MLS curved approach question

[free_hat]

Hi,

Ok, got a quick question I was hoping someone could help me with. I work on autopilot systems for military jets and am currently looking at an MLS approach system. Now im slightly confused as to why MLS allows curved approaches while ILS coupled with DME does not? If ILS gives your azimuth and elevation, with DME for range (to replace the markers on the glideslope) then aside from the increased lateral cone for the localiser (80°) on MLS, why can you not fly a curved approach with ILS?

Any help would be appreciated.

Thanks!

[kenparry]

I can really only give you a partial reply. With ILS, if flown by an autopilot, the aircraft will track the centreline (assuming everything works as advertised) and there is no capability for a curved approach. Also, my understanding is that the aid is only calibrated and checked within a very narrow approach fan. MLS, though, I think, gives the capability to track multiple defined curved approaches - how it does that, I can't answer.

Any avionics or navaid experts around today?

[Capot]

Quite a good write-up here (http://en.wikipedia.org/wiki/Microwave_Landing_System#Principle)

[safetypee]

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.

[320 driver]

BA are now usin MLS on all runways at Heathrow. It gives a significant spacing advantae in LVOs and seems to be working well so far.

[NavMonkey]

Don't think that any of the current MLS avionics is capable of curved approach guidance anyway, so wouldn't worry about it.

Not sure whether the LHR MLS have the capability to uplink curved waypoints either (not that they would want to) so for now the MLS curved approach stuff isn't going anywhere.

[forget]

BA are now usin MLS on all runways at Heathrow. It gives a significant spacing advantae in LVOs and seems to be working well so far.

Are you able to expand on that. Ta.

[Swedish Steve]

Are you able to expand on that. Ta.

BA is fitting all but 5 of their A32x with MLS. When it is approved, hopefully in Feb 09, they will able to do approaches closer together as the MLS beam is not affected by other aircraft. As long as LHR Controllers cooperate, then if BA can land a row of A32x in sequence, they can land more aircraft at LHR/hour in Cat 3 conditions.

[NSEU]

Don't think that any of the current MLS avionics is capable of curved approach guidance anyway, so wouldn't worry about it.

British Airways (for one) has had MLS fitted to their 747-400's for quite some time, but deactivated and gathering dust.

Just wondering if the average MMR (Multimode Receiver) has ILS, GPS and MLS incorporated?

Cheers.
NSEU

[Megaton]

MLS approaches approaches approved for BA ops as from 8 Dec but to Cat I only.

[forget]

Smarter brains than mine will have done the sums but when would anyone start to see a pay-back here? Benefits only accrue when Heathrow is Cat III and ATC has managed to get two or more BA 320s in trail. You might argue that MLS will ‘catch on’ and soon everyone will be doing it – but I wouldn’t believe you. :hmm:

From 2003.

Avionics Magazine :: MLS: Back to the Future? (http://www.aviationtoday.com/av/issue/feature/807.html)

…… none of the three organizations or Thales, which supplies both avionics and ground stations, is prepared to discuss costs, independent estimates put the initial NATS/BAA investment, for four advanced MLS precision landing systems to be installed at London’s Heathrow airport, at around $20 million. BA’s MLS avionics bill for receivers in its 60-plus new Airbus A320 variants has been estimated at between $20 million and $50 million. The first airplane is scheduled to be delivered to BA in April 2004.

[JenCluse]

Such a pleasure.

Thank you, safetypee.

[keith smith]

MLS would be doing fine by now, if US hadn't prematurely switched allegiance to GPS. We are still awaiting CatIII LAAS thirty years later, but hopefully it will appear before long.
MLS avoids the traffic throughput limitation(higher frequency means less multipath), but it is no good unless everybody is fitted.
These days FMS can generate any curved approach path, provided it is within the MLS "cone"---no need to have "a series of waypoints with straight sectors in between"
Just to set the record straight:
1.The ICAO-chosen MLS version(TSRB time-referenced scanning beam)is an Australian invention.
2.The loser (doppler) is an entirely British show. There was little difference between the performance of either system, except that doppler was easier to check for integrity with ground signal monitor. You can do it from a single monitor, whereas TRSB needs checking across the whole cone.
Keith

[Sir George Cayley]

May I offer two other points for consideration please?

ICAO Annex 14 and CAP168 both specify safeguarded approach surfaces. The manual for designing instrument approaches also has limiting surfaces. They look a tad like a square funnel narrowing towards the threshold. There is no such internationally agreed equivalent for a wider cone as required by MLS in the format desired. Hence obstacles that would be suppressed from penetrating the APPS to a runway could potentially infringe a crved approach.

Secondly, in the UK at least and at Heathrow in particular, non standard flight paths whether on approach or departure invite negative comment from local consultative committees, so instigating approach paths that flew over previously "dead air" would be a monumental struggle.

Also, how would ATC sequence a BA 319 say coming in left field?

