Par 4 Page 35 these systems may only be approved if the system can meet the performance, integrity, and availability standards equivalent to those established for currently approved types of operations (e.g., ILS, FMS, etc.), to corresponding minima. Operational authorization for use of enhanced vision systems requires successful completion of a proof of concept demonstration acceptable to FAA.
4.3.10.11. Circling Approaches. When instrument approach design criteria or operational factors do not permit a “straight-in” approach to the landing runway, circling procedures may be used. U.S. criteria require SIAP publication of circling maneuver minima if the inbound course is offset more than 30 degrees from the runway centerline, or when a specified descent gradient for a straight-in approach is steeper than a maximum value allowed by instrument procedure design criteria.
a. Use of circling minima, however, does not preclude a pilot making a straight in landing if the requirements of section 91.175 can be continuously met below MDA(H), to touchdown, for adequate visual reference and for normal landing maneuvering. Typically, circling approaches are based only on an MDA(H). Use of a DA(H) for circling is addressed because certain procedures using a DA(H) may apply to “sidestep” maneuvers, or may be used with very high values of DA(H), such as in mountainous areas that otherwise may require a circling maneuver to position to land after reaching minimums.
b. The circling maneuver can be initiated from any instrument approach procedure where circling is authorized, and may be continued below MDA(H) or beyond the missed approach point (MAP) only when the specified visual reference exists, and when in a position for a normal descent to landing. Electronic course or glidepath information, or FMS flight path presentations are only considered supplementary information to visually accomplishing the circling maneuver. The pilot must keep the aircraft’s position within the established maneuvering area for the approach speed and category specified for the procedure while performing the circling maneuver. An altitude at or above the circling MDA(H) must be maintained until an aircraft (using normal maneuvers) is in a position from which a normal descent can be made to touchdown within the touchdown zone. A missed approach must be executed when external visual references are lost or sufficient visual cues to manually maneuver the aircraft cannot be maintained.
c. It is important to note that the published missed approach procedure may not provide obstacle clearance when below DA(H) or MDA(H), or when past the published missed approach point (MAP). If it is necessary to conduct a missed approach from below the DA(H) or MDA(H) or from past the published MAP (e.g., as a result of a balked landing, rejected landing, loss of visual reference, not in a safe position to land, blocked runway, or other similar reason for a go-around), reference to the associated IFR departure procedure for the applicable runway(s) usually provide help to the pilot in determining a safe course of action to climb back to procedurally protected airspace (adequate obstacle clearance) as specified by the published missed approach procedure.
d. When a missed approach from a circling maneuver is executed from below DA(H) or MDA(H) such as when visual reference is lost after passing DA(H) or MDA(H), or when initiating the missed approach from beyond the missed approach point such as when not able to maneuver to be able to accomplish a normal landing in the touchdown zone, the direction of the initial missed approach turn should typically be in a direction toward an appropriate runway, to ensure obstacle clearance. This is to keep the aircraft within the maneuvering area, until climb above the DA(H) or MDA(H), and intercept of a published segment of the missed approach procedure can be accomplished. Pilots should be aware of the applicable radius of protected airspace for the respective approach category used for the circling maneuver, and attempt to maneuver the aircraft within that protected airspace radius from the airport.
e. Operators may be authorized to perform circling approaches as published, or may choose not to train flightcrews to accomplish circling maneuvers and accept corresponding high minima limitations regarding circling approaches. If an operator chooses not to train for circling approaches, a 1000 ft HAT DA(H) or MDA(H) and 2 mile visibility limit, or greater, is typically included in OpSpecs to limit use of circling minima for that operator or aircraft type.
Page 36 Par 4
f. It is recommended that unless special circumstances exist, wide body (long wingspan) aircraft or aircraft needing to accomplish circling maneuvers at speeds in excess of 165 KTS ground speed should not typically be authorized circling minima below 1000 ft. HAT and 3 miles meteorological visibility.
4.4. RNAV/Flight Management Systems (FMS). An FMS provides a means to navigate along a flight path based upon earth referenced waypoints. These waypoints can define a flight path that originates or terminates at a runway or at other relevant fixes located in terminal or en route airspace. This type of system may be approved for low visibility approach and missed approach operations IAW criteria in pertinent appendices of this AC and standard OpSpecs.
a. FMS systems eligible for use must meet criteria of AC 25-15, AC 20-129 and AC 20-130, or subsequent criteria, or equivalent criteria. Equivalent systems are considered to be those systems previously shown to meet AC 90-45 which predated the above references, but would have otherwise been capable of meeting essential elements of the later criteria (e.g., B757, B767), or other aircraft that have subsequently been determined to be capable of meeting essential elements of the above criteria even though they were not specifically certificated using that criteria
(e.g., certain non-U.S. manufactured aircraft such as the A320).
b. For RNP operations, additional information is provided below and in paragraph 4.5 and Appendix 5.
