# Maintaining straight and level altitude [duplicate]

just a completely random question that I suddenly thought - doesn't straight and level flight prove that the earth's surface is not a globe? Once cruising altitude is reached, the plane remains at the same altitude for the entire flight until it starts to descend for landing. If earth was a globe, we would need to drop the nose of the plane every couple of minutes to compensate for the curve of the earth. Is this correct? Because on my most recent flight, we reached 2000ft altitude and my instructor told me to maintain this altitude for 30 minutes (which I managed to do). I asked why we don't need to compensate for the curve of the earth and he couldn't answer me. Sorry for the random question but it's been bothering me.

• I would say off topic as this is about debating whether earth is round using airplane. – vasin1987 Oct 22 '18 at 15:27
• I disagree that it is off-topic. The OP's reference to "earth's surface is not a globe" is rhetorical when read in the context of his entire post. Could be a duplicate, but not off topic. – 757toga Oct 22 '18 at 15:53
• @757toga please try to avoid to provide answers, however partial, in the comments. – Federico Oct 22 '18 at 16:04
• All else aside, unless you have a REALLY fast airplane, you won't travel far enough in a couple of minutes for the curvature to be noticable. – jamesqf Oct 22 '18 at 17:29
• Just observing that the exact same principle applies to ships on the sea. – Cpt Reynolds Nov 16 '18 at 17:19

Set aside the barometric stuff. The simple answer is that what we think of as altitude, on a sphere, is really distance from the gravitational source, the center of the sphere. Therefore, a "constant altitude" is not maintaining the same plane in 3 dimensional space, but the same distance from the center of the earth.

Once cruising altitude is reached, the plane remains at the same altitude for the entire flight until it starts to descend for landing. If earth was a globe, we would need to drop the nose of the plane every couple of minutes to compensate for the curve of the earth. Is this correct?

You're right, except for one thing: the "every couple of minutes" part.

Since the earth is a globe, aircraft need to pitch down in order to maintain a constant altitude. So, for example, if an airplane makes a flight which covers a 15 degree arc across the earth's surface (which is a distance of about a thousand miles), then the airplane will need to pitch down by 15 degrees over the course of the flight.

However, in order to accomplish this, pilots don't "drop the nose of the plane every couple of minutes". Instead, they watch the altimeter, and raise or lower the nose as needed to maintain a constant altitude, or to climb or descend as necessary. (Or they set the autopilot to do the same thing.) This means that they're adjusting the plane's pitch every couple of seconds, and the adjustments that they use to maintain altitude are much, much larger than the adjustments that would be needed to merely match the curvature of the earth.

The short version:

Question: Why don't pilots need to pitch down occasionally in order to follow the curvature of the earth? Answer: They do.

• It looks like this answer has been downvoted, but I'm not aware of anything wrong with it. – Terran Swett Oct 22 '18 at 18:04