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4. Required Navigation Performance (RNP)

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Required Navigation Performance (RNP)

RNP is a statement of navigation performance (Note 1) necessary for operation within a defined airspace. The RNP types (e.g.: RNP 5, RNP 1, RNP 0.3) specify the navigation performance accuracy of all the user and navigation system combinations within an airspace.
RNP types specify the minimum navigation performance accuracy required in an airspace and is used for airspace planners to design the routes.

    CAUTION: It may be possible that an aircraft with a level of navigation performance more accurate than specified may not meet the requirements of a less stringent airspace due to lack of appropriate navigation equipment. E.g.: An RNP 1 certified aircraft based on DME/DME updating which is not fitted with long range aids is unable to operate in RNP 12,6 (MNPS) airspace.

The concept of RNP applies to navigation performance and therefore affects both the airspace and the aircraft. The RNP type describes the navigation performance that is expected to be achieved within that airspace for at least 95% of the time by the population of aircraft operating within that airspace.
Although modern, multi sensor, RNAV systems are generally assumed to be very accurate for most of the time, the 95% containment level allows for an error budget of 5%, theoretically without any specified maximum value but contained within an assumed statistical distribution.
RNP containment is based on the total navigation system error (TSE) which is a square mean combination of:
− Navigation system error
− Flight Technical Error
− RNAV computation error
− Display system error

NOTE: Errors are not simply added to each other but combined mathematically.

For example, if the RNP is 4, the approval process must show that the TSE in each (horizontal) dimension must be within 4 NM for 95% of the flight time on any portion of any single flight:
− The true position of the aircraft must be within 4 NM of the programmed route centreline, and
− The true distance to waypoints must be within 4 NM of the displayed distance to waypoints.

Since RNP is defined by a statement on navigation performance, there is an obligation on the part of the State and the aircraft operator to provide the required navigation performance.
Compliance with RNP requirements can be achieved in different ways and neither the State nor the operator is restricted as to how RNP is achieved.

    Note 1: Performance is defined in accuracy, integrity, availability and continuity of service. All must meet a minimum acceptable level.

The RNP type is specified by the accuracy value associated with the RNP airspace and the following types are specified:

    − RNP 1 or less: Intended to support Terminal Area operations. Practical implementation is not expected before 2010 in designated TMAs in the European Airspace.
    − RNP 4: Supports ATS routes and airspace design based on limited distance between navaids and is normally associated with continental airspace.
    − RNP 10: Supports navigation in oceanic and remote areas where the availability of navigation aids is limited (based on IRS only navigation for a limited time period of 6.2 hours).
    − RNP 12.6: Supports limited optimised routing in areas with a reduced level of navigation facilities.
    − RNP 20: Describes the minimum capability considered acceptable to support ATS route separations and is expected to be met by any aircraft in any controlled airspace at any time.
    Some States have implemented RNP 5 (Europe: BRNAV) for an interim period as a derivative of RNP 4, in order to permit the continued operation of present navigation equipment without modification of existing route structures.

RNP may be specified for routes or areas of any defined dimension. An RNP type should be selected with respect to air traffic demand in that airspace. The specified RNP will then determine the required aircraft equipment and airspace infrastructure. It is the responsibility of States to assure the accuracy of position data, which should be referenced to a common geodetic system (WGS-84).

Because of the nature of RNP operations, specific equipment is not normally specified. Instead, compliance may be met by various types or combinations of navigation equipment, such as e.g. VOR/DME, DME/DME, GNSS, etc. Specific equipment may be required for certain types of operation like e.g. IRS for RNP 10 or FMS and electronic map displays for RNP 1 or more stringent operations.
Equipment capability may be demonstrated by flight trials but is normally done by means of a statement in the Aircraft Flight Manual by the aircraft/equipment manufacturer, based on aircraft certification.
Equipment certification in itself does not constitute operational approval for RNP operations, as an Operational Approval is required as well.

The State of the operator is the authority responsible for approval of RNP operations by an operator. Approval will be indicated on the operator’s Air Operator Certificate (AOC).
Requirements for obtaining such approval depend on the desired RNP type but generally consist of: Appropriate aircraft equipment,
Adequate crew operating and monitoring procedures,
Adequate flight crew training.

Flight crew should notify ATC of contingencies, equipment failures or weather conditions that affect its ability to maintain navigation accuracy. If so, state intentions and obtain a revised clearance.
If unable to contact ATC, follow the established contingency procedures as indicated in the route documentation and obtain a (revised) clearance as soon as possible.

Source: FCRG

See also:
History of RNAV
Area Navigation Definitions
RNAV System Description
Required Navigation Performance (RNP)
RNAV Development
RNAV System Limitations
Global Positioning System (GPS)

The presented material is for training purpose only!