4.4.1. FMS Use for xLS Procedures
a. ILS, MLS, or GLS approaches or procedures are typically flown with FMS only to the extent that the FMS:
• Serves as a means to display the ILS, MLS, or GLS procedure (e.g., as on a navigation map display);
• May be used to tune appropriate ILS, MLS, or GLS NAVAIDs or radio frequencies;
• May be used to define and display and fly various LNAV or VNAV segments to intercept the final approach path or segment, or glideslope; or
• May be used to define, display and fly various LNAV or VNAV segments for a missed approach path.
b. Use of FMS to fly ILS, MLS, or GLS approaches when ILS, MLS, or GLS navigation aids are out of service
(e.g., localizer or glideslope inoperative, or GNSS GBAS facility inoperative) may be authorized only in conjunction with RNP criteria (See paragraph 4.4.4 below).
4.4.2. FMS Use for Procedures Other Than xLS or RNAV. FMS may be used to conduct VOR, VOR/DME, NDB, NDB/DME, LOC, and LOC Back Course approaches when suitable navigation position updating which provides required accuracy and integrity is used by the FMS (e.g., DME-DME-IRS, or scanning DME, or VOR/DME, or GNSS position updating, or Localizer (LOC) updating, etc.).
4.4.3. FMS Use for RNAV. FMS may be used as a 2D or 3D RNAV system, to conduct RNAV instrument approaches.
a. RNAV procedures may be authorized based on one or more “procedure specified” NAVAID(s) (e.g., the FMS data base identifies a specific VOR/DME “Procedure tuned (“P” tuned)” NAVAID, or a combination of specific DME facilities to use as a basis for the procedure).
b. GPS approaches are considered to be RVAV approaches when flown by an FMS. GPS approaches may only be flown by those FMS systems which are capable of suitable GPS position updating and have appropriate navigation data base information to properly load and display the procedure to the flightcrew. Not all GPS approaches may necessarily be suitable for use with FMS because of procedure design, vertical path definition, an inability to “call up” or “load” the procedure from a data base, because the FMS may not be able to appropriately recognize “GPS” as a type of approach classification, or because the airplane AFM may not suitably provide for GPS procedure use. Operators intending to fly “GPS approaches” using FMS should treat such procedures as
Par 4 Page 37 RNAV procedures, and ensure that the FMS can properly fly each procedure or each type of procedure to be used
(e.g., LNAV/VNAV or LNAV only).
c. RNAV procedures may also be authorized based on use of a “NAVAID rich environment” in which specific “procedure identified” NAVAIDs may not be identified, but rather the FMS is permitted to select optimum NAVAID’s from those available. When such RNAV and NAVAID updating procedures are used, the NAVAID service provider, authority, or operator must ensure that the normally selected NAVAID(s) and the alternately selected NAVAID(s) suitably support the procedure to an acceptable level of accuracy and availability (e.g., at ranges, at altitudes, and along the expected flight paths relevant to achieving appropriate system approach performance). For an FMS which uses DME-DME or VOR-DME-based NAVAID sensors in conjunction with IRS, in a NAVAID rich environment, this can typically be accomplished by analysis, or by in-flight assessment (usually during line operations) to show suitable NAVAID reception for normal facilities to be used and for the first alternate facilities anticipated to be used for a particular system and procedure if the normal facility(s) become unavailable.
For equivalent RNAV procedure assessments for RNP-qualified aircraft, see paragraph 4.4.3.3 below.
d. RNAV procedures that do not use “procedure tuned facilities” may be authorized for use with multi-sensor FMS based on use of “DME-DME” updating, “VOR/DME” updating, “scanning DME” updating, or “GNSS (GPS)” updating. These methods may be used individually, or may be used in combination, or may be used in conjunction with inertial position filtering.
NOTE: For purposes of this paragraph, any 14 CFR part 97 procedure with a specified DME limitation must be reviewed and resolved by the POI prior to the operator's use of that procedure.
4.4.3.1. Use of a Single RNAV Airborne System. Other than following an in-flight failure of one of several installed airborne RNAV systems (e.g., failure of one FMS), instrument procedures based on RNAV may be flown using a single airborne RNAV system in lieu of two RNAV systems (reference section 121.349) under the following conditions:
a. The operator is authorized to conduct procedures using a single RNAV (FMS) system, NOTE: Authorization for use of a single RNAV may be for a specific procedure, a group of procedures, for an operator’s particular fleet of aircraft (e.g., B737 fleet), for all of an operator’s aircraft, or for a geographic region (e.g., within the United States and U.S.
territories), as applicable to the operator’s route structure, and fleet.
b. Instrument procedures requiring simultaneous use of more than one RNAV system are not authorized, unless approved for that operator and each specific procedure,
c. In the event of failure of the airborne RNAV system, or other essential element of the airborne RNAV navigation or display system, or associated NAVAID(s), the approach can be safely discontinued at any point during the approach to touchdown, or at any time during a missed approach, and
d. Following initiation of the missed approach or rejected landing, a transition can be made to use some other NAVAID or NAVAIDs, other than the failed RNAV system or facility(s) used by that system, to complete a safe missed approach and subsequent flight to an alternate.
