Qualifying systems must have the ability to fly accurate tactical offsets; P-RNAV routes must be extracted directly from the FMS data base and must be flown by linking the R-NAV system to the FMS/autopilot. In addition, flight crews are restricted from manually adding waypoints to the route.
This level of navigation accuracy can be achieved using DME/DME, VOR/DME or GNSS. It can also be maintained for short periods using inertial reference systems (IRS) and the length of time that a particular IRS can be used to maintain P-RNAV accuracy without external update is determined at the time of equipment certification.
It should be noted that if GNSS is not used as a source then two independent ground-based sources are required to meet P-RNAV minimum requirements apart from specified short periods of INS ‘backup’, which is a more stringent requirement than for some older flight management system (FMS).
P-RNAV is used to provide more routes and terminal area procedures and may be used down to the final approach fix (FAF) on designated approach procedures.
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