TCAS reliability

[Matt-YSBK]

TCAS Reliability

I was recently on a flight from a regional airport back home when on departure from the airport and climbing through the E Class the centre controller was nice enough to pass us traffic (we were vfr) on a Jet * flight that was inbound to our departure point. I quickly located the Jet * flight that was on a heading roughly parallel and opposite to us around the same level and about 5 – 8 miles (by my poor estimate) away. The pilot of the jet * aircraft queried us as to if we were transponder equipped and squrking mode charley, Which we were. The centre controller also indicated that she had us on primary and secondary radar and we had also verified the accuracy of our encoders output. It seemed obvious that the RPT’s TCAS had not detected us yet I clearly had it in sight. I thought these things were to be better then our eyes. But I guess not. Could it have been as we were on climb and the other aircraft on decent they were above us for most of the flight and the transponder antenna being on the bottom of our aircraft it was shielded from the apposing TCAS. Is it usual to not detect aircraft on TCAS ? Any input from anyone more experienced than I would be great.

Thanks.

[Chimbu chuckles]

Depends on the TCAS fitted as to the 'range' they have above and below the aircraft altitude...the version fitted to the 767300ER I spend my working days in won't pick up aircraft until within 2000' vertically....Airbus ones are better.

[Cougar]

At 5-8 miles away, heading towards each other at co-altitude, i would assume that they would definitely have you on TCAS if it were working properly. TCAS systems are known to have faults and failures every now, but 8 miles should be in all TCAS systems envelopes co-altitude.

[FlareArmed]

Some TCAS systems have selectable vertical range. The normal range of most systems is about 2500' above and below. Some systems have a selection which expands the vertical range to about 10,000' above and below.

BTW. TCAS threat analysis and resolution is determined purely on range and relative altitude. The bearing is only for pilot display and has no role in collision avoidance as far as the hardware is concerned. I have witnessed quite a lag between the relative bearing of a visual sighting and that displayed on the screen, particularly with high angular velocity.

Have a look in your TCAS manual or in the simulator at a 'no-bearing' RA. You will see no reduction in performance, just different information on the situation display.

[Capn Bloggs]

Matt,

You said:
the RPT’s TCAS had not detected us yet I clearly had it in sight. I thought these things were to be better then our eyes. But I guess not.

That is exactly why I, and many others, have been saying to the Smith family all along: TCAS IS NOT INVINCIBLE and must not be used as a primary means of defence against collisions, as it is currently made out to be. If you had been at Mount Isa (no radar)and not seen the B117 coming straight at you instead of being laterally displaced, what would have happened?

I have on many occasions had the TCAS not pick up other aircraft. Smith doesn't understand that. All very good in theory, but in the real world, which he doesn't have a lot of experience in, things are not as rosey as the glossy brouchures.

[itchybum]

Even the Honeywell K-mart version TCASII unit predicts issues warnings on traffic out to 15-35 seconds and cautions on traffic 20-48 seconds from the collision area (from the manual). This equates to more than 5-8 miles and most sets are better than this.

Switch-selectable limits allow display of traffic from "2700ft above or below to 9900ft ".

Not teaching anyone to suck eggs but if the J* was more than a couple of thousand feet below you, out at "5-8" miles he would appear to be about the same level. Maybe you were just outside his vertical range.

[newbe200]

Gday all,

I regularly am not picked up by TCAS by both the dash and 146. Not sure what types they have, and would have to check the books to see how they work, but this would happen maybe a third of the time.

Now again, I dont use one, but I just expected that they were used as a secondary means of identification. However, instead of looking out the window I keep getting asked in a derogitory manner "Have you got your transponder turned on" to which I reply "YES, I am visual with you, in your two o'clock level about three miles blah blah".

Anyway, they dont appear to be the be-all end-all.

Cheers
Nb20:ok:

[itchybum]

They'd love that, especially the poorer-cousin DASH guys!

