Tecas warning how close

[magpienja]

Hi chaps listening to the scanner yesterday saturday, to an unnamed approach freq, there was a 737 being vectored to the ILS when I heard the pilot of the 737 call out in a quite exited voice, tecas tecas climb climb, just wondering how near do they have to be to each other to be in that position.

Regards Nick.

[Scott Voigt]

Hi Magpie;

If varies on the closing geometry as to exactly how close aircraft get before the TCAS goes off... But then quite often it also goes off when there is legal IFR and VFR separation, the machine just doesn't know the intent of the aircraft and alarms. It is also known to alert against ghost targets... We are having more and more of those over here.

regards

Scott

[Findo]

It is not so much a matter of distance but time before the invasion of the safe "bubble" of airspace around the aircraft. head on at cruise speed will trigger a lot further away than a slow close in descent at a few hundred feet a minute.

TCAS is pretty damned good and as far as I know very few spurious warnings here in the UK. :ok:

[APP Radar]

TCAS ( Traffic Colision Avoidance System ), as previously said, works on time and not on distance.

The TCAS considers a safety bubble around the aircraft and alerts :

a) when the bubble is likely to be 'intercepted' : TA - Traffic Advisory - "Trafic, Traffic"

b) when the bubble is broken : RA - Resolution Advisory - "Climb, Climb" or "Descend, Descend" or ...

Normally there's no RA without a previous TA, but sometimes it happens ...

There is no RA if the other traffic has no Mode C information ... sometimes TA are 'false' because traffic has no Mode C indication but is well bellow ...


Altough pilots sometimes ask for aditional information when receiving TA, only when receiving/acting based on an RA is mandatory to advise ATC.
Phraseology is "TCAS climb" or "TCAS descend".
When acting on a RA, ATC responsability of providing separation ceases.

TCAS minimum separation is smaller than ATC.

[alfie1999]

APP Radar,
Are you sure about a TA being generated by lack of Mode C? It was explained to me that only Mode S and Mode C are interrogated either directly or by polling. As I understand it an empty Mode C data block will generate a TA but if it's switched off completely then the aircraft is 'invisible' to TCAS.

I await shooting down!

[tobzalp]

I am fairly sure that nil mode C but still mode A wil generate a TA. Mebbe wrong as usual though.

[Chaos Controller]

Yes, App Radar is correct - mode A alone is enough to generate a TCAS TA, but it will never trigger a RA.
Having a few British Sea King helikopters stationed here at my airfield we hear a lot of that nowdays, as they all only have mode A on their transponder.
And of course, if the transponder is completely switched off, it will not show on any others TCAS - just imagine how that would have been flying into a larg airport with 50 aircrafts on the ground, all sending out warnings to the TCAS=)

[van der vart]

if one aircraft is climbing to, say, FL310 with one already at FL 320.
ATC passed traffic to both aircraft. there is a big chance that it will still trigger RA, am i right? if so will pilots follow RA knowing there will be 1000ft separation?

[LostThePicture]

there is a big chance that it will still trigger RA, am i right? if so will pilots follow RA knowing there will be 1000ft separation?

I would say it's likely to trigger a TA, but in my experience unlikely to generate an RA, if circumstances are as you suggest. If ATC have passed traffic to the pilot of the a/c climbing to FL310, then hopefully s/he will reduce the rate of climb until level (which s/he should be doing in the last 1000ft anyway).

On the rare occasions when an RA is triggered, then hopefully the pilot WILL follow it. The whole system is dependent on the pilot taking action if an RA is triggered. What if the a/c at FL320 suddenly starts to descend (who knows why) and the frequency is blocked?

Sure, if circumstances are as you suggest, then any RA is likely to be a nuisance alert. But pilots must always follow an RA. It's when pilots fail to follow the TCAS that real incidents occur.

LTP

[Lon More]

van der vart from personal experience, if the rate of climb/descent is great enough an RA will be generated and followed, sometimes causing a second RA with other traffic in busy airspace.
A frequent problem was with fighters crossing the airways climbing and then making an abrupt level off 1000ft below. Another is inbounds to EHAM who, when requested to cross EEL at FL 260, make a last minute dive. At the same time, fortunately seldom, traffic at FL250 crosses EEL to WSR an RA occurs leading to a potentially dangerous situation if other aircraft are in the vicinity, particularly so when pilots also initiate a turn.

