Full disclosure: I do not believe the Earth is flat. However, a guy recently posted a YouTube video that purports to prove that the Earth is flat due to there having been no changes in the position bubble in a spirit level (possibly better known as carpenters level) that would have accounted for the plane's nose dipping to compensate for the earth's curvature.
Naturally, this led to attacks from commenters saying how stupid he is. What was interesting is that, despite it being more rational to believe that that the earth is indeed round from observations such as looking at the curvature of the horizon through a plane window or noticing the differences flight times (for those who have travelled internationally) for, say, flying to China from Europe (heading eastward, which I have done) or flying to China heading westward from the US (which I also have done), there was a lack of scientific rigor in these observations (and in the comments refuting his claim).
Could someone explain, in as plain of language as physics permits, the flaws of the spirit level experiment (and if the flaw is in the instrument itself, what would occur if a different leveling instrument were used)?
I see that there are a lot of questions about Flat-earth theory already on stackexchange so, once again, I want to emphasize that I am only concerned with the flaws of this particular experiment and am not looking to impugn Flat-earth theory altogether. As an aside, while I am not convinced that the Earth is flat, there is something respectable about an individual performing an experiment to determine something on his own rather than just accepting what books/teachers tell him.