Appendix B B-3 this baro-VNAV equipment to fly to the LNAV/VNAV DA as published on the procedure.
B.4 Operating Procedures (General). For baro-VNAV approach operations, the flightcrew should be familiar with the following operating procedures.
B.4.1 Actions at DA. The pilot/crew is expected to fly the aircraft along the published vertical path and execute a Missed Approach Procedure (MAP) upon reaching DA, unless the visual references specified in part 91, § 91.175 for continuing the approach are present.
B.4.2 Temperature Limitation. Because of the pronounced effect of nonstandard temperature on baro-VNAV systems, RNAV (GPS) approaches with an LNAV/VNAV line of minima should contain a low and high temperature limitation. Uncompensated baro-VNAV systems may not operate to the LNAV/VNAV DA when the actual temperature is below or above the temperature limitations. The temperature limitation will be shown as a note on the procedure. If the aircraft contains a temperature compensation capability, manufacturer instructions should be followed for use of the baro-VNAV function.
B.4.3 VNAV Path Mode Selection. Crews should be knowledgeable on selection of the appropriate vertical mode(s) that command VNAV via the published vertical path. Other vertical modes such as Vertical Speed (VS) are not applicable to baro-VNAV approach operations.
B.4.4 Remote Altimeter Setting Restriction. Use of baro-VNAV to a DA is not authorized with a remote altimeter setting. A current altimeter setting for the landing airport is required.
Where remote altimeter minima are shown, the VNAV function may be used but only to the published LNAV MDA.
B.4.5 Altimetry Cross-Check.
B.4.5.1 Pilots must verify the current local altimeter at the airport of intended landing is set not later than the final approach fix (FAF). Remote altimeter settings are not allowed.
B.4.5.2 Where two pilots are required, the flightcrew must complete an altimetry cross-check ensuring both pilots’ altimeters agree within ±100 feet prior to the FAF after receiving the current local altimeter setting at the airport of intended landing. If the altimetry cross-check fails then the procedure should not be conducted or, if in progress, it must not be continued. The flightcrew procedures should also address actions to take if a comparator warning for the pilots’ altimeters occurs while conducting a RNP APCH procedure.
Note: This operational cross-check is not necessary if the aircraft automatically compares the altitudes to within 100 feet.
Appendix B B-4 B.4.6 VNAV System Cross-Check. For baro-VNAV operations to the LNAV/VNAV line of minima, pilots must cross-check the vertical guidance displayed by the navigation system against other available data at the FAF. If the difference between the displayed glidepath and the published altitude at FAF exceeds 75 feet, the LNAV/VNAV line of minima may not be used. Pilots should revert to the LNAV line of minima and comply with any published stepdown fix altitude.
Note 1: This cross-check is not required when GPS/Satellite-based Augmentation System (SBAS) vertical guidance is used with the LNAV/VNAV line of minima.
Note 2: For systems that provide temperature compensation capability, the displayed temperature compensated glidepath should be compared to the temperature compensated altitude computed by system when crossing the FAF.
Note 3: This cross-check may be accomplished by ensuring that the glidepath displacement from center does not exceed 75 feet when crossing the FAF at the published crossing altitude, or if descending on a centered glidepath, ensuring that the aircraft’s altitude crossing the FAF is within 75 feet of the altitude published at the FAF.
Note 4: Aircraft approved under previous AC guidance using baro-VNAV with LNAV/VNAV minimums, the legacy vertical deviation limits of +100/-50 feet remains valid.
B.5 Pilot Knowledge. Pilot knowledge should include the following:
1. The aeronautical instrument approach chart that will promulgate LNAV/VNAV procedures including, but not limited to, temperature and altimeter source limitations for baro-VNAV operations.
2. Barometric altimeters are calibrated to indicate true altitude under International Standard Atmosphere (ISA) conditions.
3. The effect of temperature on true altitude vs. indicated altitude (i.e., warmer than ISA, the true altitude will be higher than indicated altitude and colder than ISA, the true altitude will be lower than indicated altitude).