Well-restrained reply there. :ok:

[DirtyPierre]

I was told by a Dash 8 pilot that TCAS fitted to their aircraft are limited in range, altitude, and angle of detection.

What this means is that there are "blind spots" in the TCAS for the aircraft. So at certain angles of interception, vertical or horizontal, TCAS may not see the aircraft in conflict.

Is this correct?

[lame1]

Would any of you have put the defect in the techlog.
Id like to hear your feedback

[Agent86]

Newbe200,
Not having a go at you but how long is it since your AC has been in Radar and had a positive check of your transponder?

Try swapping positions, with you being the big fat target trying to find the small target.

It is very helpful when the smaller Ac provides info as to relative range/bearing to assist the eyes.

It is NOT very helpful to hear "have you visual...well clear":hmm:

It is amazing the amount of times there is TCAS traffic (in CTA mind you :} ) within 5 miles that you just cannot see when the viz is still >10k or there is patchy cloud around.

Oh well ..can't see him...must have avoided him :p

[Knackers]

I step-climbed a B737 beneath the Flying Doc some months ago. The B737 maintained FL130 for a short time, 1000' directly beneath the PC12. The Flying Doc never appeared on the TCAS until after the aircraft had passed.

[89 steps to heaven]

We had a case last week, where a departing B717 was given details of class E traffic at A095. After departure, I updated the position and received the response,

"We are not showing the traffic on TCAS"

The traffic was in the jets 11 0'clock at 4 miles and was showing on the tower situation display. The 717 pilot elected to maintain a level below the traffic. As were passing the traffic, one thousand feet below the class E aircraft, TCAS finally picked up the traffic.

A similar situation occurred at Rockhampton the previous week.

These are a good reminders that TCAS is not infallible, only another tool

[Vref+5]

Just to expand on an earlier point, TCAS does not just use range data, but range /rate (Tau concept). This is basically the slant range divided by the closure rate. This is then used to calculate the protected volume of airspace around the aircraft, unique to each encounter. It also uses an altitude test to see if the intruder aircraft will be within the defined altitude limits within this airspace. A TA or RA will then be issued if it is calculated the intruder will enter the airspace within the Tau time thresholds.

Sensitivity Level (SL)and Distance Modification (DMOD). Thes are used to reduce nuisance alerts, and to provide protection at low closure rates. The SL is a fucntion of aircraft altitude and changes the times which trigger the alerts. For example at 2000' an RA may be issued 15 secs prior to the intruder entering, but at 20 000' this may be as great as 35 secs. The DMOD prevents aircraft getting too close at low closure rates (600-800' depending upon aircraft altitude again). This is why, I think some of the lighties think they can fly as close to a jet at altitude as they would in the circuit, because they aren't aware that these warning times are increased so much. These numbers are generic and may be modified to suit particular types of aircraft.

Version 7, which was released in 1999, included a few changes, including making TCAS RVSM compliant i.e It doesn't issue RAs for 2 aircraft mainitaining their levels 1000' apart, but it will highlight them as proximate traffic. NOTE I DON"T THINK IT IS VFR CRUISING LEVEL COMPLIANT. If a VFR aircraft maintaining it's level crosses 500' beneath an IFR aircraft, the TCAS equipped aircraft will get an RA, even though no breakdown of separation occured. Not 100% on this one, but reasonably confident.

Hope this helps. If anyone else has more info on the VFR level thingie feel free to clarify it.

I think the main problems with the TCAS are related to the antenna. The bearing limitations were due to not having a rotating antenna (like an ATC radar). This has been improved by building a bunch of little antennas into the main one, hence gaining some directional information. I'm not sure if the Whisper/shout modes have something to do with missing traffic in close either, it may just be antenna location. Is it possible to get interference from propellors?

