ATC 'Maintain present heading' instruction

[ukatco_535]

Ferris

A - I am sure DFC does not appreciate the fact you say he will not understand as he is a pilot and indeed that you think hes has NFI - you are incredibly pompous, arrogant and basically, an A**e!! DFC is obviously a customer, whether of yours or not, but he does use ATC services. His input IS valuable and two-way converation is essential in this business. To simply dismiss someone because they are a pilot as opposed to controller merely highlights your shortcomings.

B - You say that it is a waste of time to use this technique when A/C are so far apart - maybe that is so in en-route sectors, but a heck of a lot of us on Pprune work in the TMA environment, some of our sectors are 35-40 miles wide with a hell of a lot of movements. If we did not apply this technique, we would come to a standstill. Not just me saying this - it is a FACT.

Your arguments have some merits, but as DFC above explained - RNP standards are resultant standards - how you get that result is another matter (manual flying/autopilot). However, I have filed on A/C before because glitches in the software have meant that they climbed above SID levels if a pilot inputs a new heading (an Airbus A/C 6 months ago) - resulting in a change of operations by the company.

What you have to realise - but I'm afraid that you never will, is that other people on this thread (the other 98%) have valid arguments as well.


DFC - As an ex-mil aviator, I understand the drift equation and you are entirely correct, but to calculate it whilst on Radar is a little difficult and time consuming because we would need near enough exact readouts of wind, the pilots true heading, the TAS or RAS, The temperature (if we are being really picky). We would also then need the exact Closest Point of Approach of the A/C then use the 1 in 60 rule. I only mentioned it because I wanted to point out to Ferris what was involved before he started his tirade on people. I say the word exact above several times because if I was being entirely pedantic in the manner of Ferris, I would want these calculations to be as accurate as possible! I do however say the above without having any reserve on endorsing all that you have put in your post.

And I am sure I can speak for the rest of the controllers on Pprune in saying that we value very much a pilots perspective.

[DFC]

Ukatco_535,

Absolutely, the mental calculation of drift is a bit much for use at radar especially considering the multitude of tracks, levels and speeds. However you guys seem to get a good feel for the wind after a time at the dispaly and if I remember correctly, on handover the previous controller should have given you an indication of the "heading of the day" and any significant winds.

Speaking of some approach radar people I have "been a customer of", it is nice to see them being able to put us on the centerline of an SRA with a single heading in a 40Kt crosswind............RNP measured in metres!!

---

Ferris,

Overall I made two important points previously -

When separation is your responsibility then you must take responsibility.

Do not expect to keep us on our own navigation and then after a loss of separation try to shift blame from you to the pilot. Separation is your responsibility and part of the service you are contracted to provide.

You ask - do we ask before diverting round a cell when on a radar heading.

The answer is absolutely yes. We will never change heading without asking when you are navigating the aircraft.

However when on our own navigation..........we can alter heading as much as we like provided we do not get too far from track.

I have been flying during the period when one of the comets was visible (about 10 years back) - the co-pilot was an astro buff and during the cruise we had a great view. After briefing the cabin we turned off the lights and zig-zaged with gentle banks so that people could get a nice view from the windows..........All in the middle of Europe during the evening.......we never were more than 2 miles off track or so and ATC never said anything.

As for weather........well if I am 90nm from a VOR on a non-RNAV route then I can go to half scale deflection to get round without asking ATC.

Go away and find out how far half scale deflection is for a VOR at 90nm allowing for all the possible errors.

If it was a acceptable practice then controller workload in Europe would be halved and thus capacity increased.

Ferris to head one sky

Regards,

DFC

[PPRuNe Radar]

What more can I add to DFC's post ??

[ferris]

ukatco_535
If you want to lower the thread to name-calling, fine. It usually happens when people run out of valid argument. I also tend to find the proponents of this technique to be the pompous ar$e-types. As for the rest of your sycophantic outburst (customers ), I quite clearly stated that pilot input was valuable, I took issue with the how-to-do-your-job advice.

What you have to realise - but I'm afraid that you never will, is that other people on this thread (the other 98%) have valid arguments as well

Well, where are they?

If we did not apply this technique, we would come to a standstill. Not just me saying this - it is a FACT.

I assume you work in the UK. It seems, from reading this thread, that there are TMAs in the UK already using a/c tracking as sep assurance. Maybe where you work is a little behind.

before he started his tirade on people

What tirade? I thought I was sticking to the discussion quite well.

