plasticpi

We all have been trained to listen to TCAS over ATC. No confusion there. A while back, on a flight into IAH, we had a blip show up on our TCAS display at our altitude, converging. ATC had said nothing to this point. The captain (the flying pilot on this leg) and I kept an eye on the TCAS display, as well as outside, trying to find this traffic. A second or so later, we heard the "Traffic! Traffic!" alert, followed by "Descend! Descend!", and the VSI indicated it wanted 1,000 fpm down or more. The captain disconnected the autopilot and immediately started descending.

I got on the radio and told approach control that we were descending out of 9,000 for a TCAS RA. Approach immediately came back telling us that we needed to stop descending, we had traffic below us at 8,000 feet.

I responded, telling approach that we would continue to descend as long as our TCAS told us to, and that we were still descending.

The controller again insisted that we stop our descent. I again told him we were complying with our TCAS RA.

At about this point, the RA went away, and the blip that we previously saw on our TCAS display was suddenly indicating itself at 8,000 feet, not 9,000 feet like it initially showed.

We were at 8,500, and started climbing back to 9,000. I informed approach.

We were then given a phone number to call, and when we got on the ground, we explained this story to an approach control supervisor. When we got home, both the captain and I filled out ASAP reports, and nothing more has been done.

This concerns me because clearly the TCAS was wrong in this case. The controller was right. Having said that, we did exactly what we are supposed to do by obeying the TCAS before the controller.

It just scares me that by doing things by the book, we put ourselves in danger. Anybody else had something like this happen?

Is TCAS dependent on the Mode C functions of transponders? Those are very often wrong...

ogogog

ive had this happen to me as a controller in chicago,and yes i belive TCAS gets its info from Mode C.i would have given a traffic call only in your case, controllers have been told not to give clrns when a pilot tells us they are responding to an RA. yes it is scary.

wmupilot85

IIRC, with RA mode, that uses TCAS II which uses mode s. TCAS I uses mode c.

rickair7777

Do you know what the other aircraft was?

A mode-S transponder reports like any other xponder, but it can report the ALT in 100 ft or 25 ft increments. There are some known problems with ALT sources which provide only 100 ft data...if the mode-S thinks it's getting 25 ft data, it can misreport it's ALT which could cause an TA/RA for another aircraft.

TCAS I provides a TA only.

TCAS II provides a TA and RA.

Also, TCAS II will coordinate it's RA with the other aircraft, but ONLY if both aircraft have TCAS II. If the other aircraft has TCAS I or no TCAS, there will be no coordination...and the other guy might do the wrong thing.

TCAS has given erroneous RA's, due to suspected bad ALT data.

A lot of controllers were concerned when pilots were instructed to follow the RA exclusively, but there are no airliner hull losses due to correctly following an RA. The known hull losses always involved erroneous or confusing ATC inputs...and we almost had another one of those over DEN last year (I know one of the pilots involved in that one).

There have been a few near-misses due to bad RA's but so far they have a better track record than ATC.

Senior Skipper

May I suggest that you contact your airline, and more importantly, the TCAS manufacturer about this incident so they can examine it? I would classify this as a sreious incident at the very least. Maybe if the TCAS stores data, the manufacturers can recover it from both aircraft and see where the error occurred. Please do this quickly.

plasticpi

It's too late for that now, I'm sure, but I filed the ASAP and NASA reports that day when I got home. Aside from that, I'm pretty certain based on previous maintenance related things with my company that they would not have any interest at all in investigating this, as nothing is really broken, it was just a transient glitch, and it's not even certain the the problem was with my aircraft.

I don't know if it was just a standard reply, but I got an email from some commitee/board thing saying how there would be no further investigative or corrective action for our "deviation," due to our willingness to report it. It confused me a bit as I wasn't reporting something I did wrong, I was reporting a potential equipment/system flaw. But oh well.

Senior Skipper

That's deeply disturbing. After that accident in Europe we're taught to trust the TCAS over ATC. You've just been put into a situation that threatens that teaching. I've said it once and I'll say it again. NOTHING gets changed in aviation until the requisite payment in blood.

Bus Driver

One important thing also to know: Alt encoders have heaters in them. When the heater acts up or is intermittent, it gives erroneous alt reporting. This is especially true with GA a/c. If you go to Flightaware.com and pull up a known tail number and check out the flight profile, you may see the alt vary for a specific flt. That is the anomolly I am referring to. I am sure that most commercial and corporate a/c, especially those certified for RVSM probably don't suffer from this problem, so it was prolly a GA a/c.

EvilGN

question for the above post, if the heater in the encoder is the cuase of an erroneous reading, thats a problem on the "other" plane from the Original Poster...so in this case from the OP, we would want the investigation to look at their EQ, not your own. OP already said no further investigation will happen...was that his company saying that, or did the FAA say no further investigation. I agree that this is a REAL issue for this incident, and it needs to be resolved or somewhere down the road a new rule will be written in BLOOD after the fact.

To the OP, you should push this issue to your chief pilot directly, explain the situation and the subsequent rulings and stress your concern with a possible future issue, that issue being that it may not be your companies EQ thats fubar'd, but its possible someone else's is, and could cause an incident similar to the one that happened over Germany (IIRC)



Edit: The more I think about this, the more I feel this is a really really big deal....I realize we ***** on here about all sorts of stuff, company, TSA, ATC, etc, but this is one of those CRM type of situations where a single link in the chain stops a mishap. And right now WE as APC are that link. If there is a possible issue with a type of encoder that over heats and give misreadings, the Original Poster has a duty to FORCE action where ever its required to keep this from becomming the next major mishap in US airline history.