FGS requirement rationale [Flight Guidance System]

A modern business jet frequently has a Flight Guidance System (FGS) which consists of an autopilot (AP), a flight director (FD), and a yaw damper (YD). In most cases, an FGS requirement will define what the system should do, but not why the system should do it. The FGS engineer can only guess at the rationale for the requirement's existence because the rationale is not documented. Therefore it is difficult for the engineer to understand how the FGS should interact with the systems that surround it in the aircraft environment. This in turn makes it difficult to maintain the system and/or reapply the system in another aircraft. This paper introduces the documentation of requirement rationale for the FGS in a business jet. First, this paper explains some of the problems associated with inadequate requirement rationale documentation, and then it defines what requirement rationale is. Then the paper describes how requirement rationale should be written, and provide examples of requirement rationale.