TCAS question

[JuJu]

I was out flying VFR this weekend in a Skyhawk 172 at the oxygen defying altitude of 6500' and I had flight following. I heard center call me as traffic to an American Eagle Saab so I started looking (they said they were looking too). Then center called me and I had spotted the Saab about 5 miles out to my 2 oclock at 7,000' and I said traffic in sight. Center then told the Saab that the Skyhawk had them in sight and for us to maintain visual separation. I watched the Saab come closer and closer and realized he would pass right over and in front of me. Not a problem I figured. As he got closer, it looked like he was climbing, but I thought that might be an optical illusion. As he went by, I heard center call and ask him why was he climbing? The Saab responded they had received a TCAS alert and were climbing, but returning to 7,000'. Center said "fine". I continued on my way, buy kept thinking about it.

Since I had traffic in sight and kept it in sight, with 500' altitude separation and maybe 1/2 or 1/4 mile horizontal separation, was that enough? Should I have taken any different action? Center didn't seem too concerned, although I would imagine on the screen it looked pretty close. Should I have said anything before or after the Saab came cruising above me? Should the Saab have known I was there and not responded to the TCAS? How sensitive is the TCAS to situations like that? Do you think any ASRS reports are in order for anyone involved.

Questions from a humble new GA pilot to the professionals on this board. I've picked up a lot of good tips from reading here. I have no professional pilot aspirations, just enjoy boring holes in the sky and exercising freedom of movement while we still can.

 

[NEDude]

If you do not have traffic in sight and you get an RA (resolution advisary) you must follow it. It does not matter if the other guy says he has you in sight.

 

[flyhard]

If it was a TA or traffic alert it provides no direction on where to go you as
the pilot have to make that determination. It simply alerts the crew that an aircraft is inside the designated alert ring. A TA is only advisory not mandatory. A RA on the other hand is mandatory to follow whether the other aircraft is in sight or not

 

[enigma]

Neither of you did anything wrong. The only thing I wonder about is the inaction of the Saab crew that lead up to the RA. Obviously, you were showing up on their TCAS so I wonder why they waited so long to take some action to avoid traffic that they had failed to gain a visual on. (I know, poor english) If I have a target on a collision course and only 500 ft below me, I don't wait for the RA to begin some sort of evasive manuever. I usually recontact ATC and ask for a vector away until I gain visual contact. Good question.

regards,
8N

 

[501261]

If both airplanes are straight and level (not climbing or descending), you will get a TA, traffic advisory, if there is someone within 500' and 3 miles of you. That's what the TCAS computer thinks is too close. If the traffic gets any closer the TCAS will issue an RA, resolution advisory, you are required to follow the RA. That's probably what happened here, the Saab crew thought they were fine in starting a descent, but it was too close for the TCAS, and once they received the RA they were required to follow it (and for good reason too, because the crew doesn't know if the RA they are receiving is from you the "known" traffic, or someone else that just "popped" up.

 

[saabcaptain]

Another thing to note is if you had 500 feet separation a TCAS RA would not have been issued provided both you and the Saab were level. Most likely... and this happens all the time (I fly Saab but this works for all TCAS aircraft) is that the altimeter settings and calibration of the static system in each aircraft (your aircraft vs. the Saab) is a little different and the noted separation becomes 400-600 feet due to error. If it drops to 400 feet you get an RA. Often ATC points out VFR traffic "500 feet" above or below but the separation varies by 100 feet or so from what ATC said and if it varies to the low side... RA. Therefore I tend to ask ATC to allow a turn for our aircraft to avoid 500 foot low/high VFR traffic when enroute by 1 1/2+ miles plus horizontally to just avoid the chance the 500 foot separation won't hold.

 

[mar]

"Traffic in sight" = "visual separation"

Good responses from everyone, but one small point that a lot of people don't realize: When you report traffic in sight, ATC will automatically instruct you to maintain visual separation--and an agreement to maintain visual separtion means you take on wake turbulence avoidance as your own responsibility.