4. Understanding the VNAV system capabilities relating to geometric or performance-based VNAV paths.
5. Understanding the use of VNAV angles and altitude restrictions during arrival.
6. Interpretation of electronic displays and symbols.
7. Levels of automation, mode annunciations, changes, alerts, interactions, reversions, degradation.
8. Turn anticipation with consideration to speed and altitude effects.
9. Functional integration with other aircraft systems.
Appendix B B-5
10. The meaning of vertical path discontinuities as well as related flightcrew procedures.
11. VNAV equipment operating procedures, including how to determine vertical-track error/deviation.
12. Procedures for change in arrival/runway assignment.
13. Understanding of possible conflicts between aircraft performance and flight management system (FMS) functions (e.g., airspeed and descent angles).
14. Failures and mode reversions, which adversely impact the aircraft’s ability to conduct baro-VNAV approach operations.
15. Contingency downgrade actions (i.e., reverting to LNAV MDA following VNAV failures).
16. Contingency abort actions (i.e., understand which failures and performance parameters require discontinuing the approach and executing an early missed approach).
17. Altimeter cross-check procedures.
18. VNAV system-specific information, including VNAV vertical deviation scaling provided in the Final Approach Segment (FAS). Pilot and flightcrews should be knowledgeable in applying this scaling to the baro-VNAV display cross-check required at the FAF when conducting an approach to the LNAV/VNAV line of minima.
Appendix C C-1 APPENDIX C. QUALIFICATION CRITERIA FOR REQUIRED NAVIGATION PERFORMANCE (RNP) 1 (TERMINAL) OPERATIONS C.1 Introduction. This appendix provides guidance on the performance and functional requirements for systems used to conduct RNP in terminal operations. This appendix includes:
1. Instrument departure procedures (DP) RNP Obstacle Departure Procedures (ODP),
2. RNP Standard Instrument Departures (SID), and
3. RNP Standard Terminal Arrival Routes (STAR).
C.2 Aircraft and System Requirements for RNP Terminal Operations.
C.2.1 Performance and Airworthiness Guidance. Refer to Advisory Circular (AC) 20-138( ), Airworthiness Approval of Positioning and Navigation Systems, for performance and airworthiness guidance on aircraft and system criteria.
C.2.2 Statement of Compliance (SOC). Aircraft with an SOC with the criteria in this AC or AC 20-138( ) in their Airplane Flight Manual (AFM), Airplane Flight Manual Supplement (AFMS), pilot’s operating handbook (POH), or the operating manual for their avionics meet the performance and functional requirements of this AC.
C.2.3 Qualified Avionics Equipment and Airworthiness Approval. Aircraft with the following avionics equipment and an appropriate airworthiness approval automatically qualify for RNP 1 capability without further documentation by virtue of the avionics Technical Standard Order (TSO) and airworthiness approval:
1. Global Positioning System (GPS) stand-alone systems approved in accordance with TSO-C146( ) operational Class 1, 2, or 3.
2. A TSO-C115c light management system (FMS) (or later revision) with a TSO-C129( ) Class B1 or C1, TSO-C145( ), or TSO-C196( ) sensor.
Note: TSO-C129a has been cancelled, but equipment with an existing TSO-C129a Technical Standard Order Authorization (TSOA) may still be installed.
3. A TSO-C115b FMS using a TSO-C129( ) Class B1 or C1, TSO-C145( ), or TSO-C196( ) sensor with documented compliance to the RNP requirements in RTCA, Inc.’s document, RTCA/DO-236 (revision ‘B’ or later) or RTCA/DO-283A.
4. Aircraft with a Required Navigation Performance Authorization Required (RNP AR) approval per AC 90-101( ), Approval Guidance for RNP Procedures with AR, or AC 20-138( ).
Appendix C C-2 C.2.3.1 Aircraft/equipment with approval under AC 90-100( ), U.S. Terminal and En Route Area Navigation (RNAV) Operations, for use of GPS are approved under this AC for RNP 1 operations.
C.2.3.2 GPS stand-alone systems approved in accordance with TSO-C129( ) Class A1 typically supports RNP 1 operations associated with instrument approach and DPs, but might not support other operations requiring RNP 1. TSO-C129( ) avionics manufacturers should provide an RNP capabilities document stating compliance with the criteria in AC 20-138( ) for RNP 1 capability. Avionics or aircraft manufacturers may provide a Service Bulletin (SB) for a pilot or equipment operating handbook entry or other notification stating aircraft with this equipment has RNP 1 capability based on the avionics manufacturer’s SOC.
