’ chances are good they are about to say something that is technically wrong.”

    The above, however, should have no bearing on this thread or question. There is no need to invent a definition. For books I own dating back to the 50’s have defined level flight as parallel (sometimes the word horizontal is used instead) to the earths surface. As Jim points out, the rest of this discussion is useful only to confuse what need not be confusing.

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  8. pbaumer on Nov 21, 2012

    Thanks for the answers. I can look through the the terminological discussion about parallel/tangent, no worries.

    One more question though:

    Assuming calm air, and flight on ocean (so no flight path change need due to terrain), is the pilot required to trim the aircraft continually to maintain the level flight at this constant altitude? Or would the changing direction of gravity cause a path parallel to earth sphere magically?

    Thanks a lot!

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  9. Bob Watson on Nov 21, 2012

    Assuming constant meteorological and power conditions, the airplane would not need to change its trim to maintain altitude. Level flight is flying at a constant distance from a spherical reference surface (e.g. Mean sea level). More like an orbit than a straight line (in 3-D terms).

    Fortunately, this is a distinction that only things like astronauts, GPS and INS systems need to worry about.

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  10. Brian on Nov 21, 2012

    “I can look through the the terminological discussion about parallel/tangent”

    You shouldn’t have to. Unless it is interests you then don’t worry about it, it’s an unnecessary complexity to a simple question.

    “is the pilot required to trim the aircraft continually to maintain the level flight”

    With appreciable changes in aircraft weight/CG shift the aircraft will need to be re trimmed. This is common to larger aircraft burning large quantities of fuel. For anyone not flying transport or military aircraft the effects are negligible.

    I’ve always found definitions to be helpful so: Trim is defined as a condition where the aircraft is in equilibrium. Where equilibrium is defined as the forces acting on the aircraft being in balance. I.E. Lift = Weight and Thrust = Drag

    “Or would the changing direction of gravity cause a path parallel to earth sphere magically?”

    It’s not magic, it’s just centripetal force. In level flight lift tries to pull you away from the center of the earth. Gravity responds in an equal and opposite capacity keeping your distance from the earths center (your altitude) constant.

    If it helps, imagine you take a ball on the end of a string and twirl it in circles. The tension of the string (gravity) keeps the centripetal force (lift) in check. The end result is the ball remaining a fixed distance from it’s point of rotation (your hand).

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  11. Andy Neumann on Nov 28, 2012

    On a related note:

    Remember that level flight is based on altitude which is measured by pressure and is affected by temperature. Because of variations in barometric pressure and temperature, a constant altimeter reading is not necessarily a level “height” above sea level.

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  12. Micah on Nov 28, 2012

    pbaumer, maybe take a step back and think about other domains you may already “know” that have similar considerations. When you’re in a boat, is the surface of the water “flat” or “curved,” and what risk do you encounter taking the boat into the air because the surface of the body of water is curved?

    When you’re running on the earth, which you know to be round (more or less) and which has a curved surface, does that mean you’re always running downhill* (because the next step curves “down” and away from you)?

    These two examples are silly. Your question is not quite as silly, but you should probably assume that flying over the surface of the earth is very similar to walking or boating over the surface of the earth/body of water. That may quickly resolve the conceptual issue.

    To answer your specific question, Steven Porter probably provides the most apt answer. Flying “straight and level” is typically unaccelerated flight, which means that your aerodynamic forces are in balance (thrust/power, lift, drag, gravity). So long as gravity remains one of these forces, and so long as gravity is a force that acts perpendicular to the surface of a curved body (ie, the earth), “level” flight will always occur along a curved path.

    *assuming a flat surface

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