bookworm

I think you're also operating under a misconception, PPrune Radar, about the alternative to TCAS and TCAD. You're thinking like a controller.

There's no doubt that ACAS systems are unsuitable for traffic management. To have traffic jumping around the screen while trying to work a busy sector would be madness, and so the sorts of azimuthal accuracies that are obtainable with ACAS are orders of magntitude less than the requirements of radar.

But the pilot's situation is a little different. Imagine that, as usual, we're in a situation where, because of controller workload, traffic density, or both, a radar advisory service is unavailable or impractical. So some helpful controller is calling traffic under a RIS.

And he calls "G-CD, traffic 12 o'clock, 3 miles, opposite direction, height and type unknown". At a typical closing speed of 300 kt, we have a little over 30 seconds to the potential impact. So we look for a few seconds, in the 30 degree arc -- that's the resolution of the clock code too -- ahead of us. And we see nothing. "G-CD nothing seen, update please?" "G-CD say again please, I was on the landline..." "Update on the traffic you called for G-CD please?"

A number of possibilities now emerge:

"G-CD he's now in your 10 o'clock, half a mile, passing down your left hand side"
"G-CD he's now in your 2 o'clock, half a mile, passing down your right hand side"

In either of those, I hold course. I never saw the traffic and had no way to manoeuvre to increase separation. If I turn left in the first case, or right in the second case, I make matters worse.

"G-CD he's still in your 12 o'clock, half a mile, opposite direction"

Course of action now? Prayer, perhaps?

Now compare that to ACAS. I find an aircraft at 12 o'clock, 3 miles, no altitude indication. I look out, but see nothing. I look again at the display for a few seconds. The relative bearing is increasing, it's now at about 12.15, 2 miles.

What do I do? Turn to the guy next to me and say "well the azimuthal resolution of TCAS is only 30 degrees so let's just sit here and see what happens?" Of course I don't. I commence a gentle turn to the left to increase the separation, looking out all the time. And I do that because as a pilot I'm a safety manager, and for every occasion that the manoeuvre takes me closer to the other aircraft, there are 99 occasions when it really does increase the separation. Those are good odds. And without guaranteed separation from ATC, odds are all I have when the notoriously unreliable human eye fails me.

PPRuNe Radar

Compare

with

They are in effect the same thing, i.e sometimes the information will not be accurate in the horizontal plane, although you give a numerical value (backed up by a report somewhere I guess).

ACAS is nothing like a controller since they are trying to provide different things. A controller is trying to meet separation standards, which are large. ACAS is monitoring other traffic with a view to preventing a collision only. It won't care what your separation is, just as long as you manouevre out of the impending collision. It will also wait until an aircraft is potentially going to be much closer than a controller (in perfect circusmtances) would before giving a warning.

It is the last safety net you have left when ground based systems and your own eyes haven't worked. But it's still only designed for vertical resolutions. Being aware of the limitations is an essential part of the 'training' before using such kit in anger. But then no one insists on training apart from the large aircraft operators.

bookworm

Yes. I wouldn't for a moment argue with your point that pilots need to understand the limitations of ACAS, and that's particularly relevant to using it to manoeuvre in the horizontal plane.

I think there are three reasons why ACAS uses the vertical plane for resolution:

1) greater precision of the measured value (and trend) of level vs azimuth
2) easier 1-dimensional deconfliction problem in calculating an advisory
3) ATC tends to use horizontal deconfliction (vectors) so it's less likely to contradict ATC

All of those are reasons for preferring a vertical manoeuvre if possible, even as a human-initiated response to a TA. If it's not possible, using the azimuthal trend information to increase separation make sense.

IO540

Do you have a reference?

Systems such as the Avidyne 600 give you a reasonable azimuth reading, from what I have seen. Way better than nothing which is all we get most of the time it is most needed. Way better than an RIS under which the controller is not obliged to give you every contact because it is subject to controller workload.

The majority of traffic is not spotted visually even when you have a bearing (from TCAS or from RIS) even if you have four people in the cockpit all "knowing" where to look. A 30 degree turn or a climb/descent is easy enough to implement.

Don't knock a piece of technology just because it isn't perfect. One day it might be much better - when everybody is emitting GPS lat/long over Mode S 1090ES, just like airliners do already. But the altitude value will still be baro derived (instead of a precise GPS value) so it will be +/- 200ft, so if flying at 2400ft under the LTMA the smart choice will be to turn rather than a level change.

NigelOnDraft

Interesting article TCAS Traffic Display

Of note: So if we go down the route above of a "TCAS type" system, but with TAs only, it will be hard to relate to TCAS where we are not allowed to manoeuvre based on a TA (only).

I am not 100% against such systems... but the implications of bringing such systems in would need to be very carefully evaluated / applied. As above, what do you do if your "system" advises you, or indicates such that you wish, to manouevre whilst under a Radar Service? Do you disobey ATC?