C.2.3.3 RNP 1 aircraft with European Precision Area Navigation (P-RNAV) approval based on GPS capability meet the functional requirements of this AC for RNP 1 operations, such as RNP DPs, RNP SIDs, and RNP STARs. GPS equipment approved in accordance with TSO-C129a and satisfying the step-detection and health word checking contained in TSO-C129a meet P-RNAV performance requirements.
Note 1. RNP 1 operations are based on GPS positioning and, if adequate coverage is available, distance measuring equipment (DME)/DME/Inertial Reference Unit (IRU). Positioning data from other types of navigation sensors may be integrated with the GPS data provided it does not cause position errors exceeding the Total System Error (TSE) budget. Otherwise, means should be provided to deselect the other navigation sensor types.
Note 2. TSO-C145 or TSO-C146 equipment may also be authorized, in accordance with this AC, for use as the only means of navigation for conducting RNP 1 operations in the U.S. airspace where domestic air traffic control (ATC) procedures are applied. This includes RNP operations outside the navigational service volume of the Federal Aviation Administration (FAA) or the International Civil Aviation Organization (ICAO) standard ground-based Navigational Aids (NAVAID).
Note 3: Operators are encouraged to retain back-up navigation systems to guard against outages and interference events.
C.3 System Performance, Monitoring, and Alerting.
C.3.1 Accuracy. The aircraft must comply with Section 2.1.1 of RTCA/DO-236( ). During operations in airspace or on routes designated as RNP 1, the lateral TSE must be within ±1 nautical mile (NM) for at least 95 percent of the total flight time. The along-track (ATRK) error must also be within ±1 NM for at least 95 percent of the total flight time.
Appendix C C-3 To satisfy the 95 percent accuracy requirement, Flight Technical Error (FTE) should not exceed 0.5 NM for autopilot (AP) or flight director (FD) or 0.8 NM for manual control.
Note: The use of a lateral deviation indicator with 1 NM full-scale deflection has been found to be an acceptable means of compliance (AMC). An AP or FD may also be used for maintaining TSE, but roll stabilization systems do not qualify.
C.3.2 Integrity. Malfunction of the aircraft navigation equipment that causes the TSE to exceed two times the RNP value is classified as a major failure condition under airworthiness guidance (i.e., 10-5 per hour).
C.3.3 Continuity. Loss of function is classified as a minor failure condition if the operator can revert to a different navigation system and proceed to a suitable airport.
C.3.4 Performance Monitoring and Alerting. The RNP system, or the RNP system and pilot in combination, must provide an alert if the accuracy requirement is not met, or if the probability that the lateral TSE exceeds 2 NM is greater than 10-5 for RNP 1 operations.
C.3.5 Path Definition. Aircraft performance is evaluated around the path defined by the published procedure and RTCA/DO-236C.
C.3.6 Signal in Space (SIS). If using GPS, the aircraft navigation equipment must provide an alert if the probability of SIS errors causing a lateral position error greater than 2 NM exceeds 10-7 per hour for RNP 1 operations.
Note: Aircraft with an approved multisensor RNP system where Global Navigation Satellite System (GNSS) integrity is incorporated into a 2 x RNP integrity alert consistent with DO-236( ) do not require a separate GNSS integrity alert when performance cannot be met.
C.3.7 Functional Requirements of Navigation Data Displays. The following navigation displays and functions are required and must be installed in accordance with AC 20-138( ) or equivalent airworthiness installation advisory material.
C.3.7.1 Navigation data, including a TO/FROM indication and a failure indicator, must be displayed on a lateral deviation display (course deviation indicator (CDI), electronic horizontal-situation indicator (EHSI)) and/or a navigation map display. These must be used as primary flight instruments for the navigation of the aircraft, for maneuver anticipation, and for failure/status/integrity indication. A non-numeric lateral deviation display
(e.g., CDI, EHSI) with a TO/FROM indication and a failure annunciation, for use as primary flight instruments for navigation of the aircraft, for maneuver anticipation, and for failure/status/integrity indication, should have the following attributes:
Appendix C C-4
1. The displays must be visible to the pilot and located in the primary field of view (FOV) when looking forward along the flightpath.
2. The lateral deviation display scaling must agree with any alerting and annunciation limits.
3. The lateral deviation display must have a full-scale deflection suitable for the current phase of flight and must be based on the required total system accuracy; ±1 NM for RNP 1. It is also acceptable for the scaling to be more conservative (e.g., ±0.5 NM for an RNP 1 route).
