During the early-days of the British (European?) MLS development, curved approaches were considered, but the implementation required extensive ‘FMS’ navigation computation. It was 30yrs ago! Modern computation can now provide the multiple waypoints required, but the principle of ‘joining up’ closely spaced waypoints may be similar.
Most FMS can fly a curved approach between ground referenced (geographic) waypoints, but this requires a high accuracy positioning system (GPS) and inherent reliability for Cat 3 operations (not yet with GPS for commercial operations?).
MLS provides both accuracy and integrity from a single ground station. The MLS waypoints are referenced (relative) to the ‘transmitter’ (or an offset location) providing both the range and azimuth component for a waypoint.
A pure ILS does not have an azimuth element, only a deviation relative to a centerline; thus, a waypoint cannot be defined by ILS/DME alone (except on the centerline). The range component required a high accuracy DME positioned at or referenced to the threshold.
The initial development trials conducted by RAE Bedford tested ‘segmented’ approaches between a few waypoints, i.e. IAF, IMF, and FAF. The segments were joined up using the flight guidance ILS control law, which during the capture phase flew a pseudo curved approach.
With a suitable location of waypoints, the flight guidance could be ‘persuaded’ to stay in the capture mode between waypoints, thus producing a curved approach by transitioning to the next waypoint without leveling the wings – except for the final leg.
The trials also considered vertical segments using the ILS GS mode.
The results were very good, enabling a ‘curved’, steep, segmented approach from 120 deg off-centerline and a 5 deg to a 3 deg glideslope culminating in a 1000ft / 3nm straight-in autoland. There were similar ideas for flying the go-around route.
A particularly impressive demonstration was flown for ICAO and the FAA at Berne airport where an approach was flown around the Belpberg mtn, finishing with a steep approach (autoland) as required by the normal approach.
Other tests were flown at Manchester, Gatwick, and in Tehran.