In current general aviation aircraft it is not uncommon to find very old designs of electromechanical instruments next to state of the art pixel-based displays. A number of these instruments appear to be inadequate from a legibility, interpretability, and operational point of view. This paper contains a review of the relevant display reading performance literature. The key principles (luminance contrast and critical detail) of display legibility are demonstrated using a general aviation altimeter as an example. A particular safety concern is the legibility of the pressure display found on typical altimeter instruments.