And I am sure I can speak for the rest of the controllers on Pprune in saying that we value very much a pilots perspective

So do I (as stated). I don't need pilot's telling me how to do my job.

DFC
Great waffle, but your argument sucks.

When separation is your responsibility then you must take responsibility

No abrogation of responsiblity has been suggested.

Separation is your responsibility and part of the service you are contracted to provide

That contract is predicated on you following the instructions issued (your part of the contract). If you are told to "track direct XXXXX", then go on comet-watching jaunts etc, how dare you attempt to suggest that it's the controller's who carry the can. If I have determined that by you following your track, then you are separated, your part in that contract is to follow your track.

We will never change heading without asking when you are navigating

That simply isn't true. I have had, on several occasions, a/c change their assigned heading without advising me (usually due to freq congestion). There was a pprune thread on the middle east forum about one such occasion (titled 'a night to remember').

we never were more than 2 miles off track or so and ATC never said anything

Clearly the mistaken belief that you can wander around when on own nav needs to be addressed. As mentioned, the UK seem to already be at the point of basing sep assurance on it at some locations. Despite what you say, you cannot legally alter heading when on own nav, unless for track-keeping.

As for your attempt at muddying the waters with half scale deflections etc.... I thought it was axiomatic that we were discussing RNAV here.

If it was a acceptable practice then controller workload in Europe would be halved and thus capacity increased

I can only assume you are talking about some method of doing away with all vectoring. Not what this thread is about. But I do have an open mind- unlike most.

[Roger Standby]

Starting to get a little heated here! Obviously different strokes for different folks. Different techniques and theories in different parts of the world. I think along the same lines as Ferris and I'm sure it's to do with way the training is done/perceived. What works for you, works for you. I think if you sat in either environment and observed for awhile, you couldn't pick one and say it was wrong.

What I can't understand with the bulk of the responses is that in modern aircraft tracking direct to a point has to be more accurate than flying a heading (I believe the tolerances are built into the standards to allow for errors/limitations of equipment, not to go flying all over the sky at your leisure. Any deviation from track must be advised according to my books.) Blah, blah, blah about the limited impact on changing winds, the fact is, a heading is NO guarantee that the track will be maintained as required by the controller, therefore where's the assurance?

R-S.

[TrafficTraffic]

A good friend of mine (they do exist!) pointed me in the direction of this thread. I have read the thread and re-read it and I would love to have a go at Ferris and call him names and dispute his posts and theories but I am afraid I do not have a clue what this thread is about (which is actually a similar outcome to whenever London try and coordinate something).

Ferris was quite obviously beaten as a child or dropped on his head (or should have been)

He is also a little upset by the beating the US peso has taken quite recently.

Don't you agree that we are wasting everybody's time by applying this technique, when the a/c are so far apart, and the real risk of collision is so small, yet we take no preacautions about other lapses/failures, which involve the a/c being much physically closer together?

As I am sure one of your heroines(sp?) would say "PLEASE EXPLAIN?"

I agree with you Ferris u saucy minx, It is silly to waste everybody's time when the aircraft are so far apart - by that do you mean 100 miles or 5 miles? (Of course we all know that the actually separation standard is 1 mile the other 4 is just for worry warts) .

I would love to stay and chat more with Ferris but the swimming pool is calling my name...after all talking with Ferris is like Charity Work... wonder if I can claim my PPrune account back on tax ??

TTFN TT

BTW DD please pm me

[DFC]

Clearly the mistaken belief that you can wander around when on own nav needs to be addressed. As mentioned, the UK seem to already be at the point of basing sep assurance on it at some locations. Despite what you say, you cannot legally alter heading when on own nav, unless for track-keeping

I can wander round as much as I like on own nav provided that I remain within the required tracking requirements which are in the case of VOR - half scale deflection.

This means that I can go from half scale deflection right to half scale deflection left and back to half scale deflection right as many times as I like. If you don't like that wiggle across your display then lock us on a heading and you take care of the tracking.

RNAV or not RNAV makes little difference unless you are delaing with routes where the VORs are more than 120nm appart.

ICAO has published the figures I quoted for separation of aircraft on their own navigation with radar monitoring. If you want to use figures less than what ICAO specify then you better have a good safety case.

What is your minimum horizontal separation for opposite direction flights on own navigation at the same level?

What is your minimum separation of the same flights when on headings issued by you (being vectored)?

