RNP and RNAV
Complete explanation of RNP and RNAV for Part 2 of Radiotelephony exam and also for Nav and radio aids paper of DGCA
REQUIRED NAVIGATIONAL PERFORMANCE (RNP) AREA NAVIGATION (RNAV)
The ICAO Special Committee on Future Air Navigation Systems (FANS) defines RNP as a statement of required navigation accuracy in the horizontal plane (lateral and longitudinal position fixing) necessary for operation in a defined airspace. RNP types are identified by a single accuracy value, shown in the table below. For example, the statement of RNP 1, refers to a required navigation performance accuracy within 1 NM of the desired flight path at least 95% of the time flying.
Table of existing and future levels of navigation accuracy
|RNP RNAV Type||Accuracy in the designated airspace|
|RNP 1||± 1.0 NM|
|RNP 4||± 4.0 NM|
|B-RNAV (RNP 5)||± 5.0 NM|
|RNP 10||± 10.0 NM|
|RNP 12.6||± 12.6 NM|
|RNP 20||± 20.0 NM|
B-RNAV (RNP 5) is a derivate of RNP 4, allowing continued operation without modification of existing route structures and is implemented in the ECAC (European Civil Aviation Conference) Airspace.
Potential application for RNP airspace includes:
– a defined airspace, such as North Atlantic minimum Navigation performance specifications (MNPS) airspace.
– a fixed ATS route, such as between Sydney, Australia and Auckland, New Zealand.
– random track operations, such as between Hawaii and Japan.
– a volume of airspace such as a block altitude on a specified route.
The implementation of RNP allows enhancements of ATC system capacity and efficiency while retaining or establishing enhanced system safety.
RNAV is the primary means of meeting RNP requirements. RNAV operations within the RNP concept permit flight in any airspace within prescribed accuracy tolerances, without the need to fly directly over ground-based navigation facilities. The application of RNAV techniques provides a number of benefits, for example:
– establishment of more direct routes reducing the flight distances.
– establishment of dual or parallel routes to accommodate a greater flow of enroute traffic.
– establishment of bypass routes for high density traffic areas.
– establishment of contingency routes.
– establishment of optimum locations for holding patterns.
– reduces the number of ground navigation facilities.
Navigation parameters such as distance and bearing to a way point are computed from the air-craft position to the location of the way point. Course guidance is generally derived from the linear deviation from the desired track of a great circle course. The desired course may be pilot elect able or may be determined by the navigation computer through computations based on the locations of successive way points.
Precision RNAV (P-RNAV) P-RNAV is the aircraft and operator approval requirement that is introduced for RNAV procedures in ECAC Terminal Airspace. Terminal Airspace procedures that require P-RNAV approval are designed following common principles which ensure that procedure design and execution are fully compatible. (RNP 1) – shall provide a 95% containment value of ± 1 NM (± 1.85 km). This level of navigation accuracy can be achieved using DME/DME, GPS or VOR/DME. It can also be maintained for short periods using IRS (the length of time that a particu-lar IRS can be used to maintain P-RNAV accuracy without external update is determined at the time of certification.
Basic RNAV (B-RNAV) B-RNAV is the forerunner of the RNAV programme. It was introduced to enable capacity gains to be achieved through modifications to the enroute structure. (RNP 5) – shall provide a 95% containment value of ± 5 NM (± 9.26 km). This level is similar to that currently achieved by aircraft without RNAV capability on ATS routes defined by a VOR or VOR/DME, when VOR’s are less than 100 NM apart.
RNP AND RNAV REQUIREMENTS
For RNP and RNAV operations, operators have the responsibility to ensure the required level accuracy, within the notified RNP/RNAV environment, by means of appropriate equipment usage and prescribed procedures for the flight crew. It is essential that ATC receives an indication from the operator that a flight, planned along RNP/RNAV routes or in a RNP/RNAV area, has the required navigation capability.
