PDA

View Full Version : TCAS Question

eeper
12th Jun 2002, 01:04
I was flying with a captain recently and we discussed an incident in which another aircraft had been vectored to pass 500' over the top of his aircraft. The incident occurred in the US and the other aircraft was operating VFR. Aside from the legality of this clearance, what confused us was whether this would generate a TCAS RA given that both aircraft would pass each other flying straight and level.

Does TCAS only give RA's if there is an actual impending collision or does it give RAs if the clearance between you and the conflicting aircraft drops below a certain figure? If so, what is that minimum clearance?

SimJock
12th Jun 2002, 02:04
TCAS has a range of sensitivity bands that are switched in at different altitudes. It all depends what altitude you were at as to whether you would get an RA or not.

Typically,for 2 aircraft flying level below 5000ft, seperation of 300ft or less will generate an RA, anything greater and a traffic only warning will be given. Between 5000ft and 10000ft 350ft is min seperation rising to 600ft at 20,000ft to 42,000ft.

All this assumes level flight of both aircraft, if either starts climbing or descending its a whole new ballgame!

SimJock
12th Jun 2002, 02:15
I should say that the figures given are those that would generate a corrective RA (Climb/Descend), there are some other seperation figures typically within 600ft that would generate preventative RA's (Don't climb/descend monitor vertical speed) to which no positive pilot avoiding action is required. Traffic advisories would occur within 850ft seperation.

:)

411A
12th Jun 2002, 03:22
Interesting.
Here I am in my trusy aeromachine at 16,500 feet (VFR) west bound, flying straight and level, passing over AA proceeding to PHX at 16,000, and he is having a fit...because the airspace belongs to him (so he thinks) even tho both of us are talking to PHX approach.
Needless to say, AA was told..."sorry, but he is at the proper altitude, call us on the telephone after you land."
Really...some of the airline guys think that the sky belongs to them, but it certainly does NOT, at least here in the USA.

Young Paul
12th Jun 2002, 09:46
I've had a similar thing (500' separation from a VFR aircraft) in German airspace.

It always intrigued me that I could apparently be under radar control below 10000' in German airspace when there were uncontrolled VFR aircraft also below 10000'. What was I supposed to do if one tried to hit me? Ask for permission to deviate from present track?!

[Before somebody says something, don't worry, I wouldn't really do that.]

None
12th Jun 2002, 22:33
"guys think that the sky belongs to them"

411A, allow me to offer another perspective. When flying within 500 feet vertically of another aircraft who is VFR and may even be of an unknown type, it is possible to lose the normal level of comfort. There may be a question as to how predictable his flight path is...after all, he is VFR and may change altitude and/or course at any time. In this case 500 feet is too close. For this reason one might prefer a vector around such traffic. You may interpret this as operating at the highest level of safety, or staying within one's comfort zone, or something otherwise. In any event, it is another point of view.

411A wrote:
Interesting.
Here I am in my trusy aeromachine at 16,500 feet (VFR) west bound, flying straight and level, passing over AA proceeding to PHX at 16,000, and he is having a fit...because the airspace belongs to him (so he thinks) even tho both of us are talking to PHX approach.
Needless to say, AA was told..."sorry, but he is at the proper altitude, call us on the telephone after you land."
Really...some of the airline guys think that the sky belongs to them, but it certainly does NOT, at least here in the USA.

411A
13th Jun 2002, 04:31
Would certainly agree with you none, except that AA was advised of the traffic. He did not ask for a vector, just complained that there was another aircraft near him.
He had the option of course to take avoiding action if he received an RA, and he had every right to do so.
Unfortunately...as he was AA, he was a "sky God".:rolleyes:

AA76757
13th Jun 2002, 04:44
And you, 411a, while posessing a huge chip on your shoulder towards AA, but yet having no clue as to why a professional airline crew would look twice at 500' separation. Yes it may be the legal mininum, but no, it isn't a "normal" or "common" situation, so it does require a second look. It's called airmanship.

Ah, yes, it all makes perfect sense now :rolleyes:

ICT_SLB
13th Jun 2002, 05:26
Generally a Resolution Advisory will only be posted if there is a probability of collision i.e. if you're at different altitudes there must be some convergent vertical speed.

Don't forget there are two software versions out there. The older Change 6.04 expects at least 1000 ft seperation and thus is more likely to bleat if you break that (as in the original post). The newer Change 7 is designed for RVSM and 500 ft seperation (we had to get within 300 feet for an alert during some sim testing). The one thing to remember is that if you have the later software and get an alert - react immediately - you're definitely on a collision course if you don't!