NOTE: A period of dead-reckoning may be permissible between the time the RNAV system is used and reversion to another system, or following NAVAID failure, to the time alternate navigation means are established for continuing the missed approach and flight to alternate.
During this period of dead-reckoning the aircraft should not be unduly exposed to loss of obstacle clearance due to proximity to terrain or significant obstacles. Suitable navigation performance should be achievable to safely complete the missed approach, fly to the alternate, and complete a subsequent approach using a different navigation system or
Page 38 Par 4 NAVAID(s), without loss of knowledge of position, loss of appropriate obstacle clearance, or loss of terrain clearance.
4.4.4. FMS Use for RNAV with RNP. RNP operations may be based on capability as specified in a FAA approved AFM. RNP operations may also be based on “Fleet Qualification” of and individual aircraft, a group of aircraft, or an aircraft type using criteria acceptable to FAA (e.g., RTCA DO-236 Appendix D for RNP Fleet Qualification).
a. Approach or departure RNP operations for an air carrier typically require dual FMS capability for RNP.
b. See paragraph 4.4.2 above for operations and limitations that may apply for a single FMS with RNP capability. In addition, procedures for departure or approach for air traffic separation that are based on use of RNP may require use of dual RNP-capable systems, when so designated.
c. FAA may authorize other approach types for use by FMS on a case by case basis for each operator or aircraft type.
4.4.4.1. Standard RNP Qualification. FMS may be used as a 2D or 3D RNAV RNP system, as appropriate, to conduct RNAV instrument approaches based on aircraft qualification for RNP. Operations should be consistent with the approved AFM and apply appropriate RNP obstacle clearance criteria. Appendix 5 provides obstacle clearance criteria for RNP that can be used for RNAV approaches using RNP-based minima. FAA Order 8260.47, or other criteria acceptable to FAA, may be used to specify vertical obstacle clearance criteria for use of VNAV.
4.4.4.2. “Fleet Qualification” For Use of RNP. Some FMSs do not incorporate provisions for RNP as part of their type design approval. Aircraft with such FMSs may be candidates for fleet qualification for one or more RNP levels when certain provisions are met for autoflight systems, displays, annunciations, and FMSs. These aircraft may use corresponding RNP procedures and criteria (e.g., see Appendix 5 for RNP-based obstacle criteria). Criteria of Appendix 5 applicable to RNP-based RNAV approaches may be used for these FMS systems when approved by the FAA. RNP vertical criteria or vertical criteria of FAA Order 8260.47, or other criteria acceptable to FAA, may be used to specify vertical obstacle clearance requirements for use of VNAV.
a. Examples of aircraft and systems which may typically “fleet qualify” under this provision would be aircraft having IRS and dual FMS incorporating GPS updating, or dual FMS using DME-DME or scanning DME updating when the aircraft is operated in an area with a significant number of DME facilities. A significant number of DME or other NAVAID facilities are considered to be a number which provide for adequate signal coverage in the event of failure of any single facility, and with more than one facility or facility pair providing acceptable position update geometry and accuracy, considering the updating requirements for the FMS and any other relevant sensors used (e.g., IRS, IRU, ADIRU). Typically, aircraft having FMS and sensor systems such as these are considered to meet either /E or /F flight plan classification.
b. The following capabilities for aircraft and systems (e.g., for aircraft systems described, named or described differently but providing equivalent capability) should be considered for fleet qualification for RNP 0.3 or greater.
(1) Suitable autopilot or Flight Director use*,
(2) Suitable alerting; e.g., an “IRS Only” annunciation message, should suitable NAV updating not be available, and
(3) Suitable navigation display; e.g., A 10 mile (or lower) EFIS Map Scale, showing the designated flight path (such as an FMS designated green or magenta flight path line), with a suitable aircraft position symbol allowing a pilot to suitably monitor availability of a correct flight path, and aircraft path displacements (FTE)**,
(4) Suitable navigation check procedures; e.g., if not otherwise ensured by system performance or flight deck annunciation, a “reasonableness check” for acceptable position fixing error to be completed not later than passing a Final Approach Fix (FAF), and
Par 4 Page 39
(5) Suitable navigation system status assessment; e.g., a NAVAID or sensor updating capability suitability cross check, performed not later than passing a Final Approach Fix (FAF)***.
c. Additional criteria may be necessary depending on the specific fleet, and desired operations, routes, or procedures. Additional information may be found in DO 236, Appendix D.
*NOTE: Credit may be limited by Flight Technical Error (FTE) capability that can be achieved.
**NOTE: The objective is to assure that the pilot has that information, in a suitable form, necessary to conduct the operation (e.g., appropriate to the airspace/type of operation).