4. The display scaling may be set automatically by default logic or set to a value obtained from a navigation database. The full-scale deflection value must be known or must be available for display to the pilot commensurate with terminal area values.
5. The lateral deviation display must be automatically slaved to the RNP 1 computed path. It is recommended that the course selector of the deviation display be automatically slewed to the RNP-computed path.
C.3.7.2 As an alternative means, a navigation map display must give equivalent functionality to a lateral deviation display with appropriate map scales (scaling may be set manually by the pilot). To be approved as an alternative means, the navigation map display must be shown to meet the TSE requirements and be located in the primary FOV.
C.3.7.3 It is not necessary for navigation displays, particularly primary flight displays (PFD), to include an Actual Navigation Performance (ANP) or RNP value.
The displays only need to provide an alert if the RNP for the operation cannot be met.
C.3.8 System Capabilities. The following system capabilities are required as a minimum within any RNP 1 equipment:
1. The capability to continuously display to the pilot flying (PF), on the primary flight instruments for navigation of the aircraft (primary navigation display), the RNP-computed desired path and aircraft position relative to the path. For operations where the required minimum flightcrew is two pilots, a means for the pilot monitoring (PM) to verify the desired path and the aircraft position relative to the path must also be provided.
2. A navigation database, containing current navigation data officially promulgated for civil aviation, which can be updated in accordance with the Aeronautical Information Regulation and Control (AIRAC) cycle. The stored resolution of the data must be sufficient to achieve the required track-keeping accuracy. The database must be protected against pilot modification of the stored data.
Appendix C C-5
3. The means to display the validity period of the navigation data to the pilot.
4. The means to retrieve and display data stored in the navigation database relating to individual waypoints and navigation aids, to enable the pilot to verify the route to be flown.
5. Capability to load from the database into the RNP system the RNP 1 procedure and/or route to be flown.
Note 1: It is acceptable to manually load en route and terminal waypoints from the database into a flight plan page and manually set the scaling and alerting (Refer to AC 90-100( )).
Note 2: Due to variability in systems, this document defines the segment from the first occurrence of a named waypoint, track, or course to the last occurrence of a named waypoint, track, or course. Heading legs prior to the first named waypoint or after the last named waypoint do not have to be loaded from the database.
6. The capability to automatically set RNP 1 from the onboard navigation database for each leg segment of a RNP 1 route or procedure.
Note: If the navigation system does not automatically retrieve and set RNP 1 from the onboard navigation database for the entirety of the RNP 1 operation, the flightcrew’s operating procedures must manually set RNP 1.
7. The means to display the following items, either in the pilot’s primary FOV, or on a readily accessible display page:
• The active navigation sensor type,
• The identification of the active (To) waypoint,
• The ground speed or time to the active (To) waypoint, and
• The distance and bearing to the active (To) waypoint.
8. The capability to execute a “Direct to” function.
9. The capability for automatic leg sequencing with display to the pilots.
10. The capability to execute RNP 1 terminal procedures extracted from the onboard database including the capability to execute flyover and flyby turns.
11. The system must have the capability to automatically execute leg transitions and maintain tracks consistent with the following Aeronautical Radio, Inc.
(ARINC) Specification 424 path terminators:
• Initial Fix (IF),
• Course to Fix (CF),
• Direct to Fix (DF), and
• Track to Fix (TF).
Appendix C C-6 Note: Path terminators are defined in ARINC Specification 424, and their application is described in more detail in RTCA documents DO-236( ) and DO-201( ).
12. The system must have either automatic or manual capability to fly on a heading to intercept a course or to go direct to another fix after reaching a procedure-specified altitude. The system may use heading to altitude (VA), heading to manual (VM) and heading to intercept (VI) path terminators for automatic capability.
13. The aircraft must have the capability to automatically execute leg transitions consistent with Course to Altitude (CA) and Fix to Manual Termination (FM) ARINC Specification 424 path terminators, or the RNP system must permit the pilot to readily designate a waypoint and select a desired course to or from a designated waypoint.
14. For paths defined by a course (e.g., CF and Fix to an Altitude (FA) path terminators), the navigation system should use the appropriate magnetic variation value of the recommended NAVAID in the navigation database.