This software is produced by Mitre for the FAA so it doesn't matter who builds your TCAS you'll have the same results.

411A
13th Jun 2002, 07:57
Actually AA76757, it is rather common (at least in the PHX, LAS and LAX areas) especially in the class B airspace.
The AA guys will just have to get used to it. :rolleyes:

Spitoon
14th Jun 2002, 21:46
I'm getting away from the original question but eeper mentions the legality of a clearance that puts a VFR aircraft 500ft vertically away form an IFR flight.

This all depends on the class of airspace. If I recall correctly, in ICAO terms in Class D or below, there is no requirement to separate VFR from IFR flights. The only obligation on the controller is to pass traffic information although here in the UK it is quite common for controllers to restrict VFR traffic so as to ensure that there's 500ft vertical between them and an IFR flight.

Specifically on the TCAS question, is it worth mentioning that you'll only get a RA if the other aircraft is squawking mode C - if it's mode A only, all you'll get is a TA.

There endeth my knowledge of TCAS!

411A
14th Jun 2002, 22:02
Yes, quite correct. However, in class B & C airspace (and in the 30NM veil around class B, mode 'C' is absolutely required in the USA.
Someone mentioned to me today that mode 'S' would be required in 2004/5 in class 'B' (USA), can anyone confirm? This may actually be a good idea as ADB-S works good, at least so far. Perhaps some tech types here can speculate/confirm.

Tcas climb
17th Jun 2002, 06:55
:D

AlfaMike
18th Jun 2002, 05:23
this are the requirements in our A/C that may trigger and R/A or a T/A, hope this helps. :cool:

Operating transponder for both A/C.
T/A @ 40 sec. from point of closest app.
R/A @ 25 sec. from point of closest app.
R/A req. at least mode C.
Coordinated R/A req. both TCAS equip.
Alt. and Vert. motion are included if A/C is mode S or C equip.
Prox. TFC advisory if within 6 nm. & 1200’ vert.
Vertical motion info is avail if rate > 500’/min.

:cool:

Capt Pit Bull
18th Jun 2002, 21:46
ICT_SLB

There are (obviously) differences between 6.04 and 7.0, but in neither type is a vertical rate of closure required in order to generate an RA.

Both versions have a "Positive RA Threshold" (for corrective RAs) and a "Preventative RA Threshold" (for preventative RAs). Both are assessed versus predicted vertical separation at predicted time of closest approach.

So in all cases you can get RAs, with both aircraft in level flight, and at a different altitude. It just has to be a difference of altitude of less than the threshold (which 500', in this example, would be.

A vertical rate of closure is not necessary. In fact, the aircraft could be vertically diverging and an RA could still be generated.

e.g. - high altitude. Preventative threshold 800', corrective say 700, RA alerting threshold 35 seconds (these figures from top of head but approx correct). Aircraft currently co altitude. A/C 1 in level flight. A/C 2 rate of climb 600 fpm. 35 seconds to closest point of approach - predicted vertical separation 350'. Result - Corrective RAs for both aircraft (if equipped).

Spitoon

Sorry to pick hairs, but thats 'altitude reporting' rather than 'Mode C'.

TCAS can not see Mode A only (it doesn't send mode A interrogations), but what people refer to as Mode A only transponders are actually Mode A and C without altitude reporting.

CPB

Denti
18th Jun 2002, 22:42
According to Honeywell (http://www.honeywelltcas.com/faq/whitepapers/tcasoperationaldescription.html) TCAS can see Mode A-answers. Just read the article.

Denti

Capt Pit Bull
19th Jun 2002, 17:55
Denti,

it says 'Mode A and C transponders'.

Note this means:

(Mode A and C) transponders,

as opposed to:

Mode A transponders AND Mode C transponders,

or:

Mode A transponders OR Mode C transponders.

When we say Mode A and C transponders we are talking plural examples of ONE type of tranponder i.e. 'A and C', as opposed to plural types of tranponders.

There is no such thing (anymore) as a solely Mode A transponder, or a solely Mode C transponder, only Mode "A and C" (or "S").

The distinction is one of altitude reporting or not.

If your transponder does not have altitude encoding, (or altitude reporting is turned off) it will still reply to mode C interrogations, just with an empty pulse frame.

For a fuller explanation see this thread from a few months back.