For this reason, if things are very marginal (low scud, haze, any form of precipitation or just plain busy) I'll often report negative contact even if I have the traffic in sight. One reason is, I may have it in sight now but I'm afraid I may lose it, or, it may be a heavy on a rapid descent to a visual approach and as the original poster mentioned sometimes optical illusions can make you misjudge it's flight path. In this situation I'm not too excited about agreeing to avoid it's wake turbulence.

I've flown through the wake of a DC-10 and it ain't no fun.

In sum, just 'cause you have him in sight doesn't mean you have to call him sight. If you do call him in sight you agree to separate the metal and the wakes.

Fly safe.

 

[328dude]

Good Point Mar. I can't remember when but the old PSA plane that had that same problem with traffic and ran into someone else is just one example. If your busy, or your at a busy airport with arrivals everywhere, why call it in sight anyway unless your absolutly sure.

Heres a TCAS question though. Excuse my lack of knowledge on the guts of the thing.

Airplane X and Airplane Z are on a collision course at the same altitude both with TCAS. Will a RA givin by the TCAS ever be the same for both aircraft? Say yours gives you the warning to climb, how does the other aicrafts TCAS realize that giving you the order to climb would be a very very bad idea?

Just curious how this thing figuares things out.

Don't hate me cuz im stupid.

 

[501261]

Re: "Traffic in sight" = "visual separation"

One BIG thing to remember about all these ATC/TCAS traffic issues is that ATC only seperates IFR traffic from IFR traffic and TCAS/TCAD's are basically only transponder recievers (have you looked at the shape of some of those Mode C's on a C150 at the local GA airport)!

Seperating IFR traffic from VFR traffic is normally on a workload permitting basis. ATC does a great job seperating IFR from IFR, and I'm almost never worried about hitting other IFR traffic, its that VFR bugsmasher, with an INOP transponder, not talking to anybody, that scares the crap out of me!

Flying IFR, with a TCAS II mode 7, all the lights on is all great, but the most important FAR you need to remember is SEE and AVOID!

 

[JuJu]

Yup, see and avoid. Even on this trip with flight following, I watched a Mooney climb right towards us and past us. After he went by, ATC called and informed me of "traffic no factor". I know I'm on a time permitting basis etc.

But I called for flight following on this trip because I knew I was going to be passing through an area with a lot of traffic arriving into DFW (I was passing pretty close to one of the VOR's outside the area used as a reference point on one of the STARs, and I know this because my former CFI told me about it and told me to be careful and best to let ATC know what I'm doing when I'm in the area so that I'm not just another bugsmasher wandering around the sky and getting in the way.)

But back to one of my original questions: With visual contact made but I see it's not a collision but still maybe kind of close, should I have spoken up and said something like "Center, Cessna 123 has the Saab, visual separation but he will pass right over and in front of me", or would that be just tying up the freq? Note, on this day at this particular time, the frequency was active but not overly busy.

Thanks for good responses and info on TCAS. I know the basics of it but I've never been able to see one in action.

 

[501261]

But I called for flight following on this trip because I knew I was going to be passing through an area with a lot of traffic arriving into DFW

Exactly, the closer you get to a busy airspace, the busier the controller gets, the less "workload permitting" help he is to you!

But back to one of my original questions: With visual contact made but I see it's not a collision but still maybe kind of close, should I have spoken up and said something like "Center, Cessna 123 has the Saab, visual separation but he will pass right over and in front of me", or would that be just tying up the freq? Note, on this day at this particular time, the frequency was active but not overly busy.

Nope, nothing else needed. The fact that you were able to identify that it was a Saab is good enough. The only thing you might add to that is that you are level at 4500' (since your mode C might be reporting 4600' or 4600') so that everybody's happy.

Occasionally I'll have some VFR traffic on the box 300' below me (should be 500' below) and I'll have approach verify that they are level at their assigned altitude (XXX Approach, we're showing traffic 12 o'clock, 5 miles, 300' below us, could you verify that"). most of the time it's just a bad altimeter setting.