ILS or RNP

[Mike Oxbig]

A question for the controllers out there. I work for a business jet company and like to fly RNP approaches as (so I have been told) they are the future. So why are ILSs still the default approach to most airports? When flying to Geneva, Zurich, Oslo, Munich, etc, why do I need to ask for the RNP? I can't believe major airlines such as LH, Swiss, AF, BA, Easy, etc, don't have the capability. Is it just a case of more expediency and traffic management as you can drive the aircraft onto the glideslope for an ILS but for the RNP you have to be inbound to the FAF in order for it to sequence? Does it become a problem for you when I request the RNP (roll of the eyes and a silent curse before replying!)? I would have thought it is easier (certainly for me!) to fly an RNAV STAR followed by the RNP allowing the system to work. My company fly to enough smaller airports for us to practice manual handling before I am accused of being a grandfather (too old for a child) of the magenta line.

[Denti]

For airlines ILS remains the standard approach as long as it is available, as it allows pretty much the same procedures from CAVOK to 75m RVR weather. And of course, ILS is a precision approach, RNP, even though deemed a 3D approach is a non precision approach. Which matters a lot for safety numbers. And those do matter if you do several thousand flights per day as the airlines tend to do. Do they use RNP? Absolutely, where it makes sense, at airports or runways that are not served by ILSs. As well as RNP AR of course, which improves upon bog standard RNP. Again, where it makes sense. And of course they do invest in in-house coded procedures for stuff like visuals with prescribed tracks, LFMN 22 is a case in point, database selectable procedure coded to the threshold.

Flying an RNAV STAR followed by an ILS works seamless for airliners, is that not the case for business jets?

And i believe ILSs do need a certain number of hours in operation to maintain their certification standard, but i am sure ATCOs and ATC engineers know a whole lot more about that.

[Dan Dare]

For runways which are capacity constrained the current RNP procedures don't seem to reliably pack the traffic in - the odd half mile here or there soon adds up. When 4D trajectories become the norm (as MLS promised all those decades ago) I expect they will become more commonplace until ILS can be replaced.

[HershamBoys]

Controllers like vectoring to the ILS because they believe they can create a tighter arrival sequence than that offered by the RNP approach. You can, of course, vector to an RNP approach as well, you just have to integrate it with your ILS sequence. However, ATC have to replace the information that the RNP provides with more track mile guidance = more chat. However, if there is minimal traffic, you can clear someone for an RNP STAR and approach and watch it being flown without having to work out headings and CDA levels, and ensure you get a decent closing heading to establish far enough. What's not to like about that ? It would be nice if crews who can't fly an RNP approach could fly an RNP STAR that leads them onto the LOC. I believe this kind of procedure has been authorised for some airports, but does it not lead to last minute mode switching in the cockpit prior to LOC capture ? .
Unfortunately, a lot of companies are mid-fleet, and won't have the kit until they renew their fleets.
RNP approaches are good for airports in the long term, because they lose the requirement to provide the associated ground infrastructure, its maintenance costs and its minders. I'm not sure how this sits in terms of a safety case where there is a requirement for a gross error check prior to commencing an approach...maybe some legacy systems will have to be retained until the regulators are satisfied with the level of SiS accuracy and continuity ?

[ATCOEHAM]

As far as I know, most airlines do prefer ILS approaches as a company policy. At EHAM we fly quite some RNP approaches, for instance, during nighttime we fly night transitions following by an RNP approach as the standard. As mentioned, RNP approaches are still considered as non-precision and ''must'' be intercepted via the IF by most airlines (e.g. KLM as far as I know). So, from ATC perspective, short approaches, direct headings towards the FAF or not an option with RNP operations. Besides, from safety perspective RNP appproaches are even not allowed in loads of situations (e.g. converging approaches, reduced visiblity etc etc).

Interesting topic :)

[V_2]

.
RNP approaches are good for airports in the long term, because they lose the requirement to provide the associated ground infrastructure, its maintenance costs and its minders

there is some truth in that, particularly for smaller airports, but as long as the airport/airline wants the option of autoland/cat 2 operations, then the ground equipment is going nowhere. In time and huge development/cost, manufactures could perhaps use GPS and radalt, but likely this would still require a ground based augmentation facility which somewhat defeats the cost saving. The LOC equipment is also used for low vis departures.

RNAV and non precision approaches in general require independent QNH checks (or at least they should!) somewhat adding to controller workload. The missed approaches can also differ which may impact controller planning&workload.

