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.
126) The Sun’s annual journey from tropic to tropic, solstice to solstice, is what determines the length and character of days, nights and seasons. This is why equatorial regions experience almost year-round summer and heat while higher latitudes North and especially South experience more distinct seasons with harsh winters. The heliocentric model claims seasons change based on the ball-Earth’s alleged “axial tilt” and “elliptical orbit” around the Sun, yet their flawed current model places us closest to the Sun (91,400,000 miles) in January when its actually winter, and farthest from the Sun (94,500,000 miles) in July when its actually summer throughout most of the Earth.
26.) If the Earth were a globe, it would, if we take Valentia to be the place of departure, curvate downwards, in the 1665 miles across the Atlantic to Newfoundland, according to the astronomers' own tables, more than three hundred miles; but, as the surface of the Atlantic does not do so – the fact of its levelness having been clearly demonstrated by Telegraph Cable surveyors, – it follows that we have a grand proof that Earth is not a globe.
136) Many people think that modern astronomy’s ability to accurately predict lunar and solar eclipses is a result and proof positive of the heliocentric theory of the universe. The fact of the matter however is that eclipses have been accurately predicted by cultures worldwide for thousands of years before the “heliocentric ball-Earth” was even a glimmer in Copernicus’ imagination. Ptolemy in the 1st century A.D. accurately predicted eclipses for six hundred years on the basis of a flat, stationary Earth with equal precision as anyone living today. All the way back in 600 B.C. Thales accurately predicted an eclipse which ended the war between the Medes and Lydians. Eclipses happen regularly with precision in 18 year cycles, so regardless of geocentric or heliocentric, flat or globe Earth cosmologies, eclipses can be accurately calculated independent of such factors.
People have believed that the Earth is flat since the beginning of humanity, but the modern Flat Earth hypothesis stemmed from an experiment called the Bedford Level Experiment, conducted in the mid-1800s by a man named Samuel Rowbotham.[1] Rowbowtham, who wrote a book named Earth Not a Globe, started the modern movement by debating scientists publicly and accumulating followers. In the experiment, Rowbowtham attempted to measure the curvature of the earth by observing the curvatures at a local river. He took his results as disproving the theory of a round earth, but future scientists have said that the results he obtained could be accounted for by the parallax effect.[2]

