View Full Version : TCAS question

13th Nov 2006, 15:08
Please go easy on me ... I'm only a PPL - and have been googling for information on this but it doesn't seem very clear cut from what I have been able to find.

If I'm flying along squawking anything mode alpha, then this will cause my aircraft to show on TCAS displays of things flying overhead, yes? Do they receive a TA (assuming I'm within the envelope for that) from me given that their TCAS has no idea how high I am?

If I'm now squawk mode charlie, then I assume that I can at least trigger a TA as the aircraft with TCAS knows where I am. Can I trigger an RA by just having a mode C transponder turned on - I guess the answer to this depends on whether or not TCAS relies on another TCAS unit (and there is some communication between them) or if the mere presence of another transponder out there is enough for the TCAS equipped aircraft to issue an RA to its crew.

My reason for asking this is mainly as a result of a request to squawk mode A only during a recent transit of a nearby busy class D - I can't really understand why he may have wanted this, as surely this removes all of the protection TCAS could give another aircraft if I stray from my assigned altitude - and he ends up not knowing that I've done so too.

(The above is not in any way meant to be a criticism of a controller ... if if helps them move the traffic, and they want me to orbit whilst singing to the frequency, then thats OK with me :} )

Thanks in advance,


13th Nov 2006, 15:32
Almost correct.

A: No altitude info. You'd be seen on TCAS without any information and correct, not TA or RA even when you would eventually hit each other.

C (no TCAS): Altitude provided. Other aircraft's TCAS will indicate traffic, proximity traffic, and later on issue TA and RA if necessary as well. The TCAS equipped aircraft will be steered away from you. If you happen to apply your own evasive action - and it is your responsibility to do so - into the manouvering traffic you'll get close, but the TCAS on the other plane should be able to provide further corrective commands.

C (TCAS on): Further benefit is, that if both boxes issue RA (and that is normally the case) their commands-advisories will have had their sense co-ordinated. One up, the other one down.

(the un-real)

13th Nov 2006, 15:47
Your understanding pretty much correct

Mode A will not give an RA

Mode C will give a TA and RA

As one of the major factors is the closure rate of the aircrat, Ive experienced a few nusience RAs inside controlled airspace. This has usually been where one is climbing and one is descending and plan to level off 1000ft apart.

Ive also had the thing screaming its guts out at me whilst flying a visual circuit at an airfield outside of controlled airspace. (Nice to know it works!)

I recon whats probably happened is that a controller has anticipated that your intended track will result in a nusience RA and, having been previously able to confirm your level from the mode C has asked you to turn it off in order to offer you both the most expedient handling.... Id be curious to know if you were given any traffic information relating to any larger aircraft (that may have been working another frequency)?

Lastly Id like to thank you for your professionalism for using in Mode C in the first place, Im aware some hold the view that its a bit 'big brother' but it really does add lots to our situational awareness.

13th Nov 2006, 16:14
I agree, please continue to use Mode C whenever possible, in Class G airspace. Even in the circuit if you fly inside an ATZ, if it doesn't conflict with an ATC request of course.

It helps transitting TCAS equipped GA enormously - we can adjust our flight path well in advance to avoid you (that is what it was designed for) and it is safer for all.


13th Nov 2006, 16:42
I recon whats probably happened is that a controller has anticipated that your intended track will result in a nusience RA and, having been previously able to confirm your level from the mode C has asked you to turn it off in order to offer you both the most expedient handling....

Can ATC folks provide some insight on what they're allowed to request? Somehow, I just can't see their OPS Manual, or whatever, suggesting or recommending or allowing controllers to instruct pilots to turn off mode C unless malfunctionning. Although nothing against advising of possible TCAS events likely, I presume.

Having flown in NY area, ifr, I've got preventive RA's for vfr traffic 500 feet off my altitude. I just can't think that ATC would tell me or the other aircraft to turn of mode C in this case. I'd want it on!!

13th Nov 2006, 17:24
Thanks to all of you for your swift replies. Its been very informative, as always.

Am I right then in thinking that if both aircraft are TCAS equipped, then they do communicate with each other to ensure that the resolution is correct (the obvious failure mode if they don't being that one of them has incorrect altitude information)?

A bit more information - we were all on the same frequency, and there was other traffic and we were all being passed information about each other.

One was far enough away and climbing out so I doubt that he could have had a nuisance RA (and he said he had us on TCAS, and we were visual with him); but there was inbound traffic descending above me and as I was crossing the extended centreline that could well have been the potential for an unwanted RA. I suspect that being told to 'Climb Climb' when trying to descend on the ILS makes for a few curses at the technology - do you have to react to that RA if you are VMC and have the "conflicting" traffic sighted? What if you are IMC in this instance?

Its a zone I regularly transit but this has been the first time I'd been asked to squawk mode A only - I guessed the reasoning was to stop nuisance TCAS alerts, but then I'm surprised I'd not been asked this before.

Thanks for the kind words re: using the mode C - I am genuinely alarmed by the attitude of a lot of GA pilots who take the view that if they think they have infringed airspace, the correct reaction is to turn off the transponder and try to hide. I learned to fly in Australia and the attitude there was "if you think you're in the middle of somewhere you shouldn't be, make sure you are squawking mode C and contact them ASAP!" which may make for red faces and slightly stern phone calls but is unlikely to result in a mid-air.

In general, I'm happy to have mode C on all the time - in fact, I'd be surprised that I'd be let into a busy class D without it. As I fly from a field directly below the said busy class D (I'm sure you can work out both the field and the CTA now!) we do keep transponders off in the circuit generally - to avoid nuisance TAs/RAs to the larger traffic departing our southerly neighbour as it turns downwind above us.

Again, thanks very much for taking the time to reply.


Piltdown Man
15th Nov 2006, 13:50
Answering on someone else's behalf - before an RA is issued, the TCAS units agree amongst themselves who will go up and who will go down. They will also agree to change their "collective" minds if insufficient action has been taken by either party. And you are right, the obvious error is duff altitude encoding. Hopefully this is checked (apart from engineering checks) when you cross-check your altimeters (we've normally got three) and whenever you check in with ATC.

Hope this helps.


15th Nov 2006, 14:03
P Staney

in answer to your query - you are correct.... ATCOs would not ask you to switch off mode C unless it was malfunctioning.

We get lots of nuisance RAs, even when we tell both sets of aircrew that they are climbig/descending to within 1000' and they slow their rate.

It's one of those things unfortunately, it would be nice if TCAS evolved so that it could interrogate the FMS of the intruder and if it then knew it was levelling off above or below, it would not give a warning (but it would have to continuously monitor the intentions, as a further climb/descent may have been given in error since it last checked!!)

All gets a bit complicated. The main thing to bear in mind is that pilots must comply with RAs then let ATC know when they are back under their control. Pilots should never second guess TCAS. And TCAS displays should never, ever be used as a pseudo radar by pilots who then may (it has been known) turn in the misbelief that they know what is going on!!

15th Nov 2006, 14:05
If you squawk C it is always better unless aked to not and when the A/C with TCAS installed is in collision course with you, his TCAS will know that you are not TCAS equiped so it will propose an escape manoeuver to the pilot.

For more info go www.honeywelltcas.com

Hope Im clear!:8

18th Nov 2006, 19:15
Thanks to everyone for their replies. It has been very informative and the Honeywell info is comprehensive...