Ah well - seemed like a good idea at the time I guess.

Sir George Cayley

[safetypee]

SGC, as you describe, there might be problems, but even at LHR there could be advantages in using MLS.
One LHR study considered parallel glide paths, conceptually similar to the current long body PAPIs, but spaced well down the runway to avoid wake turbulence. The theory being that the jets requiring shorter field lengths could land long whilst the heavy jets landed normally. The approach sequencing would use methods similar to current procedures, but could use closer spacing not having to maintain the normal wake separation. The advantage of this arrangement was greater if the long-landing aircraft could also fly steep approaches.
Other ideas involved simultaneous off-axis helicopter approached paths enabling full IFR operation, (including Cat 3?).
Also, there were many studies and flight tests involving two stage approaches, mainly for noise benefit.

Noise preferential routings were considered for MCT where standard curved/segmented azimuth tracks would be flown by all aircraft, primarily avoiding hospitals thus meeting the local requirements.

Re approach surfaces, IIRC one MLS manufacturer (Bendix?) developed a 3D math model involving terrain and standard terrain clearance values which optimised routeings for an acceptable glidepath angle. The system may have been used with ‘private’ MLS installations in the USA e.g. Aspen, Vail, (1980s?).

[divinehover]

RNP is the future. Forget about MLS. Most states have programs to roll out RNP approaches in the near future.

[TheKabaka]

Can RNP allow an aircraft to fly a CAT3B no DH autoland?

[divinehover]

No it can't YET. But RNP is far bigger than just Cat 3. It is a future of air navigation and approach capability. One would be very silly to not get your fleet RNP ready.

[Denti]

Ground augmented GNSS landing systems are able to provide CAT III guidance, however they are not yet certified except for some very rare test installations.

The big thing about those systems is cost effectiveness as one ground installation is enough to serve nearly 60 runways with non precision and precision approaches within a radius of 40 to 50NM. That makes it far cheaper than the conventional precision approach aids that require an independent installation for each single runway direction. So there is a big drive to develop those systems although there are still some major obstacles especially in terms of reliability (jamming etc).

And RNP is commonly used for Required Navigation Perfomance which is something we all have to use throughout our normal operation for at least the last 15 years. Pure RNAV approaches are described as thus in europe and ground base augmented GNSS approaches are usually abbreviated as GBAS approaches.

[Del Prado]

Benefits only accrue when Heathrow is Cat III and ATC has managed to get two or more BA 320s in trail.

That shouldn't be a problem for ATC. In LVPs we have four full stacks of aircraft to chose from and we already work to the most efficient landing order rather than 'first come first served' when busy. I see no reason why we can't bunch BA 320s in the same way we bunch heavies or 757s at the moment.

[2close]

Apologies for resurrecting this thread from the 'hole' but does anyone know of any MLS Curved Approaches in use anywhere on the planet?

Thx

2close

[discountinvestigator]

Hi, you will find that the DME requirements for curved approaches include DME/P (precision) as opposed to the normal DME with which the civilian world is familiar.

I can also comment on when MLS would provide a benefit in Low Visibility Procedures.

You do not need a string of BA A320 family to be available, you can put a MLS equipped aircraft at approximately wake vortex + 1 nm behind anything, where wake vortex is the normal in trail separation criterion, or about 4 nm if it is not. For the mixed mode operation, then you can ignore the localiser overflight requirement as the transient is not the same. In addition, some well documented work I did in the past showed that the current landing clearance delivery points (Outer Marker, 4 nm, or 2nm exceptionally 1... and we all known that Gatwick is an exceptional place...) could be reduced down to 200 feet above TDZ elevation without a problem, but might need a bit of tinkering with the flightdeck callouts (if you are a one hundred above and CAT II).

At LHR, the most logical solution for the current mix would be BA A320 family to the departure runway and continue the ILS stream to the other. This used to be (and I am not sure about current procedures) acceptable against the runway alternation procedure because it was suspended in LVPs.

MLS LVPs have no ILS sensitive area restrictions and the critical areas are not affected by flightpaths of aircraft (but still exist for ground vehicles parking in front of transmitters etc).

As for curved MLS approaches, not sure if the Space Shuttle counts still or approaches to distinctly mobile aerodromes which float and may not operate with all of the relevant Annex 14 separations provided from other aircraft when parked. The runway lengths are also well inside the Code 1 length.... I am sure you get my drift, which is what they do with currents, tides and wind!

[kijangnim]

Greetings,

THALES Avionics is deeply involved in MLS, from ground station, to FMS2 (BA A320) MLS page and function, and gets its knowledge form military...
The problem of the civilian MLS Hibernation comes from the fact that unfortunately ONLY BA was/is fighting for it, other Airlines are just dormant:\.
to go back to the subject, why dont you contact them, they are nice, and might help you in your endeavor.
:ok: