Mode C under a CTA

[Bandit650]

How do controllers maintain seperation between IFR traffic inside a controlled zone/airway and traffic under the area but not squawking.

Surely it has to be assumed the uncontrolled traffic is infringing?

[Turn It Off]

Its actually the opposite.

You assume that it is outside unless you have any information which suggests to the contrary.

No mode C = outside
Mode C indicating below/above CAS = outside
Mode C indicating inside = avoid and assume in
Other info suggesting inside (e.g. lost ac on freq. / other pilot report) = avoid and assume inside


TIO

[NudgingSteel]

Nope, it's the opposite. Traffic operating underneath a CTA without Mode C is assumed to be outside controlled airspace, unless radar derived or other information indicates that the aircraft is lost, plus a few other cautionary bits and pieces.
Otherwise it would be impossible to provide any separation within airspace where VFR traffic can operate underneath. Solution: either make permanent operation of Modes A and C compulsory for VFR traffic, or drastically increase the amount of controlled airspace and drop it all down to ground level. Both of which would not exactly help GA!

[Bandit650]

Interesting.

I regularly fly across the very very extended centreline of EGLL 09L/09R under the CTA, which at that point is 2500' (over Henley area). So the controller would see converging blips (me and an inbound IFR) and have to assume the uncontrolled traffic is at the correct height? (if I wasn't squawking of course which is very unlikely).

There are no safeguards?

[Finals19]

It really makes one wonder how many infringers actually get away with no follow-up because they are squawking mode A (or not at all) and are therefore assumed to be below the CTA. I can think of areas around Luton where this would happen. As silly and dangerous as it would be, I am sure there are some idiots out there who would quietly squawk alpha and "cut a corner" to save a few minutes...:eek:

[AirNoServicesAustralia]

I have to say in my opinion aircraft squawking Nil Mode Charlie are more trouble than if they switched their transponder off completely. The reason I say that is the number of calls I take from aircraft in my airspace getting close to these (hopefully) low level aircraft, and being obliged to find out who they are and what level they are at, due to TCAS. When my response is I don't know who it is and I assume they are below CTA, there is a very nervous "Roger" from the jet pilots.

[AirNoServicesAustralia]

only squawking mode C?


andhave to assume the uncontrolled traffic is at the correct height? (if I wasn't squawking mode A of course which is very unlikely).

Sorry I'm a little confused. Do you mean squawking Mode A, ie I can see you on my radar, but not squawking Mode C, ie. I can't see your altitude, cos that would make more sense to me. I didn't think you could squawk only Mode C?????? Maybe I am missing something here :confused:

[Bandit650]

Yep. I got my modes mixed up. Mode C = ALT encoding, Mode A code only...thanks for pointing that out. Have clarified original post.

[AlanM]

Happens around the whole TMA area. (Bandit - there is no Heathrow CTA mate)

Although, the Manual of Air Traffic Services pt 1 (CAP493) says:

Except when aircraft are leaving controlled airspace by descent, controllers should not
normally allocate a level to an aircraft which provides less than 500 feet vertical
separation above the base of a control area or airway. This will provide some vertical
separation from aircraft operating beneath the base of controlled airspace.

[Bandit650]

So, if an non-squawking, or just Mode A, aircraft inadvertently climbed into CAS from underneath, the controller would assume it was still underneath and would not instruct the IFR traffic to take avoiding action if horizontal seperation is lost?

In which case, would the IFR aircraft's TCAS (assuming a large transport) be able to calculate straight line proximity purely based on a Mode A return..and therefore provide a last line of defence?

I find it difficult to believe the system relies entirely on such an unsafe assumption that height unknown traffic are at not infringing.

If it does, I can certainly see the case for compulsory Mode C transponding/ Mode S.

[Radarspod]

I dont think TCAS can work off of Mode A only, it requires Mode C replies to identify if the aircraft is actually a threat. The Mode S transponder on the TCAS equipped aircraft will interrogate non-TCAS equipped aircraft with Mode C only all-call.

[AlanM]

I find it difficult to believe the system relies entirely on such an unsafe assumption that height unknown traffic are at not infringing.

You got it mate. That's exactly how it is.

http://www.flyontrack.co.uk/radar02.asp

[AirNoServicesAustralia]

I know I am just a humble controller so could be absolutely wrong on this, but my understanding was that an aircraft will get a Traffic Advisory on Nil Mode Charlie aircraft but not a Resolution Advisory, so the pilot should query the NMC squawk with ATC. That seems to be the way it works here in the UAE but then again we have very few NMC lighties buzzing around as most of our stuff here is the big stuff, except for a few helicopters buzzing around the oil rigs and islands in the gulf.

[Finals19]

AirNoServices..

I believe that to be correct as well. Squawking mode A will still show you on a TCAS screen, but no altitude information. The lateral information is allowing the TCAS to provide a fix on the traffic, but no resolution advisory is given as the required data is incomplete. I am pretty sure thats the way it works.

[NudgingSteel]

It relies to a large extent on trust. You have to trust that the VFR (as it will almost exclusively be VFR) traffic knocking around under, say, the base of the LTMA, knows where it is and won't go where it shouldn't. I'd say you're on thin ice with the GA fraternity by suggesting compulsory ModeA&C, which would effectively close off swathes of Class G airspace to microlights, vintage traffic, gliders etc.

