TCAS question

[malanda]

The other day I was on a United Airlines flight out of ATL. I was listening to the "from the cockpit" channel (which almost makes up for the awful UA in-flight service). Anyway, a couple of minutes after takeoff, I heard the pilot of another a/c saying he'd just had a TCAS alert. ATC asked whether it was a TA or RA (which I understood - it was a TA, incidentally). There was then some discussion between the pilot & ATC regarding what the conflicting traffic was. The bit I didn't understand was something about "a category C transponder" (I may have got that wrong), and a comment "he could be at 500 feet, he could be at 15,500".

Explanations please?

[greatorex]

In as quick (my wife is telling me that dinner is ready :eek: ) and as simple terms as possible, a Category C (or Mode C/Charlie) transponder is basically a transponder that also broadcasts the altitude of the aircraft together with its position. If the other aircraft was not 'squawking' Mode C, they would not know its altitude and that would explain the captain's comment of "he could be at 500 feet, he could be at 15,500".

My colleagues will no doubt be able to elaborate further without their dinner 'being in the dog'!!! :D ;)

[RadarContact]

There are different transponder modes:

A - which generates a single "here I am" response to a secondary radar or TCAS query

C - which additionally sends the encoded altitude of the aircraft

S - which TCAS systems use to communicate with each other

A target operating on mode A only is hard to judge, since you don't know which altitude it's flying at. TCAS will only generate a TA when coming close, but in the worst case, the target might be flying just at your level.

When also operating mode C, the target will be identified as flying for example far lower than you and therefore not being a threat. No TA will be generated in this case.

[malanda]

Thanks guys. Makes sense.

So a mode A target would always generate a TA regardless of altitude? And you'd never get an RA? Is that right?

[Captain Stable]

Correct. :)

[NorthernSky]

Sorry Stable, but the answer is 'Incorrect'.

You need to remember antenna geometry. An aircraft at low altitude will not be detected by another at high altitude, because the transponder antenna systems are not intended to (and do not) offer extensive coverage in terms of elevation, or, traffic at significantly different altitudes, close in. Witness flying over London at FL200+ and getting no TAs at all on light aircraft around the zone.

TCAS will NEVER give an RA on a Mode A target, and will sometimes give a TA provided that the target aircraft is at a relatively similar level to the TCAS-equipped aircraft, bearing in mind its range and range by range rate.

A recent incident does however point out why we should be cautious of TAs on non-mode C targets. Basically, as I understand it, a light twin departed Cranfield for Amsterdam, failed to establish two-way contact with London Info, and so climbed to FL70 on direct track to SPL. Traffic around Stansted got TAs on the aircraft, and there is a danger that pilots in this situation think 'it's at low level'. 'It' may not be.