So, especially if you have been advised of traffic 1000ft above or below your cleared flight level, start to reduce the vertical rate earlier; please announce your intentions when accepting an RA, and lastly don't turn - TCAS is not a substitute for ATC

[magpienja]

Thanks guys for you answers very informative.

Nick.

[APP Radar]

I've been away but read that the question was already replied ... Mode A only (no Mode C or Mode C u/s) causes TA but never RA.

About the question of traffic climbing to FL 310 with traffic at FL320 :

- TCAS doesn't consider the 1000 ft as separation, even 500 ft can be considered as enough ... TA and subsquent RA will only be made if there is a collision risk.
To avoid this kind of situations there is a rule stating that aircraft shoul make no more than x ft/min when less than y ft to be leveled and reduce to x2 ft/min when less than y2 ft to be leveled. This is a recommendation and each company has its own values for x and y ...

- same thing applies if traffic is at FL 310 and other is leaving FL 380 to 320 ...

- in my experience, I usually don't give traffic information in this cases, I do expect traffic to reduce rate of climb/descend when approaching cleared level to values that don't trigger any TA or RA. If, and only if, traffic reports TA I inform about the other traffic clearance.


One true story, sometime ago an escort was planned with 2 fighters to a head of state flight flying in a commercial airline but they forgot to inform the captain ... when, soon after departure, the fighters positioned left and right they had Mode A and the pilot reported the TA but as he was overflying a local known aerodrome he thought the TA was caused by local traffic flying well below ... suddenly the escort flights switched Mode C only ... two sucessive RAs and very disrurbed voice on frequency ... when informed by ATC about the escort the pilot looked left and right and saw them ...

[mad_jock]

The recent crash over Switzerland shows what happens when pilots obey ATC instead of TCAS.

If its fitted we must obey TCAS and after we have called TCAS climb/decend thats it. ATC just have to fillout the paper work of why the 2 planes got that close. And try and work out a solution what to do with all the hardware after it has completly cocked up their cunning plan.

TA are issued all the time in the UK thanks to most light aircraft being on tatical transponder A squawks so they can't be caught for level busts which is great joy at Fl200 +

From menory it is a timing thing and its something scary like 30secs before impact when you get the warning.

And the seperation again from memory is 300ft.

MJ

[tom775257]

I have been taught regarding warnings relevant to times to closest point of approach (CPA) the following:
TA (traffic advisory) given when intruder is 35-45 seconds from CPA.
RA (resolution advisory) is given if the intruder is within 20-30 seconds of CPA.
Tom.

[Eva San]

- TCAS doesn't consider the 1000 ft as separation, even 500 ft can be considered as enough ... TA and subsquent RA will only be made if there is a collision risk.

Tcas works on two different logics, the first one being time with the concept of the CPA and the second one being a safety bubble around the aircraft when the closing speed is low. As far as the vertical separation is concerned in the 2nd case it depends of the altitude of the aircraft, but it cannot go (at least to my knowledge) below 600 ft. If an aircraft ( with C mode of course) is flying 500 ft below or above a Tcas equipped plane, it will trigger a RA.

To avoid this kind of situations there is a rule stating that aircraft shoul make no more than x ft/min when less than y ft to be leveled and reduce to x2 ft/min when less than y2 ft to be leveled. This is a recommendation and each company has its own values for x and y ...

Yes, but the trouble starts when both a/c are evolving vertically, ie one climbing and the other one descending. Indeed the two rates add up and may create a RA. Where I work, we have that type of configuration with outbound traffic climbing to FL XX and inbound descending to FL XX+10, and I can tell you that I've had some RAs more than once, even with a good traffic information and even traffic in sight...

I have been taught regarding warnings relevant to times to closest point of approach (CPA) the following:TA (traffic advisory) given when intruder is 35-45 seconds from CPA.
RA (resolution advisory) is given if the intruder is within 20-30 seconds of CPA.

Which means 20-30 seconds of a possible collision ! Not much time isn't it ?

The last point I'd like to add about the Tcas is that the information is renewed every second . Compare that to the speed of you radar and think of it !

If you want some more precise info :

http://minilien.com/?AFDkYWEA7k