Maybe his altimeter depended on the earth being flat.

I wonder what flat earthers think of horizon indicators. Or, I guess, the horizon in general.

If you mean horizon as in a "horizontal earth service, to which your plane might not be parallel to", Flat Earth is more compatible with horizon than a spherical Earth.

If you mean "where the sky meets the ground far away", it's a finite distance away but maybe you can't ever reach the end, perhaps due to increasing drag force the closer you get to the edge, and a curved prism that distorts optics.

If you allow natural forces to vary with distance from the edge of a disk, you can probably recovery most spherical phenomena.

There's a sort of equivalence between static geometry models and dynamic force models.

I meant where the ground meets the sky. If there's a measurable distance landmark there, the math of "I'm on a sphere" is hard to argue with. Especially if you repeat the exercise in many far flung places.

Based on a quick reading of the TFES website there is a notion of Universal Gravity [1] which at first glance would seem to orient a simple horizon indicator in the right direction.

There are also some articles on perspective from their POV which are easy to find. Their ideas of perspective are novel.

[1] http://www.theflatearthsociety.org/tiki/tiki-index.php?page=...

I could easily imagine he did not even realize something was wrong until it was to late to react.