wind vector may not be repeatable), requires a fly-over of a waypoint (because wind vector may not be repeatable), or occurs when the aircraft reaches a target altitude (because target altitude is dependent on engine thrust and aircraft weight). In these cases, the navigation database contains a point-to-point desired flight path, but cannot account for the RNAV system defining a fly-by or fly-over path and performing a maneuver. A meaningful PDE and FTE cannot be established without a defining path, resulting in variability in the turn. Also, a deterministic, repeatable path cannot be defined for paths based on heading and the resulting path variability is accommodated in the route design. FTE relates to the air crew or autopilot's ability to follow the defined path or track, including any display error (e.g. Course Deviation Indicator (CDI) centering error). FTE can be monitored by the autopilot or air crew procedures and the extent to which these procedures need to be supported by other means depends, for example, on the phase of flight (i.e. take-off, climb, cruise, descent, landing) and the type of operations. Such monitoring support could be provided by a map display. NSE refers to the difference between the aircraft's estimated position and actual position. Longitudinal navigation Longitudinal performance implies navigation against a position along a track (e.g. 4-D control). However, at the present time, there are no navigation specifications requiring 4-D control, and there is no FTE in the longitudinal dimension. The current navigation specifications define requirements for along-track accuracy, which includes NSE and PDE. PDE is considered negligible. The along-track accuracy affects position reporting (e.g. "10 NM to ABC") and procedure design (e.g. minimum segment altitudes where the aircraft can begin descent once crossing a fix). Designation An RNAV specification is designated as RNAV X, e.g. RNAV 1. The expression 'X' (where stated) refers to the lateral navigation accuracy in nautical miles, which is expected to be achieved at least 95% of the flight time by the population of aircraft operating within the airspace, route or procedure. There are no RNAV approach specifications. Flight planning Manual or automated notification of an aircraft's qualification to operate along an air traffic services (ATS) route, on a procedure or in an airspace, is provided to ATC via the flight plan. Flight plan procedures are specified in appropriate ICAO documents. See also VNAV LNAV Localizer performance with vertical guidance References External links RNAV