Credit for systems other than EFIS “map displays” (e.g., systems using only an HSI or lateral deviation scale display) for RNP may be permitted, but credit is limited to use of “simple procedures.” Procedures considered to be unacceptable (i.e., not simple) are those procedures involving:
• multiple short flight path segments,
• frequent or large angle turns
• critical obstacles adjacent to turns
• adjacent aircraft flight paths with turns
• adjacent significant or mountainous terrain
• use of multiple or complex VNAV gradients
• procedures requiring a high level of pilot “situation awareness” to detect and correct the consequence of flight path definition or waypoint difficulties (e.g., an FMS “Legs Page” waypoint “Bypass”)
• procedures unduly sensitive to pilot setup errors or mistakes made in programming a navigation system that could readily be detected when using a map display
• procedures that require unusual levels of attention, FTE monitoring, or
• other criticality that are aided by use of a map display ***NOTE: May be a limiting factor for the level of RNP to be authorized, considering the pilot or operator’s ability to assess position fixing errors as relate to sensors or NAVAIDs intended to be used.
4.4.4.3. Assessment Credit for RNP-qualified aircraft flying “non-RNP” based RNAV Procedures. RNAV procedure assessment credit may be based on an RNP (AFM qualified) aircraft flying non-RNP based RNAV procedures to demonstrate that acceptable system performance is achieved and that a NAVAID rich environment
(e.g., DME-DME IRS or RNAV-DME IRS updating) is capable of appropriately supporting an RNAV procedure for that aircraft and system type. For such assessments, it is acceptable for an operator to show that the demonstrated ANP (EPE) remains below an acceptable value throughout an approach, and any applicable parts of a missed approach, for the normal and first alternate FMS NAVAID facility selections expected to be used (see paragraph
4.4.3).
4.4.4.4. Assessment of Expected Levels of ANP for RNP-qualified aircraft flying “RNP” Procedures. When RNP qualified aircraft (“AFM Qualified” or “Fleet Qualified”) fly “RNP” based RNAV procedures, suitable levels of positioning accuracy (e.g., anticipated, projected, or achievable) should be available appropriate to the level(s) of RNP intended and the procedures used.
Page 40 Par 4
a. If the procedure specifies ground-based facilities to be used for the procedure, this assessment may be considered to have already been done. Otherwise, an assessment must be accomplished (e.g., by that operator, by another operator, by a designee, by an authority, or by a service provider).
b. An accuracy assessment of navigation services may apply to an airspace, areas, routes, procedures or operations planned or otherwise intended (e.g., contingency alternates). The assessment may be accomplished by any one or more of a variety of technically qualified people or organizations, including the operator, a pilot, a fleet manager or other qualified representative of the operator (e.g., dispatcher), an authority, airspace planners, procedure developers, air traffic services, charting agencies, through ICAO global or regional agreement, by technically qualified supporting contractors to any of the above entities, or by a relevant aircraft or avionics manufacturer.
c. When determining the suitability of the airplane/system to achieve the expected level(s) of accuracy, the person or organization accomplishing the assessment should refer to appropriate airplane and system material. The expected levels of accuracy should be applicable to the system or systems to be used (e.g., airborne system as well as supporting NAVAIDs or space-based system elements external to the aircraft), should be suitable to support the level(s) of RNP to be used for the time period(s) to be used, and should be compatible with the airspace or procedures to be used (e.g., consider geographic or geometric effects such as “terrain masking,” if applicable).
d. Acceptable source material for determining anticipated, expected, projected, or achievable ANP may include any one or more of the following:
• Information from an applicable aircraft AFM
• Information from an applicable aircraft operating manual
• Applicable operational navigation documents (e.g., Systems Requirements and Objective (SR&O) documents) available from the aircraft or avionics manufacturer that apply to a navigation system
• Appropriate authority or air traffic service provider assessments or airspace studies
• Appropriate published instrument procedure provisions
• Authority, ATS provider, or ICAO-specified NAVAID locations, standard NAVAID characteristics, NAVAID performance and service volume charts or plans
• Published GNSS satellite constellation characteristics or GNSS augmentation method characteristics found acceptable to FAA and the State of the Aerodrome or ICAO
• NOTAM information
• AIP or AIM, or equivalent, information
• Appropriate studies or assessments conducted by an operator found acceptable to FAA, or
• Any other source material able to help assess projected accuracy that is found acceptable to FAA
e. The primary and secondary NAVAIDs identified during this process should be determined to be operating prior to use (e.g., the operator or pilot should ensure that the pertinent NAVAIDs are not “out of service”).
4.4.5. FMS VNAV. FMS procedures typically use vertical navigation capability (VNAV) based on a barometric pressure-based VNAV path (e.g., Barometric (Baro) VNAV). FMS systems may also use a VNAV path based on a geometrically defined VNAV path which is fixed in space by “earth centered earth fixed (ECEF) coordinates” (e.g., fixed relative to earth reference and does not vary with barometric pressure - analogous to an ILS Glide Slope, except does not compensate for earth curvature). In this AC these paths are referred to as “ECEF Geometric VNAV Paths.”
a. ECEF Geometric VNAV Paths (if and when used) typically are only used for final approach segment path definition. ECEF Geometric VNAV Paths, if used in either an FMS or instrument procedure, must be clearly distinguished from Baro VNAV paths, and must have clearly defined and compatible transitions from Baro VNAV
Par 4 Page 41 paths to the ECEF Geometric VNAV Path. Baro VNAV paths may be used for all applications including final approach paths.
b. Baro VNAV paths may be defined as follows:
(1) Baro VNAV paths with constraints for “at,” “at or above,” “at or below,” or the proceeding with corresponding speed constraints.
