Aircraft Electrical Systems: A320/A321 Briefing

 

Airbus xLS Concept Briefing Document

Date: October 26, 2023

Source: Excerpts from "lets-use-xls.pdf" (Safety First, 2025)

Overview: The xLS (eXtended Landing System) concept is an Airbus innovation designed to standardize and ease the flight crew's task of flying all straight approaches across the A320 family, A330, A350, and A380 aircraft. It provides a "common and consistent Human-Machine Interface (HMI) and 3D guidance based on the well-known ILS function for all straight approaches," aiming to enhance safety, reduce workload, and improve situational awareness, particularly during Non-Precision Approaches (NPA). xLS is now the standard for all newly manufactured Airbus aircraft of these types and offers retrofit options for existing fleets.

Key Themes and Important Ideas/Facts:

1. Addressing Safety Concerns in Non-Precision Approaches (NPA):

• Problem: While Instrument Landing System (ILS) approaches are common, NPAs, though fewer in number, account for "50 % of recorded Controlled Flight Into Terrain (CFIT) during approach and Runway Undershoot accidents."
• Contributing Factors: The identified factors in these accidents include "Lack or loss of situational awareness (lateral or vertical)," "Difficulties to efficiently control, monitor or adjust the vertical flight path," and "High crew workload."
• xLS Solution: Airbus developed xLS specifically "To address these factors" by providing a consistent guidance system for all straight approaches.

2. The xLS Concept - A Common Approach for Straight Landings:

• Standardization: xLS offers "a common and consistent Human-Machine Interface (HMI) and 3D guidance based on the well-known ILS function for all straight approaches."
• Operational Consistency: Aircraft handling and "operational procedures to be almost identical for all straight approaches."
• Scope: xLS applies to both 2D and 3D straight approaches. Curved approaches still use FINAL APP (A320 Family, A330) and APP-DES I NAV (A350 and A380) guidance modes.

3. Four Core xLS Functions: The xLS concept integrates four functions, each corresponding to different approach types:

• Instrument Landing System (ILS): For traditional ILS approaches.
• GBAS Landing System (GLS): For GLS approaches, using ground-based augmentation for augmented GNSS signals, capable of CAT I (CAT II on A320 family) with autoland.
• SBAS Landing System (SLS): For RNP approaches with LPV minima, utilizing satellite-based augmentation for augmented GNSS signals, enabling approaches down to 200ft (equivalent to CAT I).
• FMS Landing System (FLS): For 2D (VOR, NDB, LOC, LOC B/C, RNP with LNAV minima) and 3D (RNP with LNAV/VNAV minima) approaches, relying on the aircraft's FMS and barometric altitude.

4. Virtual Beam Principle (GLS, SLS, FLS):

• Commonality: GLS, SLS, and FLS share a core principle: "the use of a virtual beam, similar to an ILS beam (localiser + glideslope), computed by the Multi Mode Receiver (MMR)."
• Deviation Computation: The MMR uses an anchor point (typically runway threshold), approach course, and glideslope angle, then computes and displays deviations "in a way similar to the ILS deviations" on the PFD and sends guidance commands to the Flight Director (FD) and AutoPilot (AP).
• Key Difference: The primary distinction among GLS, SLS, and FLS lies in "the source of the data (aircraft position, aircraft altitude, and virtual beam characteristics) used to compute deviations."

5. Detailed Look at FLS Function:

• Independence: "The FLS function does not depend on any ground navaid to compute deviations."
• Data Sources: It uses the "aircraft's position, computed by the FMS (A320, A330, A380) or ADIRS (A350) through mixed GPS/IRS (GPIRS), mixed IRS/radio NAVAIDs, or pure IRS, for lateral deviation computation." Vertical deviation uses the "aircraft's barometric altitude."
• Virtual Beam Data: FLS "virtual beam characteristics are stored in the FMS navigation database."
• Temperature Compensation: FLS includes a valuable feature that "uses the temperature entered into the FMS APPROACH page to compensate the glideslope for low temperatures."
• Specific Guidance Modes: FLS utilizes "specific F-G/S and F-LOC guidance modes that have similar behavior (engagement and disengagement) to the G/S and LOC modes."
• Performance Considerations: Correct FLS deviations depend on "A correct QNH or QFE selection on the FCU," "A correct temperature entry in the PERF APPR page of the FMS for cold temperature compensation," and "A good aircraft position performance."
• HMI Distinction: FLS uses "Double diamonds" instead of single diamonds "to indicate the FLS deviations," highlighting that its performance is not identical to a precision approach.

