Aircraft Electrical Systems: A320/A321 Briefing

RNP AR Briefing Document: Getting to Grips with Required Navigation Performance with Authorization Required

Date: February 2009 (Source Publication Date) Source: Airbus S.A.S. "Getting_To_Grips_With_RNP-AR.pdf"

1. Executive Summary

This document provides a comprehensive overview of Required Navigation Performance with Authorization Required (RNP AR) operations, a significant advancement in Performance-Based Navigation (PBN). RNP AR optimizes instrument procedure design by leveraging advanced aircraft navigation capabilities, enabling reduced aircraft separation en route and optimized terminal area procedures. Unlike standard RNAV approaches, RNP AR procedures are characterized by very low RNP values (≤ 0.3 NM), curved flight paths, and precisely defined protection areas. These operations require specific airworthiness certification, stringent operational procedures, and specialized flight crew and dispatcher training, necessitating a "Special Aircraft and Aircrew Authorization Required" (SAAAR) designation. Airbus provides guidance and support for operators seeking to implement RNP AR, outlining the technical, operational, and regulatory considerations involved.

2. Introduction to RNP AR

RNP AR, previously known as RNP SAAAR (Special Aircraft and Aircrew Authorization Required) by the FAA, is the "utmost development of Performance Based Navigation for approach, missed approach and departure." It differentiates itself from standard RNAV procedures through:

• RNP values ≤ 0.3 NM: Significantly higher navigation accuracy requirements.
• Curved flight paths: Before and after the Final Approach Fix (FAF) or Final Approach Point, allowing for more efficient and optimized trajectories.
• Reduced protection areas: Laterally limited to 2xRNP value without any additional buffer, demanding precise navigation.
• Baro-VNAV capability: All RNP AR approach procedures are designed to be flown with this capability, using barometric altitude for vertical guidance.

The implementation of RNP AR operations demands compliance with specific airworthiness and operational requirements for both aircraft and operators.

3. Background and Evolution of RNP

The RNP concept has a "multi-path history," leading to various definitions and performance levels. Key distinctions include:

• RNP as a level of navigation accuracy: E.g., RNP 10 in oceanic areas, RNP 1 or 2 in terminal areas (ICAO now uses RNAV-1, RNAV-2 for these).
• RNP definition of MASPS ED-75/DO-236: This standard includes requirements for accuracy, integrity of containment, and continuity of containment. Equipment complying with this can fly RNP 0.3 RNAV approach procedures (sometimes called RNP RNAV or RNP APCH).
• RNP definition of FAA SAAAR / ICAO PBN Manual: These requirements "go beyond" ED-75/DO-236, with operational requirements contributing to the overall Target Level of Safety (TLS).

SAAAR operations were first introduced in the USA for "private" or "tailored" procedures, later expanding to "public" procedures with published design criteria (FAA Order 8260.52) and operational specifications (FAA AC90-101). ICAO's Performance Based Navigation (PBN) manual aims to harmonize RNAV/RNP operations, including RNP AR.

4. The RNP Concept: Errors and Requirements

Navigation Error Components: The document defines three key error components:

• Path Definition Error (PDE): "negligible, provided there is no systematic error such as navigation database coding error or error due to inadequate geodesic reference."
• Flight Technical Error (FTE): "characteristic of the pilot performance using FD or the Auto-Pilot guidance performance in the steering of the aircraft on the FMS defined flight path."
• Navigation System Error (NSE): "error made by the navigation system in the computation of the aircraft position."

The Total System Error (TSE) is calculated as: TSE = √(FTE)² + (NSE)² + (PDE)², simplified to TSE = √(FTE)² + (NSE)² for further development.

Lateral Navigation Requirements (MASPS ED-75/DO-236):

1. Accuracy: TSE components must be less than the RNP value for 95% of the flying time.
2. Containment Integrity: Probability of TSE exceeding the cross-track containment limit (2xRNP value) without annunciation must be less than 10^-5/FH.
3. Containment Continuity: Probability of annunciated loss of RNP capability must be less than 10^-4/FH.

Vertical Navigation Requirements (MASPS ED-75/DO-236): The vertical TSE (TSEz) includes Flight Technical Error (FTEz), Horizontal Coupling Error (HCE), and Altimetry System Error (ASE). TSEz must be less than the specified performance limit (e.g., 160 ft at or below 5000ft) for 99.7% of the flying time. "There is no integrity and continuity requirement for the vertical navigation."