APPROVAL AND CERTIFICATION
A fundamental requirement for the implementation of RNP is the approval of flight operations in the various RNP type airspaces by the State of the operator. Approval will be granted individually for each operator and each individual aircraft type used by the operator. RNAV and FMS equipment also needs to obtain airworthiness approval by the national authority. The approving authority must ensure that aircraft equipment be installed and operated in a manner appropriate to the RNP type approval being sought. An approval for a certain RNP type does not mean that the aircraft may be operated wherever the RNP type applies. The RNP type approval is specific to a particular type of navigation equipment and application, and for the use of INS/IRS a time limit may apply. For example, an aircraft, having approval for RNP 5 in the B-RNAV airspace of Europe, using RNAV equipment requiring input from ground based navigation facilities such as VOR/DME may not be operated in a RNP 10 airspace where such facilities are not available.
B-RNAV CERTIFICATION AND OPERATIONAL REQUIREMENTS
In order to comply with RNAV operational requirements, aircraft must be certified for B-RNAV operation in order to file an IFR flight plan in the B-RNAV FIR/UIR in the ECAC airspace. See Minimum Equipment List (MEL) requirements and applicable aircraft procedures related to navigation performance.
Many different types of equipment are currently available to meet requirements for one or more RNP types. For example, a VOR/DME navigation system in combination with a simple RNAV computer accepting VOR/DME input is the least sophisticated equipment.
Area Navigation Equipment determines aircraft position by processing data from one or more sensors. Determination of aircraft position is dependent on such factors as sensor availability and accuracy, signal parameters (signal source strength, transmitted signal degradation). Position determination may employ such inputs as :
– distance measurements from two or more Distance Measuring Equipment (DME) ground stations (DME-DME);
– Very High Frequency Omnidirectional radio Range with DME (VOR/DME);
– Inertial systems (INS, with radio updating or limited 2 hour use after last on ground update)
– LORAN C (with limitations)
– Global Navigation Satellite System (with limitations).
GENERAL OPERATIONAL LIMITATIONS
Due to the availability and integrity of the various sensor systems, and effects of from outside sources, certain operational limitations must be imposed on the use of some types of RNAV equipment as follows:
Operational Areas — operators shall define the area(s) in which operations are intended and ensure that equipment usage is capable of performance within the defined standard.
INS — Without an automatic radio update, INS function is limited in usage for a 2 hour period from the last on ground position update. This can result in a degradation of accuracy with elapsed time. As a requirement, a linear decay value of 1.5 to 2 NM per hour must be considered.
GNSS — During the pre-flight planning phase, if 24 satellites (23 if baro aiding is incorporated into the GPS installation) are projected to be operational for the flight, then the aircraft can depart without further action. If 23 satellites or less (22 or less if baro aiding is incorporated), are projec-ted to be operational, then the availability of GPS integrity (RAIM) should be confirmed for the intended flight (route and time).
Navigation systems must demonstrate an acceptably reliable continuity of function prior to approval. National authorities may choose to rely on redundancy of systems in order to obtain an aver-age airborne system availability of 99.99% of flight time for B-RNAV. Navigation function availability may be assured by the use of the multi sensor area navigation systems which incorporate various position fixing sensors, each of which is individually usable for airborne area navigation. Some RNAV systems permit the use of combinations of systems or pilot selection of one system in preference to another, depending on factors such as reception and weather conditions.
As long as VOR/DME facilities are available, and aircraft are equipped with VOR/DME instrumentation, the carriage of a single B-RNAV system will provide equivalent safety to the average systems availability requirements. It is anticipated that the withdrawal of VOR facilities will result in a requirement to carry redundant B-RNAV systems in order to meet the average system availability requirement.
Flight Crew Inputs — Procedures shall enable erroneous flight crew inputs to be detected before the aircraft position accuracy can be degraded. It is the crews responsibility to ensure that the navigation accuracy is maintained. In particular, the following common mistakes must be avoided:
Insertion errors — Coordinates are inserted incorrectly into the system. (Particular care must be taken in case of a new ATC clearance).
De-coupling — If the pilot allows the autopilot to become de-coupled from the equipment which he thinks is providing steering output.