(2) Baro VNAV geometrically-based path defined as an approximate straight line segment from one defined WP pressure altitude to another WP pressure altitude (following earth curvature), or
(3) Baro VNAV geometrically-based path defined as two approximate straight line segments from one defined WP pressure altitude to another WP pressure altitude (following earth curvature), but using a reduced gradient for the final part of the path preceding the “to” WP to accommodate a speed constraint at the “to” WP, or
(4) Baro VNAV Performance-based climb or descent paths may be used.
(5) When used for a final approach segment, Baro VNAV paths may be based on a defined descent path angle rather than a segment between two sequential WP barometric altitudes, and
(6) For credit within this AC for use in a final approach segment (e.g., DA(H) credit) a Baro VNAV path should:
(a) Meet provisions of AC 20-129, as amended, for VNAV, or equivalent (e.g., equivalent means aircraft such as the B757 or A320 which meet AC 90-45A or other earlier international standard as a certification basis, but have systems which operationally have been determined to meet objectives of AC 20-129. Such aircraft system designs preceded issuance of AC 20-129, and were the basis for its subsequent development), and
(b) Be capable of providing vertical tracking performance within + 125 ft vertically (two sigma)
(e.g., meeting or meeting the equivalent of RNP 0.3/125 ft. for the vertical performance component), excluding temperature correction for deviation from ISA, (see 4.2.5-1), or,
(c) Alternately, FMS systems may provide for additionally more accurate vertical tracking performance within + 45 ft vertically (two sigma) or + 15 ft. vertically (e.g., meeting or meeting the equivalent of RNP x.xx/45 ft. or RNP x.xx/15 ft. for the vertical performance component), excluding temperature correction for deviation from ISA, (see 4.2.5-1), and
(d) Provide a VNAV path vertical displacement scale display showing a displacement range within at least + 550 ft. or less (with a scale of + 400 ft. recommended), unless meeting the more stringent requirement of paragraph 5.9.2 Figure 5.9.2-1 for final approach segment displays.
c. It is also recommended that the FMS systems have digital readout capability available to the pilot showing vertical displacement (e.g., FMS progress page or equivalent).
d. For “Go-Around,” when using a VNAV path for a final approach segment and a corresponding DA(H) is authorized for use, momentary descent below the DA(H) is considered acceptable while the aircraft transitions from the descent approach path to a missed approach.
4.4.6. FMS Use for International Procedures. For international operations (e.g., for instrument procedures outside the United States), equivalent criteria to the criteria specified above (e.g., ICAO PANS-OPS) may be used.
In addition, operators may use criteria of this AC, and related U.S. criteria referenced by this AC, internationally when approved by FAA, and when found acceptable by the country in which the Aerodrome is located for the procedure being used. For international operations it may be important to apply provisions of this AC regarding use
Page 42 Par 4 of an appropriate waypoint or NAVAID reference datum (e.g., WGS-84 see paragraph 6.2.17), or provisions for extreme cold temperature correction (see paragraph 8.13).
4.4.7. FMS RNAV Use for Substitution for VOR, DME, NDB, or Marker Beacon NAVAIDs or Fixes. Where suitable NAVAID updating of an FMS or GNSS navigation system is available, FMS or GNSS-based RNAV may be used to substitute for inoperative or unavailable VOR, DME, NDB, or Marker Beacon NAVAIDs or fixes for approach procedures, missed approach procedures, or departure procedures. For such substitution, except as provided in item 4 below where an authority has already specified an acceptable substitution, the operator should ensure that the navigation system used and updating method available, taken with the available remaining NAVAID(s) or sensors are suitable for the route or procedure segment to be flown.
a. FMS RNAV substitution for VOR, DME, NDB, or Marker Beacon NAVAIDs or fixes may be applied if:
(1) The operator can ensure the necessary accuracy of the aircraft's RNAV system to substitute for the desired fix, NAVAID, or waypoint, and
(2) If the aircraft’s navigation system is able to suitably depict the substitute WP, facility, or fix, and
(3) The aircraft can suitably fly any applicable leg, route, or procedure segment that otherwise would be based on the inoperative NAVAID or unavailable fix, or
(4) If the responsible authority (e.g., FAA or JAA) has otherwise established or provided for, and the operator uses, an acceptable RNAV substitution (e.g., IAW AIM GPS substitution provisions for NDB or DME, or FAA’s enroute NAVAID RNAV substitution policy, or IAW an acceptable RNAV substitution method promulgated via NOTAM).
b. Also see provisions for various specific NAVAID types within paragraph 4.3.10, such as 4.3.10.7 for inoperative DME substitution.