Instrument Approach Procedure Design Criteria:

• RNP RNAV (ICAO PANS OPS): Straight-in approaches with RNP 0.3 NM, lateral obstacle protection areas include a "buffer in addition to the 2RNP half width corridor." All Airbus aircraft with FMS and GPS are certified for "basic RNP approach" in compliance with ED-75/DO-236.
• RNP AR (FAA Order 8260.52 / ICAO RNP AR procedure design manual): Protected area is "limited to 4xRNP (2RNP on both sides of the flight path without buffer)" with RNP values as low as 0.1 NM. Required Obstacle Clearance (ROC) is linked to the aircraft Vertical Error Budget (VEB). These procedures "can include turns (Radius to Fix (RF) FMS legs) even shortly before DA."

Additional Navigation Requirements for RNP AR: Given the tighter obstacle proximity (as close as 2xRNP), the "probability to exceed this containment limit without annunciation must be lower than the 10^-5/FH of MASPS ED-75/DO-236." Authorities set the Target Level of Safety (TLS) at 10^-7/procedure. Achieving this TLS requires "special authorization" and operational mitigations, as current on-board navigation systems alone are not sufficient.

Differences between FAA and EASA:

• FTE OEI (One Engine Inoperative) Evaluation:FAA: Considers engine failure a "remote event," defers FTE OEI evaluation to operational approval. Published RNP determined with All Engines Operative (AEO).
• EASA: Requires FTE OEI evaluation during certification to demonstrate containment within ±1xRNP, determined deterministically (worst case).
• System Failures: EASA requires assessment of probable failures (within ±1xRNP) and remote failures (within ±2xRNP) during certification, facilitating operational approval by reducing the need for a Flight Operational Safety Assessment (FOSA) by the airline.

5. Certified RNP Capability

5.1. "Basic" RNP Certification (MASPS ED-75/DO-236): Airbus aircraft with GPS demonstrate compliance, with typical RNP values:

ModeAP OFF, FD ONAP ONTerminal0.51 NM0.5 NMApproach0.3 NM0.3 NMGPS is "the only Navigation sensor enabling the rigorous demonstration of containment integrity, so both basic RNP and RNP AR require Navigation solutions based exclusively on GPS."

5.2. "Limited" RNP AR with "Basic" RNP Certification: Airbus supports operating "basic" RNP-certified aircraft in "limited" RNP AR operations (e.g., RNP 0.3 for noise abatement, airspace management, or flight path optimization). However, "the upgrade of the 'full' RNP AR package... is nevertheless highly recommended." Airbus plans to offer an RNP 0.3 AR certified capability referenced in the AFM with an Airworthiness Compliance Document (ACD) by end of 2009.

5.3. "Unlimited" RNP AR with Full RNP AR Certification: "Unlimited" refers to RNP AR with RNP values < 0.3 NM. This capability is stated in the AFM with reference to an ACD, which details "assumptions, limitations and supporting information necessary to conduct safe RNP AR operations." Key aircraft modifications for "unlimited" RNP AR include:

• L/DEV indication on PFD and XTK error on ND with two digits.
• Triple click audio warning for loss of NAV or FINAL APP modes.
• TAWS/EGPWS with peaks mode and GPS position capability if both FMGCs fail.
• NAV mode automatic re-engagement on Go-Around (GA) initiation.

Demonstrated RNP Performance during Certification: The ACD provides RNP performance for normal and abnormal conditions. For the A330, examples show:

• Normal Conditions (no failure): Demonstrated TSE_CT* = 0.04 NM (FINAL APP), 0.07 NM (NAV/APP NAV). Certified RNP values: 0.1 NM (FINAL APP), 0.1 NM (NAV mode, A330).
• Failure Conditions (A330, assuming ad-hoc training):Engine Failure: RNP levels of 0.3 NM for Approach, Departure, and Missed Approach (AP ON/OFF FD ON). For RF legs, max TSE values are 0.07 NM (FINAL APP) and 0.15 NM (NAV mode).
• Probable System Failures (e.g., loss of FMS guidance): Max TSE values of 0.10 NM (straight leg) and 0.15 NM (turning leg) across all approach, departure, and missed approach modes.
• Remote System Failures (e.g., loss of NAV mode, AP roll runaway): Max TSE values of 0.2 NM (straight leg) and 0.6 NM (turning leg) across all approach, departure, and missed approach modes.
• Extremely Remote Failures (e.g., double severe FMS reset, erroneous FMS position): Aircraft "remains maneuverable for a safe extraction."

The use of these documented RNP values in abnormal conditions "suppresses the requirement to perform a Flight Operational Safety Assessment (FOSA)" for the operator.