[TAY 611]

Following numerous other aircraft across the North Atlantic I have managed to sweep the WX radar down and pick up positive returns on some of these aircraft that we also had visually. On our aircraft the TCAS tags are presented on the same Nav Display as the WX radar. It is quite interesting to see the radar target and the TCAS tag diverging by quite a lot mainly in the lateral sense. Guess thats why we don't use TCAS to separate ourselves uless its an RA and then it is only in a vertical manouvre. :}

[Uncommon Sense]

Vref+5,

Interested in your comments regarding VFR level compliance of the TCAS.

Around major airports the steps are VFR bases - i.e. 3500 is 'G 'OCTA , and 4000 is 'C' CTA - usually with inbound TCAS equipped aircraft 'sitting' on the step waiting for the next step, or rapidly descending towards it. I have probably only seen 1/2 dozen 'RA's' in that situation, normally because of excessive descent rates (excessive by the CPA calculation in the TCAS that is), but have never even had a query from the inbound aircraft when overflying with 500' level between and no ROD or ROC - not even a query on possible TA or PROXIMATE traffic.

Also, some of the equipment hanging around below the steps now is relatively high perofrmance in terms of achievable ROC - e.g. LNC4, SR20, Warbirds etc.

edited for formatting W :)

[newbe200]

Well, in radar every day and the ssr has been serviced(last 2 months). never normally have problems with other aircraft.

I normally do give distance bearing reports to the other aircraft and i do understand about the big plane/small plane problem, how it is easier to see a 146 or dash than a kingair, but in my personal experience, these guys cruising around with TCAS seem to be the ones pushing the reliance on TCAS. how many bloody times do i ask for distance bearing ref airport and get the reply "we have you on TCAS!!!!!!" well doesn't that just help me with my planning.

It may have been a joke from you, not sure, but saying "Oh well...cant see him...must have avoided him" frightens the crap out of me. I MAKE SURE EVERY TIME that I am clear, even if it annoys the big boys to keep having to answer my questions.

This is also not directed at NJS crew. Again i have had problems with mainly 146 and DASH planes, not particular companies.

Ok, all out now
cheers nb20:ok:

[TAY 611]

Interesting thread. If you do nothing other than switch on your TCAS (or ACAS) then it will do it's intended job and that is issue you with an RA by either presenting you with a Vertical speed command or, in some cases, a pitch command along with an aural command to avoid the conflict in a vertical sense. However with smart use of range controls and by scanning up or below we as pilots (good SOP's help), along with built in features like the transition from other traffic to proximate traffic to TA we can enhance out situational awareness and have a fairly good idea that a possible conflict is coming our way. If VMC we are pointed to the proximity of traffic so that we may be able to sight it. However TCAS is not designed for normal traffic separation in the way GPWS is not designed for normal terrain clearance though, by some of the radio calls I observe, I believe there is a tendancy to get mislead into doing so. I personally had an RA in the LAX area at night in IMC, a light aircraft busted altitude, it was the TA that certainly drew our initial attention out of the myriad of contacts that were displayed. After going through the drama of the following RA, proving the system worked well, an unflapped controller only made minor adjustments to the traffic sequence.

[Vref+5]

Apologies to all, I made a mistake in the previous post.
The DMOD distances are lateral distances, not vertical. There are some vertical thresholds (around 800' depending on aircraft altitude again), but I am unsure if they are triggered with no rates of climb/descent. I will do some more research.

TCAS requires an ICAO approved transponder operating mode 3A. It also requires mode C for RAs, otherwise you will only get TAs. Perhaps that is why the warbirds don't give warnings?

[404 Titan]

I’ve lost count of the times I have got a TCAS return, sighted the aircraft visually to then have it disappear from my screen. Yes the system is very good but it isn’t infallible. I have gone through all the manuals and asked our engineering, FOP’s and technical departments if they can answer why this happens. None have been able to except to say that it is probably antenna geometry between the aircraft.

[ITCZ]

Some things to remember about TCAS II (which is what is fitted in most of the TCAS equipped aeroplanes).

a.k.a. TCAS urban myths.