4.4.8. Inhibiting RNAV System Use of Inoperative or Unsuitable VOR, DME, VORTAC, TACAN, or NDB NAVAIDs. If VOR, DME, VORTAC, TACAN, or NDB updating is used in support of area navigation system (FMS) position determination, Operators and flightcrews should be aware of when and how to disable RNAV system use of an unsuitable NAVAID or NAVAID element within the navigation system. This is especially true when the unsuitable NAVAID is likely to cause a significant map shift (e.g., movement of a ground NAVAID to a new geographic location without making a corresponding update to that NAVAID’s recorded position in an aircraft’s navigation system database, thus leading to introduction of a sudden navigation system map display position error).
4.5. Required Navigation Performance (RNP). RNP is a navigation performance standard for a particular area, airspace, route, procedure, or operation. A definition of RNP is specified in Appendix 1.
a. The specification of RNP has two major aspects, the airspace (e.g., area, route, route segment, leg, procedure, or particular operation) and the airborne system. The airspace requirement is to specify airspace, routes, procedures, or operations within which the aircraft must be located with a high degree of assurance. The airborne systems requirement is to provide a level of performance that is reliable, repeatable, and predictable. The airborne system specification of navigation performance is as defined in RTCA DO-236, or equivalent (e.g., as agreed in an FAA-approved certification plan), except as otherwise found acceptable to FAA.
b. Application of an appropriate airborne specification for RNP serves as a basis to ensure that airborne system performance will match or exceed the level necessary for the area, route, route segment, leg, procedure, or operation.
RNP criteria have currently been developed and applied for area navigation standards for use with lateral types and levels of RNP (e.g., types such as addressing 95% lateral performance only, or addressing lateral performance using RNP x 2 containment areas, or various levels of RNP such as RNP .3, RNP .5, RNP 1). Extension of the RNP concept to other types or levels of RNP (e.g., levels such as RNP .15/45 ft.) represent more stringent lateral and vertical performance standards that may in the future be applied to approaches or 3D terminal arrival and departure VNAV paths. Other future applications of RNP may provide for along track performance (e.g., “Required Time of Arrival (RTA)”) and are anticipated to evolve as general navigation requirements and operational concepts evolve.
Par 4 Page 43 Hence this AC currently addresses only initial RNP applications, and recognizes that RNP criteria will continuously evolve to address other future operational requirements as necessary to define and manage evolutionary changes in the International Airspace System (INAS). Accordingly, different aircraft may meet RNP requirements in different ways regarding sensors used or criteria met (e.g., FANS 1, FANS A, RTCA DO-236, Fleet qualification).
Regardless of RNP application, however, it must be possible to determine that each specific aircraft meets the level of RNP required for the airspace application, and that a suitable identifiable standard has been applied.
c. RNP addresses the aircraft and navigation service (non-aircraft) accuracy, integrity, continuity, and availability requirements for normal and rare fault-free performance and for performance with failures. RNP specifies the nominal and limit lateral, and if applicable, vertical flight path displacements permissible for a particular procedure. RNP can be related to obstacle clearance or aircraft separation requirements to ensure a consistent set of operational procedures and design requirements.
d. The following elements of RNP, and error components, are thus considered applicable to systems and operations, as defined and described below in Figure 4.5-1.
e. A desired flight path is the path that the pilot, or pilot and air traffic service, expect the aircraft to fly. A desired flight path may be identified by the pilot, by ATS, by an airspace planner or by a procedure developer. It is typically specified in the form of a route or procedure, or is as otherwise identified by ATS in a pre-specified flight plan or clearance, or is as defined by an ATS clearance issued in “real time” (e.g., an assigned track, radial, bearing, course, arc, or heading). The desired flight path may be a simple straight segment, may be a path defined by multiple waypoints connected by straight segments, or may be a complex path defined by continuous straight and curved segments. The path may be defined in two dimensions (2D) consisting of lateral and longitudinal elements, three dimensions (3D) including vertical path elements, or may be defined in four dimensions (4D) including a longitudinal position as a function of time elements, or “time of arrival” constraints at waypoints.
f. In order for an aircraft to follow the desired flight path it is necessary that the navigation system (airborne or on the ground) generate a defined flight path. The defined flight path is the path as determined by the path definition function of an aircraft’s navigation system (Note: It may also be defined by a system external to the aircraft, and intrinsically provided, or otherwise communicated to the aircraft). While the defined flight path is typically intended to be the same as the desired flight path, the defined flight path is often only a close approximation to the desired flight path due to unavoidable path definition error factors. Factors such as non-spherical earth shape or curvature, determination of geometric altitude versus true altitude or pressure altitude, changing magnetic variation or outdated NAVAID declination, differences in “great circle” route calculations, survey errors, database resolution limitations, or other such factors can result in the defined path being slightly different than the desired path. This difference between the desired path and the defined path is called the path definition error.
g. The aircraft elements of the navigation system estimate the aircraft’s position and compare that position with the defined flight path. A deviation indication is produced which represents the calculated displacement of the airplane from the defined flight path. This deviation is typically displayed on a primary flight display, or navigation displays, for flightcrew awareness, and is provided as an input to an autopilot and/or flight director system for command guidance or automatic control. The resulting difference (i.e., non-zero deviation) between the estimated aircraft position from the desired flight path is called the path steering error. This error includes display errors and flight technical error.
h. The error in the estimation of the aircraft’s position is referred to as position estimation error, or navigation system error. The navigation system error may result in a displacement from the desired flight path.
i. The accuracy with which the aircraft is controlled as measured by the indicated aircraft position with respect to the indicated command or defined flight path position is called flight technical error (FTE). FTE does not include human performance conceptual errors (e.g., entry of an incorrect waypoint or waypoint position, selection of an incorrect procedure, selection of an incorrect NAVAID frequency, or failure to select a proper flight guidance mode). FTE can be influenced by factors such as flightcrew response to guidance (e.g., response to Flight Director information), or external environment conditions such as a wind gradient or turbulence.