15. The capability to load a RNP 1 procedure from the database, by procedure name, into the RNAV system.
16. The capability to display an indication of the RNP 1 system failure, in the pilot’s primary FOV.
17. The system must allow for a means to become established prior to the final approach fix (FAF) on an instrument landing system (ILS) or Ground Based Augmentation System (GBAS) Landing System (GLS) with minimal overshoot or undershoot.
C.3.9 Database Integrity. The operator must ensure their navigation database supplier possesses a Type 2 Letter of Acceptance (LOA) in accordance with AC 20-153( ), Acceptance of Aeronautical Data Processes and Associated Databases.
C.3.10 Path Terminators. For RNP procedures, the database supplier should not substitute path terminators in lieu of those specified in the original Aeronautical Information Publication (AIP) data.
C.4 Special Characteristics of RNP 1 Operations.
C.4.1 Operation on RNP ODPs, SIDs, STARs. DME/DME/IRU and/or GNSS are the primary navigation systems supporting RNP 1 operations. Refer to AC 20-138( ), Chapter 6, Sections 6-4, 6-6, and 6-8 for additional aircraft and system qualification criteria when using DME/DME/IRU. Pilots are not required to monitor ground-based navigation facilities used in position updating unless required by the AFM.
C.4.2 Navigation Infrastructure. The FAA will monitor the navigation infrastructure and issue timely warnings of outages and facilities designated out of service (i.e., issue Notices to Airmen (NOTAM)).
Appendix C C-7 C.4.3 Source Data. FAA source data contains the navigation performance needed to fly each RNP procedure and must be clearly designated on all appropriate charts.
C.4.4 DME/DME/IRU. If DME/DME/IRU is used, the available DME navigation infrastructure supporting the design of a procedure will be assessed and validated by the FAA or an FAA-authorized third party service provider.
C.4.4.1 The FAA will ensure all DME signals within reception distance of U.S. airspace meet ICAO standards. These could include non-U.S. DME facilities, or Department of Defense (DOD)-maintained DME facilities excluded from the U.S. National Airspace System (NAS) database. Procedure designs will only use DME facilities listed in the Chart Supplements.
C.4.4.2 DME signals will meet SIS accuracy tolerances everywhere the signals are received in that procedure.
C.4.4.3 For DME-based RNP operations where reliance is placed upon the IRU/inertial reference system (IRS), some aircraft systems revert to very high frequency omni-directional range (VOR)/DME-based navigation before reverting to inertial coasting. When the VOR is within 40 NM from the procedure, and there is insufficient DME/DME navigation infrastructure, the impact of VOR radial accuracy will be evaluated by the FAA to determine the effect on aircraft position accuracy.
C.4.5 DME Facilities. If any critical DME facilities exist for ODPs, SIDs, and STARs, they will be identified within the relevant U.S. Flight Information Publications (FLIP) and comply with applicable ICAO standards.
Note: Procedures with (GPS) in the title or with a GPS required are not evaluated for critical DMEs.
C.5 System Eligibility and Approval for RNP 1 Operations.
C.5.1 Requirements. Systems meeting the requirements in AC 20-138( ) are eligible for RNP 1 operations. Aircraft qualified by AC 90-101( ) are considered qualified for RNP 1 operations without further examination. Additionally, aircraft approved to conduct RNAV 1 procedures using GPS, according to AC 90-100( ), do not require further evaluation (see paragraph C.6.3.2).
1. Stand-Alone Systems. GPS stand-alone systems should be approved in accordance with TSO-C129a Class A1 or TSO-C146( ) Class 1, 2, or 3 and meet the functionality requirements of this appendix. GPS systems must be installed in accordance with AC 20-138( ).
2. Multisensor Systems. Aircraft with a TSO-C129 Class B1, C1, B3, or C3, TSO-C145( ), Class 1, 2, or 3, or TSO-C196 sensors installed in an FMS that meets the requirements of TSO-C115b or later and is installed for instrument flight rules (IFR) use in accordance with AC 20-138( ).
Appendix C C-8
3. Multisensor Systems That Rely on Ground-Based NAVAIDs. Multisensor systems that require the use of ground-based NAVAIDs, such as DME/DME, can be used to conduct RNP 1 operations provided that the documentation described in this appendix shows the aircraft is approved for RNP instrument flight operations, the aircraft meets the functional and performance requirements for RNP 1 (e.g., RNP performance monitoring and alerting) and the requirements of AC 90-100( ), Appendix 1, or Appendix 2, as applicable.