Even if you "relegate" the system to vertical avoidance only (as above, whilst TCAS is pretty good at bearing info, it is fallible, and can easily make a safe situation catastrophic if believed), without an RA "decision" process - it can again lead to problems... TCAS uses "crossing RAs", where an aircraft above you, but descending, might ask you to climb... so a GA system where a pilot seeing an aircraft "above" (but not appreciate / judge it's high RoD) might descend and now create a collision.

If the sole purpose of such devices is to "lead your eyes to the traffic", and then take action based on the resulting visual sighting, then it would seem a benefit (provided the accuracy is such that it does not have you looking in the wrong place ). But a device that causes you to manoeuvre, based on it's information alone, is going to generate almost (?) as many problems as it solves IMHO...

NoD

IO540

Interesting article, NoD, but it relates to the pilot acting contrary to a radar control service such as one gets in CAS / airways.

The context here is GA, OCAS, where the only radar service under which the controller is legally permitted to suggest avoidance is a Radar Advisory Service. Very few pilots use RAS because ATC sends you all over the sky, often with 90/90/90 degree doglegs flown around nontransponding contacts which for anybody cares could be 5000ft below you. It's also not available most of the time you might want it, due to controller workload because the required separation is impossible to achieve in the actual traffic density OCAS.

So, with the kind of TCAS systems available to GA today, all one is going to get is other traffic azimuth/altitude information, and since ATC giving a Radar Information Service (the most common radar service one goes for, OCAS) aren't allowed to even suggest which way to turn, the pilot is never going to be acting in conflict with ATC.

I am afraid that in GA we are pretty well on our own and have to do our own "safety management".

When I fly airways, which I do occassionally, I fly FL100-FL190 and one doesn't need TCAS there because there ain't no traffic there anyway! CAT flies way above or way below, and the routings take one many miles away from any CAT climbing/descending in the busy terminal areas. Never flown a hold on ATC instructions, either. And other GA in the airways is rare as rocking horse s***

Rod1

“The majority of traffic is not spotted visually even when you have a bearing (from TCAS or from RIS) even if you have four people in the cockpit all "knowing" where to look.”

Glorious day for flying today. Turned the GPS and the Radio off (but the transponder on C). Got the map out and did a tour of Wales. Hit the waypoints +/- 1 min, spotted lots of other aircraft enjoying the weather (had to dodge a few gliders).

Maybe we should take a really good look at the biannual flight review. We could include some really accurate nav using the map, and check people are using a proper scan/lookout. Any pilots not to up to the job would have to fly with a competent companion for 12 months, they might learn something.

Rod1

ShyTorque

Rod1, but how close did you come to those other aircraft that you never saw today?

I suspect you don't fly with a TCAS/TAS because if you did you would understand that my question isn't borne out of sarcasm. You didn't spot all the other aircraft out there, because no-one does.

PPRuNe Radar, what you say is basically correct but the limitations of TCAS/TAS equipment are outlined in the instruction handbooks and the Flight Manual.

I have to agree with IO540, sometimes pilots are obliged to make a captaincy decision based on insufficient information.

mm_flynn

I understand the point in controlled airspace, but in class G following your TCAS procedures (which are being related as a rote set of prescribed actions) is even worse. You have no idea what the other aircraft is going to do, as without the other half of the system, the other pilot is just going to use his judgement on where he thinks you are and make some manoeuvre (or just hold his course relying on the big sky)

In real life controllers seem to worry a lot about some separation but none about others. On a busy hazy Saturday in the South there are lots and lots of light GA aircraft, sandwiched into a thin slice of air being asked in some cases to navigate with positional accuracy similar to PRNAV (i.e. the gap between Biggin and Gatwick airspace - which at its tightest is only 3/4 mile wide on each side of the M25) with the warm words of comfort - 'limited information service only due to high traffic density and poor radar performance'. I am not complaining, only pointing out that the GA traffic advisory concept needs to be evaluated in a world of separation expectations much reduced to those of IFR in controlled airspace - and where high quality radar information is only provided on an adhoc basis.

There are numerous posts where some GA pilot says 'I was on a RIS and someone crossed me 50 feet above me', controllers respond with 'well it is only information and you are supposed to be looking out!' (which of course is true). Having a bleeping things saying 'traffic closer and closer same level' is a huge step up from 'Sorry, was busy'.

Most of the arguments I have heard against GA traffic detection systems have been based around a logic that most things out there to hit aren't visible to these systems anyhow and having one may give a false sense of security. The fact that it is not as good as having TCAS, turbines, and Class A from the ground up is true but is not a good reason not to take the first step. (It reminds me a bit of the logic that GPS approaches are not perfect so we should stay with NDBs )

Vino Collapso

You are joking aren't you? Follow the M25, or is that PRNAV?

bad bear

here is a link to a low cost collision avoidance system
Flarm - Homepage
It is only £400 and needs no maintenance. Gliders have fitted these in large numbers and it does work in aeroplanes or helicopters too. This week alone I have had numerous warnings of traffic approaching from my blind spot or out of sun.
I remember an Australian flying school that fitted a similar system (ADSB) to all its training aeroplanes years ago, should UK flying clubs do the same?