Page 44 Par 4
j. The sum of the path definition error, navigation system error, and the path steering error (i.e., flight technical error plus any display error) is the total system error (TSE), which is the difference between the desired flight path and the actual flight path. Figure 4.5-1 below shows the error terms considered in the cross-track dimension of the total system error.
Desired Path Actual Position Estimated Position Flight TechnicalError Defined Path Path Definition ErrorNavigation System Error Total System Error Navigation Lateral Error Components Related to RNP Figure 4.5-1.
k. Particular levels of RNP can be satisfied using various NAVAIDs such as ILS and MLS, or by the use of a combination of navigation sensors (DME/DME, VOR/DME, IRU/IRS, GNSS, etc.) using a navigation computer
(e.g., FMS). When a computed path (e.g., series of waypoints) is used as the basis for an approach operation, the desired flight path must typically be defined by a series of three dimensional earth-based coordinates for the applicable waypoints or path definition points.
l. Approach or missed approach operations can be approved by demonstration of the capability to meet the required navigation performance (e.g., accuracy, integrity, availability) for a specific approach procedure, for a set of particular procedure types, or for a set of RNP levels.
m. The transition from typical en route or terminal RNP levels to an approach RNP level is accomplished by transitioning to the required RNP level for the approach IAW the approved instrument procedure or by a point no later than the final approach fix, if an aircraft is radar-vectored to final.
n. Associated with the RNP level is a containment limit that is specified as “two times the level of RNP (2xRNP).” The system performance integrity provided by this RNP containment limit is intended to support its application as a basic element for either aircraft separation or obstacle or terrain clearance assessment. However, other considerations such as an obstacle rich environment, potential weather factors, high traffic density, limited communication or surveillance environment, or other such factors may also be appropriate to consider in determining if any additional airspace buffers may be appropriate beyond the RNP containment limit. Similarly, operations at less than 2xRNP, may be found to be appropriate, such as if an ATS communication and surveillance environment otherwise safely permits ATS management of the airspace by other means than RNP containment (e.g., where ATS radar monitoring and radar vector separation on adjacent Standard Terminal Arrival Route (STAR) transitions may be used to ensure safe separation, in lieu of use of RNP containment).
4.5.1. RNP Levels or Types. The expression “RNP Level” is used to describe a specific value or level of required navigation performance. The term “RNP Level” may be interchangeably described as “RNP Type” in some industry and FAA references. However in this AC, the term “RNP Level” is meant to apply only to a lateral RNP element
(e.g., RNP .5) or to specific paired lateral and vertical elements (e.g., RNP .3/125 ft.). The term “RNP Type” is generally reserved for future uses, in which future vertical and longitudinal elements or other conditions of RNP may additionally apply.
a. Table 4.5.1-1 provides RNP Levels that could support initial, intermediate, final and missed approach segments. These RNP levels have not yet been established as international standards.
Par 4 Page 45 Table 4.5.1-1.
RNP LEVELS FOR APPROACH RNP Level Applicability/Operation (Approach segment) Normal Performance (95%) Containment Limit (*) RNP 1 Initial/Intermediate approach +/-1 nm +/-2 nm RNP 0.5 Initial/Intermediate/Final approach [Supports limited Category I minima] +/-0.5 nm +/-1 nm RNP 0.3 Initial/Intermediate/Final approach [Supports limited Category I minima] +/-0.3 nm +/-0.6 nm RNP 0.3/125 ft. Initial/Intermediate/Final approach with specified baro vertical guidance [Supports limited Category I minima] +/-0.3 nm +/-125 ft +/-0.6 nm +/-250 ft RNP 0.03/45 ft. Final approach with specified vertical guidance[Supports Category I minima] +/-0.03 nm (**) +/-45 ft +/-0.06 nm +/-90 ft RNP 0.01/15 ft. Final approach with specified vertical guidance [Supports Category I/II minima] +/-0.01 nm (***) +/-15 ft +/-0.02nm +/-30 ft RNP 0.003/15 ft.
Final approach with specified vertical guidance [Supports Category I/II/III minima] +/-0.003 nm +/-15 ft (****) +/-0.006 nm +/-30 ft (*) (*) NOTE: For barometric VNAV, the obstacle assessment methodology described in Appendix 5 may be used to addresses vertical containment limits which consider multiple factors such as altimeter error, temperature, and “along track” fix error. Each of these factors should be considered, as necessary, in determining Required Obstacle Clearance (ROC). Nominal vertical values shown in this Table associated with various levels of RNP are intended to be used in conjunction with and considering factors described in Appendix 5, as applicable to the vertical path specified and the type or types of sensor systems used. For other forms of VNAV (e.g., when using an ECEF coordinate specified geometric path), assurance of vertical containment may be met by any FAA approved method, including the method specified by Appendix 5.
