TRACON (novel) and TCAS issues

[AnthonyGA]

I looked for some previous discussion on this but didn't find any.

I've been reading Paul McElroy's novel TRACON (published in 2000). For those who haven't read the novel, which is about ATC, a key plot device is the notion that TCAS is occasionally dangerous because it only extrapolates the current movement of an aircraft and cannot know the intentions of ATC. For example, if TCAS sees two closely-spaced aircraft approaching each other, it may issue an RA to the pilots, not knowing that ATC has instructed the aircraft to level off at altitudes that avoid any conflict. Thus, the TCAS might give instructions that conflict with ATC's plan, and might actually increase the possibility of a midair rather than decreasing it.

The novel tempers this a bit with a bit about equipment at a few airports being defective due to the machinations of an evil senator, etc., but since the problem as stated in the novel seems kinda sorta plausible, I've been wondering about it.

So, my question to real controllers is: Does TCAS ever enter into conflict with ATC? If so, what kinds of circumstances lead to the conflicts, and how are they resolved? I was under the impression that TCAS is a net gain for safety, and even the novel does not assert otherwise, but it does claim that in a few cases TCAS can greatly increase the risk of a midair. Is this just a plot device for the novel, or does it have some basis in fact?

I read that a more advanced version of TCAS that could give turn instructions in addition to climb and descend instructions was in the works, but has been largely abandoned. Also, the novel points out situations in which TCAS might order a descent, not knowing that terrain awaits below, and that this is a risk also (which seems plausible). The novel asserts that (in the U.S.) ATC is not allowed to give any instructions to a pilot who has announced that he's following a RA, and I wonder if that is true, too.

(P.S.: I have no particular fear of flying and I like aviation, so feel free to give the straight story—no need for any reassuring talk about how safe air travel is.)

[djdruid]

A well-known example of TCAS vs. ATC is the Überlingen accident, see http://en.wikipedia.org/wiki/2002_%C3%9Cberlingen_Mid-Air_Collision

[chevvron]

The MAJOR problem with TCAS is that too many pilots rely solely on it rather than looking out of the window in airspace where carriage of transponders is not mandatory, hence there is always the possibility of responding to an RA and colliding with an undetected non-transponding aircraft.

[porra]

I once questioned a DLH B744 pilot on having turned to the right after I had put him on a vector "FOR TRAFFIC" - his explanation was "I turned to avoid a RA..!"

Stupid guy actually turned TOWARDS the traffic - I had to politely remind (read teach) him that the traffic as shown on his TCAS display is not correct in Azimuth...

And, as far I know - since Uberlingen - TCAS RA supersedes any ATC instruction all over the world!

P

[1985]

The novel asserts that (in the U.S.) ATC is not allowed to give any instructions to a pilot who has announced that he's following a RA, and I wonder if that is true, too


Thats true in the UK aswell. Once the pilot is responding to a RA they become responsible for their own separtion. The standard response to a "TCAS RA" from a pilot is "Roger". We can give advice on headings technically but i wouldn't.


Does TCAS ever enter into conflict with ATC?


Yes it does. Generally when one of the aircraft has a high rate of climb or descent and TCAS responds as if the aircraft is going to pass through the level of the other, not stay level separated. It can in part be mitigated by giving traffic information but sometimes that is not possible.

[roljoe]

Hi, the subject highlighted by Anthony concerns mainly the controlled airspaces...(in uncontroled airspaces, tcas is a plus but the main rule remains "see and avoid")..

To answer to anthony, an RA becomes the rule to follow in a potential conflicting situation, (whatever the atc instructions could be) and in europe atc is more and more aware about giving smooth instructions to avoid an activation of RA..

for ex..reduce the R/D or R/C while approaching a crossing traffic..

rgds

[1985]

in europe atc is more and more aware about giving smooth instructions to avoid an activation of RA..

for ex..reduce the R/D or R/C while approaching a crossing traffic..



True, however reducing ROC or ROD is not upto ATC, thats up to the pilot. We issue ROD or ROC restrictions for separation not to avoid TCAS.

[roljoe]

Hi,

as pilot that' s my feeling ..but I'd like to add that sometimes atc is requesting you to use a R/D of 2000ft/min to loose 3000 ft..and that's where I will adapt it considering the close'in traffic on tcas to avoid such an activation..(specially under french and german atc)
rgds

[ATCO1962]

My understanding of ROC and ROD in TCAS mandatory airspace is that aircraft are to reduce their rates to 1000fpm or less as they approach 1000ft of their assigned level. I have no document to support that but have a hazy memory of a pilot in a large company telling me that that is so. Such a rate of climb or descent should prevent any kind of RA but may still result in TA being given by the onboard equipment. Can anyone confirm that?

In any case, I still always give traffic information to closing traffic when one or both aircraft are climbing/descending, letting them know that they'll be stopping 1000ft above/below the other. Works a treat and everyone seems happier:ok:

[Denti]

At least in europe that is not really mandatory, but at least in RVSM airspace it is advised to not use more than 1000fpm in the last 1000ft. But there is a lot of airspace outside of the RVSM part where there is not a requirement like that.

We have a company SOP though to not use more than 1500fpm outside RVSM in the last 1000ft before level off and not more than 1000fpm if there is a TCAS target within 5NM around us and 2000ft around the cleared level. Inside the RVSM airspace its a flat 1000fpm during the last 1000ft.

The only problem is an ATC instruction of a rate higher than 1000fpm until reaching which is of course at least a technical possibility.

[zjxgator]

As a controller in the US I always let the pilots know if I have a fast climber (military jet) coming up under them so if they get an RA they will know to ignore it. I've never had an aircraft turn into traffic with an RA, most pilots seem to know that we will let them know about the traffic around them.

[Gonzo]

As a controller in the US I always let the pilots know if I have a fast climber (military jet) coming up under them so if they get an RA they will know to ignore it.

....and they do ignore it? I thought the whole point of Uberlingen was that RAs must be obeyed.

[chevvron]

xjgator; what class of airspace are you operating in? If it's D,E,F or G I would agree with giving traffic info as your traffic could get visual contact and decide there's no need for RA action; not strictly according to the rules I know but we find this happens in our local class G; it's really up to the crew.

[Gonzo]

True....I'm too used to Class A!!!!! :hmm:

[zjxgator]

I work in all classes, we work surface and above in my area. When we tell the pilots about the traffic they then know that the RA is a false reading. We have alot of military here and the fighters climb and decend rather quickly sometimes. Also when we use visual seperation the pilots might get an RA and know to ignore that.

[West Coast]

I do appreciate the early heads up and it may take care of the situation earlier but there's no way a heads up advisory is going to cause me to disregard a RA. Even if it was my desire to disregard it, the SOP requires action.