And how does it differ from the classical A320 Final App?
By FLS I mean FMS Landing System.
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The FMS Landing System (FLS) is part of the Airbus xLS concept, which uses very similar indications and modes for any straight-in approach regardless of the underlying guidance system. This includes ILS, GLS (GBAS Landing System), SLS (Satellite Landing System) and FLS:
Airbus Strategy for Straight-In approaches
Similar displays whatever the modes
(Airbus Fleet Readiness for GBAS/SBAS)
This allows pilots to fly almost any approach in exactly the same way:
The creation of new approach modes that have lateral and vertical profiles independent of navaids followed the introduction of the Flight Management System (FMS) in the 1980s and of the GPS in the 1990s. The objective was to standardize the way of flying all approaches down to the published approach minima, whatever the airport, and whatever the equipment on the ground. The FLS (FMS Landing System) is part of that concept and today, it is an Airbus option offering a solution to fly 99% of approaches that are not ILS/MLS, with a barometric vertical profile.
It offers lateral and vertical guidance for a straight-in instrument approach, referenced from the aircraft position, along a trajectory retrieved from the FMS navigation database.
(Airbus: Safely Flying Non-Precision Instrument Approaches)
The FLS system is designed to calculate a fake localizer and fake glide-slope signal based on a non-precision approach coded in the FMS database:
FLS replicates the ILS beam concept, but using only the onboard navigation sensors with no need for additional ground aids. The FMS constructs a “pseudo beam” which has an anchor point (not necessarily aligned with the runway threshold), approach course and Flight Path Angle (FPA), and which overlays the final segment of an instrument approach with a temperature compensation on final segment for the indicated altitude.
FLS allows a pilot to fly an approach down to minima as an ‘ILS-alike approach’ thanks to the CDFA technique. In addition, the human / machine interface has been designed similar enough for the crew to capitalize on their current techniques but different enough for the crew not to mistake a non-precision approach flown with FLS for an ILS thanks to a distinctive symbology. In the end, this concept makes these approaches more simple to fly, thereby contributing to an increase in safety.
A characteristic of the FLS is that it can only be used for straight-in approaches but it is not compatible with curved RNP-AR approaches. Indeed, for curved approaches, crews need to undertake specific training and checking, and use the FINAL APP (or APP-DES on A350 aircraft) mode.
(Airbus: Safely Flying Non-Precision Instrument Approaches)
So only RNP-AR approaches with RF (radius to fix) legs on final cannot be flown using the FLS, any other straight-in approach works. For the curved RNP-AR you would use the traditional method (e.g. FINAL APP on A320).
Boeing has a very similar system called Integrated Approach Navigation (IAN). See also What is the difference between IAN and LNAV/VNAV?