1 "the TCAS tracks targets within +-2700'"

What the TCAS displays and what your TCAS is tracking are two different things. Selecting 'above' or 'below' is only to reduce clutter on the pilot's TCAS display.

A TCAS II unit will track targets +-9,200 above, level with and below you at several times the slant distance compared to the 6nm or 12nm user selectable display range, regardless of the pilots' display selections.

2. "ATC can see my SSR so his TCAS should see it too."

Not always. My TCAS relies on your TXDR squawking in response when my TCAS interrogates it, not in response to a ground based SSR interrogation. So the fact that a very powerful ground based transmitter elicits a response from your TXDR, does not guarantee that my TCAS will see you too. It SHOULD :* but it ain't always so!

Remember, the TCAS equipment is just that -- electronic gadgetry. Very clever and ingenious electronic gadgetry, but an electronic gadget designed by humans.

Using a GPS without being aware of its function and limitations, and without cross checking, is lazy and unprofessional. Using an altimeter without being aware of its basic function and limitations, is also lazy, and unprofessional. And when added to the mix on a bad day, can be fatal.

Same with TCAS. A very clever aid useful in avoiding collisions, yes. Use it in blind faith, then maybe you will hit someone!

There are a couple of excellent topics on TCAS in the current Flight Safety Aus mag --- have a read!

[Matt-YSBK]

I think i understand TCAS a little better know. As a follow up question If the TCAS equipped aircraft queries an aircrafts transponder and the transponder responds also the aircraft is responding to the SSR poll what are the limits to how many TCAS/SSR response a transponder can give in say a minute. Is this fixed or say do older transponders have performance less then newer ones.

[ITCZ]

Here is some basic information from the Honeywell site (http://www.honeywelltcas.com/support_pages/whitepapers/tcasoperationaldescription.pdf) site about ACAS/TCAS systems. There is lots of useful stuff inside.

ACAS/TCAS

AIRBORNE/TRAFFIC COLLISION AVOIDANCE SYSTEM

Understanding ACAS/TCAS II

{snip by ITCZ}

TCAS LIMITATIONS

A flight crew may visually identify an intruder that is not displayed in the cockpit or which disappears from the display even though the system is functioning properly.

Certain situations exist in which TCAS cannot provide traffic awareness, target display, or collision
protection, but these limitations are the result of the intruder aircraft not carrying functioning, altitude-reporting transponders.

In other cases, TCAS is limited by system design parameters, equipment installation configuration, and control panel settings, causing an intruder to be absent or disappearing (dropout) from the display. These will be explained in the following paragraphs.

{snip by ITCZ}

ATCRBS and Mode S Transponders will NOT reply if they do NOT hear a TCAS interrogating signal. A transponder may not hear a TCAS interrogation signal for any one of the following reasons: the signal may be blocked, too weak, or too far off center frequency; the intruder aircraft may have only a lower antenna and be shielded from TCAS interrogations from above; or the transponder may have poor receiver sensitivity. If transponders don't reply, TCAS cannot track these intruders.

Hands up all Ppruner's that think the wattage of the airborne TCAS equipment is equal to that emitted by the ATC SSR interrogator...?

Hands up those Ppruner's that can identify TCAS/SSR antennae mounted on upper and lower surfaces of their aircraft?

Hmm!

When TCAS is Limited

TCAS does not display all traffic. Even with the cockpit controls set properly, sometimes intruders will not appear. However, pilots should not be concerned. Even though the following list of items are limitations of the system, TCAS is designed to protect the aircraft. These conditions do not affect the collision avoidance function. You will receive TAs (traffic advisories) and RAs (resolution advisories) if near mid-air geometry is predicted in any of the following conditions.......

{snip by ITCZ}


"the TCAS does not display all traffic"

Remember that phrase - its from the manufacturer!

It was not designed to help a pilot "see and avoid" traffic.

That is why it does not say "Traffic! Three o'clock, below!"

It was designed to protect the aircraft.

Very different.

Use this equipment, like everything else in an aircraft, within its design limitations.