The advantages are that you don't need expensive transponders, or radar, or ATC. A complete stand alone low cost life saving device.
FLARM will also warn pilots of obstructions (masts and ski cables etc) in many parts of Europe

IO540

FLARM does not appear to have a future in mainstream European GA because it has no ICAO and thus national CAA backing.

ADS-B is "coming" but it will be implemented using the Mode S data channel, known as 1090MHz Extended Squitter.

So if you have say a GTX330 Mode S transponder, and connect it to a GPS, it will radiate your lat/long. This already happens on airliners' Mode S installations but the resulting system is classified as Enhanced Mode S which is a certification issue for GA which currently uses Elementary Mode S (which AIUI is not permitted to radiate any parameter at all).

Then you need an ADS-B receiver and I don't know how that will be done. Maybe a GTX330 can do that bit too?

ADS-B would work well because it uses the GPS position which is ultra precise, and won't need the complicated direction-sensing multiple-antenna and very expensive installations which present-day TCAS systems need. In principle, a cheap PDA fed from a 1090ES receiver could act as your "TCAS".

But it is all many years away, in Europe, and as always there will be exemptions for VFR which will make it to a large degree worthless.

bad bear

With over 11,000 aircraft currently fitted with FLARM why should European GA wait for the local " Authorities on Civil Aviation" ?
It may not ( but probably could) meet certification, but it is here, now, working and saving lives.
If light aircraft pilots fit it this week end it could save them next week, not in 5 years time when full ADSB comes along. Light aeroplanes can use this now I believe and it does not need to be "certified".
Having said that, the risk of collision remains low. Over central UK yesterday there were hundreds of gliders and aeroplanes ( possibly 700+ and certainly 250 competition gliders alone) airborne within 50nm of Oxford between 1200 and 1700 amassing thousands of hours airborne without miss hap, and the vast majority without transponders or talking to ATC. How could that number of aircraft actually talk to one ATC centre and how could anyone separate all those contacts on radar? 700 aircraft 300 minutes! FLARM is actually designed to work in a high traffic density situation and could be the correct solution, but I am no expert. All I know is that it was worth me spending my hard earned money on.
I have given up on the light weight, low power transponder malarkey, I don't think it will ever happen.
bb

NigelOnDraft

Just to check, your FLARM thing will only work against other FLARM equipped aircraft? And what % of GA aircraft are so equipped, in say, the UK?

Obviously if it works, and it gets over say 50% of aircraft equipped... it becomes useful and worth having. But there's rather a chicken and egg before that?

Gliders I'd rather leave out of it - they are somewhat unique in "congregating" in areas risking collisions, and a close community where establishing the critical mass may work (or already has?).

IO540

Do you have a breakdown by aircraft type for that figure?

NigelOnDraft

IO540 You somewhat miss the point I was trying to make (no doubt poorly). TCAS gives an "indication" of bearing, but it is sometimes poor, and variable (tragets hop about in bearing but are of course unerringly accurate in range).

The article makes the point it is very easy, either by poor interpretation of, or poor accuracy of, the "bearing" to actually reduce the separation by taking action on it.

Take a head on target at 1NM when you wish to start avoidance, and a 5 degree "uncertainty" in bearing. If you are on opposing tracks, and 0 degrees off, then you would hit each other. If you are 5 degrees off you would be 500' apart - close, but you live. Trouble is, if that accuracy is misinterpreted / wroingly displayed, you may take the lateral action and cause the 500' to become nothing As was being implied by some (inappropriate) action taken in the article.

TCAS in fact can be 30 degrees out... what sort of accuracy can the GA systems out there guarantee? As you say and on an individual basis I can agree. Trouble is, for a manufacturer, or regulator, or indeed anyone, to encourage or mandate such systems causes the big "liability" word to enter the equation... where the consequences of it creating a collision outweigh leaving people to look out and bear the responsibility.

NoD

bad bear

sorry I am not able to give a current breakdown but there was some information on the FLARM website.

Indeed FLARM only works against other FLARM equiped aircraft

Indeed, chicken and egg. What did the guy with the first phone do with it?

DX Wombat

Probably made a few more as gifts for friends and family. Whilst it might be a good idea, I can't see the manufacturers giving them away free just so that we will all be persuaded to use them in the future.

PPRuNe Radar

ICAO Annex 10 Volume IV is a good place to start. Surveillance and Collision Avoidance Systems Standards and Recommended Practices. My italic bolding added.

vanHorck

Never thought of FLARM, but if it s well under 1.000 Euros..., why would we not?

Are there GA plane in the UK that carry it? Such as one who frequently fly near glider sites? Experiences?