Course Deviation Indicator (CDI)
The HSI contains a Course Deviation Indicator (CDI), with a Course Pointer, To/From Indicator, and a sliding deviation bar and scale. The course pointer is a single line arrow (FMS1, GPS1, VOR1, and LOC1) or a double line arrow (FMS2, GPS2, VOR2, and LOC2) which points in the direction of the set course or VOR radial. The To/From arrow rotates with the course pointer and is displayed when the active NAVAID is received.
The Course Deviation Indicator (CDI) moves left or right from the course pointer along a lateral deviation scale to display aircraft position relative to the course. If the course deviation data is not valid, the CDI is not displayed.
The CDI is located in the center of the standard HSI compass card, and is duplicated using a standalone CDI bar above the HSI.
The standalone CDI bar deviation will change sides when the current aircraft heading is more than 103° from the inbound course to the station to accurately reflect the HSI CDI direction of deflection.
The CDI is capable of displaying two different sources of navigation: GPS/FMS or NAV (VOR, localizer) depending on the navigator(s) configured (refer to the AXIS™ installation manual for more information). Color indicates the current navigation source: magenta (for GPS) or green (for VOR and LOC). The full-scale limits for the CDI are defined by a GPS-derived distance when coupled to GPS. When coupled to a VOR or localizer (LOC), the CDI has the same angular limits as a mechanical CDI. If the CDI exceeds the maximum deviation on the scale (two dots) while coupled to GPS, the crosstrack error (XTK) is displayed below the white aircraft symbol.
The system supports the use of GPS to aid in the conduct of VOR and VOR/DME approaches. In this configuration, a preview of the VOR final approach course is displayed on the HSI using the same criteria as used to find the preview for localizer-based approaches if available.
The VOR approach preview is not intended to alert the crew to a pending CDI navigation source change, rather it allows the crew to monitor deviations from the VOR course as a means of complying with the published approach procedure, while still using GPS to provide a course for the aircraft.
Some VOR and NDB approaches are approved for use of GPS as a substitute means of navigation, meaning it is not necessary to tune or monitor the radio navigation source associated with the approach. These are referred to as “overlay approaches” and are indicated on the approach selection screen by the presence of ‘GPS’ after the VOR or VOR/DME prefix and LNAV or LNAV+V for NDB approaches. LNAV approaches provide lateral GPS-based guidance to legs defined by the navigation database.
The navigation source can change automatically from a GPS to ILS/LOC navigation source while transitioning to an approach depending flight plan information loaded into the Flight Management System (FMS). Refer to the FMS chapter for more information.