Using faulty equipment — The pilot might continue to use a navigation system which has become inaccurate.
Navigation equipment should be capable of enabling aircraft to be navigated within the constraints of the air traffic service to the accuracy required in a promulgated RNP type of airspace. The car-riage of RNAV equipment may be required in some regions or States and therefore the reason why frequent reference is made to the use of RNAV equipment.
NAVIGATION DATA BASE
It is the responsibility of the States to maintain the level of accuracy and thoroughness of the source material on which data bases rely. Data base providers have the responsibility to ensure that they accurately reproduce the source material as provided by the States.
Aircraft Flight Management System (FMS) software should employ the same geodetic reference datum as that used for locating ground based or earth-referenced navigational aids to avoid navi-gation errors when transferring between different geodetic reference datum application areas. The equipment shall provide an electronically updatable navigation database containing at least the following location information:
– VORs, DMEs, VORTACs and NDBs
– All named fixes
– All procedures defined by a State such as Routes, SIDs, STARs, APCH, holdings, etc.
For B-RNAV a navigation data base is optional. If provided, it shall consist of current navigation reference data officially promulgated for civil aviation use, and contain at least navigation aid and way point information covering the region of intended operation. It is desirable if storing a number of flight plans. The navigation data base installed in the aircraft must be checked for its validity before the flight.
Route planning — The system shall allow the construction and/or modification of a flight plan. The flight crew shall be able to determine the correctness of the flight plan. B-RNAV shall provide a means for the insertion or modification of data in the flight plan via the RNAV Control Display Unit (CDU).
In-flight update — Verification of the data in respect to the Flight Path being flown, and the stored data base at any time without the guidance and navigation outputs of the computer being affected, is mandatory. The route data shall consist of the names or coordinates of the way points and shall include distance and tracks between them. The present track and distance to go to the next way point shall be provided, except when operating on a non fixed leg. The flight crew shall be able to modify the flight plan at any time. An additional means of updating the flight plan by use of a ground/air data link is optional.
Navigation Mode and Annunciation
The flight crew shall be enabled to monitor navigation mode and position.
Tuning and Selection of Radio Aids
Automatic selection and tuning of VOR and/or DME channels in accordance with stored program procedures, and related aircraft position and data base requirements, is required. The selected frequencies and ICAO identifiers shall be available for display. Individual NAVAIDs shall be inhibi-ted from the automatic selection process by the crew if desired. The ability of manual tuning to/of a Radio Navigation Aid (NAVAID) or displaying the data shall be given.
(Aircraft equipped with FMS should comply with the following statements in general):
Cross Track Deviation — A continuous display of distance from the intended track shall be provided. The display resolution shall be consistent with the system accuracy.
Parallel Offsets — A system is desired which provides the ability to fly parallel tracks offset by up to 20 NM from the primary track defined by the way points. The presence of an offset shall be continuously indicated.
Operators of aircraft fitted with RNAV having a navigation accuracy meeting RNP 5 shall insert the designator ‘R’ in item 10 of the flight plan.
Operators of State aircraft not equipped with RNAV but having a navigation accuracy meeting RNP shall not insert the designators ‘S’ or ‘R’ in item 10 of the flight plan. Since such flights require special handling by air traffic control, item 18 of the flight plan shall contain STS/
If, as a result of a failure of the RNAV system or degradation of it below RNP 5, an aircraft is unable to either enter the designated airspace or continue operations in accordance with the current air traffic control clearance, a revised clearance shall, whenever possible, be obtained by the pilot.
When a verbal coordination process is being used, the sending air traffic control unit shall include the phrase ‘NEGATIVE-RNAV’ at the end of the message. The phrase ‘NEGATIVE-RNAV’ shall be also included by the pilot immediately following the aircraft call sign whenever initial contact on an ATC unit frequency is established.
The Operations manual shall describe the RNAV equipment procedures to be used for
– pre-flight, in-flight and post-flight; and
– in the event of a loss, or impairment, of RNAV navigation capability. The procedures as filed by the state authorities do strictly apply.