I can only assume you are talking about some method of doing away with all vectoring. Not what this thread is about

No we are all talking about controllers issuing headings in the enroute environment to acheive separation between flights.

Regards,

DFC

[ferris]

DFC
Why keep referring to half-scale deflection nonsense? I refer you to ICAO Rules Of The Air, Annexe 2, Chapter 3.6.2.1.1 Track-keeping. Do you not fly RNAV (and especially RNP5) aircraft? Even in the Gulf, it's RNP5 only now. I am sure the people in the UK responsible for the 'track separation assurance' would be most interested in your posts. Keep babbling about situations we are not talking about, by all means, but don't expect further responses. Clearly I am not talking about DC3s flying VOR to VOR in 1960.

What is your minimum horizontal separation for opposite direction flights on own navigation at the same level?

What is your minimum separation of the same flights when on headings issued by you (being vectored)?

In both cases, the applicable radar standard (which, where I work, is 5nm). There is no difference. I think it's important you appreciate this.

No we are all talking about controllers issuing headings in the enroute environment to acheive separation between flights.

Good try, but not quite. I am talking about specific cases where 2 a/c, based on their cleared routes, are judged to miss by an amount that the controller is happy with- say 7nm (one or both is climbing/descending). A technique often espoused by some people is to ask the a/c their headings, and lock the a/c on those headings, thereby somehow gaining separation assurance. I disagree with the assurance angle of this technique, believing that it provides no assurance, can actually cause as many problems as it relieves, and is a waste of R/T.

TT. Thanks for your input. Most valuable. I am dazzled by the luminosity that is your intelligence, shining like a beacon from your tower of golden Euros.
Good to see the pilots keeping up better than you.

[LostThePicture]

What is your minimum horizontal separation for opposite direction flights on own navigation at the same level?
What is your minimum separation of the same flights when on headings issued by you (being vectored)?

To answer the second question first, the minimum is 5 miles, where I work. The first question could throw up a multitude of answers, but on specified radar-monitored routes, the minimum is 12 miles, where I work. The wording in italics is from the manual at my unit, and to me it means the following:

1) If the aircraft's track does not appear to be following the centreline of the airway or UAR, then I will lock any aircraft at the same level on radar headings. (This is in fact a requirement if track-keeping is not satisfactory, and is stated as such in the manual).
2) If my radar fails, and I am working procedurally, then I cannot have two aircraft on the two different airways at the same level.

ferris, you say in your post that your unit requires aircraft to conform to RNP5. Therefore, an aircraft can be on its own navigation, in airspace controlled by you, and be up to 5 miles away from its "correct" track - or what appears to be correct from your perspective, looking at your radar. This traffic is conforming to the required standard in your FIR. Just because modern aircraft are CAPABLE of much better navigational performance is not the point. By your own admission, an aircraft in your sector could fly 4.9 miles left or right of track and the pilot need not tell you. As someone else said earlier, if the aircraft is not a modern glass-cockpit jet, then the pilot may not even know he's 5 miles off track.

What's my point? Well, say you have two aircraft. You want to climb one through the other. Whether they are running parallel, opposite direction, or on crossing tracks does not matter. You judge they are going to miss by, say, 9 miles. So you do the climb through without using headings. Now, the fact is that both aircraft can be conforming to the minimum navigational requirements in your FIR and have an airmiss, or worse still, hit each other. At the subsequent board of enquiry, the pilots are found to have been conforming - do you think that you would be entirely exonerated from blame?
Alternatively, you could lock the aircraft on headings. Because you're such a professional controller and have checked today's upper wind forecasts, you know all about that nasty 100kt change in windpeed in a 2000ft band of air (!!), and can therefore correct for it in your headings. So you do the climb-through on headings and the two aircraft still have an airmiss. This time, at the subsequent board of enquiry, it is found that the pilots of each aircraft turned slightly into one another to avoid weather.
In the first scenario, the airmiss is ENTIRELY your fault. In the second, the pilots have turned away from an assigned radar heading without asking, which is not permitted and they must therefore take the majority of the blame. However, you will probably still be chided for not periodically monitoring your radar headings. Such is life for a controller with responsibilities.

I grant you that my scenario is somewhat unlikely with modern aircraft, but that won't stand up in a court of law. By your own admission your state requires RNP5. Until it requires RNP2 or better, you should start using radar headings a bit more.

I'd love to be a fly on the wall at your competency exam after you've allowed two aircraft to whistle past each other on their own navigation at the same level. Suffice to say, you wouldn't have validated at my unit controlling like that (and I DO work in the UK).

