I am looking for some information on how TCAS works relatively close to the ground. I know that at some point it no longer gives RAs, but I am under the impression that when that happens could be different under different circumstances.

Specifically, a new RNAV approach goes through an unofficial VFR corridor below some class B airspace that commonly has aircraft transitioning around 1000' to 2000' AGL. Aircraft on the approach (mostly medium-sized airliners) will go through the area around 2500' AGL and below.

From an ATC standpoint, I am curious if, at those altitudes, I can expect RA's and climbs, or are pilots going to receive TCAS advisories as if they were in the pattern at the airport?

 

I can't remember the altitude I got the RA at but it was low 3000-4000 maybe and it just stated to monitor Vertical Speed since it gave us a descent down low.

 

Unofficial VFR corridor? What's that, and do IFR aircraft not get the expected (required) spacing in your TRACON?

TCAS won't fly you into the ground on an RA, but 2000' - 3000' is plenty high to get RA guidance.

 

I'm guessing that its an area near or below the class b that VFR aircraft usually use to transit the area. Since its not a formally published VFR corridor, it's unofficial (pilot/local knowledge).

IFR traffic isnt required to be separated against non-participating VFR traffic that is outside of class b airspace. That's the issue here- the procedure takes aircraft outside of the protected class-b.

To the original poster, I'd be interested in knowing what airport/approach you're referencing.

 

My airplane has a bunch of TCAS logic altitudes that I don't remember exactly. Below around 1200 ft it won't participate in an RA. Below around 500 feet it won't TA. In all cases its still communicating so another equipped airplane might get an RA.

I could be wrong on the altitudes and I don't know if they're standard (tso?)

 

This happened a few years ago, but over the course of a few months I would notice DHC8s (all from the same company) not climbing and less than 2000agl. When questioned the response was TCAS is telling us to decend. No traffic observed. I assume it was some sort of logic issue and maybe with a single aircraft. I haven't seem it since, but it seems kinda scary to me.

 

The area is below a shelf of class B airspace. There is no minimum separation responsibility between an IFR aircraft and VFR aircraft in class E airspace. The majority of VFR aircraft in the area in question are not talking to ATC.

There are two new RNAV approaches to runway 22L at MDW that are supposed to be here in January (last I heard--they were supposed to be out this past June, but were delayed, I don't know why).

Today, MDW usually does the ILS runway 31C with a circle to runway 22L. This keeps airplanes inside the shoreline and for the most part inside the class C airspace. The new RNAV-Y and RNAV-Z approaches to 22L start out over Lake Michigan (at SAILZ, which is on the FISSK2 arrival), then turn slightly to stay south of downtown Chicago and onto the final.

I don't have the charts in front of me (I've seen them, but don't have access to them right now), but the crossing fix outside the shoreline is something like 2,900 MSL and inside the shoreline is 2,400 MSL.

By unofficial VFR corridor, I mean that aircraft go up and down the shoreline underneath the class B all day long without talking to anybody. (The floor of the class B in that area is 3,000'). I was told there would be no changes to the class B or C airspace in the area.

So obviously, I can see a potential for traffic conflicts in that area. My question is on whether I am going to be expecting to issue traffic and aircraft to continue on the approach, or if there are going to be RA's and aircraft climbing. I don't know the entire impact of this approach on the overall operation at MDW, but I can see conflicts popping up now and then and I'd like to start thinking about handling those.

 

On the 737NG, Increase descent RA's are inhibited below approx. 1500 AGL. Descend RA's are inhibited below approx. 1100 AGL. RA's are automatically inhibited below 1000 ft and all TCAS voice annunciations are inhibited below approx. 500 ft.
In the scenario you describe, you will be having jets get climb RA's if the VFR traffic is climbing or if it is less than 500 ft from the jet. Most likely the jets will just get a "monitor vertical speed" TA and life will be good.

 

The TCAS takes into account the rate of change of the jet, descent in this case. On most approaches this is near 7-800 FPM. The TCAS knows what traffic may be a potential conflict, based on the target's altitude.

I often adjust the rate of climb or descent to avoid giving myself an RA. Most often for such as 'climb to 1 zero thousand, traffic at 11'.

OBTW, I'm one of those guys who fly's below the ORD class 'B' at times, along the shore. I do give a few 100' of buffer on the altitude. I also stay below the SWA traffic at the JOT VOR and the corridors for ORD.