P-RNAV
Precision Area Navigation (P-RNAV) is being introduced as the standard to use throughout ECAC Terminal Airspace, if you wish to introduce RNAV routes. The option of conventional procedures is retained.
P-RNAV is predicated upon a lateral track accuracy of 1NM for 95% of the time. There are detailed design criteria to be followed for P-RNAV routes that are published in a Eurocontrol document. However, fly-by turns are required to be used for most procedure designs when using P-RNAV. The design criteria contains a multitude of formulae and tables that let the procedure designer determine when aircraft will start to turn prior to a fly-by waypoint. Factors such as the speed of the aircraft, turn angle, bank angle are taken into account. A minimum distance between successive waypoints is required for stabilisation. This minimum distance also varies depending on speed, bank angle, turn angle, etc. The point is that P-RNAV enables procedure designers to determine when an aircraft will start to turn prior to a fly-by waypoint. All aircraft, if flying P-RNAV, should turn so as to remain within 1NM of the published track. There are several rules for P-RNAV design. One such rule is that no single turn should exceed 120 degrees. This is to avoid aircraft having to initiate the turn, even in Terminal Airspace, at a substantial distance prior to a fly-by waypoint.
Work is currently on-going to determine the required spacing that should be provided be P-RNAV routes. Initial noises from the CAA suggest that 5NM is an appropriate distance to apply between P-RNAV routes. This is presumably based on the fact that aircraft fly within 1NM of published track, and allows for a 3NM separation criteria even if both aircraft are at the edge of their respective 1NM tolerances.
It is also being suggested that in the near future the CAA will look favourably upon separation criteria such as Aircraft A on a P-RNAV route, being separated by xNM from Aircraft B that is on a radar heading. Historically in the UK both aircraft A & B would have been provided with headings to fly (or be 'locked' on).
P-RNAV is likely to be introduced on a more widespread basis as new runways, requiring revisions or additions to SIDs, STARs and new airspace come into effect in the UK. Work is on-going to ensure that P-RNAV is introduced in such a way as to ensure a safe and efficient operation continues to exist in UK Terminal Airspace.