Besides the above difficulties or incompatibilities, many cases are on record of the sun and moon being eclipsed when both were above the horizon. The sun, the earth, and the moon, not in a straight line, but the earth below the sun and moon–out of the reach or direction of both–and yet a lunar eclipse has occurred! Is it possible that a “shadow” of the earth could be thrown upon the moon, when sun, earth, and moon, were not in the same line? The difficulty has been met by assuming the influence of refraction, as in the following quotations:–
The Flat Earth model, however, places the sun and moon inside the glass dome. They are small and close, circling around and above the flat earth, creating day and night, and dividing day and night, light from darkness. The Flat Earth model is clearly described in Genesis 1:14-18. God makes it clear how God created the earth, the sky, and the sun and moon and where he put them. The Bible points us to truth and reality. The true Flat Earth points us to the Bible and Who created us.
If we take a trip down the Chesapeake Bay, in the day-time, we may see for ourselves the utter fallacy of the idea that when a vessel appears "hull down," as it is called, it is because the hull is "behind the water:" for, vessels, have been seen, and may often be seen - again, presenting the appearance spoken of, and away - far away - beyond those vessels, and, at the same moment, the level shore line, with its accompanying complement of tall trees towering up, in perspective, over the heads of the "hull-down" ships! Since, then, the idea will not stand its ground when the facts rise up against it, and it is a piece of the popular theory, the theory is a contemptible piece of business, and we may easily wring from it a proof that Earth is not a globe.
51) Antarctica is by far the coldest place on Earth with an average annual temperature of approximately -57 degrees Fahrenheit, and a record low of -135.8! The average annual temperature at the North Pole, however, is a comparatively warm 4 degrees. Throughout the year, temperatures in the Antarctic vary less than half the amount at comparable Arctic latitudes. The Northern Arctic region enjoys moderately warm summers and manageable winters, whereas the Southern Antarctic region never even warms enough to melt the perpetual snow and ice. On a tilting, wobbling, ball-Earth spinning uniformly around the Sun, Arctic and Antarctic temperatures and seasons should not vary so greatly.
31) Quoting “Zetetic Cosmogeny” Thomas Winships states: “Let ‘imagination’ picture to the mind what force air would have which was set in motion by a spherical body of 8,000 miles in diameter, which in one hour was spinning round 1,000 mph, rushing through space at 65,000 mph and gyrating across the heavens? Then let ‘conjecture’ endeavor to discover whether the inhabitants on such a globe could keep their hair on? If the earth-globe rotates on its axis at the terrific rate of 1,000 miles an hour, such an immense mass would of necessity cause a tremendous rush of wind in the space it occupied. The wind would go all one way, and anything like clouds which got ‘within the sphere of influence’ of the rotating sphere, would have to go the same way. The fact that the earth is at rest is proved by kite flying.”
The circumstances which attend bodies which are caused merely to fall from a great height prove nothing as to the motion or stability of the Earth, since the object, if it be on a thing that is in motion, will participate in that motion; but, if an object be thrown, upwards from a body at rest, and, again, from a body in motion, the circumstances attending its descent will be very different. In the former case, it will fall, if thrown vertically upwards, at the place from whence it was projected; in the latter case, it will fall behind the moving body from which it is thrown will leave it in the rear. Now, fix a gun, muzzle upwards, accurately, in the ground; fire off a projectile; and it will fall by the gun. If the Earth traveled eleven hundred miles a minute, the projectile would fall behind the gun, in the opposite direction to that of the supposed motion. Since, then, this is NOT the case, in fact, the Earth's fancied motion is negatived and we have a proof that the Earth is not a, globe.
42) In the ball-Earth model Antarctica is an ice continent which covers the bottom of the ball from 78 degrees South latitude to 90 and is therefore not more than 12,000 miles in circumference. Many early explorers including Captian Cook and James Clark Ross, however, in attempting Antarctic circumnavigation took 3 to 4 years and clocked 50-60,000 miles around. The British ship Challenger also made an indirect but complete circumnavigation of Antarctica traversing 69,000 miles. This is entirely inconsistent with the ball model.
93) The St. George’s Channel between Holyhead and Kingstown Harbor near Dublin is 60 miles across. When half-way across a ferry passenger will notice behind them the light on Holyhead pier as well as in front of them the Poolbeg light in Dublin Bay. The Holyhead Pier light is 44 feet high, while the Poolbeg lighthouse 68 feet, therefore a vessel in the middle of the channel, 30 miles from either side standing on a deck 24 feet above the water, can clearly see both lights. On a ball Earth 25,000 miles in circumference, however, both lights should be hidden well below both horizons by over 300 feet!

If we refer to the diagram in "Cornell's Geography," page 4, and notice the ship in its position the most remote from the observer, we shall find that, though it is about 4,000 miles away, it is the same size as the ship that is nearest to him, distant about 700 miles! This a an illustration of the way in which astronomers ignore the laws of perspective. This course is necessary, or they would be compelled to lay bare the fallacy of their dogmas. In short, there is, in this matter, a proof that the Earth is not a globe.
Always keep in mind that tangible science is a flat earth, a ball dangling in the sky is science fiction or Hollywood. I believe the firmament to be an ice dome, described to be glass. The hotter we make the "planet" the more that dome melts, I believe the salt beds are salt dropped out of the sky as water evaporated to become air. Gods rendition is tangible science.

97) NASA and modern astronomy say the Earth is a giant ball tilted back, wobbling and spinning 1,000 mph around its central axis, traveling 67,000 mph circles around the Sun, spiraling 500,000 mph around the Milky Way, while the entire galaxy rockets a ridiculous 670,000,000 mph through the Universe, with all of these motions originating from an alleged “Big Bang” cosmogenic explosion 14 billion years ago. That’s a grand total of 670,568,000 mph in several different directions we’re all supposedly speeding along at simultaneously, yet no one has ever seen, felt, heard, measured or proven a single one of these motions to exist whatsoever.

124) Amateur balloon footage taken above the clouds has provided stunning visual proof that the Sun cannot be millions of miles away. In several shots you can see a clear hot-spot reflecting on the clouds directly below the Sun’s spotlight-like influence. If the Sun were actually millions of miles away such a small, localized hot-spot could not occur.