So, there was a flat-earther who took a spirit level on a flight. His intention was to determine whether the plane periodically adjust its angle in order to make it level in relation to the surface of the Earth. According to him, if the Earth were really spherical, then the plane would need to make these periodic adjustments; otherwise it would fly away straight into space!
His video became viral, and predictably, the plane did not make these periodic adjustments. The conclusion, surely, is not that the Earth is flat. The following are some explanations why the plane doesn’t have to make those periodic adjustments, and still reaches its destination.