Commercial Airplane Design Principles

Appendix E

Units and Conversion Factors

The English system of units continues to be the system of choice in the aircraft industry although SI units are entering the field, but slowly. Choosing a system of units to use is primarily a matter of taste and past practice with often very little to logically encourage adherence to one system or the other. In its daily operations the commercial aircraft industry tends to employ nautical miles and nautical miles per hour, or knots, to describe range and speed, while pounds and gallons remain favored to denote weight or thrust and volume of liquids. Engineering usage in the industry tends to more fundamental units within the English system; this is called the foot-pound-second or “fps” system. The major ...