LTP

[ferris]

It's good, I think, to get all this stuff out in the open.
LTP, I think you've made a good attempt, and I think where you are coming from is exactly where the proponents of this technique reside.
You are, however, sadly mistaken in the nuts and bolts side of several issues.
Firstly, if an a/c appears to be not track-keeping, you would be an idiot to allow it to continue on own nav. Why even mention such stuff? Lets raise the level of the argument a bit, please.

Therefore, an aircraft can be on its own navigation, in airspace controlled by you, and be up to 5 miles away from its "correct" track

This, to me, is your biggest misapprehension. It is simply not true. I suggest you find out exactly what RNP means. RNP5 aircraft are extremely accurate at track-keepeing (ops normal). In particular, look at the rate a conforming a/c can drift, and you will find it is significantly less than that possible with wind and headings. It, to me, is why the "lock heading" technique is a waste of time.

By your own admission, an aircraft in your sector could fly 4.9 miles left or right of track and the pilot need not tell you

Not true. At no time have I "admitted" that. See above.

As someone else said earlier, if the aircraft is not a modern glass-cockpit jet, then the pilot may not even know he's 5 miles off track.

Not unless there is a malfunction. Not if he is RNP5 regardless of glass cockpit. Even oceanic.

I grant you that my scenario is somewhat unlikely with modern aircraft, but that won't stand up in a court of law. By your own admission your state requires RNP5. Until it requires RNP2 or better, you should start using radar headings a bit more

No, your scenario is not unlikely, it's impossible (barring system failure).

At the subsequent board of enquiry, the pilots are found to have been conforming

You see, you do not understand this bit. They couldn't have been. Your scenario is a waste of words. Suggest you do some research.

Suffice to say, you wouldn't have validated at my unit controlling like that

Like I said, maybe it's time to rethink some of these 1950s techniques, with the modern technology at our disposal. The entrenched 'old-school' is hard to shake.

[DtyCln]

Ferris, despite some good arguements , you are beginning to sound like a K**b!

There are several airlines who fly through the UK every day, some UK based some foreign, who regularly use all 10 miles of their airways whilst flying under their 'own nav'. Some of the routes that transit overhead London have 'kinks' in them at BPK,CPT, LAM etc. Air2000 as was, would regularly fly 4.9 miles to the right/left of track if it straightened out the kink. Their arguement is that they are 'own nav' in the airway that they filed to fly along! It continues today.

I personally use headings where I think they are needed. Like I said on page 1 of this thread about 15 miles lateral opposite direction, slightly less same direction. The majority of level headed responses on this thread appear to be in the same ballpark. As to why?

Aircraft entering orbits at reporting points due to incorrect FMS inputs. A/C turning at reporting points going off down routes they thought they were on and company filed them a different way. A/C turning for weather and unable to report it due to the R/T being too busy. Own nav'ving an A/C which was transferred on a radar heading from Maastricht with another 15 miles abeam at the same level, only for the A/C to commence a turn back towards Holland to a previous (undeleted) FMS fix. etc etc the list is endless.

As one of your UAE colleagues texted me the other day, "we don't need headings 'cos we don't have any conflicts!"

Ciao

[vector4fun]

I can't speak to the enroute portion of the debate, having spent my entire career in the Terminal option. However, I must weigh in that during the transition from radar vectors to pilot nav via one of the published DPs, I've observed MANY strange things on a radar scope, and it has seemed the more advanced the equipment, the stranger they are. Differend FMS systems behave differently in regard to sequencing waypoints, errors are made in programming the route, and errors are made in setting the correct nav mode.

One example, I've seen multiple occurances where the crew turned out a "radial" from the airport, rather than the VOR as published. Another example, when releasing a crew to resume own nav, it's a fair bet which fix the FMS is actually going to turn towards. The named fix closest to present heading is NOT always the FMS's first choice.

The fact is, to most controllers I know, headings are, (rightly or wrongly), a more reliable method of insuring lateral separation. In fact, the FAA allows us to use less than 3 miles separation between aircraft on diverging Headings. After 25 years, I plan to continue dancing with what brung me for the last few...

[DFC]

Hey Ferris,

What is all this super new equipment you have at your disposal. Have you a new ACC? New equipment that the rest of the ATC world would love to have perhaps so that they can stop using headings to provide separation?

Let's say I am on your frequency and you expect me to track due east 090 along an airway. You base separation on that but later discover that I am drifting off track to the south towards the other aircraft which is now at the same level...........what are you going to do?

