POSITION ESTIMATION, RADIO AIDS AND INDEPENDENT HEIGHT
The flight-management position is an estimate assembled from available references, not a label attached permanently to one receiver. The supplied Airbus briefing describes mixed inertial information, GPS-derived position where installed and radio updating. Availability determines which combinations can contribute. A route displayed on an ND establishes what the management system represents; it does not independently prove the quality of that position estimate. Reference selection, navigation accuracy and the actual relationship to a received aid remain separate observations. [87, PDF pp. 142–143] [66, PROG and radio-navigation sections, position/accuracy information]
DME, VOR and ILS contribute different measurements. DME provides distance to its station, VOR provides an angular radio reference and ILS provides localizer/glideslope approach guidance. Automatic tuning supports management functions; a manual choice can deliberately retain an aid. A selected frequency must be associated with the correct identifier, course, procedure and reception conditions. A line in the lateral plan is not the same evidence as raw localizer or glideslope deviation. This becomes especially useful during vectors or a runway change. [87, PDF pp. 143, 206–209] [66, PDF pp. 469–474, 477]
Radio altitude measures height relative to the terrain beneath the aircraft within the system’s operating conditions. Barometric altitude is derived from pressure and the selected reference. Their consumers and failure consequences therefore differ. Approach guidance, automatic landing and warning functions can depend on radio-height information even while barometric altitude remains plausible. Compare the source and geometry before treating two height indications as inconsistent. Exact capture, availability and failure thresholds belong to the installed system and current procedure, not to a generic drawing of a display. [65, Contents, section1.34.40, radio altimeter scope] [21, PDF pp. 159–160, 197–198] [60, Section 1.31.40; distinct altitude and radio-height presentation]
Standby references provide a further comparison only when their actual supply and sensor dependencies are understood. Independence of presentation is not proof that every input is independent. During a multiple-reference failure, preserve a clear record of the source being used by each indication and the capability remaining. The current Fenix options include standby-instrument and backup-speed-scale configurations, but the detailed performance and permitted use of those functions require their applicable documentation. An A320-specific unreliable-speed table is not automatically transferable to A319 or A321. [70, Available Options: standby and backup-speed-scale settings] [83]