[Denti]

GBAS or GLS approaches do need a ground station and are currently approved for CAT I operation although they actually can support CAT IIIb to at least the same level an ILS can. Additionally one ground installation is enough for all airports within roughly 50 NM and can provide several different approach trajectories at the same time, which makes it more cost effective than an ILS, which costs roughly €20 Million per landing direction if a CAT III installation is needed. Slightly cheaper for CAT I only. But of course the downside is the requirement for a compatible multi-mode-receiver onboard the aircraft which costs a lot for A320 for example, but is standard on newer aircraft like the 737NG and MAX, A350/A380.

RNP approaches are of course much cheaper for airports as they do not need ground equipment and all current tech aircraft are equipped for it.

Airlines still plan for ILS as standard and lobby for its installation wherever they base aircraft or fly a lot, simply because the upgrade to low vis operation is fairly standard and a well done exercise.

[SINGAPURCANAC]

It shouldn't be forgotten "political" point of view.

We have some level of control over ILS.
And none for GPS, Galileo or Glonass.

Even with the GBAS - owner of the signal always could turn it off /or change level if service for some customers without notice.

Not only ILS VOR/DME , but NDBs will remain in service as well.

[Gonzo]

Frankfurt recently went live with CAT II GBAS on their GAST-C (effectively CAT I only) by augmenting it with EGNOS.

I have been pushing for accelerating development of GAST-D CAT III GBAS for years, because as time wears on the advantages of CAT III GBAS over the new ILS systems reduces.

20 years ago when in ATC LVP (about 250hrs per year) Heathrow had arrival spacing of 6nm to 4DME (5.5nm to touchdown). Since then, with procedural optimisation and replacement localiser antennae, we have about 25yrs of ATC LVP per year, and potential arrival spacing of 4nm (3.5nm at T/D) behind Mediums, 5nm (4.5nm at T/D) behind Heavies, with the option of taking advantage of type-specific Localiser Sensitive Areas to reduce that even more when we go to pair-wise separation.

The advantages of GBAS in CAT II/III ops are not that much more now.

I think the much-touted coverage volume (radius 23nm) for servicing multiple airports is over-hyped.

Happy to be corrected, but as far as I know, GBAS/GLS capability is an option on all Airbus aircraft, it needs to be paid for over and above the purchase of the aircraft, which of course most airlines won’t do until it’s clear that airports are investing in GBAS, and airports won’t invest in it until airlines equip their aircraft………….

[DaveReidUK]

20 years ago when in ATC LVP (about 250hrs per year) Heathrow had arrival spacing of 6nm to 4DME (5.5nm to touchdown).

Weren't the DMEs zero-ranged to the THR in those days ? Touching down 1.5 nm further on sounds a bit hairy. :O

[Equivocal]

Weren't the DMEs zero-ranged to the THR in those days ? DMEs that are associated with an ILS are normally located abeam the mid-point of the runway and set to read zero at the thresholds. Other, en-route DMEs are zeroed at the station and will indicate the level of the aircraft when it passes overhead. Going back 30 years or so, I recall a small number of ILS approach procedures (CAT I only) which utilised a suitably located en-route DME instead of markers but I have no idea if they are still published or in use.

[Gonzo]

Sorry Dave, not sure what you mean.

The spacing requested was 6nm delivered at the point when the lead reached 4DME and started to slow down. Due to the compression then experienced a 6nm gap when the lead was at 4DME usually became a 5.5nm gap when the lead touched down.

[DaveReidUK]

Sorry Dave, not sure what you mean.

The spacing requested was 6nm delivered at the point when the lead reached 4DME and started to slow down. Due to the compression then experienced a 6nm gap when the lead was at 4DME usually became a 5.5nm gap when the lead touched down.

Apologies, I see what you mean now of course.

[Del Prado]

As has been said above, at quiet airfields an RNP arrival to RNP approach in Cat 1 conditions has significant benefits for noise and fuel saving.

At busy airfields, the movement rate would suffer drastically if all aircraft flew RNP arrival to RNP approach.
Minimum spacing is often minimum radar or wake separation and there’s no way of applying that accurately on RNP arrivals that would come close to radar vectoring.

Radar vectoring to RNP approach is okay but it may require a longer final and if you’re in a sequence and you have to fly an extra 4/5 miles for a practice RNP, then everyone behind you has to fly the extra track distance too. Again, that’s fine when it’s quiet and if it’s very busy then the final will be out beyond 11 miles anyway but if there’s a continuous stream of inbounds but airborne holding is 5 mins or less then it can be a pain to have one RNP and everyone else on ILS.