6. Navigation and Guidance System Design

6.1. Navigation System: "Only a navigation solution with IRS and GPS PRIMARY can meet all the requirements of RNP AR." Ensuring GPS PRIMARY availability and alerting for its loss are crucial.

• Availability of GPS PRIMARY: For most Airbus aircraft, GPS/IRS integrity is demonstrated at 100% worldwide with 24 (or 23) satellites. Ground-Based Prediction Programs are generally only needed for fewer satellites or RNP < 0.3, or in specific environments like mountainous terrain.
• A/C Position Computation: FMS computes position based on a hierarchy: GPS/inertial, DME/DME/inertial, VOR/DME/inertial, Inertial only. GPS/Inertial is the primary mode.
• Radio Navigation: "The FMS radio-updating is normally not suitable to support RNP AR operations." Crews should deselect radio navaids to prevent FMS radio updating if GPS PRIMARY is lost.
• Inertial only: FMS position will drift but at a rate "small enough to allow the 'safe extraction' of the aircraft."
• GPS PRIMARY and Required Accuracy: GPS/IRS mode is active when Estimated Position Error (EPE) < Required ACCURACY (1xRNP) and Horizontal Integrity Limit (HILGPIRS) < Horizontal Alarm Limit (HAL).
• GPS Augmentation ("Coasting"): Techniques exist to maintain navigation integrity during temporary loss of GPS integrity. Litton systems can "coast" for up to 7 minutes; Honeywell systems for up to 6 minutes, but not for Required ACCURACY < 0.14.
• Navigation Alerts: "GPS PRIMARY LOST" and "NAV ACCUR DOWNGRAD" (with "LOW" accuracy) on both systems in approach require a go-around. Other alerts like "FM/GPS POSITION DISAGREE" or "FM1/FM2 POS DIFF" also warrant a go-around.

6.2. Flight Guidance: Autopilot (AP) guidance is required for RNP AR, specifically in NAV, APP NAV, and FINAL APP modes.

• APP NAV engagement conditions: Non-Precision Approach selected, FMS in approach phase, APPR button selected, NAV engaged or conditions met, aircraft above 400 ft AGL.
• FINAL APP mode engagement conditions: APP NAV engaged, FINAL armed, DECEL point sequenced, and aircraft crossing a descending leg or descending toward the vertical flight path. "In FINAL APP mode, the aircraft V/S is limited to 1800 ft/min in CONF0 or CONF1 and 1200ft/min in CONF 2 or more," which can be a limiting factor with high ground speeds or strong tailwinds.
• Managed Speed vs. Selected Speed: Operators must determine the appropriate speed technique for each procedure, as "each individual RNP procedure has its own particularities."
• Bank Angle Limitations: AEO, FG limits bank to 30°. With OEI, bank limits are reduced (e.g., 15° at IAS < Vman - 10 kt).
• L/DEV and V/DEV Monitoring: Essential for pilot monitoring of AP guidance for RNP < 0.3 NM operations. Flight crew procedures must ensure that pilots do not exceed maximum allowable deviations without intervention (e.g., manual takeover, go-around).

7. RNP AR Instrument Procedures

7.1. RNP AR Instrument Procedure Design: These procedures utilize FAA Order 8260.52 or ICAO Manual for RNP Implementation criteria. Flight paths are constructed with TF-RF or RF-RF legs, allowing turns in intermediate and final approach segments. The protection area has a semi-width of 2xRNP, with obstacles only within the 4xRNP corridor considered. The ROC is based on the VEB in the final approach segment. Missed approach RNP is typically 1 NM but can be lower.

7.2. RNP AR Procedure Charting: Charts are titled "RNAV (RNP)" and include "Special Aircraft & Aircrew Authorization Required" or "Authorization Required" in the notes section, along with specific limitations (OAT, wing span, RF leg capability, missed approach climb gradient, RNP values). They cannot be flown with remote or forecast QNH. Detailed information includes:

• FAF/PFAF: Position where obstacle clearance is defined relative to an Obstacle Clearance Surface (OCS).
• Minimum altitudes: Published before the FAF with fixed Minimum Obstacle Clearance (MOC).
• Vertical Intercept Point (VIP)/Final Descent Point (FDP): Latest point for VNAV system engagement (FINAL APP mode).

8. RNP AR Operations and Training

RNP AR operations are "Aircraft & Aircrew specific," requiring tailored procedures and training, similar to CAT III but with unique airport/runway specific challenges.

8.1. RNP AR (SAAAR) Instrument Procedure Evaluation: Thorough simulator evaluation is needed to:

• Verify flyability (especially for private/tailored procedures).
• Define normal and abnormal flight crew procedures.
• Validate FMS navigation database coding.
• Evaluate absence of TAWS warnings.

