ACAS TA triggered by any a/c without mod C?

[samotnik]

Does an aircraft squawking mod A triggers a TA when it breaches (horizontal) restricted zone of another, TCAS equipped a/c? The reason I ask is because recently GA pilots flying in my area have a nasty habit to use only mod A, no idea why. I wonder if it affects aircraft cruising above. However, there's no legal requirement to use mod C here.

[Albizia]

Yes it does. TA only.

[fellman]

No it doesn't. But read on...
TCAS sends Mode C and Mode S interrogations only. It does not send Mode A interrogations and so aircraft which are truly Mode A-only are invisible to TCAS.
However, if by "Mode A" you mean Mode C but not reporting altitude then you will receive TAs. Furthermore, TCAS assumes that these aircraft are co-altitude with own aircraft, so you will get some TAs that would not occur if the aircraft reported altitude (provided the altitude was sufficient away from own aircraft's altitude).

[samotnik]

Why would the aicraft not broadcast its altitude within mod C replies?

[055166k]

On our multi-radar system I can sometimes see quite a lot of warships around the UK coast whenever they are active, and they have transponders without mode "C" [obviously]. Several times over the years I have asked aircraft if they can see anything on TCAS and the answer has always been NO.

[RaRadar]

An aircraft transponder when not providing altitude (not switched to ALT) will still reply to the Mode C interrogation but just not include any data (F1/F2 bracket pairs only). I assume a ships transponder will not reply at all.
RR

[twentypoint4]

Could the TCAS system in one aircraft determine that another aircraft with a non-altitude-reporting transponder is not a threat just by the length of time it takes for a response? Or does that not come into it at all?

[Wojtus]

Why would the aicraft not broadcast its altitude within mod C replies? Some GA transponders need separate "altitude encoder" connected to it. If it's absent, transponder (Mode C - capable) responds with "alt unknown" frame. It also will happen if you select "Mode A only" on such a transponder (for example, ATC observes wierd altitutde indications and uses phrase "ABC123, stop squawking Charlie wrong indications").

Most todays transponders are C-capable, even if they do not report altitude. The remaining old units are truly Mode A-only, and thus invisible to TCAS.

Could the TCAS system in one aircraft determine that another aircraft with a non-altitude-reporting transponder is not a threat just by the length of time it takes for a response? Or does that not come into it at all?
They do. If they are far away, they appear as "no threat" targets on TCAS. If they are closer, you will receive TA.

[DFC]

Yes it does. TA only.


Indeed.

That is why it is critical for pilots to use mode C if they have it.

Otherwise while they fly nicely along at 2000ft enjoying the good visibility and fair weather, some poor sod in the cloud 5000ft above is getting a warning "Traffic!! Traffic!!".

If you don't have a serviceable mode C then please continue to squawk as it may save your life even if lots of false warnings are also provided.

However, please think about spending a few hundred bucks on a mode C encoder because the last thing you want is to be speared by something doing 250+ knots who ignores the nuisance warnings!!

[Gulfstreamaviator]

There is a mode a response visible most times I land on 16 in Beirut, anout 8 mile finals, (max 12) and 2 miles off to the right.

I asumed it was a warship, but never actually seen it.

I know that some North Sea Platforms had active responders, but did not realise that we could see them too.

glf