PDA

View Full Version : Quick question reg. TCAS


Arrows2100
31st Jan 2010, 15:43
Hi.. studying for an oral board during training with NATS...

There are a couple of bits of TCAS knowledge I need to reinforce.

TCAS on one aircraft reads the Mode C of a non-TCAS equipped aircraft and will provide an RA to that aircraft. The other aircraft will buzz along not knowing anything has happened?

TCAS II needs Mode S because of the datalink abilities in order to co-ordinate RA's

TCAS will provide an RA against aircraft with Mode C regardless of their Mode A (e.g. TCAS RA's against A7000/A0000 etc)

Thanks for your time, I'd appreciate if anyone could tell me if I have went wrong anywhere in my studies :)

Andy

Ronand
31st Jan 2010, 16:14
From what I remember:

TCAS1: Only traffic advisory no RA transponder mode C req

TCAS2: RA only in the vertical plane transponder mode S req

TCAS3: RA in vertical and horizontal plane transponder mode S req

Permafrost_ATPL
31st Jan 2010, 16:32
What is your question???

Arrows2100
31st Jan 2010, 17:02
Im asking you to double check these little facts and correct me if I have picked it up wrong. This is my understanding and would like to make sure before I get questioned on it...

Thanks

Clandestino
31st Jan 2010, 19:50
TCAS on one aircraft reads the Mode C of a non-TCAS equipped aircraft and will provide an RA to that aircraft. The other aircraft will buzz along not knowing anything has happened?

Correct.
TCAS II needs Mode S because of the datalink abilities in order to co-ordinate RA's

Both aircraft have to be S-mode equipped (and working) to achieve coordinated RA. TCAS II can still make non-coordinated RAs against C-mode equipped aircraft and they're not moch of an issue, anyway, as chances that both TCAS computers will give same directions RA are pretty small. Even if it happens, soon there will be RA reversal in one with possible increased RA in other ac.
TCAS will provide an RA against aircraft with Mode C regardless of their Mode A (e.g. TCAS RA's against A7000/A0000 etc)

I'm not sure I have understood your question. To achieve RA, encoded altitude has to be received, otherwise it would be TA only.

poldek77
31st Jan 2010, 19:58
TCAS on one aircraft reads the Mode C of a non-TCAS equipped aircraft and will provide an RA to that aircraft. The other aircraft will buzz along not knowing anything has happened?


If you don't shout at them they will not realize:)
You are right - as the intruder is not equipped with TCAS the crew may not be aware of the traffic unless informed by radio or in visual contact.

Arrows2100
1st Feb 2010, 07:28
In regards to the final point, I am checking that TCAS provides RAs regardless of the Mode A setting. We as controllers cannot provide vertical against GA squawking 7000 because we cannot be certain the Mode C is accurate, and A0000 aircraft is definitely producing unreliable SSR data. I don't think TCAS interprets Mode A... however, due to controllers being unable to provide vertical, some of my collegues believe TCAS will do the same and only provide TA against such aircraft.

Sorry for being so confusing haha, and thanks for the replies so far,
Andy

M2dude
1st Feb 2010, 11:45
Hi Arrows2100.

Assume that the TARGET aircraft is equipped as below:

MODE A ONLY:
TCAS will not decode any Mode A info as such, but will as you say give you a TA against any Mode A ONLY traffic. (This can also be shown as PROXIMATE TRAFFIC on the aircraft’s TCAS IVSI or EFIS ND). The TCAS Computer interrogates the ATCRBS in a similar way to the ground station, but extracts no IDENT data. Using the range and relative bearing of the target, the TCAS computer calculates the threat and will display as such. TCAS transmits data on 1030Mhz and receives transponder data on 1090Mhz.

MODE C ONLY:
As you probably know, Mode C has barometric altitude GREY CODE data inserted into the pulse train and TCAS will use Mode C absolute altitude data against its own aircraft altitude (i.e. relative altitude) along with relative bearing and range to generate and display target information. This time as well as PROXIMATE TRAFFIC and TA displays, RAs can be generated and displayed in the IVSI or EFIS ND AND PFD). The target aircraft of course will have no indications or protection. (TCAS cannot function without an onboard MODE S transponder, inferring that the target aircraft is not equipped with TCAS).

MODE S:
As MODE C except that avoidance manoeuvres can be co-ordinated by both aircraft, and therefore there are no conflicting RA commands. The data protocol within Mode S is radically different to the old ATC ATCRBS and is far more sophisticated.

I hope that all this blurb is some help to you, and good luck in your NATS Oral.

Ronand
1st Feb 2010, 14:46
@clandestino TCAS1 on one aircraft reads the Mode C of a non-TCAS equipped aircraft and will provide an RA to that aircraft. The other aircraft will buzz along not knowing anything has happened?
Correct.

According to my jeppesen instrumentation book this is not correct. It says: TCAS1: system provides TA only and (no RA)

kijangnim
1st Feb 2010, 15:35
Quote:
TCAS on one aircraft reads the Mode C of a non-TCAS equipped aircraft and will provide an RA to that aircraft. The other aircraft will buzz along not knowing anything has happened?

TCAS is mandatory in NATS......

Denti
1st Feb 2010, 16:13
One abbreviation, different meanings, NATS could be the national air traffic service in the UK or the north atlantic track system...

I would think the OP means the former one.

kijangnim
1st Feb 2010, 16:28
Greetings, good point, anyway in any airspace I would like not to have one :E traffic density is just going crazy...:eek:

Clandestino
1st Feb 2010, 16:38
TCAS1: system provides TA only and (no RA)

Correct.

Original question was not about TCAS 1 specifically. Since it involved RA, I assumed that it was about the most common system: TCAS 2.

Denti
1st Feb 2010, 16:38
True, however in europe you can fly happily around for at least 3 days with your TCAS INOP. Just keep your fingers crossed that the other guys TCAS works and our colleagues on the ground dont forget about us.

Graybeard
1st Feb 2010, 16:47
Just a bit of clarification. TCAS II interrogates only for Mode C replies. Even if a transponder is not replyiing with mode C altitude, it replies with Mode C timed framing pulses.

TCAS calculations are based on range, closing speed, and altitude difference. Approaching a non altitude reporting target, closing speed is extra important. The closing speed will diminish in proportion to the altitude difference. The non-altitude target will either disappear, or appear to move in an arc around your own plane.

TCAS directional antennas only provide bearings of +/- 15 degree accuracy, and are merely to aid the pilot in looking out the window for the target. There is no bearing accuracy requirement for targets behind.

What gets interesting is when a transponder is reporting erroneous altitude. One time, a KAL 747-2 was squawking 7,000+ feet while really at 10,000, and nearly caused a collision with another TCAS equipped plane.

GB

M2dude
1st Feb 2010, 18:35
Ouch GB!!. I agree with all your points here, Transponder altitude accuracy is PARAMOUNT. (I do miss having to test Mode A & C with the ATC 600. In the pre CMC days you got to test the transponder replies for real, checking P1, P2s etc. Ahh the nostalgia of it all).
Regarding what you said about the directional antennas, I seem to remember that one of the reasons for the lack of TCAS 3 implementation was the poor azimuth resolution of the TCAS " directional antennas.
On my previous fleet I remember that when we intitially fitted TCAS. MOST of my pilot friends were extremely sceptical about it's value. However, within weeks of finally going live, one of our aircraft was climbing FAST out of JFK on it's way back to LHR when we got an RA. A KC-10 with 2 F15s in tow was where it should not have been, and a certain near miss was avoided. (Within a few months of living with TCAS operation, none of our guys would ever want to be without it).