Numerous variables influence the legibility of aircraft instrument dial characters. This situation makes it very difficult, if not impossible, to establish an exact act of rules for optimizing all installations.
Character size, one of the important considertions, can be optimized where adequate dial space exists. UIsually this is not the case and the designer is faced with placing the information in a limited space while continuing to strive for error-free legibility. Appropriate minimum size requirements have been stated herein for guidance in air transport use.