TCAS operation in TA mode

[breid]

Is there any time that anybody out there uses a perfectly serviceable TCAS in the TA mode only(parallel ILS or formation flying)?

[Ricky Butcher]

Yep, we use it in TA mode when on easterlies at CDG where 09R and the 08s approach tracks are converging to prevent nuisance RAs. The condition for use is that we must have visual contact with the conflicting traffic at all times. We also use TA mode after an engine failure in order to prevent CLIMB RAs on a single engine.

[RAT 5]

Hi,

More a question than a comment; but it follows on the previous, slightly.

On the transponder box B757 & 737) there is a setting for TA & TA/RA. On the MAP range selector is a TFC selector. With the transponder in TA/RA, but the TFC selected OFF, i.e. TFC does not appear on the MAP, but rather 'TA only' what is the status of the TCAS?

In the QRH for N-1 situations, it states "Transponder TA only." It is assumed this refers to the selector on the transponder panel, but the question has been asked that if the TFC switch is selected OFF, and the MAP shows TA only, why is this not sufficient?

I hope there are some boffins out there who can give a 100% definitive answer, rather than 90% 'ers I think "............"

No offence to anyone, but I need a 100% true factual answer to pass on.

[crj-jockey]

Hi,

we often use TA ONLY in Frankfurt, especially when operating with visual separation.

[QAVION]

"In the QRH for N-1 situations, it states "Transponder TA only." It is assumed this refers to the selector on the transponder panel, but the question has been asked that if the TFC switch is selected OFF, and the MAP shows TA only, why is this not sufficient? "

RAT 5, I'm not sure what "N-1 situations" are, but, generally speaking....

With the TFC switch off, you are not going to get any intruders at all painted on the EHSI/ND/IVSI (whatever type of display you have). That is, no diamonds, circles or squares. This is completely different to having TA ONLY selected, which will show intruders. Selecting "TA ONLY" will remove the resolution commands on the EADI/IVSI, inhibit RA aurals and, as I understand it, will show normally red targets/intruders on the EADI/PFD/IVSI in yellow. i.e. , with TA ONLY selected you will get some idea of what is around you, but the TCAS system won't tell you how to avoid conflicts. With TFC off, you lose your horizontal situational awareness (With TA/RA selected with TFC off, you have to rely purely on aural & display up/down guidance commands to get you out of trouble).

Because I don't know if N-1 situations occur on the ground or in the air, it makes it difficult to comment further. However, you shouldn't get TA ONLY annunciated if you are in the air and you have TA/RA selected).

Rgds.
Q.

[Captain Stable]

"N-1 situations" are one-engine-out situations.

[Bellerophon]

QAVION

However, you shouldn't get TA ONLY annunciated if you are in the air and you have TA/RA selected

In general, of course, you are correct, but there is at least one exception to this rule!

On one current passenger aircraft, this is in fact the case for the majority of the cruise.

Depends on how fast we're going!

Regards

Bellerophon

[aviatorpk]

Our checklist says to put TCAS to TA after one engine failure on B747:confused:

[QAVION]

"Our checklist says to put TCAS to TA after one engine failure on B747"

Would the 747 have the ability (performance-wise) to follow TCAS "Increase Climb, Increase Climb" commands during "N-1 situations" at higher altitudes/weights, "Aviatorpk"? TCAS has no way of knowing if an engine has failed.

Rgds.
Q.

P.S. Thanks to Captain Stable & Bellerophon for their comments/answers. BTW, any hints on which aircraft has this in-air TA-only annunciation at speed :)

[Full_Wings]

I put TCAS to TA only when entering US airspace ;)

Many airfields I visit have a note suggesting the use of TA only when doing parallel instrument approaches.

I have to say I think carefully about this, especially when other a/c are on 'visuals' and there are people behind/to the side of me. Even though I hear "Oh yeah, we've got him in sight", it's not difficult to overcook a turn onto finals with the runway spacing at some airfields (SFO & EWR come to mind), not to mention the sheer number of runways to mis-identify (DFW are you listening?). This could mean a very late sighting (or none at all).

Sometimes I trade the likelyhood of an uneccessary go-around (or whatever) for protection from an unwanted whack up the a$$.

OK, so in 'TA only' you will still get a TA but if you can't see them and they are very close, the display ain't much help and manoevring might cause rather than avoid a collision.

I know ten years ago we didn't have any of this but having had my underwear (at the very least...) saved on two occasions by TCAS, I am becoming reluctant to let go of my safety blanket.

[RAT 5]

Qavion.

Thank you for the replies. I'm still not clear on a couple of things. On my boxes the indications on the MAP are TA ONLY when the transponder is ON in any mode, and TFC when so selected by a seperate switch. In my understanding, with TA ONLY you will get intruder information with TA info only. With TFC displayed you will get RA information. Thus it is possible to have TA/RA selected on the box but with TFC off and the dispaly of TA ONLY. To my mind TA ONLY on the display is what it says.

However, since asking my question, and this addresses your comment about TCAS not knowing about engine failures, I've been told (opinion) that by selecting TA ONLY on the box this sends a signal to other TCAS's that this a/c can not accept RA's. Thus any RA's given to other a/c will take account of the fact that one target can not manoeuvre.

If true, then this bit of kit is smarter than I thought. It's a shame hat so little indepth information is in the AOM's these days.
Indeed, on all our avionics boxes there is no identification of the manufacturer. Thus when any notices are issued you don't know if it applies to your bit of black magic. There is no notice of if the AFDS is Collins, or Rockwell or Honeywell or Dinklydell.

[jungly]

RAT 5 - the answer is YES. The little box is smarter than you thought!

AVIATORPK - A TCAS RA is based upon you managing a minimum 1500ft/min during any aviodance manouevre..... this is often not possible in even a A340/B747 with an engine out.

By switching to TA you are telling the box (and any other acft) that you cannot meet the required performance for a RA.

To answer the original question: San Francisco requires TA as the parallel approaches are so close.

[QAVION]

"Thus it is possible to have TA/RA selected on the box but with TFC off and the dispaly of TA ONLY. To my mind TA ONLY on the display is what it says. "

Unfortunately, RAT 5. it seems that your installation is somewhat different to ours... or at least my understanding of it. You may have to ask your engineering/technical department why it behaves this way. They should also be able to tell you what system is fitted.

"I've been told (opinion) that by selecting TA ONLY on the box this sends a signal to other TCAS's that this a/c can not accept RA's. Thus any RA's given to other a/c will take account of the fact that one target can not manoeuvre."

I wasn't aware of this, but it does seem probable: When "TA only" is selected, the sensitivity level of your TCAS system is set at Level 2 (At this level, no RA's are given). This sensitivity level is relayed to the selected transponder on your aircraft and this and other data is then broadcast to other aircraft. Levels range from 2 to 7 based on Radio and Barometric Altitude.

Rgds.
Q.