Let’s Use xLS

Every day, around the world, many final approaches flown by commercial jet aircraft are Instrument Landing System (ILS) approaches. Flight crews are therefore very familiar with flying ILS approaches. In comparison, Non-Precision Approaches (NPA) represent a smaller proportion of the approaches performed daily. However, 50 % of recorded Controlled Flight Into Terrain (CFIT) during approach and Runway Undershoot accidents occurred while flying an NPA.

In most of these accidents, the identified contributing factors included:

• Lack or loss of situational awareness (lateral or vertical)
• Difficulties to efficiently control, monitor or adjust the vertical flight path
• High crew workload

To address these factors, Airbus developed the xLS concept for A320 family, A330, A350, and A380 aircraft to ease the flight crew’s task of flying all straight approaches.

This is why xLS provides a common and consistent Human-Machine Interface (HMI) and 3D guidance based on the well-known ILS function for all straight approaches. Aircraft handling with the various xLS functions is identical, allowing operational procedures to be almost identical for all straight approaches.

The xLS concept applies to both 2D and 3D approaches. xLS gathers 4 functions to fly all straight approaches (fig.1):

• Instrument Landing System (ILS) for ILS approaches, 
• GBAS Landing System (GLS) for GLS approaches , 
• SBAS  Landing System (SLS) for RNP approaches with LPV minima, and 
• FMS Landing System (FLS) for 2D (VOR, NDB, LOC, LOC B/C, RNP with LNAV minima) and 3D (RNP with
• LNAV/VNAV minima) approaches. 

FINAL APP (A320 Family, A330) and APP-DES I NAV (A350 and A380) guidance modes remain for flying curved approaches.

(fig.1) ICAO Approach types with their corresponding Airbus approach functions

GLS, SLS and FLS have a common principle: the use of a virtual beam, similar to an ILS beam (localiser + glideslope), computed by the Multi Mode Receiver (MMR) (fig.2). The MMR uses an anchor point, generally the runway threshold, an approach course, and a glideslope angle. It then uses the aircraft’s position and altitude to compute deviations with regards to the virtual beam. 

The computed deviations are then displayed on the PFD in a way similar to the ILS deviations. They are also sent to the autoflight system to compute guidance commands for the Flight Director (FD) and AutoPilot (AP).

The main difference among GLS, SLS, and FLS is the source of the data (aircraft position, aircraft altitude, and virtual beam characteristics) used to compute deviations of the aircraft’s position with regard to the virtual beam.

The FLS function does not depend on any ground navaid to compute deviations. 

FLS utilizes 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 (fig.3). Additionally, FLS uses the aircraft’s barometric altitude to compute the vertical deviation. 

FLS uses the virtual beam characteristics that are stored in the FMS navigation database.

An interesting feature is that FLS uses the temperature entered into the FMS APPROACH page to compensate the glideslope for low temperatures when the OAT is below ISA temperature. 

FLS uses specific F-G/S and F-LOC guidance modes that have similar behavior (engagement and disengagement) to the G/S and LOC modes.

Correct FLS deviations rely 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
• A good aircraft position performance

The interface is adapted to highlight that FLS does not provide the same performance as for a precision approach. For example, “Double diamonds” are used instead of single diamonds to indicate the FLS deviations.

(fig.3) FMS Landing System (FLS)

SLS uses an augmented GNSS signal provided by both the Global Positioning System (GPS) constellation and a Satellite-Based Augmentation System (SBAS) service to compute the aircraft’s position and altitude (fig.4). This increased performance enables the aircraft to fly RNP APCH with LPV minima down to 200ft (equivalent to CAT I).

The characteristics of the SLS virtual beam are stored in the FMS navigation database.

SLS uses the same G/S and LOC guidance modes as ILS.

In the event of a loss of the SLS function, the FLS function can be used as a backup to fly the same RNP approach, but down to LNAV/VNAV minima. FLS installation is therefore a prerequisite for SLS.

(fig.4) SBAS Landing System (SLS)

GLS uses an augmented GNSS signal provided by both the Global Positioning System (GPS) constellation and a Ground-Based Augmentation System (GBAS) station to compute the aircraft’s position and altitude (fig.5). This increased performance enables the aircraft to fly GLS CAT I (CAT II on A320 family) approaches with autoland.

GLS receives the virtual beam characteristics from the GBAS airport station via VHF. Consequently, if the FMS is not available, the flight crew can manually tune the GLS channel on the Radio Management Panel (RMP) to fly a GLS approach, similar to how they can manually tune the ILS frequency for an ILS approach.

GLS uses the same G/S and LOC guidance modes as ILS.

(fig.5) GBAS Landing System (GLS)

The introduction of the xLS concept is a significant improvement with minor changes in the flight crew’s routine. They need to know which xLS functions are available on the aircraft they are flying, and fly their straight approach like an ILS when the associated xLS function is available.

During Preliminary Cockpit Preparation, the flight crew can quickly identify which xLS functions are available on the aircraft by referring to the Aircraft Configuration Summary table in the FCOM/QRH (fig.6).

