Chapter 26 - Software Test and Evaluation

The concept of developing a system to give a desired control function (i.e., relationship between input and output) is far from new, and was (and still is) applied to mechanical linkages, electric, pneumatic and hydraulic circuits and analogue electronics. In general, the number of inputs that can be handled by such means is usually modest, and the control algorithms are rather simple. Nowadays, many of the systems incorporated into modern aircraft (and, for that matter, in other vehicles and branches of engineering which involve control strategies of similar sophistication) rely extensively on digital electronic control, wherein the required control algorithms are achieved by means of "software". That is, the desired response (or output) of a system to a given set of conditions (or inputs) is achieved via an electronic device ("computer") controlled by a "set of instructions" (program), rather than by direct mechanical or electrical means. The principle reason for adopting such software-controlled digital electronic systems is that they can readily be designed to accept and process data in quantities, at rates, and with a degree of complexity that cannot be matched by other types of control systems. They also tend to be physically compact, light, with low power and cooling requirements. Individual systems can be integrated relatively easily and, as system design requirements become individually and collectively more complex, the use and significance of software will increase. It will remain the technology of choice for many control functions for the foreseeable future. Software does not exist in a physical sense (although it is a component of, and pervades, the electronic system it controls) and is therefore not directly amenable to test by empirical means although, with important reservations, its behaviour can be assessed by monitoring the performance of the systems it controls. Because it is employed extensively in the majority of recent aircraft designs, and is likely to become even more dominant in future designs, it is essential that the flight test engineer should appreciate the nature of software, and the problems posed by its design, development and testing (which relies mainly on non-physical test techniques).