ACAS

[l39]

Dear All,
I have several questions concerning ACAS.

1.Can anybody tell me if the TCAS 2 requires a mod S transponder?
(is there an aircraft equipped with A/C transponder only, having a TCAS2 on board)
If yes- what happens if an ACAS equipped interrogates no mode s, but also acas equipped aircraft?
And how many aircraft out there flying without mode s in European airspace?

2. How often do the pilots switch off RA mode and use TA only,why and when?

Thanks in advance

[Radarspod]

l39,

As far as I am aware, there can be no installation of TCAS II that can work with just Mode A/C. TCAS II requires a Mode S transponder to co-ordinate RAs. The Mode S transponder is used to transmit Mode S messages on 1090MHz as the TCAS unit itself can only transmit on 1030MHz for interrogating. It is the TCAS II mandates in Europe that resulted in the early adoption of Mode S transponders prior to the European ELS mandates.

Can't help with the second question, I wasn't aware that you could seperate the TA/RA ON function.

RS.

[criss]

Ad. 2 - one situation would be an interception of a/c.

[Capt Pit Bull]

Briefly,

1.Can anybody tell me if the TCAS 2 requires a mod S transponder?
(is there an aircraft equipped with A/C transponder only, having a TCAS2 on board)


A mode S transponder is essential for the aircraft fitted with TCAS.

To be detected, the other aircraft must have either a Mode S or Mode C transponder.

To be able to issue an RA the other aircraft must be altitude reporting.

(Note: It is commonly stated that TCAS can detect Mode A, this is false. It uses the empty pulse frames of a Mode C response when the intruder has a Mode A+C transponder with altitude reporting turned off or not fitted).

If yes- what happens if an ACAS equipped interrogates no mode s, but also acas equipped aircraft?

Not sure what you mean. If the intruder has no mode s, it can't have TCAS. If it has Mode S, but its turned off or U/S then the TCAS is non funtional.

And how many aircraft out there flying without mode s in European airspace?

In the open FIR? Lots.

In CAS? Not many, but some.


2. How often do the pilots switch off RA mode and use TA only,why and when?

How often? Whenever the circumstances require it to be in TA only.

When / Why?

Engine Out. To prevent unachievable RA's being given and to give the other aircraft the best number of options for its RA instead.

Parallel Runway Operations, at some locations, to avoid nuisance warnings.

Various Technical Malfunctions, varies by type. E.G. Problems with Flying controls or anything else that might prevent accurate compliance with an RA.

Automatic TA Only. At low altitude, or during windshear / GPWS activation, or due to input failures or deselection of altitude reporting.

I recommend using the search facility, in this forum and tech log.

pb

[landedoutagain]

Contrary to previous replies - you do NOT need mode S for TCAS II to work, you will get RA's if both aircraft are fitted with mode C transponders only. What you get with Mode S is complementary RA's, ie, there is no possibility of both aircraft being given the same instruction.

Under Mode C only, it is possible for both aircraft to be given a climb or descent, but its rare and even then will resolve itself quickly as when the relative positions change one of the aircraft will then change its command.

[Capt Pit Bull]

Landedoutagain,

Contrary to previous replies - you do NOT need mode S for TCAS II to work, you will get RA's if both aircraft are fitted with mode C transponders only.

You don't need mode S in the intruder. You do need it in the TCAS aircraft.

What you get with Mode S is complementary RA's, ie, there is no possibility of both aircraft being given the same instruction.

Not true either. It's TCAS in both aircraft that provides this. If aircraft 1 is Mode S + TCAS and aircraft 2 is just Mode S, the RA's won't be complementary... because there will only be one of them.

Under Mode C only, it is possible for both aircraft to be given a climb or descent, but its rare and even then will resolve itself quickly as when the relative positions change one of the aircraft will then change its command.

No. If the intruder is non TCAS (but altitude reporting) then an RA could be generated for the TCAS aircraft that coincidentally happened to be in the same sense as a spontaneous manoeuvre by the intruder. In this case the RA would update.

I wonder what the source of your information is because you seem to have the wrong end of the stick about the architecture of this system.