Don't forget, too, that if a VFR trundles into CAS by mistake, there's still the fact that they are operating VMC and clear of cloud and maintaining a proper lookout (!) which may allow some avoiding action if required.

[spekesoftly]

If a VFR trundles into CAS by mistake, then perhaps their VMCness and lookout was not all it should have been in the first place! :uhoh:

The ability to "see and avoid" possibly compromised by the distraction of being "temporarily unsure of position" ?

[throw a dyce]

Bandit,
It happens around ABZ all the time.The military are really good at steaming around 50ft below the base of the CTA,without talking.They usually squawk 7001 with mode c,but it's not verified.We have the base of the CTA, north and south down to 1500 ft and they still perform there even over high ground.
Their party piece is low level abort,into the CTA to terrain safe 4100'.Just where we are vectoring giant jetliners.:eek: No warning,no 7700,just up they go in IMC.You file reports but they just melt away somewhere.:ugh:

[ShyTorque]

"I dont think TCAS can work off of Mode A only, it requires Mode C replies to identify if the aircraft is actually a threat. The Mode S transponder on the TCAS equipped aircraft will interrogate non-TCAS equipped aircraft with Mode C only all-call."

Not true; TCAS certainly does work against Mode A only aircraft! Non-mode C aircaft can be a bit of a headache because TCAS assumes they are at the same level and gives a "TRAFFIC, TRAFFIC! :eek: " advisory.

Then it turns out it was possibly 2700' below, for example. Very worrying if you can't see him at all, which often happens - and as we all know, it's the one you don't see that hits you - which tends to concentrate the mind more than a little :hmm:

[Bandit650]

It happens around ABZ all the time.The military are really good at steaming around 50ft below the base of the CTA,without talking.They usually squawk 7001 with mode c,but it's not verified.We have the base of the CTA, north and south down to 1500 ft and they still perform there even over high ground.
Thats interesting. All these PPL infringements getting attention, yet these military jet jockeys zipping around in and out of CAS in IMC...that has got to be a bigger threat to safety..given the closing speeds....:eek:

But I'm still curious about one thing. If no squawk/Mode A traffic inadvertently climbs into a CTA and embarks on a conflicting course with IFR traffic (assuming neither a/c has seen each other) the controller would not instruct the IFR to avoid because he/she is assuming the infringing traffic is below the base of the CTA?

Which that leads me to another question...does the conflict detection software running on the radar dispays also assume the undentified traffic is not infringing and therefore does not alert the controller??

[Radarspod]

Oh well, can't be spot on all of the time :\ Yes, Mode C altitude is required for RA, TA otherwise. Thank you for the correction.
This link is quite useful from a google search, guide to TCAS by ARINC -http://www.arinc.com/downloads/tcas/tcas.pdf
What I should have said is that those aircraft incapable of providing Mode C data (regardless of whether an altitude encoder fitted or not) cannot be detected by TCAS. The transponder must be capable of providing Mode C replies to be seen by TCAS (which will rely on detecting the F1-F2 framing pulses only if no altitude info) so those older Mode A units which were never designed to provide Mode C replies cannot be seen by TCAS. I shouldn't think that there are that many of these still around, but you never know. :bored:
Anyway, the Mode S transponder used by TCAS is only requesting Mode C information, not Mode A, to track non-Mode S equipped aircraft as I said.:=

[throw a dyce]

Bandit,
As a far as the military it leaves very small margin for error under our CTA,with the high ground.Thats why if they have a low level abort,the first we know about it is they are well in CAS.Things happen quickly at 500kts.
Thats the joys of being surrounded by class G,and a low level playground.
As far your other point,if they are not squawking,or mode A only,then the controller has no way of telling that a/c is in or out of CTA.We therefore ASSUME that the target is out,unless we have information that it's lost etc.
I have seen the military infringe the CTA,then realise what they've done,and switch off the transponder.They then get out of the CTA quick,then switch it on again.:= The only conflict alert I've used worked on mode c readouts,and also a rate of closure.However at ABZ we don't have conflict alert on the radar at all.Got to keep your eyes peeled all the time with mil under the CTA.

[Bandit650]

throw_a_dyce: fascinating insight- cheers:ok:.

But pls forgive me if I appear like a stuck record here..but I find it very hard to believe a controller would allow two radar returns to merge purely on the assumption the non-squawking/modeA aircraft is not infringing. Would you just sit there with fingers crossed under the desk hoping that the unidentified a/c is not at the same level? (I take it a verbal traffic advisory would be issued to the controlled traffic at the least?).

[AlanM]

Bandit - sorry to disappoint but we don't even cross our fingers.

[Bandit650]

Jeez!!:\
Right well, I'll insist on a window seat and keep my scan going on the holiday jets in future!!:eek:

[Finals19]

On a related topic - there are certain aerdromes that fall just within the London CTR. Obviously there are rules governing joining, circuits, departure etc, so as not to conflict with London inbound/outbound traffic. What therefore is your take on transponder use in the circuit or in vicinity of the aerodrome?

Some aerodromes advocate transponder on standby in the circuit, but mode C when departing. Would it not be a lot easier at these fields if everybody squawked charlie all of the time? Surely there is enough seperation at these places as not to invoke a TA, and wouldn't it make the picture clearer to controllers (instead of just getting primary returns?)

I understand there was also some consideration to use a generic circuit squawk code, which would sure help at these fields (again with "c") Whatever happend to that?