TCAS RA's

NovemberEcho

NovemberEcho

Dergs favorite member
at my first facility in Florida, although it wasn't something we tried to do, due to our airspace layout at times we often would have to send IFR directly or almost directly over/under VFR by 500 ft. This never elicited any problems from the airliners (RJ's and 737's). This was class E airspace.

At my present facility, it seems in these situations the airliner will take a TCAS RA 95% of the time when within 500' vertical sep of a VFR, even when I tell them that the VFR has them in sight no factor etc. this facility is MUCH busier and often what happens is the IFR airliner winds up climbing or descending into IFR traffic while avoiding a VFR. This happens in both class E and class B. Why the difference? Why in some locations will 500' sep not set off a TCAS but it will in others almost every time? It usually winds up taking a safe operation and turning it dangerous when the TCAS doesn't bother to tell you about the guy it's telling you to climb into.
 
It all has to do with the computer programming of the TCAS computer. If another aircraft gets, or has a projected path, into my "bubble" then if will follow it's protocols and give us a TCAS advisory and then a TACS RA if the path of the two airplanes does not change. Even if we have another aircraft in sight, know it's not a factor, our FOM will not allow us to not comply with a TCAS RA. If we get too close to another aircraft because of reacting to a RA, then the whole process is repeated. What's frustrating on our end is trying to climb out of the NYC area where all the traffic is being kept at an initial altitude of 5000' and then VFR traffic comes bombing down the Hudson River at 5,500'. I had the guy in sight for about 10 miles and knew he was going to give us a RA, but by the time we were able to query departure, we had already gotten the RA. If the controllers know that all the departing traffic is going to be at 5000, why do you allow for VFR traffic at 5,500? Why not send them up to 7,500 or 6,500, or down to below 3,500 to avoid all the departing jet traffic? Keeping them away from our initial assigned altitude would be greatly helpful.
 
The proper legal vertical separation between IFR and VFR in a Class B is 500' (unless a heavy is involved, then it's 1000') Why does your programming give you an RA for legally separated traffic?

As for the 7500 or 6500, keep in mind that area has departures/arrivals going to at least 3 major airports. Just about every altitude is taken. I'm assuming you're departing from LGA. EWR east departures are at 6,000 above you (plus the departure corridor/final is parallel to the Hudson aob 3000), and JFK west departures are above them. LGA arrivals are below you at 4000. At least at 5500 he is in my Bravo and does what he's told. At 7500 he can tell me to screw myself and squawk 1200. I'd rather be talking to him. I assume you'd prefer I be talking to him as well.
 
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NovemberEcho said:
The proper legal vertical separation between IFR and VFR in a Class B is 500'. Why does your programming give you an RA for legally separated traffic?

As for the 7500 or 6500, keep in mind that area has departures/arrivals going to at least 3 major airports. Just about any altitude is taken. At least at 5500 he is in my Bravo and does what he's told. At 7500 he can tell me to screw myself and squawk 1200. I'd rather be talking to him. I assume you'd prefer I be talking to him as well.
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Because the system requires IFR separation and is build for two airliners coming at each other, not an airliner and SE piston VFR traffic. This comes down to legal vs. safe. I do not trust a VFR sight seer to maintain their altitude that precisely while we are blowing by them at 250KIAS. Things can go wrong very quickly that close together and an extra layer of safety would be appreciated.
 
A lot of it can have to do with when the last time their (GA traffic) altimeter and transponder where checked and calibrated properly. A lot of times ATC will tell me they are 500' above or below but on my TCAS shows that they are 400'. If they are sightseeing the NY skyline and not maintaining a steady altitude than my TCAS will give an RA based on a trend vector of them closing that gap. It also doesn't matter if the other traffic has us in sight, we have to have them in sight to not comply with an RA. The TCAS does take into account other traffic and we practice in the sim getting an RA that causes another RA.

I can just about guarantee the problem is the GA sightseers that have no autopilot rather than an airliner that most likely has the autopilot on and is calibrated for rvsm.
 
Yes that is exactly what I'm saying. Mode C on the IFR will show level at 5000, mode c on the VFR shows level at 4500. Airliner takes a TCAS RA 95% of the time. What I found most unusual though is they did not do that at my previous facility, but here they do it almost daily.
 
I wonder if different carrier's have different rules on when they have to respond to an RA. If Skywest (for example) sees the VFR traffic and gets the RA, do they need to respond? Does that vary by airline? Would American, United, Flagship, etc., have all the same requirements for RA responses....I don't know.
 
I've worked at three different airlines and the requirement at all them is to comply with the RA unless you have the traffic in sight and they would still prefer you comply with it in case it's traffic you don't see.
 
NovemberEcho said:
The proper legal vertical separation between IFR and VFR in a Class B is 500' (unless a heavy is involved, then it's 1000') Why does your programming give you an RA for legally separated traffic?
.
Click to expand...

The system isn't designed to take atc separation into account. The system is designed to keep airplanes from hitting each other. It's the ultimate layer of protection in case of a separation error. I like it. There are ways to fly the jet to reduce RA's like reducing vertical speed if you suspect a conflict. Doesn't always work.
 
I'm not sure why the amount of RAs would increase from one area to another. Level and 500ft separation should be fine.

One level and one climbing/descending to 500ft separation will usually cause an RA.

Both aircraft climbing/descending to 1000ft separation will sometimes cause an RA.

NovemberEcho, maybe you just have bad luck in the new airspace.
 
If I can avoid it I won't put a VFR and an IFR 500 feet apart. Like DE727 said TCAS isn't based on any format of atc separation. Probably not possible in congested airspace. Best you can do is be sure to call traffic, hope the crews are savvy and reduce their rate of climb/descent.
 
I would love to be able to not have to put IFR/VFR 500' apart but in my airspace sometimes you just don't have a choice lol
 
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