Regards,

DFC

[ferris]

Ferris, despite some good arguements , you are beginning to sound like a K**b!

Fair enough. If the facts are getting too hard, I'll give up. Just because people don't like those facts coming to light, doesn't make them less factual. Unless I missed some of your reasoning? Oh, there isn't any.
You base your dismissal on crews making errors. Therefore there is no merit in my argument, right?
How simple is this? If the aircraft doesn't begin tracking where you expect, when you say "track direct XXXXX", then you don't base sep on this. How friggin basic do I have to make this? Faaarrrck. If the aircraft busts his level, do you use level separation? geeez.

The majority of level headed responses on this thread appear to be in the same ballpark. As to why?

It's called "convergent thinking".

As one of your UAE colleagues texted me the other day, "we don't need headings 'cos we don't have any conflicts

If you want to make stuff up, great.
VFF

errors are made in setting the correct nav mode

I'd bet if you did the research, mode errors would crop up far more often due to radar vectoring. Telling an a/c to "track direct XXXXX" is pretty simple really, and dismisses the errors you are alluding to.

The fact is, to most controllers I know, headings are, (rightly or wrongly), a more reliable method of insuring lateral separation

Something I am asking you to question. After all, it's not necessarily a fact , it's a perception .

Ahh, DFC. Embarrassed, and trying a different tack?

What is all this super new equipment you have at your disposal

It's called RNAV and RNP.

equipment that the rest of the ATC world would love to have perhaps so that they can stop using headings to provide separation?

They have it. In fact, in the UK, apparently they are doing so as we speak. Whether they know it or not is another thing. Re-read the thread. Obviously there is a massive gap in knowledge between what is happening at various places.

Let's say I am on your frequency and you expect me to track due east 090 along an airway. You base separation on that but later discover that I am drifting off track to the south towards the other aircraft which is now at the same level...........what are you going to do?

Since your track-keeping is inadequate, I would apply another form of separation, most probably radar vectors- as the most efficient form of a/c nav (own nav)is no longer available to me, I would have to assume responsiblity for nav. I would enquire as to whether you thought ops were normal (in case the crew were engaged in some illegal activity that could be rectified, such as a comet-watching jaunt). I would also be obliged to co-ordinate with the next agency, to see if they would accept an a/c with faulty nav. I would then take steps to have you grounded, so that your navigation equipment could be repaired. Your point?

[Guy D'ageradar]

It appears to me that so far, the only two in favour of using "track to " as a form of separation are both aussies. Must be either the VB or all that time spent upside down!!!

Bon Journée!

[AirNoServicesAustralia]

Bottom line is you lock an aircraft on present heading, you in no way ensure they stay on present track. If you aren't ensuring they stay on present track, how can you say you have separation assurance.

You will find a lot of Aussies whether in Oz or working overseas, do use own nav separation, because we have realised that aircraft can navigate more accurately and allow for extremely large wind shifts in flight (as we have in abundance in Oz with the whopping jet streams, and also here in the Middle East) and still stay on track. On too many occasions have I been on the recieving end of a hand off from a perfectly well trained pom, where two aircraft on supposedly parrallel locked headings come to me 6.5 NM abeam and rapidly converging. Each time when the annoyed call is made to the Pom, there is a great deal of head scratching made on their part and they just can't fathom how their separation assurance didn't assure separation.

I know us colonists are just little upstarts who know nothing and could never teach you anything, but maybe just once you could all have a look at the calender, see that is is 2005, and move with the times.

Finally aircraft everywhere have been known to deviate around that little cell of weather without asking, whether on heading or on their own Nav. Both practices are illegal and should be reported as such.

Again finally one point I would like to make, is the number of times I have been given aircraft that were told to maintain present heading, which was at the time the same as their own nav track, and were subsequently forgotten about. Yes, the controller should not transfer them still on that heading and the pilot should report they are on that heading on first contact with the next sector, but in alot of cases neither of these things are done. The result is the aircraft flys through the next waypoint doesn't turn and the poo can hit the fan big time. The alternative is the aircraft is on their own nav and they get the to the waypoint and make the turn as flight planned. This whole heading thing creates workload, and increases risk.

[PPRuNe Radar]

Just for the avoidance of doubt, can those who use 'own nav' track separation advise what RNP they have in force and what the minimum distance is between the tracks they are advocating separate themselves without any ATC intervention ??