Normal Conditions:

• AP use is required for RNP < 0.3 and recommended for RNP = 0.3.
• Bank angle margins for RF legs (e.g., 5° margin). Max TAS for turn radius must be considered.
• Go Around AP/FD Mode: For older aircraft, GA TRK engages, requiring "prompt re-engagement of NAV mode." Newer FMGC standards will offer automatic NAV re-engagement.

Abnormal Conditions:

• Engine Failure (OEI): Bank angle limits are reduced. Specific airspeed constraints may be needed. Acceleration during OEI turns may cause lateral deviations requiring adapted procedures.
• Aircraft OEI Climb Performance: Must be assessed against obstacles within the 2xRNP limit.
• Aircraft System Failures: Airlines must address effects of failures (loss of GPS PRIMARY, FMS failures, AP/FD failures) and validate abnormal procedures. The ACD provides critical information, potentially reducing the need for a FOSA if RNP values in abnormal conditions are used.

Navigation Database (NDB) Validation and Control: NDB coding validation is "essential" and must be done using a flight simulator when procedures are first introduced or modified. Airlines must also implement a process to ensure up-to-date NDBs and verify no unauthorized modifications at each AIRAC cycle.

TAWS Terrain Database and Flight Path Adequacy: RNP AR procedures must be assessed for "absence of nuisance warnings and cautions" on the nominal flight path, potentially requiring database adaptation, procedure redesign, or speed constraints.

8.2. RNP AR (SAAAR) Operational Procedures:

• Flight Preparation: Verify aircraft RNP AR certification, required equipment, GPS PRIMARY availability, up-to-date NDB, and compatible weather.
• Required Equipment: Comprehensive list including 2 FMGC, 2 MCDU, 2 FD, 1-2 AP, EFIS DU with L/DEV/V/DEV, 2 GPS, 3 ADIRS, EGPWS, FCU. This list informs MEL updates.
• RNP AR Departure: Includes TOW/speeds, SID/EOSID verification, RNP value insertion, and radio navaid deselection. Flight crew must monitor L/DEV and adapt cleanup to speed constraints.
• Lateral and Vertical Deviation Monitoring: No automatic callouts; crew must "continuously monitor" L/DEV and V/DEV against specified limits. Exceeding limits requires corrective action, often a go-around in approach. Altimeter cross-check (+/-75 ft) required before FAF for approaches.
• RNP AR Approach and Missed Approach: Detailed preparation includes weather checks, equipment status, FMS waypoint/constraint verification, RNP value entry, DA entry, and specific procedure review. FINAL APP mode engagement is critical. In missed approach, prompt NAV mode re-engagement (if not automatic) is essential.
• Abnormal Procedures: Crews must be trained for engine failure (keeping AP on recommended unless prohibited), and navigation/guidance system failures (e.g., dual GPS PRIMARY LOST, FMS failures, AP loss, dual FMGC loss, all require missed approach). Mitigations include conventional radio navaids, EGPWS, and specific procedure charts.

8.3. RNP Monitoring Program: Airlines must implement a program "to ensure continued compliance... and to identify any negative trends in performance," similar to CAT II/III operations. This includes statistics on successful/unsuccessful procedures and reasons for failures.

8.4. Maintenance: No specific or scheduled maintenance tasks for RNP AR, but maintenance organization must be aware of minimum equipment requirements.

8.5. Flight Crew and Dispatcher Training: Training must be "adequate to the type of operations being envisaged" (generic or specific). It includes ground school and a Full Flight Simulator segment (minimum one 4-hour session). Key topics include F-PLN verification, GPS PRIMARY availability, FINAL APP use, speed constraints, deviation monitoring, GA procedures, abnormal situations (engine failure, system failures), and EGPWS use. Recurrent training involves 2 RNP AR approaches per pilot per duty position, one with a go-around. Dispatcher training focuses on GPS availability, NDB, weather, and takeoff performance.

9. Operational Approval

The process involves an airline application to national authorities, describing the project and compliance strategy. The "ACD referenced in the AFM should be used to support the airline's operational approval." A FOSA is required if the intended RNP value is lower than Airbus's demonstrated values in abnormal conditions. Operational evaluation leads to validation flights. Airbus offers support in configuration definition, operational evaluation, compliance with requirements, and validation flights.

Key Deliverables from Airbus:

• Airworthiness Compliance Document (ACD) and FM revision.
• Updated FCOM for RNP AR certification.
• Flight Crew Training (CBT course, simulator training/check).
• Flight Operations Monitoring with AirFASE.