Use of magnetic variation and station declination for heading (VX) and course (CX) leg types

Leg types, also known as path terminators, are used in aircraft navigation to define paths as routes for RNAV equipped aircraft. Many leg types are specifically defined to cause the Area Navigation (RNAV) system to emulate the actions that an aircrew would take when flying the route manually based on the text and other depictions on the chart. Some emulate magnetic headings or courses, which, for various reasons, can lead to differences between what the procedure designer intended and what the RNAV system displays or flies. Two in particular are in common use individually and in combination, and this paper will explore the potential for unexpected differences in the resulting paths. These two are the heading type (Vx) which may be used where the procedure or airspace designer calls for a heading to some type of termination, and the Course type (Cx) which may be used where designers wish to fix the aircraft path over the ground using a course defined by a navigational aid (NAVAID). Issues arise because of the need to implement magnetic courses and headings in an RNAV system that navigates in a true north reference frame. Given that the Earth's magnetic field orientation varies with location on the Earth (hence true north and magnetic north change relative to each other), changes with time, and that NAVAIDS and runways are marked/oriented in the magnetic reference frame of the time they are surveyed, care must be taken to make the RNAV system emulate the procedure intent; this paper will describe and summarize those cautions and issues.