FLS is not mentioned in the table for A350 and A380 aircraft, as it is installed on all aircraft. 

(fig.6) The Aircraft Configuration Summary provides the list of available xLS functions

If the corresponding xLS functions are available on the aircraft, the methods to fly straight approaches and ILS are similar:

① The flight crew selects the approach in the ARRIVAL page of the FMS. The associated xLS function is automatically selected, if available. ② For an approach compatible with FLS, the selected function is displayed on the APPR page of the FMS.

(fig.7) How to fly an approach using xLS – part 1

③ The xLS information appears on the PFD and ND when the flight crew presses the LS pushbutton. ④ When cleared for the approach and on the intercept trajectory to the final approach course, with LOC deviation available, the PF presses the APPR pushbutton to arm the xLS guidance modes:

• _F-G/S I F-LOC_ modes for VOR, NDB, and RNP approach with LNAV or LNAV/VNAV minima
• _F-G/S I LOC_ modes for LOC-only or ILS with G/S out approaches
• _F-G/S I LOC B/C_ modes for LOC B/C approaches
• _G/S I LOC_ modes for GLS approaches, and RNP approaches with LPV minima flown with SLS.

(fig.8) How to fly an approach using xLS – part 2

⑤ Then, depending on the approach type, the F-LOC, LOC, or LOC B/C lateral mode engages, ⑥ followed by the F-G/S or G/S vertical mode to start the final descent.

(fig.9) How to fly an approach using xLS – part 3

As explained earlier in this article, the only difference between the xLS functions is the source of the data used for the deviations computation. The way to fly all xLS functions is identical, and the guidance is identical. The SOP for all xLS functions are therefore almost identical with a slight difference for FLS in the case of a degradation of the aircraft position performance. 

This is why only Level A (self-instruction) training is needed for SLS and GLS, and Level B (aided instruction) training is needed for FLS.

For operators wishing to add the xLS to their simulators, xLS functions are available in the following simulator data package:

Table 1: Simulator data packages for xLS

The sections below indicate for each aircraft type whether the various xLS functions are installed as a standard from aircraft delivery or if they are optional. They also list the minimum computer standards required to retrofit the functions on in-service aircraft.

FLS is installed by default on A320 family aircraft manufactured from 2022. It can be retrofitted on previously built aircraft, provided they are equipped with the minimum computer standards. 

GLS and SLS are optional. They are available in line-fit (i.e., from the production line) or as a retrofit on in-service aircraft via Service Bulletin.

Table 2: xLS functions availability on A320 family aircraft

Table 3: Minimum computer standards required for xLS activation on A320 family aircraft

FLS is installed by default on A330 aircraft manufactured from 2020. It can be retrofitted on previously built aircraft, provided they are equipped with the minimum computer standards.

GLS and SLS are optional. They are available in line-fit (i.e., from the production line) or as a retrofit on in-service aircraft via Service Bulletin.

Table 4: xLS functions availability on A330 aircraft

Table 5: Minimum computer standards required for xLS activation on A330 aircraft

FLS is installed by default on every A350 aircraft. Coexistence with APP-DES mode is also available to fly curved approaches. GLS and SLS are optional and are available as a single package.

Table 6: xLS functions availability on A350 aircraft

FLS was installed by default on every A380 aircraft. Coexistence with APP-DES mode is also available to fly curved approaches on aircraft equipped with Avionic Batch 7 and the RNP AR modification. GLS and SLS are optional.

Table 7: xLS functions availability on A380 aircraft

We have described the numerous operational and safety benefits of xLS to fly all straight approaches, so let’s use xLS!

Now that FLS is standard on every newly produced aircraft, let’s also benefit from FLS on the previously built A320 family and A330 aircraft.

38.4% of the current A320 Family aircraft fleet have all the prerequisites to activate FLS. This represents more than 4,000 aircraft that could easily activate the FLS function.

Similarly, 24.3% of the A330 aircraft fleet, which represents more than 350 aircraft, could activate  the FLS function without any additional computer update.

(fig.10) FLS installation status for A320 family and A330 fleet in April 2025

The maintenance task to activate the FSL function takes 2.5 hours for an aircraft with all the minimum prerequisites.

To encourage operators to retrofit the FLS function on their in-service aircraft, Airbus proposes very attractive commercial conditions for the FLS activation Service Bulletin. Operators can contact their Customer Support Directors for more information and to perform an analysis of their fleet’s retrofit possibilities.

As of early 2025, more than 7 000 RNP approaches with LPV minima are operational in SBAS-covered areas, including Europe, the U.S., South Korea, Japan, and India. The number of RNP approaches with LPV minima is now greater than the number of ILS approaches in the U.S.A. SBAS coverage for Africa, Australia, and China is also planned in the near future.

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 that SLS should be selected by airlines to maintain CAT I equivalent operations in Europe after June 6, 2030.

To increase the availability of automatic landings, Airbus is currently assessing the possibility of certifying an SLS CAT I autoland.

Safety first | June 2025 – Airbus S.A.S. All rights reserved. Proprietary document.