Just interested to see if your figures tie up with those laid down by ICAO when deeming routes are 'separated'

Until we also get enhanced downlinked parameters which show the FMC activated route and what mode the aircraft is being flown in, ATC can't be 100% sure where an aircraft on it's 'own navigation' will be going - just the same as when it is on a heading.

[ferris]

We have RNP5 here.

The reason behind your question intrigues me, and I think may be one source of the reluctance discussed. Do you misunderstand ICAO, I wonder, when you talk about recommendations for sep for a/c under their own nav (as DFC did)?

without any ATC intervention ??

This is the key bit. I gather you consider "ATC intervention" to be taking control of the headings? I think you are missing the fact that radar identified a/c are separated BY THE RADAR STANDARD, regardless of who is doing what. RADAR is the ATC intervention. If 2 a/c are on their own nav, the RADAR STANDARD doesn't cease to apply, or conversely, the a/c do not have to be on assigned headings in order to use the radar standard. Not anywhere I've worked, and not according to ICAO. The "route standards" you allude to, apply to procedural control . Also, if you have vectored an a/c off the airway, then you tell him to "track direct XXXXX" to regain his planned route, what standard applies while he is off the airway? Radar, of course.

Your last paragraph takes the cake.

Until we also get enhanced downlinked parameters which show the FMC activated route and what mode the aircraft is being flown in, ATC can't be 100% sure where an aircraft on it's 'own navigation' will be going - just the same as when it is on a heading

Without these 'downlinks', you are prepared to give the pilot an instruction (either headings or levels) which you will rely on for separation, based on his readback (and radar monitoring). Why, then, is that safer than having him readback "track direct to XXXXX" (and radar monitoring)?
If you acknowledge that headings are no safer (as you appear to do by your last sentance) why do people waste their breath with them? To me, the fact that the a/c are asked their headings, before being locked on them, says it all.

The world is moving towards more control from the cockpit (not neccesarily the pilot) with ADS-B, flex- tracking etc etc. The change-resistance shown here is significant. And doomed.

[PPRuNe Radar]

One day Ferris may just answer a question with a straight answer ? How far apart are aircraft when you are monitoring them on their own tracks and are happy to do so ?? Clue .. the answer should be a number of miles

I'll answer your other details when I have more time.

Incident statistics in the UK prove that 'monitoring' radar separation, rather than taking steps to ensure it (however that is achieved), is quite high on the list of causal factors in ATC culpable losses of separation. Ask the Safety Regulation Group of the CAA

Distraction, workload, complexity of other tasks .... a whole host of things can take your eyes of the ball for a period of time.

Might be fine in quieter airspace but keeping your eye on everyone when it is busy and immediately picking up errors which might erode separation when it is already close to the minimum has proven that humans are not too hot at monitoring compliance. Unless you're an Ozzie it seems

[Nomorecrap]

Aw Gee,

Did we have to play the Aussie bashing card so early in the piece?

Letting that one through to the keeper (b. Warne c. Gilchrist), I will answer PPrune Radars question from my own viewpoint:

Just for the avoidance of doubt, can those who use 'own nav' track separation advise what RNP they have in force and what the minimum distance is between the tracks they are advocating separate themselves without any ATC intervention ??

I don't care - neccessarily know or need to know, if the separation standard is Radar. (Is that not the point Ferris is making?)

Just interested to see if your figures tie up with those laid down by ICAO when deeming routes are 'separated'

You are talking route tolerances for procedural separation surely. (Love that word 'deemed' separated. How ambiguous. How ICAO!)

Until we also get enhanced downlinked parameters which show the FMC activated route and what mode the aircraft is being flown in, ATC can't be 100% sure where an aircraft on it's 'own navigation' will be going - just the same as when it is on a heading.

Scuse me? Then WHY are headings being advocated if this is the case?

Radar is used where higher densities of traffic require closer/lesser separation standards than can be provided by procedural standards - the argument about ensuring separation assurance by considering the tolerances of the stated RNAV or terrestrial NAVAID tracking seems to totally negate the advantage of the Radar in the first place, no?

What distance am I happy to monitor? greater than 3 miles, with a trend to remain greater, be that through diverging tracks, diverging or parallel headings at held Altitudes, diverging headings or tracks at level changes.

Perhaps in the interest of understanding the counter argument I should pose the following question:

At same altitudes, under what criteria / minima would you allow aircraft on own navigation to be left on own navigation? What about aircraft crossing levels?