Airbus ATC AUTO mode
 

good day
FCOM says:
AUTO: In flight : Selected transponder operates.
AUTO: On ground : Selected transponder only operates in mode S (Selective aircraft
interrogation mode).

could anyone explain what is the difference between "operates" and "only operates in mode S"? what modes are operating else rather than mode S in AUTO (in flight) or ON?
did not find anything either in OM/TM/FM or in the web.

thanks in advance

No TA/RA on ground?

While I have no direct experience with these systems as implemented on large aircraft, my understanding with respect to mode S transponders is that:
-on the ground, mode S selective responses should normally be active, and carry coded data including aircraft ident...mode S does some other neat things, including data exchange for TCAS and ADS-B
-modes A and C should not be active on the ground

I'd read that as saying that "Auto" in flight gives you modes A and C as well as S, whereas on the ground it's S only.

Mode S (known as extended squitter - ES) adds a significant amount of detail to the basic secondary radar transponder response see (http://www.garmin.com/us/intheair/ads-b/squit/ )

Why Airbus would only supply this information on the ground is a little puzzling.

Err, it doesn't! With AUTO selected, the transponder operates in ALL modes when the aircraft is airborne, but on the ground it only operates in Mode S. That's because the Advanced Surface Movement Guidance and Control Systems (A-SMGCS) used by ATC at some airports uses the Mode S 'squitter' to obtain aircraft identification and position. The other modes are disabled on the ground so the transponder doesn't respond to transmissions from the Mode A/C Terminal Area Radars.

so the purpose is to be visible only for Ground ATC, right?

Mode S responses also include a data bit which tells the surveillance system if the aircraft is on the ground or not, which can then be filtered by the surveillance system. Mode A/C does not.

Most importantly Mode S allows the radar to interrogate each aircraft selectively (using the aircraft's unique address) and can therefore provide useful ranging on the ground.

Mode A/C is non selective so everyone in the radar-beam replies - and at ranges commensurate with the size of an airport that means massive garble.

Just to clarify on the comment from Ian W.
Mode S is not Extended Squitter.
Extended Squitter is a sub-function of Mode S which carries ADS-B data. A Mode S transponder will 'squitter' whether or not it is seen by a radar. If it is seen by a radar it will also 'reply' to the radar interrogations.

The "Auto" behaviour will be the same for all aircraft types.

On the ground:

A&C responses are inhibited
No responses to Mode S "All calls" (but, as 'Dont Hang Up' says, the aircraft can be selectively called with Mode S)

Another aircraft's TCAS can selectively call your aircraft. It knows your TCAS/Mode S-equipped aircraft is on the ground because of the Mode S data being sent. The other aircraft computes if a Mode C aircraft is on the ground by other methods.

You can still make it sqwawk mode A and C on the ground by selecting "ON" instead on "Auto".

is this correct?
I was under the impression that a transponder will only reply to an interrogation unless it is ADS-B.

Transponders reply to radar. That is what they are for.

Mode S was invented as a replacement for the somewhat limited Mode A/C. Operating on the same frequencies and with Mode A/C backward compatibility built into its design, Mode S was a means of transferring much more data between the air and ground over these interrogation/reply transactions. It also provided the additional benefit of selective interrogation based a unique address assigned to each aircraft. "Selective" is actually what the "S" in Mode S stands for. This eliminates many of the problems of ambiguity and message garbling of the old Mode A and Mode C.

However, in the quite early stages of Mode S development it was recognised that its use could be significantly extended. Transmissions which contained the unique aircraft address did not necessarily have to be triggered only in response to a radar interrogation. Instead, information could be spontaneously broadcast for the benefit of anyone who wanted to listed. Thus the concept of "squitter" was born.

Initially this was only a "short" or "ID" squitter. Containing little more than the address, this was intended to make the aircraft's existence known to the TCAS of nearby aircraft.

The concept was then extended - literally "Extended Squitter". These longer and more frequent transmissions are used to carry ADS-B (provided the transponder has a suitable on-board navigation source fed into it).

In summary, a Mode S transponder looks like:
- an ADS-B target to an ADS-B receiver;
- a Mode S target to a Mode S radar;
- a Mode A/C target to a Mode A/C radar

It does the whole job. And all at the same time.

Well that's lovely but it doesn't answer the question does it?

Well in that case I have to admit I don't understand the question.

Possibly a stupid question but why have the "ON" position at all if "AUTO" does it all?
Is there ever a need to use mode A or C on the ground?
The airline I fly with now on the Bus use AUTO on the ground if required by ATC and turn it ON lining up and AUTO/OFF after landing.
Seems leaving it AUTO from pushback to chocks-on simplifies things.

