Tcas Ta

[nikolic737]

Yesterday we had wery strange situation,during approach we had indication of unknown traffic on TCAS.TCAS gave us TA but we didn't have informatin about altitude of intruder.
What should be done in situation like this.(We were in IMC conditions)

[Elliot Moose]

The intruder likely didn't have mode "C" engaged. We used to get it all the time up north where it wasn't mandatory to even have a transponder let alone mode C.

In response to your question, all you can do is keep an eye on him, look out the window and ask for ATC help. Under radar control, you cant respond to a "TA only" without a clearance. Evasive action (lateral) may only make things worse anyways, as it is very hard to determine the intruder's course and obviously vertical evasion might have you go the wrong way with no alt readout on the target.

[Charles Darwin]

It is very common to have TA when flying over a place where small aircraft are, without C mode. Of course your TCAS does not know the altitude of the aircraft in question so it passes out an advisory. Of course it needs the other altitude to figure out any RA. So, not so strange at all, but I would think that APP control should have warned you about the other traffic, since you were IMC.

The intruder likely didn't have mode "C" engaged. We used to get it all the time up north where it wasn't mandatory to even have a transponder let alone mode C.

How on earth did you get TA if the other one didn´t have transponder?:confused:

[LGB]

Usually the TAs like these are from low flying aircraft with mode A transponders. Since TCAS measures closing rate, the TA will go away when getting closer, as it only measures the relative speed of the intruder. The greater the difference in altitude, the earlier it goes away because the actual closing rate decreases. You could see the same effect when passing a DME station at altitude, where the DME ground speed decreases a minute or two before the station, depending on your altitude and speed.

As TCAS RAs are based on mode C readouts, mode A equipped threats can never trigger RAs (as it would not know whether to climb or descend when target altitude is unknown). If the TA continues until the white circle on your TA/RA display gets very close to you, then it is time to start worring (albeit a bit late then).

Lookout and ATC help (when avbl) is key here, if IMC and no radar coverage then ... turn the TCAS off :uhoh:

Some older TCAS versions also trigger TAs when in RVSM airspace, as old TCAS assumes 2000 feet separation above FL280. If you are FL340 and opposite traffic is FL350, you could get a TA if using older TCAS software.

[breid]

Since TCAS measures closing rate, the TA will go away when getting closer, as it only measures the relative speed of the intruder.



So what happens if I am a mile or two behind an aircraft at the same altitude as me(lets say mode A or mode C) with zero closure rate. Are you saying the target will disappear?

[LGB]

According to the TCAS user manuals it should be correct, but maybe the system does have distance calculations included in the algorithm. Would someone care to elaborate on this? (good question!)

TCAS warns you at a distance further away when you are head on, as opposed to when you are catching up with someone. I think distance is also considered, otherwise you could get quite close if you are catching up to someone very close to your own speed, but TCAS manuals always speak of time.

From the PDF document http://www.honeywelltcas.com/products_pages/cas81_6mcu/cas81_pilotguide.pdf :

The TCAS II system monitors the airspace sur rounding
your aircraft by interrogating the transponder of
intruding aircraft.The interrogation reply enables
TCAS II to compute the following information about
the intruder:
1) Range between your aircraft and the intruder.
2) Relative bearing to the intruder.
3) Altitude and vertical speed of the intruder, if
reporting altitude.
4) Closing rate between the intruder and your
aircraft.
Using this data TCAS II predicts the time to,and the
separation at,the intruders closest point of approach
(CPA).Should TCAS II predict that certain safe
boundaries may be violated,it will issue a Traffic
Advisory (TA) to alert the crew that closing traffic is
in the vicinity.
If the intruder continues to close,TCAS II will issue a
Resolution Advisory (RA) to obtain or maintain safe
vertical separation between your aircraft and the
intruder.TCAS II bases the alarms on a five second
crew reaction time to begin the separation maneuver.
Increase or reversal of an RA requires a reaction in
two and one half seconds

...

A symbol change to a filled yellow circle indicates
that the intruding aircraft is considered to be potentially
hazardous.Depending on your altitude TCAS II
will display a TA when the time to CPA is between 20
and 48 seconds.

...

Under normal conditions a TA will precede an RA by
10 to 15 seconds.

[Touch'n'oops]

TCAS works on time to impact or near-miss.

I believe the Traffic Advisory is given 45 to 20 seconds before impact and the Resolution Advisory follows 10-15 seconds after(RA is only given if BOTH are aircraft are S-mode equipped).

No transponder:
Aircraft can only be tracked by primary radar. Clouds and ground clutter hinder tracking of aircraft.

Mode A:
Aircraft tracked by use of secondary radar. Atc may indentify aircraft by an ATC issued code (Squawk). No RA provided

Mode C:
Aircraft tracked by use of secondary radar. Atc receives the aircraft's pressure altitude information (1013mb/2992in.) and may indentify aircraft by an ATC issued code. No RA provided.

Mode S:
Aircraft tracked by use of secondary radar. Atc receives the aircraft's pressure altitude (1013mb/2992in.) information and may indentify aircraft by an ATC issued code. Also the each aircraft has a unique code and also transmits information such as aircraft type and current performance.
RA provide if all aircraft in conflict are mode S equipped (able to handle a Max. of three aircraft in conflict at one point in time)

[LGB]

RAs will be given also for targets with Mode C only, only will they not be coordinated.

If you have TCAS II, you will have mode S yourself.

See also http://www.pprune.org/forums/showthread.php?s=&threadid=59319&perpage=15&highlight=TCAS&pagenumber=2 :



TCAS I: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and "Traffic Advisory" (TA) for nearby targets.
This TA is used to help pilots visually locate nearby
co-altitude traffic (Mode C) or unknown altitude traffic
(Mode A).

TCAS II: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and a TA for nearby targets.

For target aircraft with Mode C or S transponders, the TCAS
display can generate a "Resolution Advisory" (RA), which
commanded vertical manuever (climb/descent) to avoid nearby
co-altitude traffic.

For target aircraft with Mode S transponders *AND* TCAS II
equipment, RAs will be coordinated between aircraft
(e.g. the two TCAS processors will cooperatively agree to
send one aircraft in a climb and the other in a descent.)

Note: aircraft equipped with TCAS II must have Mode S
transponders installed.

[nats]

LGB,
Could you expand on 'if IMC and no radar coverage then ... turn the TCAS off '.Why would anybody switch off a safety function such as TCAS?,aren't you just compounding the problem against other suitably equipped aircraft that may be in your vicinity.

[LGB]

Hi nats,

It was meant as a joke ... like the procedure for single engine aircraft engine failure at night:

1. Landing imminent: Turn on landing light

2. If proper landing field is in sight, land

3. If not, turn off landing light again

(because it doesn't matter anyway!)

An instructor of mine on PPL/CPL tried this, although he didn't turn the light off, but hit a three and cut of one wing of the PA28 he was flying. Unfortunately, it was the right wing, and the aircraft ended up on that side, blocking the door. The fuel from the left wing started running from the wing down over the cabin and the cowling. Fortunately, they managed to punch out the big rear window and get out that way.