RNAV is enabled through the use of a navigation computer. Waypoints are input into the computer either manually (but this has limited capabilities) or automatically with an integrated database. The flight crew then creates a route as a series of waypoints in accordance with the flight plan. The computer estimates the aircraft position using the fitted navigation sensors and compares the estimation to the defined route. Deviation between the estimated position and the defined path creates guidance information. In order to perform RNAV, aircraft must be equipped with an RNAV system.
Types of RNAV
There are 3 types of RNAV system:
2D RNAV: Provides navigation in the horizontal plane only.
3D RNAV: Provides navigation in the horizontal plane and also has a guidance capability in the vertical plane.
4D RNAV: Provides navigation in the horizontal plane, has a guidance capability in the vertical plane and has a timing function.
RNAV Navigation Specification
The International Civil Aviation Organization’s (ICAO) PBN Manual identifies four navigation specifications under the RNAV family:
RNAV 10 designated as RNP 10 in the ICAO’s PBN Manual, is an RNAV specification for oceanic and remote continental navigation applications.
RNAV 5, also referred to as Basic Area Navigation (B-RNAV), has been in use In Europe since 1998 and is mandated for aircraft using higher level airspace. It requires a minimum navigational accuracy of ±5nm for 95% of the time and is not approved for use below MSA.
RNAV 2 supports navigation in en-route continental airspace in the United States.
RNAV 1 is the RNAV specification for Precision Area Navigation (P-RNAV). It requires a minimum navigational accuracy of ±1nm for 95% of the time. 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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