One would say so. No info in the FCOM re ON vs AUTO.

A330 standard fit

We turn ours to Auto before pushback and then later switch back to Standby after shutdown.

AUTO is AUTO, no need to select it to On.

In a slightly similar vein, why do we leave tcas off until line up?

We dont need to do that either, TCAS is always on.

It is recommended to select TCAS off while on ground (and not on a runway) because otherwise mode A and C will also be active and lots of returns from many aircraft on ground in close proximity creates problems for ATC secondary radar.

Can you back that up with an FCOM reference? I haven't found anything that supports that view, although it is expressed by some trainers.

Which transponder mode is used depends on the transponder mode switch, which should be in AUTO on the ground. In that case it transmits only in Mode S with the ground flag set, which excludes it from any TCAS and unwanted ATC display. Unlike ON which does indeed switches to Mode S/C regardless if on the ground or not and will usually lead to very fast complaints by your friendly local ATCO.

Setting TCAS to TA/RA does not override the transponder mode selector, try it out next time you're in the aircraft.

Yes, and in effect, uses up bandwidth which is limited. Currently, ATC has to null out the ac on the ground, but the bandwidth is still being used up. With increasing ground ops managing surface flow, it will become critical to turn systems off.

Where are you guys digging this stuff up from? In Australia (unless we are way out in front of everybody else), it's TCAS on TA/RA on receipt of Airways clearance ie before start, and TCAS OFF when you shut down the donks.

Bandwidth overload, maxxed out SSR?? :confused:

Hey Bloggsy.......we go further, the TCAS switch is never touched out of the TA/RA position unless directed by a non normal checklist or possibly if you desire during a PRM app in SYD to TA.

The Transponder is switched to Auto before start and back off after shutdown. Turning off the Txpd shuts down the TCAS at the same time.

Besides, in the AUTO mode the TCAS is inhibited on the ground anyway.

TCAS is not inhibited on the ground, it is in ground mode (not causing RAs).
But Mode A and C will be active.
We used to keep TCAS on before, but some years ago there was a request by ATC here in Europe to PLEASE keep it to mode S on ground only.
It is also in almost any airport briefing here.
I will try to dig out the respective notice.

ACAS Guide Airborne Collision Avoidance Systems (incorporating TCAS II versions 7.0 & 7.1 and introduction to ACAS X)
May 2016

p.42 INTERFERENCE LIMITING
p.70 TCAS II/TRANSPONDER OPERATIONS ON THE GROUND

The techical explanation is not provided in that particular document, but guidelines are very clear.

--- edited ---
And a leaflet straight out of my flightbag, where a copy is stored for no apparent reason since 2004. :E

This brochure describes transponder operating procedures from departure gate to runway line-up, and landing to gate.

Full dedicated webiste here:
EUROCONTROL Documents on SSR Mode S | Eurocontrol

Bloggs, time to evolve. If and when AUS adds ground ops, or the airspace gets closer to capacity, you will begin to see the same.

Thanks for once again, trolling my posts! I have a stalker!

A-SMGCS in operation at YMML, YSSY with TCAS/transponders going full bore. Obviously no bandwidth issues there. Stop making grandiose, sweeping statements that don't apply everywhere.

As this is a thread about the airbus TCAS, do you have any official documentation that actually confirms that? I haven't found any, and testing it with our local ATCOs actually confirmed that mode A/C are not active on the ground with TCAS on. That said, that is not official confirmation of course.

Just weird that on my previous type it was actually the OEM that changed our SOPs after a fleet audit at "my" outfit to always have TCAS on instead of switching it off and on. According to the OEM all that achieved was wearing out the switch. No complaints from any ATC unit and we are a medium sized legacy carrier in the middle of europe.

I can't recall the exact reasons given but for 6 months we selected the TCAS ON before line up and OFF after vacating. That lasted 6 months until our Fleet Tech office determined that the TCAS systems fitted were updated to the latest software/mod and we can leave it switched on all the time.

Now, as per Airbus standard, we switch the TXPDR to SBY at the gate.

A330.

While the comments are for operations where there are issues, why do you continue to comment on areas where there are not issues? As noted, IF and WHEN AUS adds ground ops, there will be bandwidth issues, like at other locations where they have.

Again, time to READ, evolve, and stop stalking, stop inserting yourself where you are irrelevant.


Did you figure out that Perth only has one AWOS, and when you get TCH winds, it is simply because they give the winds per runway end with the corresponding headwind/tailwind component? Or do you really think they measure they winds at each TCH?

Mate if you don't want others to comment on your posts then...............don't post.