Legible != Readable

Legability means you have to be able to differenciate words and letters. With a font specialized for aerospace use that probably also mean it has to retain that quality when printed on panels.

A special requirement I would think of is legability while in motion. Try taking your favourite, perfectly kerned font and reading it while shaking your head wildly in poor light conditions, then you get a hint of why this font isn't optimized for looks.