Quite simply you cannot. It is widely stated you would need to be at a height of at least 40,000 ft to get even a hint of curvature if earth were round. Commercial aircraft are not allowed to fly this high. They are only allowed to fly just under this altitude. 36,000ft might be typical. In addition, the windows on commercial aircraft are small and heavily curved. Even if they flew high enough for a person to see curvature, it would still not be visible to passengers.

35) If the Earth were truly a globe, then every line of latitude south of the equator would have to measure a gradually smaller and smaller circumference the farther South travelled. If, however, the Earth is an extended plane, then every line of latitude south of the equator should measure a gradually larger and larger circumference the farther South travelled. The fact that many captains navigating south of the equator assuming the globular theory have found themselves drastically out of reckoning, moreso the farther South travelled, testifies to the fact that the Earth is not a ball.
119) It is claimed that the other planets are spheres and so therefore Earth must also be a sphere. Firstly, Earth is a “plane” not a “planet,” so the shape of these “planets” in the sky have no bearing on the shape of the Earth beneath our feet. Secondly, these “planets” have been known for thousands of years around the world as the “wandering stars” since they differ from the other fixed stars in their relative motions only. When looked at with an unprejudiced naked-eye or through a telescope, the fixed and wandering stars appear as luminous discs of light, NOT spherical terra firma. The pictures and videos shown by NASA of spherical terra firma planets are all clearly fake computer-generated images, and NOT photographs.
59.) Mr. Proctor says.- "The Sun is so far off that even moving from one side of the Earth to the other does not cause him to be seen in a different direction – at least the difference is too small to be measured." Now, since we know that north of the equator, say 45 degrees, we see the Sun at mid-day to the south, and that at the same distance south of the equator we see the Sun at mid-day to the north, our very shadows on the round cry aloud against the delusion of the day and give us a proof that Earth is not a globe.

The first main problem with the globe model is that the next solar eclipse on August 21 is coming from the west. We have been told that the moon rotates around the earth from east to west just like the sun. But the moon during the solar eclipse is eclipsing the sun from the WEST. So how does that work on the Ball earth model? They say it’s just an optical illusion because of the angle of the sun. One scientist from NASA said its because the moon rotates from west to east! What? And another scientist said the the moon rotates around the earth twice as fast as the earth spins! Huh? When did that happen? The explanations from NASA get even more confusing and make no sense. If their “science” is so accurate, then why can’t they agree on which way the moon rotates around the earth or how fast it’s going? Something is amiss. Could it be something other than the moon eclipsing the sun?
63) In a second experiment Dr. Rowbotham affixed flags 5 feet high along the shoreline, one at every mile marker. Then using his telescope mounted at 5 feet just behind the first flag looked over the tops of all 6 flags which lined up in a perfectly straight line. If the Earth were a ball 25,000 miles in circumference the flags should have progressively dipped down after the first establishing line of sight, the second would have descended 8 inches, 32 inches for the third, 6 feet for the fourth, 10 feet 8 inches for the fifth, and 16 feet 8 inches for the sixth.
145) Heliocentrists believe the Moon is a ball, even though its appearance is clearly that of a flat luminous disc. We only ever see the same one face (albeit at various inclinations) of the Moon, yet it is claimed that there is another “dark side of the Moon” which remains hidden. NASA states the Moon spins opposite the spin of the Earth in such a perfectly synchronized way that the motions cancel each other out so we will conveniently never be able to observe the supposed dark-side of the Moon outside of their terrible fake CGI images. The fact of the matter is, however, if the Moon were a sphere, observers in Antarctica would see a different face from those at the equator, yet they do not – just the same flat face rotated at various degrees.
177) In the documentary “A Funny Thing Happened on the Way to the Moon,” you can watch official leaked NASA footage showing Apollo 11 astronauts Buzz Aldrin, Neil Armstrong and Michael Collins, for almost an hour, using transparencies and camera-tricks to fake shots of a round Earth! They communicate over audio with control in Houston about how to accurately stage the shot, and someone keeps prompting them on how to effectively manipulate the camera to achieve the desired effect. First, they blacked out all the windows except for a downward facing circular one, which they aimed the camera towards from several feet away. This created the illusion of a ball-shaped Earth surrounded by the blackness of space, when in fact it was simply a round window in their dark cabin. Neil Armstrong claimed at this point to be 130,000 miles from Earth, half-way to the Moon, but when camera-tricks were finished the viewer could see for themselves the astro-nots were not more than a couple dozen miles above the Earth’s surface, likely flying in a high-altitude plane!
41.) When astronomers assert that it is "necessary" to make "allowance for curvature" in canal construction, it is, of course, in order that, in their idea, a level cutting may be had, for the water. How flagrantly, then, do they contradict themselves when the curved surface of the Earth is a "true level!" What more can they want for a canal than a true level? Since they contradict themselves in such an elementary point as this, it is an evidence that the whole thing is a delusion, and we have a proof that the Earth is not a globe.
165) NASA claims one can observe the International Space Station pass by overhead proving its existence, yet analysis of the “ISS” seen through zoom cameras proves it to be some type of hologram/drone, not a physical floating space-base. As you can see in my documentary “ISS Hoax,” when zooming in/out, the “ISS” dramatically and impossibly changes shape and color, displaying a prismatic rainbow effect until coming into focus much like an old television turning on/off.
121) When you observe the Sun and Moon you see two equally-sized equidistant circles tracing similar paths at similar speeds around a flat, stationary Earth. The “experts” at NASA, however, claim your common sense every day experience is false on all counts! To begin with, they say the Earth is not flat but a big ball; not stationary but spinning around 19 miles per second; they say the Sun does not revolve around the Earth as it appears, but Earth revolves around the Sun; the Moon, on the other hand, does revolve around the Earth, though not East to West as it appears, rather West to East; and the Sun is actually 400 times larger than the Moon and 400 times farther away! You can clearly see they are the same size and distance, you can see the Earth is flat, you can feel the Earth is stationary, but according to the gospel of modern astronomy, you are wrong and a simpleton worthy of endless ridicule if you dare to trust your own eyes and experience.