Examples of acceptable methods other than that based on Appendix 5 would be methods where containment is considered as a “designed-in capability” of a system or aircraft (e.g., as for GBAS or SBAS), or a specific system/infrastructure/operational assessment method, acceptable to FAA, with potential corresponding operational or procedural requirements.
(**) NOTE: Performance consistent with Category I operation based on ILS performance requirements at 200 feet (***) NOTE: Performance consistent with Category II operation based on ILS performance requirements at 100 feet (****) NOTE: Consistent with landing and rollout performance (refer to AC 120-28D).
Vertical accuracy does not apply below 100 feet HAT due to the transition to a flare maneuver consistent with reduction in sink rate and landing dispersion requirements.
b. RNP is a required navigation performance level described by the specification of a numeric value indicating the required navigation accuracy for a specific operation, typically specified laterally in nautical miles (e.g., RNP 1 is a Required Navigation Performance of +/-1 nautical mile (95% Probability)).
c. RNP containment is specified as RNP (X) x 2.
Page 46 Par 4
d. RNP Levels are defined for lateral performance, or lateral and vertical performance, if applicable. Standard values for RNP for general use are as specified in RTCA’s Minimum Airspace Performance Standards (MASPS) for RNP (RTCA DO-236) as amended, this AC, related ACs, or as otherwise specified by FAA through published instrument procedures, the Aeronautical Information Manual (AIM), or by NOTAM. ICAO specified types or levels of RNP as promulgated in ICAO Manuals or ICAO Regional Supplements for International Airspace may also be considered as acceptable RNP levels for Approach operations.
e. RNP Levels typically used for various approach and missed approach segments supporting Category I procedures may be based on use of multi-sensor RNAV (e.g., FMS with IRS, VOR, DME, or GNSS inputs), or on other aircraft navigation systems having FMS-like capabilities (e.g., GPS based navigation systems). RNP Levels applicable to Category I may also take advantage of, or also be based on, sensor inputs received from specific landing systems (e.g., ILS, MLS, or GLS).
f. RNP Levels typically used for various approach and missed approach segments supporting Category II procedures may be based on the same capability specified above for Category I, except that for any portions of a final approach segment below 200 ft. HAT for Category II, use of specific landing system sensors (e.g., ILS, MLS, or GLS) may be determined to be necessary to achieve the desired level of RNP. Similarly, for portions of any FAS below 200 ft. HAT, use of a multi-sensor RNAV system should have suitable integrity and availability capability
(e.g., may require use of multiple FMS with IRS, and suitable ILS, GNSS, or GBAS inputs to achieve the necessary RNP capability).
4.5.2. Other RNP Levels or Types. Other RNP Levels or Types may include types specified by a particular Authority for specific applications (e.g., RNP 5 within certain geographic areas; RNP .15 for a particular air carrier “Special approach procedure”)
4.6. Flight Path Definition. Certain flight segments and waypoints are necessary to effectively implement approach and missed approach operations using landing systems where the required flight path is not inherent in the signal structure of the navigation aid (e.g., integrated multi-sensor area navigation systems and other RNAV systems such as satellite systems). The concepts and criteria described below may be applied to other types of navigation systems when using area navigation and RNP concepts.
a. In general, an operator must have an acceptable method to ensure that any waypoints or path points which are considered critical to an instrument procedure (if any) are correctly defined, and are loaded into each applicable aircraft’s database, initially, and at each change cycle.
b. RNP-based area navigation systems may use any leg types available and suitable for RNP path definition as specified by acceptable FAA or industry criteria (e.g., RTCA DO-236; ARINC 424) for a particular type of navigation system), or leg types as otherwise approved by FAA for use with RNP. Leg types may be specified to define a suitable path in space in conjunction with established waypoints, new waypoints, or path definition points.
c. Levels of RNP may be procedurally specified, may be specified in a data base for automatic call up for an entire procedure when a procedure is loaded, may be specified in a data base for automatic call up for each leg or segment of a procedure, may be entered by the flightcrew into the navigation system for a procedure or leg, or may be based on navigation system default settings if those default RNP settings are found to be acceptable to FAA (e.g., when using standard FMS RNP default values and standard instrument procedures with a compatible RNP level specified). When possible, it is recommended that RNP levels be specified by the instrument procedure, and automatically set for each applicable leg, to minimize flightcrew input workload and potential for FMS or navigation system input error.
d. Levels of RNP may be specified for individual path segments, for an entire procedure, or for portions of a procedure (e.g., Intermediate segment, FAS, IMAS, or an entire missed approach path).
e. The following criteria and considerations are appropriate to specify the landing and rollout flight path. A graphic depiction of the points, heights, angles or other considerations described below is shown in Figure 4.6-1.