6. Advantages of FLS over FINAL APP for Straight Approaches:

• ILS Look-alike: FLS offers "a similar interface, a similar aircraft guidance, and a similar operational procedure as ILS," unlike FINAL APP which has a "specific interface and a specific behavior that requires a different procedure."
• Flexibility: "FLS beam capture, like ILS beam capture, is independent of flight plan sequencing," providing flexibility for ATC constraints or approach optimization. FINAL APP engagement is restricted and dependent on sequencing.
• Temperature Compensation: FLS modes can be used "when the destination temperature is below the published minimum temperature" for certain RNP approaches, due to its temperature compensation. FINAL APP lacks this compensation.
• Enhanced Vertical Guidance: FLS's F-G/S can provide "managed vertical guidance during LOC-only, LOC B/C, and ILS with G/S out approaches," a "significant safety improvement" compared to the higher workload of selected vertical FPA modes.

7. SLS and GLS Specifics:

• SLS: Uses augmented GNSS (GPS + SBAS) for geometric position and altitude, enabling RNP APCH with LPV minima down to 200ft (CAT I equivalent). SLS uses standard G/S and LOC guidance modes, and FLS can act as a backup.
• GLS: Uses augmented GNSS (GPS + GBAS) for geometric position and altitude, enabling GLS CAT I (CAT II on A320 family) approaches with autoland. GLS virtual beam characteristics are received from the GBAS ground station via VHF, allowing manual tuning. Like ILS, GLS can assist with maintaining runway axis during takeoff.

8. Operational and Training Considerations:

• Minor Routine Changes: "The introduction of the xLS concept is a significant improvement with minor changes in the flight crew’s routine."
• Identifying Available Functions: Flight crews can check the "Aircraft Configuration Summary table in the FCOM/QRH" to identify installed xLS functions. FLS is standard on A350 and A380.
• Flying Straight Approaches Like ILS: The process is similar to ILS: select approach in FMS, press LS pushbutton, and press APPR pushbutton to arm guidance modes when cleared.
• Limited Training Needs: Due to the identical way of flying all xLS functions and identical guidance, "only Level A (self-instruction) training is needed for SLS and GLS, and Level B (aided instruction) training is needed for FLS."
• Simulator Availability: Simulator data packages for xLS functions are available for operators.

9. xLS Functions Availability and Retrofit:

• Standard Installation: FLS is installed by default on newly manufactured A320 family aircraft (from 2022) and A330 aircraft (from 2020), as well as on all A350 and A380 aircraft.
• Optional: GLS and SLS are generally optional (line-fit or retrofit) across all aircraft types.
• Retrofit Potential: "More than 4,000 A320 Family aircraft" (38.4% of the fleet) and "more than 350 A330 aircraft" (24.3% of the fleet) have the prerequisites to "easily activate the FLS function" with a "short grounding time to activate FLS" (2.5 hours). Airbus offers "attractive commercial conditions" for FLS activation retrofits.

10. Future of SLS and Regulatory Compliance:

• Growing LPV Approaches: As of early 2025, "more than 7 000 RNP approaches with LPV minima are operational in SBAS-covered areas." The number of LPV approaches now exceeds ILS approaches in the U.S.A.
• EASA Regulation: "The European (EU) IR 2018/1048 regulation published by EASA requests exclusive use of Performance Based Navigation (PBN) for non-CAT II/CAT IIIA/CAT IIIB operations." This means "SLS should be selected by airlines to maintain CAT I equivalent operations in Europe after June 6, 2030."
• Potential SLS Autoland: Airbus is "assessing the possibility of certifying an SLS CAT I autoland" to increase automatic landing availability.

Conclusion: The xLS concept represents a significant advancement in flight operations safety and efficiency for Airbus aircraft. By standardizing the approach guidance interface and procedures, particularly for straight approaches, Airbus aims to mitigate risks associated with NPAs, reduce crew workload, and enhance situational awareness. The widespread implementation of FLS as a standard feature, coupled with the growing importance of SLS due to regulatory changes and increasing LPV minima availability, underscores Airbus's commitment to continuous improvement in aviation safety. Operators are strongly encouraged to leverage xLS, including retrofitting FLS on eligible in-service aircraft, to maximize these operational and safety benefits.