In me experience such confusion is usually caused by teaching material that does not clearly show whether it is talking about the equipment fit for the TCAS aircraft or the equipment fit for the intruder.

pb

[l39]

Many thanks to all of you.
Capt Pit Bull-thanks for the detailed answers

[landedoutagain]

Capt Pit Bull,

I accept my reply was amibiguous, it was written on the basis that all the aircraft involved in those examples had TCAS fitted. I agree with what you state if only one of the aircraft has tcas.

However,

You don't need mode S in the intruder. You do need it in the TCAS aircraft.

I disagree with this. TCAS was around long before mode s, and will work without it. If mode s is required for tcas, how come a lot of the aircraft flying around without mode s have tcas that works for them?

Feel free to PM me your sources of information, if only for me to be enlightened, but i am quite sure of this.

Regards

LOA


ps - after thinking - are you referring to a mode s transponder requirement, or mode s as may be used within a tcas unit ie, something that isn't talking to a ground radar?

[Radarspod]

LOA,

TCAS I was around before Mode S, and works today on certain aircraft fits equipped with Mode A/C only transponders - it's fitted to a number of state aircraft types that haven't yet gone Mode S (e.g RAF Tucano fleet). TCAS I is universally discouraged due to it's impact on 1030/1090MHz and limited capability.

However, the original question from I39 was on TCAS II which DOES require a Mode S transponder to be fitted in tandem with the TCAS II equipment.

RS

[Capt Pit Bull]

Basically what Radarspod said.

You can't have X aeroplanes all shouting continuous mode C interrogations at one another without trashing the SSR frequencies.

One of the main challenges of TCAS from a technical point of view is to minimise the ammount of SSR traffic. The answer to this is a number of techniques one of which is the use of a roll call and selective addressing so Mode S is an essential building block of TCAS.

As far as a reference. Well, a quick google will show you any number of installations that have TCAS and Mode S, and plent yof references that discuss the transponder arcitecture and data formats. But of course that doesn't prove the non existance of a TCAS that works using mode C...

My basis for saying this is having done an original manufacturers course and having been the lead instructor on this system at a major airline. Everyone else here agrees with me, so no offence but I think the onus is on you to provide a credible reference that a TCAS II installation can be embodied without a Mode S transponder.

pb

[Radarspod]

LOA - check your PMs :ok:

RS

[landedoutagain]

Radarspod - I accept your invitation! :ok:

Capt Pit Bull - You could have stated your background a bit earlier...:) I will concede that elementary mode s is required for tcas2, but i promise you that from an atc standpoint, that is viewed as not having mode s. We get no information from it and it doesnt help us in any way.

Perhaps the original question should have been posed in an engineering forum rather than an atc one?!

[Radarspod]

I see now - confusion on what "Mode S" means!

To LTC, MACC, and soon Prestwick Centre controllers, Mode S Enhanced Surveillance DAP (Downlink Aircraft Parameters) display is a recent addition to the display systems, including items such as magnetic heading, IAS, vertical rate and probably most useful of all - Selected Altitude / Flight Level. This is often referred to as "Mode S" by ATC as it's all the extra information that is provided that wasn't before. The fact that the position, Mode A identity and Flight Level information has been derived / provided by Mode S rather than Mode A/C for quite some time is transparent.

Mode S transponder fit on an aircraft is different and has been an ongoing theme since the 90's. I don't know when the TCAS II mandate came in (before my time?) but when I was involved in the Mode S groundstation pre-operation trials (2001-2002) and deployment of the first operational Mode S groundstation for NATS under the RSS programme (2003 onwards), there was over 85% of all aircraft in cover already Mode S transponder equipped before CAA and other European Mandates for Mode S transponder equippage (started 2003). These were almost all due to the TCAS II requirements. A snapshot today puts Mode S transponder carriage figure at 95% ish - depending on how much GA is in the air :} You have to have at least a Mode S Elementary transponder now to enter the London TMA, apart from a few exceptions.

As a complete aside, come 2012, all SSR radars have to be Mode S and all transponders operating in airspace requiring transponder carriage have to be Mode S elementary (including Gliders :E!!!!)

For TCAS II to work, you need a Mode S transponder with an appropriate TCAS interface. You do not need to be Enhanced Mode S capable :ok:


RS