Note: Manufacturers should also ensure the equipment in paragraph C.5.1, Items 1, 2, and 3 meet all functional requirements in this AC.
C.5.2 Demonstrations. Demonstration of aircraft performance in accordance with AC 20-138( ) does not constitute operations approval to conduct RNP operations.
C.6 Operational Approval for RNP 1 Operations.
C.6.1 Aircraft Qualification Documentation. The aircraft or avionics manufacturers should develop aircraft qualification documentation that shows compliance with the applicable criteria, as appropriate. This is not required for aircraft with an AFM or AFMS which explicitly states that the RNP system is approved for IFR operations with RNP values at least as low as RNP 1 and the equipment meets the performance and reliability requirements of AC 20-138( ).
C.6.2 Performance Requirements. Prior to application, operators and manufacturers of their equipment should review all performance requirements. Installation of equipment by itself does not guarantee operational approval or permit operational use.
C.6.3 FAA Acceptance of Documentation.
C.6.3.1 New Aircraft/Equipment. The aircraft/equipment qualification documentation can be approved as part of an aircraft certification project and reflected in the AFM and related documents.
C.6.3.2 Existing Aircraft/Equipment. Previous approvals to conduct RNAV 1 procedures using GPS, according to AC 90-100( ), do not require further evaluation.
C.6.3.3 Previously Unapproved Installations/Equipment. For installations/equipment not previously approved to conduct RNAV 1 procedures, the aircraft or avionics manufacturer should submit the RNP 1 documentation in accordance with AC 20-138( ). The operator will submit the RNP operational application package (if required) to the FAA.
C.7 Operational Considerations.
C.7.1 Preflight Planning. For systems with receiver autonomous integrity monitoring (RAIM)-based integrity, RAIM prediction must be performed prior to departure. This capability can be a ground service and need not be resident in the aircraft’s avionics
Appendix C C-9 equipment. Preflight integrity checks and the various capabilities that can be utilized to perform them can be found in the Chapter 9, Flight Planning, of this AC.
C.7.2 Multisensor Systems with RNP 1 Approval. For multisensor systems with RNP 1 approval based on DME/DME, pilots must confirm the availability of critical DME for ODPs, SIDs, and STARs.
Note: Procedures with (GPS) in the title and with a GPS required are not evaluated for critical DMEs.
C.7.3 Radius to Fix (RF) Legs. For RNP 1 operations employing RF legs, confirmation of RF leg aircraft eligibility must be confirmed during preflight planning. Refer to Appendix I, Additional Capabilities, for aircraft RF leg eligibility requirements.
C.7.4 General In-Flight Considerations. Pilots must not fly an RNP 1 procedure unless it is retrievable by the procedure name from the onboard navigation database and conforms to the charted procedure. Numeric values for courses and tracks should be automatically loaded from the RNP navigation database for required leg types. However, the procedure may subsequently be modified through the insertion or deletion of specific waypoints in response to ATC clearances. The manual entry or creation of new waypoints, by manual entry of latitude and longitude or rho/theta values for fixed, published routes is not permitted. Additionally, pilots must not change any database waypoint type from a flyby to a flyover or vice versa.
C.7.4.1 During RNP 1 operations, pilots must use a navigation system textual display or navigation map display.
C.7.4.2 If the navigation system does not automatically retrieve and set RNP 1 from the onboard navigation database for the entirety of the RNP 1 operation, the flightcrew’s operating procedures must manually set RNP 1.
C.7.4.3 For RNP 1 operations, pilots must use a lateral deviation indicator, FD, or AP in lateral navigation (LNAV) mode. Pilots of aircraft with a lateral deviation display must ensure lateral deviation scaling is suitable for the RNP value associated with the procedure.
Note: If the RNP 1 operation extends beyond 30 NM from the airport and a lateral deviation indicator is used, its full-scale sensitivity must be selected to not greater than 1 NM.
C.7.4.4 Pilots are not required to monitor ground-based navigation facilities used in position updating unless required by the AFM.
C.7.4.5 All pilots are expected to maintain centerline, as depicted by onboard lateral deviation indicators and/or flight guidance during all RNP operations described in this AC unless authorized to deviate by ATC or under emergency conditions. For normal operations, cross-track (XTK) error/deviation (the difference between the displayed path and the displayed